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Selecting And Sizing Spring-AndDiaphragm Actuators And Related Accessories

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces

CONTENTS

PA

SPRING-AND-DIAPHRAGM SPRING-AND-DIAPHRAGM ACTUATOR FUNCTIONS................... FUNCTIONS.............................. ...................... ...................... .............. ... Actuator Functions......................... Functions..................................... ...................... ..................... ....................... ...................... ..................... ...................... .............. ... Position The Control Valve Closure Member ..................... ................................. ....................... ................. ...... Hold The Closure Member In The Desired Position .................... ............................... .................... ......... Provide Adequate Seat Load For Desired Shutoff ..................... ............................... ...................... ............ Provide Adequate Valve Stem Travel .................... ............................... ....................... ...................... ................... ......... Provide Sufficiently Rapid Stroking Times............................. Times........................................ ...................... .............. ... Provide A Fail Mode ..................... ............................... ...................... ....................... ....................... ...................... ..................... ........... Spring-And-Diaphragm Actuator Specifications ..................... ................................. ...................... ..................... ................ ..... Actuator Action: Direct or Reverse ..................... ............................... ...................... ....................... ....................... ............ Physical Size .................... .............................. ...................... ....................... ..................... ...................... ....................... ....................... .............. .. Operational Specifications............... Specifications......................... ...................... ....................... ..................... ...................... ...................... .......... Basic Sizing Concepts ..................... ............................... ...................... ....................... ....................... ...................... ..................... ...................... ........... Valve Forces................... Forces.............................. ...................... ...................... ...................... ...................... ...................... ....................... ................ .... Actuator Forces ..................... ................................. ....................... ..................... ...................... ....................... ..................... ................... ......... Spring Forces ..................... ............................... ...................... ....................... ....................... ...................... ..................... ...................... ........... Actuator Bench Set.................... Set............................... ...................... ...................... ....................... ...................... ..................... .............. ... Bench Set Span And Performance..................... Performance............................... ...................... ....................... ....................... ............ Effects of Valve Friction On Actuator Performance ...................... ................................. ...................... ................... ........

Band............................. Band.................. ....................... ...................... ..................... ....................... ...................... ..................... .................. ....... 15 Master your semesterDead with Scribd  Read Free Foron 30this Days Sign up to vote title  .. Effects of Dead Band On The Travel Of An Installed Control Control Valve ............ .......... & The New York TimesImportance Of Instrument Over Ranging  Useful  Not useful ..................... ............................... ...................... ....................... .............

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Effects Of Friction On Process Control .................... ............................... ....................... ...................... ............... .....

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces Overview Of The Detailed Actuator Sizing M ethod Of The Fisher Sizing Program ..................... ............................... ...................... ....................... ...................... ...................... ...................... ...................... ...................... ..................... .......... 23 Selecting The Detailed Sizing M ethod ..................... ................................. ....................... ..................... ............... ..... Calculation Screen..................... Screen................................. ...................... ..................... ....................... ...................... ..................... .............. ... 24 Migration Of Valve And Actuator Actuator Data From The Quick Sizing Method ..... Direct Entry Of Information ..................... ............................... ...................... ....................... ....................... ..................... ......... Unique Input Fields.......................... Fields...................................... ....................... ..................... ...................... ....................... ................. ...... 25 Calculated Results ..................... ................................. ...................... ..................... ....................... ...................... ..................... .............. ... 26 Bench Set Selection Guidelines..................... Guidelines................................. ....................... ..................... ...................... ................ .... Manual Sizing Option ..................... ............................... ...................... ....................... ...................... ...................... ................... ........ EVALUATING THE IMPACT IMPACT OF NEGATIVE FLUID FLUID FORCE GRADIENTS .................... ...................... Definition And Examples Of Fluid Force Gradients ...................... ................................. ...................... ................... ........ Definition Of Fluid Gradients ..................... ................................. ....................... ..................... ...................... .................. ...... Examples Of Fluid Gradients ..................... ............................... ...................... ....................... ...................... ................... ........ Preventing Valve Plug Instability Instability That Is Caused By Negative Gradients................... Gradients................... Valve Plug Stability Equation ..................... ................................. ....................... ..................... ...................... .................. ...... Determining Total Actuator Stiffness......................... Stiffness..................................... ...................... ..................... .............. ... Quantifying Negative Gradients .................... ............................... ....................... ...................... ...................... ................ .... Computer Assisted Selection..................... Selection................................. ....................... ..................... ...................... .................... ........ Relative Frequency Of Problems That Are Caused By Negative Gradients .................... ............................... ....................... ...................... ...................... ....................... ..................... ...................... .................. ...... 39 .................... ............................... ....................... ...................... ...................... ....................... ..................... ................. ....... 39 Master your semesterTroubleshooting with Scribd  Read Free Foron 30this Days Sign up to vote title SELECTING ACTUATOR ACCESSORIES TO PROVIDE THE REQUIRED  & The New York Times Useful Not useful   ACTUATOR STROKING TIMES ..................... ................................. ....................... ..................... ...................... ....................... ..................... ............... ..... 40


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Clarification Of Terminology Terminology And Application Requirements .................... ............................... ................ .....

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces Vent Cv ..................... ............................... ...................... ...................... ...................... ....................... ..................... ...................... .................... ........ 43 Performing Stroking Time Time Calculations With The Fisher Fisher Sizing Program .................. .................. Objective Of Calculations .................... ............................... ...................... ...................... ....................... ...................... ............. ... Overview Of The Calculation Procedure And The Stroking Time Calculation Screen..................... Screen................................. ...................... ..................... ....................... ...................... ..................... .............. ... 43 Actuator And Spring Entry Fields ..................... ............................... ...................... ....................... ....................... ............ Actuator Pressure Entry Fields .................... ............................... ....................... ...................... ...................... .................. ...... Air Supply .................... ............................... ....................... ...................... ...................... ....................... ..................... ...................... ................ .... 45 Calculated Times.......................... Times...................................... ....................... ..................... ...................... ....................... ..................... .......... 45 Interpreting The Results Results Of The Stroking Stroking Time Calculations ..................... ........................ ... Reducing The Stroking Time ..................... ............................... ...................... ....................... ...................... ................... ........ Increasing The Stroking Time ..................... ................................. ....................... ..................... ...................... .................. ...... Accessories And Options That May Be Selected To Reduce The Actuator Stroking Time ..................... ............................... ...................... ....................... ....................... ...................... ..................... ....................... ...................... ............ 47 Volume Boosters ..................... ................................. ....................... ..................... ...................... ....................... ..................... ............... ..... 47 Oversized Piping And Fittings .................... ............................... ....................... ...................... ...................... .................. ...... High-Capacity Supply Pressure Regulators And Filters ..................... ............................... ............ .. Quick Release Exhaust Valves .................... ............................... ....................... ...................... ...................... .................. ...... Enlarged Actuator Vents .................... ............................... ....................... ...................... ...................... ....................... ............... .... Other Stroking Time Considerations ..................... ............................... ...................... ....................... ....................... ..................... ......... Potential For Valve Damage ..................... ................................. ....................... ..................... ...................... .................... ........

Master your semesterImpact with Scribd  .... Of Response Time On StrokingRead Time ..................... ........... ...................... ....................... Free For 30this Days Sign up to vote on title ...............  & The New York Times Not useful  Useful RED SELECTING ACTUATOR ACCESSORIES TO ACHIEVE ACTUATOR THE DESIRED DESI FAIL MODE ....

Matching Stroking Times To The Requirements Of The Applications .......... ..........


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Terminology ..................... ................................. ...................... ..................... ....................... ...................... ..................... ....................... ...................... ............. ... 58

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces WORK AID 1. PROCEDURES THAT ARE USED TO PERFORM DETAILED ACTUATOR SIZING CALCULATIONS WITH THE USE OF THE FISHER SIZING PROGRAM ..................... ................................. ....................... ..................... ...................... ....................... ..................... ...................... ....................... ..................... ................. ....... 67 Work Aid 1A. Procedures That Are Used To Perform Actuator Sizing Calculations With The Use Of The Quick Sizing M ethod Of The Fisher Sizing Program ..................... ............................... ...................... ....................... ...................... ...................... ...................... ...................... ...................... ..................... .......... 67 15. Press the F2 key to display a list of potentially acceptable actuator constructions. ..................... ............................... ...................... ....................... ....................... ...................... ..................... ..................... .......... 68 Work Aid 1B: Procedures That Are Used To Perform Detailed Actuator Sizing Sizing Calculations After After Developing A Specification Specification With The Quick Sizing Method ........ Migration Of Data From The Quick Sizing Method .................... ............................... .................. ....... Performing The Actuator Sizing Calculations ..................... ................................. ....................... ............... .... Selecting A Bench Set ..................... ............................... ...................... ....................... ...................... ...................... ................... ........ 68 WORK AID 2: PROCEDURES THAT ARE USED TO EVALUATE THE IMPACT OF NEGATIVE FLUID FORCE GRADIEN TS ON ACTUATO R SIZING ............. .............. ........ Determining The Fluid Negative Gradient, K n .................... ............................... ....................... ...................... ................. ....... Balanced Valves .................... ............................... ....................... ...................... ...................... ....................... ..................... ................. ....... 69 Unbalanced Valves......................... Valves..................................... ....................... ..................... ...................... ....................... ................... ........ For Flow Down (PTTC)........................ (PTTC).................................... ...................... ..................... ....................... ...................... ..................... ................ ..... 69 Performing The Sizing Calculations .................... .............................. ...................... ....................... ..................... ...................... .............. WORK AID 3: PROCEDURES AND SPECIFICATION GUIDELINES THAT ARE USED TO SELECT ACTUATOR ACCESSORIES TO PROVIDE THE REQUIRED STROKING TIME ..................... ................................. ....................... ..................... ...................... ....................... ..................... ...................... ....................... ................. ...... 71 Work Aid 3A: Procedures That Are Used To Calculate Actuator Stroking Times Master your semester with Scribd ........ With The Use Of The Fisher Sizing Program ..................... .......... ...................... ....................... ................... Read Free Foron 30 Days Sign..................... up to vote this title  & The New York Times1. Complete The Worksheet ..................... useful ................................. ....................... ..................... ...................... .................... ........  Useful  Not Cancel anytime.

Special offer for students: Only $4.99/month. 2. Perform The Stroking Time Time Calculations.............................. Calculations.......................................... .................... ........

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces General Guidelines................... Guidelines.............................. ....................... ...................... ..................... ....................... ...................... ...................... .................. ...... 74 Specific Equipment Selection Guidelines ..................... ................................. ...................... ..................... ....................... .............. .. Guidelines For Solenoid Valve Selection............................ Selection........................................ ...................... ............... ..... Guidelines For Trip Valve Selection ..................... ............................... ...................... ....................... ................... ........ GLOSSARY .................... ............................... ....................... ...................... ...................... ....................... ..................... ...................... ....................... ..................... ................. ....... 75



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SPRING-AND-DIAPHRAGM PRING-AND-DIAPHRAGM AC ACTUAT TUATOR OR FUNCT FUNCTIONS IONS

Actuator Functions

While it is common to think of the actuator as a device that simply moves the control valve closure mem the actuator performs several critical functions. These functions are shown in Figure 1 and they are discu below. Use Word 6.0c or later to

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Fig Figure 1

Control Valve Actuator Functions Position The TheControl Valve Valve Closure Member

Master your semester with Scribd To position the control valve closure member, the actuator must Read overcome the packing friction,the seal Free For 30this Days Sign up to vote on title  and the fluid forces that act on the control valve closure member. & The New York Times  Useful  Not useful Special offer for students: Only $4.99/month.

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Hold The TheClosure Member In I n The TheDesire ired Position sition

To hold the closure member in the proper position, the actuator must overcome the forces of valve plug unbalance and the buffeting forces that result from turbulence in the flow stream. The actuator’s ability t the closure member in a fixed position is a function of the stiffness of the actuator spring.

Provide Provide AdequateSeat Load L oad For Desire ired Shutoff

The actuator must provide sufficient seat load to achieve the ANSI Class shutoff rating of the control val force that is required is a function of the co ntrol valve ANSI Class shutoff rating, the valve style, and the port diameter. dia meter.

Provide Provide AdequateValve Stem Travel

The actuator must have the ab ility to move the valve stem a distance that is equal to the rated valve trave

Prov Provide Sufficie fficiently Rap Rapid Stroking Stroking Time Times

In order to satisfy the requirements of the process system, many control valves must stroke the control va a very short time.

Provide A Fail Mode

If the supply pressure to the actuator is interrupted , the actuator provides a fail mode of operation. The co fail modes are fail open, fail closed, and lock in last position (LILP).

Spring-And-Diaphrag Spring-And-DiaphragmActua Actuator Specificat cifications Actuator Action: Dir Direct or Re Reverse

Spring-and-diaphragm actuators are are available in direct-acting direct-acting constructions and in reverse-acting const as shown in Figure 2. In a direct-acting actuator, an increase in diaphragm pressure will cause the actuato to move toward the control valve. In a reverse-acting actuator, an increase in diaphragm pressure will cau  Read Free Foron 30this Days actuator stem to move away from the control valve. Sign up to vote title


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Use Word 6.0c or later to


Fig Figure 2 Direct And Reverse-Acting Actuator Constructions

Fail Mode - The selection of an action is based on the fail mode that is desired. The fail modes t be achieved ach ieved are shown sho wn in Figure Fi gure 3. Use Word 6.0c or later to




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Physical Size

Actuator selection and sizing begins with the selection of an appropriate actuator physical size. The phys size of a spring-and-diaphragm actuator is typically denoted with a numeric designation such as size 30, 33, size 40, size 100, and so forth. The criteria for selecting a particular actuator size are shown in Figure they are discussed below.



Fig Figure 3 Criterion For Actuator Physical Size Selection

Diaphragm DiaphragmArea - As the actuator diaphragm area increases, more net actuator force is availabl operate the control valve.

Master your semester Scribd  that is Diam Diameter Of TheYwith oke-To-Val -T o-Valve veConnection - The actuator yoke must have a diameter Read Free Foron 30this Days Sign up to vote title  to the diameter of the yoke boss that is located on the control valve bonnet. & The New York Times  Useful  Not useful Cancel anytime.

ctuator StemDiame Diameter - The diameter of the actuator stem must be com patible with the diam Special offer for students: A Only $4.99/month.

the actuator stem in terms of strength and the availability of a stem connector that will connect th

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Operational Specificati cifications ons

The remaining actuator specifications relate the performance of the actuator. These specifications are sho Figure 5 and they are discussed below. Use Word 6.0c or later to


Fig Figure 4 Operational Specifications

Diaphragm DiaphragmOperating Operating Pressure Range- Actuators are rated for specific diaphragm pressure ra

The most common nom inal diaphragm pressure ranges is 3 to 15 psig. Higher pressure ranges su 6 to 30 psig are sometimes specified in order to increase the thrust of the actuator. The output pr range of the instrument that supplies that actuator loading pressure must be comp atiblewith the Read Free Foron 30this Days Sign up to vote title actuator diaphragm pressure range that is selected. 

Master your semester with Scribd & The New York Times Not useful  isUseful rated for a maximum diaphragm Maximum Diaphragm DiaphragmPre Pressure ureRating - Each actuator Cancel anytime.

Special offer for students: The Onlyupper $4.99/month. pressure limit is established to prevent excessive loads from damaging the diaphragm diaphragm casings, and the actuator stem.

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Basic Sizing Sizing Conce Concepts Valve Forces

In order to size an actuator, the specifier first calculates the total force that is required (F TFR ) to operate control valve. ( TFR means “total force required”.) F TFR includes all of the static forces that the actuator overcome in order to seat the valve plug with sufficient force to achieve the ANSI Class shutoff rating of control valve. For ease of calculation, these forces (see Figure 6) are defined as follows:

• •

For Force A Forc Forcee B

• •

Forc Forcee C Forc Forcee D

The The st stati atic un unbal balance nce fo force tha thatt is is pr produc duced by the DPshutoff . The The sea seatt loa load d tha thatt is is nee neede ded d to to ach achie ieve ve the the rat rated ed ANSI ANSI Clas Classs shu shuto toff ff the valve. The The for force ce that hat is nee neede ded d to to ove overc rcom omee the the pack packiing frict iction. ion. The The for force ce that that is need needed ed to over overco come me any any oth other er sour source ce of valv valvee fri frict ctio io the piston rings in balanced valve constructions. Use Word 6.0c or later to


Fig Figure 5

Control Valve Forces Actuator Force Forces Master your semester with Scribd Read Free Foron 30this Days Sign up to vote title The amount of force that is available from a particular actuator is a function of: & The New York Times  Useful  Not useful Special offer for students: • ThOnly e di$4.99/month. a p h ra g m f o r c e

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FD =Pb x AD Use Word 6.0c or later to


Fig Figure 6

Factors That Determine The Maximum Diaphragm Force Spring SpringForces To calculate the force that is produced by the actuator spring, the following factors must be evaluated:

• • •

The sp spring ra rate (K (K S) of the actuator spring in pounds/inch. The The amo amoun untt of of valv valvee tra trave vell (T (TV), in inches). The amount amount of init initia iall compr compres essi sion on (Fi).

Spring Spring Rate, K S - Spring rate is defined as the amount of force that is needed to compress a giv

spring 1 inch; for example, if the spring rate of a particular spring is 100 pounds/inch, 100 p oun Master your semester Scribd force will be neededwith to compress the spring 1 inch, 200 pound s force will be needed tocompres Read Free Foron 30this Days Sign up to vote title spring 2 inches, and so forth, as shown in Figure 8. Similarly, if a spring with a spring rate rat e of 10 & The New York Times  Useful ifNot pounds/inch poun ds/inch is compresse com pressed d 1 inch, inc h, it will exert 100 1 00 pounds po unds force, f orce, and, a nd, it isuseful comp ressed 2 inch 200 pounds force. Special offer for students: will Onlyexert $4.99/month.

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Fig Figure 7

Spring Rate

Spring SpringCompression Force Forces: Dir Direct-Acting ct-Acting Actuators - The components of spring compressio direct-acting direct-acting actuator are shown in Figure 9. Use Word 6.0c or later to


Master your semester with Scribd & The New York Times Fig Figure 8


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Special offer for students: •OnlyComponents $4.99/month.Of Spring Compression In A Direct-Acting Actuator

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces •

Initial Compression (Fi) - The minimum diaphragm force is equal to the minimum actuato loading pressure (P a) multiplied by the area of diaphragm (A D). To prevent the minimum diaphragm force from moving the actuator stem away from the 0 percent travel position, an and opposing force (Fi) is wound into the spring. Because the spring is preloaded, lost moti dead band is eliminated and the actuator produces a stem force as soon as the diaphragm for exceeds the force of initial compression. The distance that the spring should be compressed initial windup) is calculated as follows:

P x AD Initia Initiall Windup Windup (inches inches) = a KS •

Spring Com Compression Over Travel (Fs) - FS is the force that is required to compress the sp

distance that is equal to the rated valve travel (T V); accordingly, F S is the product of the spr rate (K S) and the rated valve travel (T V), as shown below: FS = K S x T V

Spring Spring Compression: Re Reverse-Acting -Acting Actuators - The components of spring compression for

reverse-acting actuator are are shown in Figure 10, and they are discussed b elow. Initial Compression (Fi) - In a reverse acting actuator, the force that is required to close the • control valve (F TFR ) is always wound into the spring. In some applications, F TFR is the on component of F i. If the minimum output pressure from the instrument that provides the load pressure pressu re to the actuator actuato r is greater grea ter than 0 psig, an additional add itional force fo rce that is equal equa l to P a x A D generally wound into the spring to ensure that the valve will shut off. The total amount of in windup (in inches) is calculated as follows:

F P x AD Initial Initial Windup = TFR + a KS KS •

Spring Com Compression Over Travel (Fs) - As was discussed for direct-acting constructions,

the force that is required to compress the spring a distance that is equal to the rated valve tra (TV).



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Fig Figure 9

Components Of Spring Compression In A Reverse-Acting Actuator Actuator Be Bench Set

Many actuator force relationships can be summarized with an actuator specification that is referr as the actuator bench set , or spring range. For both direct-acting and reverse-acting actuators, the set is the range of diaphragm pressures over which the actuator spring is compressed a distance equal to the rated valve travel when the actuator actuator is disconnecte disconnected d fr om all control valve forces when the actuator and valve stems are disconnected. The term bench set derives from an adjustm that is made with the spring adjuster while the actuator is on the assembly bench.

Dir Direct Acting ActingActuator Bench Set Specificati cification on - Figure 11 shows how the bench set specifica helps to identify all the actuator forces that are produced b y a direct-acting actuator on a push close valve. The forces are discussed below.



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Figure 10

Typical Direct-Acting Actuator Bench Set

For purposes of discussion, assume that the actuator operating pressure range is 0 to 18 psig, the diaphragm area is 70 square inches, and the valve travel is 1 inch. As Figure 11 shows, the lowe set pressure (BS lower ) is the diaphragm pressure at which the d iaphragm force overcomes the fo Fi; i.e., the diaphragm pressure at which the actuator stem just begins to move away from  the up Read Free For 30 Days Sign up to vote on this title stop. In this example, the force of initial compression is calculated as follows:  Fi = P a x A Not useful D Useful  Cancel anytime. Fi = 3 psig x 70 squ are inches Special offer for students: Only $4.99/month. Fi = 210 pounds force

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K S = pounds force / T V, inches K S = [(BS upper - BS lower ) x A D]/TV K S = (9 psig x 70 square inches)/1 inch K S = 630 pounds/inch As the diaphragm pressure is increased from 11 to 15 psig, there is no additional spring compres stem travel. Instead, the diaphragm force that is produced over this pressure range overcomes all valve forces (F TFR ); i.e., the forces that must be provided to overcome static unbalance, to over packing packin g friction, frictio n, and to t o seat the th e valve plug to the ANSI Class shutoff sh utoff rating. ra ting. The force that th at is av to operate the control valve is calculated as follows: Net actuato r force = (P b - BS upper ) x A D Net actuator actu ator force for ce = (18 psig- 11 1 1 psig) x 70 square sq uare inches in ches Net actuator actu ator force forc e = 7 psig p sig x 70 square inches Net actuator actua tor force = 490 p ounds force f orce

ReverseActing Actuato Actuator Bench Set Specificati cification on - Figure 12 shows how the bench set specifi

helps to identify the actuator forces that are produced by a reverse-acting actuator that is mounte push down to close valve. v alve. The Th e forces force s are discusse d iscussed d below. below .





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Assume that the actuator operating pressure range is 0 to 18 psig, the diaphragm area is 70 squar inches, and the valve travel is 1 inch. In a reverse-acting actuator, the force that is required to ov all the valve forces and to seat the valve is wound into the actuator spring. Therefore, the force o initial compression (F i) must be equal to or greater than the total force that is required to operate control valve (F TFR ) .

As the diaphragm pressure is increased from 0 psig, the actuator stem will not move until the diaphragm produces a force that is greater than F i. The net actuator force that is available to ope control valve is calculated as follows: Net actuato r force = (BS (B S lower - P a) x A D Net actuator actu ator force forc e = (6 psigp sig- 0 psig) p sig) x 70 7 0 square squa re inches inche s Net actuator actu ator force forc e = 6 psig p sig x 70 square inches Net actuator actua tor force = 420 p ounds force f orce

As the diaphragm pressure is increased from 6 to 14 psig, the diaphragm force compresses the a spring a distance that is equal to the rated valve travel. Therefore, the bench set specification is 6 psig. The slope of the plot indicates the spring rate of the actuator spring. The spring rate is calculated follows: K S = pounds force / T V, inches K S = [(BS upper - BS lower ) x A D]/TV K S = (8 psig x 70 square inches)/1 inch K S = 560 pounds/inch As the diaphragm pressure is increased from 14 psig to 18 p sig, there is no additional movemen valve stem because the actuator has engaged its upper travel stop. The additional force that is ge by the diaphragm diaphr agm ensures en sures that t hat the valve will w ill fully open.

Bench Set Span And Perf Perfor ormance mance

During the actuator selection process, the specifier will typically find that several different spring rates an bench set span s are available av ailable that th at will satisfy sa tisfy the basic force fo rce requiremen requ irements ts of the control valve. Some So me ben spans are fairly narrow (for example, 3 to 7 psig for direct-action or 12 to 15 psig for a reverse action), an  Read Free For 30 Days other bench set spans are very wide (3 to 15 psig for direct-action or 6up toto 15vote p sigon for reverse-action). Spe Sign this title  should be aware of the consequences of selecting extremely narrowUseful or extremelyNot wide bench set ranges. useful


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Special offer for students: Narr Onlyrow $4.99/month. Nar Bench Set Rang Ranges - If the specifier’s major concern is seat load and shutoff, the specif

should select a spring with a relatively low spring rate. The low spring rate will increase the forc

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While a narrow bench set span increases the force that is available for control valve shutoff, a bench set span spa n may result in poor controlla c ontrollability. bility. Figure Fi gure 13 shows s hows that a narrow na rrow bench be nch set se t span in very high static gain (i.e., a small change in the diaphragm pressure will cause a large change actuator stem position).



Figure 12

High Gain That Results From A Low Spring Rate And A Narrow Bench Set Span

WideBench Set Span - When the objective is to provide the best possible con trol, the specifier

select a very high spring rate. Scribd The higher spring rate will result in a wider bench set span becaus Master your semester with  diaphragm force is needed to compress the spring over the rated valve For Read Free Fortravel. 30this Days Sign up to vote on titlepurposes of  illustration, assume that a spring is selected that results in a bench set specification of 3 to 15 & The New York Times Useful  Not useful  shown in Figure 14.


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The static gain is low which results in good controllability. Because the spring is very stiff, the a

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Figure 13

Effects Of Wide Bench Set Span And High Spring Rate

Bench Set Guidelines Guidelines - When specifiers are faced with a choice of spring rates and bench set sp

they must consider the application and the valve specifications to make an appropriate selection following guidelines summarize the prior discussion of bench set specifications. • If shut shutof offf is is of of pri prime me impo import rtan ance ce,, a light lighter er spri spring ng and and a narro narrower wer benc bench h set set span span may may appropriate. • If valv valvee plug plug sta stabi bili lity ty is is a con conce cern rn,, and and if if tig tight ht shu shuto toff ff forc forcee is not not a req requi uire reme ment nt,, a h spring and wider bench set span should be sp ecified.  Read Free For 30 Days Sign to vote on this title • If shut shutof offf and and plug plug sta stabi bili lity ty are are bot both h crit critic ical al issu issues es,, aup lar large ger r actu actuat ator or and and/o /orr incr increa ease d sed useful diaphragm pressure may be required to achieve theUseful performance objectives.  Not


Special offer for students: Only $4.99/month. Effectsof Valve ValveFriction Friction On On Actua Actuator Performance

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces •

The actuator actuator may not begin to to stroke stroke until until a pressure pressure that that is significa significantly ntly greate greaterr than than 3 psi applied to the diaphragm.

•

The actuator actuator may may not achieve achieve the the full rated rated valve valve travel travel until until a pressure pressure that that is signifi significantl cantl than 9 psig is applied to the diaphragm.

•

As the the diaphragm diaphragm pressure pressure is is decreased, decreased, the actuat actuator or stem stem may not begin begin to to move toward toward th closed position until the diaphragm pressure is reduced to a pressure that is significantly les 9 psig.

•

The actuator actuator may not fully fully open the the valve valve unless unless the the diaphragm diaphragm pressur pressuree is reduced reduced to a pr that is significantly less than 3 psig.





 Figure 14


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Theoretical Effects Of Friction And Dead Band On Bench Set

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Figure 15

Effects Of Friction Band On Control Valve Travel Importance Of Inst I nstrument Over Rang Ranging

The effect of friction on valve travel underscores the importance of being able to supply a diaphragm pre range of 0 to 18 psig even though the actuator may be described as requ iring a nominal 3 to 15 psig load pressure. pressu re. When Wh en the expand ed diaphragm diap hragm pressure pressur e range ra nge (0 to 18 psig) p sig) is available a vailable,, an additiona ad ditional l force th  Read Free For 30 Days Sign up to vote on this title equal to 3 psig x A D is available to overcome valve friction. The additional force helps to ensure  that the will fully open and that the valve will fully close.  Useful  Not useful

Master your semester with Scribd & The New York Times Special offer for students: Only $4.99/month. EffectsOf Friction Friction On Proc ProcessControl

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces • • •

The di distinct shape of the limit cycle is determined by the time constant of the process and factors. The magnitude of the limit cycle is determined by the proportional gain of the controller. The frequency of the limit cycle is a function of any integral action in the controller or in the of process. proces s.



Figure 16

Control Valve Dead Band (Upper Plot) And Limit Cycle (Lower Plot)

Master your semester with Scribd  Read Free Foron 30this Days Minimizing Frictio Friction Effe Effects Sign up to vote title  & The New York Times  Useful  Not useful Equipment Selection To To Minimi Minimize ze Dead Band - Because limit cycle is caused by valve frictio Cancel anytime.

Special offer for students: tactic Only $4.99/month. that minimizes dead band will also help to minimize limit cycle. In terms of equipment sel the selection of any of the following may help to minimize control valve dead band

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Figure 17 Minimizing Dead Band With A Control Valve Positioner

Minimizing L imit Cycle Cycles With Controller Tuning- The controller can be adjusted (tuned) to minimize the limit cycle in an existing system as follows: •

Reducing Reducing the proportional proportional gain of of the controller controller reduces the magnitu magnitude de of the limit limit cycle. cycle.

•

Changing Changing the integral integral action. action. Increas Increased ed integra integrall action action compensates compensates for the the reducti reduction on in proportional propo rtional gain g ain and increases increas es the frequenc fr equency y of the th e cycles, cycles , while reducing re ducing the integ ral actio decrease the frequency of the cycles. Even if the limit cycles cannot be totally eliminated, th may be minimized to the extent that their effects on the process variable become very small  even imperceptible. Read Free Foron 30this Days Sign up to vote title


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PERFORMING ERFORMING DETAILE DETAI LED D ACTUATOR SIZI SIZIN NG CALC C ALCULATI ULATION ONS S Introduction ntroduction

While actuator selection and sizing is typically performed with the use of simple methods, there are insta when the specifier will benefit from from performing detailed sizing calculations. calculations. A detailed analysis may be beneficial benefic ial in the following followin g circumstance circu mstances: s: The control valve includes high-friction packing. • The instrument that provides the actuator loading p ressure provides a “nonstandard” output pressure • i.e., any pressure range that is other than 0 to 18 psig or 0 to 33 psig. It is desirable to evaluate the performance of an installed actuator or to predict the performance of an • actuator that is on-hand or available in inventory. •

Detailed sizing calculations are typically performed with the use of computer software programs such as Fisher Sizing Program. Much of the discussion that follows will be based on the features and the nomenc that are included in the Detailed Sizing Option of the Fisher Sizing Program.

Nomenclature nclatureFor A Detailed Analysis Of Actuato Actuator Pe Performa formance Bench Set, Low L ower And Upper

As described previously, the bench set is the range of diaphragm pressures over which the actuator stem a distance that is equal to the rated valve travel when the actuator i s di sconnecte . Refer t sconnected d fr om the valve Figure 19.

Stroking Stroking Pressure, Low Lower And Upper

As shown in Figure 19, the stroking pressure range is the range of diaphragm pressures over which the ac moves the valve plug a distance that is equal to the rated valve travel when the valve is install ed ; i.e., wit friction present and with pressure in the valve body. When the valve is open, the pressure unbalance from stem area may tend to open the valve or to close the valve; therefore, the lower stroking stroking pressure may be slightly higher or slightly lower than the lower bench set pressure. The upper upper stoking pr essure essure does not i

the diaphr agm press pressur e thatwith is requir ed to provi de seat seat load. Master your semester Scribd & The New York Times



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Figure 18

Bench Set And Stroking Pressure Range Friction Band

Friction band is the diaphragm pressure that is required to overcome the packing friction. Friction band i calculated as follows:

Master your semester with Scribd Friction Fricti on (pounds )30 Days Read Free Foron Sign up to vote this title Frictio Friction n Band Band ( psig) = 2 A nches  ) Not useful D (iUseful & The New York Times Special offer for students: Only $4.99/month.

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The term fri ction band band is unique to the Fisher Sizing Program. Friction band is equal to half of the total d

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Figure 19

Stroking Pressure Range Plus Friction Band Operating Operating Pre Pressure ure Rang Range

The operating pressure range is the range of diaphragm pressures that is required to fully o pen and fully c the valve and to provide the required seat load, as shown in Figure 21. The operating pressure range will be within with in the lim its of P a and P b. In the detailed sizing method, the specifier can set P a and P b to any pr value.

Supply Pre Pr essure ure Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title  The supply pressure (P S) that is required to ensure optimum controlUseful is also calculated & The New York Times  valve performance  Not useful software. As shown in Figure 21, the supply pressure should be somewhat higher than the upper opera Special offer for students: Only $4.99/month.

Cancel anytime.

pressure. pressure .

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Figure 20

Operating Pressure Range And Supply Pressure, Ps Overview Of The T heDetaile iled Actuator Sizing Sizing Method Of The T heFisher Sizi Sizing ng Prog Program Selecting cting TheDetaile Detailed Sizing Sizing Method

Master your semester with Scribd The specifier selects the Detailed Sizing Method from the main menu by first selecting the Ssact  option. ReadupFree Foron 30this Days to vote title the menu that appears, the specifier selects the Spring & Diaph under heading Deta DiaphrSign agm option iled & The New York Times  Useful  Not useful Special offer for students: Only $4.99/month.

Cancel anytime.

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Calculation alculation Screen Figure 22 shows the calculation screen for the Detailed Sizing Method.

R e v 1.4 2

Detailed Sizing

Actuator Type Valve Design Flow

657 ED DO W N

Port Diameter U n b a la n c e d A r e a Valve Travel V a l v e S t e m S iz e Valve Friction P1 M ax dP M ax S ea t L o ad U n b a l F rc e @ O p e n Pa Pb Fluid Neg. Gradient

3 .43 8 in 0 .4 00 in 2 1 .50 0 in 0 .50 0 in 230.00 lbf 300.00 psig 300.00 psid 20.00 lbf/in 0.00 lbf 0.00 psig 18.00 psig 0 .00 in

Bellows (Y/N) ?

N

Spring & Diaphragm Calculated Results -- - -- - -- - -- - -- - -- - -- -- - -- - - -- -- Actuator Size Spring Spring Rate Spring Windup Max Spring Load Min Req'd Air Sup.

-- - -- - -- - --

- lbf/in - in - lbf - psig

Actr. Outp. Thrust Req'd Valve Thrust

- lbf - lbf

L ow er B en ch Se t Lower Stroking Low Frict Band (+/-)

- psig - psig - psig

U p pe r B e nc h Se t U p p e r S tr o k i n g Up Frict Band (+/-) Lower Operating Upper Operating

- psig - psig - psig - psig - psig

F1-HELP F2-Calc F3-Option F4-Choice F5-Clear F10-E Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title  Figure 21 & The New York Times Useful Not useful   Detailed Sizing Screen For Spring-And-Diaphragm Actuators Cancel anytime.

Special offer for students: Only $4.99/month. Migration Of ValveAnd Actuator Data Data From FromTheQuick Quick Sizing Sizing Method

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Dire Direct Entry Of Of Info I nformation If you are entering the data directly, the pertinent information must be located in the appropriate valve specification bulletin or in the manufacturer’s actuator sizing documentation; e.g., Fisher Catalog 14.

Unique Inpu Input Fie Fields The unique input fields of The Detailed Sizing screen are as follows:

Unbal Frce @ Open - For some special valve constructions that throttle at reduced travel and in

applications where the actuator is operated directly by process pressure, the unbalance force at th position positi on can impact actuator actuato r sizing. sizing . For mo st installa tions, a value of zero is entered entere d in this field Diaphr Diaphrag agm Pre Pressure ures Pa and Pb - These entry fields allow the specifier to input the actual dia loading pressure range. Fluid luid Negative Gradient - This entry field asks for a coefficient that describes the fluid reaction that tend to destabilize the valve plug. Negative gradients will be discussed in the next section o Module. Bellows (Y/N) (Y/N) - This entry asks if the control valve includes a valve stem bellows. If this entry i Y (yes), the software calculates the force that results from the spring rate of the bellows.



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Calculate alculated Results

After all the inputs are entered, the specifier presses the F2 key to perform the sizing calculation s. The fie the calculated results section of the screen are shown in Figure 23 and they are explained below.

R e v 1.4 2

Detailed Sizing:

Actuator Type Valve Design Flow

657 ED DO W N

Port Diameter U n b a la n c e d A r e a Valve Travel V a l v e S t e m S iz e Valve Friction P1 M ax dP M ax S ea t L o ad U n b a l F rc e @ O p e n Pa Pb Fluid Neg. Gradient

3 .43 8 in 0 .4 00 in 2 1 .50 0 in 0 .50 0 in 230.00 lbf 300.00 psig 300.00 psid 20.00 lbf/in 0.00 lbf 0.00 psig 18.00 psig 0 .00 in

Bellows (Y/N) ?

N

Spring & Diaphragm Calculated Results - -- - - - - -- - -- - -- - -- -- - -- - - -- -- Actuator Size Spring Spring Rate Spring Windup Max Spring Load Min Req'd Air Sup.

-- -- - -- - - -40 1F1770 27 5 .00 l 0 .9 58 in 676.0 lb 2 1.0 0 p s

Actr.Outp. Thrust Req'd Valve Thrust

773.0 lb lb 566.0 lb lb

L ow e r B en ch Se t Lower Stroking Low Frict Band (+/-)

3 .82 p si 3 .82 p si 3 .33 p si

U p pe r B e nc h Se t U p p e r S tr o k i n g Up Frict Band (+/-) Lower Operating Upper Operating

9 .80 p si 1 1.5 4 p s 3.33 psig 0.49 ps psig 18.00 ps ps

Master your semester with ScribdFigure 22 Read Free For 30 Days Sign up to vote on this title Calculated Results For The Detailed Sizing Method & The New York Times  Useful  Not useful Cancel anytime.

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Special offer for students: A Only $4.99/month. ctua tor Size - After calculating all valve forces, the software automatically selects the smallest actuator that will:

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Max. ax. Spring Loa Load - This field describes the maximum spring load for the selected spring. The

maximum spring load is the sum of F i (the force of initial compression) and F S (the force of spr compression over travel). The software will not select an actuator construction in which the sprin overloaded; therefore, this field is for information purposes only.

Min Req’d Air Sup. - This field displays the minimum supply pressure , in psig, that is needed

proper prope r actuato r operation. operat ion. The minimum minimu m supply sup ply pressure pre ssure is always set to a pressure pressur e that is a fixe amount above the upper operating pressure. The additional pressure compensates for deviations plant pressure p ressure,, tolerances toleran ces in regulato re gulatorr ratings and performa p erformance, nce, and an d other conditions conditi ons that tha t could the available supply pressure.

L ower And Upper Stroking Stroking Pre Pressure ures - As previously described, the stroking pressure range

range of pressures over which the valve plug of an installed valve will move from one travel sto valve other. This pressure range does not include the pressure that is required to produce the needed se load.

L ower and Upper Friction Friction Band (+/-) /-) - This field lists the friction band (one-half the dead ban psig, that th at results result s from valve v alve friction. fric tion.

L ower and Upper Operating Operating Pressures- These pressure values define the pressure range that produces produ ces full valve travel tr avel plus plu s the seat se at load, load , plus an allowance allowan ce for friction f riction band. b and. Non-Standard Non-Standard Be Bench Sets-The Detailed Sizing Method calculates mathematically precise benc For example, bench sets such as 1.6 psig to 8.4 psig for a direct-acting actuator, or 8.75 to 16.2 p a reverse-acting actuator actuator may be displayed. Non -standard bench sets are troublesome to some specifiers who are familiar with with the standard bench set ranges such as 3 to 9 psig or 9 to 15 p however, the non-standard bench sets are simply the calculated pressure values that will ensure p operation of the control valve assembly. Bench Set Selection ction Gui Guide delines If the calculated bench set specification is non-standard, it may be possible to select a standard bench certain circumstances. The selection guidelines are as follows:

Master your semester with Scribd  Read Free 30 Days Sign up to vote on this title that provide PTFE Packing - If the control valve includes single PTFE packing, and ifFor the instrument  can produce a minimum pressure of approximately calculated bench & The Newdiaphragm York pressure Times Notthe useful  Useful 0psig, •

Cancel anytime. be shifted shif ted to a standard standa rd bench ben ch set range r ange (3 psig to x psig) psig ) with the th e same approximate appro ximate span s pan as the standard bench set.


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Manual Sizing Option

By pressing the F3 key, the specifier may view the options for the detailed sizing method. The options ar discussed below.

Automatic Sizing Sizing - When Automatic Sizing is selected, the software automatically selects the optimum actuator construction.

Manual sizing - The manual sizing option is useful when the specifier wishes to determine if an

existing construction is appropriate for a specific application. For example, when troubleshootin problematic proble matic control co ntrol valve, v alve, the actuator may be b e evaluated evalu ated to ensure that the actuator actuato r has been be en pro sized. The manual method may also be selected to determine if an actuator that is in inventory c adapted to a new application. In the manual sizing mode, the specifier may evaluate the performance of a particular actuator s spring combination. First, all the appropriate valve information must be entered. The specifier m then display a list of actuator sizes and a list of actuator springs by placing the cu rsor in the appr field and pressing the F4 key. The actuator size and spring that are to be evaluated are selected f lists. When the specifier presses the F2 key, the software attempts to calculate an actuator specif that is based on the valve information, the selected actuator size, and the selected actuator spring software will either display the appropriate actuator specification or it will display a report that e why the selected actuator size and spring are not appropriate for the application.



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EVALUATIN EVALUATI NG THE IMPACT OF NEGATI NEGATIVE VE FLUID FL UID FORCE FORCE GRADIENTS Definition finition And Exam Examples Of Flui Fluid d Force Gradie Gradients Definition Of Fluid Gradients Dynamic Ve VersusStatic Cons Considerations - When one is sizing an actuator for a specific control

one must consider both the static performance and the installed dynamic performance of the actu and control valve assembly. Mo st basic actuator sizing techniques are based on static performan criteria; i.e., ensuring ensuring that the actuator produces sufficient force to fully open the valve and to fu close the valve. A dynamic analysis is performed to ensure that the actuator will be sufficiently stiff to oppose th reaction forces that may tend to destabilize the valve travels from travel stop to the other.

Fluid Fluid Force Grad Gradients - The term “fluid force gradient” is used to describe the chan ges, over v

travel, in the forces on the valve stem that are generated by the interaction of the flowing fluid w valve plug.

Force ForceGradients - To evaluate a force gradient, one must evaluate the change in force relative t change in position. The basis for evaluation is expressed with: ∆F/ ∆x

where: ∆F

= the change in force, pounds force ∆x = the change in travel, inches Examples Of Fluid Gradients Positive Gradie Gradients - Gradients are most easily identified as being positive or negative if one

the plotting conventions that are shown in Figure 24 . The valve forces that place the valve stem tension are plotted above the baseline and the valve forces that place the valve stem in compress plotted below the t he baseline. bas eline.  Free For 30this Days SignThe up to vote on titlean unbalanced A positive valve plug force gradient is shown in Figure Read 24. plot is typical for  that is installed in the flow-up orientation. Because the  fluidUseful force gradient positive over the ra Notisuseful  Cancelwill anytime. travel of the valve, valve plug instability (from negative gradients) not be a concern.


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Figure 23 Positive Valve Plug Force Gradient

Negative Gr Gradient: Open Loo L oop System - Figure 25 shows a hypothetical plot of the valve plug

reaction forces that might be generated by a balanced valve that is installed in a flow-down orien In the flow-down orientation, the fluid pressure tends to open the valve; i.e., the valve plug force create a compressive force on the valve stem. For the discussion that follows, assume the follow •

The actuator is being stroked by manually adjusting the set pressure of a pressure regulator; there is no feedback control.

Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title  The actuator is a direct-acting type and it is somewhat undersized. & The New York Times Useful Not useful   As the diaphragm pressure is reduced from the maximum diaphragm pressure, the actuator will •

Cancel anytime.

Special offer for students: smoothly Only $4.99/month. stroke the valve plug from point A to travel point B. The actuator force that is generate

travel point B also satisfies the valve force requirements for travel point D; therefore, a small red

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Figure 24 Bi-Stable Plug In An Open Loop System

Negative Gradien Gradient: Closed Loo L oop System - If a feedback device such as a controller or position

included in the system, the feedback device will attempt to correct the stem position errors that a caused by the bi-stable valve plug. As the feedback device tries to correct the error, the plug will rapidly cycle (change position) between points D and A. In addition, depending on the gain of th feedback device, the valve stem may overshoot the bi-stable points of operation. The result may system that is wildly unstable as shown in Figure 26



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Figure 25 Unstable Plug In A Closed Loop System

Negative Gradient: Balance Balanced Valve Valve, Flow Flow Down Configuration onfiguration - While balanced valves

occasionally generate negative force gradients, unbalanced valves in a flow down orientation alw present presen t a significant signif icant negative ne gative gradient g radient as a s shown show n in Figure Figu re 27. Howeve H owever, r, because becau se larger actuato selected in order to overcome the considerable static forces at shutoff, the actuators are typically sufficiently stiff to prevent the valve plug positioning problems that are associated with negative gradients. When the actuator is not sufficiently stiff, the plug, as it approaches the seat, will slam the seat.

Master your semester with Scribd & The New York Times Use Word 6.0c or later to

Special offer for students: Only $4.99/month. view Macintosh picture.


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Pre Preventing Valve ValvePlug Plug Instability bility That That Is Is Caused By Negative Gradients Valve Plug Stability Equation

To ensure valve plug stability, the total actuator stiffness must be greater than the maximum value of is calculated for the valve plug forces. The mathematical expression for valve plug stability is as follows: K s + K a >K n ∆P where:

K s K a K n ∆P

the mechanical spring rate, pounds/inch the air spring rate, pounds/inch coefficient for the valve negative gradient, pounds/inch/psid the the flowi flowing ng pres pressu sure re drop drop acro across ss the the valv valve, e, psid psid

The basic concept of the stability equation is shown in Figure 28. The terms that are used in the equ ation discussed below.





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Determining Tota Total Actuato Actuator Stiffne Stiffness

Mechanical Spring Spring Rate - Increasing the spring rate (K s) of the actuator spring can h elp to min

the effects of negative gradients. However, the relatively high spring rate that would be required overcome a large negative gradient may lead to the selection of a very large actuator that is impr in terms of its physical size and its cost.

Air Air Spring ringEffects- The volume of air in the actuator casing provides an “air spring” effect tha

helps to stabilize the valve plug. The air spring d erives from the compressibility of the fluid. The of the air spring effect is referred to as K a. In most instances, the spring rate of the air spring is m greater than the spring rate of the mechanical spring. Determining ining TheAir Spring Rate - The spring rate of the air spring, K a, is calculated as follow 2

Ka

=

kPA V

where:

K a k P A2 V

the spring rate of the air spring, pounds force per inch the ratio of specific heats of the actuating media (typically air), dimensi the the avera average ge pres pressu sure re that that is appl applie ied d to the the act actuat uator or dia diaphr phragm agm,, psig psig the area of the diaphragm, inches2 the the vol volum umee of of the the actu actuat ator or casi casing ng,, inc inche hess3

Fisher Controls publishes plots that allow the specifier to quickly determine the air spring rate. F 29 shows the air spring effect, K a versus the diaphragm pressure for a size 45 actuator.



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Figure 28

Air Spring Effect Versus Diaphragm Pressure For A Fisher Type 657 or Type 667 Size 45 Actuato

Diaphragm DiaphragmPre Pressure ures That Ar AreUsed To To Calcul Calculate ateK a - The value of K a is computed with th

of the average diaphragm pressure. The average diaphragm is the mean pressure between the low stroking pressure and the upper stroking pressure.

Role Of A Position itioner - The values of K a that are calculated with the use of the methods that are

described above assume that a positioner is included in the control valve assembly. If a positione not included in the control valve assembly, the value of K a must be reduced by one half. The rea for the reduction is explained as follows:

Master your semester with  frequ Any form of feedback controlScribd adds significantly to the air spring effect at very low cycling Read Free Foron 30this Days Sign up to vote title If the actuator is to be occasionally operated in the manual mode; i.e., the controller will  be set to & The New York useful in the control isNot manual,Times the air spring effect will be greatly diminished. If aUseful positioner included Cancel anytime.

a feedback loop will be maintained even if the controller is set to manual; therefore, th Special offer for students: assembly, Only $4.99/month.

benefit o f the air-sp ring effect effe ct will be realized. If I f a control contro l valve that t hat does not include a position

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Quantifying uantifying Negative Gradients

Coefficient fficient For Ne Negative Gradients, K n - Manufacturers, through the use of appropriate labora

tests, can determine the magnitude of the negative gradient that will be produced by a specific v For example, Fisher Controls tests each valve in the laboratory, identifies the maximum negative gradient, and publishes a value of the coefficient K n for each valve style that is likely to be affec negative gradients. The maximum negative gradient is normalized for a 1 psid flowing pressure Therefore, specifiers may estimate the impact of the negative gradient at any flowing pressure dr The nomenclature for the coefficient K n is as follows: ∆F

Kn =

∆x

1 psid

Typical Values Of K n For Balance Balanced Valve Valves - For balanced valve constructions, the values of

published publis hed in tabular format f ormat in Fisher Catalog C atalog 14. 1 4. A table tab le of typical ty pical K n values is shown in Figure

Port Diameter, I nches 1/4 thru ¾ 1 1-1/2 1-7/8 2-7/8 3-5/8 5-3/8

Negative Gradient, K n L inear Equal Perce Percentage Class1500 Class 1500 - -- -- -2 .0 4 .0 4 .0 5 .4

0 0 .1 1 .6 1 .4 3 .1 2 .5 3 .6 Figure 29

K n Values For Design HPD and HPT Valves, Flow Down

Values Of K n For Unbalanced Valves, PTTO - For unbalanced valves where pressure tends t Master your semester  the valve (flow up),with the valueScribd of K n is 0. Read Free Foron 30this Days Sign up to vote title tends t Values Of K n For Unbalanced Valves, PTTO - For unbalanced valves where pressure & The New York Times Not useful theUseful following: the valve (flow down), the value of K n is estimated with use of the Cancel anytime.


2

Kn

=

2 (unbalance area, inches )

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ValvePlug Stabili Stability ty Equa Equation - To ensure valve plug stability, the value of K n times the maxi

flowing pressure drop must be greater than the mechanical spring rate plus the air spring rate. Th mathematical expression for stability is as follows: K s + K a >K n ∆P where:

K s K a K n ∆P

the mechanical spring rate, pounds/inch the air spring rate, pounds/inch negative gradient, pounds/inch/psid the flowing flowing pressur pressuree drop drop acros acrosss the the valve valve,, psid psid

Determining The TheFlow Flowing Pre Pressure ureDrop Drop - The flowing pressure drop that is used in conjuncti

with the value of K n could be the pressure drop at the minimum flow condition, the normal flow condition, or the maximum flow condition. Negative gradients tend to cause the most significan problems proble ms und er high pressure pressu re drop conditions; conditi ons; therefore, the refore, the largest larg est flowing flow ing pressure pre ssure drop d rop is th pressure pressu re drop dro p that should s hould be closely clos ely evaluated eva luated



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Computer Assisted Selection ction The Detailed Sizing Method within the Fisher Sizing Program includes the ability to select actuator constructions that are adequately sized to prevent the valve plug instability that can occur as a result of gradients. In order to account for negative gradients, the appropriate data must be entered in two input The fields are shown in boldface in Figure 31 and they are discussed below.

R e v 1.4 2

Detailed Sizing:

Actuator Type Valve Design Flow With Side MO? (Y/N) Port Diameter U n b a la n c e d A r e a Valve Travel V a l v e S t e m S iz e Valve Friction P1 M ax dP M ax S ea t L o ad U n b a l F rc e @ O p e n Pa Pb Fluid Neg. Gradient dP Flowing Bellows (Y/N) ?

667 ED DO W N N 2 .31 3 in 0 .2 70 in 2 1 .12 5 in 0 .50 0 in 50.00 lbf 300.00 psig 300.00 psid 20.00 lbf/in 0.00 lbf 0.00 psig 18.00 psig 1 .80 in 150.00 psid N

Spring & Diaphragm Calculated Results -- - -- - -- -- - - -- -- - -- - -- - -- - -- - -Actuator Size Spring Spring Rate Spring Windup M ax Spring Load M in R e q'd A ir S up .

- - -- - -- - -- -- - - 40 1 E 8 05 3 736.00 lbf/ 0.375 in 1 1 0 4.3 lb f 2 0 .0 0 p sig

Actr. Outp. Thrust Req'd Valve Thrust Air Spring Rate L o we r B en c h S et Lower Stroking L o w F r ic t B a n d ( + / - )

2 7 6.3 lb f 276.3 lb lbf 1 4 08 .5 8 lb f 4 .0 0 psig 2 .8 3 psig 0 .7 2 psig

U p p er B en ch S et 1 6 .0 0 p sig Upper Stroking 1 6 .0 0 p sig Up Frict Band (+/-) 0 .7 2 psig Lower Operating 0 .0 0 psig Upper Operating 1 6 .7 3 p sig  Read Free Foron 30this Days Sign up to vote title

Master your semester with Scribd 30  Useful  Not useful & The New York Times Detailed Sizing ScreenFigure Of The Fisher Sizing Program Special offer for students: Only $4.99/month.



Cancel anytime.

Fluid Fluid Ne . G adient - Here the specifier enters the value of K that has been determined for the

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Relative lativeFrequency Of Proble ProblemsThat Are AreCaused By Negative Gradie Gradients

Experienced specifiers have observed that negative gradients are not a p roblem in a very high p ercentage control valve installations. However, when negative gradients do cause problems, the problems can be significant in terms of poor valve performance. In order to prevent problems, many specifiers evaluate th potential potenti al for negative ne gative gradients gradien ts whenever when ever the th e following follow ing valve va lve constructio con structions ns are being b eing co nsidered: nsidere d: • • •

Flow down unbalanced valves Large unbalanced valves (valve sizes > 6-inches) High pressure applications (∆P > 300 psid)

Troubleshooting

As mentioned previously, many specifiers completely ignore the potential impacts of negative gradients; they do not solve the stability equation prior to valve selection. As a result, performance problems are occasionally encountered after the valve is installed. When one is called upon to troubleshoot an instabil problem, proble m, and when the instability inst ability problem p roblem cannot canno t be readily rea dily attributed attr ibuted to excessive exce ssive lo op gain, gai n, to limit lim it cy to other causes, the stability equation may be solved in order to determine if a control valve negative grad the source of the instability problem.



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SELEC EL ECTI TING NG ACTUATOR ACCES ACCE SSORIES ORIE S TO PROVIDE PROVIDE THE REQUIRED REQUIRE D ACTUATOR STROKIN TROKI NG TIMES TI MES Clarif larification ication Of Te T erminology And Application Application Requir Require ements Clarification Of Terminology

Stroki Stroking ng Speed - Specifiers commonly describe the amount of time that is required for a valve t

travel from one travel stop to the other as “stroking speed”. The term stroking speed implies that speed of actuator stem travel will be measured in units such as inches per second. However, actu performance perform ance is rarely described de scribed in terms of a distance dista nce per unit of time. t ime. Therefore, Ther efore, the term “stro speed” is a misnomer and it should not be used use d to describe de scribe the speed spe ed of control c ontrol v alve operation op eration

Stroking Time - The correct term for describing the speed of operation of control valves is “stro

time”. For example, a control valve specification may require a stop-to-stop stroking time of one second or less.

Clarifi larification cation Of Applicat lication Require uirements The requirement for a short stroking time dep ends upon the specific requirements of each application.

Fast Re Response toNorma Normal Operating Operating Tr Transients - For many applications, the primary objectiv

ensure that the control valve quickly responds to any normal operating transient. Short stop-to-s stroking speeds are not critical for the successful control of most systems.

Short Stop-To-Sto -T o-Stop Stroking troking TimeFor Critical itical Applicat Applications - For some applications such as

emergency shutdown, compressor surge, and p ump bypass, the successful and safe operation of system may require short stroking times.

Stroking Tim TimeVs. Con Control Accuracy (Overshoot) - Stroking times are often shortened throug

use of equipment that increases the loop gain. Increased loop gain can also cause system instabil Therefore, specifiers must balance the requirements for a short stroking time with the requiremen  system stability. Read Free For 30 Days

Master your semester with Scribd Sign up to vote on this title  & The New York Times Useful Not useful   Extended Stroking Stroking Time Times - In some instances, extended stroking times are desirable. Extended Cancel anytime.

Special offer for students: stroking Only $4.99/month. times help to minimize pipeline surges, water h ammer, and other hydraulic events that occur as a result of rapid changes in pressure and flow conditions.

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Param Parameters That Influe nfluenceActuator Stroking Stroking Time Time

The major influences that impact actuator stroking times are shown in Figure 32 and they are discussed b





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Supply Pre Pressure ureAnd Capacity Capacity

The supply pressure and capacity have a direct bearing on stroking time. Factors that influence the suppl

factors tha pressure are the pressure of the plant air system and the set pressure of the filter regulator. The factors influence the supply capacity include the capacity of the plant air system and the capacity of the filter reg

Cv Of Of The TheL oading Ins Instrument

Supply C v - The supply capacity of the instrument that loads the actuator diaphragm has a direc

bearing bearin g on stroking s troking time. The Th e instrument instru ment co uld be an I/P transduc tra nsducer, er, a positione po sitioner, r, or a volume v olume Exhaust Cv - Each instrument also has an exhaust capacity. The stroking time is directly influen the ability of the instrument to exhaust pressure from the actuator diaphragm casing.

Tubing And Fitting Size Reduced stroking times can be achieved by specifying oversized components for the tubing, piping, and that are used to connect the instrumentation and the actuator diaphragm casing.

Diaph Diaphragm Are Area

The diaphragm area has a significant bearing on stroking time. In general, a larger diaphragm area will re more time to p ressurize; therefore, larger actuators will typically provide longer stronger times than small actuators.

Actuator Volume The volume between the diaphragm and the pressurized diaphragm casing influences stroking time. The actuator volume at each extreme of v alve travel must be considered.

Clearance Volume, Vo - The clearance volume is the volume between the diaphragm and the pressurized pressu rized actuator ac tuator casing ca sing when wh en the control v alve is at a t 0 percent perce nt travel. trave l.

Total Volume, Vm - The total volume is the volume between the diaphragm and the pressurized Master your semester with Scribd  actuator casing when the valve is at 100 percent travel. The total volume includes the clearance v Read Free Foron 30this Days Sign up to vote title plus the th e displaceme disp lacement nt volume volu me (the volume volum e that is displaced displac ed by the actuator act uator diaphra d iaphragm gm as the t he val & The New York Times Useful Not useful   stokes from 0 percent travel to 100 percent travel). Special offer for students: Only $4.99/month.

Cancel anytime.

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Fill Filling ing And And Exha Exhausting Pressures

Initial Fil Filli ling ngPressure, Pi Fill - The initial filling pressure is the pressure at which the actuator

starts to move away from the 0 percent travel position; i.e., the diaphragm pressure that is requir overcome the force of initial compression and the control valve friction forces. Final Filling Pressure, Pf Fill - The final filling pressure is the pressure at which the valve stem reaches the 100 percent travel position.

Exhaust Pre Pressure ures

Initial Exhaust Pre Pressure, Pi Exhau E xhaust - Pi Exhaust is the pressure at which the valve stem begin

move from the 100 percent travel position to the 0 percent travel position. Final Exhaus E xhaust Pressure, Pf Exhaust - Pf Exhaust is the pressure in the diaphragm casing when control valve reaches the 0 percent travel position.

Vent Cv

The actuator stroking time is also influenced by the capacity of the vent that is located in the non-pressur diaphragm casing. As stroking times become shorter, a standard v ent may create a restriction that will tra the non-pressurized diaphragm casing, thereby slowing the movement of the actuator stem.

Performing rforming Stroking Time TimeCalculations With With The TheFish Fisher Sizing SizingProg Program Obje Objective Of Calculations C alculations

The Fisher Sizing Program includes a means for estimating the stroking time of a control valve assembly calculations can be performed to ensure that a selected valve an d actuator will meet the application requirements or the calculations may be performed in order to troubleshoot a co ntrol valve assembly that already in service.

Overview Of TheCalculation Proce Procedure And The T heStroking Time TimeCalculation Screen

Master your semester with Scribd Prior to calculating the actuator stroking time, one must first size an actuator with the use of theDetailed Read Free Foron 30this Days Sign up to vote title  Method of the Fisher Sizing Program. M uch of the data that is required to perform the stroking time & The New York Times Useful Not useful   calculations is available from the calculated results section of the Detailed Sizing Method screen. The Sp Cancel anytime.

And-Diaphragm Actuator Stroking Time screen is shown in Figure 33. The fields are discussed on the Special offer for students: Only $4.99/month. following pages.

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Selecting and Sizing Spring Diaphragm Actuators and Related Acces R ev 1.42

Spring & Diaphragm Stroking Time

Actuator & Spring ---------------------------------- ------------------------------------------------Area at top 69.00 in2 Area at midpoint 69.00 in2 Area at bottom 69.00 in2 Vo 57.0 in3 Vm 1 4 2.0 in3 Spring Rate 736.0 lb lbf/in Actuator Pressures -------------------------Pi Fill Pf Fill Pi Exh aust Pf Exhaust Air Supply F1-HELP F2-Calc F3-Option

Air Supply Cv's --------------------------------------- ----------------------------------Air Cv Fill 0 .14 0 Air Cv Exhaust 0 .24 0

-------------------------3 .5 5 p s i g 1 6 .3 3 p s i g 1 4 .8 9 p s i g 2 .1 1 p s i g 2 0 .0 0 p s i g F5-Clear

Calculated Times ----------------------Prestroke Fill M oving Fill Total Fill Time

0 .1 6 8 s e c 3 .3 4 8 s e c 3 .5 1 6 s e c

Prestroke Exhaust M oving Exhaust Total Exhaust Time

0 .3 8 0 s e c 2 .4 9 6 s e c 2.876 sec

F8-Unit

F10-Exit

Figure 32 Spring-And-Diaphragm Actuator Stroking Time Screen

Actuator And Spring Entry Fields Area At Top Top, Ar Area At Midpo Midpoint, Ar Area At Bottom - The area of an actuator diaphragm is not

constant over the rated travel of the control valve. If the area of the diaphragm does change over rated valve travel, and if the diaphragm area at b oth travel extremes and at the midpoint of valve are known, the values may be entered in the appropriate fields. If the diaphragm areas at various points are not known, known , a single sing le entry is all that tha t is required. requ ired. The Th e diaphragm diaph ragm areas a reas of v arious actuat a ctuat sizes are listed in tabular format in the Help Screens of the Fisher Sizing Program.

Master your semester with Scribd clearance volumes of various actuator types and sizes are Vo (Clearance (Clearance Volume ) - The  listed i Read Free For 30 Days Sign up to vote on this title tabular format in the Help Screens of the Fisher Sizing Program.  various types and sizes are liste V m (Cas (Cas ing Volume) - The actuator casing volumes of & The New York Times Usefulactuator useful  Not Cancel anytime.

format in the Help Screens of the Fisher Sizing Program. Casing volumes vary as a funct Special offer for students: tabular Only $4.99/month. valve travel. The value of Vm includes the clearance volume (Vo).

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Actuator Pre Pressure Entry Entry Fields Fields

Pi Fill , Pf Fill, Pi Exhaust, Pf Exhaust- These values may be calculated from the actuator stro

pressure pressu re range and the actuator friction band that th at are determined de termined with the use of the t he Detailed Detail ed Siz Method of the Fisher Sizing Program. The values of Pi Fill , Pf Fill, Pi Exhaust, Pf Exhaust are calculated as follows: Pi Fill Pf Fill Pi Exhaust Pf Exhaust

= lower stroking p ressure + friction = upper stroking pressure + friction = upper stroking pressure - friction band = lower stroking pressure - friction band Ps, Supply Pre Pr essure ure - The supply pressure is the maximum pressure that is available to the instruments that load the actuator diaphragm; i.e, the set pressure of the filter/regulator. This val displayed in the calculated results section of the Detailed Sizing Method as the Minim inimum Req

Air Supply. Air Supply

Air Cv Fill And Air C v Exhaust - The filling and exhaust C v‘s of the instrument or instrument

load the actuator diaphragm are entered in these fields. The fill and ex haust C v ratings of variou instruments are located in the Help Screens of the Fisher Sizing Program.

Summing Summing C v’s - If pneumatic instruments are piped in series, the C v’s of the appropriate instru must be summed. For example, if a Fisher Type 67AFR filter/regulator provides the supply pres a Fisher Type 3582 positioner, the total C v for filling or exhausting the actuator casing will be substantially less than the loading or filling C v of either of the two instruments. A utility is inclu the Fisher Sizing Program that will sum the C v’s of two or more instruments. Calculated Time Times Prestroke Fill - This value is the time that is required to pressurize the actuator casing prior to actual movement of the actuator stem.

Master your semester with Scribd  Read Free Forto 30 Days Signvalve up tostem vote on this title move a distance that is Moving Fill - This value is the time that is required for the  to the rated valve travel. & The New York Times  Useful  Not useful Cancel anytime.

Special offer for students: Only $4.99/month. Total Fill Time - The total fill time is the sum of the prestroke fill time and the moving fill time

the total time that is required to stroke the valve from the 0 percent travel position to the 100 per

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Pre PrestrokeExhaust - This value is the time that is required to exhaust the diaphragm casing pres prior to any actual ac tual movemen mo vementt of the actuator a ctuator stem.

Moving Exhaust - This value is the time that is required for the valve stem to move a distance t equal to the rated valve travel.

Total Exhaust Time - The total exhaust time is the sum of the prestroke exhaust time and the m

exhaust time; i.e., the total time that is required to stroke the valve from the 100 percent travel po to the 0 percent travel po sition.

Interpreting The TheResults Of TheStroking Time TimeCalculations

L imits Of Accu Accuracy - The stroking time equations that are included in the Fisher Sizing Progra

generally accurate to within +/- 25 percent of the actual stroking time. However, when the calcul stroking is 1 second or less, the error has been observed to be as much as 50 percent of actual st time. Exclusion Of Shutoff Force Forces - The stroking times that are calculated are the times that are requ the actuator to overcome packing friction, overcome the forces of valve unbalance, and move th plug from f rom one on e travel trave l stop to the other. oth er. The calculations calculat ions do not account for the time that is requir generate the maximum seat load.

ReducingTheStroking Time Time

If the stroking time that is calculated is longer than the desired stroking time, one may select different act and instrument options that are designed to reduce stroking times. Then, the stroking time calculations ar repeated in order to determine if the selected options will provide the d esired results. The options that are commonly selected to shorten stroking times will be discussed in the next section of this Mo dule.

IncreasingTheStroking Time Time

If the stroking time that is calculated is shorter than the desired stroking time, one may select different ac and instrument options that will increase the stroking time. For example, one may select: A larger actuator. •  Read Free Foron 30this Days Sign up to vote title An actuator loading instrument with reduced C v ratings. •  A filter regulator with a reduced capacity. • Not useful  Useful  Cancel anytime. optional needle valve that can be installed in the piping that supplies the actuator diaphr Special offer for students: •OnlyAn $4.99/month. pressure. pressur e.


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Accessorie ories And Options Options That That May Be Be Selected To To ReduceTheActuator Stroking Stroking Time Volume Boosters

As shown in Figure 34, volum e boosters accept a pneumatic input signal and a supply pressure, they provide a pneumatic output pressure . The input signal modulates the supply pressure in ord produce produ ce an output outpu t pressu re that is i s proportiona prop ortionall to, but bu t independen indep endentt from, the pressure pressur e of the inp signal. The output pressure is at a much higher capacity than the input signal.

VolumeBooster Operation - Volume boosters typically provide a 1:1 ratio of output pressure t pressure, pressu re, but they t hey include inc lude larger lar ger valv e ports for the purpos p urposee of increasing inc reasing the capacity of the out pressure. pressu re. Most Mo st volume volu me boosters bo osters are a re dead-band volume boosters. As shown in Figure 34, dead b volume boosters include a bypass circuit and a bypass valve. During normal throttling when the changes in the control signal are relatively small, dead band boosters direct the input pressure to downstream device through the bypass circuit. If the input pressure suddenly increases, the capa the bypass circuit is insufficient to pass the flow and the pressure within the booster body begins build u p. The increase i ncrease in pressure press ure pushes pu shes the th e diaphragm diap hragm downward downw ard which wh ich in tu rn opens ope ns the l supply valve. When the valve op ens, the full supply pressure is directed to the booster output connection. The magnitude of the change in the input signal that must o ccur to initiate a booster response is adjusted with a needle valve. Until this magnitude of change occurs, there is no boos response, hence the name dead band booster. booster. Use Word 6.0c or later to




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Figure 33


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Instrument Gr GradeVolumeBoosters - Instrument grade volume boosters provide an output pre

that is approximately equal to the input pressure. As a result, instrument grade boosters provide throttling control. In order to achieve accuracy of the output pressure, the relative capacity of the instrument be limited (valve port sizes must be relatively small). High-Capa igh-C apacity city Volume Boosters - High-capacity boosters include very large valve ports for th purpose purpo se of initiating in itiating large and a nd rapid rap id changes chan ges in the output ou tput pressure. pre ssure. However, Howev er, high capacity capacit y volu boosters booste rs are not n ot as accurate ac curate as instru ment grade gr ade volume vo lume boosters b oosters;; i.e., the output pressure pressur e of a h capacity volume booster may temporarily rise or fall to a value that is significantly above or belo input signal pressure. Two potentially negative consequences must be carefully evaluated when considering the use of a high-capacity volume booster. 1. 2.

The high gain of the booster can cause valve plug instability instability if if the bypass valve is not preci adjusted. The action of the the valve plug slamming into into a travel stop can result in damage to the plug, th or the actuator.

Because of the potential for instability and equipment damage, high-capacity volume boosters ar typically selected only when very short stop-to-stop stroking times are required in order to prote equipment from damage or to ensure the safety of operating personnel.

Relative lative Capacitie cities Of Volume VolumeBoosters - The capacities of several different types of instrume

shown in Figure 35. Note that volume boosters provide considerably greater capacity than other pneum atic instru ments. Also A lso note the difference diffe rence in capacity between betwee n the high-cap hi gh-capacity acity booster bo oster (t Fisher Type 2625) and the instrument grade volume booster (the Moo re Type 61H).

Instrument

Typical Filling C v

Fisher Type 546 I/P Transducer Fisher Type 3582 Positioner Moore Type 61H Volume Booster Fisher Type 2625 Dead Band Volume Booster with 1/2 inch ports and tubing

0 .3 9 0 .1 9 0 .7 0 7 4.98

Master your semester with Scribd Read Free Foron 30this Days Sign up to vote title Figure 34 & The New York Times  Useful  Not useful Relative Capacities Of Volume Boosters And Other Instruments Special offer for students: Only $4.99/month.

Cancel anytime.

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VolumeBooster Appli Application cation Guide Guideline lines - Several guidelines that should be considered during selection of a volume boo ster are illustrated in Figure 36 and they are d iscussed below. Use Word 6.0c or later to


Figure 35 Volume Booster Application Guidelines

Select Volume Boosters Only When Necessary - High-capacity dead band boosters can produce large and sudden changes in pressure. Such large and sudden chang es can result in erratic and u loop performance if the booster is not applied and adjusted properly. As a result, high-capacity d band boosters boost ers should sho uld be selected selecte d only when there is a legitimate legitimate requirement for a very short time, and when the installation and calibration of the booster can be supervised by experienced personnel. perso nnel.

Connect To Large Volume Devices Only - The ou tput of a dead band volum e booster should on connected to the input of a large volume device such as a pneu matic actuator. Because volume b produce produ ce high-capa hig h-capacity city ou tput signals, sig nals, they th ey may easily over drive d rive the low-volume low-vo lume input in put sections se ctions other instruments.

Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title  Install A Booster In Combination W ith A Positioner - H igh-capacity dead band boosters are typ & The New York Times Useful  Not useful  used only in combination with a control valve positioner. The combination of a high-capacity bo Cancel anytime.

Special offer for students:and Onlya $4.99/month. control valve po sitioner provides extremely fast response while preserving some measure positioning positio ning accurac a ccuracy. y.

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Oversized Piping Piping And Fittings Fittings

Size Cons Considerations For Pneumatic Tubing Tubing And Fittings Fittings - If high-capacity instruments are se

all of the tubing and the fittings that are located downstream of the h igh-capacity instrument sho at least as large as the output p ressure connection of the high-capacity instrument. If any of the that are used to connect the instruments and the actuator are smaller than the tubing size, the pub instrument C v is effectively reduced by one third for each reduction in standard pipe size. The im of the size of fittings on the rated C v of a loading instrument is shown in Figure 37.

I nstrum trument Connection ction Size, Size, Inche I nches

Pipe ipe Fit Fitting ing Size ize, Inc Inches

3/4 3/4 1/2 3/8

1/2 1/4 1/4 1/4

Mult Multip iply ly The The Ra Instrument Cv . 67 . 29 . 44 . 67

Figure 36 Impact Of Fitting Size On The C v Of The Loading Instrument

Actuator L oading Pre Pressure ureConnections - A common error is to connect a high capacity loadi

pressure pressu re to a standard standar d size actuator a ctuator fitting as a s shown show n in Figu re 38. To T o avoid this com mon error, er ror, o should specify an optional, enlarged actuator connection when high capacity instruments have b selected. Enlarged actuator connection ports such as the o ne that is shown in Figure 38 are avail options for most actuators.


Master your semester with Scribd & The New York Times view Macintosh picture.



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High-Capacity igh-Capacity Supply Pre Pressure ureRegulators ulators And Filters Filters

High-Capacity igh-C apacity Supply Pre Pressure ureRegulators ulators - When high-capacity instruments are selected, th

capacity of the supply pressure regulator must be considered. In most applications that include h capacity instruments, the standard regulator must be replaced with a high-capacity model; e.g., t standard Fisher 67AFR (Cv = 0.28) is commonly replaced with a type 98H (Cv = 1.9 to 4.4 depe on connection and port size). Refer to Figure 39. High-Cap igh-Capacity Supply Pre Pressure Filte Filters - The supply pressure filter can have a limiting effect o capacity of the supply pressure. For ex ample, the C v rating of the standard Fisher 67AFD filter i while the C v rating of the Fisher Type 262 filter is 10. Refer to Figure 39. Use Word 6.0c or later to


Master your semester with Scribd & The New Times QuickYork Release E xhaust Valves Valves

Figure 38

Typical C v Ratings Of Standard And High Cap acity Air Filters And Regulators



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As shown in View A of Figure 40, the diaphragm casing volume typically exhausts through the instrume

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I nstrument

Exhaust Cv

Fisher Type 546 I/P Transducer Fisher Type 3582 Positioner Moore Type 61H Volume Booster Fisher Type 2625 Volume Booster (1/2 inch port) Rexroth Quick Release Valve - 3/8 inch port Rexroth Quick Release Valve - 1/2 inch port Rexroth Quick Release Valve - 3/4 inch port Rexroth Quick Release Valve - 1 inch port

0 .3 6 0 .1 9 1 .0 2 3 .3 2 3 .0 8 5 .5 1 0 .5 5 1 3 .4 5

Master your semester with Scribd Free Foron 30this Days Sign up to vote title Figure 39 Read Installation And Relative C v Ratings Of Release Valves & The New York Times Useful useful Quick  Not

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Special offer for students: Only $4.99/month. Quick Re R elease Exhaust Valve ValveOperation - The quick release valve is essentially a pressure sw

that is triggered by differential pressure. When the output p ressure from the loading instrument i

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Figure 40 Operation Of A Quick Release Valve

Actuator Action Action Versus Stroking Stroking Time Reduction - Because quick release valves can only

the time that is required to exhaust the diaphragm casing volume, they are useful only when ther need to reduce the time that is required to move the valve plug in the direction of the spring forc shown in Figure 42, a quick release valve that is used in conjunction with a direct-acting actuato push-down-to push- down-to -close control c ontrol valve v alve will wil l reduce the time that is required re quired to open the valv e. Similar quick release valve that is used in conjunction with a reverse-acting actuator and a push-down-to control valve will reduce the time that is required to close the valve.



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Figure 41

Impact Of Quick Release Valves On The Stroking Time Of Direct And Reverse-Acting Actuators

Quick Quick Re Release Valves And The ThePotential For For Ins I nstability ility - Because quick release valves provi

Master your semester withInScribd to cause system instability. order to provide a means for tuning the relative speed of exhaust, Read Free For 30 Days Sign up to vote on this title needle valve is typically installed in a bypass circuit as shown in Figure 43. As the needle valve & The New York Usefulis forced opened,Times an increasing amount of the diaphragm casing  p ressure to useful exhaust through the  Not high-capacity path for exhausting the actuator, the actuator may change position with sufficient

Cancel anytime.

Because of the limited capacity of the instrument, opening the needle valve has the e Special offer for students: instrument. Only $4.99/month. of increasing the time that is required to exhaust the actuator casing.

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Figure 42 Needle Valve A nd Bypass Byp ass For Adjusting Adjustin g The Exhaust E xhaust Time

Enlarg nlarged Actuator Ve Vents

When very short stroking times are required, the capacity of the actuator vent passage must be sufficientl Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title to ensure unrestricted venting. If the vents are un dersized or obstructed, the diaphragm will have to push  a pressure or pull against a partial vacuum. As a general guideline, capacity theuseful actuator vent shou & The New York Times Useful  the ofNot approximately equal to or greater than the C of the instrument that provides the actuator loading pressur


v

Cancel anytime.

In a direct-acting actuator, the standard vent path is along the actuator stem and through the clearance be

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In a reverse-acting actuator, the standard vent path is through the threaded connection on the upper diaph casing. To prevent weather, insects, and debris from entering the casing, a vent cap is typically threaded connection as shown in View C of Figure 44. When a larger vent C v is required, optional, high capacity may be specified.

When extremely short stroking times are specified, the valve manufacturer typically calculates the approp vent size. Use Word 6.0c or later to


Figure 43 Standard And High Capacity Actuator Vents

Other Stroking Str oking Time Conswith iderations iderations Master your semester Scribd & The New York Potential For Valve VTimes alve Damage Special offer for students: Only $4.99/month.


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One of the inherent problems of a very short stroking time is that the valve plug, the valve plug stem, the

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Matching Stroking Stroking Times To TheRequire quirements Of The TheApplications pplications

There is a tendency among specifying engineers to overstate the stroking time requirements. In In order to p the unnecessary selection of accessories that can introduce control system complexity and the potential fo control, specifiers should always verify the need for short stroking times.

Impact Of Re ResponseTimeOn Stroking Stroking Time

An influence that is rarely recognized is the response time (time constant) of the transmitter and the contr as well as the response characteristics of the instruments that supply and control the actuator loading pres The stroking time calculations typically do not consider the lags that can be introduced by one or more upstream instruments.



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SELECT EL ECTING ING ACTUATOR ACT UATOR ACCESS ACCESSORIES TO ACHIEVE ACHIE VE THE DESIRED FAIL MODE MODE Terminology Definition Of A “Failure “

The selection of a particular fail mode is based on an evaluation how a lo ss of suppl y pr essure essure would aff process. proces s. Consider Cons ider the example examp le of a control c ontrol valve v alve that th at contro ls the flow flo w of steam ste am to a heat exchang ex changer. er. One concern is that high temperatures can damage the heat exchanger tubes. To prevent overheating, a fail-cl fail mode may be selected. However, if a fail-closed fail mode is specified, and if the material in the exch would solidify if cooled, the potential for equipment damage and downtime could be great. In this applic fail-open fail mode may be more appropriate, especially if the heat exchanger can be protected from high temperature by other means such as a relief valve.

Inherent Vs. Vs. Eng Engineered Fail Fail Mode

Inherent Fail Mode- Actuators that include a spring provide an inherent fail mode. In the even loss of supply pressure, the spring will move the valve plug to one travel extreme or the other, depending on the actuator action and the valve style, as previously discussed.

Engineered Fail Fail Mode Mode- When an actuator does not include a sp ring; e.g., a double-acting pisto

actuator, or when it is desirable to initiate a fail mode that is opposite of the actuator’s inherent f mode, a fail mode must be engineered. An engineered fail mode is implemented with the use of accessories such as solenoid valves, switching valves, and trip valves. This Module will discuss application of the accessories that are commonly selected to implement an engineered fail mode

Solenoid Valves Valves Description cription And Function Solenoid valves are small, on-off valves that are activated by en ergizing an electrical solenoid. Solenoid may be included in a control valve assembly to manipulate the actuator loading pressure.

Master your semester with Scribd Configurations & The New York Times


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hre hree -Way -Way Solenoid Valves Valves- Solenoid valves are available in many different types and styles. Special offer for students: T Only $4.99/month.

very popular configuration is the three-way solenoid valve that is shown in Figure 45. Note that

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Figure 44

Typical 3-Way Solenoid Valve

Normally Ope Open Or Or Normally Close C losed Ve Versus Universal Type T ypes - Some solenoid valves are des

as normal ly open , and other solenoid valves are designated as universal type open or normal ly closed closed Normally Norma lly open ope n and normally norma lly closed clos ed solenoid sole noid valves v alves are a re desig ned to control contro l flow in one specific sp ecific direction; e.g., from the common port to port A or B, or vice-versa. Universal types are for bidirectional flow. To allow proper venting of the actuator casing back through the supplying instr universal types must be selected for throttling applications.

Applications

Initiating The Inherent Fail Mod M ode- In some control strategies, it may be beneficial to force the

actuator to initiate its inherent fail mode. For example, a valve may be selected that will provide closed inherent fail mode that will protect the downstream process in the event that there is a  conditio . However, failures in other systems mayRead alsoup cause unsafe or unwanted supply pr essure essure Free For 30this Days Sign to vote on title example, the failure of a downstream device may cause an overpressure condition in the  piping Useful Not useful   downstream of the control valve. To protect against the over pressure condition, a pressure switc Cancel anytime. Special offer for students: installed Only $4.99/month. in the downstream piping. If the downstream pressure increases to a value that is above limit, the switch will trip thereby completing a circuit that energizes a solenoid valve. The inher


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Figure 45

Initiating The Inherent Actuator Fail Mode With A Solenoid Valve

Initiating A Fail Fail Mod M ode That Is Opposite TheInherent Fail Mode- The control valve assemb be forced for ced to fail, f ail, throu gh the installation installati on of a solenoid soleno id valve, valv e, in the d irection that is o pposite fail mode. Figure 47 show s the basic concept. In normal operation, the pressure from the current pressure pressu re (I/P) tran sducer supplies s upplies the actuator actu ator loading loa ding pressure. pr essure. If I f the solenoid sole noid co ils are energize en ergize full regulated supply pressure is sent directly to the actuator.


Master your semester with Scribd & The New York Times view Macintosh picture.



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L IL P From FromA RemoteSignal - A solenoid valve may be configured to initiate a lock-in-last-po

(LILP) fail mode, as shown in Figure 48 . During normal operation, the transducer controls the ac diaphragm pressure. When the solenoid is energized, the actuator is isolated from the transducer is connected to the solenoid valve port that is blocked with a plug. As a result, the actuator diaph pressure pressu re is "locked "l ocked"" into the diaphragm dia phragm casing .



Figure 47

Initiating The LILP Fail Mode With A Solenoid Valve Trip rip Valves Description

A trip valve, or switching valve, is actually a special type of pressure regulator that has the ability to sens failure in the supply pressure. Trip valves can be configured to respond to a failure in a variety of ways. A common application for a trip valve is to provide a lock-in-last position (LILP), or fail in place,fail mode Read Free Foron 30this Days Sign up to vote title

Master your semester with Scribd & The New York Times Single-A le-Acting ctingTrip rip Valve Valve Special offer for students: Only $4.99/month.

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If the regulated supply pressure falls below the trip valve set pressure, the trip valve spring will force the valves downward. As the lower valve seats, the existing actuator loading pressure is "locked in" the diap casing, and a lock-in-last-position fail mode is achieved.



Figure 48

Typical Trip Valve (Switching Valve) Configuration Double DoubleActing Trip Trip Valve Valve

Definition - A double-acting trip valve includes two pressure switching circuits that are triggere whenever the supply pressure falls below the trip pressure. The trip pressure may be adjusted to pressure pressu re value. value .

Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title  f Application Application - Because a springless, double-acting piston actuator does not provide an inherent & The New York Times Useful  Not useful  mode, and because both the upper cylinder pressure and the lower cylinder pressure must be con Cancel anytime.

Special offer for students: in Only $4.99/month. order to achieve the desired stem position, a double-acting trip valve is typically selected to cr engineered fail mode of operation for a springless, double-acting actuator.

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Figure 49

Typical Trip Valve (Switching Valve) Configuration

Supply Pressure > Trip Pressure - When the supp ly pressure is greater than the trippressure Master your semester with Scribd Free Foron 30 Days Signves up to vote this title shown in View A of Figure 50, the supply pressure Read mo the valve spool upward, thereby  exhaust valve. Because the exhaust valve is closed, the supply loads the upper s & The New YorktheTimes Not useful  Useful pressure •

Cancel anytime. of the lower diaphragm, thereby forcing the main valves downward. When the main valves forced downward, port A is conn ected with port B and port C is sealed off. Similarly, port D connected with port E and p ort F is sealed off.


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Fail Down Operation - Figure 51 shows a double-acting trip valve that is configured for a fail-d

mode of operation. During normal operation, the positioner outputs pass throug h the switching v the upper cylinder and to the lower cylinder. The supply pressure is piped through a check valve pressurize pressu rize a volume v olume tank. The Th e volume volu me tank provides provid es the pressure p ressure that is needed n eeded to operate ope rate the in the event of a failure in the supply pressure.

When the supply pressure falls below the trip pressure, the main valves move upward. The lowe cylinder pressure is vented to atmosphere through ports E and F and the supply pressure (from th volume tank) is directed to the upper cylinder through ports C and B. The pressure unbalance causes the actuator piston and stem to move downward. When the supply pressure becomes greater than the trip pressure, the main valves move upward normal operation is resumed.





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Fail Up Operation - Figure 52 shows a do uble-acting trip valve that is configured for a fail-up m

of operation. The system is the same as the fail-down configuration that is shown in Figure 51 e that, during a failure, the volume tank supplies pressure to the lower cylinder through ports F an and the upper cylinder is vented to atmosphere through ports B and C . The pressure unbalance c the actuator piston and stem to move upward. When the supply pressure becomes greater than the trip pressure, the main valves move upward normal operation is resumed.





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When the supply pressure falls below the trip pressure, the main valves move upward. Ports B a are connected and ports E and F are connected. However, because ports C and F are plugged, the cylinder pressures are “locked” in. When the supply pressure becomes greater than the trip press the main valves move upward and normal operation is resumed.



Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title  Figure 52 & The New YorkLock-In-Last-Position Times Useful  Not useful  Trip Valve Configuration In The Tripped Mode Special offer for students: Only $4.99/month.

Cancel anytime.

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WORK AID AI D 1. PROCEDURES PROCEDURES THAT ARE USED USED TO PERFORM DETAIL DETAI L ED ACTUATOR ACT UATOR S CALCUL CAL CULATI ATION ONS S WITH THE T HE USE USE OF THE FISHER FISHER SIZING PROGRAM

Note: W hile the Detailed D etailed Sizing Method M ethod calculates calculate s the most m ost precise prec ise actuator actua tor specification spec ification,, the method me thod re entry of detailed valve information that must be located in a variety of locations. To expedite the en try of information, one may first perform the actuator sizing calculations with the use of the Quick Sizing meth and, then, migrate the data to the Detailed Sizing Method. Work Aid 1A, below, gives the procedural ste the use of the Quick Sizing Method. Work Aid 1B gives the procedures for migrating data to the Detailed Sizing Method and the procedures for performing a detailed sizing calculation.

Work Aid 1A. Pr Procedures That Are Are Used To To Perform Actuator Sizing Calculations CalculationsWith With The TheUse Quick Quick Sizing Sizing Method Of TheFish Fisher Sizing SizingProg Program 1.

With the Fisher Sizing Program running, running, press the ESCAPE key to return to the the main menu if necess From the main menu, select Ssact. 2. From the menu that appears, select Spring & Diaph Diaphragm under the heading Quick-Sizing Method 3. Press and hold the Alt key, and, then, press the F5 key to clear any existing values. values. 4. Place the cursor on the Actuator Typefield, press the F4 key to view the pull-down menu, and selec appropriate actuator type. If necessary, refer to Fisher Catalog 14, page B-27, Table 12 in order to se actuator type (657 or 667) that will provide the desired fail mode. 5. Place the cursor on the Air To Diaphragm field, press the F4 key to view the pull-down menu, and the appropriate pressure range. Note: If the appropriate pressure range is not listed as a choice, select nearest pressure range. The precise values of Pa and Pb may be changed after the data has been migr the detailed sizing method. 6. Place the cursor on the With Side MO field, press the F4 key to view the pull-down menu, and selec (for yes) or N (for no) to indicate whether or not a side-mounted manual operated is included in the assembly. 7. Place the cursor on the Valve alveDesign field, press the F4 key to view the pull-down menu, and select appropriate valve design. 8. Place the cursor on the Body Size Size field, press the F4 key to view the pull-down menu, and select the appropriate valve body size. 9. Place the cursor on the Class field, press the F4 key to v iew the pull-down menu, and select the appr  ANSI Class rating of the valve body. Read Free Foron 30this Days Sign up to vote title  10. Place the cursor on the the Flow field, press the F4 key to view the pull-down menu, and select either Fl Useful Not useful   or Flow Down. Note: On e may also select either PTTO (pressure-tends-to-open) (pressure-tends-t o-open) or PTTC (pressure-t Cancel anytime. Special offer for students: Only $4.99/month. to-close). 11. Place the cursor on the the Trim field, press the F4 key to view the pull-down menu, and select the appr


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Place the cursor on the Seat Type Type field, press the F4 key to view the pull-down menu, and select the appropriate seat option. 12. Place the cursor on the the Packing field, press the F4 key to view the pull-down menu, and select the appropriate packing option. 13. Place the cursor on the the P1 field and enter the maximum value of P 1. 14. Place the cursor on the the dP Max field and enter the value of the shutoff p ressure drop. 15. Press the F2 key to display a list of potentially acceptable actuator actuator constructions.

16. Select the first actuator that is displayed in the list. The first entry that is listed listed will always be the sma actuator that will mount to the control valve and it will include the widest bench set span. 17. Press the ENTER key to display the actuator actuator specification. Note: If the data is to be migrated to the Detailed Sizing Method, the actuator specification that is calcula above may be ignored.

Work Aid 1B: Proc Procedures That That Are Are Used To To Perform rform Detailed Actuator Actuator Sizing Sizing Calculation Calculations After Developing lopingA Specificati cification on With The TheQuick Quick Sizing Sizing Method Migration Of Data Data From FromTheQuick Quick Sizing Sizing Method

If an actuator specification has been calculated with the use of the Quick Sizing Method, the data m ay be migrated to the Detailed Sizing Method as follows: 1. While in the Quick Sizing Method, press the ESCAPE ESCAPE key. 2. From the main menu, select the Spring-And-Diaphragm option under the heading that is titled Det

Sizing. Performing rforming TheActuator Sizing SizingCalculations 1. 2. 3.

If the data in the input fields of the Detailed Sizing Sizing Methods does not accurately describe the control the values of P a and P b , or any other information, adjust the entries as necessary. Place the cursor on the Unb @ Ope Open field and enter a value of 0. Place the cursor on the NegGrad field and enter a value of 0. 

Master your semester with Scribd 4. Press the F2 key to calculate an actuator actuator specification. specification. & The New York Times Special offer for students: $4.99/month. Selecting ctingOnly A Bench Set


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PTFE PTFE Packing Packing- If the control valve includes single PTFE packing, and if the instrument that provid

•

diaphragm pressure can produce a minimum pressure of approximately 0 psig, the calculated bench be shifted shif ted to a standard standa rd bench ben ch set range r ange (3 psig to x psig) psig ) with the th e same approximate appro ximate span s pan as the standard bench set. Graphite Packing - If the control valve includes high-friction packing, the calculated bench set sho be shifted shifte d to a standard s tandard bench set rang e. An actuator ac tuator with w ith the calculate c alculated d bench benc h set should sh ould be b e select

WORK AID 2: 2: PROCEDURES PROCEDURES THAT ARE USED TO EVAL UATE THE T HE IMPACT I MPACT OF NEGAT NEGAT FLUID FLUI D FORCE GRADIENTS GRADI ENTS ON ON ACTUATOR SIZING

The impact of negative gradients is evaluated through the use of the Detailed Sizing Method of the Fishe Sizing Program. In order to account for the impact of negative gradients, one must have previously enter the data that is required to develop an actuator specification. Refer to Work Aid 1. Then, the appropriate entered in the following fields: Fluid Fluid Neg Neg. Gr Gradient - The value of K n is entered in this field.

dP Flow Flowing - This field is not visible unless a non-zero value is entered in the Fluid Fluid Neg. Gr Gradient field value of the maximum flowing pressure drop, psid, is entered in this field.

Determining The TheFluid luid Negative Gradien Gradient, K n Balance Balanced Valves Valves

For all balanced valves, the value of K n may found in tables 19 through 21A on page B-85 of Fisher Cat 14.

The K n values for many control valves are also listed in the Help Screens of the Fisher Sizing Program. T view the appropriate screens, press the F1 key twice, press the N key to navigate to the appropriate sectio the Help Screen Index, and, then, select the entry Negative Gradients. Press the Page Down key to view t Help Screens that display the tables of K n values.

Unbalance Unbalanced Valves Valves

Master your semester with Scribd  Read Free Forand 30this Days Sign up to vote on title The K n values for some balanced valves are also listed in the Catalog 14 tables the Fisher Sizing Pro  Help Screens that are referenced above. If the value of K n for an unbalanced valve not located in the a & The New York Times Useful  Notisuseful tables, calculate the value of K with the use of the following: Special offer for students: Only $4.99/month.

n

For Flow Flow Up (PTTO) (PTTO) K = 0

Cancel anytime.

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Performing rforming TheSizing Calculations Calculations

After the values of K n and the maximum flowing ∆P have been entered (and after all of the other inform has been entered), press the F2 key to perform the calculations.

If the fluid negative gradient has an impact on the actuator specification, the software will select an If Note: actuator with a higher spring rate.



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WORK AID AI D 3: PROCE PROCEDU DURES RES AND AND SPECIFI SPECIFICAT CATION ION GUIDELINES GUIDEL INES THAT ARE USED TO SE ACTUATOR ACT UATOR ACCES ACCE SSORIES ORIE S TO PROVIDE PROVIDE THE T HE REQUIRED REQUIRE D STROKING TROK ING TIME TI ME

Work Aid 3A: Proce Procedures That Are Are Used To To Calculate CalculateActuator Stroking Time Times With The TheUseOf Fish Fisher Sizing Prog Program 1. Complete The Workshe Worksheet In order to simplify the process of gathering the needed data and calculating the stroking time, all of the necessary data can be entered on the worksheet that is shown on the following page. To complete the worksheet, perform the following: a)

With the use of the Detailed Detailed Sizing Method of the Fisher Sizing Sizing Program, select and size an appropri actuator. Refer to Work Aid 1.

b)

With the th e use the th e calculated calcu lated results res ults that tha t are obtained ob tained in step 1., above, above , enter the actuator actu ator size, size , the valv travel, and the appropriate values for Worksheet Item 1d and Worksheet Items 2a through 2e.

c)

Refer to the appropriate FSP Help Screens to locate the data that is needed to complete Worksheet It though 1c and Worksheet Item 3.

2. Perform The Stroking TimeCalculations a)

Press the ESCAPE key until until the Fisher Sizing Sizing Program Main Menu is displayed. From the Main Me select sTroking. From the menu that appears, select Spring & Diaphragm.

b)

Refer to the com pleted workshe w orksheet et and enter e nter the appropriate appro priate values va lues in the entry entr y fields of the stroking st roking calculation screen.

Note: If the diaphragm area at only one travel position is known, enter the diaphragm area in the fiel is titled Area At Top. The calculations will proceed with only one entry for the diaphragm area. c) Press the F2 F2 key to calculate calculate the stroking roking time. time. Master your semester with stScribd & The New York Times



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Hint

If the calculated stroking times are too short or too long, one may determine the supply and exhaust C are required to achieve the desired stroking times by increasing or decreasing the values in the Air Cv Su and the Air Air Cv Exh Exhau aust entry fields. When the C v values that will provide the specified stroking time h been d etermined, etermin ed, one may then th en select selec t the instrumen in strumen ts that provide p rovide the needed ne eded Cv’s.

Worksh Worksheet For For Calculating Str Stroking Time Times

Actuator Size: Size: Valve Travel:

inches

Item

FSP Entry

1.

Actuator An And Spring

a.

d.

A re a A t T o p (Diaphragm Area) Vm, Casing C asing Volume V o, C learan ce Volume Spring Rate

2

Actuator Pressures

a.

Pi Fill

b. c.

Data Source

FSP FSP Hel Help p Scr Screeen

Value

Pres Presss the the F1 ke key twi twicce, press the T key, select

Table: Diaph Diaphr ragm Area. A s A b o ve A s A b o ve F S P D e t a i le d Sizing Method

b. Pf Fill Master your semester with Scribd & The New c. York Pi ExTimes haust F S P D e t a i le d Special offer for students: Only $4.99/month.

ProcedureOr Calculation

Actuator Sizing Method

As above; values are listed as Casing Volumes. As above; values are listed as Clearance Volumes Spring Rate

lower stroking pressure + friction band upper uppe r stroking strok ing pressur p ressuree + Free Foron 30this Days Sign up to vote title frictionRead band upper stroking pressure  Useful  Not useful Cancel anytime. friction band

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Work Work Aid Aid 3B: Selection And Specificati cification on Guide Guideline lines That Are AreUsed To To Select Actuator A Acce ccessori Provide ProvideTheRequire quired Stroking Time Time General Equipme E quipment Selection Guidelines Guidelines •

To minimize the potential for control problems, specify short stroking times only when they are requ for safe or efficient operation of the process.

•

To minimize control and operational problems, d o not specify accessories that will provide stroking that are shorter than the required stroking times.

•

To prevent equipment damage, the output of a volume booster should be connected to a large volum device only; never to the small volume input section of another instrument.

•

To preserve good control, volume boosters, if selected, should be installed in combination with a positioner. positio ner.

Equipment That That May Be Selected To To Achie AchieveA Shorter Stroking Stroking Time • • • •

Volume boosters Quick release valves Smaller actuators High capacity filter/regulators

Equipment That That May Be Selected To To Achie AchieveA Longer Stroking Stroking Time • • • •

Instruments with reduced capacity Larger actuators Reduced capacity filter/regulators Reduced piping and/or fitting sizes

Master your semester Scribd  Instructions To The TheValvewith Vendor Or Valve Manufacturer Read Free For 30 Days Sign up to vote on this title  & The New York Times Useful Not useful   When stroking times are critical, the specifier should request that the valve vendor or manufacturer perfo following: Special offer for students: Only $4.99/month.

Cancel anytime.

Verify the actual stroking time through the use of laboratory tests or other methods.

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WORK AID AI D 4: GUIDELINES GUIDEL INES THAT ARE USED USED TO SEL SELEC ECT T ACTUATOR AC TUATOR ACCES ACC ESS SORIES ORIE S TO ACHIEVE THE DESIRED DESIRED FAIL MODE MODE General Guideline lines 1.

2.

Whenever an engineered fail mode is designed to provide the fail mode that is opposite of the inhere mode of the actuator, consider all the upstream and downstream consequences of a failure. Ensure th engineered fail mode that is being considered will not lead to an u nsafe situation. Ensure that the selected filter/regulator filter/regulator will provide sufficient sufficient capacity to operate all all of the the accessori

Specifi cific Equipme Equipment Selection Guideli Guideline nes Guideline uidelines For Solenoid Valve Valve Selection 1. 2.

Only universal solenoid valves should be specified. Solenoid valve seats should be made of Viton to prevent against seat leakage.

Guideline lines For Tr T rip Valve Valve Selection 1.

Select single-acting single-acting trip trip valves for spring-and-diaphragm spring-and-diaphragm actuators. (Double-acting devices may be u but they are not required). require d). 2. Select double-acting trip valves for springless piston actuators. 3. Provide the vendor or manufacturer with with the precise precise trip pressure; pressure; i.e., i.e., the pressure pressure at which the trip will initiate the fail mode. 4. If a volume tank is selected, selected, request the the vendor or manufacturer to provide a tank tank with sufficient sufficient cap stroke the valve through three complete stop-to-stop stroking cycles.



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GLOSSARY actuator

A mechanism that uses pn eumatic, hydraulic, or electronic signals to position a final control c ontrol element elemen t such as a s a control con trol valve. va lve.

actuator, spring-anddiaphragm

A compressible fluid-powered (usually by air) device in which the acts upon a flexible member, which is the diaphragm, in order to pro linear motion to the actuator stem.

air spring effect

The increase in the stiffness of a pneumatic actuator that results from actuator diaphragm or piston moving against the pressure that is trap in the actuator casing or cylinder.

bench set

The diaphragm pressure range over which the actuator stem moves a distance that is equal to the rated valve travel, with no valve forces b present (i.e., ( i.e., on th e bench); bench ); also referred re ferred to as spr ing r ange. ange.

dead band

The range through which an input can be varied without initiating an observable response.

dire direct-acting actuator

An actuator construction in which an increase in the diaphragm press causes the actuator stem to extend toward the control valve body.

dynamic force force

The valve stem force that is produced by the fluid pressure acting on control valve closure member while the valve is throttling.

dynamic instability ility

A buffeting of the closure member that results when the fluid reactio forces that act on the closure member are greater than the actuator fo See negative . negative gr adient

dynamic unbalance

The net force that results from the fluid pressure acting on the con valve closure member when the valve is throttling. A fail mode that is established through the use of pneumatic accessor

engineersemester ed fail fail mode Master your with Scribd  Read Free Foron 30this Days Sign up to vote title  Refers to the combined action ofUseful anduseful the control valve w fail-closed & The New York Times  the actuator  Not upon loss of diaphragm pressure, the actuator moves the valve closur Special offer for students: Only $4.99/month.

Cancel anytime.

member to the closed position.

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friction band

The force that is required to overcome valve friction. The friction ban equal to one-half of the total dead band that results from valve frictio

FS

The force that is produced by the actuator spring when the spring is compressed a distance that is equal to the rated valve travel.

FTFR

The sum of all the valve forces that must be overcome by the actuato

hysteresis

The maximum difference in an o utput value, excluding errors that ar caused by dead band, for any single input value during a calibration cycle.

inherent fail mode

The fail mode that is established by the action of the spring that is included in a spring-and-diaphragm or spring-and-piston actuator.

initial initial compression

The distance that an actuator spring is pre-compressed; also referred and and windin . windup

initial force

The force that is produced by the initial compression of the actuator spring; F i.

initial windup

The distance that the actuator spring is compressed before the actuato placed into i nto service. serv ice. For d irect-acting irect-actin g actuators, actua tors, the initial win dup is typically the amount of spring compression that produces a spring fo that is equal to the minimum diaphragm pressure (P a). For reverse-ac actuators, the initial windup is the amount of spring compression tha produces produc es a force for ce that is equal to the sum of the control c ontrol v alve forces for ces (FTFR ).

K a

A coefficient that quantifies the air spring effect, pounds/inch..

K n

The coefficient for the maximum negative gradient of a particular co valve, pounds/inch, normalized for 1 psid.

Master your semester with Scribd  Read Free Foron 30this Days Sign up to vote title  The spring rate, given in pound Useful of the actuator spring. K S York Times & The New useful  s force/inch,  Not Special offer for students: Only $4.99/month. lock-in-last-position

Cancel anytime.

A failure mode that is also known as LILP or fail-in-place, in in which t control valve closure member is h eld in the position that it was in

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Pb

The maximum diaphragm pressure.

PDTC

See push-down-to-close.

PDTO

See push-down-to-open.

pressure-tends-to-close

Also known as PTTC; describes a valve plug geometry and flow direction in which the fluid pressure tends to close the valve.

pressure-tends-to-open

Also known as PTTO; describes a valve plug geometry and flow direction in which the fluid pressure tends to open the valve.

PTTC

See pr essure essure - tends-totends-to- close.

PTTO

See pr essure essure -t ends-toends-to- open.

push-down-to-close

Also known as PDTC; a valve construction in which movement of th valve stem towards the body tends to close the valve.

push-down-to-open

Also known as PDTO; a valve construction in which movement of th valve stem towards the body tends to open the valve.

quick release valve

A simple pneumatic switching valve that is used to decrease the time is required to exhaust the casing or cylinder of a pneumatic actuator. the event that the input pressure to the q uick release valve falls below output pressure of the quick release valve, the output pressure is quic vented to the atmosphere.

reverse-acting -acting actuator

An actuator construction in which the actuator stem retracts away fro the control valve body as the diaphragm pressure is increased.

seat load load

Generally expressed as pounds force per lineal inch of port circumference, the force that is needed to seat the closure member wi  Read Free For 30shutoff Days sufficient authority to achieve theup ANSI Class Sign to vote on this titlerating of the  control valve. Useful Not useful

Master your semester with Scribd & The New York Times Special offer for students: $4.99/month. solenoidOnly valve valve

spring rate





Cancel anytime.

A small on-off valve that is operated by an electrical solenoid. Generally expressed in terms of pounds/inch, the amount of force tha

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stiffness

A measure of an actuator's resistance to the fluid reaction forces that to destabilize the valve plug. In a spring-and-diaphragm actuator, stiffness is determined by the spring rate of the mechanical spring (K and the air spring effect (K a).

stroking stroking time

The time it takes for an actuator to stroke a control valve closure mem from one stated position to another stated position.

travel

The amount of movemen t (linear or rotational) of the control valve closure member between the closed and open positions. Linear movement is typically expressed in inches or as the percentage of the rated travel. Rotation is typically expressed in degrees of rotation.

trave travel stops

Mechanical obstructions, often adjustable, that limit the actuator trav one or both of the travel extremes.

trip trip valve

A pneumatic switching valve that, upon sensing that the supply press has fallen below a preset trip pressure, opens and/or closes its interna valves to redirect fluid flow. Trip valves are often selected to establis engineered fail mode.

Tv

The rated control valve travel, generally expressed in inches.

unbalanceforce force

The net force that is produced on the valve plug stem or shaft as a res re of a pressure differential across the closure member or as a result of t geometry of the closure member.

vent, actuator

An opening in the non-pressurized casing of a spring-and-diaphragm actuator. The vent allows unrestricted movement of the actuator diaphragm.

volume volume booster

A pneumatic instrument that modulates a high-capacity supply press in direct proportion to the changes in a low capacity signal pressure.

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actuator in the event of a failure of the normal supply pressure system

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Selecting And Sizing Spring-And-Diaphragm Actuators And Rela.pdf

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