quection on piping material selection

Revised 03/ 06to conform wit the 2004ASME Extrac

CHAPTER 10

Piping

Here is what you will be able to do when you complete each objective: 1. Explain selection selection criteria for piping piping materials. 2. Calculate the the required thickness and maximum allowable allowable working pressure of piping. 3. Describe typical inspection procedures for piping installations and repairs. 4. Describe a typical routine inspection procedure and schedule for highenergy piping. 5. Explain the effects of high temperature on piping strength. 6. Describe the design and installation criteria for a piping system layout. 7. Explain the theory and effects of water hammer.

Revised 03/ 06to conform wit the2004ASME Extra Extrac

481 483

Chapter 10 • Piping

OBJECTIVE 1 Explain selection criteria for piping materials.

PIPING PIPING MATERIALS MA TERIALS SELECTION

The selection election of materials for for piping piping applica pplications tions is is a process that requires considera consideration tion of of material characteristics cteristics appropriate ppropriate for the required service. Materials are suitable for for the flo flow w medium and the given iven operating ting conditi conditions ons of of temperature and pressure pressure safety during the intended design design lif life e of the product. Mechanical streng strength must be fac factored tored in for for long long term service and the resistance resistance to operational operational variables such as therma thermal or mechanical cycling. cycling. Extremes in the process Extrem process temperatures influence influence the material capabil capabilit ities ies ranging from: • Brittl Brittle e fra fracture toug toughness at low low temperatures • Creep strength at the higher higher operating temperatures The operating ting environme environment surrounding the pipe or piping piping components must must be factored tored into the the design. Corrosi Corrosion on and erosion erosion can cause degradation tion of of the properties properties of the material. The The products that that are contained in the the piping are are also an important fac factor. tor. The foll following owing properties contribute to the the attractivene ttractiveness and economy conomy of a given iven pipe material: • Abi Ability to be bent or for formed • Suitability for for welding or other methods of joining joining Ease of heat treatment • • Uniformit Uniformity y and stabilit tability y of the resultant resultant microstructure icrostructure The piping used used must be of the correct correct size size in order to prov provide ide the required flow low and must have suffici sufficient ent strength to to withstand the pressure pressure and tempera temperature of the fluid fluid being transferred. In addition addition to this, this, the piping piping system must include include provision provision for for expansion and contraction, proper support, support, insulation and drainage. The design, manufac nufacture, testing and installa installation tion of of power piping piping systems for for steam plants is covere covered in the the AS ASME Code B 31.1 “P “Power ower Piping” Piping” and in the ASME Code Section I “Po “Power Boilers.” Conforms with the 2004 ASME Extract • Revised 03/06

483

484

B1 • Second Class • SI Units

PIPING PIPING MATERIAL S Steels are the most freq frequently used mate materials for for powe power piping piping systems. The The general clas classificatio sifications ns or stee steels are: • Low Low carbon stee steels • All Alloy steels • Austenitic stainless steels Table 1A in the AS ASME Code Sectio ection n II II, Pa Part D, D, lists lists the allowable stress stress values for these these materials for for various various tempera temperatures up to 900°C. Low Carbon Steel

Low Low carbon steel is is the lowe lowest priced steel steel and it is used extensiv extensively ely for for steam, water, fuel fuel oil oil and compressed compressed air piping piping for for tem temperatures below low 400°C. Abov Above e 400°C, it is not not recommended as graphitization graphitization may occur within within the pipe pipe material at these elevated elevated temperatures. Gra Graphitiz phitization ation is the breaking down of steel into iron iron and carbon graphite. phite. Failure of the material occurs alo along ng lines lines where thereis a concentration concentration of of graphite. Pipe Pipe made from from low low carbon steel is seamless electric resistance welded or butt butt welded. Specificatio cification n numbers of some examples of low low carbon steel steel pipe, pipe, as listed in Table 1A, are: SA SA-53 E/ B, SA SA-106 B and SA-178 A.. Allo Al lo y Steels St eels

All Alloy steels, su such as the chrome-molybdenum types, are used for for temperatures above 400°C. 400°C. An An applica pplication tion would be for for use in the central boiler boiler station stea steam pipi piping ng at 540°C or or more. Superheaters are norma normally made from rom chrome molybdenum molybdenum tubes and headers. The uses of some types, such as 1 chromium chromium ½ molybdenum olybdenum or 1¼ chromium chromium ½ molybdenum molybdenum where graphitiz raphitizatio ation n can be a problem problem, are limited limited to 525°C. 2¼ chromium 1 molybdenum (or (or higher % chrome alloys lloys up to 9Cr-1M 9Cr-1Mo) is usually lly used above 460°C. 460°C. All Alloy steel pipe may be seamless or welded and some examples, as listed in Table 1A, are: SA-213T12, SA-335P11 and SA-335 P22. Aust Au steni eniti ti c Stain St ainles less s Steels St eels

Austenitic stainless steels are a special class of high alloy steels which range fro from 18% chrome - 8 % nickel nickel to 25% chrome - 20% nickel. They are also alloyed alloyed with chromium, molybdenu molybdenum and sometimes with ith copper, tit titanium anium, niobi niobium um and nitrogen. nitrogen. Allo Alloying ying with ith nitrog nitrogen raises the yield strength strength of the steels.

Conforms with the 2004 ASME Extract • Revised 03/06


They are highly resistant to corrosi corrosion on and maintain intain high strength at high temperatures. This This piping piping is available as seamless or welded pipe and tubing tubing. App Applications are high temperature loop tubes in once-th -through boilers. Some specificatio pecification n numbers as listed listed in Table 1A of ASME Section ection II II Material Specificatio pecifications ns are: • SA-213TP304 – seamless tube, • SA-268 – welded pipe, • SA-213TP310S – seamless pipe Other Materials

Materials other other than steel which may be used in in powe power plant plant piping piping are cast iron iron and nonferrous nonferrous materials such as copper and brass. brass. However, However, these these materials are limited imited by the code in regard to pressure pressure and temperature. Acc According to th the ASME Co Code Section I, I, PG PG 8.2.2 cast ir iron ca can be be used fo for steam pressures up to 1700 kPa providi providing ng the steam temperature does not not exceed 230°C, but in no case, se, can be use used d for for boil boiler er blow-of blow-off connections. tions. Cast Cast iron iron is is not used where shock loading loading may may occur. The ASME Code Code Sectio ection n I, I, PG 8.4 also specifies cifies that nonfer nonferrous rous pipe or tubes shall not be used used for for blowblow-of offf piping or for for any other other service service wher where the temperature exceeds 208°C. PG 8.4.2 8.4.2 states that materials terials SB 61, SB 62, and SB 148 may be used only only for for parts of safety valves or safety relief valves valves subject to the limitatio limitations ns of PG 67.7 at allowa llowable stress values not to exceed those those given iven in Section ection II II D table 1B with ith a maximum ximum allowa allowable temperature of 290°C for SB 61 and SB 148 and 208°C for SB 62 In cases where the use of nonferrous nonferrous materials (any (any metal other other than iron and its alloys alloys such as aluminium, luminium, copper or copper nickel) is allowed, allowed, there is a possibil possibilit ity y of galvanic corrosio corrosion n occurring when these materials are used in conjunctio conjunction n with ith steel or or other metals. The The galvanic corrosion corrosion occurs occurs where the dissimilar dissimilar metals come in contac contact.


485

486


OBJECTIVE 2 Calculate the required thickness and maximum allowable working pressure of piping.

COMMERCIAL PIPE SIZES Commercial pipe pipe is made in standard sizes with dif different wall thicknes thicknesses or weights. Up to and including including 300 mm pipe, the the size is is expressed as nominal (approxima (approximate) inside inside diameter. Abov Above e 300 mm, the the size is is given as the actual outside outside diameter. For example, For ple, if if a pipe was designated as 152 mm size size this this would mean that it has has a nominal or approximate approximate inside diameter of 152 mm. The The outside diam diameter is 168 mm and this this is is a constant value value no matter what the wall thicknes thickness is. is. The The actual inside inside diameter of the pipe wil willl depend upon upon its its wall thickness thickness. Fo Forr a standard wall thi thickness ckness (Schedule 40), the actual insi inside de diameter of 152 mm pipe pipe is 154 mm. For For an extra strong w wall all thickness thickness (Schedule (Schedule 80), the actual inside inside diameter is 146 mm. There are two systems systems used to designate the various wall thicknes thicknesses of diff different sizes sizes of pipe. The The older method thod lists lists pipe as as standard (S), extra strong (XS (X S) and double double extra strong (XX (XXS). The newer method, which is is superseding the older method, uses schedule numbers to designate wall thicknesses. thicknesses. These These numbers are: 10, 20, 20, 30, 40, 60, 80, 100, 120, 120, 140 and 160. In In most sizes sizes of pipe; • Schedule 40 corresponds corresponds to standa standard • Schedule 80 corresponds to extra strong Table 1 lists lists the dimensions nsions and the mass per metre of diff different sizes sizes of steel pipe with var varying wall thicknesses thicknesses..



487

TABLE 1 Dimensions and Masses of Steel Pipe

Note: Note: Upper figure figures in in each square denote note wall thicknes thickness in in mm Lowe Lower fi figures denote mass per metre in kilo kilogra grams


488


STRENGTH OF PIPING The strength of of a pipe depends upon: Wall thickness thickness • Material from from which it it is is made • • Temperature to which it is subjecte subjected • Method thod of of its its manufac nufacture (whether (whether seamless or welded)

REQUIRED THICKNESS To determine the maximum ximum wall thickness thickness necessary for for a pipe to withstand a certain pressure and temperature, the fol follo lowing wing formula from from the B-31.1 B-31.1 Power Power Piping Piping Code, Code, Paragraph 104.1.2 (S (Straight Pipe Pipe under Interna Internal Pressure) is used. This is is essentially entially the same formula as given iven in the AS ASME Code Section ection I PG-27.2.2. tm

=

P Do

2SE + 2YP

+

A where

tm = Minimum required wall thickness in milli illime metres. (As (As pipe manufa nufacturing processes do not not produce absolutely bsolutely uniform uniform wall thickness thicknesses, the value of tm as determined by the the formula formula is usually iincrea ncreased by 12.5% 12.5% to provide provide a manufac nufacturing toleranc tolerance). P = Maximu ximum Allowa llowable Working Pressure (kPa may be rounded up to the nearest 10 unit) Do = Outside diameter of pipe in millimetres millimetres SE = Maximum ximum allowable allowable stress value in MPa at the operating operating temperature as listed in Appendix A Tables A-1 and A-2 in the Power Piping Code 31-1 The stress values in these these tables take into account the effici efficiency ency of the long longitudinal itudinal seam of welded pipe ‘E’. ‘E’. Seamless Pipe Pipe has an E of 1.0 The values found in AS ASME Code Code Sectio ection n II II, Part D. do not conta contain in the weld joint joint efficiency efficiency factor factor which is given iven in the follo ollow wing table. Factor Factor E

TABLE 2 Factor E

1 2 3 a b c d

Furnace butt weld Electric ectric resistance resistance weld Elec Electric Fusion Single butt without without filler iller Single butt with filler iller Doub Double le butt butt without fille iller Double ouble butt with filler iller

Straight seam Straight or spiral spiral seam

0.6 0.85 100% radiograph diograph

Straight or spiral seam Straight or spiral seam Straight ight or spira spiral seam Straight or spiral seam

0.85 0.8 0.9 0.9

1 1 1 1



489

A = Allowa llowance for threading and structural stability, stability, millimetres millimetres Threaded steel or nonferrous nonferrous pipe A = depth of thread, For machined surfa surfaces or grooves grooves A = A = depth of of machining chining with ith specified specified toleranc tolerances A = A = depth of machini machining ng plus 0.4 mm if no specified specified tolera tolerances For pla plain end pipe A = 0 Plain Plain end pipe is that which does not have its wall thickness thickness reduced when joini joining ng to another pipe. For For example, pipe pipe lengths welded tog together rather than joini joining ng by thread threading. y = Temperature coeff coefficient icient having values as given in Table 3

Temperature ºC Ferritic Steels Au st eni ti c Steels

482 and below 0.4

510 0.5

538 0.7

566 0.7

593 0.7

621 and above 0.7

0.4

0.4

0.4

0.4

0.5

0.7

For y values between the temperatur temperatures listed listed in Table Table 2, interpolati interpolation on may be used. Example 1 Calculate the required thickness ffor or 304.8 mm nominal size size plain end steam pipe to operate at 10 250 kPa and 500°C. The The material is is seamless alloy lloy stee steel SA-335P12. Solution t m

=

P Do S E y A

= = = = = =

P Do

2(SE + P y )

+ A

10.25 MPa (given) 323.85 mm (Table 1) 88.3 MPa (Table 1A in the ASME Code Section II, Part D) 1 (SA-335-P12 is listed as seamless pipe) 0.464 (Table 3 – Ferritic steel by interpolation) 0.000 (plain end pipe)


TABLE 3 Values of y

490


tm

=

t m

=

t m

=

10.25 × 323.85 2 [ (88.3

1) + (10.25 × 0.464)]

×

3319.46 2(88.3 + 4.756) 3319.46

+

A

+0

+0

186.112 t m = 17.84 mm

Using a manufa nufacturer’s toleranc tolerance allowa allowance of 12.5%, the required wall thickness thickness is: = 17.84 × 1.125 = 20.07 mm

( Ans.)

MAXIMUM ALLOWABLE WORKING PRESSURE To calculate the value of P for for a given iven value of tm, the formu formula la is tra transposed nsposed to solve for P as follows: P=

2 SE (tm − A) Do − 2 y (tm

−

A)

Example 2 Calculate Calculate the maximum allowa lowable ble workin orking g pressure, in in MPa, for for a 203.2 mm nominal size size plain end steam pipe with a minimum thickness of 18.24 mm. The The average operating ting temperature is 500°C. 500°C. The pipe material is is a ferritic ferritic steel SA213-T11. Solution Where: t m Do S E y A

= = = = = =

18.24 mm 219.08 mm (Table 1) 76.7 MPa (Table 1A in the ASME Code Section II, Part D) 1 (SA-213-T11 listed as seamless pipe) º 0.464 (Table 3 by interpolation for 500 C) 0.000 (See previous page)



P= =

=

=

2 SE (tm − A) Do

− 2y

(tm − A)

2 × 76.7(18.24 -0) 219. 219.0808- 2 × 0.46 0.464( 4(18 18.2 .24 4 - 0) 2798.016 219. 19.08 − 16.927 2798.016 202.153

= 13.84 MPa (Ans.)


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492


OBJECTIVE 3 Describe Typical Inspection Procedures For Piping Installations And Repairs.

INSPECTION PROCEDURES

Whenever new piping piping is installed installed or repairs ar are e made to existing existing piping piping, the the piping piping is tested to ensure it will withstand its maximum ximum allowa llowable opera operating ting pressure. The The majority jority of of piping piping is joi joined ned toge together with ith welding. The The welding process may cause anumber of defects which in include clude the foll following: owing: • Incomplete fusion • Undercutting • Porosity Slag inclusio inclusion n • • Cracking. Various methods of non-de -destructive examination (NDE) DE) are used to discover these these defects. ND NDE is the testing of materials without destroying destroying the integrity of of the material or lowering lowering its its abilit bility y to perform perform its its primary function. function. These tests include: Visual • Vis Magnetic particle particle • Liquid Liquid penetrant penetrant • • Radiographic • Ultrasonic • Leak Time-of-Flight Diffraction (TOFD) • Visual

Vis Visual inspection is the most cost-effe ffective method, but it must take place prior to, during during and after welding. The The ANSI/ AWS D1.1, (Ame (American National Standards Instit Institute/ ute/ American Welding lding Society) ociety) Structural Welding Welding Code-S Code-Steel, states, "Welds "Welds subj subject ect to nonnon-destru destructive ctive examination mination shall have been found found acceptable by visua visual inspection. inspection."" Befo Before re the first welding arc is struck, materials are examined to see if they meet specificatio specifications ns for for quality, lity, type, size, size, clea cleanliness nliness and freedom from from defects. Grea Grease, paint, int, oil, oil, oxide oxide film film or heavy scales are removed. Conforms with the 2004 ASME Extract • Revised 03/06


The pieces to be joined joined are examined for: for: • Flatness • Straightness • Dimensional accuracy Alignment • Ali • Fit-up Joint preparation • Jo Process Process and procedure variables are are verified, verified, including including electrode size size and type, equipm equipment ent settings settings and and provisions provisions for preheat or or postheat. All of these these precautions utions apply regardless of the inspection inspection method thod used. During During fabrication, vi visual examination of a weld bead and the end crater may reveal problems such as cracks, inad inadequate penetration, tration, and gas or slag inclusions. inclusions. On simple welds, inspecting inspecting at the beginning inning of each operation tion and perio periodicall dically y as work progr progresses is adequate. Howe However, where more than one la layer of filler iller metal is deposited, deposited, each layer is inspected before depositing depositing the next. The The root pass of a multipass ltipass weld is the most critica critical for for weld soundne soundness. It It is is especially lly susceptible ptible to cracking, and because it soli solidif difies ies quickly, it may trap gas and slag. On subsequent subsequent passes, conditi conditions ons the shape of the weld bead causes or changes changes in the joi joint nt conf configura iguration tion can cause further further cracking as well as undercut and slag trapping. Aft After we welding, visual inspection de detects a variety of surface fla flaws, including cracks, porosity porosity and unfil unfilled led craters regardless of subsequent inspection inspection procedures. Magnetic Magnetic Partic Partic le

Magnetic tic particle rticle testing (MT) (MT) is is used to detect surfac surface or subsurface fl flaw aws. An An electric current produce produces a magnetic flux that attracts attracts magnetic particles particles to the cracks in the metal. In In the presence presence of disconti discontinuiti nuities, es, the magnetic flux flux in in a material is distorted. This distortion is a function of the orientation of the discontinuity to to the magnetic field field (f (flux lines). lines). The The distortion distortion is greatest when the discontinuity is is perpen perpendicula dicularr to the magnetic field. field. When distorti distortion on of of the magnetic tic fiel field d is great enough, a pair of magnetic poles poles that act as small magnets, areestablished blished at the disconti discontinuity. nuity. Fig. 1(a) 1(a) shows how magnetic tic particle testing testing is used to loca locate cracks in ferromagnetic netic materials. terials. Magnetic netic particles particles are attracted ttracted to the pol poles and gather at the crack, Fig. Fig. 1(b), indica indicating ting a surface or subsurface flaw. This This technique technique can can only only be applied pplied on ferroma ferromagnetic tic materials. Mag Magnetic particle particle testing is often often used for findi finding ng cracks in piping, piping, vesse vessels, and the storage tanks of deaerators. tors.


493

494


FIGURE 1 (a) (b) Magnetic Particle Testing

(a)

(b)

Magnetic particles, particles, applied wet or or dry, are available vailable in va various rious colors: colors: • Silver-grey Black • • Red Yellow • Yel • Green • Fluorescent Various co colours ar are ne necessary to to obtain th the ma maximum co contrast be between th the surface of the component and the discontinui discontinuity. ty. Fluoresce Fluorescent particles are extremely visibl visible e when view viewed under ultra ultraviol violet et light light and have a high high contra contrast with the the surface being examined. Liqui d Penetrant Penetrant

Surface cracks and pinholes pinholes that are are not visibl visible e to the naked eye can be located located using liquid penetrant inspection. inspection. This method is widely used to locate locate leaks in welds and can be applied pplied with austenitic austenitic steels steels and nonf nonferrous materials terials where magnetic particle inspe inspection is not not eff effectiv ective. e. Two types of penetrating liquids liquids are used: • Fluorescent Visible dye • Vis



495

Fluorescent With fluorescent luorescent penetra penetrant inspe inspection, a highly highly fluoresc fluorescent ent liquid liquid with good penetrating qualiti qualities es is applied to the surface of the part to be examined. Capillary Capillary action tion draws the liquid liquid into into the the surface openings, and the excess is removed. oved. A developer is is then used to draw the penetrant to the surface, and the resulting indica indication tion is is view viewed under ultraviolet violet (blac (black) k) lig light. The high contrast between the fluo fluoresc rescent material and the obj object ect makes it possibl possible e to detect minute traces of penetrant that indica indicate surface def defects. Vi Visible Dye Dye penetran netrantt inspe inspection is similar similar,, except that that vivi vividly dly coloure coloured d dyes visible visible under ordinary light are used (Fig (Fig. 2). A white developer veloper is used with ith the dye penetrants that create create a sharply contrasting contrasting background to the vivi vivid d dye color. color. This allows allows greater portabilit portability y because it eliminate eliminates the need for for ultraviolet ultraviolet light. light. FIGURE 2 Dye Penetrant

The part to be inspected is cl clea ean and dry beca because any foreign foreign matter could could close close the cracks or or pinhol pinholes es and exclude the penetrant. penetrant. Penetrants can be applied by dipping, spraying or brushing with sufficient sufficient time time allowe llowed for for the liquid liquid to be fully absorbed into the discontinuities. discontinuities. This This may take take an hour or or more of very exacting ting work. Liquid Liquid penetrant inspection iis s widely used for leak detection. tion. A common procedure is to: to: 1. App Apply flu fluorescent material to one side of a joint 2. Wait an adequ adequate time for for capillary capillary action ction to take place 3. Vie View the other side of the joint with ultraviolet light


496


Radiographic

Radiog iography (X(X-ray) is is one of the most important, important, versatile and widely accepted of all the non-de non-destruc structive tive examination tion methods. XX -ray is used to determ determine the interna internal soundnes soundness of welds. Radiogr iography is based on the ability ability of of X-rays and gamma rays to pass throug through metal and other other materials opaq opaque to ordinary ordinary light and produce photogr photographic records of the transmitted itted radiant energy. All All materials absorb kno known wn amounts of this this radiant radiant energy. There Therefore, X-rays and gamma rays can be used used to show disconti discontinuiti nuities es and inclusio inclusions ns within ithin the opaque material. The The permanent film film record of the the interna internal conditi conditions ons shows the basic information information that determines weld soundness. High-v High-vol oltag tage generators tors produce x-ray x-rays. As As the high volt voltag age applied pplied to an x-ra x-ray tube is increa increased, the wavelength of the emitted itted XX-ray becomes shorter and provides provides more penetrating ting power. The atomic tomic disinteg disintegration tion of of radioiso ioisoto topes pes produces gamma rays. The The radioac dioactive tive isoto isotopes pes most widely used in industrial industrial radiog iography are Cobalt 60 and Iridium ridium 192. Gam Gamma rays emitted itted from from these isotopes isotopes are are similar to xx-rays except that their wavelengths wavelengths are are usually shorter. shorter. This This allows allows them to penetrate to greater depths than X X--rays of the same power. Howe However, exposure exposure times times are considera considerably longer longer due to the the lower intensity. When XX-rays or gamm gamma rays are directed at a section tion of weldment, not not all of the radiation diation passes throug through the metal. Various ious materials, depending on their their density, thickness thickness and atomic number absorb dif different wavelengths of radiant energy. The degree to which these these materials absorb the rays determines the intensity intensity of the rays penetrating penetrating through through the material. terial. When variatio variations ns of these rays are recorded, recorded, there is a means means of seeing inside inside the material available. The image on a developed photosensiti photosensitized zed film film is known as aradiogr iograph (Fig. (Fig. 3). The opaque material absorbs a certain amount of radiation, diation, but but where there is a thin section or or a void (slag (slag inclusion inclusion or porosity), porosity), less absorption ta takes kes place. These areas appear darker on the the radiogra diograph. Thi Thicker cker areas of the specimen or higher density nsity material (tung (tungsten inclusio inclusion), n), absorb more radiati radiation on and their correspond correspondin ing g areas on the the radiogra diograph are lighter. lighter.



497

FIGURE 3 Radiograph

The reliabili liability ty and interpretive interpretive value of radiog diographic images are a function unction of their sharpness and contra contrast. The sharpness of an image and its its contrast contrast wit with h the background enables the observer to detect a flaw flaw. To To be sure that the the radiogra radiographic phic exposure exposure produces acceptable results, results, a gauge called called an Ima Image Quality lity Indica Indicator (I (IQI) is is placed on the part so that its ima image is produced on the radiograph. Image quality lity indicators, indicators, used to determine determine radiographic radiographic quality, quality, are also called lled penetrameters. A standard holehole-type type penetrameter is a rectangular piece of metal with three drilled drilled holes holes of set diameters. The thicknes thickness of the piece of metal is a percentage of the thicknes thickness of of the specimen being radio radiog graphed. The The diameter of each hole hole is diff different and is a given multiple ultiple of the penetrameter thickness thickness. A penetrameter is not not an indica indicator tor or gauge to measure the size of of a discontinuity discontinuity or the minimum detecta detectable flaw flaw size. It It is is an indicator indicator of of the quality lity of the radiogr iographic technique technique. Surface rface defects defects show up on tthe he fil film m and must be recognized. cognized. Beca Because the angle of exposure also influenc influences the radiograph, it it is is diff difficult or or impossible to evaluate luate fillet illet welds using thi this s method. thod. Beca Because a radiog diograph compresses all the defects that occur throughout the thickness thickness of the weld into into one one plane, it it tends to give an exaggerated impression o off scattered-type ttered-type defects such as porosity porosity or inclusions. An x-ra -ray image of the interior of a weld can be viewed on a flu fluorescent screen as well as on developed developed film. film. The The screen makes it possible possible to inspect parts faster and at lo lowe wer cost than with ith fil film. m. Linking Linking the fluoresce fluorescent screen with ith a video video camera overcomes many of the shortcomings shortcomings of radiogr diographic imaging. Instea Instead of waiting iting for for fil film m to be developed, the the images are viewed in real time. This This improves improves quality lity and reduces costs on on production production applica applications, tions, such as pipe welding, lding, where a problem can be identif identified ied and corrected quickly. quickly. Conforms with the 2004 ASME Extract • Revised 03/06

498


Radiog iographic equipment produces radiation diation that is harmful to body tissue tissue in excessive sive amounts, so safety safety precautions utions are follo ollowe wed closely. All All instructio instructions ns are follo ollowe wed carefully refully to achieve satisf tisfac actory tory results. Only Only personnel who who are trained in radiation diation safety and qualif lified as industrial radiographers radiographers are permitted to do radiogr diographic testing. testing. Ultrasonic

Ultrasonic inspe inspection (Fig. 4) is is a method of detecting detecting discontinuities. discontinuities. A highfrequency sound beam, at an angle of of about 70°, is is directed throug through the base plate plate and weld on on a predictable predictable path. These These sound waves pass through the material bouncing bouncing off off the the inner and outer walls. lls. A defect reflects reflects part of the sound back to the source (a quartz crystal transducer). The The sound pulses are shown on an an oscillo oscilloscope scope together with ith the reflected signal signal from from the defect. When the sound be beam am's path strikes an interruption interruption in the the material conti continuity, nuity, some of the the sound is reflected back. The The instrument collects collects the sound which is is then amplif plified and display displayed as a vertical tra trace on a video video screen. screen. FIGURE 4 Ultrasonic Inspection

Both surface Both surface and subsurface defects in in metals are detected, lo located cated and measured using ultrasonic ultrasonic inspection, inspection, including including flaw flaws too small to be detected with other other methods. thods. The The ultra ltrasonic sonic unit contains a crystal of of quartz or other other piezoelectric piezoelectric material encapsulated encapsulated in a transducer transducer or probe. When a voltag voltage is applied, the crystal vibrates vibrates rapidly. pidly. As As an ultrasoni ultrasonic c transducer is held against the metal metal to be inspected, it it impa imparts mechanical vibratio vibrations ns of the same frequency as the crystal crystal through a couplant material into into the base metal and weld. The couplant tra transfers nsfers the ultrasonic ultrasonic waves better than than air does. For For relatively flat, flat, smooth smooth surfac surfaces, a mixture of glycerin and water may be used as a couplant. couplant. Fo For rough rough surface surfaces, light light motor motor oil oil with a wetting etting agent may be used. Waves are propaga propagated through the material until until they reach a discontinuity discontinuity or or change in density. At these points (discontinuities) some of th the vibration energy is refle flected back. As the current that causes the vibration is shut off and on at 60-10 -1000 times per second, the quartz quartz crystal intermittently intermittently acts as a receiver iver to pick pick up the reflecte reflected vi vibrations. Thi This causes pressure on the crystal and generates an electrical current. Fed to a video screen, screen, this this current produces vertical deflectio deflections ns on the horizont horizontal al base line. line. The The resulting sulting pattern on the the face of the tube represe represents the reflected reflected signal and the discontinuity. discontinuity.



Compact, portable ult ultras rasonic onic equipment is available vailable for fiel field d inspection and is commonly used on on bridg bridge and structural structural work as well as for for checking the thickness of piping. Ultra Ultrasonic sonic testing is not not as suitable as other NDE NDE methods for for determining ining porosity porosity in welds because because round gas pores pores respond to to ultrasonic ultrasonic tests as a series of singlesingle-point reflectors. This This results results in low low ampli mplitude tude resp responses onses that are easily confuse confused with ith "bas "base line noise" noise" inhere inherent with testing parameters. Howe However, it it is the preferred test method for for detecting common types of disconti discontinuiti nuities es and laminations. Portable Portable ultrasonic equipment is available available with ith digital opera operation tion and microprocess microprocessor control controls. s. These These instrum instruments may have builtilt-in in memory and provide provide hard copy printouts printouts or video video monito onitoring ring and recording. cording. They are interface interfaced with compute computers which allow allow further analysis, lysis, docum documentation tion and archiving, hiving, much as wit with h radiographic radiographic data. Ultrasonic Ultrasonic examination ination requires expert interpretation tion from from highly skilled skilled and extensively xtensively trained personnel Leak

Leak testing, to verif verify y the integ integrity of of a piping piping system, iis s performed rformed in accordance with ith AS ASME B31.1 Power Pipi Piping ng Code. The The testing methods, methods, most widely used, are: • •

Hydrostatic Pneumatic

Hydrostatic It is mandatory that that the design, fabrica fabrication, tion, and erectio erection n of power piping, piping, constructed under this AS ASME Code demonstrate demonstrate leak tightness. A hydrostatic leak test prior to initial initial oper operation tion meets meets this requirement. A non-compr non-compres essible sible liquid, liquid, such as water, is is usually lly the test medium used. used. Water is is inexpensive inexpensive and readily available. available. A glycol/ water mixture ixture or methanol is used if the testing testing is performe performed when the ambient temperatur temperature is near or or below below free freezing. zing. The hydrostatic test pressure pressure of a piping system is not not less less than 1.5 time times the design pressure, but does not not exceed the maximum test pressure of any vessels or components components in the piping piping system. The The test pressure is maintained for for sufficient sufficient time timeto inspect all joints, joints, with a minimum minimum time of ten minutes. Hydrostatic Hydrostatic testing is the the preferred method because because it is is very saf safe. Liquids Liquids are not compressibl compressible. e. When a leak occurs, the pressure is gone. Compre Compressible ssible fluids fluids continue continue to expand, creati creating ng asafety hazard.


499

500


Pneumatic Pneumatic testing of piping piping systems invol involves ves the pressurizatio pressurization n with ith a compressibl compressible e gas, such as air or or nitrogen. nitrogen. Air Air is is an inexpensive and readily available test test medium. Nitroge Nitrogen is selecte selected if there there is the possibili possibility ty of combustible combustible gases being present. This type type of test is only only used when the design of piping piping systems does not allow allow the complete removal oval of water. The primary hazard with compressed compressed gases is the amount of stored stored energy contained. The The results are cata catastrophic strophic if a failure failure occurs. occurs. Pneumatic tic testing is done with all nonessential personnel removed oved from from the immediate area. Time-of-Flight Diffraction (TOFD)

TOFD is a type of ultrasonic inspection that uses diffraction signals instead of reflection signals. The TOFD technique is an effective, fully computerized inspection tion method method for for the the detectio tection n and sizi sizing ng of flaws with a high high rate rate of accuracy. The The loca location, tion, geometry geometry or orienta orientation tion of of the the anomalies is irrelevant for for detection tion and sizing. sizing. In In the TO TOFD technique, a transmitter itter and a receiver iver are placed equal distances from from the weld. The The scanner with ith the probes probes is moved moved parallel to to the weld. TOFD is utilized utilized over over the entire leng length of the weld to cla classify ssify inherent flaw flaws and and creep damage. The The small, high intensity intensity beam spot used in thi this s inspection inspection is effective effective in in detecting ting creep damagedue to an early form form of cavitatio vitation. n. Fig. 5 shows the typical TO TOFD arrangement for for the detection tection of deep-sea p-seated damage, with the probes set broadly. The intersection intersection point point of the beam centres lies lies at a depth of approximately pproximately 2/ 3 wall thickness. thickness. This This inspectio inspection n is done in a single single scan pass with ith transducers straddling straddling the weld.

FIGURE 5 TOFD TOFD Transduc er Configuration fo r Deep Deep Coverage



OBJECTIVE 4 Describe a typical routine inspection procedure and schedule for high-energy piping.

HIGH-ENERGY PIPING HighHigh-energ energy piping piping includes includes main steam and hot reheat pipi piping ng systems designed to operate at hi high gh temperatures and pressures. Main steam piping piping has design temperatures betwe between 510ºC and 565ºC and operating operating pressures betwe between 8.6 MPa up to supercriti supercritical. cal. Hot Hot reheat pipi piping ng systems operate between 510ºC and 565ºC but at lowe lower pressures than the main steam steam pipi piping. ng. For For example, ple, a Combusti Combustion on Engineerin Engineering g steam generator with with a main stea steam pressure of 17.4 MPa has a reheat pressure of of 4.05 MPa. The AS ASME B31.1 Powe Power Piping Piping Code Code prescribes recommended practices tices for for the inspection inspection of of high-energ high-energy piping systems. systems. High-ene High-energy piping piping systems, part of the feedw feedwater and steam circuit circuit of of a steam generating power plant, include nclude runs of piping piping and supports, restraints and and all valves. valves. This This also includes includes all systems systems under two-phas two-phase flow flow conditions. conditions. A record keeping program is developed developed to analyze lyze piping piping systemdistorti distortions ons and potential potential failures. failures. The fol follo lowing wing procedures are established and implemented: implemented: intenance programs • Operating and maintenance Piping and pipe support iinspection nspection progr program • Piping

OPERATING AND MA INTENANCE INTENANCE PROGRAMS Written Written procedures procedures include include the qualificatio alifications ns of personnel and material history history and records. Each plant files files and maintains the follo ollowing wing documentation: ntation: Flow diag diagrams • Flow • Valve data Welding procedures procedures and records • Welding pport drawings • Support Pipe drawings • Pipe ting records that that document document cases of exceeding piping piping design • Operating criteria Conforms with the 2004 ASME Extract • Revised 03/06

501

502


• • • •

Piping Piping drawings (isometric (isometric piping piping drawings) Construction Construction drawings that identif identify weld loca locations tions Pipe Pipe specificatio cifications ns that outline outline the material, outside outside diameter diameter and wall thickness Material certificatio certification n records

PIPING AND PIPE SUPPORT INSPECTION PROGRAM The piping and pipe support support inspection inspection prog program identif identifies the initial initial hanger positi positions ons at the time time of installatio installation n and unit startup. Routine visua visual surveys are scheduled to identify identify any any changes in positio position n of piping and and setting of pipe hangers, slide slide supports and shock suppressors. Att Attaching markings or or po pointers to to th the piping components allows fo for pe periodic positi position on determinations and and permanent identificatio identification. n. These These observations observations include: Any interference fro from other piping or equipment • Any • Piping vibrations General conditi condition on of the supports, pports, guides, anchors, supplementary • steel and attachments Procedure Procedures are are developed developed for for corrosion corrosion control and evaluation of of the piping components for for corrosi corrosion on damage. These These procedures include include the periodic periodic visua visual inspection of the following: Condition of of the paint on the piping piping to resist exter externa nal ambient mbient • Condition corrosion Condition of the insulation and/ and/ or wra wrappings ppings for for winter freeze freeze • Condition protection Thickness testing for pipe pipe elbows elbows and welded joint joints s • Check superheater and reheat piping piping for for sig signs of creep. This This is is done after a period of of operation such as 10, 15 or 20 years. Samples of metal are taken for for metallurgical inspection inspection or leng lengths of of pipe are measured to detect increa increase in length.



OBJECTIV OBJ ECTIVE E5 Explain the effects of high temperature on piping strength.

HIGH TEMPERATURE EFFECTS ON PIPING Piping Piping in powe power plants and process plants is of often ten subjected to high operating temperatures. The The operating temperature has an effect effect on the tensile tensile strength of of the metal and may also cause creep. Tensile Tensile Strength

As the temperature is increased, the properties of th the pipe material change. The The tensile tensile strength of of the material terial rapidly rapidly decreases above bove a certain temperature. This is indicated indicated in Ta Table 1A 1A of the ASME Code Code,, Sec Sectio tion n II II, Part D. For For materials list listed ed in this this table, the working stress allowe allowed decreases as the temperature increases. For example, steel pipe pipe of material SA-53E/ SA-53E/ B is allowed allowed a working working stress of 118 000 kPa at 325°C. But, at a temperature of 425°C, the the working stress allowed llowed is only only 75 300 kPa. The ultima ultimate strength of of carbon stee steel and a number of alloy alloy steels as as determined ° by short short time time tensile tensile strength tests tests over over a temperature range of 38C to 816°C is shown in Fig. 6. The results of of these tests indicate that the strength decreases with an increa increase in temperature. There There is a temperature region for the austenit austenitic ic ° alloy alloy steels steels between 204 and 482Cwhere the strength is fairly fairly constant. constant. The The strength of of carbon and many low alloy alloy steels steels increa increases between the ranges of 38 ° to 316 C.


503

504


FIGURE 6 Tensile Tensile Strength o f Various Steels

CREEP

In addition dition to immediately reducing reducing the tensil tensile e strength of a material, high temperatures cause the pipe material to to creep. creep. This This is is a condition where the pipe material gradually lly stretches or undergoes plas plastic tic deforma deformation. tion. This occurs occurs if the material terial is is subjected subjected to stress stress under high temperature and can become a long long term gradual decrease in tensile tensile strength. Ev Eventua entually lly the material will will fail fail if if the stress at the elevated temperature is maintained for for a suffici sufficient ent length of time time. For powe power plant piping, piping, an elongati elongation on or or stretching rate of 1 percent in 100 000 hours is is considere considered acceptable. To determine the rate of creep of a material, a creep test test is conducted. conducted. A specimen of the material terial is held held at constant tempe temperature in a furnace and, using a system of levers, a dea deadweight is applied. applied. The The deformation of of the specimen is measured periodicall riodically y throug throughout the the test and a curve is plot plotted ted showing the percent creep throug throughout the the time of the test.



505

Fig. 7 shows the creep curves for for a material tested tested at low low stress and at hig high stress. The rate of creep is divi divided ded into three stages. During During the first first stage, the creep rate decreases (the slope slope of of the curve decreases). During the second stage, stage, the rate is constant constant (the slope slope of the curve does not change). During During the third stage, the rate increases (the curve sl slope ope becomes steeper) until until the specimen specimen ruptures. Ano Another adverse effe ffect of hi high temperature on pipe material is that it promotes oxidatio oxidation n and corrosion. corrosion. A low low carbon steel heated in air for for a certain period can experience over 50 times as much oxi oxida dation tion at 800°C as it did when heated for for the same period period at 500°C. In addition ddition to the above bove problem oblems, if the operating ting temperature of the the pipe is high, then then the pipe expands when when coming up to that temperature. Movement of the pipe due due to expansio expansion n is allowe allowed for for when installing installing the pipe. FIGURE 7 Typical Creep Curves


506


OBJECTIVE 6 Describe the design and installation criteria for a piping system layout.

PIPING SYSTEM LAYOUT Piping Piping systems, used to trans transfer fluids fluids such as as water, steam, oil oil,, gas and air from from one loca location tion to another, must must include include: support • Proper support Provisions for expansion and contraction • Cold Cold springing • Anchors • Anc • Drainage • Insulation Piping Supports

Piping Piping is supported so that that the equipment to which it is is attached does not carry the weight of the the piping. The supports used used prevent excessive ssive sagging of the the pipe and, at the same time time, allow allow free free movement of of the pipe due to expansion and contraction. contraction. Howe However, unlike unlike a pipe guide, the pipe support does does not control control the direction direction of the pipe line movement. The supporting supporting arrangement is is designed to carry the weight of of the pipe, valves, valves, fitting ittings and and insulation plus plus the weight of the fluid fluid contained contained withi within n the pipe. Fig. 8 ill illustra ustrates two types of adjustable pipe hangers which are suspended from from overhea overhead beams. Fig. Fig. 8 (a) shows shows an adjustable adjustable strap hanger whil hile Fig. 8 (b) illustrates illustrates an an adjustable roller roller hanger.



507

FIGURE 8 Pipe Hangers

The roller roller stands in Fig Fig. 9 may be bolted bolted to brackets, structura structural supports supports and floors. loors. Four adjustment adjustment screws which raise or lower lower the roller roller the pipe rests on control the vertical adjustment djustment of the pipe position position in in the adjustable justable stand.. stand.. FIGURE 9 Pipe Roller Stands

In the case of a horizontal pipe pipe where the action of other parts of the piping system cau causes vertical movem movement, the the rigid type hangers or supports supports in in Figs Figs. 8 and 9 are not suitable. In In this this situa situation, tion, variable spring ha hange ngers are used permitting permitting the pipe to move up or or down without ithout disturbing the loa load distribution. distribution. Fig. 10 shows a type of a variable spring hanger.


508


FIGURE 10 Variable Spring Hanger

If the amount of vertical movem movement of the supported pipe is large, then a constant support hanger hanger (Fig (Fig. 11) is is used. This This type fea features a coiled coiled helica helical spring spring which hich is arranged to move move as the pipe pipe moves and maintains aintains a constant constant supporting supporting force force on the spring. Roll Roller er bearings with sealed lubrica lubrication tion are used used to reduce frictio friction n between the moving moving parts of the hanger. The constant support hanger is fac factory adjusted and tested tested to support support the specified specified load load throug throughout a definite inite range of travel. The The spring compression ssion can be adjusted in the fiel field d to give a plus or minus 10% 10% variati variation on in in the load setti setting. ng. FIGURE 11 Constant Support Hanger



509

Expansion of Piping

Expansion control Expa control in in pipelines that that carry carry hot or cold cold fluids fluids or are exposed to large variations variations in in ambient temperature can be a major jor problem. problem. As As the metal temperature of the pipe increa increases or decreases, its its leng length also varies due to thermal expansion or contraction. Therefore efore,, unless unless provision provision is made for for these changes in in leng length, excessive ive stresses are induced iin n the piping piping and large forces forces are transmitted transmitted through the system to anchors anchors and connected connected equipment. Severa everal diff different erent methods are available for for controlling controlling pipeline expan expansion. sion. Two of the most comm common are: Expans pansion ion bends • Ex • Expansion joints Expansion Bends

With this this method, the pipe is fa fabricated with special special bends bends or loo loops. ps. Flexing Flexing or springing of the bends or loops loops ta takes up the increase due to expansion in the the length of pipe. Fig. Fig. 12 shows some typical shapes shapes of expansion ion bends. Length Length and height dime dimensions nsions are used to insta install the bend that will ill withstand withstand the required amount of expansion. nsion. FIGURE 12 Expansion Bends

Adv Advantages of expansion bends are: Easily added to piping piping systems and fit fit on pipe pipe racks and high lines lines • Easily trouble-ffree method thod as there is no maintena intenance involv involved ed • Most troubleunlikely • Leakage is unlikely Conforms with the 2004 ASME Extract • Revised 03/06

510


•

Any Any temperature, pressure or flu fluid can be handled with proper selection selection of material and thickness thickness

Disadvantages of of expansion nsion bends are: are: • Require a larger amount of space Produce Produce ahighe higher pressure drop and heat loss loss • Produce Produce higher end thrusts thrusts which can present probl problems ems when • connecti connecting ng to equipment equipment such as turbines turbines and pumps Expansion Expansion Joints

Two types in use are: Slip expansion joint • joint nt • Corrugated expansion joi Slip Expansion Expansion Joint This type, ill illustra ustrated in Fig.13 Fig.13, fea features a slip slip pipe which is welded to an adjoining pipe. The slip pipe fits into the main body of the joint which is fastened to the the end of the other adjo adjoini ining ng pipe. When the pipe line expands, expands, the slip slip pipe moves within ithin the joi joint nt body. To To prevent leakage between the slip slip pipe and the joint joint body, body, packing is used around the outside outside of the slip slip pipe and the slip slip pipe moves oves within the the packing. In the joint joint illustrated, illustrated, the packing consists of two sections ctions of packing separated by a section ction of of plastic packing. A Additi dditional onal pla plastic stic packing may be added using a packing plunger plunger while hile the joint joint is is in service. Grea Grease fitti fittings ngs are used to provi provide de lubrication. FIGURE 13 Slip Expansion Expansion Join t



511

Adv Advantages of slip expansion joints are: Simple and rugged • Simple of handling a larg large amount of of expansion ion • Capable of minimum space • Require minimum Produce littl little e pressure drop and heat heat loss loss • Produce Disadvantages of slip expansion joints joints are: re: moving parts and possibili possibiliti ties es for leaks • More moving located where the packing cking can be given iven attenti ttention on • Must be located arise if the joint joint is poorly poorly aligned aligned or if if it becomes omes • Problems may arise corroded Joint are installed and maintained according to manufacturer’s • Jo instructions Proper packing is used • Proper lubrication tion two two or three times a year unless selflf-lubrica lubricating ting • Require lubrica packing is is used. Corrugated Expansion Joint Joint A simple design su suitable fo for on only lo low pressures is is il illustrated in in Fi Fig.14 and is is available ilable with ith either flang flanges or welding ends. This This type of expansion nsion joint joint has a flexible lexible corrugated section tion which can absorb a certain amount of endwise movement of of the pipe. They are often often seen at the exhaust end of a steamturbine. FIGURE 14 Low Pressure Pressure Corrugated Expansion Joint

For hig higher pressures, the corrugated joint joint uses control control or or reinforci reinforcing ng rings which surround the corrugations as illustrated illustrated in Fig Fig. 15.


512


FIGURE 15 Reinforced Reinforced Corrugated Expansion Joint

The bellows llows type corrugated expansion xpansion joint joint,, shown in Fig. 16, is suitable for for pressures up to 2070 kPa. It It is is equipped with an internal safety sleeve wit with h a limit limit stop to to prevent undue extension or or compression. ssion. Because this this sleeve is closely closely fitted, it it prevents prevents excessive leaka leakage if failure ilure of the bell bellows ows section occurs. This This type may be supplied supplied with ith or or without without anchor bases. FIGURE 16 Bellows Type Corrugated Expansion Joint

Adv Advantages of corrugated expansion joints are: less space • Require less Produce less pressure pressure drop and heat loss loss than than the expansion xpansion bends or • loops Do not not require maintenance as in the case of the slip slip type •



513

Disadvantages of corrugated expansion nsion joi joints nts are: of movement th the be bellows or or co corrugations pr provide is is le less • Amount of than than the slip expansion expansion joi joint nt provide provides s Vulnerable to condensate corrosion during shutdown periods as the • Vul condensate condensate does not drain effectiv effectively ely Fig. 17 illustra illustrates the vario various us diff different erent designs of bellows or corrugations. tions. FIGURE 17 Types Types of Bellows

Cold Springing

Cold Cold springing or pre-stressing pre-stressing of a piping piping system is applied applied to reduce the effect effect of thermal expansion in in the piping piping system. system. Lea Leaving ving a gap at an appropriate ppropriate loca location tion in the piping system and "pulli "pulling ng up cold" cold" during during the erection/ installation of the piping piping achieves this. Cold Cold pull pull,, usually lly 50% of the expansion ion of the pipe run under under considera consideration, tion, has no eff effect on the code stress stress but can be used used to reduce the nozzl nozzle e load loads on machinery or vessels.

EFFECT OF COLD SPRINGING Cold Cold springing introduces introduces a predetermined stress in the the pipe and reduces the maximum ximum thermal loads loads and stresses in a system when the pipe is is cold. cold. Its main purpose is to to reduce the peak loading on connecting connecting equipment. Howe However, it it does not affect affect the overa overall stress range, and therefore therefore cannot be used in the stress range equations. In piping piping systems well below below the creep range, any cold cold spring spring should should stay for for lif life. Pipes in the creep range eventually full fully y relax out, so they become 100% cold sprung regardless of how much is applied at original original build build stage. Some codes make use of cold cold spring spring to reduce the maximum ximum hot hot stress (deadweight + pressure +thermal expansio expansion).


514


Cold spring is used to: 1. Minimize the off offset set of of a piping syste system from the neutra neutral positi position on (installed position without cold spring) to the operating condition. For example, if if a pipe moves 50 mm from from the neutral to to the hot hot positi position on and it is is cold sprung 25 mm, the off offset from from the neutral position position when cold will ill be -25 mm and in the hot position position +25 mm. 2. Minimize Minimize the forces forces on an end point point which may be at a piece of equipment. Beca Because a negative force force is is put on on the equipment equipment in in the cold positi position, on, the pipe passes through through a neutra neutral force condition condition during heat up and has a reduced force force in the the hot or operating position. 3. Reduce the stress in the hot hot posit positio ion. n. Beca Because a negative tive stress is placed on the pipe pipe when installed installed with ith cold cold spring and during heat up, the pipe pipe relieves lieves this init initial ial stress and passes through through a neutral stress conditi condition. on. The final stress in the hot positio position n is reduced duced.. 4) Minimize Minimize hanger movement. For example, if if a hanger is on on a pipe that moves oves 50 mm horizont horizontally, ally, the the hanger is dislocated dislocated from from its neutral positi position on 50 mm without ithout cold cold spring spring. The The hanger off offset and rod leng lengths are such such that the hange hanger rod is is not off offset more than than 4 degrees. If 25 mm of cold cold spring is install installed ed and the hanger is moved moved -25 mm from its neutra neutral positio position n and and in the hot positi position on it it is is +25 mm from the neutral positi position, on, then the rod can half the length and sti still ll be within the 4 degree limit. limit. If the hanger off offsets more than 4 degrees, the uplif uplift becomes a factor tor and induces more load load and stress at the hanger poin pointt and possibl possibly y at equipment connections. onnections. Good ood judgment is necessa necessary when applying pplying cold old spring. The The cold spring spring becomes a vital vital part of the design. Extra Extra precautions utions and field field verif verificatio ications ns are used when actually lly installing installing the pipe to ensure that the cold cold spring is installed installed as designed. Piping Anchors

Anc Anchors are important in any piping system but there are some special considera considerations tions necessary necessary when expansion nsion joint joints s are used. No expansion expansion joint joint operates properly unless the pipeline pipeline is secure securely anchored. In In addition, ddition, the pipeline pipeline has enough guides or supports to prevent buckling or bowing of the pipe.



When guides are installed installed near an expansion joint joint they hold hold the pipe in the the proper position for best operation of the joint. With the slip type joint, this prevents prevents misalignment of the sleeve in the the joint. joint. With With the bell bellows ows type joint, joint, the guides prevent excessive stress on the the bellows bellows which results from misalignment of the pipe. A pipe alignment guide is a for form of sleeve or fra framework, fastened to a rigid part of the installa llation, tion, which permits rmits the pipe to move fre freely ely in one directio direction n only, along the axis xis of the pipe. It It allows allows suff sufficient icient clearance between the fixed ixed and moving moving parts to give give proper guidance without excessive ffricti riction. on. Anc Anchors are installed to: tabilize the piping piping at certain points, points, such as valves valves or other • Stabilize equipment off tw two o or more pipes • Support junctions o • Terminal points With expansion expansion joints, joints, anchors serve serve to divide divide the syste system into sections, sections, so that each expansion joi joint nt absorbs absorbs only only the expa expansion nsion of of its ow own section. tion. If only one expansion joint is used, it is placed in the middle of the pipeline. If it is not not fitted fitted with an anchor, the line line is anchore anchored d at ea each end. end. If If the single joi joint nt is is fitted itted with ith an anchor then it is is placed at the end of the line. line. When several expansio expansion n joi joints nts are used in a pipe line, line, the pipe may may be anchored midway between the joint joints s or at the joint joints s themselves if they are fitted fitted with anchor bases. Drainage

All All piping sy systems that have a possibility of for forming li liquids need to have provisions provisions for for the liquid to drain to low low spots. spots. From the low low spots, the liquid is removed oved using traps and low low point drains.

STEAM TRAPS

Steam traps are automa utomatic tic valves valves that discharg discharge condensate fro from m a steam line line without without discharg discharging steam. Steam traps are an essential essential part of of a steam system. Without Without them the steam pipes and heat exchangers quickly ickly fil filll with condensa condensate that prevents the fl flow ow of steam and transfer of heat. Steam traps are placed along distributio distribution n piping piping and after all heat exchangers.


515

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There are four four types of steam traps: Inverted bucket bucket • Float Float and thermostati thermostatic c • • Thermostatic • Thermodynamic Inverted Inverted B ucket Traps Traps

In inverted inverted bucket traps (Fig (Fig. 18), steam is contained within within an inverted bucket floating loating in condensate. As the level of condensa condensate rises, it is discharged. Inverted bucket tra traps require water, called called the prime, withi within n the bucket to operate. This trap is most appropriate ppropriate for steady loads loads such as on distributio distribution n systems. Condensate is discharged interm intermittently. ittently. FIGURE 18 Inverted Bucket Trap (Courtesy of Spirax Sarco)

Float and Thermostatic Traps

In flo floa at and thermostatic ostatic traps (Fig (Fig. 19), condensate is discha discharged when the rising rising level of condensa condensate lifts lifts a floa loat a attac ttached to a level level valve. A thermosta thermostatically operated vent discharges air from from the top of the trap. Float Float and thermostatic traps have superior rior air removal charac characteristics. teristics. Howe However, the interna internal valves valves and seats are matched to steam pressure or the the trap can fail in closed closed positi position. on. Condensate is discha discharged continuo ontinuously usly as it coll collects ects in the trap trap body.



517

FIGURE 19 Float and Thermostatic Trap (Courtesy of Spirax Sarco

Thermost Thermost atic Traps

Thermosta ostatic tic traps (Fig. (Fig. 20) operate on the dif difference in temperature temperature between steam and condensate. When condensate reaches the trap, the ffiilled lled thermal element opens a pilot pilot valve to allow llow limited limited flow flow. The The main va valve stays clo close sed d until the condensate load exceeds the capacity ity of the pilot pilot valve. valve. Then the pilot pilot va valve opens the main valve, and both discharge at full capacity. At startup, both the pilot pilot valve valve and the main valve are open for for hig high-ca h-capacity ity discharge of air and condensate. condensate. In In standard operation, the pilot pilot valve valve may drain condensa condensate continuously, closing only only in the abse absence of condensa condensate. Alt Although condensate is discharged continuously, thermostatic traps always cause some condensate to remain in the the systemso steam is not blown blown through the trap.


518


FIGURE 20 Thermostatic Thermostatic Trap Trap

Thermody Thermody namic Traps Traps

Thermodynamic traps (Fig (Fig. 21) have a disk situate situated on a central central orif orifice. As As condensate pressure builds, builds, it lif lifts the disk, passes through the orif orifice at the centre of the disk and exits exits through smaller orif orifices surrounding the disk. Flash Flash steam builds ilds up pressure on top top of the disk and closes closes the orif orifice. Condensa Condensate is discharged intermittently. intermittently. FIGURE 21 Thermodynamic Thermodynamic Trap Trap (Courtesy of Spirax Sarco)

Piping Insulation Insulation nsulation is materials or combinations combinations of materials that that retard the flow low of heat energy. Substances with a large number of microscop microscopiic air pockets dispersed dispersed throughout the material make the most effici efficient ent insulators. insulators. These These extremely



small air spaces restrict the forma formation tion of of convection convection currents and the air is is a poor poor conductor conductor of of heat. Piping Piping is cover covered ed with insula insulation tion to: loss and condensation tion • Reduce heat loss uncomfortably hig high ambient temperatures within ithin the power • Prevent uncomfortably plant injury to personnel from from contact with hot surfa surfaces • Prevent injury sweating ting of cool cool pipe pipe surfac rfaces • Prevent swea A material suitable for for use as an insulation has the fol following characteristics: High insulating value • High • Long life • Vermin proof corrosive • Non corrosive Ability to retain its shape and insulating value when wet • Abi installa ation tion • Ease of application and install Thermal conductivit conductivity y or K value of a material is is a way of measuring the quantity ntity of heat that passes through through ametre thickness thickness per square metre per time unit with one degree dif difference in tempe temperature between the face faces. The The units units of measure are watts per square metre per temperature diff difference (W/ m²K). m²K). K va value (W / m ² K ) =

Energy Area × ΔT ( ° K ) × Time

Therma hermal conductivity tivity (k (k value) is important important in determining ining a material’s abilit ability y to resist the flo flow w of hea heat. The The lower thek the k fac factor, the higher the materials insulating insulating power and thus lowe lower overa overall heat transfer and operating ting costs. osts. The The value of thermal conductivi conductivity ty is used: formance during operation • As a benchmark of a material’s perfor determine a utility’s utility’s savi saving ngs in the consumption consumption of of steam or fue fuell • To determ return on on investme investment • To measure the return


519

520


PIPE INSULATION MATERIALS The foll following owing are types of pipe insulation insulation materials materials used in comm commercial and industrial industrial installations: Diatomaceous ceous silica silica • silicate • Calcium silica • Fibreglass • Cellular fibre (rock (rock and slag wool) wool) • Mineral fibre silica, or perlite lite • Expanded silica, • Elastomeric plastic tic • Foamed plas Refractory fibre fibre • Refractory Insulating cement cement • • Reflective metal insulation Diatomaceous Diatomaceous Silica

Diatomaceous silica silica is combined with ith a hydraulic ulic binder to form form asbestos free free block block insulation. insulation. These item items are versatile tile products available in a range of sizes sizes and thickness thicknesses up to 18 cm. Because of its its low low thermal conductivit conductivity y (0.09 – 0.15 W/ m²K), m²K), this this type of insulation is an economical, economical, energy saving ving insulation. It It exhibits exhibits minima minimal shrinkage at its its 1040°C tem temperature limit, limit, and does not readily decompose even when exposed exposed directl directly y to flame. Calcium Calcium Silicate

Calcium silica silicate is a granular insulation insulation made of lime lime and silica ilica reinforced inforced with ith organic and inorg inorganic fibres fibres and molded olded into rigid forms. forms. Servi Service ce temperature range covere overed is is 37.8ºC to 648.9ºC. Calcium silicate silicate insulation insulation has the foll following owing features: features: Lig Light weight • Low Low therma thermal conductivity conductivity of 0.049 – 0.095 W/ m²K • High High temperature and chemical resi resistance stance • • Water absorbent • Non-combustible Easily Easily cut and installed installed • Ideal materials for for insulatio insulation n applica pplications tions in in power and chemical • plants.



Fibreglass

Fibreg Fibreglass insulation insulation is is available vailable as flexible lexible blanket, blanket, rigid board, pipe insulation insulation and other pre-molded pre-molded shapes. shapes. Service temperature range is -40.0ºC to 250ºC. Thermal conductivity conductivity of fibreglass is 0.039 0.039 – 0.045 W/ m²K. m²K. Fibre Fibreglass is neutra neutral. Howe However, the binder binder may have a pH factor. It is is non-combustibl non-combustible e and has good sound absorptio absorption n qualities. lities. Cellular

This is is available vailable in board form and can be fabricated into into pipe pipe insulation insulation and va various shapes. Service temperature range is -26 -267.8ºC to 482.2ºC. The Thermal conductiv conductivity ity of cell cellula ularr glass is 0.043 – 0.045 W/ m²K. m²K. This product has the follo follow wing fea features: ood structural strength • Good Poor impact resistance • Poor • Non-combustible • Non-absorptive • Resistant to many chemicals Mineral Fibre (Rock And Slag Wool)

Rock and/ or slag slagwool fibres are bonded together with ith a heat resistant binder to produce mineral fibres. Upper tempera temperature limit limit can reach reach 1037.8ºC. The The 2 thermal conductivity conductivity of mineral fibre fibre is 0.05 0.05 to 0.17 W/ m K. The The material has a practicall ctically y neutra neutral pH, is non-combu non-combustible, and has has good sound sound control qua qualities. lities. Expanded Silica (Perlite)

Perlite Perlite is made from from an inert siliceous siliceous volcanic volcanic rock combi combined ned with ith water. The The 2 thermal conductivity conductivity of perli perlite te is 0.04 0.04 to 0.06 0.06 W/ m K at 24°C. The material has low low shrinkage and high resistance resistance to substra substrate te corrosion. corrosion. Perlite Perlite is nonnoncombustib combustible le and operates in the intermediate intermediate and high high temperature ranges. The The product is is available vailable in rigid pref preformed ormed shapes and blocks. blocks. Elastomeric

Foamed resins combined with elas elastome tomers produce a flexibl flexible e cellular llular material. Ava Available in prefor formed shapes and sheets, elastomeric insulations possess good cutting cutting characteristics cteristics and low low water and vapour permeability. ability. The upper temperature limit limit is 104.4ºC. The The thermal conductivi conductivity ty of elastomeric elastomeric insula insulations tions 2 is 0.036 W/ W/ m K. Elastom Elastomer eric ic insulation is is cost efficient efficient for for low low tempera perature applications applications with no jacketing necess necessary.


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Foamed Plastic

Insulation nsulation produced from foaming foaming plastic resins creates predominantly closed closed cellular cellular rigid materials. "K "K" values decli decline ne after initi initia al use as the gas trapped trapped within the cellular cellular structure structure is eventuall eventually y replaced by air. Foamed plastics are light weight with excellent moisture moisture resistance resistance and cutting cutting characteristics. cteristics. The The chemical content content varies varies with ith each manufac nufacturer. Avail Available able in prefo preforme rmed shapes and boards, foamed foamed plastics are generally lly used in the the low low and lowe lower interme intermediate diate service service temperature range -182.8ºC -182.8ºC to 148.9ºC. The therma rmal conductivity conductivity of elastomeric insulations is 0.03 - 0.04W/ m2K. Refractory Refractory Fibre

Refractory fibre fibre insulations insulations are mineral or ceramic ceramic fibres, fibres, including including alumina and silica, silica, bound bound with ith extremely high high temperature binders. The The material is is manufac nufactured in blank blanket or rigid form. form. Tem Temperature limits limits reach reach 1648.9ºC. The The 2 therma thermal conductivity conductivity of ref refractory fibre fibre insulations is 0.019 - 0.038 W/ mK. The material is nonnon-combu combustible. Insulating Cement Cement

Cements may be appli pplied to high high temperature surface surfaces. Fini Finishi shing ng cements or or onecoat cements are used in the the lowe lower interme intermediate range and as a finis inish h to other other insulation applica applicatio tions. ns. The The therma rmal conductivity conductivity of ref refractory fibre fibre insulatio insulations ns is 2 0.011 - 0.022 W/ m K. Opera Operating ting temperaturelimits limits reach reach 982.0ºC. Reflective Reflective Metal Metal Insulation

This is is a new new type of insulation constructed constructed of metal reflective reflective sheets of stainles inless s steel, spaced and baffled to form isolated isolated air chambers around the piping. piping. The The highly polished polished reflective reflective sheets sheets reflect reflect the heat and prevent loss loss due to radiation radiation but absorb littl little e heat throug through conduction. conduction. The The k factor tor varies from 0.53 to 0.66 2 W/ m K. Appl Ap plic ic ation ati ons s

The foll following owing indicates the general application of of various piping insulatio insulations ns for for diff different temperature ranges: Above 1040oC - refractory fibres fibres are generally lly used or in some cases • Abo ref reflective metal insulation 650oC - 1040oC - double double layer constructio onstruction n is used with the inner • layer diatomaceous silica silica and the outer llay ayer calcium lcium silicate silicate o o 150 C - 650 C - calcium lcium silica silicate is generally used with ith double double layer • o construction construction for for pipe pipe temperatures over 316 C o fibre is is most commonly used as it is generally the • 0 - 260 C - glass fibre most economica economical and has good resistance to no norma rmal abuse Conforms with the 2004 ASME Extract • Revised 03/06


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The effectiveness of a particular rticular insula insulation tion is is expressed as an effici efficiency ency E where: E =

Heat loss from bare pipe

−

heat loss from insulated pipe

heat loss from bare pipe

The heat losses losses are expressed sed in kJ/ kJ/ h/ linea linear metre. Piping insula insulatio tion n is usually lly fabricated in halflf-cylindrical cylindrical sections for for fitti fitting ng over the pipe. The sectio sections ns are held toge together with metal metal wire or bands and then a surf surfac ace fini finish, sh, usually lly a canvas type, is is applied. applied. Special shapes and arrangements of insulation insulation are used for for fitti ittings ngs such as elbows, flang flanges, and valves valves such as shown in Fig. Fig. 22. FIGURE 22 Insulation of Fittings


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OBJECTIVE 7 Explain the theory and effects of water hammer.

WATER HAMMER

Water hammer is a series ries of hammer blowblow-llike shocks shocks produced produced by a sudden change of velocity velocity of water or other liquid liquid flo flow wing within ithin a pipeline. Thes These shocks may have suffici sufficient ent magnitude to rupture the pipe or pipe pipe fitti fittings ngs or to damage connected connected equipment. equipment. The sudden change of veloci velocity ty necessary to pro produce duce water hammer may be caused by the foll following: owing: Rapid oper operation tion of a valve lve • Sudden stoppag stoppage in flo flow w dueto a pump trip trip • ithin the the pipe • Rapid condensing of a pocket of steam within Valve Valve Operation

In the case of a valve lve being quickly closed closed in a pipeline throug through which water is flowing lowing, the first first eff effect is the sudden sudden decrease in the velocity velocity of of the water and a correspondi corresponding ng increa increase in pressure at the valve. valve. Thi This s causes a pressure wave to travel back upstream to the inlet inlet end of of the pipe pipe where it reverses and surges back and fo forth through through the pipe, pipe, gettin tting g weaker with each successive successive reversal. This his pressure wave due to water hammer is in in addition addition to to the the normal water pressure withi ithin n the pipe pipe and depends upon the magnitude nitude and rate of change in ve velocity. Complete stoppage of fl flow is not necessary to produce water hammer as any sudden change in velo velocit city y may bring bring it about to some degree depending pending upon the above conditions. conditions. Where too rapid closing closing of a valve is is the cause of the water hammer, the the remedy is to ensure that that the valv valve e is closed slo slow wly. The The period of of effec effective tive closing of a gate valve takes takes place in the the last 20% 20% of the valve travel and this this portion portion is is undertaken as slowly as possible. possible. If If the valve is is equipped with a bypass, the bypass is opened opened to equalize alize the pressure on both both sides sides of the valve. valve. The The bypass va valve is closed afte fter the main valve has been closed. When opening a gate valve, the the first first 20% of the valve travel is the the most criti critica cal portion. portion. If If so equipped, the bypass should be opened to allow llow for for pressure pressure equalization alization.. Then Then the main valve valve is opene opened as slowly lowly as possible. possible. As As a general rule, all valves valves are opened and closed closed slowly and cautiously. utiously. Conforms with the 2004 ASME Extract • Revised 03/06


Sudden Stoppage in Flow

When water hammer is is due to the sudden sudden stopping stopping of of a motorotor-driv driven en pump due to a power failure, failure, the the pressure drops at the the pump discha discharge. The The water in the discharge line line stops and then reverses reverses direction. direction. Subsequent rapid closing closing of the check valve valve at the pump causes severe shock shock when the energy of the reverse flo flow w is vio violentl lently y expended against the the check valve disc. A pu pump tr trip may also ca cause water hammer in in th the pump suction li line in cases where the water flo flows ws to the pump through a long line line by gravity vity or or under pressure from from another pump. The maximu ximum intensity intensity of the wave can be calcula lculated using Joukowsky’s Joukowsky’s Law: H wh

=

cv g

Where: H wh c v g

= = = =

head of water hammer, m velocity of sound in the liquid, m/ s instantan instantaneous eous velocity velocity change in liqu liquid id (m/ s) 2 acceleration tion due to gravity, 9.81 9.81 m/ s

Example 3 A pu pump delivers water to a tank 75 m above the pump. Dur During a power failure, the pump discharge check valve gets gets stuck in the the open positi position on for for a few few moments and then slams shut. shut. Before Before the check valve closes, closes, water begins to flow low backwards through the pump pump with a velocity velocity of 15 m/ s. If If the speed of sound in in water is 1469 m/ s at 15.6°C, what is is the the water hammer head produced? produced? Solution H wh

=

H wh

=

cv g

1469 m/s

×

15 m/s

9.81 m/s 2 H wh = 2246.18 m

A water hammer surge of 2246.18 m, added to the normal running head of 75 m, would create a total head of of:: 2246. 46.18

+ 75 =

2321.18 .18 m


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Converting Converting this head to pressure: pressure: Pressure = ρ gh Pres Presssure ure = 1000 1000 kg/ kg/m3

2

9.81 m/s × 9.81

2321.1 .18 8 × 2321

m

Pres Presssure ure = 22 770 770 776 776 N/m2 Pressure = 22 770 770 776 776 Pa Pressure = 22 771 kPa (Ans.)

This may be suffici sufficient ent to destroy any weak point point in the the system. The The above bove example is for for instantaneous closing. closing. If If the valve closing closing time time is increa increased, the shock shock wave is is greatly decreased. Devi Devices ces which hich can be used to reduce the shock shock in a pump discha discharge line line are air chambers, relief relief valves valves or check valves valves with ith a built-in built-in dashpot to prevent rapid closing closing of the disc. disc. Steam Condensing

In the case of a steam line, line, water hammer may occur occur if condensate is is present in in the line. line. As As the steam passes through the line line above bove the surf surface of the condensate it may raise up behind it a mass of the condensate condensate (water). Thus an isolated isolated pocket of steam is formed. formed. Because it is is in contact contact with ith the cooler cooler water, the steam suddenly condenses and a low low pressure is formed formed in the pocket. pocket. Water rushing into this this low low pressure pocket pocket causes severe shock to the pipe and pipi piping ng fittings. Water hammer can also occur in a steam line line that is horiz horizont ontal al or pit pitched ched upward from the source of steam. It It is is most viol violent ent when a blank or a closed va valve dead ends the steam flo flow in the pipe. To avoid avoid water hammer in stea steam lines lines they are properly properly pitched pitched and drainag inage points points installed installed between valves valves and at pockets pockets in the line line where water can accumulate. The The drainage points points are equipped wit with h drip legs, free free-blow blow drain va valves, and traps. In addition, gate valves in the line are not installed with their stems below below the horizont horizontal al because the valv valve e bonnets act as pockets. When warming up a steam line line all drain drain valv valves es are opened wide before before stea steam is admitted. The The steam admission dmission valve valve should only only be cracked cracked open. If If equipped with a bypass, it is is slowly opene opened to pressuriz pressurize e the line line on both sides sides of the main isola isolation tion valve. The The main va valve lve is slowly and carefully open opened ed fully fully af after the line line has been suffici sufficiently ently warmed up. The The drain valves valves are left open open until all of the warm-up rm-up condensate condensate has been discharg discharged and drains are blo blowing wing dry steam. The The trap is then then able to handle the condensa condensate that forms forms under stand standar ard opera operating ting conditions.



CHAPTER QUESTIONS 1.

List List the properties properties that contribute contribute to the suitabilit suitability y and economy of a given iven pipe material. material.

2.

(a) Calculate the required thicknes thickness for 406.4 mmnominal size size plain end steam pipe pipe to operate operate at 17 250kPa and 540°C. The material terial used is seamless alloy alloy steel SA-335P12. (b) Calculate the maximu ximum allowa llowable working pressure, in MPa, MPa, for for the nominal size size plain end steam pipe in in the above example.

3.

With the aid of a simple sketch, show how the probes are loca located in relation to the weld in time-of-flight diffraction.

4.

Explain Ex plain how high temperatures affect the tensile tensile strength of piping. piping.

5.

Give ive the advantages and disadvantages of the follo ollowing wing: (a) Ex Expa pansion bends (b) Slip lip expansion joints joints (c) Corrugated expansion ion joint joints s

6.

Explain xplain how how the sudden closi closing ng of a valve valve can cause water hammer in a pipe.


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quection on piping material selection

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