CONTROL VALVE CHARACTERISTICS
Foreword Welcome to value-conscious company. We are proud of the advanced engineering and quality construction of each equipment we manufacture. This manual explains the working of equipment. Please read it thoroughly and have all the occupants follow the instructions carefully. Doing so will help you enjoy many years of safe and trouble free operation. When it comes to service remember that K.C. Engineers knows your equipment best and is interested in your complete satisfaction. We will provide the quality maintenance and any other assistance you may require. All the information and specifications in this manual are current at the time of printing. However, Because of K.C. Engineers policy of continual product improvement we reserve the right to make changes at any time without wit hout notice. Please note that this manual explains all about the equipment including options. Therefore you may find some explanations for options not installed on your equipment. You must follow the instructions and maintenance instructions given in the manual carefully to avoid possible possible injury or damage. damage. Proper maintenance maintenance will help ensure ensure maximum performance, greater reliability and longer life for the product.
K.C. Engineers
Contents 1. Objective
……………. 1
2. Introduction
……………. 1
3. Description
……………. 2
4. Utilities Required
……………. 3
5. Precaution & Maintenance Instructions
……………. 3
6. Troubleshooting
……………. 3
Experiment No. 1: Study the Control valve Flow Co-efficient (Cv) 1. Aim
……………. 4
2. Theory
……………. 4
3. Experimental Procedure
……………. 5
4. Observation & Calculation
……………. 5
5. Nomenclature
……………. 6
6. References
……………. 6
Experiment No. 2: Study the Inherent Characteristics of Control Valve 1. Aim
……………. 7
2. Theory
……………. 7
3. Experimental Procedure
……………. 8
4. Observation & Calculation
……………. 8
5. Nomenclature
……………. 9
6. References
……………. 9
Control Valve Characteristics (Two Valves)
Experiment No. 3: Study the Installed Characteristics of Control Valve 1. Aim
……………. 10
2. Theory
……………. 10
3. Experimental Procedure
……………. 10
4. Observation & Calculation
……………. 11
5. Nomenclature
……………. 11
6. References
……………. 11
Experiment No. 4: Study the Hysteresis of Control valve 1. Aim
……………. 12
2. Theory
……………. 12
3. Experimental Procedure
……………. 12
4. Observation & Calculation
……………. 13
5. Nomenclature
……………. 13
6. References
……………. 13
Experiment No. 5: Study the Rangeability of Control Valve 1. Aim
……………. 15
2. Theory
……………. 15
3. Experimental Procedure
……………. 15
4. Observation & Calculation
……………. 16
5. Nomenclature
……………. 16
6. References
……………. 16
Control Valve Characteristics (Two Valves)
CONTROL VALVE CHARACTERISTICS (TWO VALVES)
1.
OBJECTIVE: To study Control Valve Characteristics
2.
INTRODUCTION: VALVE ACTION & ACTUATOR MECHANISM If a control valve is used to control fluid flow, some mechanism must physically open or close the valve. Different types of actuators are used to control the stem travel of the valve, like electrical actuators, pneumatic actuator, Hydraulic actuators etc. In present set-up, pneumatic actuators are used for Control Valves. Spring opposed diaphragm actuator positions the valve plug in response to the controller signals. Mostly the controller signals are in the range of 3-15 psig. There are two types of actuators described below: i) Direct acting actuator (Air to Close): Direct acting actuators basically consist of a pressure tight housing sealed by a flexible fabric reinforced elastomer diaphragm. A diaphragm plate is held against the diaphragm by a heavy compression spring. Signal air pressure is applied to upper diaphragm case that exerts force on the diaphragm and the actuator assembly. By selecting proper spring rate or stiffness, load carrying capacity, and initial compression, desired stem displacement can be obtained for any given input signal. ii) Reverse acting actuator (Air to open): In case of reverse acting actuators the stem gets retracted with increase in pressure.
TYPES OF CONTROL VALVE: Valve is essentially a variable orifice. Control valve is a valve with a pneumatic, hydraulic, electric (excluding solenoids) or other externally powered actuator that automatically, fully or partially opens or closes the valve to a position dictated by signals transmitted from controlling instruments. Control valves are used primarily to throttle energy in a fluid system and not for shutoff purpose. Depending upon the valve plug design the control valves can be divided in three categories as under: i) Equal % Type.
Flow changes by a constant percentage of its instantaneous value for each unit of valve lift. Q = be ay Q
=
flow at constant pressure drop
y
=
valve opening
e
=
base of natural logarithms
a and b=
constants
Constants a and b can be evaluated to give more convenient from
log R x y y max
Q = Qo e Qo
=
Flow at constant drop at zero stroke
R
=
Flow range of valve, maximum to minimum at constant drop.
ymax
=
maximum rated valve opening
ii) Linear Type. Flow is directly proportional to valve lift. F F max
=
m mmax
Q = k y Q
=
Flow at constant pressure drop
y
=
Valve opening
k
=
Constant
ii) Quick Opening Type (On/Off Type). Flow increases rapidly with initial travel reaching near its maximum at a low lift. It is generally not defined mathematically.
3.
DESCRIPTION: The present set-up consists of two Nos. Control Valves with Pneumatic Actuators. One Control Valve is with Equal % Characteristics (air to close type) and second is with Linear Characteristics (air to open type). Apparatus is self-contained water recirculating unit (except compressed air supply, is to be provided from user end). Water from Sump Tank is sucked and delivered to a Constant Level Overhead Tank by means of a Centrifugal Pump. Water flows back from Overhead Tank to Sump Tank through the control valve.
Each Control Valve is provided with a Ball Valve at the inlet. Flow rate of water, passing through the Control Valve is measured with the help of Rotameter. A common Water Manometer is provided for the measurement of Water Pressure Head at the inlet of Control Valve. As outlet of Control Valve is open to the atmosphere, the pressure at inlet of Control Valve is considered as Pressure Drop (∆P) across the valve. Change in the Stem Travel/Position of Control Valve results in change in flow through the Control Valve. This adjustment is done by regulating the Air Pressure inside the diaphragm of Control Valve (3 to 15 psig). For this purpose a Pressure Regulator with Pressure Gauge is provided in Pneumatic Line. Pressure Gauge facilitates to get direct reading of Pressure inside the diaphragm of Control Valve. Individual ball valves are provided to actuate the Pneumatic line for individual Control Valve. For detecting the Stem Travel/Position, scale is provided on each Control Valve.
4.
UTILITIES R EQUIRED: Electricity Supply:
Single phase, 220 VAC, 50 Hz, 5-15 amp socket with earth connection.
Compressed Air Supply: 1 CMH at 2 Bar Water Supply Drain
5.
PRECAUTIONS & MAINTENANCE INSTRUCTIONS: 1. Never run the apparatus i f power supply is less than 180 volts & 230 volts. 2. If apparatus will not in use for more than one month, drain the apparatus completely & fill pump with cutting oil. 3. To prevent clogging of moving parts, run pump at least once in a fortnight. 4. Always use clean water. 5. Always keep apparatus free from dust.
6.
TROUBLESHOOTING: 1.
If pump gets jam, open the back cover of pump & rotate the shaft manually.
EXPERIMENT NO. 1:
STUDY OF CONTROL VALVE FLOW CO-EFFICIENT (Cv)
1.
AIM: To determine flow coefficient Cv of the control valves.
2.
THEORY: A Control Valve is used to control the flow rate in a fluid delivery system to control the process. There is a close relation between the pressure and the flow rate in fluid stream passing through pipe so that if pressure is changed, the flow rate will also be changed. A Control Valve changes the flow rate by changing the pressure in the flow system because it introduces the constriction in the delivery system. So we can say that the flow rate through the constriction can be given as: Q=
K
∆ P
A most important factor associated with control valve is the correction K of the above equation. This correction factor allows selection of proper size of valve to accommodate the rate of flow that the system must support. This correction factor is called as valve coefficient and is used in valve sizing.
CONTROL VALVE FLOW CO-EFFICIENT: A correction factor is known as valve coefficient and is measured as the number of USGPM (US Gallon per Minute) that flows through a fully open valve with a Pressure Drop of 1 psig. 1 US Gallon = 3.785 Liters. In SI Units the formulae for calculating Cv is: Q = K v
∆Ρv G
Where: Q = flow rate, m3/hr
∆P v
=
Pr. drop across valve, kgf/cm2
Relation between K V & Cv is:
K V = 0.856Cv
G
C v = 1.16 Q
∆ P
Where: Q
=
Discharge of Fluid in m3/h
∆P
=
Pressure Drop in Bar.
G
=
Specific Gravity of Fluid
To convert ∆P in mm of H2O in Bar = ∆P mm of H2O/(10.33X103) Bar.
3.
EXPERIMENTAL PROCEDURE: 1. Start the set up for Control Valve of Equal % Characteristics. 2. Open Pneumatic Line for the Control Valve. 3. Open the Control Valve fully. As the Control Valve is Air to close so pressure in diaphragm should be 0 psig. 4. Adjust the Rotameter for 500 LPH flow by regulating the valve provided at the inlet line of the Control Valve and Wait for 5 minutes to steady the flow. 5. Record the Manometer reading in mm of water. 6. Record the Rotameter reading. 7. Increase the pressure slowly and record the manometer & Rotameter reading up to 15 Psi.
4.
OBSERVATION & CALCULATION: OBSERVATION TABLE: S.No.
∆P, mm of H 2O
Q, LPH
CALCULATIONS: C v = 1.16 Q
G
∆ P
= ----------------------
Conduct the experiment for Linear Control Valve. As the Linear Control Valve is Air to Open, so Pressure in diaphragm should be more than 15 psig. Adjust this pressure in diaphragm by operating the Pressure Regulator. Do not exceed this pressure more than 20 psig.
5.
6.
NOMENCLATURE: Cv
=
Flow coefficient of control valve
G
=
Specific gravity of fluid
∆P
=
Pressure drop, Bar
Q
=
Flow rate, LPH
R EFERENCES: 1. Donald R. Coughanowr, “Process System Analysis and Control”, 2nd ed., McGraw Hill, NY, 1991, Page 305-309.
EXPERIMENT NO. 2:
STUDY OF INHERENT CHARACTERISTIC OF LINEAR CONTROL VALVE
1.
AIM: To study the Inherent Characteristic of Linear Control Valve and to determine the flow coefficient Cv of control valve
2.
THEORY: VALVE CHARACTERISTICS: The amount of fluid passing through a valve at any time depends upon the opening between the plug and seat. Hence there is relationship between stem position, plug position, and the rate of flow, which is described in terms of flow characteristics of a valve. Inherent and installed are two types of flow characteristics of a Control Valve.
I NHERENT CHARACTERISTICS: The inherent flow characteristic of control valve is the relation between the flow and the valve travel at constant pressure drop across the valve. Following are the inherent characteristics for different types of valves.
100
Flow % of Maximum
90 80
Quick Opening
70 60
Linear
50 40 30
Equal
20 10 0 0 10
20 30 40
VALVE LIFT % OF FULL LIFT
50 60 70
80 90 100
3.
EXPERIMENTAL PROCEDURE: 1. Start the set-up for Linear Control Valve. 2. Open Pneumatic Line for the Control Valve. 3. Open the Control Valve fully. As the Control Valve is Air to open so pressure in diaphragm should be 15 psig. 4. Adjust the Rotameter for 500 LPH flow by regulating the valve provided at the inlet line of the Control Valve. 5. Wait for 5 minutes to steady the flow. 6. Record the Manometer reading in mm of water. 7. Record the Rotameter reading. 8. Now slowly decrease the air pressure by regulator so that the stem travel towards closing position becomes 2 mm. 9. The pressure drop across the valve will increase. Throttle the regulatory valve at the inlet of the Control Valve to maintain Pressure Drop constant. 10. Again note down the reading of Rotameter and stem travel. 11. Repeat the procedure and take the readings at each 2mm stem travel till the valve is fully closed (Pressure upto 0 psig). 12. Plot the graph of valve coefficient versus lift to show inherent characteristic of the control valve. 13. Conduct the same procedure for other two valves too.
4.
OBSERVATION & CALCULATION: OBSERVATION TABLE:
Stem lift in mm
14
12
10
Q, LPH ∆P,mmH2O
CALCULATIONS: C v = 1.16 Q
G
∆ P
= ----------------------
8
6
4
2
0
Conduct the experiment for other two valves. As the Linear Control Valve is Air to Close, so Pressure in diaphragm should be 0 psig. Repeat the experimental procedure same as above, but pressure in the diaphragm will be increased gradually.
5.
6.
NOMENCLATURE: Cv
=
Flow coefficient of control valve
G
=
Specific gravity of fluid
∆P
=
Pressure drop, Bar
Q
=
Flow rate, LPH
R EFERENCES: 1. Peter Harriott, “Process Control”, 19th ed., McGraw Hill, NY, 2000, Page 194198.
EXPERIMENT NO.3:
STUDY OF INSTALLED CHARACTERISTIC OF CONTROL VALVE 1.
AIM: To study Installed Characteristics of Control valve and to determine the flow coefficient Cv of control valve
2.
THEORY: VALVE CHARACTERISTICS: The amount of fluid passing through a valve at any time depends upon the opening between the plug and seat. Hence there is relationship between stem position, plug position, and the rate of flow, which is described in terms of flow characteristics of a valve. Inherent and installed are two types of flow characteristics of a Control Valve.
I NSTALLED CHARACTERISTICS: The Installed characteristics of the valves described are subject to distortion due to variations in pressure drop with flow. Line resistance distorts linear characteristics towards that of quick opening valve and equal % to that of linear.
3.
EXPERIMENTAL PROCEDURE: 1. Start the set-up for Equal % Control Valve. 2. Open Pneumatic Line for the Control Valve. 3. Open the Control Valve fully. As the Control Valve is Air to Close so pressure in diaphragm should be 0 psig. 4. Adjust the Rotameter for 500 LPH flow by regulating the valve provided at the inlet line of the Control Valve. 5. Wait for 5 minutes to steady the flow. 6. Record the Manometer reading in mm of water. 7. Record the Rotameter reading. 8. Now slowly increase the air pressure by regulator so that the stem travel towards closing position becomes 2 mm. 9. Wait for 5 minutes to steady the flow and note down the reading of Rotameter, Manometer and stem travel.
10. Repeat the procedure and take the readings at each 2 mm stem travel till the valve is fully closed (Pressure upto 15 psig). 11. Plot the graph of valve coefficient versus lift to show Installed characteristic of the control valve. 12. Plot the graph of the flow versus lift.
4.
OBSERVATION & CALCULATION: OBSERVATION TABLE: As valve is air to close lift is measured from 14 to 0 mm
Stem Lift in mm
14
12
10
8
6
4
2
0
Q, LPH ∆P, mm of H 2O
CALCULATIONS: C v = 1.16 Q
G
∆ P
= ----------------------
Installed characteristics of the linear valve slightly approaches to the characteristic of quick opening valve and that of equal % valve approaches to linear characteristic because of the pipe friction and other resistance to the flow.
5.
6.
NOMENCLATURE: Cv
=
Flow coefficient of control valve
G
=
Specific gravity of fluid
∆P
=
Pressure drop, Bar
Q
=
Flow rate, LPH
R EFERENCES: 1. Donald R. Coughanowr, “Process System Analysis and Control”, 2nd ed., McGraw Hill, NY, 1991, Page 309-310.
EXPERIMENT NO. 4:
STUDY OF HYSTERESIS OF CONTROL VALVE
1.
AIM: To study the hysteresis of the control valve
2.
THEORY: Hysteresis is a predictable error resulting from the differences in the transfer functions when a reading is taken from above and below the value to be measured. In case of control valves for same actuator signal different stem travel (hence valve coefficients) are obtained depending upon the direction of change in the signal. The maximum error in stem travel (or valve coefficient) expressed in % for same actuator pressure while opening and closing the valve is indicated as hysteresis.
3.
EXPERIMENTAL PROCEDURE: 1. Start the set-up for Equal % Control Valve. 2. Open Pneumatic Line for the Control Valve. 3. Open the Control Valve fully. As the Control Valve is Air to Close so pressure in diaphragm should be 0 psig. 4. Adjust the Rotameter for 500 LPH flow by regulating the valve provided at the inlet line of the Control Valve. 5. Wait for 5 minutes to steady the flow. 6. Record the Manometer reading in mm of water. 7. Record the Rotameter reading. 8. Now slowly increase the air pressure by regulator upto 3 psig. 9. Wait for 5 minutes to steady the flow and note down the reading of Rotameter, Manometer and Pressure in psig. 10. Repeat the procedure and take the readings at +3 psig till the valve is fully closed (Pressure upto 15 psig). 11. Now increase the pressure upto 20 psig and start decreasing the pressure gradually upto 15 psig. 12.
Wait for 5 minutes to steady the flow.
13.
Record the Manometer reading in mm of water.
14.
Record the Rotameter reading.
15.
Repeat the procedure and take the readings at –3 psig till the valve is fully opened (Pressure upto 0 psig).
16.
Calculate valve flow coefficient for actuator pressure for every reading.
17.
Plot the graph of actuator pressure versus flow coefficient.
The ratio of
maximum difference between flow coefficients at same actuator pressure to that of maximum flow coefficient is termed as hysteresis. 18.
4.
Repeat the experiment for the other two valves.
OBSERVATION & CALCULATION: OBSERVATION TABLE: Pressure
Increasing Pressure
Decreasing Pressure
(psig)
∆P, mm Q, LPH
∆P,mm of Q, LPH
of H20
H20
3 6 9 12 15
CALCULATIONS: C v = 1.16 Q
G
∆ P
Hysteresis % =
= -----------------
C V at decreasing pressure − C V at increasing pressure Maximum C V
x 100 = --------
Repeat the experiment for linear valve. As the Control Valve is Air to Open so keep in mind this parameter during conducting the experiment.
5.
NOMENCLATURE: Cv
=
Flow coefficient of control valve
G
=
Specific gravity of fluid
∆P
=
Pressure drop, mm of H2O
Q
=
Flow rate, LPH
6.
R EFERENCES: 1. Donald R. Coughanowr, “Process System Analysis and Control”, 2nd ed., McGraw Hill, NY, 1991, Page 315.
EXPERIMENT NO. 5:
STUDY OF R ANGEABILITY
1.
AIM: To study the rangeability of equal % valve.
2.
THEORY: Equal % valve has a characteristic such that flow changes by a constant % of its instantaneous value for given % change in stem position. Generally this type of valve does not shut off the flow completely in its limit of stem travel. The rangeability (R) is defined as the ratio of maximum to minimum controllable flow. R =
F max F min
Where F max is the flow when the valve stem is at nearly extreme open position for maximum controllable flow. Fmin is the flow when valve stem is at nearly extreme closed position for minimum controllable flow. Fmax, Fmin represents flow rates measured at constant pressure drop across control valve. Hence rangeability R also can be defined as ration of Cv max to Cv min. For Equal % valve flow has an exponential characteristics of rangeability, F = R m-1. R is the rangeability of the valve and m is its fractional stem position.
3. EXPERIMENTAL PROCEDURE: 1. Start up the setup for equal % control valve. 2. Adjust the Rotameter valve and set 500 LPH flow. 3. Set actuator air pressure to 3 psig. 4. Note down the flow rate and pressure at inlet of control valve. 5. Set actuator air pressure to 15 psig 6. Note down the flow rate and pressure at inlet of control valve.
4.
OBSERVATION & CALCULATION: OBSERVATION TABLE: Pressure
∆P, mm
(psig)
of H2O
F,LPH
C v
Remarks
Nearly 3
max
Nearly 15
min
CALCULATIONS: R =
R =
C v max C v min F max F min
= ---------------
= --------------
[Repeat the experiment by keeping constant pressure drop across the control valve and note the flow rates.
5.
6.
NOMENCLATURE: R
=
Rangeability
Cv
=
Flow coefficient of control valve
Fmax =
Maximum controllable flow rate
Fmin =
Minimum controllable flow rate
∆P
Pressure drop, mm of H2O
=
R EFERENCES: 1. Donald R. Coughanowr, “Process System Analysis and Control”, 2nd ed., McGraw Hill, NY, 1991, Page 309.