Exp. No: 1
Group No.: 3
CONTROL VALVE CHARACTERISTICS Roll No.
Name
CH11B080
Naveen
CH11B081
Neeraj
CH11B089
Sahithi
Signature
Date of experiment:
11th September 2014
Date of submission:
18th September, 2014
Signature of TA: Name of TA:
Process Control Laboratory Department of Chemical Engineering Indian Institute of Technology Chennai 600 036
Objective: To study the inherent characteristics of a control valve Theory: Pneumatic control valves are commonly used as final control elements to manipulate the flows of material and energy into and out of a process. The control valve components include valve body, plug, seat and actuator. Normally a pressure of 3-15 psig is given to the diaphragm. 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. All control valves have an inherent flow characteristic that defines the relationship between 'valve opening' and flow-rate under constant pressure conditions. In general, the flow of the fluid through the valve can be described as follows: q=f(po,p1,L) where p0 and p1 signify the upstream and downstream pressures and L denotes the stem position. The inherent valve characteristics are determined for constant upstream and downstream pressures. Hence q=f(L) The Cv coefficient of control valve depends on the valve position. It varies from zero when the valve is closed, to a maximum value, Cv, max, when the valve is fully opened, that is when the fraction valve position is unity. It is this variation in the C v that allows the valve to regulate the flow continuously. The particular function relating the C v coefficient to the valve position is known as the inherent valve characteristics. Valve manufacturers can shape the valve characteristics by changing the shape of the plug and there by changing the opening area. 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 a relationship between stem position, plug position and the rate of flow, which is described in terms of flow characteristics of a valve. The two types of valve characteristics are inherent and installed flow characteristics.
Inherent flow 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. The inherent flow characteristics of the valve are determined by the valve orifice and plug geometry. There are three types of inherent flow characteristics, namely, linear, quick opening and equal percentage. The flow characteristics are represented below.
A. Linear flow Characteristics : Linear flow characteristics show that flow rate is directly proportional to the valve travel. Control valves with linear flow characteristics are commonly specified for liquid level control. The sensitivity of a control valve with linear flow characteristic is always constant. Mathematically, it can be described as Q=ky, where Q is the flow at constant pressure drop, y is the valve opening and k is the constant. B. Equal percentage flow characteristics : In this flow characteristic, equal increments of valve travel produce equal percentage changes in the existing flow. The change in flow rate is always proportional to the flow rate just before the change in valve stem position. This can be represented as Q=beay, where Q is the flow at constant pressure drop, y is the valve opening and a and b are constants.
C. Quick opening flow characteristics : A valve with this characteristic provides the maximum change in flow rate at low stem travels and small changes when the valve stem is in maximum. Control valves with quick opening flow characteristics are often used for on-off applications where significant flow rate must be established as the valve begins to open. They are often used in relief valve applications.
Inherent valve characteristics curve (pressure drop across valve is constant) I linear, II increasing sensitivity (e.g. equal percentage valve), III decreasing sensitivity (Quick opening) Installed Characteristics: The inherent 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 percentage to that of linear. Hysteresis of control valve: 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.
Experimental set up: The experimental set up of control valve consists of on-off, linear and equal percentage valve respectively. The desired valve is taken into the line and the line pressure drop is maintained constant (1 kg/cm 2) during the measurements by adjusting the bypass valve (HV1) and the operational control valve. Various opening and closing conditions are obtained by regulating the pressure G2 by means of the vent-valve. The corresponding rotameter readings were noted down which correspond to the flow rate.
Procedure I.
ON-OFF valve 1. Before conducting the experiment, make sure the availability of water in reservoir tank. Fill clean and soft water in the reservoir. 2. Switch on the unit and the compressor. Now the pump should be in the off position. 3. Keep the bypass valve (HV1) in the fully open position. 4. Set the output pressure of the air regulator (G2) to 15 psi by using the air regulator valve and the vent valve (HV8). 5. Choose the valve. 6. For the case of on-off valve, open the inlet regulating valve (HV2) and pressure supplying valve (HV5). 7. Switch on the pump. 8. Now set the backpressure (G1) to 3psi by adjusting the bypass valve (HV1) and inlet regulating valve (HV2). 9. Set the maximum flow in the rotameter by adjusting the bypass valve (HV1) 10. Now set the backpressure (G1) to 3psi by adjusting the bypass valve (HV1) and inlet regulating valve (HV2).
11. Maintain this pressure of 3psi in G1 throughout the experiment. 12. Never disturb the hand valve (HV2), once it is adjusted. 13. Observe flow in the rotameter and inlet pressure variation. Note down the air regulator pressure (G2), rotameter flow and stem position in control valve. 14. Decrease the pressure in air regulator to 12 psi, at the same time pressure across the control valve slightly increases; adjust hand valve (HV1) to maintain the predefined pressure in G1. 15. Note flow in rotameter and stem position in control valve, air regulator pressure. 16. Slowly decrease/increase the air pressure regulator and vent valve (HV8) for achieving different stem positions till the valve is fully closed/open. 17. Tabulate the rotameter flow, air regulator and stem position. 18. Plot the graph between rotameter flow in the y-axis and stem position in x-axis. 19. When the experiment is complete in one valve, set G2 to zero by fully opening vent (HV8) and air regulator valve (G2) and switch off the pump. 20. For the case of equal percentage valve repeat the procedure by opening the inlet regulating valve (HV3) and pressure supplying valve (HV6) and for linear valve the procedure is repeated with inlet regulating valve (HV4) and pressure supplying valve (HV7). II.
Equal percentage control valve: Repeat the procedure by selecting the inlet regulating valve (HV3) and pressure supplying valve (HV6).
III.
Linear Valve Repeat the procedure by selecting the inlet regulating valve (HV3) and pressure supplying valve (HV6).
Observations and Calculations: The flow through the three control valves are found out at constant pressure drop across the Control Valve. The measurements taken are tabulated below. The flow rate (%) vs stem lift (%) is plotted in the graph as shown below.
I. ON-OFF valve: Pressure drop: 3 psi Data: Actuated pressure (psi)
Valve Closing Stem Position Flow Rate (mm) (lph)
15 12 9 6 3
24 21 12 3 2
Valve Opening Stem Position Flow Rate (mm) (lph)
1000 1000 1000 500 0
Valve Closing Stem Lift %
1000 1000 1000 0 0
Valve Opening
Flow Rate %
100.00 87.50 50.00 12.50 8.33
23 16 8 2 2
Stem Lift %
100.00 100.00 100.00 50.00 0.00
Flow Rate %
100.00 69.57 34.78 8.70 8.70
100.00 100.00 100.00 0.00 0.00
Graph:
On-Off Valve 120.00 100.00 % e t a R w o l F
80.00 60.00
Valve Closing
40.00
Valve Opening
20.00 0.00 0.00
20.00
40.00
60.00
Stem Lift %
80.00
100.00
II. Equal percentage control valve: Pressure drop: 3 psi Data: Valve Closing Stem Position Flow Rate (mm) (lph)
Actuated pressure (psi)
15 12 9 6 3
27 19 13 4 0
Valve Opening Stem Position Flow Rate (mm) (lph)
1000 900 400 150 0
27 19 11 3 0
Valve Closing Stem Lift %
1000 850 350 100 0
Valve Opening
Flow Rate %
100.00 70.37 48.15 14.81 0.00
Stem Lift %
100.00 90.00 40.00 15.00 0.00
Flow Rate %
100.00 70.37 40.74 11.11 0.00
100.00 85.00 35.00 10.00 0.00
Graph:
Equal Percentage Valve 120.00 100.00 % e t a R w o l F
80.00 60.00
Valve Closing
40.00
Valve Opening
20.00 0.00 0.00
20.00
40.00
60.00
Stem Lift %
80.00
100.00
III. Linear control valve: Pressure drop: 3 psi Data:
Actuated pressure (psi)
Valve Closing Stem Position Flow Rate (mm) (lph)
15 12 9 6 3
22 18 12 3 0
Valve Opening Stem Position Flow Rate (mm) (lph)
1000 950 750 150 0
Valve Closing Stem Lift %
22 19 14 3 0
1000 950 750 50 0
Valve Opening
Flow Rate %
100.00 81.82 54.55 13.64 0.00
Stem Lift %
100.00 95.00 75.00 15.00 0.00
Flow Rate %
100.00 86.36 63.64 13.64 0.00
100.00 95.00 75.00 5.00 0.00
Graph:
Linear Valve 120.00 100.00 % e t a R w o l F
80.00 60.00
Valve Closing
40.00
Valve Opening
20.00 0.00 0.00
20.00
40.00
60.00
Stem Lift %
80.00
100.00
Conclusions: 1. Hysteresis is the dependence of the output of a system not only on its current input, but also on its history of past inputs. These differences in values observed during valve opening and valve closing for each of the valves is because of this property of values. 2. On-Off Valve shows the characteristics of typical on-off feedback systems. The values are not same along both ways because of the hysteresis which might be caused due to fluctuations in flow rate, variation in pressure drop and the presence of non-ideal components like springs, membranes etc. 3. Equal Percentage Valve follows a relationship between valve stem position and the flow rate described by a curve called the valve’s flow characteristic curve. It can be seen that equal increments of valve travel produce equal percentage changes in the existing flow. But, we can also observe some deviations along the path. Those deviations may be due to faulty apparatus, fluctuations in flow rate, variation in pressure drop or hysteresis property of controllers. 4. Linear Valve – the flow capacity increases linearly with valve travel. The deviation from the theoretical linear curve might be because of prolonged usage of non-linear systems in the control value i.e., hysteresis or fluctuations in flow rate, variation in pressure drop, etc. In each case parallax error is also involved. References: 1.
http://www.engineeringtoolbox.com/control-valves-flow-characteristics-d_485.html
2.
http://blog.opticontrols.com/archives/994
3.
http://en.wikipedia.org/wiki/Control_valves
