Description of hysteresis study of equal % control valve and linear control valve using experimental setup:
The set up should contain two control valves with pneumatic actuators. One control valve is with equal % characteristics (air to close type) and the other is with linear characteristics (air to open type). Water from receiving tank is pumped to supply tank and re-circulated.Water passes from supply tank comes out of the control valve. The water flow-rate is measured with the help of rotameter and to know about the inlet pressure of water at control valve a water column can be used. The outlet of the control valve is open to atmosphere. Stem movement of the control valve can be changed with the air regulator which changes the outlet area of the control valve body. A scale is also provided to measure the stem travel (in mm) from fully open to fully close.
Theory of hysteresis of control valve:
A Control valve regulates the flow rate in a fluid delivery system. The control valve is a valve with a pneumatic, hydraulic, electric or other extremely powered actuators that automatically, fully or partially opens or closes the valve to a position dictated by signals transmitted from controlling instruments. Most commonly, pneumatic actuators are used for control valves. A pneumatic control valve is an air-operated valve which controls the flow through an orifice by positioning appropriately a plug. The plug is attached at the end of a stem which is supported on a flexible fabric reinforced elastomer diaphragm at the other end. If air pressure is applied on the upper side of the diaphragm, the stem moves down and consequently, the plug restricts the flow through the orifice. It is known as ‘air to close’ or ‘equal %’ valve. If air pressure is applied on the bottom side of the diaphragm, the stem moves up and consequently, the plug frees the flow through the orifice. It is known as ‘air to open’ or ‘linear’ 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. The friction in the packing and guiding surfaces of a control valve causes a control valve to exhibit hysteresis. Presence of hysteresis is not desirable since it produces cycling and causes wear of the valve plug and seat.
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| Control valve diagram |
Procedure for doing an experiment on control valves:
- Start up the set up for air to close control valve.
- Rotate the regulator valve of the control valve to maintain the flow rate, 400 Liter per hour.
- Set actuator air pressure to 3 psig.
- Note the flow rate and pressure at the inlet of the control valve.
- Gradually increase the actuator pressure in the steps 2 psig up to 15 psig and note the readings.
- Gradually decrease the actuator pressure in the steps 2 psig from 15 psig to 3 psig and note the readings.
- Calculate valve flow coefficient for actuator pressure for every reading.
- Calculate hysteresis as the ratio of the maximum difference between flow coefficients at same actuator pressure to that of maximum flow coefficient.
- Repeat the same experiment for air to open (linear) valve.
Note down the observations for equal % valve and linear valves separately in the following format.
Pressure(psig) Increasing Pressure Decreasing Pressure Pressure Drop Flow, LPH
3
5
7
9
11
13
15
Hysteresis calculation:
Valve Coefficient, Cv = 1.16 Q √(Sp.G/∆P)
Where, Q = Flow ( m3 per hour) = Q in LPH / 1000
∆P = Pressure drop across valve (bar) = ∆P in mm of H2O / (10.33 x 103)
Sp.G = Specific gravity = 1 for water.
Hysteresis % = (CV at decreasing Pressure – CV at increasing pressure) X 100
Maximum CV
Tabulating the results:
Pressure(psig) CV(increasing pressure) CV (decreasing pressure) Hysteresis %
3
5
7
9
11
13
15
Similar result table should be presented for air to open valve also.
To develop a graph: Plot the graph of actuator pressure versus flow coefficient.
Result: Hysteresis behaviour of both valves is observed and % hysteresis is calculated.
Average hysteresis, % for air to close valve is ________.
Average hysteresis, % for air to close valve is ________.