Study of Hysteresis of Control Valve | Study of Inherent Characteristics of Control Valve

Description of hysteresis study of equal % control valve and linear control valve using experimental setup:

The setup 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 the supply tank and re-circulated. Water passes from the supply tank and comes out of the control valve. The water flow rate is measured with the help of a rotameter and to know about the inlet pressure of water at the control valve a water column can be used. The outlet of the control valve is open to the 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 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 that controls the flow through an orifice by positioning appropriately a plug. The plug is attached at the end of a stem which is supported by 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 the ‘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 an ‘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 the case of control valves for the 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 the 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 the control valve to exhibit hysteresis. The presence of hysteresis is not desirable since it produces cycling and causes wear of the valve plug and seat.

Control valve diagram

Procedure for doing an experiment on control valves:

  • Start up the setup for air to close the control valve.
  • Rotate the regulator valve of the control valve to maintain the flow rate, of 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 steps 2 psig up to 15 PSIG and note the readings.
  • Gradually decrease the actuator pressure in steps 2 psig from 15 psig to 3 psig and note the readings.
  • Calculate the valve flow coefficient for actuator pressure for every reading.
  • Calculate hysteresis as the ratio of the maximum difference between flow coefficients at the same actuator pressure to that of the maximum flow coefficient.
  • Repeat the same experiment for air to open the (linear) valve.

Observation of the experiment:

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

A 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 behavior 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 ________.

Study of Inherent Characteristics of Control Valve

To study the inherent characteristic of the control valve, equal %, and linear experiment modes are conducted by a unique experimental setup:

With two control valves, one operates based on air-to-close mode and the other with air-to-open mode. when air is supplied to the diaphragm of the control valve it results in closing then it has equal percentage characteristics and when in the vice-verse it has linear characteristics. Pneumatic actuators are used to control the air supply to the control valves. In general tap water is circulated with a pump from the bottom of the receiving tank to the supply tank. By opening the manual gate valve water from the supply tank is passed to receiving tank through the rotameter and control valve. Inlet pressure at the control valve can be measured in terms of the water column. By the air regulator stem of the control, the valve is moved and adjusted for the required flow rate. Stem opening in terms of mm can be observed by the scale fitted near the stem.

Theory of inherent characteristic determination: 

The fluid flow rate in a pipeline is controlled with the help of automated control value in modern industries. Extremely powered actuators and pneumatic singles by means of pressurized air, hydraulic, etc allow the control room operator to open or partially open and close the valve. The amount of fluid passing through a valve at any time depends upon the opening between the plug and the seat. Hence there is a relationship between stem position, plug position, and the rate of flow. The relation between the flow through the valve and the valve stem position (or lift) is called the valve characteristic.

In general, the flow through a control valve for a specific fluid at a given temperature can be expressed as Q = f1(L,p0,p1)
Where Q is the volumetric flow rate, L is valve stem position or lift, and p0 and p1 are upstream and downstream pressures.

The Inherent flow characteristic of the control valve is the relationship developed between the flow of fluid and the valve movement at constant pressure drop across the valve ( fixed upstream and downstream pressures). Hence, the inherent characteristic is, Q = f2(L)
It can also be written as m = Q/Qmax = f(L/Lmax )
m = f(x)
Where Qmax is the maximum flow when the valve stem is at its max lift Lmax (valve is fully open),
m is fraction of maximum flow, Q/Qmax and x is the fraction of maximum lift, L/Lmax

Procedure to determine the inherent characteristics of the control valve:

Valve Sizing Calculator 

  • Open the manual plug valve of equal percentage (air-to-close) control valve.
  • Open the valve up to 14 mm travel (fully open).
  • Adjust the regulatory valve at the inlet of the control valve to maintain the flow at 400 LPH. Note down the pressure drop.
  • Slowly increase the air pressure by the air regulator and close the control valve to travel the stem by 2 mm.
  • The pressure drop across the valve will increase. Maintain the pressure drop by adjusting the regulatory valve. Observe the flow rates.
  • Take the observations at each 2 mm stem travel till the valve is fully closed by repeating the above step.
  • Plot the graph of flow % of maximum versus valve lift % of full lift.
  • Repeat the experiment for the linear valve (air to open).

Parameters observation:

Stem lift, mm                     Air to Close                                               Air to Open
                            Pressure in mm     H2O Flow in LPH        Pressure in mm       H2O Flow in LPH
14
12
10
8
6
4
2
0

Valve coefficient calculations:

G = Sp.g = 1 for water.
Q = m3/hr = LPH/1000.
Cv = 1.16 Q √(G/∆P)
∆P = ∆P in mm of H2O / (10.33 x 103)

Results in table form:

Stem lift, mm                   Air to Close                                   Air to Open
                         Flow in LPH        ∆P, mm H2O          Flow in LPH      ∆P, mm H2O

14
12
10
8
6
4
2
0
RESULT: The inherent characteristics of the air-to-open and air-to-close valves are verified

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