Study of Hysteresis of Control Valve

Introduction

Control valves are crucial components in process control systems, regulating fluid flow to maintain desired process conditions. However, control valves can exhibit hysteresis, a phenomenon where the valve's response to a given input signal differs depending on the direction of the signal change. This study aims to investigate the hysteresis behavior of a control valve and its implications on process control.

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 has equal % characteristics (air to close type) and the other has linear characteristics (air to open type). Water from the 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 closed.

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 an ‘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 in control valves arises from various factors, including:

1. Friction: Mechanical friction between moving parts, such as the valve stem and seat.

2. Elastic deformation: Deformation of valve components, like the valve plug and seat.

3. Fluid forces: Asymmetric fluid forces acting on the valve plug.

Hysteresis can be categorized into two types:

1. Rate-dependent hysteresis: Hysteresis that depends on the rate of change of the input signal.

2. Rate-independent hysteresis: Hysteresis that is independent of the rate of change of the input signal.

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 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 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 (linear) valve.

A templet for 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: 

The experimental results show:

1. Hysteresis loops: The valve stem position versus input signal plots exhibit hysteresis loops, indicating rate-dependent hysteresis.
2. Frequency dependence: The hysteresis loop area increases with increasing input signal frequency.
3. Amplitude dependence: The hysteresis loop area decreases with increasing input signal amplitude.

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 ________.