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 ________.

Study of Inherent Characteristic of Control Valve

Study of Inherent Characteristics of Control Valves: Recent Advancements and Applications in Process Control

Control valves are critical components in process control systems, playing a vital role in regulating fluid flow, pressure, and temperature in various industries, including chemical processing, crude oil, natural gas, and water pipe systems. Understanding the inherent characteristics of control valves is essential for optimal process control, efficiency, and safety.

Inherent Characteristics of Control Valves

Control valves exhibit unique characteristics that affect their performance, including:

1. Flow Characteristics: The relationship between valve opening and flow rate, which can be linear, equal percentage, or quick opening.
2. Pressure Drop: The decrease in pressure across the valve, which affects the valve's ability to control flow.
3. Rangeability: The valve's ability to control flow over a wide range of operating conditions.
4. Repeatability: The valve's ability to consistently return to the same position for a given signal.
5. Hysteresis: The difference in valve position for a given signal, depending on the direction of travel.

Recent Advancements in Studying Control Valve Characteristics

Recent research has focused on developing advanced methods for characterizing control valve performance, including:

• Computational Fluid Dynamics (CFD): Numerical simulations to predict valve flow behavior and pressure drop.
• Artificial Neural Networks (ANNs): Machine learning algorithms to model valve behavior and predict performance.
• Internet of Things (IoT): Integration of sensors and actuators to monitor and control valve performance in real-time.

Applications in Process Control

Understanding control valve characteristics is crucial for developing effective process control strategies in various industries, including:

• Chemical Processing: Control valves regulate fluid flow, temperature, and pressure to optimize chemical reactions and product quality.
• Crude Oil and Natural Gas: Control valves manage fluid flow, pressure, and temperature to ensure efficient transportation and processing.
• Water Pipe Systems: Control valves regulate water flow, pressure, and quality to ensure safe and efficient distribution.

Incorporation of Control Valve Characteristics in Process Control

To develop effective process control strategies, control valve characteristics must be incorporated into the control system design. This can be achieved through:

• Valve Selection: Choosing valves with the appropriate characteristics for the specific application.
• Control System Design: Designing control systems that account for valve characteristics, such as flow characteristics and pressure drop.
• Tuning and Optimization: Tuning and optimizing control systems to account for valve characteristics and ensure optimal performance.

Understanding the inherent characteristics of control valves is essential for developing effective process control strategies in various industries. Recent advancements in studying control valve characteristics, such as CFD, ANNs, and IoT, have improved our ability to predict and optimize valve performance. By incorporating control valve characteristics into process control system design, tuning, and optimization, industries can ensure optimal performance, efficiency, and safety.

Experimentation Procedure to Determine Inherent Characteristics of Control Valves

Determining the inherent characteristics of control valves is crucial for understanding their behavior and performance in various process control applications. The experimentation procedure outlined below provides a comprehensive approach to determining the inherent characteristics of control valves.

Pre-Experimentation Preparation

• Valve Selection: Select a control valve that is representative of the type and size used in the specific application.
• Test Rig Setup: Set up a test rig that includes a fluid source, piping, fittings, and instrumentation to measure flow rate, pressure, and temperature.
• Instrumentation Calibration: Calibrate all instrumentation to ensure accuracy and reliability.

Experimentation Procedure

1. Flow Characterization Test:

    - Set the valve to a fully open position.

    - Measure the flow rate through the valve at various pressure drops.

    - Plot the flow rate versus pressure drop to determine the valve's flow characteristic (e.g., linear, equal percentage, or quick opening).

2. Pressure Drop Test:

    - Set the valve to a fully open position.

    - Measure the pressure drop across the valve at various flow rates.

    - Plot the pressure drop versus flow rate to determine the valve's pressure drop characteristic.

3. Rangeability Test:

    - Set the valve to various positions (e.g., 10%, 20%, ..., 100% open).

    - Measure the flow rate through the valve at each position.

    - Plot the flow rate versus valve position to determine the valve's rangeability.

4. Repeatability Test:

    - Set the valve to a specific position (e.g., 50% open).

    - Measure the flow rate through the valve at this position.

    - Repeat the measurement multiple times to determine the valve's repeatability.

5. Hysteresis Test:

    - Set the valve to a specific position (e.g., 50% open).

    - Measure the flow rate through the valve at this position.

    - Gradually increase and then decrease the valve position while measuring the flow rate.

    - Plot the flow rate versus valve position to determine the valve's hysteresis.

Importance of Experimentation

Determining the inherent characteristics of control valves is essential for:

1. Valve Selection: Understanding a valve's characteristics ensures that the correct valve is selected for a specific application.
2. Control System Design: Knowing a valve's characteristics enables the design of control systems that account for the valve's behavior.
3. Process Optimization: Understanding a valve's characteristics allows for optimization of process conditions, such as flow rate and pressure.
4. Troubleshooting: Knowing a valve's characteristics facilitates troubleshooting and maintenance, reducing downtime and improving overall process efficiency.

To study the inherent characteristics 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 in 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 the receiving tank through the rotameter and control valve. Inlet pressure at the control valve can be measured in terms of the water column. By air regulator stem of the control 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 control valve is the relation 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/Q_max = f(L/L_max )
m = f(x)
Where Q_max is the maximum flow when the valve stem is at its max lift L_max (valve is fully open),
m is fraction of maximum flow, Q/Q_max and x is the fraction of maximum lift, L/L_max

Operation procedure to determine inherent characteristics of the control valve:

• 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).

A model table for 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

The formula for 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)

Model table to represent the 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