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:
- Flow Characteristics: The relationship between valve opening and flow rate, which can be linear, equal percentage, or quick opening.
- Pressure Drop: The decrease in pressure across the valve, which affects the valve's ability to control flow.
- Rangeability: The valve's ability to control flow over a wide range of operating conditions.
- Repeatability: The valve's ability to consistently return to the same position for a given signal.
- 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:
- Valve Selection: Understanding a valve's characteristics ensures that the correct valve is selected for a specific application.
- Control System Design: Knowing a valve's characteristics enables the design of control systems that account for the valve's behavior.
- Process Optimization: Understanding a valve's characteristics allows for optimization of process conditions, such as flow rate and pressure.
- 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/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
Operation procedure to determine inherent characteristics of the control valve:
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
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/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
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
