The major components of the industrial control system would be actuators, control valves, Sensors, etc. We can describe the control valve as the final control element of a control system. The major process variables in an industrial process are temperature, pressure, level, and flow. So the process variable should be kept in a specified range for the proper operation.
The control valve would open or change the direction of flow based on the signal which is received from the controllers.
The controllers in a control system would compare the process variable to the set point. If the process variable is not similar to the set point, then the controller would send a signal to make the process variable similar to the set point. The sensors in the control system would check the process variable and give the details to the controller.
What are the major parts of a control valve?
The control valve is composed of two major components they are valve body and diaphragm actuator. The valve body consists of the bonnet, valve plug, and seat ring, and it also has trim parts. Bonnet would be assembled to the valve body with bolts, nuts, etc, to provide pressure. The valve plug in the valve body would be moved up and down by the actuator through the valve stem.
Diaphragm & Stem
This Is a flexible element that is responsive to pressure and functions to transmit the diaphragm plate force and stem.
concentric plate with a diaphragm to transmit the force to the actuator stem.
This is a spring or a combination of several springs that are in the yoke that functions to move the actuator stem in the opposite direction to the diaphragm pressure.
This is the part that connects the actuator to the valve stem and transmits motion from the actuator to the valve.
This is the part that functions to regulate the compression of the spring, usually in the form of a thread on the actuator or yoke stem.
This is a rigid, immovable part that serves as a support and connection to the valve actuator unit.
How does a control valve work?
The operation of the control valve is done by the actuator and the positioner. The actuator is the part that is utilized to open or close a valve. The positioner in the control valve would determine the exact position of the valve and it would help the valve to reach its proper position by applying proper pressure to the actuator diaphragm. The controller would send the control signal and based on this signal the valve position would be varied.
What are the major functions of a control valve?
- It would be able to start and stop the flow
- The speed of the flow can be controlled
- Flow direction can be controlled
- Pressure regulation can be done by controlling the flow
- It would be able to reduce the pressure in a pipeline
What are the major requirements of a control valve?
- It should be able to keep the process fluid inside it without any leakage
- Should be able to handle the proper capacity of the fluid which is required for the process
- Should be able to handle the process fluid without any erosion or corrosion
- It must be able to handle the temperature of the process fluid
- Must be able to connect with the pipelines
How should we select a control valve?
- Make sure that the selected valve is capable to operate properly in the service
- Select the valve according to the required type of trim
- Noise and cavitation levels must be considered
- The size of the trim and the valve body must be checked
- Determine if this valve can be used in hazardous areas
- You should consider the type of actuator
- Check if the valve is fire safe
- Stroke time must be considered
What are all the tests that must be conducted in a control valve?
- Valve body testing
- The closing and opening times of the control valve must be checked
- Positioner testing must be done
- Seat leakage must be checked
- Accuracy and repeatability of the positioner
- Conduct a partial stroke test in the control valve
Installation of the control valve
Prior to installation, make sure that scales, welding chips, or some other unfamiliar materials have been cleared out of the cycle line to which the control valve is to be connected. While installing the control valve guarantee the interaction of liquid stream heading coordinates with the stream course demonstrated on the associating spines of the valve body. Install the control valve, in a position such a place that its actuator is situated vertically. Make sure that there are no edges of the gaskets associated with the line extruder into the cycle pipe. Ensure the chosen gasket material is appropriate for the interaction liquid.
Mostly the control valve is the final process control in a system. So if this device is not installed properly then it won’t be able to do the proper control of the process. While installing a control valve we should make sure that it has proper space for the safe operation and maintenance of the valve. Control valves should not be installed in a place where there is vibration and external stress because it can badly affect the operation of the control valve.
It would be good if there is support for the valve so that the piping won’t be affected by the weight of the valve. If a control valve is installed to handle flammable fluids then it must not be installed near hot pumps or with any other flammable equipment. It should be installed in an environment in which it can handle the temperature and pressure so that the control valve would operate properly.
How should we inspect a control valve?
The control valve should be inspected periodically because it is the final control element in a system. So if it’s not operating properly then it could affect the whole process.
- Make sure there is no leak from the diaphragm
- Make sure that there is no leakage of process fluid from the bonnet and flanges
- Make sure that the yoke clamp bolt is tightened
- Make sure that there is no air leak from the air piping and also in the actuator
- Make sure that the valve lift is proper
- Make sure that there is no sound or vibration from the control valve during its operation
- Make sure that there is no damage to the valve stem and the actuator stem
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