The control valve selection for the process plant requires a particular combination of the valve body, and trim material.
The capacity requirements and system operating parameter ranges must be considered in selecting a control valve to ensure the satisfactory operation of a particular process.
The valve flow characteristic is the most important thing to be considered in selecting a control valve.
Improper size and type of the valve may cause some serious operational issues within the valve or in the system.
The control valves are the essential devices to control or regulate the fluid flow varying the size of the flow passage as directed by the controller.
The control valves are found in all industrial process control applications including power plants, food processing industry, chemical industry, and so on.
Control valves have a capacity to handle all kinds of fluids at temperatures up to 1000°F (538°C).
In selecting the appropriate control valve for the required application the manufacturers and their marketing representatives advise their clients on the right control valve by providing the following information.
The selection information of the right control valve for the required application is shown in the below table:
|CONTROL VALVE SELECTION INFORMATION
|Type of fluid to be controlled
|Water, Natural Gas, or Steam,
|The temperature of fluid
|In Degree Fahrenheit or Celsius
|Viscosity, Specific gravity of fluid
|Specify the viscosity or specific gravity of the fluid.
|Flow capacity of the control valve
|Maximum and Minimum flow rates.
|Valve inlet pressure
|Maximum and minimum inlet pressures are handled by the control valve.
|Valve outlet pressure
|Maximum and minimum outlet pressures are handled by the control valve
|Max permissible noise of 55 dB A
|Inlet and outlet pipeline size and schedule.
|Specify the line size to which the control valve is installed.
|Body material of the control valve
|ASTM A216, WCC, ASTM A217, WC9, ASTM A351 CF8M.
|End connections and valve ratings
|Is it going to be screwed? Class 600 RF flanged, Class 1500 RTJ flanges and so on..
|Valve action is desired when instrument air fails.
|Is the valve to fail open, fail to close, or retain in its previous state.
|Instrument Air Availability
|State whether the valve requires instrument air.
|Valve type and number
|Only when required by manufacturer
|Valve body construction
|Is the valve body going to be the angle? Double-port? Butterfly? etc.
|Valve port size
|Specify whether the valve port size is full or restricted.
|State whether the flow will tend to open valve or close valve
|Specify the size of the actuator required for the control valve.
|Weather the bonnet style is plain, with extension, bellows seal, etc.
|Valve packing materials
|Specify which type of valve packing material is used, Poly Tetra Fluoro Ethylene V-ring, laminated graphite, environmental sealing systems, and so on.
|State the control valve accessories required- positioner, hand wheel, and so on.
What are the factors to be considered during the selection of a control valve?
The major factors to be considered during the selection of a control valve are
1. Fluid type:
The process industry includes a variety of fluids such as water, steam, gas, some liquid food products, and so on… but for every fluid type, the same control valve doesn’t function properly due to different properties of fluids. Depending upon the fluid type the control valve must be selected.
2. Fluid parameter:
The selection of the control valve and the valve sizing is the most important step to be considered for the particular application.
In addition to this, the fluid parameters like temperature, pressure, and flow rates of the particular fluids must be considered in selecting the valve. The metal valves of higher protection can withstand higher temperature and pressure values.
The term functionality defines the actual purpose of the valve.
The 2-way valve is selected only for ON/OFF control.
The 3-way valve is selected for ON/OFF control and diverting the fluid path.
4. Actuation systems:
The actuation system defines the operation of the control valve.
In some cases, the valve needs to operate manually by hand.
And the valve will need to be actuated to TURN ON or TURN OFF by a pneumatic or hydraulic supply.
5. Valve size:
The valve size is the main thing to be considered, if the control valve is of larger size for the required application then the valve travel must be of a very small percentage. A small change in valve position results in a large effect on flow which in turn makes the valve hunting.
What is the flow characteristic of the control valves?
The flow characteristics of the control valve define the inherent relationship between the fluid flow and valve opening. This allows a certain amount of flow through the valve at a particular opening.
The following are the most common flow characteristics of the control valve:
- Linear: Here the flow characteristic exhibits a linear relationship between fluid flow rate and percentage of valve position.
- Equal percentage: Here the flow capacity varies exponentially with valve trim travel. Here the equal increments or decrements of the valve travel will produce equal percentage changes in the existing valve coefficient.
- Quick opening: Here the flow characteristic for a quick opening valve exhibits a large change in the flow for a relatively small initial change in valve stem travel, in this valve type the maximum flow rate is achieved at a relatively low percentage of the valve stem travel.
What size of the valve must be selected?
According to the thumb rule,
The low profile rims with a diameter of around 25mm require a valve stem of 40mm.
The wheelsets with a diameter of around 45mm require a valve stem of 60mm.
While anything taller or larger than 50 mm demands a valve stem of 80mm.
What are the various types of control valves?
The different types of flow control valves are Butterfly valve, Ball valve, Gate valve, Globe valve, Pinch valve, and Diaphragm valve.
- Butterfly valve: In a butterfly valve, the flow is regulated through a disc-type element placed in the center of the valve by a rod. The disc is rotated to 90 degrees to open or closing of the valve. This valve has a lesser pressure drop.
- Ball valve: Here the fluid flow is controlled using a hollow perforated ball called a floating ball. The ball valves are considered high recovery valves with a low-pressure drop.
- Gate valve: In the gate valve, the flow is controlled by raising or lowering the valve stem.
- Globe valve: Flow is controlled by the lift of the valve plug. The needle-type globe valves are particularly well suited for accurate control of fluid flow rate. The globe valve has a higher pressure drop because the passageway is S-shaped.
- Pinch Valve: This valve is a linear motion type and is used to start, regulate, and stop fluid flow. Its design is simple.
- Diaphragm valve: Diaphragm valves use a ‘pinching’ method to regulate the fluid flow using a flexible diaphragm.
How the control valves are classified based on the number of plugs?
Based on the number of plugs there are two types of control valves
1. Single-seated valve:
The valve has a single plug and a single seat within a single valve body. Large amount of force is required to drive the stem.
2. Double-seated valve:
The valve has two plugs and two seats within a single valve body. It requires a small amount of force to drive the stem.
How to select a Control Valve for a particular application?
Though the control valves are available in a variety of types
The control valve selection depends on their ability for a particular application
1. It must be able to control the fluid’s flow rate.
2. It should have a lack of turbulence to flow when the valve is fully opened as turbulence reduces head pressure.
3. It must have a quick opening and closing mechanism
4. Tight shut-off in the control valve prevents leakage against high pressure.
5. It should have the ability to flow in one direction
6. The valve must have the ability to handle abrasive fluids.
How the control valves are classified based on Action?
Control valves that are operated through pneumatic actuators can be either
- Air to Open: Air to open valves are normally closed by the spring action and require air pressure (a control signal) to open them.
- Air to Close: Air to close valves are valves that are open by the valve spring action and require air pressure to close the valve.