Selection criteria for control valve
- Type of fluid to be controlled.
- Temperature, viscosity, specific gravity of fluid.
- Flow capacity (maximum and minimum).
- Inlet and outlet pressure( minimum and maximum).
- Pressure drop at normal and shutoff condition.
- Degree of flashing, if possible.
- Desired action against failure(Fail to open, fail to close).
- End connections and body material ( ASTM A216 grade WCC, ASTM A217 grade WC9, ASTM A351 CF8M etc.)
- Valve size
- Valve body ( angle, double-port, butterfly etc)
- Valve plug guiding(cage, port guiding etc)
- Port size(full or partial restriction)
- Valve trim material.
- Actuator size required.
- Bonnet style( plain, extension bonnet required).
- Stem travel versus flow characteristics.
- Ability to cope with high pressure.
- Valve sizing ( valve must operate between 40% to 70% of full operating range).
Rule of thumb:
1. Equal percentage:
- Used where large changes in process drop is expected.
- Used where small % of total pressure drop is permitted.
- Used in temperature & pressure control loops.
- Used in Level or flow loops.
- Used in system where pressure drop across valve remains constant.
3. Quick opening:
- Used for ON/OFF services.
- Used where instantly " large flow " is needed( safety or cooling system).
Read about Control Valves : Check Here