Selection criteria for control valve


#1

images

  • 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.
  • Range-ability.
  • 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.
  1. Linear:
  • Used in Level or flow loops.
  • Used in system where pressure drop across valve remains constant.
  1. Quick opening:
  • Used for ON/OFF services.
  • Used where instantly " large flow " is needed( safety or cooling system).