Why is Reduced Trim required in Control Valves?

  • Control valves are sized according to the application requirements and must satisfy both Cv and velocity criteria.
  • Reduced trim is used where it is necessary for the valve to have a Cv capacity smaller than the maximum possible in that size of valve.
  • The most common reason for reduced trim is that the flow rate is low for the size of valve required – particularly where 25mm valves have been specified as the smallest size to be used. Some plants stipulate that no control valve should be less than two sizes smaller than the line size, other that the valve should not be less than half the line size.
  • The second reason is that on high pressure drop gas or vapour applications the valve invariably is sized on the outlet port velocity limits and the Cv required is much less than the full bore Cv.