Glossary · Process instrumentation and analytical measurement
Control-valve sizing
Also known as: Control valve sizing, Valve sizing
German: Stellventilauslegung
In process engineering, control-valve sizing is the calculation of the required flow coefficient (Kv or Cv) of a control valve for the specified operating conditions, as the basis for selecting valve size, trim and characteristic.
- Process instrumentation
- Standards
- Process industry
In one sentence
Control-valve sizing calculates the required flow coefficient (Kv or Cv) for the operating conditions to select the right valve size and trim.
Example
The sizing calculation for a steam control valve shows a required Kv of 25 at maximum load, so the team selects a valve with Kv 40 to keep it within a good control range.
How it applies
- Engineering: IEC 60534-2-1 gives equations for incompressible and compressible fluids, including effects such as choked flow and cavitation. Inputs are flow rates, pressures, temperatures and fluid properties for minimum, normal and maximum operating cases.
- Selection: An oversized valve works near the closed position, controls poorly and wears quickly; an undersized one cannot reach maximum flow. Noise prediction and material selection belong to the same work.
- Change: When process conditions change, the original sizing may no longer hold; management of change should include a check.
- Documentation: Keep the sizing calculation with the valve data sheet, including all operating cases and assumptions. It is the evidence behind the valve selection and is needed for later troubleshooting.