Instrumentation
Control Valve Sizing
Preliminary Cv/Kv sizing for liquid, gas, and steam service. Verify final selection against the manufacturer's sizing software.
Standard method — stated scope limits apply
Enter values and calculate.
This application is intended for engineering education, preliminary calculations, estimation, and reference purposes. Results should be verified against approved engineering standards, manufacturer data, plant design documents, calibrated instruments, and qualified engineering personnel before being used for operational, safety, or design decisions.
About This Calculation
A control valve’s flow coefficient (Cv, or Kv in metric units) describes how much flow it passes for a given pressure drop — it’s the valve equivalent of a pipe’s diameter, but empirically measured rather than geometric, since actual flow path shape varies by valve type and trim. Sizing a valve means finding the Cv the process needs at its worst-case (usually minimum-pressure-drop, maximum-flow) condition, then picking a valve whose Cv range comfortably covers it — oversized and the valve operates too close to shut where control is poor and unstable (“valve hunting”); undersized and it can’t pass the flow the process needs at all.
Frequently Asked Questions
What is the difference between Cv and Kv?
They measure the same thing in different unit systems. Cv is US units (gpm of water at 60°F per psi½ differential); Kv is metric (m³/h of water per bar½ differential). Kv ≈ 0.865 × Cv.
What is valve authority and why does it matter?
Valve authority is the fraction of total system pressure drop that occurs across the valve itself at full-open. Low authority (valve is a small part of total drop) gives poor, non-linear control near the closed position — a valve sized purely for Cv at one operating point can still control badly if authority is too low.
What is choked (critical) flow in a control valve?
Past a certain pressure ratio, flow through the valve stops increasing even if downstream pressure keeps dropping — the fluid reaches sonic velocity at the vena contracta. Sizing calculations for gas/steam service must check for choked flow, since the simple square-root Cv equation over-predicts flow once choking occurs.
Should I size for maximum flow or normal flow?
Neither alone — the valve needs enough Cv to pass maximum required flow, but sized so NORMAL operating flow doesn’t leave the valve running too close to fully open (poor control margin) or too close to closed (poor rangeability). A common target is normal flow around 60–80% of valve travel.