Gas and steam are compressible. Their density changes through the valve, and critical-flow behavior can limit capacity. The basic water-like liquid equation is therefore not a valid sizing method for these services.
Define the flow basis
A gas flow stated in normal or standard cubic metres per hour is referenced to a defined pressure and temperature. It is different from actual volume at the valve inlet. State the reference basis, composition, molecular weight and compressibility information. For steam, provide pressure, temperature and whether the inlet is superheated, saturated or wet.
Pressure ratio matters
In compressible flow, the downstream-to-upstream pressure relationship influences expansion and the onset of choked conditions. Reducing downstream pressure further does not always increase mass flow proportionally. The selected valve’s relevant factors and attached piping need to be included in an appropriate sizing method.
Check more than capacity
Noise, outlet velocity, vibration, material temperature and actuator loading may control the selection. A valve that meets a calculated coefficient can still be unsuitable for the energy level. Ask for a documented gas or steam sizing calculation and an explicit statement of the supplied configuration’s limits.
Practical checklist
- State mass flow or the exact volumetric reference conditions.
- Use absolute pressures for compressible-flow calculations.
- Provide composition and inlet thermodynamic state.
- Include required noise limits and downstream piping.
Discuss your application
Send the process conditions and required valve function for a configuration review. Request a technical quotation ↗