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The flow coefficient can be calculated as: C v = C v_ saturated (0.95) 1/2 = 0.97 C v_saturated. Flow Coefficient - C v - Air and other Gases. Note! - there is a difference between critical and non critical pressure drops. Critical pressure drop is approximately 47% of inlet p i absolute pressure.
The adjusted Cv (or Kv) value should be based on the Cv or Kv formula for sub critical gas pressure drop. A simplified version of the Cv formula for sub critical gas pressure drop is shown below: Where: Cv = flow coefficient. SCFH = flow rate in ft 3 /hr (NTP) D n = the gas density in lbs/ft 3 at 0.00 psig. o F = gas temperature
The coefficient of flow (Cv) is a formula which is used to determine a valve’s flows under various conditions and to select the correct valve for a flow application. The Cv was designed for use with liquid flows, it expresses the flow in gallons per minute of 60o F water with a pressure drop across the valve of 1 psi.
The equations for compressible fluids are for use with gas or vapor and are not intended for use with multiphase streams such as gas-liquid, vapor-liquid or gas-solid mixtures. For compressible fluid applications, this part of ISA-75.01.01-2007 is valid for all valves.
Based on M = 0.96, the critical pressure coefficient was found to be −0.0697. A pressure coefficient below this critical pressure coefficient constitutes the supercritical region or the region of supersonic flow on the surface of the wing.
notion is that of “choked flow”, also referred to as “critical flow”. In gas flow through an orifice there is an occa-sion where the gas velocity reaches sonic conditions. This occurs for air flow when the absolute pressure ratio is .528, i.e. when the downstream absolute pressure (P2) is 52.8% of the upstream absolute pressure (P1).
Sizing of control valves and pressure regulators can be performed through calculation of flow coeficient, analysis of flow characteristic curves, nomographs or other empirical methods. The IEC 60534-2-1 is typically used for such tasks, most commonly for control valve sizing.
