Question 1

What is the difference between Cv and Kv?

Accepted Answer

Cv (flow coefficient) uses US customary units: it is the flow rate in US gallons per minute (GPM) of water at SG=1 that passes through the valve at a pressure drop of 1 psi. Kv uses metric units: m³/h of water at ΔP = 1 bar. The conversion is Kv = 0.865 × Cv. North American and global instrumentation standards typically use Cv; European and international standards often use Kv.

Question 2

What is choked flow and when does it occur?

Accepted Answer

Choked flow occurs when the velocity at the vena contracta of the valve reaches the speed of sound (gas) or the vapor pressure limit (liquid), after which increasing ΔP no longer increases flow rate. For gases, choked flow typically occurs when ΔP > 0.5 × P1 (approximately). In this condition the standard flow equation no longer applies and a choked flow correction must be used. For liquids, the equivalent condition is cavitation or flashing.

Question 3

What safety margin should I apply to the required Cv?

Accepted Answer

The common practice is to select the next standard valve size whose rated Cv is at least 1.3× the calculated required Cv (30% margin). Oversizing beyond 2× the required Cv is undesirable because it leads to poor controllability and hunting (instability). For control valves, the design target is typically 60–80% of rated Cv at normal operating conditions.

Question 4

Why are the Cv equations different for liquids and gases?

Accepted Answer

Liquids are essentially incompressible, so the flow–pressure relationship follows the simple formula Cv = Q√(SG/ΔP). Gases are compressible: density changes with pressure and temperature, and an expansion factor Y (ranging from 0.67 to 1.0) accounts for gas expansion through the valve: Cv = Q/(963·Y·√(P1·ΔP·MW/T)). At high pressure ratios, Y approaches its limiting value of 2/3 at the choked flow condition.

Valve Sizing & Cv Flow Coefficient Calculator

Theory Notes (ISA/IEC 75.01)