Kv for Liquid Flow

The flow coefficient Kv (European standard) is defined as the flow of water in m³/h through a valve at 1 bar pressure drop:

• Kv — flow coefficient (m³/h at ΔP = 1 bar)
• Q — volumetric flow rate (m³/h)
• SG — specific gravity (water = 1.0; hydraulic oil ≈ 0.87)
• ΔP — pressure drop across valve (bar)

Kv for Gas Flow

For compressible gas flow (subcritical, P2 > 0.53 × P1):

• Q_n — flow rate at normal conditions (Nm³/h, 0°C, 1.013 bar)
• P1, P2 — upstream and downstream absolute pressures (bar)
• T — gas temperature (K)
• Z — compressibility factor (1.0 for ideal gas)

Kv ↔ Cv Conversion

Cv is the US/imperial equivalent: flow in US GPM of water at ΔP = 1 psi and 60°F.

Valve Sizing Guide

Valve Type	Typical Kv Range	Notes
Ball valve DN15	10 – 40	Full bore preferred for hydraulics
Ball valve DN25	50 – 200	Low pressure drop at full open
Ball valve DN50	250 – 800	Most common in process
Globe valve DN25	8 – 15	Good for throttling/control
Globe valve DN50	25 – 50	Higher ΔP than ball
Butterfly DN100	300 – 600	Compact, large flows
Needle valve	0.1 – 5	Fine flow adjustment

Practical Example