The control valve must be selected large enough to pass the desired steam flow rate with the desired pressure drop. The capacity of the valve is expressed by the coefficient Kv (m³/h, Europe) or Cv (US gpm). This tool finds the required Kv/Cv value or calculates the steam flow rate passing through a known valve.
Formula (ISA/IEC 60534, steam)
Critical flow (x ≥ Fγ · xT): ṁ = (2/3) · 2,73 · CV · √(Fγ · xT · p₁ · ρ₁)
p: kPa absolute, x = Δp/p₁, Fγ = κ/1.4 (κ: true isentropic exponent of steam, IAPWS-IF97), xT: valve pressure ratio factor (0.72 typical in globe valve), Cv = 1.156 · Kv.
Example
The value required for a valve that will pass 1,500 kg/h of steam from 10 bar(g) to 8 bar(g) is Kv = 15.4 (Cv = 17.8). With a 30% margin in valve selection, the valve with at least 20 Kvs ≈ is selected.
The valve size may be smaller than the pipe diameter; the speed and noise after the valve should be checked separately. To evaluate your steam system as a whole, you can take a look at our steam boiler guide and get a free quote for your project.