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1. Cv and Kv definition

2. Calculation of Cv and flow
through a valve - Case of 2 phases flow - metric units

Please refer to the valve flow coefficient page of Process
Engineer's tools.

When a valve **is operated in a bi-phasic flow** *at the
valve inlet*, the Cv required for a given mass flow rate can be
estimated thanks to the following formula [Masoneilan] :

With

m=Flow rate (kg/h)

F_{p} = piping
geometry factor (reducer correction), it is = 1 if the valve
size is equal to the pipe size

C_{v}=valve flow coefficient (GPM)

f_{f} = weight fraction of liquid in 2 phases flow (-)

f_{g} = weight fraction of gas in 2 phases flow (-)

ΔP_{f} = pressure drop of liquid phase (bar)

ΔP_{g} = pressure drop of gas phase (bar)

γ_{f} =mass density of the liquid phase at inlet conditions
(kg/m^{3})

γ_{g} =mass density of the gas phase at inlet conditions
(kg/m^{3})

Y = expansion factor = 1-x/(3*F_{k}*x_{T})

- x = pressure drop ratio = ΔP/P
_{1}(-) - P
_{1}= upstream pressure (bar abs) - F
_{k}= ratio of specific heat factors = k/1.40 - k = gas specific heat ratio
- x
_{T}= pressure drop ratio factor (this value is supplied by the constructor of the valve in product brochure). Note the value of xT can be taken as such only if the valve is used without reducers ; if used with reducers it should be corrected (not detailed in this page)

**How to calculate ****ΔP _{f} and **

ΔP_{f}=F_{L}^{2}*(P_{1}-F_{F}*P_{v})

ΔP_{g} = F_{k}*x_{T}*P_{1}

**With**

ΔP_{f} = pressure drop of liquid phase (bar)

F_{L} = critical flow factor (given by the valve
manufacturer)

P_{1} = upstream pressure (bar abs)

F_{F} = liquid critical pressure factor =
0.96-0.28*(Pv/Pc)0.5

P_{v} = vapor pressure of liquid a flowing temperature (bar
abs)

P_{c} = pressure at thermodynamic critical point (bar abs)

F_{k} = ratio of specific heat factors = k/1.40

k = gas specific heat ratio

x_{T} = pressure drop ratio factor