Liquid, Steam and Gas - Flow Factor Kv
By using flow factors the capacities of valves with different sizes, different types and from different manufacturers - can be compared.
The flow factor Kv is in general determined experimentally expressing
- flow capacity in metric units - m3/h - that a valve will pass for a pressure drop of 1 bar
Kv Liquids
Volumetric flow:
Kv = q (ρ / (1000 (pu - pd)))1/2 (1)
where
Kv = flow coefficient
q = volume flow (m3/h)
ρ = density (kg/m3)
pu = upstream pressure pressure (bar abs)
pd = downstream pressure pressure (bar abs)
Mass flow:
Kv = m / (1000 ρ (pu - pd))1/2 (1b)
where
m = mass flow (kg/h)
Example - Control Valve for Water and Kv Value
The Kv value for a valve with water flow 3000 kg/h and upstream pressure 10 bar and downstream pressure 7 bar can be calculated with (1b) as
Kv = (3000 kg/h) / (1000 (1000 kg/m3) ((10 bar) - (7 bar)))1/2
= 1.7
Kv Saturated Steam
pd > pu / 2:
Kv = 0.032 m (vd / (pu - pd))1/2 (2)
where
m = mass flow (kg/h)
pu = upstream pressure (bar abs)
pd = downstream pressure (bar abs)
vd = specific weight of downstream steam and actual temperature (m3/kg)
pd < pu / 2:
Kv = 0.032 m (2 vd2 / pu )1/2 (2b)
where
vd2 = specific volume for steam with pressure pu / 2 and actual temperature (m3/kg)
Kv Gases
pd > pu / 2:
Kv = 0.0019 q (ρg Tu / (pd (pu - pd)))1/2 (3)
where
q = volume flow of gas at 0oC and 1013 mbar (m3/h)
ρg = density of gas at 0oC and 1013 mbar (kg/m3)
Tu = upstream temperature (K)
pu = upstream pressure (bar abs)
pd = downstream pressure (bar abs)
pd < pu / 2:
Kv = 0.0039 q (ρg Tu)1/2 / pu (3b)
Related Topics
• Control Valves
Control Valve terminology, bodies, trim, flow characteristics, Cv and Kv sizing, noise, actuators and positioners
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