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Liquid, Steam and Gas - Flow Factor Kv

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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)


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)


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)


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)


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)


q = volume flow of gas at 0oC and 1013 mbar (m3/h)

ρ= 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)

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