Valve Design and the Physical Properties of Fluids

Please refer to the following sites:

Fluid Valve Notes

Piping Design Central: Manual Valves

Valve Selection Handbook - Engineering Fundamentals for Selecting

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Flow Factors
It is clear that different sized valves have different flow rate capacities and it is very important to be able to assess the flow through a valve for a fluid at certain conditions. The flow characteristics for an on-off valve are fixed and can be evaluated directly using the relevant flow factor. The flow through flow control valves, relief valves, pressure reducing valves and check valves depend on the operating condition for the valve and require a more detailed evaluation..


The most general method of identifying the flow capacity of a valve is the C_v factor..
The C_v factor based on american imperial units and is defined as follow

C_v = The flow of water through a valve at 60 ^oF in US gallon/minute at a pressure drop of 1 lb/in²

The metric flow factor (K_v) is used throughout outside of america and is defined as follows

K_v = the flow of water through a valve at 20 ^oC in m³/hr with a pressure drop of (1 bar)

The conversion between the two factors is K_v = 0.865 C_v

Also k_v is defined as follows...

k_v = the flow of water through a valve at 20 ^oC in litres/min with a pressure drop of (1 bar)

The conversion between k_v and C_v is ...k_v = 14.42 C _v


Liquid Flow
To establish the flow ( Q ) in litres /m at a differential pressure ( Dp )in bar for a liquid with a specific gravity relative to water ( γ_w ).

Note: This relationship only applies for liquids similar to water at reasonable flows (sub-sonic).

Gas Flow
For gases and supersonic flows more complicated formulae are required....

The formulae below must be used only for rule of thumb estimates. The assumption that the critical Dp is at P₁ /2 does not hold for all valves. For accurate flow calculations the valve manufacturers data sheets must be used


For gases flowing at sub-sonic velocities the following relationship holds..

T₁ = Inlet gas temperature deg K = Deg C + 273
γ_a =Gas specific gravity relative to air
q _n = gas flow at normal conditions ...
P₁ = Inlet Pressure (absolute)

For gases flowing at super-sonic velocities the following relationship holds..