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Gas Flow

10 February 2009

Gas flow calculations are slightly more complex because gases are compressible fluids whose density changes with pressure. In addition, there are two conditions that must be considered—low-pressure drop flow and high-pressure drop flow.

Equation 2
Equation 2

Equation 2 applies when there is low-pressure drop flow—outlet pressure (p2) is greater than one half of inlet pressure (p1):

Equation 3
Equation 3

The low-pressure drop air flow graphs show low pressure drop air flow for a valve with a Cv of 1.0, given as a function of inlet pressure (p1) for a range of pressure drop (Δp) values.

Air flow chart
Air flow chart

When outlet pressure (p2) is less than half of inlet pressure (p1)—high-pressure drop—any further decrease in outlet pressure does not increase the flow because the gas has reached sonic velocity at the orifice, and it cannot break that “sound barrier.”

Air flow chart
Air flow chart

The equation (equation 3) for high-pressure drop flow is simpler because it depends only on inlet pressure and temperature, valve flow coefficient, and specific gravity of the gas:

The high-pressure drop air flow graphs (pages 10 and 11) show high pressure drop air flow as a function of inlet pressure for a range of flow coefficients.


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