# Product Forum

More product forum articles

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

##### 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

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

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.