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Low and High Pressure Drop Gas Flow

11 February 2009

The basic orifice meter illustrates the difference between high- and low-pressure drop flow conditions.

In low-pressure drop flow—when outlet pressure (p2) is greater than half of inlet pressure (p1)—outlet pressure restricts flow through the orifice: as outlet pressure decreases, flow increases, and so does the velocity of the gas leaving the orifice.

When outlet pressure decreases to half of inlet pressure, the gas leaves the orifice at the velocity of sound. The gas cannot exceed the velocity of sound and—therefore—this becomes the maximum flow rate. The maximum flow rate is also known as choked flow or critical flow.

Any further decrease in outlet pressure does not increase flow, even if the outlet pressure is reduced to zero. Consequently, high-pressure drop flow only depends on inlet pressure and not outlet pressure.

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