Sonic Flow Nozzle

Definition - What does Sonic Flow Nozzle mean?

A Sonic Flow Nozzle, also known as Critical Flow Nozzle or Critical Flow Venturi, is a tool which consists of a smooth rounded inlet section converging to a minimum throat area and then diverging along a pressure recovery section or exit cone. It is one of the important equipment used by the natural gas marketing industries. As natural gas accelerates through the nozzle, its velocity increases and its density decreases. The maximum velocity is achieved at the throat, the minimum area, where it reaches the desired speed of sound, i.e., 1 Mach.

Petropedia explains Sonic Flow Nozzle

The Sonic Flow Nozzle is operated by pressurizing the inlet of the venturi or evacuating the outlet in order to achieve an Inlet to Outlet pressure ratio of 1.4 to 1 or greater. This ratio maintains the Nozzle in a "choked" or "sonic" state. In this state, only the upstream pressure and temperature influence the flow rate of gas through the Nozzle. The flow rate through the Nozzle becomes nearly a linear function of the inlet pressure which increases the thereby doubling the flow rate of gas.

Some of the advantages of using sonic flow nozzle are:

  • Mass Flow Varies Linearly with Inlet Pressure
  • Minimal Upstream Piping Required
  • Long Term Accuracy
  • Excellent Repeatability
  • No Moving Parts
  • Differential Pressure Measurement Not Required
  • Flow Rate Is Not Affected by Downstream Flow Disturbances
  • Mass Flow is Constant with Varying Downstream Pressure
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