Calculating the sonic velocity is necessary for different design activities. It is also handy to be able to determine under which pressure drop a gas flow will reach sonic conditions.
What is the sonic velocity ?
The sonic velocity is the maximum velocity that can be reached by a gas in a pipe, or exiting from the system through an orifice or a nozzle for example. Knowing the sonic velocity is very important as the velocity of the gas will be "capped" by the sonic velocity, it cannot go quicker, which will induce some limitations for pressure safety valves for example.
The sonic velocity of a gas can be calculated thanks to the following equation :
Critical pressure ratio = P2/P1 = the ratio
of pressure upstream and downstream an orifice or nozzle
K = Cp/Cv around 1.4 for most diatomic gases
If the critical pressure ratio is exceeded, then the gas sonic velocity will be reached, and the flow will be sonic.