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# Gas Venting Query - Flow Rate?

## Gas Venting Query - Flow Rate?

(OP)
Hello,

I am new to this forum, i have created an account as i am hoping for some advice on a project i am working on. I am an Acoustics Engineer and am looking at noise impacts associated with Gas Venting. I am wondering whether anyone here could help me with either calculating or predicting some characteristics for the scenario detailed below.

I recently observed a venting operation which took place for approximately 19 minutes. the operation involved the release of natural gas through a circular nozzle of 25mm diameter, from a starting pressure of 60 bar. the temperature of the venting gas was approximately 10 degree celcius, while air temp was 7 degree celcuis. i am not sure of the volume of gas that was released but need to try to determine the flow rate at the start of the venting operation.

many thanks

### RE: Gas Venting Query - Flow Rate?

In this case your flow is "choked" (i.e., flowing at the speed of sound). Take a look at FAQ378-1201: Mass flow rate of a gas through an orifice during choked conditions for a discussion of choked flow. The velocity is Mach 1.0 and the speed of sound is a function of gas composition and upstream temperature (not pressure). The mass flow rate of the gas is a function of upstream density, the speed of sound, and the flow area. If you divide mass flow rate the theoretical density at "standard" conditions you get a volume flow rate at standard conditions which I expect is the number you need.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

### RE: Gas Venting Query - Flow Rate?

(OP)
Hello zdas04, the starting pressure was 60 bar, will such a high starting pressure, re. to atmospheric pressure, not mean that the jet exit velocity could exceed the speed of sound?

### RE: Gas Venting Query - Flow Rate?

Nope. Upstream pressure is irrelevant to velocity, it could be 6,000 bar and the exit would still be Mach 1.0. The reason that the velocity is limited (choked) is that there is no way for the information about the pressure drop to get back to the upstream sink faster than the speed of sound so the flow stream develops an unbounded shock wave.

You can design supersonic devices because as velocities exceed about Mach 0.6 the gas becomes compressible (relative to short lengths of pipe). The Bernoulli equation will tell you that as you increase pipe diameter, you must decrease velocity. Bernoulli has an assumption of constant density which is not valid above Mach 0.6, so you have to go to compressible flow arithmetic where increasing the pipe diameter increases the diameter of the standing wave and allows velocity to increase. The math to show this starts out really messy and gets worse quickly.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

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