I am working on a project which is down a concrete oil platform leg.
I want to vent off CO2 through a needle valve. The gas pressure is 8 bar and the valve is 9mm needle valve discharging to a 1 inch vent tube.
How much noise will be created?
The valve has a Cv of 2.9, and discharges to atmospheric pressure in the 1" tube.I also assumed the temp of the CO2 to be 80F.  Plugging this into the Flowserve sizing program, and using the IEC noise routine, I get a prediction of 101 dB 1 meter down and one meter away from the valve. 
Insulation is good for several dB, and sticking sch80 pipe downstream of the valve also is good for several dB. 
An insulated sch 80 pipe calculates to 87 dB. 
It will be noisy where it discharges, too, so vent the pipe out a wall where people will not be.

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