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Pipeline Venting: Mach Number In Tailpipe


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#1 billybedeveril

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Posted 27 October 2016 - 01:58 AM

I'm currently carrying out a validation exercise for the manual venting of a 500 psi, 10km 24" NB natural gas pipeline via a manual blowdown station.  The valve station comprises a 6in manual valve and associated downstream R.O.  The take-off line and manual tail-pipe are vertically oriented directly off the top of the pipeline.

Initially it seems that the RO is somewhat oversized and flow is supersonic in the tailpipe downstream of the RO. The tailpipe is only a couple of metres long.

My concern is the sound pressure level in the vicinity of the tailpipe.  My initial feeling is that the high flow rate and high mach No. will lead to potentially damaging sound levels in the region of the blowdown station.  Has anyone got any experience on this?  Are there any "rules of thumb" for this sort of thing to determine the sound level?  Or should a silencer be specified as a rule in these conditions? 

Most of the literature considers either tailpipes which are connected to a flare header, where fluid momentum is limited to prevent piping damage.  In this case there aren't any bends so vibration should (??) not be an issue.

 

My question really is whether sonic flow can be tolerated in the (short) tailpipe, or is this something which will have to be validated by some accoustic simulation work to determine sound pressure levels and determine whether silencing is required.

 



#2 senthil13

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Posted 27 October 2016 - 07:52 AM

Are there any hydrate formation issues by venting natural gas from such high pressure to atmospheric pressure? just curious.

 

A silencer will be required since the sound level will be really high. My experience says, it will be very noisy without a silencer.



#3 Bobby Strain

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Posted 27 October 2016 - 09:05 AM

It is irresponsible to vent such large quantities of methane. You should use a temporary flare instead of venting. Methane is one of the worst greenhouse gases.

 

Bobby



#4 shan

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Posted 27 October 2016 - 10:34 AM

You may consider installing a supersonic vent tip at the end of the tailpipe.



#5 flarenuf

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Posted 28 October 2016 - 02:20 AM

I totally agree with Bobby , venting of quantities of methane is not acceptable.
Install a portable flare and burn it off.



#6 billybedeveril

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Posted 28 October 2016 - 05:20 AM

Thanks for the responses.

 

I'm aware that a portable flare is the best solution, and will most likely be the solution in the end.

 

My question though was really on whether anyone has experience for sound pressure levels around a cold vent with supersonic flow.



#7 Bobby Strain

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Posted 28 October 2016 - 09:09 AM

You can't get supersonic flow without a specially designed nozzle. So, when you calculate velocity as supersonic after the pipe discharge, the calculation is meaningless. You can achieve sonic velocity at the end of the pipe, but the velocity beyond the pipe is subsonic. All the kinetic energy in excess is dissipated as noise.






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