Static Electricity in Natural Gas Piping

[nhcf]

I have an application a pressure relief valve vents into a vessel which is vented to atmosphere.

During initial opening of the relief, there is the potential that the vessel will contain both oxygen (ambient air) and natural gas. Both the piping and the vessel are steel and bonded together and to ground.

Is it still possible for this NG flow to generate a static charge where there is risk of ignition? Does the conductive pipe/tank prevent accumulation of charge? Most online resources I am seeing are related to static discharge in liquid filling operations, and/or with PE pipe. Is this still a concern in gas flow/conductive pipe?

 

[Compositepro]

It does seem that you could very well get static sparks generated in the system you describe. Gas flow can create static. Your vessel should be kept purged of O2 using N2 or natural gas. All metal parts should be bonded and grounded.

 

[LittleInch]

I've never heard this mentioned for a gas vapour before.

The other issue to consider is that at atmospheric pressure, the gas will probably sweep out the potentially explosive mixture very quickly, but this might depend on the size of the tank and the volumes / flow involved.

However it seems a little far fetched to me, but I'll stand corrected if shown otherwise. I can't see how the vapour would be able to hold an electric charge unless you start to get a mist or liquid droplets.

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[hacksaw]

static charging is always an issue, in refinery or gas processing, specific design cautions are manditory

 

[nhcf]

thanks for replies
Is there a good reference document which provides guidance? NFPA 77 seems more geared towards liquid piping applications. I have not come across a document with guidance on gas (vapor) flow considerations/mitigation recommendations.

 

[zdas04]

You have to think about relative velocities. A stream flowing at sonic velocity will NOT mix with the air in the tailpipe--it will push it out of the way and you don't get any appreciable mixing until the process stream slows to something below 0.3 Mach, which happens a couple of meters away from the end of the pipe--physically the static that will build up in the pipe cannot connect a hot spark with an explosive mixture. We see static problems in piping, but never associated with a choked flow.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist