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Hydrogen Vent Sizing And Purging Rate

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#1 go-fish


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Posted 19 March 2021 - 03:44 PM


I have to come with a high level estimate of nitrogen demand for continuous purging of hydrogen vents which are open to atmosphere. I have a couple of questions:

1. I know the venting rate and have to calculate the line size of the vent pipes. Is there any typical velocity criteria for sizing the vent tail pipes d/s of the pressure controlled spill-off control valves? For PSV tail pipes joining into a flare header I have come across 0.5 Mach No. but is there any limiting velocity criteria for open vents in hydrogen service?

2. What should be the basis for nitrogen purging rate? Is there a sweeping velocity criteria?

#2 Pilesar


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Posted 19 March 2021 - 06:42 PM

The purpose of nitrogen purging is make sure oxygen does not enter the vent system. Five volume turnovers per minute is reasonable for this purpose. Vent velocities should be constrained so that the piping can be supported and vibration does not become a problem. Design vent velocities should be based on your vent rate, pipe size and wall thickness.

See if you can find a copy of NFPA 68 'Standard on Explosion Protection by Deflagration Venting' for reference.

Links to other possibly useful documents:




#3 go-fish


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Posted 22 March 2021 - 01:56 PM

Many thanks. I will see these documents.

#4 breizh


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Posted 22 March 2021 - 04:49 PM


A few documents to support .

Good luck.


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