Need to purge discharge hydrogen relief valve to prevent detonation?

[rojans]

I am a bit puzzled. Someone told me that a discharge pipe of relief valve for pure hydrogen should be continiously purged with an inert in order to prevent a flammable mixture during venting. Without sufficient purging, air will be present in the discharge pipe. A flammable mixture could then be formed by leaking of the relief valve or during a discharge at the hydrogen/air interface.
Auto ignition of hydrogen (pressures 10-40 bar) is quite realistic. When ignition occurs the deflagration can transition into a detonation as the discharge lines are quite long. For a deflagration the situation is safe as the relief takes place at a safe location; however a detonation is something to prevent.
I have checked NFPA 68/69 but not clear to me what to do.
A few situations exist: In general the discharge pipe itself is quite long: 10-40 meters. The diameters vary from 15mm to 100mm. In all cases L/d (length / diameter ratio) is > 100 and higher.

Please give any guidance.

 

[RaRo]

You are relieving pure hydrogen to the atmosphere, is that it?
Autoignition is indeed plausible for high pressure hydrogen relieving to an oxygen rich open space (e.g. atmosphere).
Most of the work I know of has been made for automotive technology - simulation works done shows it is possible starting at 37barg and above.
Many mechanism have been proposed and studied. What I prefer to recall is that the shockwave heats up the hydrogen and air above the autoignition temperature of a low (<20%) hydrogen-air mixture present at the interface, going boom.

Sweeping the line with nitrogen seems a good idea.
If you are ready to accept the current uncertainty about having flame arrester downstream relief devices, you could use one if you can find one that works.


Reference:
HYDROGEN SAFETY RESEARCH: STATE-OF-THE-ART

http://www.see.ed.ac.uk/feh5/pdfs/FEH_pdf_pp028.pdf

HYDROGEN SELF-IGNITION IN PRESSURE RELIEF DEVICES
conference.ing.unipi.it/ichs/images/stories/papers/224.pdf
Physics of spontaneous ignition of high-pressure hydrogen release
conference.ing.unipi.it/ichs/images/.../ID265-Bragin_Molkov.pdf

 

[RaRo]

also Spontaneous Ignition of Pressurized Releases of Hydrogen and Natural Gas into Air

http://www.princeton.edu

 

[rojans]

yes, releassing hydrogen to the atmosphere. My concern is whether a detonation is possible when the discharge line are not sweeped with an inert

 

[RaRo]

The answer would be yes.