N2 purge and Reduced Husa method

[mawells]

I am trying to get to a min purge rate in our vent header to keep the O2 conc below the LEL. As I understand, Reduced Husa allows a flammable mixture to exist for some distance in the vent stack from the tip. I guess the assumption is that, if an ignition were to occur then the flame would only propagate for a short distance. I am used to flared rather than non-flared systems, however I don't know if it is good practice to use Reduced Husa on a flared system as the likelihood of a flame existing in the stack at low flow rates must be great?

I received some useful articles from Flareman on the subject (thanks Flareman). I am trying now to evaluate the effect of adding nitrogen to the purge gas stream. Part of the purge gas is fuel gas, and part is N2 (approx. 70% by vol. of required purge rate is made up of fuel gas). There are a couple of things which I would like to clear up and would appreciate feedback on:

1) It seems that N2 is better than fuel gas (Mr=18) at keeping air from entering the stack but I haven't seen a method for quantifying this. I have included the N2 as an equivalent quantity of fuel gas and assumed it to be conservative.

2) Small amounts of O2 are present in the N2 (it comes from a PSA unit). Can you use Reduced Husa directly in this case, or will the presence of O2 increase the purge rate? What is the latest thinking on this?

Thanks
Mark

 

[Flareman]

Mark
I responded to this at thread798-66873.


David