Vent header purge rate

[mawells]

I am looking at the purge rate of fuel gas in our vent header. I have used the Shell DEP to calculate the required min purge rate, which is a function of diameter of the vent tip and the Mr of the purge gas. The purge gas is injected at the far end of the vent header.

It appears the same method is used to calculate the min purge rate for both flared and non-flared systems. Without going outside the DEP, I am interested to know if there is justification for reducing the purge rate for a non-flared system.

Thanks,
Mark

 

[Flareman]

The most definitive approach to purging was developed by Husa (send me your address I'll email a paper expanding the ideas and the formula .. flareman_xs@netzero.net)
It basically gives the oxygen concentration down into a stack as an exponential function of diameter, depth and gas composition and is probably quite accurate.
O2% = 21 * exp{- U * L /0.0036 / F / D^1.46 }
U = velocity in fps
L = distance from top - ft
F = gas buoyancy factor (a formula in the paper I reference)
D = stack dia - inches

Your concern is primarily how far into the stack do you allow a flammable concentration to exist and still expect it to be safe. That depends on the gas. Husa used 6% O2 at 25 ft down but I think you can probably manage UEL at 10 diameters for a vent.

I checked my copy of DEP 80.45.10.10 but, unfortunately, the purge formula is illegible so I can't compare it with the Husa paper. The Shell web site says they use Husa so it's probably the same.

The devices known as "gas seals" have an application relative to purge reduction in just this type of situation. You could look into that (see also

http://www.geocities.com/flareman_xs/Systems/seals.html)

David