6 replies to this topic

[processji]

I am currently working on modification to a Process oil and gas platform. We are adding launchers and receivers. Main drive behind this project is to stop flaring and divert gas to onshore. Currently, the gas is flared. The gas is 80 mol% methane 8% ethane and 5% propane.

We have a 24" flare header to the scrubber. A 20" line from scrubber to sub sea flare. Gas flow rate is 72 mmscfd.

Once flaring is stopped we need to provide a continuous purge.

I am looking for a way to estimate purge rate with the 80% methane stream. Purge gas is at 65 psia and 94 Deg F. I read in API-521 that there is an article by H. W. Husa, "HOW TO COMPUTE SAFE PURGE RATES", published in Hydrocarbon Processing and Petroleum Refiner -1964.
Any inputs regarding this will be useful.

i can be reached at goldsuresh@hotmail.com

regards
K Suresh

[gvdlans]

Following I copied from a discussion on eng-tips.com (post was by Flareman), see http://www.eng-tips....d.cfm?qid=15647

There is a paper by Husa which basically says that the oxygen profile which establishes in a vertical open pipe to atmosphere varies as an exponential relationship of the pipe diameter, purge rate and the distance down the pipe.

This formula can be expressed as

O2% = 21 * exp{- U * L /0.0036/Fb/(D^1.46)}
U = purge velociy - fps
L = distance from top - ft
Fb = Gas buoyancy factor
D = diameter - inches

If the purge is a single component, then
Fb = exp{0.065*(28.96 - MW)}
If the gas is multicomponent, Husa's formula becomes a little top heavy and I recommend using
Fb = 6.25*[ 1 - 0.75*((MW/28.96)^1.5)]
MW = Mol wt of purge gas (mixture)

In the same discussion thread, there was a quote from API RP 521:

"Alternatively, the continuous introduction of purge gas can be used to prevent flashback. Studies [13] have shown that a safe condition exists in situations that involve hydrocarbon-air mixtures if a positive flow of oxygen-free gas is maintained, allowing the oxygen concentration to be no greater than 6 percent at a point 25 feet (7.6 meters) from the flare tip."

So you can find required purge velocity U by entering O2% = 6 and L = 25 feet, as well as the applicable MW and D for your particular situation into the above mentioned formulas.

[processji]

GVDLANS

Thanks a lot for reply. Can i get soft copy of the article by HUSA if possible.
Thanks

K Suresh
goldsuresh@hotmail.com

[gvdlans]

I may have a hardcopy of Husa's paper somewhere, but this will take time to find... From what I remember it did not contain much more than the above mentioned formulas.

Please check out appendix C of following document: http://igs.nigc.ir/i...d/BP/RP44-3.DOC
The method as it is described in this appendix is similar to what I used in the past, apart from the Ki values for heavier-than-air gases.

[gvdlans]

After some digging I did find the 1977 Husa's paper, being a presentation to the fire/safety engineering subcommittee of the API, Fall 1977 meeting. It contains the same formulas as included in the BP standard (see the link in my previous post), but he used Imperial units instead of metric. He gave the following K values:

Gas/K
Hydrogen/+5.783
Helium/+5.078
Methane/+2.328
Nitrogen/+1.067 (no wind)
Nitrogen/+1.707 (wind)
Ethane/-1.067
Propane/-2.651
CO2/-2.651
Butane+/-6.586

So the K values for heavier-than-air gases are negative and different from the ones given in the BP standard. It's hard to say which K values are better, although it seems like BP has further developed the method. As indicated in the BP standard: "In view of the uncertainties involved in purging a flare, the calculated purge rates should be multiplied by factors ranging from 2 for light gases, to about 5 for gases similar in density to air in high wind conditions."

[jha]

How do they get the following values from the BP RP 44-3 equation
Ethane / +0.94
Propane / +0.38
Carb.diox / +0.38
Butan+ / +0.15

Shouldn't these be the same as for the HUSA formula except with oppsite signs?

-jha-

[Martin Murray]

Perhaps someone could confirm that using this formula to compare a minumum flowrate of a fuel gas as purge vs nitrogen purge results in a lower flowrate requirement for nitrogen than a fuel gas containing 50% Methane 30% Ethane 15% Propane and 5% Heavier than propane.