5 replies to this topic

[Jimmayy]

I am working on calculating the minimum purge rate for a 3inch flare stack using the H.W. Husa method. I have the API Standard 521 Section 7.3.3.3.3 that I am working with.


I have attached a spreadsheet to show you what Im doing and the results that i am getting. The numbers in the end do not make sense and I am out of toubleshooting ideas.

Please Help,

Thanks.

Attached Files

•  Flare Purge Pilot Gas Calculation 3Inch.xlsx   18.37KB   807 downloads

[zyllaron]

Hi Jimmayy,

In the past I've discovered a similar problem with Husa - the results are very small. In the past I've just specified a larger flow from experience. However, this week I've asked my EIT to get to the bottom of this. It would also be good he hear feedback from experienced members on this topic. Our rough assessment of the problem for and 8" flare tip is as follows:

Husa recommended by API 521: ~7 scf/h
EPA/452/B-2-1: 628 scf/h
EPA minimum flare tip velocity flow: ~4800 scf/h

I'll try to post the results of our current investigation.

[kkala]

Probably http://www.cheresour...ic/12530-flare/ '> http://www.cheresour...ic/12530-flare/ can be useful, including comments on Husa recommendation.

[Jimmayy]

Thanks for the Response,

Yes the values given by HUSA are very small and not even close to comparible to what flare suppliers use for purge gas rates. Mactronix and Tornado industries use a purge constants 0.04ft/s for flares with a velocity seal and 0.5ft/s for flare without a velocity seal.

Both use the following formula to calculate their purge flow rates:

Q=(0.04ft/s/144in²/ft²)*3600*Area of Tip (ft² ) with velocity seal

Q=(0.5ft/s/144in²/ft²)*3600*Area of Tip (ft²) - no velocity seal

For a 3in flare Purge rates are calcuated to be:

7scfh - with velocity seal

88 scfh - no velocity seal

HUSA values are for a flare with a plain end (no seal) and the values calcuated are nowhere near what these vendors use for purge rates. Both vendors will not specify how they came to use these purge constant velocites.

Please help I am not sure why HUSA values are so small.

Thanks.

[ankur2061]

Jimmayy,

Refer the attachment in the link below for the purge gas rate based on Husa correlation.

http://www.cheresour...9-sweeping-gas/

Regards,
Ankur.

Edited by ankur2061, 22 January 2013 - 12:18 AM.

[zyllaron]

Jimmayy,  I realize that this discussion is very old but the simple answer to your question is that, as API 521 points out, the Husa formula only applies to purge gas that is lighter than air.  Notice that the K values of the components that have molar masses greater than that of air (28.97) are negative.  Your resultant weighted average K is negative so the formula does not apply.

Apart from that have you or any one else noticed how small the values are from Husa?  Husa did the work for Amoco.  Tornado Technologies claims to have a rate from Imperial and Home Oil studies, a rate that is 100 times that of Husa.