5 replies to this topic

[Genii]

Hello,

I like to confirm about the basis of purge gas in flare headers;

Its flow is based on the flash-back velocity of flame ie efflux (flare effluent + purge gas) velocity should be atleast equal or greater than the flame front to prevent the back fire. Its flow estimation is also a vendors trade.

With the above basis its regulating valve should be FCV but I have seen PCV on purge lines. please comment.

There is also some criteria about the purge line requirement. This is required on flares which are intermittant. But in case of continuous flow certain minimum flow is required?

Also comment on the location of this purge gas. I have a horizontal flare and purge gas is fuel gas of MWt in between 20 and 22. This purge gas should be some 25 ft away from flare tip or this should be at the start of header?

Regards.

[JoeWong]

With the above basis its regulating valve should be FCV but I have seen PCV on purge lines. please comment.

Both actions are to maintain a positive pressure in the flare during NORMAL operation to avoid air ingress. There are advantages and disadvantages using FCV caompare to PCV. FCV may always maintaining a constant flow into flare. However, it does not always gurantee positive pressure in the flare escpecially during raining, cold weather, strong wind... PCV will try his best to maintain a positive pressure in the header, however you may consume a lot of purge gas. Pressure in the header is difficult to measure and difficult in selecting the exact location of pressure transmitter.

Sometime people consider a globe valve with flowmeter or manual block valve with RO...

There is also some criteria about the purge line requirement. This is required on flares which are intermittant. But in case of continuous flow certain minimum flow is required?

Easy question but difficult to answer. If you do some literature studies, you will find that there are many equations e.g HUSA, TAN, etc to estimate the purging rate...the calculated purge rate can range from factor of 1 to >10...e.g. 0.02 ft/s to 0.2 ft/s...all suject to line size, purge gas MW, use of velocity seal, type of gas, etc Suggest you to read latest API 521 (2007 edition), e.g. 6.4.3.6.2, 7.3.3.3..3, etc...it discuss a lot on flare purge. Please obtain purging rate from flare tip vendor for verification.

Also comment on the location of this purge gas. I have a horizontal flare and purge gas is fuel gas of MWt in between 20 and 22. This purge gas should be some 25 ft away from flare tip or this should be at the start of header?

Yes. Always consider to purge the header from the furthest remote area.

Hope above helps.

JoeWong

[Genii]

There is also some criteria about the purge line requirement. This is required on flares which are intermittant. But in case of continuous flow certain minimum flow is required?

Easy question but difficult to answer. If you do some literature studies, you will find that there are many equations e.g HUSA, TAN, etc to estimate the purging rate...the calculated purge rate can range from factor of 1 to >10...e.g. 0.02 ft/s to 0.2 ft/s...all suject to line size, purge gas MW, use of velocity seal, type of gas, etc Suggest you to read latest API 521 (2007 edition), e.g. 6.4.3.6.2, 7.3.3.3..3, etc...it discuss a lot on flare purge. Please obtain purging rate from flare tip vendor for verification.

Thnx...

I have heard that if minimum continuous flow in flare header is 10 scfh then you dont need purge line. is that correct?

Regards.

[JoeWong]

I have heard that if minimum continuous flow in flare header is 10 scfh then you dont need purge line. is that correct?

I don't think this statement is correct...10 scfh for 10" compare 10 scfh for 36". Obviously velocity for 10" header is much larger than 36" and potentially former meeting criteria but not for second...

JoeWong

[Genii]

Thnx Joe Wong.

Can anyone give the reference of slope and drain requirement in purge line of flare?

[JoeWong]

Thnx Joe Wong.

Can anyone give the reference of slope and drain requirement in purge line of flare?

Genii,
Again...i would like to advise you to open a new post for different subject for easy management & search in future...

Answer to your question :
Is your purge gas dry (free from condensate at minimum operating & ambient condition) ? If so, i don;t any requirements on sloping and draining. Chekc for both upstream and downstream of your throtling device...However, Good engineering proctice...NO LOW POCKET, FREE DRAINING, NO LIQUID ACCUMULATION, LESS BEND...



Hope this help.




JoeWong