5 replies to this topic

[tsrc8204]

Dear all,

Normally, we design a water seal durm to prevent the back firing from the flare stack. But, if we want to increase the vented gas to flare for burning, do we need to enlarge this seal drum for safety concern? (assume the flare capacity is enough for this demand) Pleaese provide your comments, thank you.

tsrc8204.

[AKsrey]

I would review API RP521.

[tsrc8204]

Dear Aksrey,

Thanks for your kindly help, I also checked the ISO-23251 & API 537 but there was no further information could be found.

If anyone want these 2 files, please let me know I can mail its to your box.

tsrc8204.

[Homayun]

I would say that the design of the seal drum is probably determined and governed by the maximum flare load and not the "normal" venting. Therefore, if you are only increasing the vent gas rate, I don't think you also need to resize the seal drum.

Edited by Homayun, 18 November 2009 - 03:45 PM.

[kkala]

Normally, we design a water seal durm to prevent the back firing from the flare stack. But, if we want to increase the vented gas to flare for burning, do we need to enlarge this seal drum for safety concern? (assume the flare capacity is enough for this demand) Please provide your comments, thank you

API 521 gives criteria for sizing a seal drum:
(a) Vessel free area for gas flow above liquid level should be at least three times the inlet pipe cross sectional area to prevent surges (D at least = 2d for vertical drum).
( For vertical drums (usual case) height of vapor space above liquid should be 0.5 - 1.0 times D (=drum diameter), or 3 ft (1 m), whichever is greater.
Supposing that existing pipe diameter is acceptable for the new max flow conditions (and this has to be checked), seal drum configuration is not affected (i.e. can remain same).

API calls for max exit back pressure allowable in the vent header to set max distance h, that the inlet pipe is submerged. This is not quite clear to me (flow will not occur till header pressure overcomes h?), but at any case back pressure will increase under new conditions so existing h shall be < max h resulting. It would be useful, if another member could clarify this point.

Special measures may be needed if new conditions include large hot vapor discharge, as indicated by API 521.

Edited by kkala, 16 December 2009 - 02:05 AM.

[Joyy]

Dear tsrc8204,

If you are increasing the flaring load then i suggest that you need to thoroughly review the entire flare network (back-pressure on PSV / Flare PCV, mach no in tail pipe & header, adequacy of KOD/Water seal drum, radiation and dispersion).
But let us say that we presume that since the increased flare load is below the 'design' capacity, your system is ok.

Coming to your question on whether seal drum need to be changed for increased gas load. First, you MUST read through API PR 521, there is a section on seal drum which gives all the guidelines that you need to follow.
Second, my initial view to your answer is no, change not required. But to understand the real picture, we need to know the purpose of your seal drum and check factors related to sizing.

A water seal drum can have various applications (prevent air infiltration, act as flame arrestor, act as a staging device between multiple flares, provide back-pressure to flare gas recovery system), and you needs to know which one is yours. Based on the purpose of water seal drum in your case, the height of water seal i.e., dip pipe inside the water, will be set. The seal will not change for different flare loads in one system. As long as the purpose of your seal drum is not changing, it is ok.
Another aspect is of sizing the seal drum is to meet its performance objective. It should allow the gas to pass through the seal with minimum fluctuations in gas flow/pressures. Whether your seal drum was originally designed properly at the design flow rates should be verified in accordance with guidelines given in API RP 521 for sizing of seal drum. This will check sufficient gas space area, internals and other aspects.

Hope this gives to the direction.

If you need information on any other aspect of flare network, i suggest you first search this forum.

Best Regards,
Joyy