Hi, there!

I am a Process Engineer with 3 years experience. I need to verify a flare system capacity after adding more equipment to it. My worst case scenario is 3 phase production separator PSV relief. I calculated required PSV orifice area with current production rates and got 0.33 sq. in. while installed 20 years ago PSV has orifice area 11.05 sq.in. With the orifice area 11.05 sq. in. maximum relief rate would be 160 MMscfd while flare capacity is only 55 MMscfd. I understand that maximum relief rate will last only few seconds but how can I put it into numbers to prove the flare system is safe/unsafe to operate?

Thank you for your advice!

NewInProcess,
Welcome.

Required area is 0.33 sq.in compare to PSV orifice area of 11.05 sq.in. You may anticipate a PSV chattering. Change of PSV may required. Please discuss with PSV vendor and consider to change it to smaller PSV.

Please refer to API STD 521 (2007), section 7.2.1, table 12 for design basis for PSV lateral & header assessment.

I am expecting the existing PSV tail pipe has gone through the detail check based on correct flow as per code 20 years ago. But it may not inline with API STD 521 (2007), section 7.2.1, table 12. Please check tail pipe with rated flow to ensure mach no, rho*V2, noise level, etc are within the limit. As for header, i don't see a problem as you only consider relieving flow.

Thank you, JoeWong!

1. When I will be calculating mach no, etc. should I use the maximum instantaneous flow rate?

2. Could you share with me copy of API STD 521 (2007)? I use the 1997 version of it.

It subject to type of PSV. Please refer to table 12.
Generally your PSV tail pipe shall be good for maximum instantaneous flow rate.



API STD 521 (2007) is a copyrighted document. I am sorry for not distributing it. You may get a copy from IHS.

NewInProcess:

You don't mention anything about the other reliefs that are part of this flare system nor do you say anything about the sizing criteria for this particular vessel. So, it now comes time to assume some stuff.

The odds are fire was the controlling scenario for this particular relief valve back then. If this is so, then production rate has nothing to do with the relief valve size, only how full the vessel was and the liquid properties used in the sizing calculation. If the service is pretty much the same then there should be no difference in the calculated size of the PSV. So now I am suspicious that either the "old" valve was calculated grossly wrong our you are sizing this valve grossly improperly.

Can you shed some light on the old and new scenario basis?

And for the tail pipe size, you must use the rated (stamped) flow in the calcualtion. If this relief valve is part of a system where there can be simultaneous relief events, then for the header you should be using the required relieving rates of all the PSVs that will simultaneously relief.

JoeWong:

Thank you for you recommendation!


To Phil:



The reason I did not mention the other reliefs is in this project we do not consider double jeopardy and the relief from this particular PSV gives the largest instantaneous flow rate. That is why my assumption is the maximum flaring rate would be constant flaring rate plus instantaneous flow from the relief device with the largest flow rate.



I do not know what the controlling scenario was 20 years ago for the sizing of separator PSV. I sized PSV assuming that gas outlet will be blocked but all liquid outlets will be functioning. Later today I will size same PSV for external fire emergency to see if it gives me orifice size closer to 11.05 sq in.



Are you saying that no matter what is the orifice size and conditions in vessel the flow rate through PSV and therefore through tail pipe will not exceed rated (stamped) flow?

Thank you for helping!!!

What's the service of the 3-phase separator ? Is it Slugacatcher or inlet separator or first separator receiving offshore fluid from pipeline ?

Is so i don't surprise with large PSV installed previously and small relief flow 20 years later.



Above scenario is valid case.

Is there analysis done on number of vessel exposing to fire simultaneously within a fire zone ? Have you considered those PSVs within a fire zone lifting during fire case ?



No. You have interpreted Phil's statement differently. Phil (as well as me) have the same opinion. Your tail pipe shall be designed for rated (stamped) flow (Note 1) and header may be designed for required flow.

Note 1 : Exception for NOT considered rated flow is subject to type of PSV and clearly indicated in table 12.

It appears that there is a small communication problem here.

If this PSV is a standard, pop open type valve (Farris 2600 series for example) then the tail pipe is sized for the PSV rated (stamped) flow. I can't be more clear than this.

And as Joe pointed out, checking to see if there are simultaneous reliefs happening is not double jeopardy. Relief events can affect multiple vessels. Fire, as Joe points out, is one of these.

Simultanious relief should always be considered for your flare, as mentioned earlier fire is one, but one can also think of total cooling water failure, total power failure or total instrument air failure.

Before you can look at your flare you have to calculate all the possible relief scenario's of the equipment connected to the flare.

Don't forget to look at flare back pressure once you have determined the largest simultanious relief scenarios.

I do not know exact service of the sparator but it is the first one receiving fluids from other platform.

I also calculated the size of PSV for relief in case of external fire but got only 1 sq in. I get close to 11 sq in only if I consider 2 phase relief through the PSV. Does it mean that this case would give me the required flow rate?



I do not know if such analysis was done but I will look into it. Good you have mentioned multiple PSVs relief during fire!



It is exactly Farris 2600! By the way, thank you for explaining how to recalculate rated flow in one of previous topics.



Is there a list of scenarios that one should look at or you learn it during years of experience?

Thank you all for comments and advices!!!

You have to understand the real service of the separator. Ask for it !




Your wording looks like you are not very sure if single or 2-phase relief is possible scenario.
You have to be firm if you will have vapor relief only or there is a potential of 2-phase relief. Analyse it. End of the job, you needs to prepare a report / calculation note / Dossier to state all your basis, input, assumption, justification, etc...you have to justify if 2-phase relief is possible. If not, why NOT ? If yes, Why YES ? And your document will potentially be audited, submission for authority approval, insurance company audit, etc...



Please check out in API STD 521. It basically cover most (if not all) of the scenarios...