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7

Psvs Relieving To Closed Drain System


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#1 _1angelia23

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Posted 12 January 2025 - 09:27 AM

Dear engineers,

In the FEED of a project I am supervising, the consultant designed the oil and gas processing facility so that some PSVs (PSVs in the inlet manifold and for the dehydrator and desalter) are connected to a closed drain system other than to the flare system. The set pressure of PSVs in the inlet manifold is 60 barg, so what are the consequences if we go with this option, especially for PSVs in the inlet manifold? Many standards do not recommend connecting PSVs to closed drain systems. For me, I have never seen such design before . When we asked Consultant about the reason for the connecting PSVs of dehydrator and desalter to CDS, they said it is possible since the governing case for them liquid blocked outlet and the liquid is relieved not gas.

I appreciate your opinions and suggestions,

Thank you in advance..

Edited by _1angelia23, 12 January 2025 - 09:29 AM.


#2 Pilesar

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Posted 12 January 2025 - 09:37 AM

All relief cases must be accommodated -- not just the governing case for PSV sizing.



#3 Dacs

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Posted 13 January 2025 - 01:53 AM

The issue with PSVs discharge to CDS vessel is you take away the inherent capacity of the vessel to account for the inventory discharged from PSVs.

 

For the liquid discharge: Unless you have a CDS vessel that can accommodate all expected drainage when doing T&I AND able to accommodate the expected inventory from the PSVs for a certain amount of time (maybe 30 minutes), then it's quite unwise to do this. 

That's the reason most standards don't recommend doing this, maybe except for thermal relief.

 

I'd actually challenge them for the sizing basis of the CDS vessel and see if this PSV is listed as a source for liquid holdup.

 

Another inherent problem with this is if the CDS vent line (to flare or to whatever location) is adequately sized for the maximum expected vapor relief from the same PSV (as Plesar has touched on). Isn't there a fire case for the vessel in question? 



#4 Ammaniya

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Posted 13 January 2025 - 09:45 AM

Honestly, connecting PSVs to a closed drain system instead of a flare isn’t something you see often. For the PSVs in the inlet manifold set at 60 barg, there could be some risks. Like, if there’s a pressure spike, the PSV might relieve liquid instead of gas. This could create a bunch of problems downstream or even in the closed system. I get the consultant's point about liquid blockage for the dehydrator and desalter, but still… it kind of raises red flags. What if there’s a blockage in the closed drain? Will pressure buildup be an issue? It looks to be a gamble because you're relying on something unusual. Standards are often created for a cause, so it's surprising that they're defying them. Maybe double-check their calculations and reasoning? Even if the questions appear insignificant, they are vital to ask. Better safe than sorry, right? Just my two cents.



#5 shvet1

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Posted 14 January 2025 - 11:31 PM

@angelia

What is the difference between the options the relief will be discharged to an aboveground flare knock-out separator instead of an underground drain vessel?

Note that in an alternative case the PSV elevation is an issue as the PSV discharge shall be routed continiously downward to KO separator. Are you ready to solve the problems resulted?

 

Was the underground vessel's vapor space and a vent (I presume to the flare) designed for large vapor relieves like a dehydrator or desalter firecase?


Edited by shvet1, 14 January 2025 - 11:31 PM.


#6 breizh

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Posted 15 January 2025 - 02:22 AM

Hi,

As pointed out, what is going to happen if there is no room in the CDS? How do you manage the volume of liquid to be minimum?

To me you should route your PSVs discharge to a separate vessel prior to flare.

Hazop study should confirm.

Safety first, 

Breizh  



#7 _1angelia23

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Posted 15 January 2025 - 01:06 PM

All relief cases must be accommodated -- not just the governing case for PSV sizing.

As l know PSV is sized based on the credible scenario which is blocked outlet here so I do not understand how we can accomadate all relief cases for PSVs,
Please clarify further

Edited by _1angelia23, 15 January 2025 - 01:06 PM.


#8 latexman

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Posted 15 January 2025 - 05:02 PM

 

All relief cases must be accommodated -- not just the governing case for PSV sizing.

As l know PSV is sized based on the credible scenario which is blocked outlet here so I do not understand how we can accomadate all relief cases for PSVs,
Please clarify further

 

I'll add one word to Pilesar's advice to clarify it for me, "All credible relief cases must be accommodated -- not just the governing case for PSV sizing.

 

The PRD record should show all relief cases considered (checklist?), and which of those are credible.  



#9 _1angelia23

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Posted 19 January 2025 - 08:39 AM

The issue with PSVs discharge to CDS vessel is you take away the inherent capacity of the vessel to account for the inventory discharged from PSVs.

For the liquid discharge: Unless you have a CDS vessel that can accommodate all expected drainage when doing T&I AND able to accommodate the expected inventory from the PSVs for a certain amount of time (maybe 30 minutes), then it's quite unwise to do this.
That's the reason most standards don't recommend doing this, maybe except for thermal relief.

I'd actually challenge them for the sizing basis of the CDS vessel and see if this PSV is listed as a source for liquid holdup.

Another inherent problem with this is if the CDS vent line (to flare or to whatever location) is adequately sized for the maximum expected vapor relief from the same PSV (as Plesar has touched on). Isn't there a fire case for the vessel in question?

There is no fire case for closed drain vessel because of being underground vessel.
in my opinion the reason for connecting PSVs to closed drain system instead of flare system is to avoid the accumulation of liquid in vapor flare collection header and high high liquid level in Knockout drum which would result in burning rain.
So they are trying to avoid this by collecting liquids in closed drain vessel.
I am asking here, for fire case is fluid released only liquid or two phase.

Edited by _1angelia23, 19 January 2025 - 09:05 AM.


#10 latexman

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Posted 19 January 2025 - 09:20 AM

I am asking here, for fire case is fluid released only liquid or two phase.


It depends on the components, properties, and PSV set pressure. We are not privy to that information.

#11 Dacs

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Posted 19 January 2025 - 09:57 PM

in my opinion the reason for connecting PSVs to closed drain system instead of flare system is to avoid the accumulation of liquid in vapor flare collection header and high high liquid level in Knockout drum which would result in burning rain.

 

 

You're right, but CDS vessel is not the proper equipment to handle this. Flares have their own KO drum, specifically for this purpose.



#12 breizh

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Posted 20 January 2025 - 07:28 AM

Hi,

Standard set up for flare system attached (drawing).

Breizh

Attached Files



#13 DavidPorras99

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Posted 10 February 2025 - 07:03 PM

If your company does not have design criteria for this type of case, check the thermodynamic properties of your fluid to see if steam can be generated in the event of a fire in the pipes “not in the tank”. If you do not have steam, your tank will work like a KO Drum but larger, just make sure you design it well to avoid emergencies due to overfilling. If you have steam, you must recover it to avoid emissions during emptying. You can use any container to store it, as long as you have a good design and a Hazop workshop.






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