5 replies to this topic

[Ghasem.Bashiri]

Dear Process Experts

Refer to API 521:
In some situations, a single pressure-relief device can be desirable to protect several equipment components in a process system. For this system to be adequately designed, the following four criteria should be satisfied.
a) No means can exist for blocking any of the equipment components being protected from the installation of the single pressure-relief device unless closure of these valves is positively controlled.

My question: I have a pump with exchanger and vessel in its outside. Vessel is for two phase liquid liquid separation. between exchanger and vessel I have just one manual valve. Then is it sufficient to use a PSV in vessel for blocked outlet protection for exchanger and vessel(I did not shut off pressure as design case for vessel and exchanger).

Also with vessel that have more than 1 liquid outlet or generally more than one stream outlet, what is basis of flowrate for blocked outlet. Becasue for liquid liquid separation a blocked outlet in one stream will not close other output stream. Therefore it is not possible to assign inlet stream flowrate as blocked outlet condition.

Ghasem Bashiri

[paulhorth]

Ghasem,

Your query is not clearly expressed and is difficult to undestand.Could you draw a diagram of your system and send it in please?
What I think you are describing is as follows:
(1) a pump
(2) this pump discharges into a heat exchanger
(3) the exchanger discharges the liquid into a vessel which is a liquid-liquid separator, that is, it is full of liquid with an interface between the light phase and the heavy phase.
(4) between the exchanger and the vessel there is a single manual block valve.

You are asking (a) if the vessel needs protection from overpressure caused by the pump if the outlets are blocked and ( if the exchanger also needs protection.

Have i understood you correctly?

My response is that a PSV on the vessel will serve to provide protection for both the vessel and the exchanger, provided the manual block valve is locked open. This is what is meant by "administrative controls" in API RP 521 5th edition para 4.3.2 (can't copy it here, sorry).: That paragraph also warns you to first check whether "administrative controls" will be safe enough.
The sizing basis for the PSV on the vessel should then be Blocked Outlet. There are two liquid outlets, for the light phase and the heavy phase, and you can take credit for one of these being open at normal flow, UNLESS the closure of the first outlet will actually cause the closure of the second. This could happen in the vessel you describe. If one outlet is closed, the interface will move up or down (towards the open outlet) then the interface level controller or a level trip will act to shut the open outlet. Then you will have both outlets closed, just through the correct action of the control system.

So, the PSV should be sized for the TOTAL liquid inflow.

If you are following the recommended practice API RP 14C, you should also fit a high pressure trip on the vessel acting to stop the pump. Thhis will then provide two independent means of protection (PSHH and PSV) as called for by RP14C.

Of course, this problem could be avoided by choosing the design pressure of the exchanger and the vessel to be higher than the maximum shut-in pressure of the pump.
Paul

Edited by paulhorth, 14 February 2011 - 10:18 AM.

[Ghasem.Bashiri]

Dear Process Expers
Refer to Paul feedback, with min flow line of pump that equiped with control valve:
Is it required to consider pump shut off pressure?
GB

[fallah]

Dear Process Expers
Refer to Paul feedback, with min flow line of pump that equiped with control valve:
Is it required to consider pump shut off pressure?
GB

Dear Ghasem,

No,if the PSV on the downstream vessel/exchanger is set below pump shut off pressure.

Fallah

[paulhorth]

Ghasem,

Thanks for your reply, after three months.
It would help us to consider the problem if you can send us a diagram of the system, because I still cannot clearly understand what you are describing.

I would remind you of my last point in my earlier reply:

Of course, this problem could be avoided by choosing the design pressure of the exchanger and the vessel to be higher than the maximum shut-in pressure of the pump.

Why not do this? then you can eliminate a lot of your problems and make the system more secure, for little extra cost.

Paul

[Ghasem.Bashiri]

Dear All
Refer to attachment for your information.
Let me know your feedback.
GB

Attached Files

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