If I read you correctly, first both vessels must be protected with their own relief valves because they can be isolated from each other.

Now, it is extremely difficult, if not impossible, for any respondant to address a particular scenario without seeing the P&IDs. So, as far as I can tell by what you post, and guessing what a "blow-by" scenario actaully means: if the first (higher pressure) vessel's relief valve is set at a higher pressure than the relief valve on the lower pressure vessel, then yes, you should consider this blow-by scenario from both vessels in analyzing the relief valve size for the low pressure vessel's relief valve. This is because during normal operation, they will be opened to each other.

If the relief valve's are set at the same pressure (limited by the lower pressure vessel), then you can assume that both relief valves will take about 1/2 the relieving rate from this scenario.

One final note that may be obvious but for completion sake, the final size of the relief valves will depend on the worse case scenario which may not necessarily be what you describe.

Remember theres also a control valve between the HP and the LP separator.

This could mean that the entire flow could not go though to the LP separator. your PSV may only need top be sized for the max flow through the CV. remember to allow for increased pressure both upstream and downstream.

Best regards

Morten

If this is a "blow-by", and by my response I am assuming what the definition of a blow-by is, the control valve in-between is irrelevant. The presence of a control valve and the emergency shut off valve is precisely why I say that BOTH vessels MUST be supplied with their own relief valves.

When evaluating the relief valve size on the lower pressure vessel, I agree that for a fail opened control valve scenario, one should consider not only the fully opened CV of the control valve but of the emergency valve as well. One of these will be controlling the amount of flow.

But there must be some sort of valve between the two vessels? This valve will have a CV and thus limit the max flow.

Naturally the valve has no safety function - a PSV must be present at both vessel - but sizing can allow for the presence of this valve.


Best Regards

Morten

1.  What is being seperated & how many phases?

2.  What is the design/operating pressure of the Hp/Lp
    seperators?

3.  The seperated streams where are they routed to; more
    specifically what other pressure vessels/ storage ?

4.  Is there a blow down line to flare?

5.  How many partions are on the header that is feeding
    the seperators? is the system Hp/Lp exclusively?

6.  Are the header incomers segragated frequently?

Thanks for all your help, to answer some of the questions above

The separation is oil & gas, HP only 600psi, LP 160psi (operating). Gas goes eventually to compression via heat exchangers, oil goes to oil treater vessel. There is a blowdown line to flare on the separators. The system is HP/LP exclusively

To MortenA: Please read my post more carefully...I believe I said that!

If the flow is always from the HP vessel to the LP vessel and is not by-passed, then the only "blow-by" scenario that I can see would be the control valve in-between the two vessels failing to the fully opened position. The flow rate used in determining the relief valve on the LP side would most definitely be affected by what can pass through this control valve. However, there is also mention of an emergency shut-off valve that would be in the fully opened position during normal operation. The Cv of this valve may even be less than the control valve. If this is the case, then the emergency shut-off valve will be more limiting. In either case, "chrisrich" can take credit for the restricted flow when evaluating the size of the LP relief valve.

chrisrich,

Single PSV minimum for each PV.

1.  Consider blow-by and fire case; the most  
    demanding should be the basis for your PSV sizing.
    440-600 psi blowing by to LP Seperator normally  
    operating at 160 psi :) check your pressures please

2.  Consider nos. 2 PSV both able to fully relieve worest
    upset condition; just in case one fails.

3.  Consider, isolating any PSV for maintenance purposes
    while taking into account pressure drops and isolaton  
    on both sides (blow-down side and PV side).

4.  I keep on considering an Hp Gas Scrubber. (To knock
    Hp Gas from Hp Seperator prior to dispatch or
    compression.  Or Compression/Cooling PLant that diverts
    Hp Gas to a Hp Gas Scrubber.  I would envisage that the
    operating pressure is high.  Please consider if any a
    case by which such a PV is isolated; would that over
    -pressure your seperators.

5.  The pressures you outlined have a considerable
    difference.  440 psi is knocked at Hp ? please confirm


Cheers,
 

Well figured, you must be good, I did have the HP separator wrong - it is 450psi - the LP was correct at 160.

Of interest, would you allow a check valve upstream of any PSV or would you insist on a block valve. I know a block is obviously better, but check valves have been fitted and will be very expensive to change

Why would a check valve be placed upstream the PSV; for what purpose. (between the PV and PSV)

PVRV/chririch

I still find that you should only size the LP PSV for a flowrate equal to the max gas flowrate through the CV on the liquid line between the HP and the LP sep. If this valve has a larger capacity than the max flowrate then of course for max flowrate.

Make allowance for increased pressure upstream/downstream of the CV.

Best Regards

Morten

PVRV (Mechanical) Feb 23, 2003
chrisrich,

Single PSV minimum for each PV.

1.  Consider blow-by and fire case; the most  
    demanding should be the basis for your PSV sizing.