Is it mandatory for a liquid gas separator to have a low low pressure protection?

Hi Bahjat, I don't believe it is mandatory. It is good practice to provide a low pressure trip to in conjunction with gas detection to indicate any significant loss of containment. I believe this is also a recommendation in API 14C but again this is only a recommendation unless of course you have stated in the BOD that you will comply fully with API's, etc. So if you believe that your gas detection is extremely reliable and the assocated risk is manageable with only gas detection then you may have a case for not providing PSLL.
(Note, I would need to understand the scenario in detail before offering any personal position)

Thanks John!
The scenario is as follow.
A typical onshore oil and gas separation facility with two stage separation, the wells flow into the first stage only. The question came during a HAZOP where the designers didn’t want to install a PSLL while I insisted that we should have. I have noticed from my previous projects that designers tend to have PSLL for almost 90% of the vessels however, some of the vessels don’t have PSLL. The only scenario that I envisaged is loss of containments as you mentioned in your reply as other scenarios for low low pressure can happen but following other action such as low low level (for example a stuck open liquid control valve) which is safeguarded by an independent switch LSLL.

I hope the above explains the situation


Cheers,
Bahjat

Thanks Bahjad, I note that your case in onshore for which the gas
detection is not as assured at indentifying large releases – simply because
any release could be projected in a single direction without necessarily
forming a sufficiently large cloud to pick up 2 or more detectors. In this
scenario I would be tempted to stick with the PSLL.

Regards,

John

Hi Bahjat,
I think the concern is what you will do with your PSLL if you have it. As you mentioned, the only scenario need PSLL is the loss of containment, PSLL is act as a mitigation measure and in this case, PSLL does not guarantee reducing casualties, or prevent escalated asset damages, etc. and possibly can only act as an alarm.
Then, what is difference between pre-alarm such as PSL or F&G detectors.
In this scenario, I would prefer to emphasize corrosion risk assessment or preventive maintenance program.

It isn't just a question of is it mandatory; also consider how robust a PSLL will be in reducing risk: e.g. if you have a major accident with complete vessel failure then the PSLL will likely not help mitigate the consequences. If you have a major accident with a continuous release e.g. from a 50mm hole, then the pressure in the vessel may not reduce anyway given the continued inflow rate and response of controls such as PCV on the gas outlet. (Also, will the released material lead to escalation in this scenario?).

You may also want to consider the risks introduced by a PSLL: if it is tied into your shutdown system with the intent that it provides an action on being activiated, then how will the PSLL be managed in a start-up situation? It may need an override, which will need an administrative procedure to control. The override could be accientally left in place. If it is not tied into the shutdown system, then the Operators will need to be trained in how to react. Both these are procedural safety and are less robust. Data suggests personnel will only react in a correct way under stress 50% of the time.

Will a PSLL really provide you with a barrier and reduce risk? Is there something else you can do to make the design more inherrently safe? (e.g. material selection, pressure rating, equipment spacing, operating pressure, less inventory, removing ignition sources). Can the protection provided by the active systems (F&G) be improved? (e.g. reducing the spacing between detectors so a smaller release can be detected).

Thanks all for your helpful replies!
@ Caroline: A study has been performed to check the dispersion model and plotting the plumes for LFL for different hole sizes 25, 50, 75 and 150 mm (full piping rupture), location of gas detectors then have been selected based on the results. For start up you absolutely need to bypass lots of switches not only PSLL, I remember when I was an operator we had to hang a tag on the switch that it is bypassed and register it in the override logbook to make sure it is not left forgotten providing that there is an efficient auditing system in place

Cheers,
Bahjat

The likely reason why you have found that a PSLL was implemented in some cases and not in others is that code compliance (API 14c) will state that you require one, but if the design was risk assessed (SIL assessed) and code compliance was not mandatory, you'd almost certainly find that it provided little or no useful additional protection/mitigation as already mentioned - and so could be removed

Excellent discussion! Let's re-examine the original re-phrased question: For a typical onshore oil and gas separation facility with two-stage separation, wherein the wells flow into the first stage only, is it mandatory for a liquid gas separator to have a low-low-pressure protection?

Why will there be low pressure? Either there is less gas coming in from the source or the gas flow leaving the separation vessel is greater than the incoming gas flow.

Why do you need low-low pressure protection? To prevent pulling vacuum into the system!

If flow rate of gas leaving the separator cannot be reduced to maintain the pressure in the separator, as in the case of a downstream compressor that is pulling gas out at a constant flow, then you do need a low-low-pressure protection. This signal will either open the recycle line valve to return the compressed gas into the inlet separator or shutting down the compressor.

If the reduced gas flow from the separator does not affect the downstream equipment operation, then you do not need a low-low-pressure protection.

@ Yuv: Are you suggesting that if the gas being flared then we don't need a PSLL?

@ Yuv: Are you suggesting that if the gas being flared then we don't need a PSLL?