Eliminate Gas Blow-by PSV

[patchlam]

Hi fellow engineers,

May I ask if we are allowed to eliminate Gas Blow-by PSVs by having a level transmitter and a shutdown valve which closes during low level detected?

Thank you for your time.

 

[LittleInch]

Not as a blanket rule no.

You may be able to argue this is an equivalent level of safety, but each design is its own.

So depending on what would happen if the blowby occurs, what the consequences of that are and what SIL level is required to prevent it.

PSVs are very reliable and relatively cheap. SIL 3 or 4 loops require multiple sensors, a logic solver rated to the same level of redundancy / reliability and similar for the shutdown valve itself.

For me you should have one of these (TX and S/D valve) already to prevent the issue and then the PSV takes care of what happens if it all goes wrong

 

[shvet]

@patchlam

We practice such regularly for similar issue for governing cases as all industry practicies/codes prohibit double jeopardy.
We install a shutoff valve upstream of a large column reboiler connected to a 2oo3 pressure sensor at the column overhead. This allows us to justify ignoring uncontrolled heat input case.
Sometimes we practice such for liquid level control (as you described), but only for minor cases, as we have a consensus that a level trip is not equial to a PSV from the point of ultimate reliability of the whole system.

As per my experience in industry such issues are site specific and case specific both and are subject to a local practice or philosophy. As per my experience some companies fully refuse such practice while some practice constantly.

 

[patchlam]

Apologies for my late reply. Thank you so much

LittleInch​

shvet​

for your comments. I was trying to find if SIL3 Level indicator ESD function will be enough to prevent the blowby and eliminate the PSV. But I found nothing.

 

[MortenA]

Generally PSV's are needed for code reasoms - so you may elimnate the scenario as described, hvorever the blow-by vase may not be your worst case? Wrt SIL remember that its not only about getting the gadgets - they also needs to be serviced and teststed. For SIL 3 you will have to test quite often and partial strockes will not be enough - now productions is going to hate you too!

 

[patchlam]

MortenA​

Thank you for your comment. I agree with your opinion, I was trying to reduce the relief load of the gas-blowby PSV by putting a mechanical stop, so that our top guys could accept from cost perspective.

 

[shvet]

Apologies for my late reply. Thank you so much

LittleInch​

shvet​

for your comments. I was trying to find if SIL3 Level indicator ESD function will be enough to prevent the blowby and eliminate the PSV. But I found nothing.

What exactly are you looking for? Can you describe?
SIL3 is enough per para. 4.2.6 API 521-2020 to consider this case incredible.
An interlock isolating the HP system on low LL from the LP system is enough to mitigate this case as per annex G API 521-2020.

 

[LittleInch]

MortenA​

Thank you for your comment. I agree with your opinion, I was trying to reduce the relief load of the gas-blowby PSV by putting a mechanical stop, so that our top guys could accept from cost perspective.

Well a mechanical stop can reduce max flow for sure, but the risk is that someone will remove it to get a bit more flow (this does happen), so often the safety reviews don't like them unless the stop is welded on.

 

[patchlam]

What exactly are you looking for? Can you describe?
SIL3 is enough per para. 4.2.6 API 521-2020 to consider this case incredible.
An interlock isolating the HP system on low LL from the LP system is enough to mitigate this case as per annex G API 521-2020.

shvet​

I tried to read through the ANNEX G. Do you mean the High-Integrity Protection System (HIPS) to either isolate the flow from HPS or progressively remove liquid from LPS?

 

[patchlam]

Well a mechanical stop can reduce max flow for sure, but the risk is that someone will remove it to get a bit more flow (this does happen), so often the safety reviews don't like them unless the stop is welded on.

Thanks for the insight. I thought this should be considered as operator intervention?

 

[LittleInch]

Indeed, but its hard to enforce. A welded mechanical stop would satisfy most HAZOP chairmen I think, but a bolted one maybe not.