Elimination of overpressure scenario on ASME pressure vessel
Elimination of overpressure scenario on ASME pressure vessel
(OP)
Hi all,
I was hoping someone had some guidance on something that frequently comes up. Say you had an ASME VIII pressure vessel with a relief valve set at 150 psig. The vessel has an ASME relief valve which meets the requirements of ASME VIII including inlet/outlet piping requirements etc.
Now say there is a pump feeding into the vessel, and the pump is capable of generating 300 psig. However the pump has its own relief valve which is set at 150 psig, relieving back to the pump suction. However it is not an ASME relief valve. It is just a simple proportional relief valve sized for the capacity of the pump. The pressure the pump normally puts out is just the backpressure from the vessel plus whatever frictional losses are in the line.
My question is, there is the potential to block in the outlet of the vessel and have it become liquid-full if the level controls fail. Do you have to consider the case for the ASME relief valve on the vessel where the pump could overpressure the vessel? Or can you just assume that the proportional relief valve on the pump will do its job, and eliminate the scenario? Does the code allow you to do this? Seems like technically you're using a non-ASME valve to protect an ASME vessel against a certain scenario?
I was hoping someone had some guidance on something that frequently comes up. Say you had an ASME VIII pressure vessel with a relief valve set at 150 psig. The vessel has an ASME relief valve which meets the requirements of ASME VIII including inlet/outlet piping requirements etc.
Now say there is a pump feeding into the vessel, and the pump is capable of generating 300 psig. However the pump has its own relief valve which is set at 150 psig, relieving back to the pump suction. However it is not an ASME relief valve. It is just a simple proportional relief valve sized for the capacity of the pump. The pressure the pump normally puts out is just the backpressure from the vessel plus whatever frictional losses are in the line.
My question is, there is the potential to block in the outlet of the vessel and have it become liquid-full if the level controls fail. Do you have to consider the case for the ASME relief valve on the vessel where the pump could overpressure the vessel? Or can you just assume that the proportional relief valve on the pump will do its job, and eliminate the scenario? Does the code allow you to do this? Seems like technically you're using a non-ASME valve to protect an ASME vessel against a certain scenario?





RE: Elimination of overpressure scenario on ASME pressure vessel
1) Failure of the vessel level controls;
2) Failure of the pump discharge relief valve.
These are two independent systems and considering simultaneous failure of both would be a double jeopardy. So unless we are talking about really MAJOR consequences in terms of HSE or Economics if the combined event occurs, there should be no additional layers of protection required for this system.
In addition to very low likelihood of such a combined event, process safety time (PST) required for the vessel to be completely filled with liquid and consequently overpressured should be calculated. There are various guidelines out there, but as a general note if the calculated PST is more than 10-15 minutes, there is sufficient margin for operator intervention and mitigation of the top event.
If you want a documented decision process, LOPA will tell you what is the expected frequency and risk of unmitigated event (likelihood would differ for ASME- and non-ASME certified relief valve on the pump, hence the risk calculated in both cases would be different).
Dejan IVANOVIC
Process Engineer, MSChE
RE: Elimination of overpressure scenario on ASME pressure vessel
The constant pressure valve is not part of the system safety equipment. It could fail and it is likely that no one would notice for an extended period (the pump discharge pressure is actually controlled by the backpressure on the system). If you block the outlet then you could overpressure the vessel with the pump.
Those valves are notorious for running for years without anyone noticing that they are broken, so I don't think that this is a double-jeopardy situation. I wouldn't ever consider one of those valves to be a contributing portion of the safety equipment (I use them for control, but ignore their existence when I'm doing a credible scenario evaluation).
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Elimination of overpressure scenario on ASME pressure vessel
RE: Elimination of overpressure scenario on ASME pressure vessel
It depends on the probability of the liquid to form a pre-existing condition that could go undetected.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Elimination of overpressure scenario on ASME pressure vessel
Requirements for additional safeguards will depend on what is the consequence of vessel ovepressure (likely results in a flange leak). Again, unless it is a very toxic compound or perhaps a flammable liquid in an offshore environment, I doubt this will call for further assessment but LOPA should be used to confirm this anyway.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Elimination of overpressure scenario on ASME pressure vessel
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Elimination of overpressure scenario on ASME pressure vessel
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Elimination of overpressure scenario on ASME pressure vessel
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Elimination of overpressure scenario on ASME pressure vessel
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Elimination of overpressure scenario on ASME pressure vessel
What I was saying in previous posts is that arriving to vessel overpressure conditions requires quite a few things to go wrong, and simultaneously: pump controls to fail, vessel level control to fail, and operator intervention to fail. Everything in the universe is possible, but in this industry we are interested in actual risk (= consequence X likelihood) of a hazardous event, and see if that risk is acceptable or not. So, in reality, the question is not whether the overpressure can happen or not (because everything can happen, it is just that likelihood of various events is different), it is how big is the total risk and whether that risk requires further reduction or not.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Elimination of overpressure scenario on ASME pressure vessel
We don't know what other credible scenarios have been considered. I would call this one credible with the flow rate equal to the pump capacity.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Elimination of overpressure scenario on ASME pressure vessel
RE: Elimination of overpressure scenario on ASME pressure vessel
Dejan IVANOVIC
Process Engineer, MSChE
RE: Elimination of overpressure scenario on ASME pressure vessel
I take that to mean that (absent any other scenario) this was heading towards removing the PSV.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Elimination of overpressure scenario on ASME pressure vessel
The only time I would sign off on the removal of the only relief device connected to an ASME VIII pressure vessel is if it could be demonstrated that a relief event or leak caused by a faulty relief device would represent a substantial risk of harm that cannot practically be mitigated by design, and even then I would do so extremely reluctantly. Even in systems which are protected "by design", i.e. by setting the MAWP above the maximum pressure expected in a worst-case failure event, I would STILL insist on installing a relief device unless as noted above.
If the question is not one of removing the only relief valve on the vessel, but rather relying on a proportional relief valve on the only legitimate source of pressure to that vessel to take the place of the code-required overpressure protection device: I might consider that on a small UM stamped tank full of a low hazard material or the like where the hazard to people is very small, and then only if it can be demonstrated that the installed relief device (though non ASME) is sized such that it will keep the overpressure within 10% under all cases. In our business we do frequently find ourselves in the situation of having to install properly sized proportional relief devices to protect small UM stamped tanks, because the smallest commonly available ASME relief valve (a C or D orifice)is disproportionately large and an oversized PSV would have negative consequences for the downstream relief handling equipment. There are people who peddle A and B orifice ASME type relief valves and there are cases where we're forced to use them, but in many cases a properly sized proportional relief device is a safer and more practical solution.
If the question is rather whether or not the ASME relief device installed on the vessel needs to be sized for a case generated by the failure of another relief device such as one on a pump, that's a matter for a proper, systematic hazard evaluation process. I couldn't make a hard and fast rule on that which was suitable for all circumstances.
RE: Elimination of overpressure scenario on ASME pressure vessel
Now, when re-reading the last paragraph in his post, "there is the potential to block in the outlet of the vessel and have it become liquid-full if the level controls fail" I am getting a bit confused because if the outlet is blocked then it does not matter if level control works or not, because the outlet is blocked anyway.
As usual, we would need a sketch of the system and a bit of clarification what the OP meant...
Dejan IVANOVIC
Process Engineer, MSChE
RE: Elimination of overpressure scenario on ASME pressure vessel
Documents like API 521 fill this gap by providing safe and practical guidance for assessing scenarios, but these documents also leave the final decision to the user.
So, there's no right or wrong answer to the question of whether the PSV should be sized for overpressure due to the pump. For me, this is clearly in the grey zone - I could go either way. To make a decision I would look at the particular fluid involved. How hazardous is this material if it is spilled, and how much could be potentially spilled? I would also look closely at how many failures would have to occur in order for this scenario to play out. I sense that this scenario would require more than just a failure of the relief device on the pump. The downstream vessel is probably level controlled, and it may have a pressure transmitter that would alarm if the outlet was blocked. If so, there's a good argument that this is double-jeopardy, which is not typically regarded as a credible sizing scenario. Regardless, the final assessment is for the plant owner to make based on their analysis of the specific risks of this case, and their risk tolerance.
RE: Elimination of overpressure scenario on ASME pressure vessel
I guess to give more background, this is a vapor service valve that is only certified for vapor service. So when sizing for a liquid scenario, I'm forced to apply the correction factor (Kp) from API 520 5.9.1. which derates the valve to the point that I would have to step up an orifice size. So if I count the scenario (assuming the pump external proportional relief valve isn't functioning), then I have to go up an orifice size. But if I don't count the scenario (assuming the pump relief valve is functioning properly), then I don't have to change the relief valve.
The relief device on the pump only has to meet the B31.3 requirements, but the vessel relief device has to meet ASME VIII. API 520/521 are now standards, so I wasn't sure which path to take here. The company requires that all applicable scenarios are documented, and put into the database.
RE: Elimination of overpressure scenario on ASME pressure vessel
RE: Elimination of overpressure scenario on ASME pressure vessel
It's better to consider possibility of liquid relief for the PSV on the pressure vessel. It might it has been certified for liquid as well as for vapor.
Then, to determine if mentioned PSV is certified for liquid too, refer to NB and look at NB-18, Pressure Relief Device Certifications.
RE: Elimination of overpressure scenario on ASME pressure vessel
So if this is a critical process for your plant I would not exclude it from sizing. If it is a auxiliary piece of equipment I would discuss this with stake holders and come to a decision as a group. You can be surprised by the insights brought on when you talk to the production folks instead of sitting at your desk looking at your P&IDs.
If your vessel is limited with nozzles, or has a large relief valve I would consider installing a 'real' PSV on the pump discharge. The PSV can return material either back to the suction side or back to the tank. These often will be fairly small and in addition to eliminating an over pressure scenario can help reduce the amount of material in the vessel should this over pressure scenario present itself.