We have two PSVs protecting the same high pressure steam line at same time. Both have a iso valve in front of and they are locked open. Set pressures of them are staggered, meaning one is at 810psig (design pressure of the line), and one is at 850psig. We don't want to set them at same pressure because they would fight against each one when the pressure is close the MAWP.As per the codes (B31.3 para 322.6.3, which refers to VIII-1), allows the additional PSV's pressure set at higher than MAWP(design pressure for piping) as long as <105% MAWP and the first PSV is <=MAWP). So our settig is OK. Both PSVs have sufficient capacity to relief the line by each self. The reason we have two is to avoid shutting steam line down when the lower PSV lifted and not reseat, we can close its iso valve and only using the second PSV for a short period, while we are sending out the 1st PSV for service or installing a spare PSV. My question is in that case we will only have one PSV w/ higher than design pressure setting, is that allowaled? My understanding is YES as per B31.3 para 322.6.3 (b)(1) allows this with limit:"(1) With the owner’s approval the set pressure may exceed the limits in Section VIII, Division 1, provided that the limit on maximum relieving pressure stated in (c) below will not be exceeded". But I'd like to hear your option or experience with this situaiton.
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Set pressure higher than design pressure when using multiple PSVs
- Thread starter wangyao98
- Start date
We have redundant PSV's in locations but we keep one blocked in and one open. If there are issues we switch. They are both set at the same set pressure. I don't think that you are following the intent of the code if you remove the PSV with the lower setpoint.
Regards
StoneCold
Regards
StoneCold
I am forced to concur with wangyao98's reading of B31.3, with one caveat - the total time [if any] spent above the Max. Allowable Stress for that pipe has to be recorded, because of these requirements in B31.3 302.2.4 (f)
(a) No more than 10 hours at 33% above Max, and 100 total hours in one year.
(b) No more than 50 hours at 20% over Max, and a total of 500 hours in one year.
But for anything I do, I won't exceed the Max. Allowable Stress. It is bad engineering. If you need a higher pressure rating, use thicker pipe.
(a) No more than 10 hours at 33% above Max, and 100 total hours in one year.
(b) No more than 50 hours at 20% over Max, and a total of 500 hours in one year.
But for anything I do, I won't exceed the Max. Allowable Stress. It is bad engineering. If you need a higher pressure rating, use thicker pipe.
There are some specific statements in codes and regulations that I feel are ill advised, so let's get back to basics. You are obligated to provide over-pressure protection, by both law, design standards, and engineering practice.
So what do all these things have in common? They all are intended to prevent an over-pressure scenario that subjects the vessel to more pressure than the MAWP plus an allowance for transients (10% of MAWP for most transients, 22% for others). If your over-pressure protection strategy meets that obligation then it is OK. I had a case once where I connected some ANSI 150 piping into an ANSI 300 system, I reset the PSV on the ANSI 300 vessel to values consistent with the ANSI 150 piping. That decision was absolutely in accordance with the intention of of the laws, standards, and engineering practices.
I don't know your process, but I'm going to go out on a limb and suggest that normal operating pressure is less than 810 psig. Probably a lot less. If I was in your shoes and that was true, I would set the lower one at 750 psig and the upper one at 810 psig. In my mind this does not improve "safety" (the way you're doing it is perfectly safe), but it reduces the amount of time you have to spend explaining an 850 psig set point on an 810 psig system.
David
So what do all these things have in common? They all are intended to prevent an over-pressure scenario that subjects the vessel to more pressure than the MAWP plus an allowance for transients (10% of MAWP for most transients, 22% for others). If your over-pressure protection strategy meets that obligation then it is OK. I had a case once where I connected some ANSI 150 piping into an ANSI 300 system, I reset the PSV on the ANSI 300 vessel to values consistent with the ANSI 150 piping. That decision was absolutely in accordance with the intention of of the laws, standards, and engineering practices.
I don't know your process, but I'm going to go out on a limb and suggest that normal operating pressure is less than 810 psig. Probably a lot less. If I was in your shoes and that was true, I would set the lower one at 750 psig and the upper one at 810 psig. In my mind this does not improve "safety" (the way you're doing it is perfectly safe), but it reduces the amount of time you have to spend explaining an 850 psig set point on an 810 psig system.
David
The key question is what is it that you're protecting? Are you protecting just pipe or are you also protecting one or more pressure vessels with this PSV.
If the PSV is protecting just the pipe, then what you have is perfectly OK. B31.3 allows the pipe to be overpressured by up to 33% for 10 hrs at a time and no more than 100 hrs per yr. That provision is also applicable to any overpressure events.
If this PSV is also protecting a pressure vessel, then the set pressure must conform to the ASME Sec VIII rules for those vessels(max of 100% MAWP, or 105% MAWP for a second device).
If the PSV is protecting just the pipe, then what you have is perfectly OK. B31.3 allows the pipe to be overpressured by up to 33% for 10 hrs at a time and no more than 100 hrs per yr. That provision is also applicable to any overpressure events.
If this PSV is also protecting a pressure vessel, then the set pressure must conform to the ASME Sec VIII rules for those vessels(max of 100% MAWP, or 105% MAWP for a second device).
- Thread starter
- #8
To Zdas04:
Unfortunately this line's operating pressure is 750PSI. I can't set the first PSV's pressure so close to the opr. pressure. The margin between 750psi and 810psi is already narrow.
To don1980:
These PSVs are protecting piping only. The vessel are protected by the designated PSVs.
To Duwe6:
I was in consulting with our regulatory body. Unfortunately they don't allow this ASME 31.3 exemption. So we have to follow the <=design pressure rule. I guess that's one of the cases that local authority/regulation put more restrictions over ASME's.
So StoneCold: we will probably end up doing your way. Using only one PSV at a time and change the 850psi to 810psi. Just need to be careful when operator opening the block valve on the run. They need to slowly crack open it to prevent thermal impact. Do you have similar procedure in place for that?
And zdas04, I have same question for Yuze
Unfortunately this line's operating pressure is 750PSI. I can't set the first PSV's pressure so close to the opr. pressure. The margin between 750psi and 810psi is already narrow.
To don1980:
These PSVs are protecting piping only. The vessel are protected by the designated PSVs.
To Duwe6:
I was in consulting with our regulatory body. Unfortunately they don't allow this ASME 31.3 exemption. So we have to follow the <=design pressure rule. I guess that's one of the cases that local authority/regulation put more restrictions over ASME's.
So StoneCold: we will probably end up doing your way. Using only one PSV at a time and change the 850psi to 810psi. Just need to be careful when operator opening the block valve on the run. They need to slowly crack open it to prevent thermal impact. Do you have similar procedure in place for that?
And zdas04, I have same question for Yuze
With the operating pressure of 750 psig (93% of set pressure), you cannot lower the set pressure without causing the PSV leaking. There are two scenarios: 1, you take out the PSV with lower pressure setting, resulting in relief pressure higher than MAWP which may be OK with ASME 31.3 exemption for piping. However, the key point is the relief capacity. You don't have enough relief capacity! 2. You take out the PSV with higher setting pressure, the result is the same that you don't have enough relief capacity. You have to have an operating procedure to relief the pressure during emergency.
Yuze
wangyo98 did not say that one relief valve did not have enough capacity to serve the piping, you made that statement.
wangyo98 After seeing the operating pressure is at 93% of the relief valve set pressure I am surpised that you are not having problems all the time. You are really in a rough spot.
wangyo98 did not say that one relief valve did not have enough capacity to serve the piping, you made that statement.
wangyo98 After seeing the operating pressure is at 93% of the relief valve set pressure I am surpised that you are not having problems all the time. You are really in a rough spot.
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