Dear All,
Please provide basis of PSV sparing for multiple relief valves. In case for a gas blowby case due to very high relief capacity there are multiple PSVs mounted on a vessel e.g. 3 number on line (two of one orifice type while the other of different orifice type) with staggered set pressure, then whether only one spare PSV can be considered and what should be the set pressure of the spare PSV.
Regards,
somen
Full Version: Psv Sparing Philosophy
Read section 5.21.4 of the latest API Standard 521 for a full explaination of why you might consider using multiple relief devices.
Some reasons are:
1. Some times multiple relief devices are used as redundant spares. In this case, they have the same set pressure.
2. Sometimes the required relieving rate is just so big that it is impracticle to find a single relief deivce big enough to handle it so you use multiple relief devices to handle the totla flow. In this case one relief device can be set at MAWP and the second can be set at 105% of MAWP.
3. Sometimes you want to use multiple relief devices where one may be set at a lower pressure to handle a lesser relieving scenario. For example, you can use one for all scenarios but fire and have a second dedicated to fire. In this case the first may be set at MAWP and the one for fire only can be set up to 110% MAWP.
4. Sometimes you need to minimize the reactive thrust you would get at one big relief so you use multiple relief devices to open at different pressures to minimize these forces.
There are a couple of more, which you can get from the reference.
There is no rule for minimum staggered set pressures other than practical engineering design. There is a maximum as I shown above.
Some reasons are:
1. Some times multiple relief devices are used as redundant spares. In this case, they have the same set pressure.
2. Sometimes the required relieving rate is just so big that it is impracticle to find a single relief deivce big enough to handle it so you use multiple relief devices to handle the totla flow. In this case one relief device can be set at MAWP and the second can be set at 105% of MAWP.
3. Sometimes you want to use multiple relief devices where one may be set at a lower pressure to handle a lesser relieving scenario. For example, you can use one for all scenarios but fire and have a second dedicated to fire. In this case the first may be set at MAWP and the one for fire only can be set up to 110% MAWP.
4. Sometimes you need to minimize the reactive thrust you would get at one big relief so you use multiple relief devices to open at different pressures to minimize these forces.
There are a couple of more, which you can get from the reference.
There is no rule for minimum staggered set pressures other than practical engineering design. There is a maximum as I shown above.
QUOTE (pleckner @ Jul 3 2007, 06:20 PM) 
Read section 5.21.4 of the latest API Standard 521 for a full explaination of why you might consider using multiple relief devices.
Some reasons are:
1. Some times multiple relief devices are used as redundant spares. In this case, they have the same set pressure.
2. Sometimes the required relieving rate is just so big that it is impracticle to find a single relief deivce big enough to handle it so you use multiple relief devices to handle the totla flow. In this case one relief device can be set at MAWP and the second can be set at 105% of MAWP.
3. Sometimes you want to use multiple relief devices where one may be set at a lower pressure to handle a lesser relieving scenario. For example, you can use one for all scenarios but fire and have a second dedicated to fire. In this case the first may be set at MAWP and the one for fire only can be set up to 110% MAWP.
4. Sometimes you need to minimize the reactive thrust you would get at one big relief so you use multiple relief devices to open at different pressures to minimize these forces.
There are a couple of more, which you can get from the reference.
There is no rule for minimum staggered set pressures other than practical engineering design. There is a maximum as I shown above.
Some reasons are:
1. Some times multiple relief devices are used as redundant spares. In this case, they have the same set pressure.
2. Sometimes the required relieving rate is just so big that it is impracticle to find a single relief deivce big enough to handle it so you use multiple relief devices to handle the totla flow. In this case one relief device can be set at MAWP and the second can be set at 105% of MAWP.
3. Sometimes you want to use multiple relief devices where one may be set at a lower pressure to handle a lesser relieving scenario. For example, you can use one for all scenarios but fire and have a second dedicated to fire. In this case the first may be set at MAWP and the one for fire only can be set up to 110% MAWP.
4. Sometimes you need to minimize the reactive thrust you would get at one big relief so you use multiple relief devices to open at different pressures to minimize these forces.
There are a couple of more, which you can get from the reference.
There is no rule for minimum staggered set pressures other than practical engineering design. There is a maximum as I shown above.
Dear Phil,
Is it possible for you to give us a numerical range for relieving loads about item 2 that we use 2 PSVs because of high relieving loads?
Also I confused with item 3 because we usually consider all the credible scenarios and size safety valve for the worst one but item 3 is totally new for me! could you please explain more about this item and its application?
And finally about item 4: Should the process engineer consider this facility? it seems that mechanical engineer should calculate this reactive force and warn about its high value.Am I right?
Thanks in advance.
Question #1: From my point No. 2, only you and your instrument engineer can determine this. You calculate the required relieving load as if you were going to use one PSV and see if you can find one to fit. If not or it will be too big then you need to break it up into smaller PSVs.
Question #2: From my point No. 3, you can size a relief device for a fire case only, if you want to and at a higher set pressure than MAWP and still size the other relief device for the worst case of the non-fire cases at a set pressure at MAWP. This technique is more typically used when there is a chance for run-away reactions at excessive heat.
Question #3: From my point No. 4, maybe. I would let a piping designer do it if it was up to me. However, if you were using some software, say SuperChems for instance, it could give you the reaction forces as part of the output.
Question #2: From my point No. 3, you can size a relief device for a fire case only, if you want to and at a higher set pressure than MAWP and still size the other relief device for the worst case of the non-fire cases at a set pressure at MAWP. This technique is more typically used when there is a chance for run-away reactions at excessive heat.
Question #3: From my point No. 4, maybe. I would let a piping designer do it if it was up to me. However, if you were using some software, say SuperChems for instance, it could give you the reaction forces as part of the output.
QUOTE (pleckner @ Aug 5 2007, 07:29 PM)
Question #1: From my point No. 2, only you and your instrument engineer can determine this. You calculate the required relieving load as if you were going to use one PSV and see if you can find one to fit. If not or it will be two big then you need to break it up into smaller PSVs.
Question #2: From my point No. 3, you can size a relief device for a fire case only, if you want to and at a higher set pressure than MAWP and still size the other relief device for the worst case of the non-fire cases at a set pressure at MAWP. This technique is more typically used when there is a chance for run-away reactions at excessive heat.
Question #3: From my point No. 4, maybe. I would let a piping designer do it if it was up to me. However, if you were using some software, say SuperChems for instance, it could give you the reaction forces as part of the output.
Question #2: From my point No. 3, you can size a relief device for a fire case only, if you want to and at a higher set pressure than MAWP and still size the other relief device for the worst case of the non-fire cases at a set pressure at MAWP. This technique is more typically used when there is a chance for run-away reactions at excessive heat.
Question #3: From my point No. 4, maybe. I would let a piping designer do it if it was up to me. However, if you were using some software, say SuperChems for instance, it could give you the reaction forces as part of the output.
For point #1, it could be restricted by API 526 (some company use only API 526 standard PSV)...and further restricted by COMPANY requirements (e.g. SHELL) for whatsoever reasons (failure history, standardization, cost effective, handling, etc)
JoeWong
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