Multiple relief valves when reliability of single valve is questionable?
Multiple relief valves when reliability of single valve is questionable?
(OP)
Dear all
I need your help please.
I believe/read multiple relief devices are installed when
1) sparing (duty and standby) philosophy is required (maintenance purpose)
2) release rate is large and is to be compensated by more than one relief valve/device
3) two very different release rate are required say 2" valve for vapor phase release and 12" bursting disc for two phase release.
I have a situation where it is very critical to protect the vessel and we have 1off relief valve and there was a discussion on its reliability I.e. Whether it will be activated during relief scenario or not and wondering if another identical relief valve could be installed?
I need your help please.
I believe/read multiple relief devices are installed when
1) sparing (duty and standby) philosophy is required (maintenance purpose)
2) release rate is large and is to be compensated by more than one relief valve/device
3) two very different release rate are required say 2" valve for vapor phase release and 12" bursting disc for two phase release.
I have a situation where it is very critical to protect the vessel and we have 1off relief valve and there was a discussion on its reliability I.e. Whether it will be activated during relief scenario or not and wondering if another identical relief valve could be installed?





RE: Multiple relief valves when reliability of single valve is questionable?
If you intend to install another PSV (of the same size) in parallel and keep both relief valves in service, you will likely end up with both valves damaged in case of an overpressure event, due to chattering. For applications with two PSV's in parallel, total relief capacity should not exceed 140% of the required capacity and the PSV set points should be staggered. Obviously, with 2x100% valves you are far above the 140% figure.
On the other hand, it is a good practice to consider parallel (2x100%) PSV's for maintenance and reliability reasons. Normally only one PSV is in service and the other one is isolated. This allows you to perform the required maintenance without shutting down the equipment/process. I found this arrangement to be justifiable on many occasions.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Multiple relief valves when reliability of single valve is questionable?
Best regards, Morten
RE: Multiple relief valves when reliability of single valve is questionable?
But pls note that rupture discs are known to burst when not required to - it is typical to install rupture sensors on these RDs' for this reason. Also you will have to shutdown if a RD bursts, which is not the case with a PSV.
If you can describe the concerns you have stated, that will help
RE: Multiple relief valves when reliability of single valve is questionable?
RE: Multiple relief valves when reliability of single valve is questionable?
Dependant on application, you can also have a pilot operated safety relief valve with more than one pilot controller. 1 Main valve is controlled by a pilot at any one time.
Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.
RE: Multiple relief valves when reliability of single valve is questionable?
RE: Multiple relief valves when reliability of single valve is questionable?
If corrosion products or the fluid itself might cause blockage, a single PSV is an incorrect application in the first place. You can install 10 of them in parallel but the problem will remain. If a lot of dirt/rust gets accumulated upstream of the rupture disk, relief capacity of the downstream PSV may become compromised as well during an overpressure event.
Definitely more information is required, unless we have given the answer already.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Multiple relief valves when reliability of single valve is questionable?
Thanks for your valuable input.
Dejan Ivanovic's first post answered all I wanted. Thanks. It was just a concern without any reason of corrosive or viscous fluid etc. Agreed, regular maintenance and testing of relief should be done properly. Is there any guide on frequency of testing the installed relief valves?
Thanks
KS
RE: Multiple relief valves when reliability of single valve is questionable?
Quote from API RP 576 Inspection of Pressure-Relieving Devices: "The inspection of pressure-relieving devices provides data that can be evaluated to determine a safe and economical frequency of scheduled inspections. This frequency varies widely with the various operating conditions and environments to which relief devices are subjected. Inspections may usually be less frequent when operation is satisfactory and more frequent when corrosion, fouling, and leakage problems occur. Historical records reflecting periodic test results and service experiences for each relief device are valuable guides for establishing safe and economical inspection frequencies. A definite time interval between inspections or tests should be established for every pressure-relieving device on operating equipment. Depending on operating experiences, this interval may vary from one installation to another. The time interval should be sufficiently firm to ensure that the inspection or test is made, but it should also be flexible enough to permit revision as justified by past test records."
Some additional information is pasted below. You can also refer to Marc Hellemans' "The safety relief valve handbook", Chapter 10 "Maintenance and testing".
https://www.nationalboard.org/SiteDocuments/NBIC/N...
http://www-group.slac.stanford.edu/esh/eshmanual/r...
http://frame.leser.com/engineering/file/EHB_en_6.2...
http://valves.pentair.com/valves/Images/PVCMC-0296...
http://www.icheme.org/communities/subject_groups/s...
Dejan IVANOVIC
Process Engineer, MSChE
RE: Multiple relief valves when reliability of single valve is questionable?
Also I couldn't find any reference for the statement where you said total relieving capacity (of more than one valve?) should not be increased more than 140% of required capacity.
Thanks in advance.
KS
RE: Multiple relief valves when reliability of single valve is questionable?
Arrangement of 2x100% relief valves would result - in the ideal world - in active 50% capacity of each installed PSV, during overpressure event. In reality, due to possible differences in friction losses in the associated pipework and differences in the actual set points of each PSV (+/- 3% tolerance on the set point), one of the two PSV's would start to experience "starvation of flow", and subsequent chattering.
This cyclic operation happens because of "over-relief" (remember that there is effective 200% relief capacity or even more than 200%, depending on how much the actual relief capacity is higher than the required capacity, based on the orifice size chosen).
Dejan IVANOVIC
Process Engineer, MSChE
RE: Multiple relief valves when reliability of single valve is questionable?
a) Installing a PSH with SDV to isolate the source of high pressure
b) That PSH setpoint is no more than 90% of PSV setpoint for a conventional PSV; or 95% for a pilot operated PSV
Do you already have this ?
In addition, it is also good practice to check that the increase in pressure in the vessel after PSH has triggered as a result of the time taken for the Auto isolation valve to close is still within the 90% of PSV setpoint (or 95% whichever type of PSV you have) - this time taken is called the process safety time.
That way, we can reduce the demand frequency on the PSV.
RE: Multiple relief valves when reliability of single valve is questionable?
- Instantaneous overload of the flare network due to both valves being open (for a short time but the incipient flows can be quite excessive);
- Unavoidable chattering if the valves pop open under any other than the governing relief scenario (which results in much smaller relief flows) - see attachment.
Having two PSV's (one online, one standby) is generally a good practice and in many occasions the best practice which can often save you time and money. Having two PSV's (2x100%) online is a bad and unnecessary practice and it may result only in bad things.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Multiple relief valves when reliability of single valve is questionable?
I do, however, still think that from a logical point of view if your though that something would cause the reliability of the PSV to be low then this would affect the 2. valve as well?
Best regards, Morten
RE: Multiple relief valves when reliability of single valve is questionable?
For process overpressure cases (other than fire), the 1st valve would have the set point at or below the MAWP. The 2nd valve can then have the set point at or below maximum 105% of the MAWP but to ensure maximum overpressure does not exceed 116% of MAWP. Difference between set points should be higher than the available tolerance, if the concept is to have staggered opening of PSV's (e.g. sized for multiple relief cases which include wide variations of relief flows).
You are correct on the second issue. It is hard to imagine any increase in reliability if two online relief valves are installed in parallel and the service is corrosive or dirty. Then the right choice is what you have proposed in your post - either a rupture disk followed by a PSV, or a rupture disk in parallel.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Multiple relief valves when reliability of single valve is questionable?
A disk ahead of a PSV is only safer if a truly reliable telltale system is installed between the two. In my book, that means that the telltale system MUST be capable of both detecting leaks across the disk leading to pressurization of downstream side of the disk, AND of venting the space between the disk and the valve inlet such that the disk can still burst at or near its bursting pressure. The telltale system must perform BOTH functions automatically and reliably. If you don't need an automatic telltale or venting, you likely will gain no benefit from the disc itself if you think about it a little more.
The services that scare me most are the ones where materials can solidify in the relief nozzle neck and do so without warning. In those services, a flush-mounted rupture disc is one of the few solutions which can be relied upon, but it also can have lots of downsides.
RE: Multiple relief valves when reliability of single valve is questionable?
The buckling pin can be changed inline in a few minutes and has many advantages in comparison to an RD
http://www.valve-world.net/pdf/rpt_rupture_pin_ver...