Multiple relief valves when reliability of single valve is questionable? 2

[chemks2012]

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?

 

[EmmanuelTop]

The concern that the PSV "will not open" is not a legitimate concern - there are inspection and certification requirements in each country, to ensure that PSV will operate when needed.

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

 

[MortenA]

@chemks2012, if your concern is cause by the medium (e.g. its agreesive) then you could consider installing an upstream bursting disc with the same SP as the PSV. The bursting disc could be of some exsotic material or just replaced once in a while.
Best regards, Morten

 

[georgeverghese]

There are some cases where a rupture disc is installed to enable very fast relief - one example described in API 520 is that for tube rupture on a HX where the diff press between tubeside and shellside is >1000psi.

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

 

[MortenA]

I didnt recommend a ruputere disc INSTEAD of a PSV, but in series (and upstream) since it will "protect" the PSV from the medium and thus mean that risk of e.g. corrosion is managed

 

[The Obturator]

Per comments making one PSV isolated. A changeover valve is a solution (1 valve active, 1 valve closed. Easy to switch between the 2 without shutdown. Or even 1 PSV active, 1 bursting disc).

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.

 

[MortenA]

Try and re-read the post - its not a spare for maintenance hes asking about - its IF you can reduce the risk of failure to work on demand by installing a parallel active PSV. I think if you want to go "all in" you should review this as a LOPA exercise. IMO the reason that one PSV might fail might be a "root cause". E.g. if its an abrasive fluid that might cause internal corrosion - that would affect the other PSV as well. Add to this then fact mentioned that you will gt much higher capacity - and then its not a good idea. Here my suggestion to add an upstream bursting disc might do the trick - but again it depends on why hes concerned with the reliability of the PSV. A parallel bursting disc e.g. with a higher SP. But operations might be concerned if the maximum operation pressure is close to the PSV SP.

 

[EmmanuelTop]

Perhaps the OP should come back and put some more light on his original query.

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

 

[chemks2012]

Hello all
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

 

[EmmanuelTop]

This depends on the PSV application/service and local or international codes which are mandatory to be followed. The frequency of testing may be increased based on first-hand experiences with PSV performance or manufacturer's recommendations.

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/SiteD...re Relief Device Inspection Guide 1-19-10.pdf

http://www-group.slac.stanford.edu/esh/eshmanual/references/pressureReqInspectMaintRepair.pdf

http://frame.leser.com/engineering/file/EHB_en_6.2-Installation-of-the-Safety-Valve.pdf

http://valves.pentair.com/valves/Images/PVCMC-0296-US_tcm106-35825.pdf

http://www.icheme.org/communities/s...chive/S110 - Preventing Accidents/S110-13.pdf

Dejan IVANOVIC
Process Engineer, MSChE

 

[chemks2012]

Thanks very much for that Dejan Ivanovic

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

 

[EmmanuelTop]

The 140% figure is a rule of thumb and it is not explicitly given in ASME (and other codes), if I remember well.

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

 

[georgeverghese]

It is possible to reduce the frequency of operation of a PSV by ensuring BOTH of the below:

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.

 

[EmmanuelTop]

The arrangement with 2x100% relief valves also poses other problems such as:

- 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

 

[MortenA]

Dejan, could you then install the 2. valve with a 6% higher SP (and allow a max 116% overpressure? API 520 compared to the low SP valve allows this but for cases where a single valve is not big enough). I assume that the low SP PSV must then have a SP below MAWP?

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

 

[EmmanuelTop]

Hi Morten

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

 

[moltenmetal]

If the mode of failure expected is common between the two devices selected, adding a 2nd one will not improve the reliability. Adding the 2nd device and increasing the testing and maintenance frequency, which may be made safer and easier by the presence of the 2nd device, would actually help reduce the risk of failure on demand. Adding a different type of device or improving the device installation may also improve the reliability.

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.

 

[georgeverghese]

Another supplementary / alternate device to a PSV ( other than a RD ) is the buckling pin operated relief valve - this is now generally accepted as an ultimate overpressure protection device also.

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_versus_disc.pdf