Relief piping configuration question

[knapee]

Dear eng-tips member

I have a simple question about pressure relief piping question.

1. We often see the single relief valve, but sometimes the spare pressure relief valve
is required and I would like to know how to determine the spare PSV required or not.

2. Each pressure relief valve have inlet block valve (CSO/CSC) and discharge block valve
to allow easier and safter isolation for removal, inspection and test. But sometimes
I didn't see inlet block valve and outlet block valve but only rupture disk. And sometimes
there is inlet block valve and repture disk before the PSV. Could you please tell me how
to determine when the repture disk and inlet block valve shall together and sometimes
only rupture disk is enough ?

3. The bypass valve often connected upstream of the inlet block valve and downstream of the
discharge block valve. But, sometimes double block with an intermediate drain valve
betweem block valve is used. Could you please guide me when the double blocks with intermediate drain
should be provided and sometimes bypass line don't required to be set up?


Thank you for your help and consideration.

 

[zdas04]

There are a lot of answers to your questions and I'm sure that there will be folks who disagree with mine.

First the applicable code doesn't specify any of the things you mentioned. They are all the adaptations of individual engineers to try to address specific issues in specific facilities.

1. The code does say that you must provide over pressure protection that will handle all the credible scenarios and prevent exceeding the overpressure limitations (10% for most cases). Sometimes your most likely scenarios will have vastly different relief requirements. It is common to size a PSV for the smaller scenario and set it at or below MAWP, then size another PSV for the bigger scenario and set it below the allowable overpressure. Since lifting a 6X8 PSV in a thermal overpressure scenario would dump far more fluid than required, this can be a really good idea. On the other hand setting two PSV at the same pressure is pretty stupid since both of them will have a tolerance and you never know which one will open. If you only have one credible over pressure scenario or all of them have similar relieving requirements then you should generally provide the protection with a single valve (no installed spare).
2. Generally, you put a rupture disk under a PSV if you are worried about your fluid being inappropriate to hold stagnant against the PSV (e.g., if the fluid has significant scaling tendency, is very corrosive, or is poisonous). If you have a rupture disk under a PSV then you need to reduce your relieving capacity by a percentage laid out in the code. I never treat a rupture disk as isolation. If I only have a rupture disk under a PSV then I'll depressurize the vessel before pulling the PSV. If I want to leave the vessel in service then I'll have a lockable block valve under it. Block valves on the PSV outlet are a specific adaptation to flare headers--they allow you to test a PSV without exposing the operating personel to the other streams that may enter the flare header.
3. Bypasses leak. It is unfortunate, but it happens. If you have a vessel operating at a very low pressure with a PSV going into a flare header, then another PSV opening can cause the direction of the dP on a bypass to change which will put fluid from the flare header into the low pressure vessel. If this is intolerable, then it can be pretty much prevented by venting between a pair of block valves. When you think about it, a "PSV bypass" is simply a method to blow the vessel down into the flare header. May be needed for operational reasons, but they are never "required" by code.

David

 

[MortenA]

With regards to item 1 - if you are thinking about 100% sparing of valve (with interlocking the criteria for installing this could/should be:

Is the vessel in continious service and will the plant (rembeber the the flare/flare hearder) "never" shut down for inspecing/maintenance. Then a 100% spare is practival since inspection of the PSV will require that the vessel is taken out of service and isolated - including the flare connection.

If you dont have a spare with interlocking valves then you would need that plant to be shut down (including the flare header) before you could inspect the PSV. If you have a spare then you can inspect the PSV while the plant/train continues.

Save the spare if your vessel only see intermittant service. I know that ASME dont like (prohibits?) block valves on PSV's. As far as i know this is no problem in Europe. But interlocking or "locked open" is imo better than car sealed. Without a block valve you will still have to have the flare header shut down prior to inspection of the PSV/vessel.

Best regrads

Morten