10 replies to this topic

[UETLHR]

Dear All,

I have a very simple question, but i'm confused.

We are operating a Natural gas plant where normal plant pressure is 1400 psig. As per design, each pressure vessel contains two PSV's with same design flow rates, whereas even a single PSV suffices the requirement.

We have different openions on keeping both PSV's in service. Some says that both PSVs must be online for enhanced plant safety, whereas to my openion only one PSV must be online while keeping the other on standby or offline.

I want your valuable suggestions/openion, whether to keep spare PSV online or standby? Kindly refer/share any standard practice going on, in the case of spare PSV's.

Best Regards,

Edited by UETLHR, 19 December 2010 - 04:54 AM.

[fallah]

There is no reason for confusing!

Two PSV's, each of them can handle maximum relief load to prevent relevant vessel to be pressurised exceeding to design pressure/MAWP, no need to be online because of chattering possibility and one online PSV would be adequate. Another PSV should be offline for using in the case of maintenance,....

[Jack Follick]

We have many similar installations in our plant, and we always run with just one of the PSV's on line. We lock open the inlet and outlet block valves on the PSV that is in service.

Jack Follick

[fallah]

And for another PSV, the inlet/outlet block valves would be in LC (lock closed) position........

[djack77494]

I know it seems to be common practice to do so, but I see no need to lock closed the "spare" PSV. Just close the inlet valve and it's out of service, as long as there is no problem with having the outlet side exposed to the discharge system (and there shouldn't be). Furthermore, closing the outlet block valve could be dangerous if the outlet piping and valves cannot withstand the inlet conditions. In my opinion, locking a valve should be reserved for avoiding a hazardous situation, and locking closed a PSV does not meet this criterion.

[fallah]

djack77494:

For outlet block valve (of spare PSV) you are right; and not only should it be opened, but also should be CSO.

But for the inlet block valve, at least it should be CSC.

[djack77494]

For outlet block valve (of spare PSV) you are right and not only it should be opened but also should be CSO.

We are agreed on this point.

But for inlet block valve at least it should be CSC.

Why? What hazardous situation are you avoiding? To justify the onerous efforts to maintain administrative control over the position of the inlet block valve, I would say that you should be avoiding a hazard. Yet I cannot identify a hazard that would exist if the inlet block valve is "accidentally" left open.

[fallah]

djack77494:

Please note that we are discussing about spare PSV and you already accepted the above in your post dated Dec. 28, 2010!

Anyway, if the inlet block valve of the spare PSV is left open, in the case of overpressure there is a possibility of simultaneous lifting of the two PSVs leading to chattering that will result in PSVs' damage.

[murthy.nelakanti]

It is very common practice to keep one in service and the other in stand by when each PSVs is designed to handle required relief rate. rather than practice it would be company's philosophy during design stage whether to design each PSV for full load and keep one as standby or no standby. when a single PSV is not available to take the full laod, then will go for two PSVs and both will be online.

Hope this is clear.

regards,
Murthy

[paulhorth]

UETHR,

In my experience it is standard practice of all the major oil companies for all relief valves in hydrocarbon service to have 100% standby valves installed. This has been the case on every plant design I have done. The reason is to allow a valve to be taken out of service for regular retesting,or in case of leakage, while the equipment remains on line. To do this requires isolation valves at the inlet and outlet which must be locked open on the on-line valve. It is usual practice to fit mechanical interlocks to the isolation valves, so that the standby PSV must be correctly opened before the other valve can be isolated. The interlocks work by a key interchange system, forcing the operator to open and close the isolation valves in a predetermined sequence.
The reason for leaving open the outlet of the standby valve is to allow any leakage from upstream to flow away without building up pressure.
Without the interlocks, you have to rely on the operator to ensure the setup is safe, and the operator will get it wrong one day.

Exceptions to the 100% standby rule might be (1) when the vessel itself is duplicated and can be taken off line without a plant shutdown, (2) in air or water service, or in liquid thermal expansion service, where the risk when the single PSV is removed is considered to be low. However this is not really worth taking a chance on. The Piper Bravo disaster, in 1989, when about 160 men were killed, started because a single PSV on a pump which was off-line was isolated, and the pump was put back on line. The flange leaked, starting a fire.

Paul

[paulhorth]

Sorry, I should have said Piper Alpha disaster.

Piper Bravo was the platform built to replace Piper Alpha. I worked on that project. Occidental insisted on duplicate relief valves everywhere, with no exception.

Paul