PSV Inspection on Pentane Tank

[RJB32482]

Hello,
We have installed a full port gate valve before a PSV on one of our pentane storage tanks (about 7'x7') for maintenance purposes. There is only one PSV on this vessel. We have administrative controls to have this tank empty and shutdown before the relief valve is taken off by closing the gate valve and opening a vent after the gate valve.

Is there anywhere is API or ASME that shows that this is incorrect? I know the installation of 2 PSV's with a 3-way valve is mentioned in the standards but this is to do a valve inspection while running.

Thanks.

 

[JoeWong88]

RJB32482 (Chemical),

First, PSV inlet block valve for single PSV installation has served no purpose.

Second, don't see anything "wrong" from code perspective but common "good engineering practice" is to LOCK OPEN the gate valve to avoid inadvertently closure of gate valve.

Third, installation of the gate valve shall ensure broken internal does not block the flow path... Is the gate valve handle incline and facing downward ?

Forth, i think cost for emptying inventory, availability of the service, purging cost, etc would be much more than CAPEX for a spare PSV. Please study and consider a standby PSV for your case.

Fifth, check and see if online testing is possible...

 

[RJB32482]

PSV has been "locked" open by car seal procedure during normal operation.

We have looked at the purging etc and have seen that the cost of a second PSV has bee more than just purging and putting the tank OOS while doing PSV testing for a couple of hours. Administrative procedures only allows the PSV to be off the veseel for a minimum of 4 hours.

Just wanted to know if the 4 hours breaks any codes for pressure vessels.

Thanks for the feedback.

 

[djack77494]

RJB,
I don't believe 4 hours or any other time period is allowable by code. You may be able to substitute an administrative procedure(s) for added protection when the PSV is out of service, but I hardly think that arbitrarily limiting the OOS time period to a MAXIMUM of 4 hours is acceptable. As Joe points out, the gate valve serves no purpose. It merely blocks in the tank. (Or is that your intention - it's not clear.) Also, you don't indicate what you are trying to do. You could test the PSV in place with the gate valve, but this would take just minutes. If you want to remove the PSV for (say) maintenance, why not just purge the tank and then leave it open to atmosphere until the system can be returned to normal? Tell us more about what you're trying to do please.
Doug

 

[RJB32482]

Thanks again for the reply.

The purpose is to take the PSV off of the tank without having to purge the tank. It could be a difficult procedure (this was requested by maintenance).

We do have a conservation vent that opens at 10 PSIG (40 PSIG below MAWP of vessel). This is not locked out during PSV maintenance (operational). I believe it can't relieve the total vapor in the fire case for pentane in the vessel but I don't know off by heart if it can relieve the fire case with only N2 in the vessel.

I have read the code and I have seen the dual PSV service. I took the section as testing the PSV while the vessel was still in service.

Thanks,
Rob B.

 

[dcasto]

There is nothing in the "code" that prevents you from operating with the valve under the relief valve closed. It is your (or as the code says "the owner operators") decision on how you want to handle the risk. I've taken relief valves off and repaired them while product was in the vessel. We call it risk management and all work is by permit issued from the proper authority.

We would empty the vessel as much as possible. Isolate it. then have a person with his and on a vent valve ready to open it in an emergency.

Think about this. During an inspection you find your psv is defective in place. It won't open. Now what do you do. Its full of product and it'll take 8 hours to empty. Have you broken some 4 hour rule?

 

[djack77494]

dcasto makes some good points, and I am pretty much in full agreement with him. My earlier comment, "You may be able to substitute an administrative procedure(s) for added protection when the PSV is out of service" essentially says something very similar to what dcasto is saying (i.e. risk management).