***** System Pressure Relief

[trong40]

Hi,

I am looking at a commercial refrigeration unit and wonder why the plant has a dual pressure-relief valve on every single piece of equipment. I could not not come up with a whole lot of relieving scenarios for this two-stage system.

Could you please help to explain? Thank you

[Prof Sporlan]

Pressure relief valves may be required if the system uses isolation valves. If one is able to trap liquid refrigerant using isolation valves in a manner that would create hydrostatic pressures, then a relief valve must be used.

You'll need to provide more information about your system.

[trong40]

Hi Prof.,

The system I looked at is a R-22 unit part of a chemical unit. The system cosists of a high pressure and a low pressure system.

The HP system has RV's on the condenser, the reciever, oil still, and all chillers, etc.

The HP compressor max discharge pressure (225 psig) is below the MAWP (300 psig) of all vessels involved. The R22 vapor is on the shellside of the CW condenser.

About the liquid being trapped when all outlet valves closed, did you mean the potential source of overpressure is the trapped liquid being heated by solar energy?

Thank you.

[Andy]

Hi
dual reliefs are so the system can be put back into safe operation should one relief lift the other will be out of circuit, ready for operation when the valve is turned to take the used relief out of circuit and the new PRV into circuit.
Regards. Andy

[trong40]

Thanks Andy,

I understood why it has to be a dual pressure-relief valve. What I am looking for is the root cause of overpressure in the system so that I could justify the needs for having RV's on every single piece of equipment in the system. I wonder if this is required by Codes.

Thanks again.

[Andy]

Hi
each vessel requires a PRV, that way if a vessel is isolated from the system with refrigerant liquid within it the liquid will be heated up, and may vaporize craeting higher than design pressures. In effect the PRV stops the component from being suject to pressures that it was not designed for.
Hope this helps. Regards. Andy.

[adacus1]

Each section of isolated pipewrk or component that could contain liquid isolated fromeach requires a pRV or similar. This is to prevent the system becoming a bomb in the event of unaccountable situations. Would you want to be a fireman walking into a supermarket freezer section knowing there are 13 case bombs waiting to go off around you at the weakest point in the system

[trong40]

The key point is that if a piece of eqipment or piping being isolated. In my particular case, every piece of equipment has one small PSV regardless of isolation..

The unit was built more than 15 years ago. I wonder if this is required by CODES?