Centrifugal Air Compressor RV cases

[Bill3752]

I keep running into a similar set of circumstances related to multi-stage centifugal compressor gas systems. While I have seen this on all gas systems, I run into this mainly on air compressors.
Typically, the only RVs are located downstream of the final compressor stage. Often the performance curves indicate that the intercoolers cannot be overpressured due to block flow, which is good. However, my concern is that the coolers be protected in a fire event (system is shut down, fire occurs). Also, since the vessels are ASME, aren't RVs required.
A related issue occurs when the performance curves indicate the intercoolers could be overpressured, but the unloaders/software on the compressors would operate to keep the pressure lower than MAWP. Do I still need RVs?
Thanks in advance for your input.

 

[TD2K]

I question how much the PSVs on the coolers protect you in a fire. Yes, you can calculate the heat input and vapor load as condensed water in the exchanger vaporizes. However, the amount of water in the exchanger is likely to be very little and it will be quickly vaporized. Once that water is vaporized, there's little to keep the metal cool so it heats from the fire and fails once its strength is no longer sufficient for the internal pressure. A PSV won't change that, you need to depressure the system. Code just require you to provide a PSV if there is a way to overpressure the vessel, just don't expect that PSV to protect your vessel during a fire.

I wouldn't depend on unloaders, you are depending essentially on instrumentation to protect against overpressure and that unlikely to be sufficiently reliable. API 520 doesn't allow you to take credit for instrumentation in reducing the relief load (but if the expected response of the instrumentation makes the relief requirement worse you assume it functions as designed).

 

[don1980]

Code compliance requires an RV, HIPS, or UG140 documentation. Code doesn't allow you to omit that just because you have no credible causes for overpressure.

As TD2K say, an RV won't protect this vapor-filled system from fire. So, just for code compliance I'd stick a small RV on the intercoolers and be done. No need to size it for vapor expansion due to fire - that a waste of time and money.