Compressors and Reverse/Misdirected Flow

On a compressor shutdown there are 2 main scenarios to consider for the reverse/misdirected flow deviation. Compressor settle-out and reverse flow from a high-pressure source connected to the discharge of the compressor.

First let’s take a look at compressor settle-out:

The compressor above is a fairly typical set-up with a suction scrubber, aftercooler, suction and discharge XV’s, and a recycle control valve. In a typical settle-out case, on a shutdown, the inlet and outlet XV’s close, blocking in the volume of gas between the valves. The pressure from the discharge side of the compressor flows back into the suction piping either through the compressor or through the recycle valve and equalizes. The equalized pressure is called the settle-out pressure. There can be a hazard here if there is a pressure spec break from the suction to the discharge. If the settle-out pressure is above the inlet design pressure, it can be overpressured on a compressor shutdown. Typically, this is controlled with either a blowdown on the compressor discharge to quickly relieve the trapped pressure to flare, or a pressure control valve on the suction relieving pressure into the flare system. As a final protection there is also typically a PSV on the suction scrubber sized for the settle-out case.

Other ways to mitigate the hazard are to increase the suction design pressure to match the discharge equipment or to delay the closing of the inlet XV to allow the discharge pressure to equalize into a larger upstream system.

The other case is reverse flow from a high-pressure source connected to the discharge of the compressor:  

Looking at the compressor above, it is injecting into a high-pressure system with a large inventory of high pressure gas downstream. On a compressor trip, pressure from the discharge will begin to push back into the suction system and potentially overpressure the suction scrubber or upstream equipment with lower design pressure. Cases like these are typically controlled by having a discharge XV which closes on compressor suction, and a PSV sized for the reverse flow case, potentially with redundant check valves on the discharge of the compressor.