Reciprocating Compressor: Differential Pressure

[RefineryRR]

Does a recip compressor (which takes suction from a header and discharges into a header) maintain a fixed differential pressure across the cylinder if the speed of the machine and suction temperature of the gas remain constant?
Or does the compressor react to the pressure of the discharge header? That is, if the suction pressure drops from 20 bar to 10 bar but the discharge header pressure is constant at 40 bar, does the pressure ratio of the cylinder change from 2 to 4?
In my mind, the discharge pressure should be relatively fixed as the discharge valve will not open until the pressure in the cylinder reaches the discharge header pressure. But if the suction pressure is lower, the DP across the cylinder increases, and therefore the discharge temperature of the gas will increase substantially.

 

[zdas04]

Both the suction and discharge valves are simply spring-loaded check valves. Downstream pressure must be lower than upstream pressure plus spring force for them to open. When suction pressure drops, the piston must move lower in the cylinder to open the check. When suction pressure rises the suction valve will open earlier in the piston travel. Same with discharge. An increase in discharge header pressure will cause the discharge check to open later and vice versa. The discharge temperature is a good indication of where the valves are opening. Big changes in either suction or discharge header pressure can result in very high temperatures.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[georgeverghese]

You also need to look at the process capacity controls for this compressor to see how it would behave.