Suction throttling vs discharge throttling

[roteng518]

Anyone can tell me the difference of suction throttling and discharge throttling? In the centrifulg compressor, what does them really mean? In API-617 compressor datasheet 1st page, suction throttling from ? pisa to ? pisa is a process control method you can chose, what are these two pressures and how to determine them?

Thanks

 

[dcasto]

A centrifugal compressor is a constant head system. The compressor will "throw" the gas up in the air a fixed number of feet. You then multiply that number of feet times the density of the gas to get psi difference across the compressor.

so if the compressor will put up 5000 feet of head on a 3.8 lb/ft^3 gas, thats 5000 ft * 3.8 lb/Ft^3 / 144 in^2/ft^2 = 131 psi. if the suction is 700 psig, the the discharge will be 831. If the delivery pipeline is 800, the compressor will move on its volume vs head curve to a new volume where the head is 100 psig, this may be more volume than you have available.

SO, you can cut the pressure on the outlet to 800 or you can cut the pressure on the suction to 770 psig to get the 800.

If the gas is dry, no water, not condensibles, then cut the inlet pressure to control. If not, then cut the outlet pressure.