Some concluding comments on piping layout for pumps and compressors

4 Some concluding comments on piping layout for pumps and compressors 

Irrespective of considerations of NPSH, the pump suction line should direct the liquid smoothly into the pump suction without any bends or flow disturbances close to the pump, and without pockets where vapours or air can be trapped, eventually to be sucked into (and cause damage to) the pump. Very long suction lines should be avoided. 

In the case of compressors, the fundamental performance parameter is pressure ratio, which has a direct relationship to the head. A compressor of any type tends to be rated to achieve a certain pressure ratio for a given flowrate. A restriction in the suction system is more detrimental to performance than a restriction in the discharge. If a compressor is rated to compress a gas from 1 bar absolute to 10 bar absolute, it will be able to maintain a pressure ratio of 10 under varying suction pressure. The temperature ratio (often the limiting machine characteristic) corresponds to the pressure ratio. Thus a pressure loss of 0.2 bar in the suction line will result in a pressure of only (1.0 - 0.2) X 10 = 8 bar in the discharge for the same flow, or for the same temperature rise per stage. 

As vapour bubbles tend to damage pumps, so do liquid inclusions tend to damage compressors suction systems must avoid any possibility of liquid build-up or carry-over. 

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