Q - We have an application for pumps in NGL recovery system where the liquid has entrained gas. What are the effects of entrained gas on pump performance? What precautions should we take?
A - Studies show that entrained gas up to [ * ]% by volume has no detrimental effect on pump performance, about [ * ]% to [ * ]% causes a small amount of performance impairment, and above [ * ]% causes severe performance degradation.
The studies were made over short periods of time so the long term effect of entrained gas on pump reliability is still uncertain. Also the studies were done on entrained gas, as opposed to dissolved gas.
Some precautionary measures for pumps handling liquids with entrained gas are:
Select a pump with rated flow close to best efficiency point (BEP.) Avoid operating at low flow condition; provide a low flow by-pass if needed.
Avoid a unit with high suction specific speed (NSS); use double suction impeller, or reduce speed, if possible. It is recommended that the NSS should be less than [ * ] - the lower the NSS, the better it is.
Maintain ample NPSH margin. Normally, an NPSHA margin of [ * ] %, or [ * ] feet, whichever is higher, is acceptable. For liquids with entrained gas it is recommended to maintain [ * ] as much NPSH margin, than normal, to provide ample vapor-suppression suction head.
Entrained gas may cause the pump to develop less head than normal. Increase the impeller diameter a bit to compensate for the head loss. Or, size the impeller diameter for positive head tolerance allowed by the user.
Use materials that are more cavitation-resistant. Some typical casing and impeller material options are [ * ]