Effects of entrained gas



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 [ * ]



[ * ] Some information are excluded.
Read more.



R: 0110-EFOF
C: design, operation
F: entrained gas


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