Pump Cativation Caused By Entrained Gas



Here’s how the problem was solved on an FCC main fractionator.



When refinery engineers think of pump cavitation, what first comes to mind is usually insufficient net positive suction head (NPSH) available. Symptoms include erratic pump flow and discharge pressure, a distinct crackling noise coming from inside the pump, and severe suction line vibration. Cavitation is caused when vapor bubbles formed inside the pump or suction line implode as pressure increases inside the pump. Much less frequently, entrained gas in the fluid being pumped causes the same symptoms.

A case study presents troubleshooting an FCC main fractionator revamp including the LCO pumparound (PA) pump that was cavitating due to entrained gas. Here pump cavitation and column flooding caused reduced column heat removal, decreased column capacity, degraded fractionation, lower-than-design unit capacity and high endpoint gasoline. The short-term remedy and longer-term modifications to correct the root cause are discussed.

An FCC revamp failed to meet two of its processing objectives: higher FCC charge and better gasoline/LCO fractionation. After startup, poor fractionation, low LCO PA heat removal, lower than design charge rate and unstable operation were the norm. Gasoline endpoint could not be controlled and logical operating variable changes had no influence on gasoline quality. Attempts to increase LCO PA rate resulted in erratic pump flow and rapidly oscillating discharge pressure, both common symptoms of pump cavitation. Because the LCO PA is used to reboil the gas plant deethanizer, a consequence was poor hydrogen sulfide and C2 removal from the debutanizer feed. Yet prior to the revamp, the LCO PA pump had operated at much higher flow without showing any cavitation symptoms...


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