Reflux pump, cavitation

Naturally, we do not want to overpressure the tower, and pop open the safety relief valve. Alternatively, if the tower pressure gets too low, we could not condense the reflux. Then, the liquid level in the reflux drum would fall and the reflux pump would lose suction and cavitate. But assuming that we have plenty of condensing capacity, and are operating well below the relief valve set pressure, why do we attempt to fix the tower pressure Further, how do we know what pressure target to select ... 

Once a liquid level reappears in the reflux drum, the condenser capacity has been exceeded. The level in the condenser will continue falling, until the drum empties, and the reflux pump begins to cavitate. 

Reflux drum level indicator, level gage, and level alarm were cormected to the same tape. The ui )er t plugged, and all became erratic. This caused liquid to flow into the flare because of excessive level. Later, the reflux pump blew a seal because of cavitation resulting ftom low level... 

Later, I met with unit operating personnel. They were more specific. They observed that the pumparound circulating pump (see Fig. 18-8) was defective. Whenever they raised hot oil flow to the debutanizer reboiler, the gas plant would become destabilized. Reboiler heat-duty and reflux rates would become erratic. Most noticeably, the hot-oil circulating pump s discharge pressure would fluctuate wildly, indicating cavitation. They felt that a new pump requiring less net positive suction head was needed (see Chapter 11). 

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