Sub-atmospheric Suction Pressure

Liquids in storage tanks are almost always subcooled. This is so because otherwise the ambient vapor losses from the tank s vent would be excessive. This creates the potential for a negative pump suction pressure. 

For example, the pump shown in Fig. 36.7 is sufFering from a partially plugged in-hne suclion filter. The positive pressure to the filter of 5 psig is due to the 15-ft head of liquid in the tank. The filter AP is 12 psi. Hence, the pressure at the suction of the pump is —7 psig. Since atmospheric pressure happens to be 15 psia on this particular day, the pressure at the suction of the pump is 8 psia. 

The liquid being pumped is methanol, which has a vapor pressure, at the pumping temperature of 100°F, of 3 psi. The physical pressure at the suction of the pump (8 psia) minus the vapor pressure of the liquid at the suction of the pump (3 psi) ecjuals 5 psi. To convert the 5 psi to feet of head  

77 is the density of methanol. The 15 ft of head is the available NPSH to this pump. Does this mean that pumps may have a substantial amount of available NPSH even when their suction pressure is under a partial vacuum Yes, if we are pumping a subcooled liquid. But this is quite common, because the liquid stored in an ordinary atmospheric-pressure storage tank is almost always well below its boiling point—that is, the liquid is subcooled. 

The most common pump used is the sump pump shown in Fig. 36.8. The vast majority of the pumps in the world are of this type. They are the sort of pumps used to pump water out of shallow wells and from irrigation ditches. In New Orleans, we use thousands of these pumps to push rainwater over the levees and into the mighty Mississippi River. 

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The majority of industrial slurry-transport installations tend to be free of cavitation and related problems with pump suction performance, but in many other applications cavitation is important and must be considered in system design. In disposal pipelines operated by the dredging industry and also in pipeline applications such as those used by the phosphate extraction industry (for example in Florida, U.S.A.), the first (upstream) pump is susceptible to highly sub-atmospheric suction pressures (see Fig. 1). In these cases, cavitation is a common occurrence, and may be significant in determining the costs of operation. 

To fix this problem in Port Arthur, 1 connected an external source of seal flush water to the pump, from a nearby washwater station. Pumps that have sub-atmospheric suction pressures, and that are not in continuous service, should not be self-flushed pumps. They should have an external source of seal flush material connected to the mechanical seal. 

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