Subatmospheric 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. 31.7 is suffering from a partially plugged in-line suction 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 

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) equals 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. 31.8. 

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Subatmospheric Suction Pressure Pump suction under vacuum... 

Figure 25.7 Pumping liquids with subatmospheric suction pressure.

To fix this problem in Port Arthur, I connected an external source of seal flush water to the pump, from a nearby washwater station. Pumps which have subatmospheric suction pressures, and which 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. 

An important factor in the design of the liquefaction unit is the pressure of the process gas. This is demonstrated in Fig. 6, which shows how much heat must be removed from a pound of methane at 100°F in order to convert it to liquid at 250°F, and at the same time shows the temperature level at which the heat must be removed. Temperature levels of -34 F and -120 F have been indicated as the design limits of propane and ethane refrigeration, respectively, to eliminate the possibility of subatmospheric suction pressures at the refrigerant compressors. 

The need to use suction to produce TMP means that the permeate side of the membrane is under subatmospheric pressure. This has the potential to cause bubble formation as dissolved air comes out of solution. If the bubbles so formed are free to flow with the permeate, the effect on pressure drops will be neghgible. However, if bubbles become stagnant and/or attach to the lumen wall, they will present a restriction to flow causing an added resistance. 

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