Liquid is forced into the pump's suction when a partial vacuum, or a pressure reduction, is created from the fluid discharge out of the centrifugal pump housing by the impeller. Atmospheric pressure, or line pressure, causes fluid to flow into the pump because of the pressure reduction within the pump. A positive head on the suction side of the centrifugal pump is desirable to prevent the partial vacuum from becoming too low. In reciprocating pumps, this pressure reduction is created by the piston, in rotary pumps by the gear action, and in centrifugal pumps by the liquid forced out of the pump discharge.
One pound per square inch of water pressure is equivalent to 2.31 feet of head in a water column. Atmospheric pressure equals 14.7 pounds per square inch at mean sea level:
14.7 x 2.31 = 33.9 feet (one atmosphere of pressure, on earth, is equivalent to 33.9 feet of head of water)
This is the maximum theoretical suction lift a pump should attain. There are, however, various losses that reduce this theoretical lift including losses through pistons, pump cams, stuffing box leakage, and so forth. Losses in positive-action pumps are less than losses in centrifugal pumps. The maximum practical suction lift in positive acting pumps—based on "water-like" fluid viscosity—is 22 feet and 15 feet with centrifugal pumps.
A low-speed pump will operate safely with a greater suction lift than one with a higher speed. If the suction lift is very high (over 15 feet for water), slower pump impeller speeds and larger pumps are necessary. Increased speeds without proper suction conditions may cause serious problems from cavitation, which can ruin a pump very quickly. Head losses from fluid flowing through the suction lines reduce the pressure, or head, at the impeller. This produces the same effect as attempting to lift fluid from a level below the pump.
Was this article helpful?