Page 135 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Chapter 3

Reciprocating and Liquid Ring
Vacuum Pumps*

The justification for using a reciprocating pump in a petrochemical
plant instead of a centrifugal or rotary pump must be cost-not just the
initial cost but total cost, including costs for power and maintenance.
Some applications are inherently best suited for reciprocating units.
Such services include high-pressure water cleaning (typically 20 gpm at
10,000 psig), glycol injection (typically 5 gpm at 1 ,000 psig), and ammo-
nia charging (typically 40 gpm at 4,000 psig). Another application that
practically mandates a reciprocating unit is abrasive andor viscous slur-
ries above about 500 psig. Examples of these services range from coal
slurry to peanut butter.
The best feature of the power pump is its high efficiency. Overall effi-
ciencies normally range from 85 to 94 percent. The losses of approxi-
mately 10 percent include all those due to belts, gears, bearings, pack-
ings, and valves.
Another characteristic of the reciprocating pump is that capacity is a
function of speed, and is relatively independent of discharge pressure.
Therefore, a constant-speed power pump that moves 100 gpm at 500 psig
will handle very nearly 100 gpm at 3,000 psig.
The direct-acting pump has some of the same advantages as the power
pump, plus others. These units are well suited for high-pressure low-
flow applications. Discharge pressures normally range from 300 to
5,000 psig, but may exceed 10,OOO psig. Capacity is proportional to
speed from stall to maximum speed, regardless of the discharge pressure.

* From “Reciprocating Pumps,” Chemicui Engineering, Sept. 21, 1981 by T. L. Hen-
shaw, Union Pump Co. Adapted by permission of the author.
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