Page 136 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating and Liquid Ring kmum Pumps 119

Speed is controlled by throttling the motive fluid. The unit is normally
self-priming-particularly the low clearance-volume type.
Direct-acting pumps are negligibly affected by hostile environments
such as corrosive fumes, because of the absence of a bearing housing,
crankcase, or oil reservoir (except for units requiring a lubricator). Some
direct-acting pumps inadvertently inundated by flood-water have contin-
ued to operate without adverse effects. Direct-acting pumps are quiet,
simple to maintain, and their low speeds and rugged construction lead to
a very long life.
Both power and direct-acting pumps with special fittings and operating
at low speeds have been successfully applied to abrasive-slurry services.
The low thermal efficiency of the direct-acting pump is sometimes
used to advantage. When steam is the motive fluid, very little heat is lost
from inlet to exhaust. The exhaust temperature is the same as that ob-
tained by throttling. In those cases where high-pressure steam is throttled
to a lower pressure for heating (such as for deaerating boiler feedwater),
the steam can be used to drive a direct-acting pump, with the exhaust
steam used for heating. In this circumstance, the drive end (piston rings,
valves, etc.) is made to operate without lubrication, so that the exhaust
steam will be oil-free.

Pump Classification

Reciprocating pumps are usually classified by their features:

Drive end, i.e., power or direct-acting.
Orientation of centerline of the pumping element, i.e., horizontal or
vertical.
Number of discharge strokes per cycle of each drive rod, Le., single-
acting or double-acting .
Configuration of the pumping element, i.e., piston plunger or dia-
phragm.
Number of drive rods, Le., simplex, duplex, or multiplex.

Figure 3-1 illustrates this classification in chart form.

Figure 3-2 shows two examples of reciprocating pumps.

Cross-sectional drawings for power and direct-acting pumps are shown
in Figures 3-3 and 34, respectively.

The size of a power pump is normally designated by listing first the
diameter of the plunger (or the piston), and second the length of the
stroke. In the United States, the units are inches. For example, a pump