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9

©1999 CRC Press LLC

The Advantages of

Rotary Pumps

The advantages of rotary pumps are the following:

 Efficiency  — 20 to 5% (depending on pressure) higher efficiency for most

typically pumped liquids, with high viscosities.

Viscous Fluids Handling — Above approximately 300 to 1000 SSU (such as

DTE light oil), a centrifugal pump simply cannot be used, as viscous drag

reduces efficiency to nearly zero. PD pumps continue to pump at high

efficiency, with no problems.

Pressure Versatility — As an example, a 11 / 2 × 1 to 6 centrifugal pump can

only produce approximately 20 psi system pressure at 1800 RPM. A similar

PD pump goes to much higher pressures. This goes back to the inherentcharacteristic of PD pumps as “flow generators,” practically unaffected by

pressure, within a wide range.

Self-Priming — a typical ANSI-dimensioned centrifugal pump that is com-

monly found in many chemical plants cannot lift liquid. A standard PD

pump, such as a gear pump, can easily lift liquid in the range of 1 to 20

ft. Centrifugal pumps can be made self-priming, by adding a priming flood

chamber in the inlet, however, this adds expense and violates dimensional

interchangeability.

 Inlet Piping  — Centrifugal pumps are extremely sensitive to inlet piping

details. Improper piping may cause an increased NPSHR, cavitation, high

vibrations, and possible damage to seals and bearings. PD pumps are less

sensitive to inlet piping and can be a real solution for many difficult

installations with space constraints, since piping modifications to the exist-

ing setups are very costly. Bi-directional — By simply reversing the direction of motor rotation, many

PD pumps will pump in reverse, which can be advantageous in many

processes. Centrifugal pumps can pump in only one direction. In some

installations, two centrifugal pumps are used: one for loading, and another

for unloading, which doubles the piping runs, valves, and auxiliaries. Asingle rotary pump would do the same job. (Note: Relief valves should be

installed in both directions in such cases.)

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Flow Maintainability — PD pumps produce almost constant flow, regardless

of fluids’ properties and conditions (viscosity, pressure, and temperature).

For centrifugal pumps, a change in fluid properties and external conditions

would result in a definite change in performance. Metering Capability  — PD pumps can be used as convenient and simple

metering devices. Centrifugal pumps have no such capabilities.

 Inventory Reduction — Since PD pumps can pump a wide variety of fluids in

an extreme range of viscosities, the same pump parts inventory is required

for a wide range of applications throughout the plant. Centrifugal pumps

require a greater multitude of sizes for different applications, which results

in increased inventory of parts.

Nevertheless, centrifugal pumps also have numerous advantages and operate verywell, especially at very low viscosities (such as water, etc.) when applied properly,

as will be seen from the examples that follow.