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POSITIVE DISPLACEMENT RECIPROCATING PUMP FUNDAMENTALS—

POWER AND DIRECT ACTING TYPES

design is required to make allowance for thermal growth to

vary between 1 to 5 percent based upon pump speed and

valve design. In general, most pump designs will

maintain good alignment between the pumping element and

stuffing box seal.

typically have a 3 percent loss.

•

Any pump speed limits imposed by user specifications—This is

Mechanical Efficiency

needed to determine the pump plunger/piston size, and actual

The mechanical efficiency (ME) of a reciprocating machine,

pump speed to comply for the application.

as previously mentioned, is an important consideration. This

•

Fluid specific gravity—This is used in determining the pump

section outlines the influencing factors with respect to mechanical

plunger/piston size, pump speed, and what modifications may be

efficiency and illustrates both power and direct acting pumps.

required for a low enough NPSHR.

Power Pumps

•

Fluid compressibility factor or bulk modulus at pumping

temperatures—This is used to determine the pump volumetric

As shown in Figure 27, mechanical efficiency varies as a

function of frame load, which is simply the pressure applied times

efficiency and in turn pump speed for a given plunger/piston size

the cross-sectional area of the plunger/piston. Percentage of frame

within a fluid end.

load is actual frame load from the application divided by the frame

•

Fluid vapor pressure at pumping temperatures—Needed to

load rating established by the pump vendor. Higher frame loading

determine pump plunger/piston size, pump speed, and any

increases ME. Therefore, the plunger/piston should be sized to

modifications required for low enough NPSHR.

provide as high as possible frame load within the pump’s frame

•

Fluid viscosity at pumping temperatures—Allows determination

load rating to achieve the highest possible ME.

of pump plunger/piston size, pump speed. Starting at 300 SSU

(~72 cP) pump speed needs to be reduced as indicated in API-674

(1995) Second Edition Figure 1 and ANSI/HI 6.1-6.5 (1994). Also,

helps to determine what or if modifications may be required for

low enough NPSHR.

•

Does fluid contain any substances or chemicals that will cause

corrosion and/or erosion within the pump selected?—This aids in

determining the pump fluid end size, pump speed, pump valve type

and size, stuffing box seal design (as required for personnel and/or

environment protection), and other modifications needed to

Figure 27. Mechanical Efficiency Versus Frame Load.

provide low NPSHR.

As shown in the above, having all of this important information

helps determine key aspects of the pump selected and used, in

Power pumps have mechanical efficiencies up to 87 percent with

addition to many important pump performance and construction

sleeve type power end bearing pumps, and up to 90 percent for

characteristics, including materials of construction, stuffing box

roller type power end bearing pumps. Contributing to the losses are

seal type, and fluid end design.

the pump bearings, stuffing box seals, and the pump valves.

Additional losses from drive components (i.e., driver, belts, gears,

RECIPROCATING PUMP EFFICIENCY

couplings, etc.) should be determined and accounted for in the

While centrifugal pump efficiency is understood in terms of

overall efficiency of the complete pump unit.

leakage, mechanical, and hydraulic losses, the efficiency of a

Direct Acting Pumps

reciprocating machine is somewhat different. The main constituents

of reciprocating pump efficiency are covered below.

Mechanical efficiency or pump efficiency is a ratio of force

applied to the fluid pumped by the piston/plunger by the force

Volumetric Efficiency

transmitted by the drive piston from the drive media (steam, gas, or

As shown in the following equation reciprocating pump

air), minus losses due to friction from drive piston drag, stuffing

volumetric efficiency (VE) can be determined with reasonable

box packing, and pump valves. This ratio is based upon differential

accuracy (within 1 percent), if all factors are known. Also evident is that

pressures at both the pump’s drive and fluid ends, as determined

VE is dependant upon the fluid compressibility, application pressure,

using the following equation.

pump C/D ratio (pumping chamber clearance to displacement ratio),

and pump valve slip. Therefore, since fluid compressibility, pump C/D

ratio, and pump valve slip are known by fluid properties and pump

dimensions and characteristics, the actual fixed displaced volume per

where:

complete cycle (rpm) is dependant upon pressure and not pump speed.

= Cross-sectional area of fluid piston/plunger

= Differential pressure across the pump fluid end

= Cross-sectional area of fluid piston/plunger

= Differential pressure across the pump drive end

where:

= Differential pressure (psig) = P

PUMP AND SYSTEM INTERACTION

= Compressibility factor of fluid to be pumped at pumping

temperature reciprocal (inverse) of fluid bulk modulus at

One significant factor in the successful operation of any pump is

pumping temperature

an appropriately designed piping system. A poorly designed system

= Ratio of total volume between the suction and discharge

can cause problems such as:

valves inside the pumping chamber when the plunger

•

Fluid flashing—Entrained gases in the fluid coming out when

(or piston) is at full forward stroke divided by the plunger

pressure in piping or pump falls below fluid vapor pressure

(or piston) displacement volume (area × stroke length); also

•

called C/D ratio

Cavitation—Free gases in a fluid being forced back into the

= Valve loss or VE loss from fluid slippage back past the

fluid. These implosions cause severe pressure spikes that pit and

pump valves before they can close and seal. This will

damage pump internal parts.

"Positive Displacement Reciprocating Pump Fundamentals - Power And Direct Acting Types" By Herbert H. Tackett, Jr., James A. Cripe, Gary Dyson Page 9

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