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PROCEEDINGS OF THE TWENTY-FOURTH INTERNATIONAL PUMP USERS SYMPOSIUM • 2008

RECIPROCATING PUMPS—

Power or Direct Acting Machines

DEFINITION AND OVERVIEW

The difference between a power pump and a direct acting pump

can be illustrated by the following statements:

By definition, a reciprocating pump is any machine using

reciprocating motion to cause fluid to be moved from one location

•

A power pump drives a pumping element(s) through the

to another. The most common form of reciprocating pump is the

reciprocating motion using either a crank and slider mechanism or

positive displacement type.

camshaft to convert rotary motion and power from an electric

This type of pump traps a fixed volume (Figure 1) of fluid and

motor, engine, or turbine into reciprocating motion and work

displaces it from suction conditions to discharge conditions by

energy inside the fluid end of the pump.

means of check valves placed in series, at least one on the suction

•

A direct acting pump drives pumping element(s) through the

side and at least one on the discharge side. These check valves

reciprocating motion by use of an integral reciprocating engine

ensure fluid movement is in one direction from pump suction

(similar to a steam engine) to directly transfer engine reciprocating

toward the pump discharge. Since a fixed volume of fluid is

motion and power into work energy at the fluid end pumping

displaced the rate of flow is directly proportional to speed.

element(s). Because of the integral reciprocating engine, this type

Capacity can also be increased by using a pump with multiple

pump can be driven by steam, air, or gas.

plungers or pistons. Pump speed and number of plungers/pistons

•

available is limited by mechanical considerations.

Both power and direct acting designs can be either “horizontal”

or “vertical” in design (Figure 3) and the direction of reciprocating

motion is used to define their configuration. Horizontal pumps

operate with the reciprocating motion in a horizontal plane parallel

to the ground while vertical pumps operate with the reciprocating

motion in a vertical plane perpendicular to the ground.

Figure 1. Fixed Displaced Volume.

As a result of the fixed displaced volume per pump revolution

and the fact that pressure is independent of pump speed and

flow rate, there is no capacity-head curve for these machines as

with centrifugal pumps. If a capacity curve were to be drawn, it

would simply be a straight line from zero capacity and speed to

maximum capacity and speed like shown in Figure 2 at 100 percent

(theoretical) volumetric efficiency and another line at actual

application volumetric efficiency.

Figure 3. Different Pump Configurations.

Differences Between Single and Double Acting Pumps

When the pump starts on suction stroke, the pumping element

begins to withdraw from the pumping chamber (area between

suction and discharge valves), and pumping chamber pressure

drops below suction manifold pressure. The suction valve then

opens allowing flow to enter the pumping chamber, and then closes

at the end of the suction stroke. The pumping element begins to

re-enter the pumping chamber on its discharge stroke, compressing

Figure 2. Fixed Displaced Volume Capacity Curve.

trapped fluid until pressure in the pumping chamber is above that

in the discharge manifold causing the discharge valve to open,

Within the sphere of reciprocating pumps there are a number of

allowing a fixed volume of fluid to flow into the discharge

fundamental configuration differences that warrant explanation.

manifold. Each time this happens, the pump element is “acting”

These include the difference between power and direct acting

upon the fluid causing pumping. Therefore, pumps designed to do

machines, the difference between single and double acting pumps,

this once per complete cycle or revolution are single acting, while

and the differences between plungers and pistons. This basic

those designs causing this to happen twice per complete cycle or

configuration knowledge is the foundation of the subject.

revolution are double acting.

American Petroleum Institute standard API-674 (1995) and

A single acting pump displaces fluid only in one direction of its

Hydraulic Institute standards cover many aspects of pump limitations

and the features outlined in this tutorial and therefore should be

stroke as illustrated in Figure 4. A double acting pump displaces

viewed against the background of these two publications.

fluid in both directions of its stroke shown by Figure 5.

"Positive Displacement Reciprocating Pump Fundamentals - Power And Direct Acting Types" By Herbert H. Tackett, Jr., James A. Cripe, Gary Dyson Page 2

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