"Positive Displacement Reciprocating Pump Fundamentals - Power And Direct Acting Types" By Herbert H. Tackett, Jr., James A. Cripe, Gary Dyson Page 3
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14Table of Contents
POSITIVE DISPLACEMENT RECIPROCATING PUMP FUNDAMENTALS—
47
POWER AND DIRECT ACTING TYPES
Figure 4. Reciprocating Pump Sequence—Single Acting.
Figure 7. Piston.
COMPONENT OPTIONS
Correct component selection is an important part of ensuring long
term reliability. The hydraulic requirements and the constituents of
the pump products dictate a number of important design features.
Understanding these features and selecting the appropriate design
is essential to achieving long mean time between failure (MTBF).
While the pump manufacturer is crucial in advising the correct
configuration for service, it is prudent to understand the features
and benefits of some key components. The following provides a
general overview of key fluid end components and where they are
best suited for use.
Valve Designs
Valve units (suction and discharge) in essence are check type
valves that are opened by differential pressure of the fluid, and
Figure 5. Reciprocating Pump Sequence—Double Acting.
allow flow in one direction only. The majority of valve designs are
spring-loaded to rapidly return the valve back onto the seating
Differences Between Plungers and Pistons
surface and minimize the valve slip (reverse flow through the valve
before it seats). The most widely used valve designs are illustrated
The terms plunger and piston are often used interchangeably, as
in Figures 8 through 14.
if they were the same thing. This can lead to confusion. It is
The standard wing guided valve, Figure 8, has wings extending
important to understand the difference between the two and use the
from the valve head that guide it as it opens and closes. The seating
correct term.
surfaces between the valve and valve seat conventionally are
A plunger type pumping element, Figure 6, is a smooth rod
beveled and lapped together to create a tight seal.
attached to the slider mechanism/crosshead, which imparts energy
into the pumped fluid to cause pressure to be developed within the
fluid cylinder. A stationary seal is used around a plunger and the
plunger is allowed to move through the seal rings. Plungers are
classified as single acting, therefore, when used on a double acting
pump two are required. Plungers are normally used when smaller
capacity and high pressure are required.
Figure 6. Plunger.
Piston type pumping elements, Figure 7, consist of a cylindrical
disc equipped with a seal at the outer diameter (O.D.) attached to a
smooth rod (piston rod). The piston imparts energy into the
pumped fluid to cause pressure to be developed within the
fluid cylinder. The small smooth rod (piston rod) acts to transfer
reciprocating motion and power into work energy at the fluid end.
The seal at the piston O.D. moves with the piston. This technology
Figure 8. Wing Guided Valve Standard.
is conventionally applied to low pressures and higher flows.
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