"Positive Displacement Reciprocating Pump Fundamentals - Power And Direct Acting Types" By Herbert H. Tackett, Jr., James A. Cripe, Gary Dyson Page 7
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14Table of Contents
POSITIVE DISPLACEMENT RECIPROCATING PUMP FUNDAMENTALS—
51
POWER AND DIRECT ACTING TYPES
Plunger and Piston Pumping Elements
These components displace the fixed volume of fluid and
generate pressure and work with the stuffing box seal. Due
consideration should be given to the needs of the fluid being
pumped, operating conditions, customer requirements, and any
EPA or other regulations for a given application. Pump vendors
have extensive experience in this area and should be consulted for
assistance in finalizing the appropriate design.
Plunger Type Power and Direct
Acting Pump Pumping Elements
These components are smooth rods that move in and out of the
fluid cylinder to cause differential pressure and transmit force to
Figure 25. Piston with Pump Cup Seal.
develop these pressures inside the fluid end.
Plungers can vary in basic design, as shown in Figure 22, and are
made of a base metal compatible/suitable for the service with a
hard face coating. Most commonly used coatings include chrome
plating, tungsten carbide, nickel/chrome/boride, or one of several
grades of ceramic. This coating provides wear resistance to the
stuffing box packing seal and extends plunger life.
Figure 22. Plunger Designs.
Figure 26. Piston with Bull Ring and Metallic or Phenolic Ring Seal.
Piston Type Pumping Elements
Figure 23 piston design has a hydraulic packing seal that is
These elements consist of a cylindrical disc normally equipped
normally an elastomer and woven fabric (cotton, nylon, aramid
with a seal of some type at the O.D. attached to a small smooth rod
fiber, etc.) laminated and cut to form rings. The seal is energized
(piston rod). Pistons are available in several designs for application
by the elastomer used and/or swelling, and also by pumping
differential pressures up to 1500 psig (97 barg), and all rely on the
pressures. This design is well suited for various pumped fluids like
fluid pumped for lubrication to the piston bearing surfaces and
lube oil, grease, and slops services, but it is most popularly used on
seal. Design selection is dependent on the best O.D. seal for the
water applications like boiler feed.
application and ease of maintenance. The body and follower
The piston design in Figure 24 has a metallic three ring seal, self
designs shown in Figures 23 through 26 are the most common types
energized by spring action. It consists of two outer seal rings and an
because the piston O.D. seal can generally be replaced without
inner expander ring to ensure an O.D. seal is maintained. The most
having to remove the entire piston assembly from the pump.
common materials are cast-iron and ni-resist, but a few applications
require stainless steels. Long seal life is achieved when used on a
metallic liner and pumping a fluid with good lubrication properties.
The Figure 25 pump cup seal piston design has a self
energized cup seal that is enhanced by pumping pressures.
Normally cup seals are a laminated elastomer/polymer reinforced
with fabric (cotton, nylon, aramid fiber, etc.), or can be made of
polytetrafluoroethylene (PTFE). This design is well suited for
various pumped fluids like lube oil, grease, and slops services,
and water applications like boiler feed, just a few examples.
The Figure 26 piston design is the least popular due to the
thinner seal cross-section and the additional “bull ring” part to be
maintained or replaced. Seal rings are typically made of hard
Figure 23. Piston with Hydraulic Packing Seal.
plastic or metal, and require a piston liner material with suitable
wear resistance. This design relies solely upon pumping pressures
to create a seal, and can be used on pumped fluids from lube oil,
grease, slops services, and water applications. When correct
piston, seal, and liner materials are used for an application, long
seal life can be achieved.
SPECIFICATION CRITERIA
Identifying the service that is ideal for a reciprocating pump is
not always a straightforward exercise. Multiple variables must be
evaluated and assessed. To make the best choice between pump
types it is important to understand how various pump types behave.
Positive displacement reciprocating pumps are generally used for
Figure 24. Piston with Metallic Three Ring Seal.
the following reasons:
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