"Positive Displacement Reciprocating Pump Fundamentals - Power And Direct Acting Types" By Herbert H. Tackett, Jr., James A. Cripe, Gary Dyson Page 8

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Table of Contents
52
PROCEEDINGS OF THE TWENTY-FOURTH INTERNATIONAL PUMP USERS SYMPOSIUM • 2008
Hydraulics
flow reduces as the viscosity increases. The positive displacement
reciprocating pump increases flow due to the increased volumetric
Where the hydraulic requirements of a process system make a
efficiency caused by higher viscosity liquids filling the clearances
kinetic or rotary pump unsuitable for the service. Typically, lower
inside the pump fluid end.
3
capacities (up to about 600 gpm (137 m
/hr) and higher pressures
Pumps have a tendency to shear fluids as speed is increased
(up to 20,000 psig (1379 barg).
and the centrifugal is a high speed pump. Lower speed positive
Because it is well suited for lower flow rates. Certain combinations
displacement reciprocating pumps are better when handling shear
of flow rate and pressure make centrifugal pumps inherently less
sensitive fluids. Shear rates in positive displacement reciprocating
efficient. These applications are around 100 gpm and lower, but
pumps vary by design, but they are low shear devices. Pump
especially under 50 gpm. Centrifugals, do well at higher
manufacturers have proprietary information for specific shear rates
flow rates.
and applications and can provide some recommendations.
Positive displacement reciprocating pump net positive suction
Like all other pump designs, reciprocating positive displacement
head required (NPSHR) varies as a function of flow, which is deter-
pumps have their own set of disadvantages, with the most common
mined by speed. The lower the positive displacement reciprocating
being the pulsations. The pulsating flow and pressure have specific
pump speed, the lower the NPSHR.
suction and discharge piping systems requirements to ensure
A positive displacement reciprocating pump can be considered
proper pump performance and to protect the piping and other
as an alternative to a centrifugal pump operating away from
system components.
best efficiency point (BEP). The farther away from BEP a
SPECIFICATION METHODOLOGY
centrifugal pump operates, the more viable a positive displacement
reciprocating pump solution becomes. This typically happens at
As with all pump types (i.e., centrifugal, rotary, screw, etc.)
low flow conditions, moderate to high pressure conditions, or
having correct and complete application information is essential to
with elevated viscosity.
selecting the most suitable model and size of reciprocating positive
displacement pump. The following application information should
Capacity and Pressure Control
be provided to the pump vendor to allow the pump model selection
and size to be determined.
Because of the fixed volume of fluid displacement at a
given speed, a more precise capacity can be achieved. A positive
What is the fluid to be pumped, i.e., water, gasoline, triethylene
displacement reciprocating pump has a constant flow regardless
glycol (TEG), ammonia, etc.?—This tells the pump vendor the
of pressure; pressure and flow are independent of each other. A
fluid type (Newtonian, thixotropic, colloidal, or dilliant). Many
centrifugal pump has varying flow depending on pressure or head.
commonly pumped fluids are Newtonian, which some pump
vendors have solely establish their pump selection and sizing
Using a reciprocating pump variable capacity can be achieved by
upon. However, other fluid types have special considerations for
changing the pump speed.
pump selection and sizing. The pumped product is used to give a
A positive displacement reciprocating pump can be used if the
general indication of the pump fluid end design and materials of
application has variable pressure conditions. A centrifugal pump
construction required.
will be forced up and down the performance curve thus varying the
Is the fluid to be pumped toxic, flammable, or otherwise
flow. This can cause process problems. A positive displacement
hazardous to personnel and/or the environment?—This gives a
reciprocating pump will give near constant flow making it possible
general indication of the pump fluid end design, materials of
to match pump flow rate to the process requirements. Using a
construction, and stuffing box seal design.
reciprocating pump variable capacity can be achieved by changing
the pump speed.
Required delivered capacity—A reciprocating pump is a
positive displacement type, and as such, it does not have a rated
When a constant flow is required, a positive displacement
flow; it only has a given flow at a given speed. Therefore, a range
reciprocating pump is the pump of choice, especially for
of flows can be provided simply by varying the pump speed. This
metering applications.
can be achieved by an electric motor with a variable frequency
Reciprocating pumps can be considered self-priming with the
drive on power pumps, or by throttling the drive media at the inlet
right suction conditions.
on a direct acting pump.
Efficiency
Suction pressure—This indicates if a standard pump design can
be used, or if the pump size needs to be modified to handle elevated
Pump types are very different when considering mechanical
or high suction pressure (equal to or greater than 5 percent of the
efficiency. Pressure changes have little effect on the positive
discharge pressure). Also, tells the pump vendor what pressure
displacement reciprocating pump because pressure and flow are
category the suction side of the pump must to be rated for.
independent of each other. Centrifugal pump characteristic
Discharge pressure—Used to determine pump plunger/piston
performance curves have interdependency between flow and head.
size, frame/rod load to achieve best mechanical efficiency, and
Energy efficiency—As a positive displacement reciprocating
what fluid end design pressure is required.
pump has a high mechanical efficiency, a smaller driver can be
Net positive suction head available (NPSHA)—Aids in
used, resulting in lower power/energy consumption versus that of a
determining pump plunger/piston size, pump speed, suction valve
kinetic or rotary pump.
type and size, and what other modifications (if possible) may be
required to provide a low enough NPSHR.
Special Fluid Requirements
Fluid pumping temperatures: normal, minimum, and
Because of their lower operating speeds, reciprocating pumps
maximum—Provides indications of what pump fluid end design is
can be used on certain types of fluids containing solids that would
required, materials of construction, and stuffing box seal design
quickly erode the internals of a high speed kinetic or rotary pump.
needed. This also indicates if any pump speed reduction may be
required. For pumping temperatures between 350 F (177 C) and
Another major difference between pump types is the effect
viscosity has on the capacity of the pump. A centrifugal pump’s
850 F (455 C) pump speed must be reduced and a special fluid end

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