Editor's Note: This is the second in a series of
five articles based on the Hydraulic Institute's new Positive
Displacement (PD) Pumps: Fundamentals, Design and Applications
e-Learning course.
Positive displacement (PD) pumps are used in a myriad of applications
across multiple industries. Users have found them to be the solution to
many specific pumping challenges; however, due to their size, simplicity
and ruggedness, they often are not as well understood as other pump
types.
Technologies within the extensive positive displacement family cover a
broad range of horsepower, fluid and pressure applications. These
products merit increased consideration in a user's pump selection
process. To assist pump users with a proper understanding of
definitions, applications, installation, operation, maintenance and
testing procedures, the Hydraulic Institute publishes ten ANSI/HI
Standards covering PD pumps including: Air Operated, Controlled Volume
Metering, Reciprocating and Rotary.
ANSI/HI standards perform a vital function in pump industry commerce
and serve important roles in minimizing misunderstandings in the
marketplace. The Hydraulic Institute has extended its mission to include
the development of a pump knowledge and education portfolio in response
to member and pump user needs. Among the first key elements are a
re-launch of the Centrifugal Pump e-Learning course and the development
of a new Positive Displacement Pump course covering fundamentals, design
and applications.
Last month, we provided an overview of the curriculum and an overview
of positive displacement pumps, as well as the 12 benefits of PD pumps.
This installment will focus on "Positive Displacement Pump
Hydraulics," which introduces the fundamental physical concepts and
fluid properties that affect positive displacement pump selection and
operation.
Since many of these properties affect positive displacement pumps
differently than centrifugal pumps, it is critical to understand the
interaction between the pump and the fluid, and how the operation of
positive displacement pumps differs from centrifugal pumps. Without this
foundation of fundamental principles, it would be difficult to
effectively learn about the myriad of positive displacement pumps (to be
presented in the next three articles).
On the most basic level, pumps are used to provide pressure and/or
flow so that the pump user can accomplish a specified task. With this
premise in mind, note that positive displacement pumps create flow, not
pressure. The pressure on the pump is a function of the system's
reaction to the delivered flow due to pipe losses, restrictions and
elevation changes. See Figure 1 for an explanation of gauge versus
absolute pressure; these two different reference points have caused
confusion through the years.
Figure 1. Gauge versus Absolute Pressure
Since positive displacement pumps theoretically generate flow
independent of discharge pressure, Figures 2 and 3 show how the
delivered flow rate is affected by the differential pressure and speed.
This is a fundamental difference between positive displacement pumps and
centrifugal pumps. The delivered flow rate is the theoretical flow rate
minus the internal slip of the pump. The slip is the internal leakage
that occurs in the pump due to clearances, viscosity and differential
pressure, and will vary between pump types and applications.
Figures 2 and 3.
Of course, nothing comes for free, and every pump requires a certain
amount of power to perform work. The pump input power is comprised of
the theoretical liquid horsepower and the internal power losses at the
operating point. The theoretical liquid horsepower is the work done to
move the theoretical volume of fluid from inlet to outlet pressure and
is solely based on the physical dimensions of the pumping elements, the
operating speed and the differential pressure. On the other hand, the
internal power losses account for the mechanical and viscous losses that
occur as the pump operates. It is typical that the mechanical loss is
the major component when operating at low viscosities, while the viscous
loss is larger at high viscosities. These losses in turn affect the
overall efficiency of the pump.
Fluid characteristics play a major role with positive displacement
pumps since most of these pumps are used to handle products other than
water. The most important fluid property is viscosity, which is the
fluid's ability to resist a shearing force, or how easily the fluid will
flow. Viscosity affects pump selections due to its impact on operating
speed, allowable differential pressure, suction capability and input
power. Since viscosity is typically temperature dependent, it is
important to know what the viscosity values will be over the entire
operating temperature range, from cold start-up to maximum upset
condition. Viscosity can be expressed in many different units, the most
common being SSU, SSF, centipoises (cP) and centistokes (cSt). Units
depend on which viscometer instrument was used to evaluate the different
fluids. These values are tabulated in many texts, and several examples
are provided in the module (see sidebar for more information).
Along with viscosity, it is essential to understand how a fluid
reacts to being pumped. A fluid can have a constant viscosity (at a
particular temperature and pressure) regardless of the rate of shear
(known as a Newtonian fluid), or the viscosity can vary with the shear
rate and shear stress (known as a Non-Newtonian fluid). Non-Newtonian
fluids are divided into five types: plastic, pseudo-plastic, dilatant,
thixotropic and rheopectic. The first three are time independent since
the viscosity is simply a function of the shear stress, while the last
two are dependent on both time and shear stress. To accurately determine
the viscosity of non-Newtonian fluids, it is usually necessary to take
measurements at several points. If the apparent viscosity is not
accurate, it can result in an improper pump selection.
Beyond viscosity, several other fluid characteristics relate to
positive displacement pump selection and operation. One of these
characteristics is vapor pressure. The vapor pressure is the absolute
pressure at which a liquid changes to a gas (see Figure 4). The primary
influence of vapor pressure is on the Net Positive Inlet Pressure (NPIP)
required for the pump. The NPIP is the absolute pressure above vapor
pressure required to get the fluid into the pumping elements. As can be
seen with a fixed inlet pressure, the higher the vapor pressure, the
less pressure available to get the fluid into the pump.
Figure 4. Vapor pressure at 60-deg F in PSI absolute
Net Positive Inlet Pressure plays a key role in pump selection and
system design. Since pumps cannot pull fluid into them, they rely on the
difference in pressure between the liquid source and the pump inlet to
get the product into the pump. Sufficient pressure must be available at
the pump inlet to fill the pumping chambers and prevent gas from being
released from the fluid. Therefore, the inlet pressure available must
always be greater than the inlet pressure required.
The inlet pressure available is a system characteristic that is
determined from the following factors: atmospheric pressure, elevation
of the fluid level (above or below the pump inlet), inlet line friction
losses, vapor pressure, and in the case of reciprocating pumps,
acceleration head. Acceleration head, a unique factor for reciprocating
pumps, is the pressure required to accelerate the liquid column at the
beginning of each stroke. Depending on the suction line length, average
line velocity, pump rotational speed, number of pistons and liquid
elasticity, the acceleration head can easily be the largest factor of
the available inlet pressure calculation.
The inlet pressure required is a pump characteristic that the
manufacturer calculates. It is affected by such factors as pump design,
viscosity and speed. Once the available inlet pressure is known, it can
be compared to the inlet pressure required by the pump to determine if
the pump will operate properly. Sometimes several iterations between the
user and supplier are required to select the correct pump.
Once all of the important factors affecting pump selection are
understood, hydraulic selection becomes easier. However, other factors
external to the pump need to be evaluated to ensure that the entire pump
system is properly selected. These factors include environmental
conditions such as installation location and electrical requirements,
the control philosophy of how the pump will operate, the utilities
available and the operating energy requirements.
All of these factors flow into the life cycle costs of a pumping
system, which nearly always exceed the initial cost of the machine. The
life cycle costs, which can be fully explored in the Hydraulic Institute
publication, Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping
Systems, are explained in detail and consist of the initial cost plus
such items as installation, power, operation, maintenance, downtime and
environmental considerations.
This information on PD hydraulics forms the knowledge base for PD
pumps (and the HI e-Learning course), so users can properly evaluate any
pumping system and select the best technological solution based on the
entire spectrum of process conditions and system limitations.
In future issues, look for articles devoted to specific positive displacement pump technologies:
-
Rotary Pumps, including: Vane, Rotary Piston, Flexible Member, Lobe, Gear, Circumferential, Piston, Progressing Cavity, Timed Screw, Untimed Screw
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Reciprocating Pumps, including: Power, Direct Acting, Power Diaphragm, Air Operated Double Diaphragm, Air Operated Piston
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Metering Pumps, including: Torque Sources, Drive Mechanisms, Capacity Control, Liquid End Reviews
The Hydraulic Institute is the largest association of pump
producers in North America and the "value-add" standard-setting resource
for member companies and pump users worldwide.
This course has been created by renowned industry experts from the following HI Sponsor companies:
ARO/Ingersoll Rand
Colfax-IMO & Warren Pump
CLYDEUNION
Flowserve Pump Division
Grundfos Pumps Corporation
Iwaki America Incorporated
Leistritz Corporation
LEWA Inc.
Milton Roy Americas
Moyno, Inc.
Roper Pump Company
Siemens Water Technologies
Written by Hydraulic Institute Positive Displacement Pump Sponsors
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