Without pumps, there would be no hydrocarbon processing
industry.1 Pumps are simple machines that lift,
transfer, or otherwise move fluid from one place to another.
Process pumps are usually configured to use the rotational
(kinetic) energy from an impeller to impart motion to a fluid.
The impeller is located on a shaft; together, shaft and
impeller(s) make up the rotor. This rotor is surrounded by a
casing. Located in this casing (or pump case) are
one or more stationary passageways that direct the fluid to a
discharge nozzle. Impeller and casing are the main components
of the hydraulic assembly; the region or envelope containing
bearings and seals is called the mechanical assembly, or power
end (Fig. 1).2
Fig. 1. Principal
components of an elementary
However, many process pumps are designed and constructed to
facilitate field repair. On these so-called back pullout pumps
(Fig. 2),3 shop maintenance can be performed while
the casing and its associated suction and discharge piping are
left undisturbed. Although operating in the hydraulic
end, the impeller remains with the power end during removal
from the field. The rotating impeller (Fig.
3)4 is usually constructed with swept-back
vanes, and the fluid is accelerated from the rotating impeller
to stationary passages in the surrounding casing.
Fig. 2. Typical process
pump with suction flow entering
horizontally, and vertically oriented
leaving the casing tangentially.
Fig. 3. A semi-open
impeller with five vanes.
As shown, the impeller is configured for
counter-clockwise rotation about the
Greater spacer lengths reduce angles of misalignment
in case of the unavoidable differential
temperature-related parallel offset between the
centerlines of pump and driver. Greater-length
spacers may cost a bit more, but, by reducing bearing
loads and vibration severity, they beneficially
affect the likely cost of future maintenance.
In this manner, kinetic energy is converted to potential
energy, and the fluid (often called pumpage) moves from the
suction (lower) pressure side to the discharge (higher)
pressure side of a pumping system. As the fluid leaves the
impeller through the pump discharge, more fluid is drawn into
the pump suction, where, except for the region immediately
adjacent, the pressure is lowest.4
Reliability tip: Pipe elbows located
too close to the pump inlet nozzle may save money
initially, but they often create flow disturbances,
which tend to reduce pump life.
Pump performance expressed as head and flow
Pump performance is always described in terms of head,
H, produced at a given flow capability, Q,
and hydraulic efficiency, h, attained at any particular
intersection of H and Q. Head is usually
plotted on the vertical scale or vertical axis (the left of the
two y axes) of Fig. 4; it is expressed in feet
or meters. Hydraulic efficiency is often plotted on another
vertical scale, the right of the two vertical scales; i.e., the
y axis in this generalized plot.
Fig. 4. Typical
H-Q performance curves are sloped
as shown. The BEP is marked with a small
power and other parameters are often
on the same plot.
Head is related to the difference between discharge pressure
and suction pressure at the respective pump nozzles. Head is a
simple concept, but this is where consideration of the impeller
tip speed is important. The higher the shaft rotations per
minute (rpm) and the larger the impeller diameter, the higher
the impeller tip speed will bei.e., its peripheral
The concept of head can be visualized by thinking of a
vertical pipe bolted to the outlet (the discharge nozzle) of a
pump. In this imaginary pipe, a column of fluid would rise to a
height H. If the vertical pipe would be attached to
the discharge nozzle of a pump with higher impeller
tip speed, the fluid would rise to a greater height
(H+). Note: The height of a column of
liquid, H or H+, is a function of only the
impeller tip speed. The specific gravity of the liquid affects
power demand, but it does not influence either H or
H+. However, the resulting discharge pressure
does depend on the liquid density (specific gravity).
For water (with a specific gravity of 1), an H of 2.31
ft equals 1 psi, while, for alcohol, which might have a
specific gravity of 0.5, a column height or head H of
4.62 ft will equal 1 psi. Therefore, if a certain fluid has a
specific gravity of 1.28, a column height (head H) of
2.31/1.28 = 1.8 ft will equal a pressure of 1 psi.
For reasons of material strength and reasonably priced
metallurgy, the head per stage is usually limited to about 700
ft. This is an important rule-of-thumb limit to remember;
however, when too many similar rule-of-thumb limits combine,
pump reliability cannot be expected to be at its highest. As an
example, a particular impeller-to-shaft fit is required to have
a 0.0002-in. to 0.0015-in. clearance on average-sized impeller
hubs. With a clearance fit of 0.0015 in., a somewhat greater
failure risk can be anticipated if this upper limit is found on
an impeller operating with maximum allowable diameter.
Reliability tip: The higher the peripheral
impeller velocity, the greater the rate of erosion in
solids-containing pumps. High-tip velocity pumps manage
with fewer impellers than would be needed for
equivalent-head low-tip velocity pumps, but beware:
lowest installed cost today usually equates with higher
maintenance outlays tomorrow.
In Fig. 4, the point of zero flow (where
the curve intersects the y axis) is called the shutoff point.
The point at which operating efficiency is at a peak is called
the best efficiency point (BEP). Head rise from BEP to shutoff
is often chosen around 10%15% of differential head. This
choice makes it easy to modulate pump flow by adjusting control
valve open area based on monitoring pressure. Pumps
operate on their curves, and knowledge of what
pressure relates to what flow allows technicians to program
Reliability tip: Operation at
locations too far from BEP comes at a price.
Inefficient operation increases power consumption, or
maintenance frequency, or
The generalized depictions in Fig. 4 also
contain a curve labeled NPSHr, which represents net
positive suction head required. This is the head of
liquid that must exist at the edge of the inlet vanes of an
impeller to allow liquid transport without causing undue
vaporization. NPSHr is a function of impeller geometry and
size; it is determined by factory testing. The NPSHr of an
impeller can range from a few feet to a three-digit number. At
all times, the head of liquid available at the impeller inlet
(NPSHa) must exceed the required head of liquid (NPSHr).
Reliability tip: Some non-hydrocarbon process fluids have
properties that make it advisable to provide an
available NPSH well in excess of the published required
NPSHr. Investigation of the applicable process
experience is advised.
Operation at zero flow.
The rate of flow through a pump, in gallons per minute
(gpm), is labeled Q, and is plotted on the x axis.
Note: For a given speed and for every value of
head H read from the y axis, there is a corresponding
value Q on the x axis. This plotted relationship is
expressed as the pump is running on its curve. Pump
H-Q curves are plotted to commence at zero flow and
highest head. Process pumps need a continually rising curve
inclination; a curve with a hump somewhere along its inclined
line will not serve the reliability-focused user.
Reliability tip: Operation at zero
flow is not allowed and, if over a minutes
duration, could cause temperature rise and internal
recirculation effects that might destroy many pumps.
A published pump curve is valid only for this particular
impeller pattern, geometry, size and operation at the speed
indicated by the manufacturer or entity that produced the
curve. Curve steepness or inclination has to do with the number
of vanes in that impeller. Curve steepness is also affected by
the angle each vane makes relative to the impeller hub.
In general, curve shape is verified by physical testing at
the manufacturers facility. Once the entire pump is
installed in the field, it can be retested periodically by the
owner-purchaser for degradation and wear progression. Power
draw may have been affected by seals and couplings that differ
from the ones used on the manufacturers test stand.
Reliability tip: Occasionally, high
efficiencies are alluded to in the manufacturers
literature when bearing, seal and coupling losses are
not included in the vendors test reports. Make
sure to install a large-enough motor.
Impellers and rotors
Regardless of flow classification, centrifugal pumps range
in size from tiny to very big pumps. The tiny ones might be
used in medical or laboratory applications; the extremely large
pumps may move many thousands of liters or even gallons per
second from flooded lowlands to the open sea.
All six of the impellers in Fig. 5 are
shown with a hub fastening the impeller to the shaft, and each
of the first five impellers is shown as a hub-and-disc version
with an impeller cover. The cover (or shroud)
identifies the first five as closed impellers;
recall that Fig. 3 had depicted a semi-open
impeller. Semi-open impellers are designed and fabricated
without the cover. Finally, open impellers come with
freestanding vanes welded to, or integrally cast into, the hub.
Since the latter incorporate neither disc nor cover, they are
often used in viscous or fibrous paper stock
Fig. 5. General flow
of process pump impellers.
To properly function, a semi-open impeller must operate in
close proximity to a casing internal surface, which is why
axial adjustment features are needed with semi-open impellers.
Axial location is a bit less critical with closed impellers.
Except on axial flow pumps, fluid exits the impeller in the
radial direction. Radial-flow and mixed-flow pumps are either
single-suction or double-suction (double-flow)
designs. Once the impellers are fastened to a shaft, the
resulting assembly is called a rotor.
In radial- and mixed-flow pumps, the number of impellers
following each other, typically called stages, can
range from one to as many as will make such multistage pumps
practical and economical to manufacture. Horizontal shaft pumps
with up to 12 stages are not uncommon; using more than 12
stages on a horizontal shaft risks causing the rotor to
resonate or vibrate at a critical speed. Vertical shaft pumps
have been designed with 48 or more stages. In vertical pumps,
shaft support bushings are relatively lightly loaded; they are
spaced so as to minimize vibration risk.
The meaning of specific speed
Pump impeller flow classifications and the general meaning
of specific speed deserve further discussion. Moving from left
to right in Fig. 5, the various impeller
geometries reflect selections that start with high differential
pressure capabilities and end with progressively lower
differential pressure capabilities. Differential pressure is
simply discharge pressure minus suction pressure.
Specific speed calculations are a function of several
impeller parameters; the mathematical expression includes
exponents and is found later, in Fig. 6.
Staying with Fig. 5 and again moving from left
to right, we can reason that larger throughputs (flows) are
more likely achieved by the configurations to the right of the
illustration, whereas larger pressure ratios (discharge
pressure divided by suction pressure) are usually achieved by
the impeller geometries closer to the left.
Fig. 6. A pump-specific
allows for quick estimations.
Impellers toward the right are more efficient than those
near the left, and pump designers use the parameter-specific
speed, Ns, to bracket pump hydraulic efficiency
attainment and other anticipated attributes of a particular
impeller configuration and size. A similar-sounding parameter,
pump suction-specific speed, Nss or Nsss,
should not be confused with specific speed, Ns.
As an example, observe the customary use where, with
N and Q as the typical given parameters that
define centrifugal process pumps, a pivot point can be
determined. Next, with pivot point and head H, Ns can
be easily determined. In Fig. 5, Ns
is somewhere between 500 and 15,000 on the US scale. In this
range, a pump is known to exist, and the general impeller shape
can even be anticipated from Fig. 5.
Remember that thousands of impeller combinations and
geometries exist. Impellers with covers are the most prominent
in hydrocarbon processes, and uneven numbers
of impeller vanes are favored over even numbers of vanes, for
reasons of vibration suppression.
Pump manufacturers often use modular casing construction. A given casing
size may, however, accommodate several different
impeller sizes or geometries. Once an existing plant
has determined actual operating flows and heads, it may
be cost-effective to purchase custom-built, optimized
upgrade impellers from knowledgeable
Pump-specific speed (Fig. 6) might be of
primary interest to pump designers, but average users will also
find specific speed helpful. On the lower right of Fig.
6 is shown the equation for Ns; it is easy to
see how Ns is related to the shaft speed, N
(rpm), flow, Q (gpm), and head, H (ft).
As an example, consider a particular flowrate Q and
what could happen at some other speed. A straight line is drawn
to establish the pivot point. Then, drawing a line from
H through the pivot point and to Ns, those
pumps with Ns outside of the rule-of-thumb range (500
to 15,000) can be identified; these pumps should not be
In another example, after establishing the pivot point, an
impeller with a maximum head capability of 700 ft can be
selected by drawing a line through the pivot point. If the
resulting Ns is too low, a higher-speed N can
While there are always fringe applications in terms of size
and flowrate, this article considers centrifugal pumps in
process plants. These pumps are related to the generic
illustrations in Figs. 1 and
2, and others in this article. All of these
pumps would typicallybut by no means
exclusivelyrange from 3 hp to perhaps 300 hp (2 kW to 225
Process pump types
The elementary process pumps illustrated in Figs.
1 and 2 likely incorporate one of the
radial vane impellers, as shown in Fig. 5. If
a certain differential pressure is to be achieved combined with
higher flows, such a pump is often designed with a double-flow
impeller (Fig. 7).5 One of the side
benefits of double-flow impellers is very good axial thrust
equalization (axial balance). A small thrust bearing will often
suffice; it is shown in the left bearing housing in
Fig. 7. Note that the two radial bearings are
plain, or sleeve-type. Certain sleeve bearings have relatively
high speed capability.
Fig. 7. Double-flow
impellers are used
for higher flows and relatively
(balanced) axial thrust.
If elevated pressures are needed, several impellers can be
lined up in series on the same pump rotor. Of course, this
would then turn the pump into a multistage model, as shown in
Fig. 8. A six-stage
centrifugal process pump.
Process pump response to flow changes
Finally, it is useful to examine changes in
flowrate and how these changes can affect the mechanical
response of centrifugal process pumps. After the pumpage leaves
at the impeller tip, it must be channeled into a stationary
passageway that merges into the discharge nozzle. Many
different types of passageway designs (single or multiple
volutes, vaned diffusers, etc.) are available. Their respective
geometry interacts with the flow and creates radial force
action of different magnitude around the periphery of an
impeller (Fig. 9).6 These forces
tend to deflect the pump shaft; they are greater at partial
flow than at full flow.
Fig. 9. Direction and
magnitude of fluid
forces change at different flows.
A so-called recirculation phenomenon often aggravates the
problem at low flows. Recirculation is a flow reversal near
either the inlet or the discharge of a centrifugal pump. This
flow reversal can produce cavitation-erosion damage that starts
on the high-pressure side of an impeller vane and proceeds
through the metal to the low-pressure side.6
Reliability tip: Operation outside the
design range will have some repercussions. There are no
exceptions to this immutable rule.
Pump internal recirculation can cause surging and
cavitation, even when the NPSHa exceeds the manufacturers
published NPSHr by considerable margins.79
Also, extensive damage to the pressure side of the impeller
vanes has been observed in pumps operating at reduced
flowrates. These are the obvious results of recirculation;
however, more subtle symptoms and operational difficulties have
been identified in pumps operating in the recirculation zone
for extended periods.
The pumps behavior can be likened to that of a manual
transmission in an automobile. While a car can be driven 80 mph
in first gear, it would be a mistake to do this for an hour or
so. Machines are built for operation within a finite design
range. As a final analogy, a small Piper Cub aircraft can be
flown 50 mph for as long as it has fuel; however, that speed is
not possible with a superbly designed Boeing 787at least
not at cruising altitudes. HP
1 This article is synthesized, with permission,
from: Bloch, H. P., Pump Wisdom: Problem Solving for
Operators and Specialists, John Wiley and Sons, Hoboken,
New Jersey, 2011.
2 SKF USA Inc., Bearings in centrifugal
pumps, Version 4, Kulpsville, Pennsylvania, 2008.
Excerpted or adapted by permission of the copyright
3 Emile Egger & Cie., Salt Lake City, Utah, and
4 ITT/Goulds Pump Corp., Installation and
Maintenance Manual for Model 3196 ANSI Pump, Seneca Falls,
New York, 1990.
5 Mitsubishi Heavy Industries Ltd., Publication
HD30-04060, Tokyo, Japan, and New York, New York.
6 World Pumps, February 2010, p. 19.
7 Fraser, W. H., Avoiding recirculation in
centrifugal pumps, Machine Design, June 10,
8 Ingram, J. H., Pump reliabilityWhere
do you start? ASME Petroleum Mechanical Engineering
Workshop and Conference, September 1981, Dallas, Texas.
9 Bloch, H. P. and A. Budris, Pump Users
Handbook, 3rd Ed., Fairmont Press, Lilburn, Georgia,
Heinz P. Bloch is HPs
reliability/equipment editor. A practicing consulting
engineer with 52 years of applicable experience, he
advises process plants worldwide on failure analysis,
reliability improvement and maintenance cost avoidance.
He has authored or co-authored 18 textbooks on
machinery reliability improvement and over 550 papers
or articles dealing with related subjects. More
up-front reviews of process compressors can be found in
the most recent Bloch-Geitner book, Compressors:
How to achieve high reliability and
availability, McGraw-Hill, New York, New York,