CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP
FOR YOUR APPLICATION.
NOTE: The blue links do not work on this
version of the tutorials. They only work on the CD
We'll begin by deciding what operating conditions our pump has to
meet, and then we'll approach pump suppliers to see how closely they
can satisfy those needs. Unfortunately no comprehensive theory which
would permit the complete hydrodynamic design of a centrifugal pump
has evolved in the many years that pumps have been around, so the
pump manufacturer will be doing the best he can with the information
you supply to him.
To clearly define the capacity
and pressure needs of our system
we'll construct a type of graph called a system
curve. This system curve will then be given to the pump suppliers and
they will try to match it with a pump curve that satisfies these
needs as closely as possible.
To start the construction of the system curve I'll assume you want
to pump some fluid from point "A" to point "B." To do that
efficiently you must make a couple of decisions:
- Decide the capacity you'll need. This means the gallons per
minute or cubic meters per hour. You must also consider if this
capacity will change with the operation of your process. A boiler
feed pump is an example of an
application that needs a constant pressure with varying capacities
to meet a changing steam demand. The demand for boiler water is
regulated by opening and closing a control valve on the discharge
side of the pump with a discharge re-circulation line returning
the unneeded portion back to a convenient storage place, or the
suction side of the pump. Remember that with a centrifugal
pump if you change its capacity you change the pressure (head) also. A
rotary or positive displacement
pump is different. It puts out a constant capacity regardless of
- For other centrifugal pump applications, you're going to have
to calculate how much pressure will be needed to deliver different
capacities to the location where you'll need them. You'll need enough
pressure to :
- Reach the maximum static
head or height the fluid will have to attain.
- Over come any pressure that might be in the vessel where
the fluid is discharging, such as the boiler we just discussed.
This is called the pressure
- Overcome friction resistance in the lines, fittings and any
valves or hardware that might be in the system. As an example:
high-pressure nozzles can be tricky, especially if they clog
up. This resistance is called the friction
- Will you need any special materials for the pump components?
- The pump manufacturer will try to choose pump metal
components that are chemically compatible with what you're
pumping, as well as any cleaners or solvents that might be
flushed through the lines. If the temperature of the pumpage
changes the corrosion rate can change also. His choice of
materials could have a serious affect on your spare parts
inventory. Will he be selecting universal and easily obtainable
materials? Unless you have a great deal of experience with the
product you're pumping, do not select the metal components by
using a compatibility chart. Metal selection is a job for
metallurgists, or your own experience.
- If the product you're pumping is explosive, or a fire
hazard, you should be looking at non-sparking materials for the
pump components. Do not depend totally upon the pump
manufacturer to make this decision for you. If you're not sure
what materials are compatible with your product, how will the
pump man know? Also, keep in mind that some of the fluids
you'll be pumping could be proprietary
products known only by their trade name.
- Dangerous and radioactive materials will dictate special
- Food products require high-density seal and pump materials
that are easy to clean and sterilize.
- If there are abrasive solids in the pumpage, you'll need
materials with good wearing capabilities. Hard surfaces and
chemically resistant materials are often incompatible. You may
have to go to some type of coating on the pump wetted
parts or select an expensive duplex
- Occasionally you'll find an application where metal is neither
comparable nor practical. There are many monomer and
polymer materials available for these applications, but their cost
is generally higher than comparable metal parts. Be aware that if
you're using a mechanical seal in a non-metallic pump, the seal
can't have metal parts in contact with the fluid, for the same
reasons the pump was manufactured from non-metallic materials. Use
a non-metallic seal in these
Since we're just getting into the subject, one of the first things
we should learn is that centrifugal pump people do not use the word
pressure. They substitute the
word "head", so you'll have to
calculate the three kinds of head that will be combined together to
give you the total head of the
system required to deliver the needed capacity. Here are the three
kinds of head you'll be calculating:
- The static head or maximum height that the liquid will reach.
We must also learn how to compensate for the siphon
affect from down running pipes on the discharge side of the pump.
Remember that if you fill a tank from the bottom instead of the
top the static head will continually increase. This is not a good
application for a centrifugal pump because the capacity is
decreasing with an increasing head. If you must fill from the
bottom, or if you'll be using the pump as an accumulator, a
rotary positive displacement pump
will be your best choice as long as it can meet the needed
- The pressure heads are next if the container we're pumping to,
or from, is pressurized. We'll have to learn how to convert
pressure units to head units because later on we'll need this
conversion knowledge to read the manufacturers pump curve. Pump
gages are labeled in psi or bar. Pump curves
are labeled in feet, or meters of head.
- The friction head is the last one that we'll have to
calculate. This head tells us how much friction or resistance
there is in both the suction and discharge piping, along with the
fittings and valves in the piping system. And to make the job a
little tougher this head changes dramatically as the pump capacity
You'll be calculating these heads on both the suction and
discharge side of the pump. To get the total head you'll subtract the
suction head from the discharge head to learn the head that
the pump must produce to satisfy the application. It'll become
obvious in the calculations, but I should mention here, that if the
suction head is a negative number, the suction and discharge heads
will be added together to get the total head. If you subtract a minus
number from a positive number you must add the numbers together. As
an example: 4 - (-2) = + 6
The total head of a pump seldom remains static. There are a number
of factors that can change the head of a pump while it is operating,
and you should become familiar with most of them.
All of this head information is calculated from piping, valve, and
fitting, friction graphs you'll
find in the index.This head data will be plotted on a set of
coordinates called a system curve.
Since we'll not be operating at a single point all of the time we'll
make the calculations for a range of different capacities and heads
that we might expect to encounter. This range is described as the
operating window we'll need to
satisfy the application.
Making these calculations is not an exact science because the
piping is seldom new, pipe inside diameters are not exact, and the
graphs you'll be consulting cannot
compensate for corrosion and solids built up on the piping, valve and
Life is never simple. This is the point where most people start
adding in safety factors to compensate for some of the unknowns.
These safety factors will almost always guarantee the selection of an
oversized pump that will run off of
its best efficiency point (BEP) most
of the time.
The final calculations are then plotted on the system curve that
describes what the pump has to do to satisfy the requirements of the
application. You can learn to do all of this by referencing the
- Calculating the total head in metric
- Calculating the total head in USCS
- Making a system curve, S111
The pump manufacturer requires a certain amount of net positive
suction head required (NPSHR) to
prevent the pump from cavitating. He shows that number on his
pump curve. When you look at the
curve you'll also note that the net positive suction head required
(NPSHR) increases with any increase
in the pump's capacity.
You'll also be calculating the net positive suction head available
(NPSHA) to be sure that the pump you
select will not cavitate. Cavitation
is caused by cavities or bubbles in the fluid collapsing on the
impeller and volute. In the pump business we recognize several
different types of cavitation. :
Pump cavitation is recognized in several different ways
- We can hear cavitation because it sounds like the pump is
pumping rocks or ball bearings.
- We can see the damage from cavitation on the pump's impeller
- The operator can sometimes tell if the pump is cavitating
because of a reduction in the pump's capacity.
- The main problem with cavitation is that it shakes and bends
the shaft causing both seal and bearing problems. We call all of
this shaking and bending shaft
Remember that the net positive suction head required (NPSHR)
number shown on the pump curve is for fresh water at 68°
Fahrenheit (20°C) and not the fluid or combinations of fluids
you'll be pumping.
When you make your calculations for net positive suction head
available (NPSHA) the formula you'll
be using will adjust for the specific gravity of your fluid.
- In some cases you can reduce
the NPSH required. This is especially true if you're pumping hot
water or mixed hydrocarbons.
- You may have to install an inducer
on the pump, add a booster pump,
or go to a double suction pump
design if you don't have enough net positive suction head
When the pump supplier has all of this in-exact information in his
possession he can then hopefully select the correct size pump and
driver for the job. Since he wants
to quote a competitive price he is now going to make some critical
He might begin with the type of pump he'll recommend:
- If the capacity were going to be very low he would recommend a
rotary, or positive displacement
- Between 25 and 500 gpm (5 m3 /hr - 115
m3/hr) he'll probably select a single stage
end suction centrifugal pump. It all depends upon the
supplier. At higher capacities he may go to a double suction
design with a wide impeller, two pumps in parallel
or maybe a high-speed pump.
- You might need a high head, low
capacity pump. The pump supplier has several options you
should know about.
- Will he recommend a self-priming
pump? These pumps remove air from the impeller eye and suction
side of the pump. Some operating conditions dictate the need for a
self-priming design. If you do not have a self-priming pump and
you're on intermittent service, will priming become a problem the
next time you start the pump?
- How will the pump be operated?
- If the pump is going to run twenty-four hours a day, seven
days a week and you're not going to open and close valves;
you will not need a heavy-duty
pump. It's easy to select a pump that'll run at its best
efficiency point and at the best efficiency point (BEP)
there's very little shaft displacement
- Intermittent service is the more difficult application
because of changing temperatures, vibration levels, thrust
direction, etc. Intermittent pumps require a more robust,
heavy-duty design with a low L3/D4
- How important is efficiency
in your application? High efficiency is desirable, but you pay a
price for efficiency in higher maintenance costs and a limited
operating window. You should be looking for performance,
reliability, and efficiency in that order. Too often the engineer
specifies efficiency and loses the other two. The following
designs solve some operation and maintenance problems, but their
efficiency is lower than conventional centrifugal pumps.
- A magnetic drive or
canned pump may be your
best choice if you can live with the several limitations they
- A vortex or slurry
pump design may be needed if there are lot of solids or
"stringy" material in the pumpage.
- A double volute
centrifugal pump can eliminate many of the seal problems we
experience when we operate off the pump's best efficiency
point. The problem is trying to find a supplier that will
supply one for your application. Although readily available for
impellers larger than 14 inches (355 mm) in diameter they have
become very scarce in the smaller diameters because of their
less efficient design.
- The supplier should recommend a centerline
design to avoid the problems caused by thermal expansion of the
wet end if you're operating at temperatures over 200°F
- Will you need a volute or circular casing? Volute casings
build a higher head; circular casing are used for low head and
- Do you need a pump that meets a standard?
ANSI, API, DIN, VDMA or ISO are some of the current standards. You
should be aware of pump standards problems that contribute to
premature seal and bearing failures. An ANSI
(American National Standards Institute) standard back pullout
design pump has many advantages but presents problems with
mechanical seals when the impeller clearance is adjusted, unless
you're purchasing cartridge seals.
- The decision to use either a single or multistage
pump will be determined by the head the pump must produce to meet
the capacities you need. Some suppliers like to recommend a high
speed small pump to be competitive, other suppliers might
recommend a more expensive low speed large pump to lessen NPSH and
There are additional decisions that have to be made about the type
of pump the supplier will recommend:
- Will the pump be supplied with a mechanical seal or packing?
If the stuffing box is at negative pressure (vacuum) a seal will
be necessary to prevent air
- If he's going to supply a mechanical seal will he also supply
an oversized stuffing box and any
environmental controls that might be needed?
- Will he specify a jacketed stuffing box so that the
temperature of the sealed fluid can be regulated? How does he
intend to control the stuffing box temperature? Will he be using
water, steam or maybe a combination of both? Electric heating is
sometimes an option.
- How will the open or semi-open impeller
be adjusted to the volute casing or back plate? Can the mechanical
seal face loading be adjusted at the same time? If not, the seal
face load will change and the seal life will be shortened.
- If the pump is going to be supplied with a closed
impeller you should have some means of knowing when the wear
rings have to be replaced. If the wear
ring clearance becomes too large the pumps efficiency will be
lowered causing heat and vibration problems. Most manufacturers
require that you disassemble the pump to check the wear ring
clearance and replace the rings when this clearance doubles.
- Will he supply a "C" or "D" frame adapter,
or will the pump to motor alignment
have to be done manually using dual indicators or a laser aligner
to get the readings? A closed-coupled
design can eliminate the need for an alignment between the pump
- What type of coupling will he
select to connect the pump to its driver? Couplings can compensate
for axial growth of the shaft and transmit torque to the impeller.
They cannot compensate for pump to driver misalignment as much as
we would like them to. Universal joints are especially bad because
they have to be misaligned to be lubricated.
- He may decide to run two pumps in parallel
operation if he needs a real high capacity, or two pumps in
series operation if he needs a
high head. Pumps that run in parallel or series require that they
are running at the same speed. This can be a problem for some
- An inline pump design can solve many pipe strain and thermal
- The pump supplier must insure that the pump will not be
operating at a critical speed or
passing through a critical speed at start up. If he has decided to
use a variable speed drive or motor this becomes a
- We all want pumps with a low net positive suction head
required to prevent cavitation problems but sometimes it's not
practical. The manufacturer has the option of installing an
inducer or altering the pump
design to lower the net positive suction head required, but if he
goes too far all of the internal clearances will have to be
perfect to prevent cavitation problems. This modification of the
impeller to get the low net positive suction head required (NPSHR)
and its affects will be explained when you learn about
suction specific speed.
- The difference between
specific speed and suction specific speed can be confusing but you
should know the difference.
- Shaft speed is an important decision. Speed affects pump
component wear and NPSH requirements, along with the head,
capacity, and the pump size. High
speed pumps cost less initially, but the maintenance costs can
be staggering. Speed is especially critical if you're going to be
specifying a slurry pump.
- The ratio of the shaft diameter to its length is called the
shaft L3/D4number. This ratio will have a
major affect on the operating window of the pump and its initial
cost. The lower the number the better, but any thing below 60 (2
in the metric system) is acceptable when you're using mechanical
seals. A low L3/D4 can be costly in a
standard long shaft pump design because it dictates a large
diameter shaft that is usually found only on expensive heavy-duty
pumps. A short shaft with a smaller outside diameter would
accomplish the same goal, but then the pump would no longer
conform to the ANSI or ISO
standard. We often run into L3/D4problems
when you specify, or the pump supplier sells you a low cost,
corrosion resistant sleeve, mounted on a steel shaft rather than a
more expensive solid, corrosion resistant shaft.
There are multiple decisions to be made about the impeller
selection and not all pump suppliers are qualified to make them:
- The impeller shape or specific
speed number will dictate the shape of the pump curve, the
NPSH required and influence the efficiency of the pump.
- Has the impeller configuration been iterated
in recent years? Impeller design is improving with some of the
newer computer programs that have become available to the design
- The suction specific speed
number of the impeller will often predict if you're going to
experience a cavitation problem.
- The impeller material must be chosen for both chemical
compatibility and wear resistance. You should consider one of the
duplex metals because most
corrosion resistant materials are too soft for the demands of a
- The decision to use a closed
impeller, open impeller, semi-open, or vortex design is another
decision to be made.
- Closed impellers require wear
rings and these wear rings present another maintenance
- Open and semi-open
impellers are less likely to clog, but need manual adjustment to
the volute or back-plate to get the proper impeller setting and
prevent internal recirculation.
- Vortex pump impellers are great for solids and "stringy"
materials but they are up to 50% less efficient than conventional
- Investment cast impellers are
usually superior to sand cast versions because you can cast
compound curves with the investment casting process. The compound
curve allows the impeller to pump abrasive fluids with less vane
- If you're going to pump low specific
gravity fluids with an open impeller, a non-sparking type
metal may be needed to prevent a fire or explosion. You'll be
better off choosing a closed impeller design with soft wear rings
in these applications.
- The affinity laws will
predict the affect of changing the impeller speed or diameter.
You'll want to be familiar with these laws for both centrifugal
and PD pumps..
Either you or the supplier must select the correct size electric
motor, or some other type of driver
for the pump. The decision will be dictated by the specific gravity
of the liquid you'll be pumping, along with the specific gravity of
any cleaners or solvents that might be flushed through the lines. The
selection will also be influenced by how far you'll venture off the
best efficiency point (BEP) on the capacity side of the pump curve.
If this number is under-estimated there is a danger of burning out
some electric motors.
- How are you going to vary the pump's capacity? Are you going
to open and close a valve or maybe you'll be using a variable
speed drive, or maybe a gasoline or diesel engine. Will the regulating
valve open and close automatically like a boiler feed valve, or
will it be operated manually? The variable speed motor might be an
alternative if the major part of the system head is friction head
rather than static or pressure head.
- The viscosity of the
fluid is another consideration because it'll affect the head,
capacity, efficiency and power requirement of the pump. You should
know about viscosity and how the viscosity of the pumpage will
affect the performance of the pump. There are some viscosity
corrections you can make
to the pump curve when you pump viscous fluids.
- After carefully considering all of the above, the pump
supplier will select a pump type and size, present his quote and
give you a copy of his pump curve. Hopefully you'll be getting his
best pump technology. To be sure
that's true you should know what the best
pumping technology is.
- At this stage it is important for you to be able to read the
pump curve. To do that you must
- Best efficiency point (BEP)
- Shut off head.
- How to convert pressure to head so you can reference pump
gage readings to the pump curve. When you learn the three
formulas you'll get the conversion information.
- Brake horsepower (BHP)
- Water horsepower (WHP)
- Net positive suction head required (NPSHR)
- How to calculate the net positive suction head available
(NPSHA) to the pump to insure
you'll not have a cavitation problem.
If all of the above decisions were made correctly, the pump
supplier will place his pump curve on top of your system curve and
the required operating window will fall within the pump's operating
window on either side of the
best efficiency point (BEP). Additionally, the motor will not
overheat and the pump should not cavitate.
If the decisions were made incorrectly, the pump will operate where
the pump and system curves intersect and that will not be close to,
or at the best efficiency point, producing radial
impeller loading problems that will cause shaft deflection, resulting
in premature seal and bearing failures. Needless to say the motor or
driver will be adversely affected also.
With few exceptions pump manufacturers are generally not involved
in mechanical sealing. You'll probably be contacting separate seal
suppliers for their recommendation about the mechanical seal.
Recent mergers between pump and seal companies unfortunately does
not produce the instant expertise we'd like our sales and service
people to posses.
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CD with over 600 Seal & Pump Subjects
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