LEARNING ABOUT CENTRIFUGAL
This mini course is all about centrifugal
pumps. I 've chosen this type of pump to talk about because,
unlike positive displacement pumps (PD pumps), they can be altered
and modified to just about satisfy what ever pumping needs you might
I've decided the best way to handle this subject is to write an
outline narrative that will get the pumping terms and concepts into
some type of perspective. Once you become familiar with the terms
you can turn to the individual subjects for a detailed explanation.
The phrases colored in blue are links to
a more detailed explanation of that particular subject. Here we
You want to pump some liquid from point "A" to point "B". To do
that you must first make a couple of decisions:
- Decide what materials you'll need that are chemically
compatible with the fluid you're pumping, as well as any cleaners
or solvents that will be flushed through the pump and the process
lines. The pump manufacturer is not always the best qualified to
make these decisions. Many times your fluid is a proprietary
product and the knowledge of compatible materials is only
available from your own people.
- Decide the capacity you'll need (the gallons per minute or
cubic meters per hour). Will this capacity vary at times? If it
does, the head will also change with the capacity and your pump
must be ruggard enough to operate at various points on the pump
- You're going to have to calculate how much pressure (psi. or
bar) will be needed to deliver this capacity to the point where
you'll need it. You will need enough pressure to :
- Reach the maximum static height the moving fluid will have
- Overcome the pressure in a pressurized tank or boiler.
- Overcome resistance to the fluid moving in the lines,
fittings and any valves or hardware that might be in the
- We'll begin by learning that centrifugal pump people do not
use the word pressure. They substitute the word "head" (measured
in feet or meters) instead, so you'll have to calculate the
three kinds of head that will be required to deliver the needed
- The static head or maximum height that the liquid will
reach. We must also learn how to compensate for the siphon
affect from any down running pipes on the discharge side of the
- The pressure head is next if the container we are pumping
to or from is pressurized. A pressurized boiler would be a good
example. You'll have to know how to convert pressure units to
head units. We also need this conversion knowledge to read the
manufacturers pump curve.
- The friction head is the one that we have to calculate. It
tells us how much friction or resistance head there is in both
the suction and discharge piping, along with the fittings,
strainers, valves etc. And to make the job a little tougher,
this head changes as the pump speed or capacity changes. Since
the suction and discharge piping are normally different
diameters you'll be doing a lot of "walking the piping,"
counting the fittings and valves, looking at charts and graphs,
as well as making a lot of calculations.
Some of this information is calculated from charts and graphs you will find in this web site and various other
publications. Since we'll not be operating at a single pumping
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 data will then be plotted on a set of coordinates that we call a system curve. This pumping range
is often described as the "operating window" we'll need for the
Making these calculations is not an exact science because the piping is seldom new, diameters are not
exact, and the charts and graphs you'll be consulting cannot
compensate for corrosion and solids built up on the piping and
fitting walls. Life is never simple. This is where most people start
adding in safety factors to compensate for some of the unknowns.
These safety factors will almost guarantee the selection of an
oversized pump that will operate
off of its best efficiency point (B.E.P.) adding to its operating cost and making the pump
vulnerable to premature seal and bearing problems.
When the pump supplier has all of this inexact information in his
possession he can then try to select the correct size pump and driver
for the job. Since he wants to quote a low price he has to make some
- If the capacity is very low he'll probably recommend a
positive displacement (PD)
- Between 25 and 500 gpm (5 m3/hr - 115
m3/hr) he will probably select a single stage end
suction centrifugal pump. It all depends upon the supplier.
- At higher capacities he may go to a double ended design with a
- How will the open or semi-open impeller be adjusted for
efficiency? Can the mechanical seal be adjusted at the same time?
Impeller adjustments have to be made for both thermal growth and
- Will he supply a "C" or "D" frame adapter to simplify the
alignment process, or will the pump to motor alignment have to be
- Will he supply a centerline pump design to avoid the problems
with thermal expansion of both the pump and the piping at
operating temperatures over 200°F (100°C).
- Magnetic drive and canned
pumps have become very popular in recent years, maybe you should
be using that type and eliminate the need for mechanical seals or
- What type of coupling will he select to connect the pump to
- How will he handle the head/capacity requirement ?
- He may decide to run two pumps in parallel if he needs a
real high capacity.
- Or he might run two pumps in series if he needs a high
- The decision to use either a single or multiple stage pump
is another decision that has to be made.
- Will you need a volute or circular casing? Volute casings
build head, circular casing are used for lower heads and higher
- Should he specify a pump that meets the ANSI or ISO standard? It is one more decision to be made. These
standard pumps lack important
features that prevent problems with mechanical seals.
- He 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 motor this becomes a possibility.
- The affinity laws will
predict the affect of changing the impeller speed or diameter.
You will want to be familiar with these laws because in many
cases they will dictate if you should be using a variable speed
- Will he recommend a self priming pump? These pumps remove air
from the 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 are on intermittent service, will
priming become a problem?
- If he is supplying a self priming pump, is it equipped with
a mechanical seal that can seal a vacuum? If it is not, you are
going to have troubles keeping the prime.
- The ratio of the shaft diameter
to its length is called the L3/D4
number. This ratio will have a major affect on the operating
window of the pump and its initial cost. The lower the number the
wider the window, but any thing below 60 (2 in the metric system)
is acceptable. A low L3/D4 can be costly in
a standard pump design because it dictates a large diameter shaft
that is usually found only on expensive, heavy duty pumps. A short
shaft would accomplish the same goal, but then the pump would no
longer conform to the ANSI or ISO standard. We often run into
L3/D4 problems when you accept corrosion
resistant sleeves rather than a solid, corrosion resistant
- How much net positive suction
head (NPSH) will you need to prevent cavitation problems? We
all want pumps with a low net positive suction head required, but
sometimes it's not practical. The manufacturer has the option of
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
alteration is explained when you learn about suction
- A double suction design, or the installation of an inducer
on an end suction centrifugal pump is sometimes a sensible
solution to NPSH problems.
- Maybe the suction piping can be altered in some manner to
solve the problem.
- He must choose pump materials that are chemically compatible
with what you're pumping as well as any cleaners or solvents that
might be flushed through the lines. Be sure to remember that if
the temperature of the pumpage increases, the corrosion rate will
- The supplier must choose the correct size electric motor or some other type of driver. His
decision will be affected by the specific
gravity and the viscosity of
the liquid you'll be pumping. It will also be influenced by how
far you'll venture out of the operating window on the capacity
side of the pump curve. If this number is miscalculated there is a
danger of burning out the electric motor.
- If there are abrasive solids in the pumpage you will 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
incorporate "Duplex Metal" materials. Cartridge seals will
probably be necessary to compensate for the frequent impeller
- Shaft speed is an important decision. Speed affects pump
component wear as well as the pump size. High speed pumps cost
less initially, but the maintenance costs can be staggering.
- Will this application be affected by the OSHA
1910 regulation for hazardous
chemicals? If it is, you will have to address special "fugitive
emission" sealing problems.
- How will the bearings be lubricated? The choices are grease,
oil level and oil mist. Which should you use, and how are you
going to seal the bearing case to prevent the ingress of moisture
and solids? A simple grease or lip seal is not a very good choice
and developing fugitive emission laws might dictate your final
selection. You may want to look at other options that include
labyrinths and positive face seals that will add to the initial
- There are lots of decisions to be made about the impeller
- The impeller design or specific
speed number will dictate the shape of the pump curve and
influence the efficiency of the pump.
- The suction specific
speed number of the impeller predicts if you are going to
have a cavitation problem.
- The impeller material must be chosen for both chemical
compatibility and wear resistance. You might want to consider
- Investment cast impellers can be designed with compound
curves that work better in abrasive service. Sand cast versions
lose this option.
- The decision to use a closed impeller, open impeller,
semi-open, or vortex design is another decision to be made.
Closed impellers need wear rings and that presents another
After carefully considering all of the above, the pump supplier
will present his quote and supply you with a copy of his pump curve.
At this stage it is important for you to be able to read
the curve and to do that you must be able to understand what is
meant by :
- The pump's best efficiency point (BEP)
- Pump efficiency
- Shut off head.
- How to convert pressure to head so you can reference gage
readings on the pump and piping to the pump curve.
- Brake horsepower (BHP)
- Net positive suction head required (NPSHR)
- Net positive suction head available (NPSHA)
- How to calculate the NPSH
available to the pump to insure you will not have a cavitation
If all of these decisions were made correctly, the supplier will
place his pump curve on top of the system curve you supplied and
these curves should intersect at the pump's best efficiency point. At
this point the pump will experience minimum vibration, the motor will
not overheat, and the pump will 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 at the best
efficiency point , causing shaft
deflection. Needless to say the motor or driver will be adversely
affected along with the bearings and seal.
There are a number of additional conditions that'll affect the
performance and reliability of the pump. You will need to learn about
all of them:
- Alignment between the pump and its driver.
- The piping layout. Especially elbows and fittings at the pump
- The causes and affect of pipe strain.
- The base plate, grouting and
the pump pedestal where the pump and driver will be mounted.
- Dynamic balancing of the
rotating pump components.
- Is the piping new or has calcium or some other type of solids
been building up on the piping walls? The smaller the pipe inside
diameter the more the resistance we get in the piping.
- The differences between constant running and intermittent
- Many constant running pumps use a bypass line connected
from the discharge of the pump back to the suction side that
will open when the discharge is throttled. This could cause
problems with the heating of the liquid at the pump
- Intermittent service is the most difficult for the
mechanical seal. At shut down many fluids become viscous,
solidify or crystallize clogging up the seal and causing a
failure at the next start up.
- Will the lines be cleaned with some cleaner or solvent?
- If this cleaner has a different specific gravity than the
pumpage, the electric motor could be damaged.
- The rubber parts in mechanical seals are very sensitive to
cleaners and solvents. You may have to use one of the super compounds.
- If the application requires a varying capacity, remember that
the head will change with the capacity. In some applications like
a boiler feed pump, that may not be desirable.
- Maybe you should install monitoring
systems that will give you the running information you
- The proper way to start a centrifugal pump.
- Is the mass of the pump and its driver no more than 25% of the
mass of the pedestal the hardware will rest upon?
It would be nice if new pumps were trouble free. They seldom are.
When we describe pump problems we usually mean:
At some point you're going to have to become familiar with all of
these problems and their solution.
Knowing some basic rules of thumb about pumps helps.
When it comes to analyzing these pump failures you have several
opportunities that require different troubleshooting techniques:
- You can analyze the problem while the pump
is still operating. Here you get to use your senses of sight,
smell, feel and hearing
- You can look at the various components after the pump has been
disassembled in the shop. You will only be able to see evidence of rubbing and damage, and that is
usually enough information to troubleshoot the problem.
- There are some pump problems that have no apparent cause, but you still have to solve them .
If you're lucky, your company will purchase
the correct pump the first time, but that is not going to happen
so you will probably have to modify your existing pump to get the life you require from the bearings
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