SUBJECT: The pump is not producing enough
head to satisfy the application 10-9
Let me begin by pointing out that there are a couple of things you
must keep in mind when troubleshooting centrifugal pump problems:
- The centrifugal pump always pumps the difference between the
suction and discharge heads.
- A centrifugal pump always pumps a combination of head and
capacity. These two numbers multiplied together must remain a
constant. In other words, if the head increases the capacity must
decrease. Likewise if the head decreases, the capacity must
- The pump will pump where the pump curve intersects the system
- If the pump is not meeting the system curve requirements the
problem could be in the pump, the suction side including the
piping and source tank, or somewhere in the discharge system.
- Most pumps are oversized because of safety factors that were
added at the time the pump was sized. This means that throttling
is a normal condition in most plants, causing the pump to run on
the left hand side of its curve.
PROBLEM COULD BE IN THE PUMP ITS SELF
- The impeller diameter is too small.
- The impeller is running at too slow a speed
- You are running an induction motor. Their speed is
different than synchronous motors. It's always slower. The pump
curve was created using a variable frequency motor that ran at
a constant speed. Put a tachometer on your motor to see its
- Your pulley driven pump is running on the wrong pulley
- A variable frequency motor is running at the wrong
- Check the speed of the driver if the pump is driven by
something other than an electric motor.
- There is something physically wrong with the motor. Check the
- Check the voltage of the electric motor. It may be too
- The impeller is damaged. The damage could be caused by
excessive wear, erosion, corrosion or some type of physical
- Physical damage often occurs during the assembly process
when the impeller is driven on or off the shaft with a wooden
block and a mallet. Many impeller designs do not have a nut
cast into the impeller hub to ease removal.
- Erosion occurs when solids enter the eye of the impeller.
The solids can chip off pieces of the ceramic that are
passivating the impeller, causing localized corrosion.
- Damage can occur if the impeller to volute, or back plate
clearance is too small and the shaft experiences some type of
deflection. The original clearance could have diminished with
thermal growth of the shaft. Keep in mind that some open
impellers adjust to the volute (Goulds) while other designs
adjust to the back plate (Duriron).
- In an ANSI and similar design centrifugal pumps, the normal
thrust towards the volute has bent the snap ring designed for
bearing retention. This can allow the rotating impeller to hit the
- Here are some examples of shaft displacement:
- Operating the pump too far off the BEP.
- Pulley driven applications.
- Pipe strain.
- Misalignment between the pump and driver.
- The shaft could be bent.
- The rotating assembly was probably not dynamically
- The impeller is clogged. This is a major problem with closed
impellers. With the exception of finished product, most of what
you will be pumping contains entrained solids. Remember also that
some products can solidify, or they can crystallize with a change
in fluid temperature or pressure.
- Impeller balance holes have been drilled between the eye and
the wear rings of a closed impeller. The reverse flow is
interfering with the product entering the impeller eye. A
discharge recirculation line should have been used in place of the
balance holes to reduce the axial thrust.
- The double volute casting is clogged with solids or solids
have built up on the surface of the casting.
- The open impeller to volute clearance is too large. 0.017"
(0,5 mm) is typical. This excessive clearance will cause internal
recirculation problems. A bad installation, thermal growth, or
normal impeller wear could be the cause.
- A large impeller to cutwater clearance can cause a problem
called discharge recirculation. Wear is a common symptom of
- If the impeller is positioned too close to the cutwater you
could have cavitation problems that will interfere with the
- The impeller specific speed number is too high. Lower specific
speed numbered impellers are used to build higher heads.
- An impeller inducer was left off at the time of assembly.
Inducers are almost always needed with high specific speed
impellers. Leaving off the inducer can cause cavitation problems
that will interfere with the head.
- The impeller is loose on the shaft.
- The impeller is running backwards
- The shaft is running backwards because of a wiring
- The pump is running backwards because the discharge check
valve is not holding and system pressure is causing the reverse
rotation. This is a common problem with pumps installed in a
parallel configuration. Check valves are notoriously
- The impeller has been installed backwards. This can happen
with closed impellers on double ended pumps
- The second stage of a two stage pump is wired backwards. The
pump reverses when the second stage kicks in. You should have
heard a loud noise when this happened.
- The wear ring clearance is too large.
- This is a common problem if the shaft L3/D4
number is greater than 60 (2 in the metric system).
- You should replace the rings when the original clearance
doubles. Needless to say this can only be determined by
- If you are pumping a product at 200°F (100°C) or
more you should use a centerline design volute to prevent
excessive wear ring wear as the volute grows from the base
straight up, engaging the wear rings.
- A wear ring is missing. It was probably left off during the
- A high suction tank level is reducing the differential
pressure across the pump increasing its capacity. The pump pumps
the difference between the suction and discharge heads.
- A bubble is trapped in the eye of the impeller. The eye is the
lowest pressure area. When this bubble forms it shuts off all
liquid coming into the pump suction. This could cause the pump to
lose its prime.
- You cannot vent a running pump because centrifugal force will
throw the liquid out the vent leaving the air trapped inside.
- Air is coming directly into the pump. This happens with a
negative pressure at the suction side. Negative suction happens
when the pump is lifting liquid, pumping from a condenser hot well
- Air is coming into the stuffing box through the pump
- Air is coming into the stuffing box through an unbalanced
mechanical seal. As the carbon face wears the spring load
holding the faces together diminishes.
- If you are using mechanical seals in vacuum service, they
should be of the O-ring design. Unlike other designs, o-rings
are the only shape that seals both pressure and vacuum.
- The pump was not primed prior to start up. With the
exception of the self priming version, centrifugal pumps must
be full of liquid at start up.
- Air can enter the stuffing box if the gasket between the
two halves of a double ended pump is defective or does not
extend to the stuffing box face. Any small gaps between the
face of the stuffing box and the split at the side of the
stuffing box will allow either air in, or product out.
- Air is coming into the suction side of the pump through a
pin hole in the casing.
- Air is entering the stuffing box between the sleeve and the
shaft. This happens if you convert a double ended pump from
packing to a mechanical seal and fail to install a gasket or
o-ring between the impeller hub and the sleeve.
- The open impeller was adjusted backwards and now the close
fitting "pump out vanes" are creating a vacuum in the stuffing
- You need a volute casing instead of a concentric casing.
Volute casings are much better for producing head.
- You have the wrong size pump. It cannot meet the system curve
- The pump was not selected to meet the system curve
requirements because no system curve was given to the pump
- At replacement time the same size pump was purchased because
no one had calculated losses in the system.
- The pump was sized from a piping diagram that was thirty five
years old. There have been numerous piping changes and additions
since the original layout. In many instances additional pumps have
been installed and this pump is running in parallel with them, but
nobody knows it.
PROBLEM IS ON THE SUCTION SIDE OF THE PUMP. THE PUMP COULD BE
- Air is entering the suction piping at some point.
- Air is being pumped into the suction piping to reduce
- Fluid returning to the sump is being aerated by too far a
free fall. The return line should terminate below the liquid
- The fluid is vortexing at the pump inlet because the sump
level is too low and the pump capacity is too high.
- Air is coming into the system through valves above the
water line, or gaskets in the piping flanges.
- The liquid source is being pumped dry. If this is a problem
in your application you might want to consider a self priming
pump in the future.
- The vapor pressure of the fluid is too close to atmospheric
pressure. When it rains the drop in atmospheric pressure causes
the inlet fluid to vaporize.
- There is a problem with the piping layout. It is reducing the
head on the suction side of the pump.
- There is too much piping between the pump suction and the
source tank. You may need a booster pump or an inducer. The
higher the pump speed the bigger the problem.
- There is an elbow too close to the pump suction. There
should be at least ten diameters of pipe between the elbow and
the pump suction. Suction piping should never run parallel with
the pump shaft in a double ended pump installation. This can
cause unnecessary shaft thrusting.
- A piece of pipe of reduced diameter has been installed in
the suction piping.
- Piping was added on the inlet side of the pump to by-pass a
piece of equipment that was installed on the floor.
- A piping to pump reducer has been installed upside down
causing an air pocket. Concentric reducers can cause the same
- Multiple pump inlets are too close together.
- The pump inlet is too close to the tank floor.
- The suction lift is too high.
- A gasket with too small an inside diameter has been installed
in the suction piping restricting the liquid flow.
- A gasket in the suction piping is not centered and is
protruding into the product stream.
- A globe valve has been substituted for a gate valve in the
suction piping. The loss of head in a globe valve is many times
that of a gate valve.
- Two pumps are connected in series. The first pump is not
sending enough capacity to the second pump.
- The piping inlet is clogged.
- A filter or strainer is clogged or covered with
- Intermittent plugging of the suction inlet.
- Loose rags can do this.
- If the suction is from a pond, river, or the sea, grass can
be pulled into the suction inlet.
- A foot valve is stuck.
- A check valve is stuck partially closed
- The foot valve is too small.
- A small clam or marine animal cleared the suction screen, but
has now grown large on the pump side of the screen.
- The suction piping diameter has been reduced.
- The suction piping collapsed when a heavy object either hit
or ran over the piping.
- Solids have built up on the piping walls. Hard water is a
good example of this problem
- A liner has broken away from the piping wall and has
collapsed in the piping. Look for corrosion in the piping
caused by a hole in the liner.
- A foreign object is stuck in the piping It was left there
when the piping was repaired.
- The suction is being throttled to prevent the heating of
the process fluid. This is a common operating procedure with
fuel pumps where discharge throttling could cause a fire or
- The pump inlet temperature is too high.
- The tank is being heated to deaerate the fluid, but it is
heating the fluid up too much. Look for this problem in boiler
feed pump applications.
- The sun is heating the inlet piping. The piping should be
insulated to prevent this problem.
- The operating temperature of the pumped fluid has been
increased to accommodate the process requirements.
- A discharge recirculation line is heating the incoming
fluid. You should direct this line to a reservoir rather than
the pump suction.
- Steam or some other hot cleaner is being circulated through
- The problem is in the tank connected to the suction of the
- The pump capacity is too high for the tank volume.
- The tank float is stuck, showing a higher tank level that
does not exist.
- The tank vent is partially shut or frozen, lowering the
- There is not enough NPSH available for the fluid you are
pumping. Maybe you can use an inducer or booster pump to
increase the suction pressure.
- A high suction tank level is reducing the differential
pressure across the pump, increasing its capacity and lowering
ON THE DISCHARGE SIDE OF THE PUMP INCLUDING THE PIPING
- Two pumps are in connected in series. The first pump does not
have enough capacity for the second pump. They should be running
at the same speed with the same width impeller.
- The pump discharge is connected to the bottom of the tank. The
head is low until the level in the tank increases.
- Units in the discharge piping should not normally be shut off,
they should be by-passed to prevent too much of a change in the
- If too many units are being by-passed in the discharge system
the head will decrease as the capacity increases. This can happen
if an extra storage tank farm is being by-passed because the
storage capacity is no longer needed.
- A bypass line has been installed in the pump discharge
increasing the capacity and lowering the head.
- Piping or fittings have been removed from the discharge side
of the pump reducing piping resistance.
- Connections have been installed in the discharge piping that
have increased the demand that increases capacity.
- The pump is acting as an accumulator, coming on when the tank
level drops. The head will be low until the accumulator is
- Consider the possibility of a siphon affect in the discharge
piping. This will occur if the pump discharge piping is entering
into the top of a tank and discharging at a lower level The pump
must build enough head initially to take advantage of the
- A discharge valve (manual or automatic) is opened too
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