The centrifugal pump always pumps the difference between the suction and discharge heads. If the suction head increases, the pump head will decrease to meet the system requirements. If the suction head decreases the pump head will increase to meet the system requirements. 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 increase.

The pump will pump where the pump curve intersects the system curve. 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.


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 actual speed.

Your pulley driven pump is running on the wrong pulley diameter.

A variable frequency motor is running at the wrong speed.
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 bearings etc.

Check the voltage of the electric motor. It may be too low.

The impeller is damaged. The damage could be caused by excessive wear, erosion, corrosion or some type of physical damage. 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 stationary volute.

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 balanced.
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 cut water clearance can cause a problem called discharge recirculation. Wear is a common symptom of this condition. If the impeller is positioned too close to the cutwater you could have cavitation problems that will interfere with the head.

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 problem.

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 unreliable.

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 inspection.

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 installation process.

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 etc.

Air is coming into the stuffing box through the pump packing.

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 box.

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 requirements:

The pump was not selected to meet the system curve requirements because no system curve was given to the pump supplier.

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.


Air is entering the suction piping at some point.

Air is being pumped into the suction piping to reduce cavitation problems.

Fluid returning to the sump is being aerated by too far a free fall. The return line should terminate below the liquid level.

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 problem.

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 something. 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 explosion.

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 lines.

The problem is in the tank connected to the suction of the pump.

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 suction pressure.

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 the head.


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 pump's capacity.

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 recharged.

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 siphoning action.

A discharge valve (manual or automatic) is open.

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