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The pump is not producing enough capacity to satisfy the application 10-10

This is the second paper in a four part series about pump troubleshooting. 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. 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 width is too narrow
  • The impeller speed is too slow. Check the voltage and frequency
  • The impeller is damaged.
  • The impeller is clogged.
  • The open impeller clearance is too large.
  • The impeller to cutwater clearance is too large.
  • The impeller specific speed number is too low.
  • The impeller has been installed backwards
  • The shaft is running backwards.
  • The wear ring clearance is too large.
  • A wear ring is missing.
  • The second stage of a two stage pump is wired backwards.
  • A bubble is trapped in the eye of the impeller.
  • A low suction tank level is increasing the differential pressure across the pump decreasing its capacity.
  • Air is coming into the pump suction through the packing.
  • Air is coming into the pump suction through an unbalanced mechanical seal.
  • The pump was not primed prior to star up.
  • You may need a concentric casing rather than the volute design.
  • You are using a variable speed motor trying to produce a flat curve. Remember that both the head and capacity change with speed. /
  • The pump is the wrong size. Someone gave the pump distributor a wrong system curve




  • There is too much piping between the pump suction and the source tank.
  • There is an elbow too close to the pump suction.
  • A filter or strainer is clogged.
  • Intermittent plugging of the suction inlet. Loose rags can do this.
  • A foot valve is stuck
  • The tank float is stuck. Showing a higher tank level that does not exist.
  • The tank vent is partially shut or frozen.
  • A globe valve has been substituted for a gate valve.
  • A check valve is stuck partially closed
  • Solids have built up on the piping walls.
  • A liner has broken away from the piping wall and has collapsed in the piping.
  • The piping was collapsed by a heavy object that hit the outside of the piping.
  • A foreign object is stuck in the piping It was left there when the piping was repaired.
  • A small clam cleared the suction screen, but has now grown large on the pump side of the screen.
  • The sun is heating the inlet piping. It should be insulated to prevent this problem.
  • Piping was added on the inlet side of the pump to compensate for a piece of equipment that was installed in the shop.
  • A reducer has been installed upside down.
  • A discharge recirculation line is heating the incoming fluid.
  • The pump capacity is too high for the tank volume.
  • Multiple pump inlets are too close together.
  • The suction lift is too high.
  • There is not enough NPSH available for the fluid you are pumping. Maybe you can use an inducer to increase the suction pressure.
  • Air is coming into the system through valves above the water line or gaskets in the piping.
  • 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 fluid is vortexing at the pump inlet because the sump level is too low.
  • The tank is being heated to deaerate the fluid, but it is heating the fluid up too much.
  • Two pumps are connected in series. The first pump is not sending enough capacity to the second pump.
  • The operating temperature of the pumped fluid has increased.
  • 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.
  • The suction is being throttled to prevent the heating of the process fluid.


  • Extra piping has been added to the system to accommodate extra storage capacity.
  • A bypass line has been installed in the pump discharge.
  • Piping or fittings have been added to the discharge side of the pump.
  • An orifice has been installed in the discharge piping to reduce the capacity or produce a false head.
  • A gate valve has been substituted for a globe valve in the discharge piping.
  • A check valve is stuck partially closed.
  • An orifice has been installed into the piping to restrict flow.
  • The piping was collapsed by a heavy object that hit the outside of the piping.
  • The discharge valve is throttled too much.
  • There is a restriction in the discharge piping.
  • Extra pumps have been installed into the existing piping They are connected in parallel, but are not producing the same head.
  • Two pumps are in parallel. The larger one is shutting the check valve of the smaller pump.
  • 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 increasing and the capacity is decreasing as the tank fills.
  • The pump is acting as an accumulator&emdash;coming on when the tank level drops. The head is too high when the tank fills.

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