Air is entering the suction piping at some point.

    • Air is entering the stuffing box between the pump sleeve and shaft. Many times this gasket is left off when converting from packing to a mechanical seal.
    • 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.
    • 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.
    • 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 gasket that seals the halves of a horizontally split case must penetrate the face of the stuffing box if a mechanical seal is installed or there will be leakage at some point in the stuffing box near the face.

 There is a problem with the piping layout.

    • There is too much piping between the pump suction and the source tank. You may need a booster pump.
    • 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.
    • A piece of pipe of reduced diameter has been installed in the suction piping.
    • 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 piping to pump reducer has been installed upside down causing an air pocket. Concentric reducers can cause the same problem..
    • The pump capacity is too high for the tank volume.
    • 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. 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.
    • Intermittent plugging of the suction inlet. Loose rags can do this.
    • A foot valve is stuck.
    • A check valve is stuck partially closed
    • A small clam or marine animal cleared the suction screen, but has now grown large on the pump side of the screen.
    • If the suction is from a pond or the sea, grass can be pulled into the suction inlet.

The 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.
    • A liner has broken away from the piping wall and has collapsed in the piping.
    • 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 common with fuel pumps where discharge throttling could cause a fire or explosion.


    • Extra piping has been added to the system to accommodate extra storage capacity.
    • A bypass line has been installed in the pump discharge increasing the capacity.
    • Piping or fittings have been removed from the discharge side of the pump reducing piping resistance.
    • A globe valve has been substituted for a gate valve.  This will increase the resistance in the piping.
    • Solids building up on the piping inside wall will restrict flow.
    • A check valve is stuck partially closed.
    • An orifice has been installed into the piping to restrict flow or to create a false head.
    • 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.
    • Units in the discharge piping should not be shut off, they should be by-passed.
    •  Be sure to consider the siphon affect of piping extending into the top of the tank when calculating the total 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 size impeller.
    • The pump discharge is connected to the bottom of the tank. The head is constantly changing.
      • The pump is acting as an accumulator, coming on when the tank level drops.


  • On February 16, 2018