Maximum allowable pump speed


A frequently asked question among centrifugal pump users is: what is the
maximum allowable speed for their pumps? Many users are disappointed that,
oftentimes, pump vendors can not give them specific answers.


Indeed, there is no simple answer to this question because there are many
factors that limit the maximum permissible pump speed, and each installation is
unique in itself. Among these factors are:


  • Maximum allowable working pressure (MAWP) - Increasing the RPM will
    increase the pump differential pressure by the square of the speed ratio.
    The maximum RPM will be limited to a point where the shut-off pressure at
    maximum suction pressure will not exceed the pump's MAWP.

  • B-gap - The B-gap may no longer be adequate for the increase in energy
    density - this may result in high vibration level.

  • NPSHR - Increasing the RPM will increase the NPSHR. The increase in
    RPM will be limited to a point  where the NPSHR at the higher speed will
    remain below the NPSHA.

  • Shaft size - Increasing the RPM will increase the required brake horse-
    power (BHP). The shaft, coupling, mounting keys, etc., should be good for
    the increase in BHP. In some instances, where these parts are not good
    for the increase in BHP, it might be possible to upgrade to a stronger
    material, or to increase the size of these parts.

  • Bearing size - Higher speed will result in higher axial and radial thrust
    loads. The existing bearings may become inadequate to carry the increase
    in thrust loads.

  • Ball bearing maximum speed - The increase in RPM should not exceed
    the allowable maximum speed of the thrust ball bearings. Bigger ball
    bearings have lower maximum allowable RPM. For example, an oil-
    lubricated 7310 ball bearing has a maximum allowable speed of 5000
    RPM; that of 7313 is 4000 RPM.

  • Bearing lubrication requirement - The bearing lubrication requirement
    increases with RPM. At higher RPM oil-ring lubrication may no longer be
    sufficient, and forced-feed lubrication may be required.

  • Impeller peripheral speed - The impeller tensile strength will limit its
    maximum peripheral speed. Example, at ambient temperature, the
    allowable maximum RPM for 15" diameter impeller is:
    3130 RPM for Class 40, ASTM A-278 cast iron
    4120 RPM for alloy 952, ASTM B-148 aluminum bronze
    4425 RPM for ASTM A-216, Grade WCB cast steel

  • Auxiliary piping may need modification - Auxiliary pipings - such as
    mechanical seal flush, cooling piping, balance line - may require changes
    to pipe size, flange rating, orifice size, etc., due to the higher differential
    pressure.

  • Others - request a full version of this article [ * ]


Whether a pump will operate at higher speed continuously, or intermittently, is
also a factor. An example is a pump running at higher speed that results in
excessive liquid velocity at the volute throat area. The excessive velocity may be
tolerable in intermittent operation but not in continuous operation where it may
cause rapid erosion of the volute lips.


Because of many variables that limit the maximum allowable pump speed, a
better approach is to identify the intended operating conditions, and then ask the
vendor if the pump can meet those conditions, or how close it  can be. This may,
or may not, require some pump modifications.


[ * ] Some information are excluded.
Read more.


R: 0110-MAAL
C: design, operation
F: speed


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