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.
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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.