Impeller Setting


Some impeller clearance settings are made between the impeller and the pump volute. In other designs the clearance is set between the impeller and the pump back plate.

Ideally there would be very little clearance between the impeller vanes and the pump volute or back plate, but any deflection of the shaft would cause the impeller to hit this stationary piece.

To prevent this contact the pump manufacturer has opened the impeller to volute clearance as far as he can without causing internal recirculation problems.

This clearance must be set when the pump is at operating temperature and must be re-set to compensate for any vane wear during operation. If the clearance becomes too large the pump will experience internal recirculation problems causing a rise in the pump’s internal temperature and a loss of some of its efficiency.

Most clearances are specified somewhere between 0.015 inches and 0.020 inches (0,4 mm to 0,5 mm) when the pump is at operating temperature, but be sure to check with your pump manufacturer for the correct number.

  • A typical setting technique is to tighten the nuts on the back of the pump power end until the impeller rubs against the volute. You can see these adjusting nuts on the left side of the illustration.
  • You then back off the nuts the proper clearance, and turn the jack bolts (located between the adjusting nuts) inward until the shaft assembly bottoms against the adjusting nuts

Remember that all impeller clearance settings must be done when the pump is at its operating temperature.

Most open impeller pumps will reduce their capacity about one percent for every 0.002 inches (0.05 mm) you open this clearance, because of internal recirculation problems.

You can tell that the impeller needs adjusting when you notice that the pump’s amperage requirement is the same, but the capacity is falling off.


  • On February 16, 2018