HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – SINGLE STAGE

During operation of centrifugal pump, the hydraulic imbalance is caused due to the different values of pressures in the pump like discharge pressure and suction pressure acting on either side of the impeller and around circumferential direction of the impeller.

The following are the two hydraulic forces acting on the impeller.

Ø  Radial Thrust

Ø  Axial Thrust

Bearing life of the centrifugal pump also depends on the above two hydraulic forces.

Radial Thrust

The force generated in radial direction due to unequal pressure generation in volute is called radial thrust.

Radial thrust can be reduced by

ü  Double volute casings

ü  Diffuser type casings

 

 Axial Thrust

The force generated in axial direction due to unequal pressure generation on either side of the impeller is called axial thrust.

Axial thrust can be carried away by thrust bearings.

  Axial thrust can be reduced by

ü  Provision of balancing holes on pump impeller

ü  Provision of back vanes or back plate on the impeller

Provision of balancing holes on pump impeller

·         Drilling hole through impeller back shroud into front portion of impeller. There by pressure distribution is in such a way that axial thrust is balanced up to some extent.

Figure below shows the pressure distribution of the fluid and reduction in axial thrust in the centrifugal pump when balancing hole is provided. 

·         Leakage through holes is directed against the flow in the impeller eye causing disturbances.

·         But balancing by this method will not fully reduce axial thrust.

Provision of back vanes or back plate on the impeller

·         Radial vanes are used on back shroud of the impeller to reduce the pressure in the space between the impeller and pump casing.

·         Radial back vanes provided at back shroud of impellers acts as an auxiliary impeller which restricts the entry of the liquid into the clearance between impeller back shroud and casing cover.

·         Balancing by this method reduces axial thrust in a better way when compared to provision of balancing holes.

Figure below shows the pressure distribution of the fluid and reduction in axial thrust in the centrifugal pump when back vanes are provided. 

Pic credits

Axial Thrust in centrifugal pumps - Experimental analysis (15th international conference on experimental mechanics)