Recirculation occurs at reduced flows and is the reversal of a portion of the flow back through the impeller. Recirculation at the inlet of the impeller is known as suction recirculation. Recirculation at the outlet of the impeller is discharge recirculation. Suction and discharge recirculation can be very damaging to pump operation and could be avoided for the continuous of pumps of significant energy level or pressure rise per stage.
Diagnosis from Pump Operation
Suction recirculation will produce the previously mentioned loud cracking noise and around the suction of the pump. Recirculation is of greater intensity than the noise from low NPSH cavitations and is a random knocking sound. Discharge recirculation will produce the same characteristic sound as suction recirculation except that the highest is in the discharge volute or diffuser.
Diagnosis from Visual Examination
Suction and discharge recirculation product evaporation damage to the pressure side of the impeller vanes. Viewed from the suction of the impeller, the pressure side would be the invisible, or underside, of the vane. This is unlike cavitations damage from inadequate NPSH that occurs on the low pressure surface of the inlet vanes. Damage to the pressure side of the vane from discharge recirculation is shown in the drawing. Similarly the casing tongue or diffuser vanes may show cavitations damage on the impeller side from operation in discharge recirculation.
Instrumentation
The presence of suction or discharge recirculation can be determined by monitoring the pressure pulsation in the suction and in the discharge areas of the casing. Piezoelectric transducer installed as close to the impeller as possible in the suction and in the discharge of the pump can be used to detect pressure pulsation. The data may be analysed with a spectrum analyser couple to an XY plotter to produce a record of the pressure pulsation versus the frequency for selected flows.
Corrective Procedure:
Every impeller design has specific recirculation characteristics. This characteristics are inherent in the design and cannot be changed without modifying the design. An analysis of the symptoms associated with recirculation should consider the following as possible corrective procedures:
- Increase the output capacity of the pump
- Install a by pass between the discharge and the suction of the pump
- Substitute an improve material for the impeller that is more resistant to cavitation damage
- Modify the impeller design
