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Centrifugal Pump Operating Range |
- All types of pumps have operational limitations. This is a consideration with any pump whether it is positive displacement or centrifugal. The single volute centrifugal pump ( the most common pump used worldwide) has additional limitations in operating range which, if not considered, can drastically reduce the service life of pump components.
- "BEP" - Best Efficiency Point ( refer to fig.3 below ) is not only the operating point of highest efficiency but also the point where velocity and therefore pressure is equal around the impeller and volute. As the operating point moves away from the Best Efficiency Point, the velocity changes, which changes the pressure acting on one side of the impeller. This uneven pressure on the impeller results in radial thrust which deflects the shaft causing:
- Excess load on bearings.
- Excess deflection of mechanical seal. or:
- Uneven wear of gland packing or shaft / sleeve.
The resulting damage can include shortened bearing / seal life or a damaged shaft . The radial load is greatest at shut head.
- Outside the recommended operating range damage to pump is also sustained due to excess velocity and turbulence. The resulting vortexes can create cavitation damage capable of destroying the pump casing, back plate, and impeller in a short period of operation. Refer to fig.3 which indicates range of operation ( between approximately 50% and 120% of Best Efficiency Point )
- When selecting or specifying a pump, it is important not to add safety margins or base selection on inaccurate information. The actual system curve may cross the pump curve outside the recommended operating range. In extreme cases the operating point may not allow sufficient cooling of pump, with serious ramifications !
- The best practice is to confirm the actual operating point of the pump during operation ( using flow measurement and / or a pressure gauge ) to allow adjustment ( throttling of discharge or fitting of bypass line ) to ensure correct operation and long service life.
fig. 3
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