Tuesday, May 10, 2011

Centrifugal pumps (3)


The kinetic energy of the fluid flowing through the impeller is converted to pressure energy by the shape of the volute casing. For high pressure pumps such as boiler feed pumps a diffuser ring is fitted in the casing which converts a greater portion of the pressure energy allowing greater pressures to be generated.
A scroll type inducer may be fitted to the inlet which improves the efficiency of unit and allows the pump to operate with low suction pressures.

Wear rings

For efficient operation it is important to ensure that leakage from the high to low pressure side is kept to a minimum. This is achieved by the use of wearing rings. Traditionally these are fitted to the casing,to increase the longevity of the impeller wear ring tyros may be fitted.
The clearance given for wear rings is often a source of contention especially when dealing with on-ship made rings. A clearance of 1/1000 of the diameter of the bore is often quoted although this may be very difficult to achieve in practice.

Axial force

Without careful design an axial force is created by the action of the impeller. This is due to the low pressure acting on the suction eye whilst the rest of the impeller is subjected to discharge pressure.
One solution is shown above where radial blades are cast into the back (stuffing box side) of the impeller. These blades are commonly called pump-out vanes, and are meant to increase the centrifugal force of the fluid trapped behind the impeller. This causes the fluid to be "thrown" outwards, reducing the pressure behind the impeller for the same reason that the impeller causes a reduction of pressure at the suction eye.
Another method which may be found in conjunction with the pump-out vanes are the balancing holes. These are holes drilled near the center of the impeller, connecting the space in the back of the impeller with the suction eye. This reliefs the pressure behind the impeller by allowing the high pressure fluid trapped there to flow to the low pressure region at the suction eye. In order for this to be effective, there must be a tight clearance between the impeller and the casing to reduce the flow of fluid into the back of the impeller.
Alternately dual back to back impellers may be fitted in common with a double casing.
Materials suitable for general service 
Shaft Stainless steel Aluminum bronze  
Impeller Casing Bronze or cast iron 
Wear ring Aluminum bronze or brass . continued....


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