Reduction of Cavitation Damage

After the pump has been built and installed, there is little that can be done to reduce cavitation damage. As previously mentioned, sharpening the leading edges of the blades by filling may be beneficial Stepanoff has suggested cutting back part of the blades in the impeller eye together with sharpening the tips, for low specific speed pumps, as a means of reducing the inlet velocity c, and thus lowering parameter s. Although a small amount of prerotation or prewhirtl in the direction of impeller rotation may be desirable, excessive amount should be avoided. This may require straightening vanes ahead of the impeller and rearranging the suction piping to avoid changes in direction or other obstructions. The cavitation damage to the impeller was believed to have been at least partly due to bad flow conditions produced by two 90o elbows in the suction piping. The planes of the elbows were at 90o to each other, and the arrangement should be avoided.

Straitening vanes in the impeller inlet may increase the NPSH requirement at all flow rates. Three or four radial ribs equally spaced around the inlet and extending inward about one quarter of the inlet diameter are effective against excessive prerotation and may require less NPSH then full length vanes. This is very important with axial flow pumps, which are apt to have unfavourable cavitation characteristic at partial flow rates. Operation near the best efficiency point usually minimizes cavitation.

The admission of a small amount of air into the pump suction tends to reduce cavitation noise. This rarely is done, however because it is difficult to inject the right amount to mixing air with the liquid pumped.

If new impeller is required because of cavitation, the design should take into account the most recent advances describe in the literature has suggested:

1. The use of ample fillets where the vanes join the shrouds
2. Sharpened leading edges of vanes
3. reduction of b, in the immediate vicinity of the shrouds
4. raking the leading edges of the vanes forward out the eye. Increasing the number of vanes for propeller pumps lower s for a given submergence.

Typical Part of Centrifugal Pumps

 

A centrifugal pump is a relatively simple pump. Design, types and numbers of parts vary depending on centrifugal pump brand, type and configuration.

 

Typical main pump parts:

Casing/back plate:

• Contains impeller where fluid is transferred from inlet to outlet.
• Includes inlet and outlet ports.
• Typically flexible port orientation.
• Typically fitted to an adapter.

Shaft:

• Rotates impeller which is fixed to it.
• Is fixed to the motor and rotates with it.

Impeller:

• Transfers fluid from inlet to outlet with increased capacity and pressure is fixed on the shaft and rotates with it.
• Typical types are open, semi-open or closed.

Shaft seal:

• Seals between rotating shaft and stationary casing.
• Typically a mechanical seal, external or internal.
• Typically available as single, single flushed and double flushed seal.

Adapter:

Fixes pump casing to the motor.

Motor:

• Rotates shaft (impeller) which is fixed to it.
• Typically a 3-phase electrical motor.
• Typically available for various electrical site supplies (voltage and frequency).
• Typically available in various protection classes (flameproof etc.).

Other parts: Seals, motor cover, seal flushing, coupling/ base (base-mounted pump).

Typical materials:

• Steel parts of 316L or 304 stainless steel.
• Elastomers of NBR, EPDM, FPM, PTFE.

Self Priming Pump

The basic requirement for a self priming centrifugal pump is that the pumped liquid must be able to entrain air in the form of bubbles so the air will be removed from the suction side of the pump. This air must be allowed to separate from the liquid after the mixture of the two has been discharged by the impeller, and the separated air must be allowed to escape or to be swept out through the pump discharge. Such a self priming pump therefore requires, on its discharge side, an air separator, which is a relatively large stilling chamber, or reservoir, either attached to or built into the pump casing. Alternatively, a small air bleed line can be installed from the discharge pipe between the pump and the discharge check valve back to the suction source.

There are two basic variation of the manner in which the liquid from the discharge reservoir makes the pump self priming.

1. recirculation from the reservoir back to the suction and
2. recirculation within the discharge and the impeller itself

Recirculation to Suction
In such a pump, a recalculating port is provided in the discharge reservoir, communicating with the suction side of the impeller. Before the first handles whatever liquid comes to it through the recalculating port plus a certain reservoir, where the two elements are separated, the air passing out of the pump discharge and the liquid returning to the suction of the impeller through the recirculation port. This operation continuous until all the air has been exhausted from the suction line.

Recirculation at Discharge
This form of priming is distinguished from the preceding method by the fact that the priming liquid is not returned to the suction of the pump but mixes with the air either in the impeller or at its periphery. The principal advantage of this method, therefore, is that it eliminates the complexity of internal valve mechanisms.

Regenerative Turbine Pumps
Because these pumps can handle relatively large amounts of gas, they are inherently self-priming as long as sufficient liquid remains in the pump to seal the clearance between the suction and discharge passages. This condition is usually met by building a trap in the pump suction.