Why does the cavitation performance of a centrifugal pump deteriorate after cutting the impeller?

Impeller cutting refers to mechanically reducing the outer diameter of the centrifugal pump impeller to change its hydraulic performance. Impeller cutting is a common method to adjust pump performance, usually used to reduce the pump head and flow to meet the actual working conditions. However, although this method can adjust the pump head and flow to meet the new working conditions, it will destroy the matching of the original hydraulic design and lead to a series of problems, one of the most common and easily overlooked problems is that the cavitation performance of the pump will deteriorate.

 

 

1. Increased flow velocity at the blade inlet causes a sudden drop in pressure

1) After the impeller is cut, the outlet diameter of the impeller is reduced, and the flow velocity at the impeller inlet will increase due to the redistribution of the flow (Q=A⋅v). According to the Bernoulli equation, the flow velocity is inversely proportional to the static pressure, and the local pressure will be reduced to a level closer to or even lower than the saturated vapor pressure of the liquid, significantly increasing the probability of bubble generation, which may cause cavitation.
2) Key formula: NPSHr is proportional to the square of the inlet flow velocity, and an increase in flow velocity will significantly increase NPSHr.

 

2. Changes in the blade inlet angle of attack

1) After the impeller is cut, the outer diameter of the impeller is reduced, but the inlet diameter usually remains unchanged (only the outlet part is cut). This will change the relative flow velocity direction (angle of attack β) at the impeller inlet, leading to flow separation or increased turbulence, further reducing local pressure and worsening cavitation performance.

2) If the angle of attack deviates from the design value, flow separation will form a low-pressure vortex area, promoting bubble generation.

 

3. Head-flow characteristic curve deviation

After the impeller is cut, the head-flow curve of the centrifugal pump moves downward, but the best efficiency point will move toward the direction of small flow. If the actual operating flow is not adjusted synchronously, the pump may deviate from the BEP operation, resulting in inlet backflow or unstable flow, increasing the risk of cavitation.

 

4. Proportional effect of NPSHr

NPSHr is inversely proportional to the square of the impeller diameter (empirical relationship). After the impeller is cut, the diameter decreases and NPSHr increases. For example, according to the similarity law, if the impeller diameter is cut from D1 to D2, then:

 

A reduction in impeller diameter results in a significant increase in NPSHr.

 

5. Changes in blade load distribution

After cutting the impeller, the working section of the blade is shortened, but the unit load (pressure gradient) of the inlet section may increase. The local low-pressure area at the leading edge of the blade expands, and the cavitation initiation point is advanced.

 

6. Deterioration of flow channel stability

The impeller outlet width after cutting is unbalanced with the volute throat area ratio, forming secondary backflow and a vortex. These unstable flows will propagate back to the impeller inlet area, superimposing the original pressure pulsation to form a compound cavitation.

 

7. Actual impact and countermeasures

1) Cavitation performance deterioration: NPSHr increases, and the pump's NPSHa needs to be larger to avoid cavitation.

2) Solution:

- Limit the cutting amount: Usually, the cutting amount of the centrifugal pump impeller diameter does not exceed 20%, otherwise the NPSHr needs to be re-evaluated.

- Optimize inlet conditions: Increase the inlet pipe diameter or reduce the inlet flow rate.

- Adjust operating conditions: Avoid operating in the low flow area and operate as close to the best efficiency point as possible.

 

Cutting the impeller directly leads to the expansion of the inlet low pressure area and the increase of NPSHr by changing the velocity distribution, attack angle and blade load. Although this geometric modification can adjust the performance of the centrifugal pump, it is necessary to carefully evaluate the cavitation risk and redesign the impeller inlet or adjust the system NPSHa if necessary.