What is cavitation?
When the local pressure of the liquid in a pump drops to the critical pressure, bubbles will form in the liquid. Cavitation is the entire process of bubble aggregation, movement, breakup, and elimination. The critical pressure is generally close to the vaporization pressure.
What are the hazards of cavitation?
01
Corrosion of flow components
There are two causes of corrosion:
First, the high-frequency (600~25000HZ) impact generated when bubbles collapse, with pressures as high as 49MPa, causes mechanical erosion on the metal surface;
Second, the heat released during vaporization, along with the hydrolysis caused by the thermoelectric effect, produces oxygen that oxidizes the metal, resulting in chemical corrosion.
02
Decreased pump performance
During pump cavitation, the energy exchange within the impeller is disturbed and disrupted. This manifests in the external characteristics as a decrease in the Q-H curve, Q-P curve, and Q-η curve. In severe cases, it can interrupt the liquid flow in the pump, rendering it inoperable.
For low specific speeds, due to the narrow and long flow channels between the blades, once cavitation occurs, bubbles fill the entire flow channel, causing a sudden drop in performance curves.
For medium to high specific speeds, the flow channel is short and wide, so the development of bubbles into filling the entire flow channel requires a transition process. The corresponding performance curve initially declines slowly, then drops sharply only after reaching a certain flow rate.
The most cavitation-prone areas in centrifugal pumps:
The front cover plate with the largest impeller curvature, near the low-pressure side of the blade inlet edge;
The low-pressure side of the volute tongue and guide vanes near the inlet edge in the discharge chamber;
The sealing gap between the outer circle of the blade tip and the casing, and the low-pressure side of the blade tip, in high specific speed impellers without a front cover plate;
The first-stage impeller in a multistage pump.
Measures to improve cavitation resistance:
01
Measures to improve the cavitation resistance of the centrifugal pump itself:
Improve the structural design from the pump suction inlet to the vicinity of the impeller. To reduce the rapid acceleration and pressure drop of the liquid flow, the following measures can be taken: Increase the flow area; increase the radius of curvature of the impeller shroud inlet section to reduce the rapid acceleration and pressure drop of the liquid flow; appropriately reduce the thickness of the blade inlet and round it to make it more streamlined, which can also reduce the acceleration and pressure drop around the blade head; improve the surface finish of the impeller and blade inlet sections to reduce drag loss; extend the blade inlet edge towards the impeller inlet to allow the liquid flow to receive work earlier and increase pressure.
Use a pre-induced impeller to allow the liquid flow to perform work earlier in the pre-induced impeller, thereby increasing the liquid flow pressure.
Use a double-suction impeller, allowing the liquid flow to enter the impeller from both sides simultaneously, which doubles the inlet cross-section and halves the inlet velocity.
Use a slightly larger positive angle of attack in the design operating conditions to increase the blade inlet angle, reduce the bend at the blade inlet, reduce blade blockage, and increase the inlet area; improve the working conditions under high flow rates to reduce flow losses. However, the positive angle of attack should not be too large, otherwise it will affect efficiency.
Use cavitation-resistant materials. Practice shows that the higher the strength, hardness, and toughness of a material, the better its chemical stability and the stronger its resistance to cavitation.
02
Measures to Improve the Effective Net Positive Suction Head (NPSH) of the Inlet System
Increase the pressure of the liquid surface in the upstream storage tank to improve the effective NPSH.
Reduce the installation height of the suction pump.
Convert the suction system to a reverse-flow system.
Reduce flow losses in the upstream pipeline. For example, shorten the pipeline as much as possible within the required range, reduce the flow velocity in the pipeline, reduce bends and valves, and maximize valve opening.
Lower the temperature of the working medium at the pump inlet (when the working medium is close to its saturation temperature).
The above measures should be comprehensively analyzed and applied appropriately based on pump selection, material selection, and the conditions of the pump's operating site.