What is Cavitation?How to improve the measures of anti-cavitation？

 

• What is cavitation?

Cavitation is a detrimental condition that often occurs in centrifugal pumping units. Cavitation can reduce pump efficiency, cause vibration and noise, and lead to severe damage to the pump's impeller, pump housing, shaft, and other internal parts. Cavitation occurs when the fluid pressure in the pump drops below the vaporization pressure, causing vapor bubbles to form in low-pressure areas. These vapor bubbles can violently collapse or "implode" when they enter the high-pressure area. This can cause mechanical damage within the pump, create weak points that are susceptible to erosion and corrosion, and impair pump performance.

 

Understanding and implementing strategies to mitigate cavitation is critical to maintaining the operational integrity and service life of centrifugal pumps.

 

• Types of Cavitation in Centrifugal Pumps

 

1. Vaporization cavitation. Also known as "classic cavitation" or "net positive suction head available (NPSHa) cavitation," this is the most common type of cavitation. Centrifugal pumps increase the velocity of the fluid as it passes through the impeller suction hole. The increase in velocity is equivalent to a decrease in the fluid's pressure. The pressure reduction may cause some of the fluid to boil (vaporize) and form vapor bubbles, which collapse violently and produce tiny shock waves when they reach the high-pressure area.

 

2. Turbulent cavitation. Components such as elbows, valves, and filters in the piping system may not be suitable for the amount or nature of the pumped liquid, which can create eddies, turbulence, and pressure differences throughout the liquid. When these phenomena occur at the pump inlet, they can directly erode the pump interior or cause the liquid to vaporize.

 

3. Blade syndrome cavitation. Also known as "blade pass syndrome," this type of cavitation occurs when the impeller diameter is too large or the pump casing internal coating is too thick/the pump casing inner diameter is too small. Either or both of these conditions will reduce the space (clearance) within the pump casing to below acceptable levels. The reduction in clearance within the pump housing causes an increase in fluid velocity, which results in a decrease in pressure. The decrease in pressure can cause the fluid to vaporize, creating cavitation bubbles.

 

4. Internal Recirculation Cavitation. When a pump is unable to discharge fluid at the required flow rate, it causes some or all of the fluid to recirculate around the impeller. The recirculated fluid passes through low and high pressure areas, generating heat, high velocity, and forming vaporization bubbles. A common cause of internal recirculation is running the pump with the pump outlet valve closed (or at a low flow rate - Pump Salon Note 1).

 

5. Air Entrainment Cavitation. Air can be drawn into the pump through a failed valve or loose fitting. Once inside the pump, the air moves with the fluid. The movement of the fluid and air can form bubbles that "explode" when exposed to the increased pressure of the pump impeller.

 

• What are the hazards of cavitation?

 

1. Corrosion of flow-through components：

(1) Due to the high-frequency (600~25000HZ) impact generated when the bubbles burst, the pressure is as high as 49Mpa, causing mechanical erosion on the metal surface.

 

(2) Since heat is released during vaporization and hydrolysis occurs due to the temperature difference battery effect, the generated oxygen oxidizes the metal and causes chemical corrosion.

 

2. Pump performance declines:

When the pump cavitation occurs, the energy exchange in the impeller is disturbed and destroyed, and the external characteristics are manifested as the Q-H curve, Q-P, and Q-n curves decline. In severe cases, the flow in the pump will be interrupted and it will not work.

 

For low specific speeds, since the flow channel between the blades is narrow and long, once cavitation occurs, the bubbles fill the entire flow channel and the performance curve will drop suddenly.

 

For medium and high specific speeds, the flow channel is short and wide, so it takes a transition process for bubbles to develop from occurrence to filling the entire flow channel. The corresponding performance curve starts to decline slowly, and then drops sharply when it increases to a certain flow rate.

 

• Measures to improve anti-cavitation

 

1. Measures to improve the anti-cavitation performance of the centrifugal pump:

(1) Improve the structural design from the pump suction port to the impeller. Increase the flow area; increase the radius of curvature of the impeller cover inlet section to reduce the rapid acceleration and pressure drop of the liquid flow; appropriately reduce the thickness of the blade inlet and round the blade inlet to make it close to the streamline shape, which can also reduce the acceleration and pressure drop of the flow around the blade head; improve the surface finish of the impeller and blade inlet to reduce resistance loss; extend the blade inlet edge to the impeller inlet so that the liquid flow receives work in advance and increases the pressure.

 

(2) A front inducer is used to make the liquid flow work in advance in the front inducer to increase the liquid flow pressure.

 

(3) By using a double-suction impeller, the liquid flow enters the impeller from both sides of the impeller at the same time, so the inlet cross-section is doubled and the inlet flow rate can be halved.

 

(4) The design working condition adopts a slightly larger positive angle of attack to increase the blade inlet angle, reduce the bending at the blade inlet, reduce blade blockage, and increase the inlet area; improve the working conditions under large flow to reduce flow loss. However, the positive angle of attack should not be too large, otherwise it will affect the efficiency.

 

(5) Use cavitation-resistant materials. Practice shows that the higher the strength, hardness, and toughness of the material, the better the chemical stability, and the stronger the cavitation-resistant performance.

 

2. Measures to improve the effective cavitation margin of the liquid inlet device:

(1) Increase the pressure of the liquid surface in the liquid storage tank in front of the pump to increase the effective cavitation margin.

 

(2) Reduce the installation height of the suction device pump.

 

(3) Change the upward suction device to a reverse irrigation device.

 

(4) Reduce the flow loss in the pipeline before the pump, such as shortening the pipeline as much as possible within the required range, reducing the flow rate in the pipeline, reducing the number of elbows and valves, and increasing the valve opening as much as possible.

 

(5) Reduce the working medium temperature at the pump inlet (when the conveying medium is close to the saturation temperature).

 

The above measures can be appropriately applied after comprehensive analysis based on the pump type, material selection and pump usage site conditions.