In the modern pipeline processes, using centrifugal pump is a common practice. However, there are a major problems that threaten the service life of this pump: “Cavitation”.

What's Cavitation?

Normally, Pump operation stable requires a unique flow rate, we call this flow "minimum flow (Qmin)", when pump entrance flow (Qm) is lower than the required minimum flow (Qmin) the pump needs. The fluid pressure will drop, the liquid will vaporize and it will become air molecule, this process was called Flash evaporation.

Why Cavitation happen?

There are so many reasons that cause the the cavitation, we list some main points:

• The drag force of the pump inlet pipeline is too large or inlet pipeline size too small.
• The temperature of the inlet pipeline medium is too high
• Excessive flow deviation or the outlet flow rate is too large
• The Installation of the pump is too high, which affect the pump suction power.
• Selection issue, select wrong model, wrong material.
• The inlet flow is not enough, that is, what we usually say that inlet flow is smaller than the minimum flow.

Where did cavitation happen?

Impeller

Mostly at the front and back of the impeller or at the root of the impeller.

Body

The pump internal unevenness surface and outlet high pressure area.

The harm of Cavitation

• When cavitation occurs, a lot of bubbles will be produced. When there are a large number of bubbles in the water, the normal law of water flow will be destroyed, and the effective area of impeller will be reduced, so the flow direction will be changed, and the energy loss will be increased, which will lead to the rapid decline of pump flow.
• Damaging component: Under the repeated action of the impact force of high-strength water molecules, the inner wall of the pump and the impeller cause local deformation and hardening and brittleness of the metal surface, causing the metal to rupture and peel off. In addition to the mechanical action, it is also mixed with the chemical corrosion of the metal by the deep and active gas (such as oxygen) escaping from the water body and the electrochemical corrosion of the metal by the water body. Under the combined effect, the inner wall of the pump initially has pitting, and then becomes a honeycomb shape. In severe cases, the shell will be penetrated in a short period of time.
• Vibration and nois: When bubbles burst, liquid molecules collide with each other. At the same time, they collide with metal surfaces, such as pump body and impeller, resulting in various frequencies of noise and vibration. Under the repeated action of huge impact on the impeller part, the surface of the impeller appears spots and cracks, and even gradually drops off in a spongy shape, which reduces the service life of the pump.

PS: Noise and vibration are also used to judge whether cavitation occurs or disappears.

The preventive measure of Cavitation

The effective measure to reduce cavitation erosion is to prevent bubble formation.

1. Selecting appropriate materials can improve the anti-cavitation ability. Usually metal materials with high strength and toughness have better cavitation resistance.
2. Clean up foreign bodies in the inlet pipeline to make the inlet unobstructed, or increase the size of the pipe diameter.
3. For the pump manufacturers, it is necessary to improve the anti-cavitation ability of the centrifugal pump itself, such as improving the structure design from the suction port to the impeller; increasing the blade inlet angle, reducing the bending of the blade inlet, so as to increase the inlet area, but this is a long job.
4. Keep the pump close to the fluid supply ASAP.

All of the above methods are to make the pump flow bigger than the minimum flow, thus avoiding the occurrence of cavitation.

But If all these methods do not work, we need to design a system to ensure the pump running under the minimum flow, that we call this system "Recirculation System".We call this valve Automatic Recirculation valve.