1、 Cavitation phenomenon

When the pressure of the liquid is reduced to the vaporization pressure at a certain temperature, the liquid will produce bubbles. This phenomenon of producing bubbles is called cavitation. When the bubble generated during cavitation flows to the high pressure, its volume decreases and burst. This phenomenon of bubbles disappearing into the liquid due to pressure rise is called cavitation collapse.

During the operation of the pump, if the absolute pressure of the pumped liquid decreases to the vaporization pressure of the liquid at the current temperature in the local area of the overflow part (usually somewhere later at the inlet of the impeller blade) for some reason, the liquid begins to vaporize there, producing a large amount of steam and forming bubbles. When the liquid containing a large amount of bubbles passes forward through the high-pressure area in the impeller, The high pressure liquid around the bubble causes the bubble to shrink sharply and even burst. While the bubble condenses and breaks, the liquid particles fill the hole at a high speed, produce a strong water hammer at this moment, and hit the metal surface with a high impact frequency. The impact stress can reach hundreds to thousands of atmospheres, and the impact frequency can reach tens of thousands of times per second. In serious cases, the wall thickness will be broken down.

The process of generating bubbles and bubble rupture in the water pump and causing damage to the overflow components is the cavitation process in the water pump. After the cavitation of the water pump, in addition to damaging the flow-through parts, it will also produce noise and vibration, and lead to the decline of the performance of the pump. In serious cases, the liquid in the pump will be interrupted and can not work normally.

2、 Basic relation of pump cavitation

The cavitation condition of the pump is determined by the pump itself and the suction device. Therefore, the study of cavitation conditions should be considered from both the pump itself and the suction device. The basic relationship of pump cavitation is

NPSHc≤NPSHr≤[NPSH]≤NPSHa

Npsha = NPSHr (npshc) - the pump starts cavitation

Npsha npsha > NPSHr (npshc) - the pump has no cavitation

Where npsha - NPSH of the unit, also known as effective NPSH, the greater the NPSH, the less likely it is to be subject to cavitation;

NPSHr - pump NPSH, also known as necessary NPSH or pump inlet dynamic pressure drop. The smaller the NPSHr, the better the anti cavitation performance;

Npshc - critical NPSH, which refers to NPSH corresponding to a certain value of pump performance decline;

[NPSH] - allowable NPSH, which is the NPSH used to determine the service conditions of the pump, usually [NPSH] = (1.1 ~ 1.5) npshc.

3、 Calculation of NPSH of unit

NPSHa=Ps/ ρ g+Vs/2g-Pc/ ρ g=Pc/ ρ g±hg-hc-Ps/ ρ g

4、 Measures to prevent cavitation

To prevent cavitation, npsha must be increased so that npsha > NPSHr. The measures to prevent cavitation are as follows:

1. Reduce the geometric suction height Hg (or increase the geometric backflow height);

2. Reduce the suction loss HC, so try to increase the pipe diameter and minimize the pipe length, elbows and accessories;

3. Prevent long-time operation under large flow;

4. At the same speed and flow, double suction pump is adopted, which is not easy to cavitation due to reducing the inlet flow rate;

5. In case of cavitation, the flow shall be reduced or the speed shall be reduced;

6. The condition of pump suction pool has an important impact on pump cavitation;

7. For pumps operating under harsh conditions, cavitation resistant materials can be used to avoid cavitation damage.