First of all, we must understand clearly what cavitation is. Cavitation makes the pump noise, as if pumping gravel. Cavitation damages the pump faster than any other accident. The liquid entering the pump must flowAfter the impeller, the liquid suddenly turns an angle and then enters the rotating blade space. If the pressure of the inlet pipe is not too high, bubbles will form, but it is not air. These onesBubbles reach the high pressure zone along the impeller, where they are easily compressed in a short distance. The high-speed water flow crushes the bubbles, and the water flows into the space originally occupied by the bubbles.
There was a crackling sound, as if a rock hit the pump casing. If many bubbles burst at the same time, the noise will be like mixing gravel in a pump.
The ruptured bubbles hit the metal surface to produce a very high pressure shock wave, and the grain structure of the metal on the inner wall of the pump casing will become loose and peel off. Thickness appears on the inner surface of the pump casing and the impeller blades
The grainy surface is damaged by cavitation. The smooth wear surface is mostly caused by erosion. Cavitation and erosion work together, and the pump will be severely worn in a very short time. Inner wall of pump housing
The coarse-grained surface formed by cavitation is subsequently eroded by the solid matter in the mud and becomes smooth. It should be pointed out that the only similarity between cavitation and air lock is that both have bubbles at the impeller.
But they are caused by different reasons, and produce different causes.
1. Causes of cavitation
When the liquid is at a certain temperature, when the pressure is reduced to the vaporization pressure at that temperature, the liquid produces vapor bubbles. This phenomenon of generating bubbles is called cavitation. The bubbles generated during cavitation flow to high pressure
At the time, its volume is reduced to burst. This phenomenon of bubbles disappearing in the liquid due to pressure rise is called cavitation collapse.
When the impurity pump is in operation, if the partial area of the overflow part (usually somewhere behind the inlet of the impeller blade) for some reason, the absolute pressure of the pumped liquid is reduced to the liquid at the current temperature
At the vaporization pressure, the liquid begins to vaporize there, generating a large amount of steam and forming bubbles. When the liquid containing a large number of bubbles passes through the high-pressure zone in the impeller, the high-pressure liquid around the bubbles causes
The bubbles shrank sharply and even burst. While the bubbles are condensed and ruptured, the liquid particles fill the cavity at a very high speed. At this moment, a very strong water hammer is generated, and the impact frequency is very high.
Hit the metal surface, the impact stress can reach hundreds to thousands of atmospheres, the impact frequency can reach tens of thousands of times per second, and the wall thickness will be broken down in severe cases.
The process of generating bubbles and bubble bursting in the water impurity pump to destroy the flow parts is the cavitation process in the water impurity pump. After the water impurity pump produces cavitation, it will not only damage the flow parts
In addition to the effect, noise and vibration will be generated, and the performance of the impurity pump will decrease. In severe cases, the liquid in the impurity pump will be interrupted and cannot work normally.
2. The basic relationship of impurity pump cavitation
The conditions of impurity pump cavitation are determined by the impurity pump itself and the suction device. Therefore, when studying the conditions of cavitation, we should consider both the impurity pump itself and the suction device.
The basic relationship of pump cavitation is
NPSHc≤NPSHr≤[NPSH]≤NPSHa
NPSHa=NPSHr(NPSHc)-impurity pump starts cavitation
NPSHaNPSHa>NPSHr(NPSHc)-impurity pump without cavitation
In the formula, NPSHa——The cavitation allowance of the device is also called the effective cavitation allowance.
NPSHr-impurity pump cavitation margin, also called necessary cavitation margin or impurity pump inlet dynamic pressure drop, the smaller the better the anti-cavitation performance;
NPSHc——Critical NPSH, refers to the NPSH corresponding to the performance drop of the impurity pump by a certain value;
[NPSH]——Allowable NPSH is the NPSH used to determine the operating conditions of the impurity pump, usually [NPSH]=(1.1～1.5)NPSHc.
3. Calculation of device cavitation margin
NPSHa=Ps/ρg+Vs/2g-Pc/ρg=Pc/ρg±hg-hc-Ps/ρg
4. Measures to prevent cavitation
To prevent cavitation, NPSHa must be increased so that NPSHa>NPSHr can prevent cavitation from occurring as follows:
1. Reduce the geometric suction height hg (or increase the geometric backflow height);
2. To reduce the suction loss hc, try to increase the pipe diameter and minimize the pipe length, elbows and accessories;
3. Prevent running under large flow for a long time;
4. At the same speed and flow rate, a double-suction impurity pump is used, and the impurity pump is not prone to cavitation due to reduced inlet flow rate;
5. When cavitation occurs in the impurity pump, the flow should be reduced or run at a reduced speed;
6. The condition of the impurity pump suction pool has an important influence on the impurity pump cavitation;
7. For impurity pumps operating under harsh conditions, to avoid cavitation damage, cavitation resistant materials can be used.