How to deal with cavitation

No cavitation has been encountered in production. Found some information online: 1. Cavitation Phenomenon: When a liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature, the liquid will generate bubbles. This phenomenon of bubble generation is called cavitation. When the bubbles generated during cavitation flow to high pressure, their volume decreases and they burst. This phenomenon of bubbles disappearing into the liquid due to rising pressure is called cavitation collapse. When the pump is running, if for some reason the absolute pressure of the pumped liquid drops to the vaporization pressure of the liquid at the current temperature in the local area of ​​the overflow part (usually somewhere behind the impeller blade inlet), the liquid will begin to vaporize there, generating a large amount of steam and forming bubbles. When the liquid containing a large number of bubbles passes forward through the high-pressure area in the impeller, the high-pressure liquid around the bubbles will cause the bubbles to shrink sharply and even burst. At the same time as the bubbles condense and collapse, the liquid particles fill the holes at a very high speed. At this moment, a very strong water hammer effect is produced, and the metal surface is hit at a very 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 severe cases, the wall thickness will be broken down. The process of generating bubbles and bursting bubbles in the water pump, causing damage to the flow-passing components, is the cavitation process in the water pump. After cavitation occurs in a water pump, in addition to damaging the flow-passing components, it will also produce noise and vibration, which will lead to a decrease in pump performance. In severe cases, the liquid in the pump will be interrupted and the pump will not work properly. 2. Basic relationship between pump cavitation The conditions for pump cavitation to occur are determined by the pump itself and the suction device. Therefore, when studying the conditions for the occurrence of cavitation, we should consider both the pump itself and the suction device. The basic relationship of pump cavitation is NPSHc ≤ NPSHr ≤ ≤ NPSHa NPSHa = NPSHr (NPSHc) - the pump starts to cavitate NPSHa NPSHa > NPSHr (NPSHc) - the pump has no cavitation. In the non-cavitation type, NPSHa - the device cavitation margin is also called the effective cavitation margin. The larger it is, the less likely it is to cavitate. ； NPSHr——pump cavitation margin, also called necessary cavitation margin or pump inlet dynamic pressure drop, the smaller the better the anti-cavitation performance ； NPSHc - critical NPSHc, which refers to the NPSH that corresponds to a certain value of reduced pump performance. ； ——The allowable NPSH is the NPSH used to determine the operating conditions of the pump. It is usually taken as = (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. Measures to prevent cavitation are as follows:: 1． Reduce the geometric suction height hg (or increase the geometric backflow height) ； 2． To reduce the suction loss hc, you can try to increase the pipe diameter, minimize the length of the pipe, elbows and accessories, etc. ； 3. Prevent long-term operation under heavy traffic ； 4. Under the same speed and flow rate, a double-suction pump is used to reduce the inlet flow rate and the pump is less likely to cause cavitation. ； 5. When cavitation occurs in the pump, the flow rate should be reduced or the speed should be reduced. ； 6． The condition of the 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.

What should I do if it is cavitation of the liquefied gas pump? I am so depressed that I made a fool of myself today. What should I do if cavitation of this kind of pump occurs during production?

It should be easier to lower the critical temperature of the medium and increase the critical pressure of the medium.

How do you know the critical temperature and pressure of LZ? Such experiments are rarely done in actual operations, and how to improve this. I have never thought about it before, but now I encounter this problem, it really gives me a headache.

The critical temperature and critical pressure of the medium are determined by the composition of the material system. There should be no problem of decreasing or increasing.

Increase the pump inlet medium pressure, stop the pump, refill the pump, and fully exhaust it, and it will be fine.

Increase the pressure of the liquefied gas tank, lower the temperature of the liquefied gas, and vent the pump inlet and outlet.~~~~

Reduce the critical temperature of the medium and increase the critical pressure of the medium. You can also reselect the pump, increase the inlet pressure, increase the inlet liquid level, and re-adjust the amount.

If it is started normally, the tower pressure is above the saturated vapor pressure of the liquefied gas to ensure that the liquefied gas is in a liquid state, indicating the feasibility of restarting the pump. . . Then restart in the following order: 1. Close the pump outlet valve. . . 2. Open the pump pressure relief line and vent the gas at the pump inlet and outlet to the torch. . . 3. Reopen the pump inlet valve. If it is a centrifugal pump, start the pump, and then slowly adjust the normal pressure or flow according to the centrifugal pump starting procedures. . . 4. If it is a reciprocating positive displacement pump, the gas still needs to be vented. Generally, a positive displacement pump is similar to a compressor and can be started and operated with partial gas. . . There is no cavitation phenomenon. . . Positive displacement pumps can achieve mixed gas-liquid transmission in a short period of time, but centrifugal pumps cannot. . . :lol

Exhaust air from the pump inlet and outlet to the torch line to increase the liquid level and pressure of the liquefied gas tank and the pump inlet temperature. If the pump has severe cavitation and cannot be turned on for a long time, the temperature of the pump inlet will rise. You can try to connect the belt and use water to cool down outside the pump inlet pipeline.

Your experience is very good. We have encountered such a problem. It cannot be started for a long time. The conventional method is to close the outlet valve immediately after judging cavitation, stop the pump, and refill the pump.