First, the cavitation phenomenon of the gear oil pump At a certain temperature, when the pressure is reduced to the vaporization pressure at the temperature, the liquid will generate bubbles. This phenomenon of generating bubbles is called cavitation. When flowing to a high pressure, its volume is reduced to collapse. This phenomenon of bubble disappearing in the liquid due to pressure rise is called cavitation collapse.
When the pump is in operation, if a partial area of ​​the overcurrent portion (usually somewhere later in the impeller blade inlet), for some reason, the absolute pressure of the pumped liquid is reduced to the liquid vaporization pressure at that temperature, the liquid is in the At the beginning, vaporization starts to generate a large amount of steam to form bubbles. When the liquid containing a large amount of bubbles passes forward through the high pressure zone in the impeller, the high pressure liquid around the bubble causes the bubble to sharply shrink and rupture. At the same time as the bubble condenses and ruptures, the liquid particle is Filling the cavity at a high speed, it produces a strong water hammer at this moment, and hits the metal surface with a high impact frequency. The impact stress can reach several hundred to several thousand atmospheres, and the impact frequency can reach tens of thousands per second. Once, in severe cases, the wall thickness will be broken down.
The process of generating bubbles and bubble rupture in the pump to damage the overcurrent components is the cavitation process in the pump. In addition to the damaging effect on the overcurrent components, the pump generates noise and vibration, and causes noise and vibration. The performance of the pump is degraded. If it is serious, the liquid in the pump will be interrupted and it will not work properly.
Second, measures to prevent cavitation to prevent cavitation must increase the NPSHa, so that NPSHa> NPSHr can prevent cavitation:
1. Reduce the geometric suction height hg (or increase the geometric backflow height);
2. Reduce the suction loss hc, for which you can try to increase the pipe diameter, minimize the length of the pipe, elbows and accessories;
3. Prevent long-term operation under high flow rates;
4. At the same speed and flow rate, a double suction pump is adopted, and the inlet flow rate is reduced, and the pump is less prone to cavitation;
5. When the pump cavitation occurs, the flow should be reduced or reduced;
6. The condition of the pump suction basin has an important influence on the pump cavitation;
7. For pumps operating under harsh conditions, to avoid cavitation damage, use cavitation resistant materials.
Third, the basic characteristics of the gear oil pump cavitation cavitation conditions are determined by the pump itself and the inhalation device. Therefore, the study of cavitation conditions should be considered from both the pump itself and the inhalation device, pump cavitation The basic relationship is NPSHc≤NPSHr≤[NPSH]≤NPSHa
NPSHa=NPSHr(NPSHc)——The pump starts to cavitation NPSHa NPSHa>NPSHr(NPSHc)——The pump has no cavitation. The NPSHa in the pump——the NPSH of the device is also called the effective NPSH.
NPSHr——the NPSH of gear oil pump, also called the necessary NPSH or the dynamic pressure drop of the pump inlet. The smaller the anti-cavitation performance is, the better;
NPSHc——critical NPSH is the NPSH corresponding to a certain value of pump performance degradation;
[NPSH]——The allowable NPSH is the NPSH used to determine the pump conditions. Usually, [NPSH]=(1.1~1.5) NPSHc.
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