The noise of piston vacuum pump is mainly caused by impact. Whether the impact of pump oil or mechanical impact, the noise is related to the pump speed. With the development of pump speed to high speed, this problem is more prominent. Therefore, EVP vacuum pump manufacturers think it is necessary to reduce the noise from the aspects of sound conduction and shielding.
Oil tank is the main part of sound conduction. Compared with steel tank, the noise of cast iron tank can be reduced by (2 ~ 3) dB (a). The main reason is that the structure of cast iron is slightly looser than that of steel plate, so it has certain sound absorption effect. Moreover, the cast iron oil tank is thicker than steel plate oil tank, and its sound insulation effect is better. For example, if a film is sprayed on the inner wall of the oil tank, the effect will be better. The structural shape of the inner surface of the oil tank shall be designed to be asymmetrical to avoid resonance sound.
The exhaust valve cover is connected with the oil-gas separator, which has the function of sound insulation and noise elimination. If the exhaust valve cover is removed, the noise of the pump will be much higher. If the oil-gas separator is designed properly, it will also have certain noise reduction effect. For example, the “secondary cyclone oil-gas separator” designed by us has two collisions through the oil-gas separator during the transmission process, resulting in a certain loss of sound energy, which is conducive to noise reduction.
Cavitation exists objectively in the piston vacuum pump. If there is cavitation, there will be noise. However, due to the low rotating speed of the general piston vacuum pump, cavitation is not enough to damage the parts, so it has not been found. In the trial production of h150ab pump with speed up to 920 R / min, we found abnormal noise with high noise frequency and sound level, with sound power up to 100 dB (a). After disassembly and inspection, we found that there were many shallow pinholes near the exhaust port on the slide valve (ring), which were removed and then operated for several hours, and the above phenomenon appeared again. After many tests, we confirmed that this is Cavitation erosion. However, the operation time is short and cavitation damage only appears on the surface.