First, the phenomenon of cavitation liquid at a certain temperature, reduce the pressure to the temperature of the vaporization pressure, the liquid will produce bubbles. The phenomenon of bubbles is called cavitation. Cavitation bubble generated when flowing to high pressure Department, the volume decreases so as to burst. This phenomenon of bubble disappearing in liquid due to pressure increase is called cavitation collapse. When the pump is in operation, if the localized area of ​​the overcurrent portion (usually somewhere later in the impeller vane inlet) for some reason causes the absolute pressure of the pumped liquid to drop to the liquid vaporization pressure at the current temperature, Began to vaporize, produce a lot of steam, the formation of bubbles, when the liquid containing a lot of bubbles forward through the high-pressure area within the impeller, the bubbles around the high-pressure liquid bubbles so sharply reduced to rupture. At the same time as the bubble is broken and condensed, the liquid particles fill the cavity at a very high speed, causing a very strong water hammer moment at this moment, and striking the metal surface with a high impact frequency with an impact stress of several hundreds to several thousands Atmospheric pressure, the impact frequency of up to tens of thousands of times per second, will be severe wall breakdown. Cavitation in a pump is the process by which bubbles in the pump and rupture of the bubble damage the overcurrent component. Pump cavitation in addition to the over-current components will have a damaging effect, but also produce noise and vibration, and lead to the decline in the performance of the pump, serious pump liquid will be interrupted, can not work properly.
Second, the basic relationship between pump cavitation Pump cavitation conditions are caused by the pump itself and the inhalation device determined in two ways. Therefore, to study the conditions of cavitation should be considered from both the pump itself and the inhalation device, the basic equation for the pump cavitation is NPSHC≤NPSHR≤NPSH≤NPSHANPSHA = NPSHR (NPSHC) -the pump starts cavitation NPSHANPSHA "NPSHR (NPSHC) - No cavitation in pumps NPSHA - Equivalent NPSH, also known as effective NPSH, is more difficult to cavitate; NPSHR - NPSHR, also called required NPSH Or pump dynamic pressure drop, the smaller the better anti-cavitation performance; NPSHC - critical NPSH, which corresponds to the decline in pump performance NPSH; NPSH - allowable NPSH , Is to determine the conditions for the use of pumps NPSH, usually take ã€NPSH】 = (1.1 ~ 1.5) NPSHC.
Third, the device NPSHA = PS / ÏG + VS / 2G-PC / ÏG = PC / ÏG ± HG-HC-PS / ÏG
Fourth, to prevent the occurrence of cavitation measures To prevent the occurrence of cavitation must improve NPSHA, so NPSHA "NPSHR prevent cavitation measures are as follows: 1. Reduce the geometric suction height HG (or increase geometric inversion height); 2. Reduce the inhalation loss HC, for which you can try to increase the diameter, minimize pipe length, elbows and accessories, etc .; Prevent long run at high flow rate; 4. In the same speed and flow, the use of double suction pump, due to reduce the inlet flow rate, the pump is not prone to cavitation; Cavitation occurs when the pump, the flow should be reduced or reduced speed operation; 6. Pump suction pump cavitation has a significant impact; 7. For pumps operating under harsh conditions, cavitation-resistant materials may be used to avoid cavitation damage.
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