What is the pump cavitation

First, the cavitation phenomenon Liquid at a certain temperature, reduce the pressure to the temperature vapor pressure, the liquid will produce bubble. This phenomenon of generating bubbles is called cavitation. Bubbles generated during cavitation decrease in volume when they flow to high pressure. This phenomenon of bubbles disappearing in the liquid due to pressure rise is called cavitation collapse. In the operation of the pump, if the partial area of ​​the overcurrent part (usually somewhere behind the inlet of the impeller blade) for some reason, the absolute pressure of the pumped liquid is reduced to the vaporization pressure of the liquid at the current temperature, the liquid will be At the beginning of the vaporization, a large amount of steam is generated and bubbles are formed. 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 causes the bubbles to drastically shrink and break. At the same time when the bubble condenses and breaks, the liquid particle fills the cavity at a very high speed. At this moment, it generates a strong water hammer and hits the metal surface with a high impact frequency. The impact stress can reach several hundred to several thousand. At atmospheric pressure, the impact frequency can reach tens of thousands of times per second, and in severe cases, the wall thickness will breakdown. The process of generating bubbles and bursting bubbles in the pump and damaging the overflow components is the cavitation process in the pump. After the cavitation of the water pump, in addition to damaging the flow components, noise and vibration are also generated, and the performance of the pump is reduced. In severe cases, the liquid in the pump is interrupted and cannot work normally.

Second, the pump cavitation basic relationship Pump cavitation conditions are determined by the pump itself and inhalation device. Therefore, the conditions for the occurrence of cavitation should be considered from both the pump itself and the inhalation device. The basic relationship of pump cavitation is NPSHC≤NPSHR≤[NPSH]≤NPSHANPSHA=NPSHR(NPSHC)—Pump Start NPSHPSHA”NPSHR (NPSHC) - NPSHA in pumps without cavitation - NPSH is also known as effective NPSH. The larger the NPSH, the less likely it is to cavitation. NPSHR - pump NPSH, also known as NPSH Or the dynamic pressure drop of the pump inlet, the smaller the better anti-cavitation performance; NPSHC - critical NPSH, refers to the NPSH corresponding to a certain drop in pump performance; [NPSH] - Permissible NPSH margin , It is the NPSH used to determine the pump's operating conditions. Usually NPSH = (1.1~1.5)NPSHC.

Third, the device NPSH calculation NPSHA = PS / ρG + VS / 2G-PC / ρG = PC / ρG ± HG-HC-PS / ρG

Fourth, to prevent the occurrence of cavitation measures To prevent cavitation must be improved NPSHA, so NPSHA "NPSHR can prevent the occurrence of cavitation measures are as follows: 1. Decrease the geometric suction height HG (or increase the geometric backfill height); 2. Reducing the inhalation loss of HC. For this reason, it is possible to increase the pipe diameter, minimize the pipe length, elbow and accessories, etc. 3. Prevent long-term operation under heavy traffic; 4. In the same speed and flow rate, the use of double suction pump, because of reducing the inlet flow rate, the pump is not easy to cavitation; 5. When the pump cavitation occurs, the flow should be reduced or reduced speed operation; 6. The condition of the pump suction pool has important influence on pump cavitation; 7. For pumps operating under severe conditions, cavitation resistant materials may be used to avoid cavitation damage.