Cavitation in pumps and protective measures

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Cavitation is one of the common faults in hydraulic machinery. Due to improper design of the inlet tank or pipelines, as well as failure to take into account changes in atmospheric pressure, temperature, and the vaporization pressure of the fluid, cavitation often leads to premature failure of pumps. Pumps that have been installed and are in use have little ability to completely overcome the cavitation damage caused by their poor cavitation resistance (Volume 1 of the Pump Handbook). This article will mainly introduce methods to reduce cavitation damage in operating pumps, all of which have achieved significant results in practical applications. I. Causes of cavitation Cavitation is caused by the vaporization of a liquid; the process in which liquid molecules escape from the surface of the liquid and turn into gas molecules is known as \"vaporization\". The degree of vaporization of a liquid is related to the pressure and temperature. Gases dissolved in a liquid are also released when pressure and temperature change, forming vapor bubbles. When the internal pressure of the liquid drops below its saturated vapor pressure at that temperature, bubbles or cavities form in localized areas ； Where the pressure increases, the bubbles are suddenly crushed by the surrounding pressure; due to inertia, the liquid flow rushes toward the center of the bubbles at extremely high speeds, causing hydraulic shock to the equipment. The entire process of the formation and collapse of these microbubbles, as well as their physical and chemical effects on the flowing surface, is known as cavitation. If the liquid contains no impurities, cavitation will not occur even at very low pressures. Researchers on cavitation abroad have concluded through experiments that the tensile strength of ultra-pure water (i.e., the limit at which cavities are formed) is far higher than that of ordinary metal materials. However, ordinary liquids always contain gases or solids, and these impurities act as cavitation nuclei that induce the formation of cavities under certain conditions. In sand-containing water flows, due to the difference in specific gravity between water and sand, the trajectory of the sand particles deviates from the flow lines, which may accelerate the occurrence of cavitation. The author provided a detailed description in the paper “Cavitation in Mud Pumps and Materials Resistant to Wear and Cavitation” (Lubrication and Sealing, 1993). II. Cavitation diagnosis methods for in-service pumps: Pump users usually cannot use the method of monitoring the decrease in head at a constant flow rate provided by the manufacturer to determine whether cavitation is occurring. To determine whether an operating pump is suffering from cavitation, in addition to the observation method after cavitation damage occurs, (1) the ultrasonic method can be used ； (2) Pump housing noise method ； (3) Judgment by methods such as vibration testing. Observation method: The surface damage observation method is a post-event observation technique that makes judgments based on the shape of the damaged surface. Cavitation, casting pores, erosion wear, corrosion, and other factors can all cause the shape of the metal surface to deviate from its ideal shape. The metal surface damaged by cavitation usually appears honeycombed; this is caused by local high-speed water impacts on the metal, leading to fatigue damage of its surface. As a result, the honeycomb pores are generally connected to the outside, and most of the pits are perpendicular to the metal surface. The porosity associated with casting defects is often found deep within the metal. Sometimes, due to the action of water flow, this porosity and voids come to the surface, leading to a mistaken assumption that it is cavitation. However, when we use mechanical methods to remove the material from the surface, we find that voids still exist inside. Erosion wear marks often show grooves in the same direction as the flow of water, but it should be noted that water vortices may occur sometimes. Noise method: The external noise method for the pump body is relatively simple and does not require contact with the pump body. However, since the noise method is greatly affected by ambient noise, this occurs when its intensity is at its highest. Generally, cavitation in water pumps has reached a very severe stage; at this point, the intense sound of cavitation explosions can be heard by the human ear, allowing one to detect the condition of cavitation. Therefore, the pump body noise method is not very suitable for on-site monitoring of cavitation occurrence. Vibration method: This is a method that measures the vibration frequency of the pump body using an accelerometer probe; it is simple to use, but its sensitivity is low. Especially for large pumps, the pump body has high stiffness. The response to the excitation caused by bubble collapse due to local cavitation inside the pump is sluggish, and there are numerous vibration sources on the pump. Vibrations caused by cavitation are often masked by other vibrations. Therefore, the vibration method is only suitable as an auxiliary means for on-site cavitation monitoring. Ultrasonic method: The ultrasonic method for measuring cavitation is simple to use, easy to adjust, and not affected by other environmental noises; it is highly sensitive to the occurrence and development of cavitation. Therefore, monitoring cavitation at the pump station site is a relatively ideal method. In addition, Zhao Huijun from the Chemical Engineering Machinery Research Institute of Fushun Petroleum Institute introduced the use of electrical measurement methods to predict the cavitation performance of centrifugal pumps.