Working principle of water (liquid) ring vacuum pump

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The water ring vacuum pump (abbreviated as water ring pump) is a type of rough vacuum pump; the ultimate vacuum it can achieve is 2000–4000 Pa, and with an atmospheric ejector in series, this value can reach 270–670 Pa. Water ring pumps can also be used as compressors, known as water ring compressors; they are low-pressure compressors with a pressure range of 1~2×105 Pa gauge pressure. 　　Water ring pumps were initially used as self-priming pumps, and later came to be employed in many industrial sectors such as petroleum, chemicals, machinery, mining, light industry, pharmaceuticals, and food processing. Water ring pumps are widely used in many industrial processing procedures, such as vacuum filtration, vacuum water pumping, vacuum feeding, vacuum evaporation, vacuum concentration, vacuum rehumidification, and vacuum degassing. Due to the rapid advancement of vacuum application technology, water ring pumps have always been highly regarded for achieving rough vacuums. Since the gas compression in a water ring pump is isothermal, it can be used to extract flammable and explosive gases, as well as gases containing dust or moisture; therefore, water ring pumps are being used more and more frequently. 　　Other suitable liquids can also be used as the working fluid in water ring vacuum pumps; regardless of the type of working fluid used, they are all referred to as water ring vacuum pumps. The water ring vacuum pump is the most common and widely used type among liquid ring vacuum pumps. 　　As shown in the figure: an appropriate amount of water is placed inside the pump body as the working fluid. As the impeller rotates clockwise as shown in the diagram, water is thrown outward by the impeller; due to centrifugal force, the water forms a closed ring of approximately uniform thickness, shaped according to the geometry of the pump chamber. The inner surface of the lower part of the water ring is in perfect tangent with the impeller hub, while the inner surface of the upper part of the water ring is in contact with the tips of the blades (in fact, the blades extend to a certain depth within the water ring). At this point, a crescent-shaped space is formed between the impeller hub and the inner surface of the water ring, and this space is further divided by the impeller blades into several small chambers, one for each blade. If 0° at the lower part of the impeller is taken as the starting point, then when the impeller has rotated 180°, the volume of the small chamber increases and it becomes connected to the suction port on the end face; at this point, gas is drawn in. Once suction is complete, the small chamber is separated from the suction port. As the impeller continues to rotate, the volume of the small chamber decreases, which compresses the gas. When the small chamber becomes connected to the exhaust port, the gas is expelled outside the pump. 　　In summary, a water ring pump relies on changes in the volume of its pumping chamber to achieve suction, compression, and exhaust, and therefore it belongs to the category of variable-volume vacuum pumps. Compared to other types of mechanical vacuum pumps, water ring pumps have the following advantages: simple structure, low requirements for manufacturing precision, and ease of fabrication. It has a compact structure, with a high rotation speed; it can generally be connected directly to the motor without the need for a reduction gear. Therefore, by using a small structural size, a large displacement can be achieved with a reduced floor area. Compressed gas is essentially isothermal, meaning that the temperature changes very little during the compression process. . Since there are no metal friction surfaces inside the pump chamber, no lubrication is required for the pump, and wear is minimal. The sealing between the rotating part and the fixed part can be achieved directly by a water seal. . Even air intake, stable and reliable operation, simple to use, and easy to maintain. 　　Water ring pumps also have their disadvantages: low efficiency, generally around 30%, with better ones reaching up to 50%. The vacuum level is low, not only due to structural constraints but more importantly due to the saturated vapor pressure of the working fluid. Using water as the working fluid, the ultimate pressure can only reach 2000~4000 Pa. Using oil as the working fluid, a pressure of 130 Pa can be achieved. 　　In short, since the gas compression in a water ring pump is isothermal, it is possible to pump out flammable and explosive gases. Due to the absence of exhaust valves and friction surfaces, it is possible to extract gas containing dust, condensable gases, and gas-water mixtures. Thanks to these prominent features, it is still widely used despite its low efficiency. Cavitation in liquid ring vacuum pumps When a liquid ring vacuum pump is in operation, at a certain temperature, if the liquid pressure in the low-pressure area falls below the vaporization pressure at that temperature (i.e., the saturated vapor pressure), the liquid begins to vaporize and bubbles are formed. As these bubbles move into the high-pressure area, they burst, and the surrounding liquid quickly fills in the space left by those bubbles, resulting in hydraulic shock. The formation, growth, and collapse of such bubbles is known as cavitation. 　　During the operation of a liquid ring vacuum pump, the absolute pressure in the suction area is equal to the absolute pressure of the system being evacuated. In other words, the higher the vacuum level of the system being evacuated, the higher the vacuum level in the suction area as well. 　　A liquid ring vacuum pump requires a liquid as the working medium. Each liquid has a corresponding saturated vapor pressure at a certain temperature. The closer the absolute pressure in the suction area is to the saturated vapor pressure of the liquid, the closer the liquid is to the boiling state. At this point, a large number of bubbles are formed on the surface of the working fluid in the suction area. Since the vapor generated by the working fluid within the working chamber occupies part of that chamber’s space, the pump’s ability to draw in air is reduced. When the pressure in the suction area reaches the saturated vapor pressure of the working fluid, it can be assumed that the working chamber is completely filled with working fluid vapor; at this point, the pump’s ability to draw in fluid is nearly zero, and cavitation in the pump becomes most severe. 　　If the working fluid is water, the higher the water temperature, the more the pumping performance of the pump declines. Page 72 of the manual. The cavitation damage in liquid ring pumps operates on the same principle as that in centrifugal water pumps; in both cases, pitting occurs on the metal surface at the locations where bubbles are formed and burst, and in severe cases, honeycomb-like damage can result. If the impeller of the vacuum pump has high residual stress at the cavitation site, it can also lead to stress release and the formation of cracks. 　　Noise and vibration are generated due to the continuous sudden collapse of bubbles in the high-pressure area during pump cavitation, along with the resulting severe water hammer. A cracking sound, like beans exploding, can be heard. Experimental results show that the vibration frequency range caused by cavitation is 600–25,000 Hz, with a pressure of 49 MPa. 　　If the aforementioned bubbles burst on the metal surface, the metal surface will be subjected to continuous and intense water hammer forces, resulting in pitting; the metal grains will become loose and flake off to form a honeycomb pattern, and in some cases, holes may even form. In addition to mechanical effects, cavitation damage is accompanied by various complex processes such as electrolysis and chemical corrosion. The actual damage pattern shows that the cavitation damage in the pump’s flow components occurs exactly at the locations where the bubbles disappear. 　Preventive measures and methods to eliminate cavitation When a liquid ring vacuum pump operates under cavitation conditions for an extended period, it can lead to increased noise and vibration in the pump, and may even cause damage to it. Therefore, it is necessary to avoid operating the liquid ring vacuum pump under such conditions. This can be achieved through the following methods: 1. Lowering the temperature of the working fluid By reducing the temperature of the working fluid, while keeping the operating point of the liquid ring vacuum pump constant, it is possible to prevent cavitation. 　　2. Keep the vacuum pump operating within a safe range to improve the accuracy of the selection process. This requires that, during the selection phase, the suction pressure and operating fluid temperature of the liquid ring vacuum pump be chosen in such a way as to avoid the pressure ranges where cavitation is likely to occur in the pump. 　　3. Install cavitation protection circuits or check valves. When the bubbles generated during cavitation burst under compression, introducing high-pressure non-condensable gases from the outside can fill the space created by these bubble bursts. This helps to **reduce the damage caused by cavitation to the pump, as well as the noise and vibrations resulting from cavitation. 　　2BV and 2BE1 are equipped with cavitation protection interfaces. When cavitation occurs in the pump, introducing non-condensable gas through the cavitation protection port on the exhaust side of the pump can eliminate the cavitation noise and **reduce the damage caused by cavitation to the pump. 　　4. Equipping with an atmospheric ejector: If the suction port of a liquid ring vacuum pump is equipped with an atmospheric ejector, cavitation is less likely to occur in the liquid ring pump. 　　5. Use a liquid with a lower saturated vapor pressure as the working fluid. 　　6. The impeller and disk of the pump are made of materials with strong cavitation resistance.