Solutions to air entrapment and cavitation in fluoroplastic chemical centrifugal pumps

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Solutions to air locking and cavitation in fluoroplastic chemical centrifugal pumps. Air locking in chemical centrifugal pumps: Before operating a chemical centrifugal pump, it is necessary to fill the pump with liquid first; then the pump is started, and the impeller begins to rotate rapidly. The blades of the impeller drive the liquid to move, and as the liquid rotates, it flows toward the outer edge of the impeller due to inertia. At the same time, the impeller draws in liquid from the suction chamber. During this process, the liquid circulates around the blades, and as it does so, it exerts an upward force on the blades. In turn, the blades exert a force on the liquid that is equal in magnitude but opposite in direction to this upward force. This force does work on the liquid, giving it energy and causing it to flow out of the impeller. As a result, both the kinetic energy and pressure energy of the liquid increase. Chemical centrifugal pumps transfer the mechanical energy of the prime mover to the liquid by means of the action of the rotating impeller on the liquid. In a chemical centrifugal pump, as the liquid flows from the inlet to the outlet of the impeller, both its kinetic energy and pressure energy increase. The liquid discharged by the impeller passes through the discharge chamber, where most of its kinetic energy is converted into pressure energy; this pressurized liquid is then sent along the discharge pipeline. At the same time, the evacuation or low pressure created at the impeller’s inlet due to the discharge of liquid causes the liquid in the reservoir to be forced into the impeller’s inlet under the effect of surface pressure. Thus, the rotating impeller continuously draws in and discharges liquid. When a chemical centrifugal pump is started, if there is air inside the pump, the low density of air results in a weak centrifugal force generated upon rotation; as a result, the low pressure created in the center area of the impeller is not sufficient to draw liquid from the storage tank into the pump, and thus the pump cannot transport liquid even when it is started. This phenomenon is known as gas entrapment. It indicates that the chemical centrifugal pump does not have self-priming capability, so the tank must be filled with liquid before starting. To prevent gas entrapment, fill the shell with liquid before starting up. Ensure proper sealing of the housing; the valves and faucets used for filling water must not leak, and good sealing is essential. The suction line of the chemical centrifugal pump is equipped with a foot valve to prevent the liquid filled in before startup from flowing out of the pump. The filter screen prevents solids in the liquid from being drawn in. The discharge pipeline is equipped with a control valve for starting and stopping the pump as well as for regulating flow rate. By placing the suction inlet of the chemical centrifugal pump below the level of the liquid to be transported, the liquid will flow into the pump automatically. Cavitation in chemical centrifugal pumps: When the installation height of a chemical centrifugal pump is inappropriate, it results in too low pressure at the pump inlet. When the inlet pressure drops to the saturated vapor pressure of the liquid being transported at that temperature, vaporization occurs. The bubbles formed undergo rapid condensation as pressure increases significantly during their flow from the inlet toward the periphery, leading to a phase change and the instantaneous destruction of these bubbles. It causes the surrounding liquid to rush toward the center of the bubble at high speed, generating impacts with very high frequencies and intense instantaneous pressures, which leads to fatigue and corrosion on the surface of the equipment; this phenomenon is known as cavitation. Generally speaking, while a pump is in operation, if the absolute pressure of the liquid being pumped in a certain local area of its flow path drops to the vaporization pressure of the liquid at that temperature for some reason, the liquid begins to vaporize there, generating large amounts of steam and forming bubbles. When this liquid containing many bubbles moves forward through the high-pressure areas inside the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly until they burst. As the bubbles coalesce and burst, liquid particles fill the voids at high speeds; this creates a very strong water hammer effect, which strikes the metal surface at a high frequency. The impact stress can reach several hundred to several thousand atmospheres, while the impact frequency can be in the tens of thousands of times per second. In severe cases, this can cause the wall thickness to be penetrated. Harm of cavitation: The shock waves transmitted to the impeller and pump casing during cavitation, combined with the chemical corrosion of the metal caused by trace amounts of oxygen dissolved in the liquid, can over time lead to the formation of scars and cracks on its surface, eventually causing it to peel off in a pattern similar to ocean waves ； When cavitation occurs, noise is also generated, which in turn causes the pump body to vibrate, potentially leading to a decline in the pump’s performance ； At the same time, the generation of steam reduces the apparent density of the liquid, which in turn leads to a decrease in the actual flow rate of the liquid, the outlet pressure, and the efficiency; in severe cases, this can result in no liquid being able to be discharged at all. The main causes of cavitation: 1. Excessively high temperature of the medium being pumped. 2. The flow rate is too high, which means the outlet valve is opened too wide. 3. Selection issues, including the selection of the pump and the material used for the pump. 4. The resistance in the inlet pipeline is too high, or the pipeline is too narrow. 5. The installation height is too high, affecting the pump’s suction capacity. Solutions to cavitation: 1. Reduce the installation height. 2. Lower the temperature of the conveying medium. 3. Re-select the pump, or improve certain components of the pump, such as using materials resistant to cavitation, etc. 4. Remove debris from the inlet pipeline to ensure unobstructed flow, or increase the pipe diameter.