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This post was last edited and explained by hai8487 on 2017-7-11 20:58, covering the phenomena of air entrapment and cavitation in centrifugal pumps and how to avoid them Answer: Cavitation phenomenon and principle: The cavitation phenomenon in centrifugal pumps is a mechanical effect resulting from an instantaneous change in the phase state of the fluid. When pumping a liquid medium, the pump creates a minimum pressure near the inlet of the impeller blades. When this pressure is below the critical pressure of the fluid (usually the saturated vapor pressure), the fluid vaporizes (or dissolved gases flash), forming tiny bubble clusters that continue to grow; these bubbles disappear as the pressure increases (due to the dissolution of gases and the condensation of steam), thereby causing water hammer effects in certain areas, with stresses that can reach several thousand atmospheres. As a result, it is destructive (caused by continuous high-frequency impacts), which appears to generate vibrations and noise, as well as fluctuations in the pressure at the pump outlet ； In severe cases, flow breakdown occurs, leading to channel blockage and a decline in the performance of the water pump. To avoid cavitation, it is necessary to take into account an adequate margin against cavitation in the design and installation of the centrifugal pump, ensuring that there is sufficient pressure at the suction inlet ； In operation, the inlet valve should be fully open; the inlet filter screen should be cleaned promptly based on the pressure difference. The storage tank (pump suction inlet) must have an adequate liquid level, and the medium being transported should not be saturated with dissolved gases (to avoid low-pressure flashing inside the pump). The phenomenon of \"air locking\" occurs when, during operation of a centrifugal pump, the suction pipeline and the pump shaft are under negative pressure. If the sealing of the pipeline and shaft seals is poor, air can leak in, resulting in a decrease in the average density of the fluid inside the pump. If the average density drops significantly, the impeller will be unable to throw the liquid medium out, and insufficient negative pressure will not be generated within the pump, preventing it from drawing in liquid; this phenomenon is known as \"air locking\". When starting a centrifugal pump, it is necessary to first fill the pump with liquid; this process is known as priming the pump. Priming must ensure that all gas is removed. During inspection, it was confirmed that the connections such as the pump body flanges and packing seals were in good condition ； Ensure that the suction inlet is below the liquid level during operation.
\"Air locking\": The phenomenon in which the pump cannot draw in liquid after being started, due to air present inside the pump, is known as \"air locking\". “After the \"air binding\" phenomenon occurs, the pump does not discharge any liquid, and there is no noise or vibration. To prevent the occurrence of \"air locking,\" the vessel should be filled with liquid before starting. “\"Cavitation\": This is an erosion phenomenon that occurs when the suction height of a pump is too high, causing the pressure inside the pump to equal or fall below the saturated vapor pressure of the liquid being transported. As a result, the liquid vaporizes, bubbles form and burst, leading to cavitation. When cavitation occurs, the liquid generates noise and vibration due to the impacts, the flow rate decreases, and in severe cases, no liquid is discharged at all. To prevent the occurrence of \"cavitation\" ； The actual installation height of the pump should not be higher than the allowable suction lift.
\"Air locking\": The phenomenon in which the pump cannot draw in liquid after being started, due to air present inside the pump, is known as \"air locking\". “After the \"air binding\" phenomenon occurs, the pump does not discharge any liquid, and there is no noise or vibration. To prevent the occurrence of \"air locking,\" the vessel should be filled with liquid before starting. “\"Cavitation\": This is an erosion phenomenon that occurs when the suction height of a pump is too high, causing the pressure inside the pump to equal or fall below the saturated vapor pressure of the liquid being transported. As a result, the liquid vaporizes, bubbles form and burst, leading to cavitation. When cavitation occurs, the liquid generates noise and vibration due to the impacts, the flow rate decreases, and in severe cases, no liquid is discharged at all. To prevent the occurrence of \"cavitation\" ； The actual installation height of the pump should not be higher than the allowable suction lift.
\"Air locking\": The phenomenon in which the pump cannot draw in liquid after being started, due to air present inside the pump, is known as \"air locking\". “After the \"air binding\" phenomenon occurs, the pump does not discharge any liquid, and there is no noise or vibration. To prevent the occurrence of \"air locking,\" the vessel should be filled with liquid before starting. “\"Cavitation\": This is an erosion phenomenon that occurs when the suction height of a pump is too high, causing the pressure inside the pump to equal or fall below the saturated vapor pressure of the liquid being transported. As a result, the liquid vaporizes, bubbles form and burst, leading to cavitation. When cavitation occurs, the liquid generates noise and vibration due to the impacts, the flow rate decreases, and in severe cases, no liquid is discharged at all. To prevent the occurrence of \"cavitation\" ； The actual installation height of the pump should not be higher than the allowable suction lift.
Air entrapment: Air is already present inside the pump before it is started, and this air isn’t removed, resulting in insufficient vacuum levels and an inability to draw up liquid. Cavitation: After the machine is started, the centrifugal force generated by the high-speed circular motion of the liquid together with the impeller creates a low pressure at the inlet. As a result, the liquid vaporizes in this area of low pressure, which can damage the impeller. It mainly involves adjusting the height, valve size, flow rate (pipe size), liquid temperature, etc.
Gas entrapment phenomenon: If the pump casing is filled with gas before startup, when the gas at the center of the impeller is expelled after startup, a sufficient vacuum cannot be created there, and as a result, the liquid in the tank cannot be drawn in. This phenomenon is called gas entrapment. Cavitation: It is an erosion phenomenon that occurs when the suction height of a pump is too high, causing the pressure inside the pump to equal or fall below the saturated vapor pressure of the liquid being transported. As a result, the liquid vaporizes, bubbles form and burst, etc. This phenomenon is known as \"cavitation.\" When cavitation occurs, the liquid generates noise and vibration due to the impacts, leading to a reduction in flow rate, or even no liquid being discharged at all. Preventive measures: Cavitation: The actual installation height of the pump should not be higher than the allowable suction lift. Air lock: The liquid to be transported must be filled completely before starting the centrifugal pump.
When the absolute pressure at the pump inlet is lower than the saturated vapor pressure of the liquid, the liquid boils at the pump outlet, generating numerous bubbles that strike the impeller, pump casing, and pump body, causing vibration and abnormal noise; in severe cases, this can even lead to the impeller losing its integrity, cracking, or being damaged. This phenomenon, characterized by a sharp drop in the pump’s head, flow rate, and efficiency, is known as cavitation. To prevent cavitation in the pump, it is necessary to take into account the pump’s installation height and the liquid temperature, so that the pressure at the inlet is higher than the saturated vapor pressure of the liquid during pump operation
\"Air locking\": The phenomenon in which the pump cannot draw in liquid after being started, due to air present inside the pump, is known as \"air locking\". “After the \"air binding\" phenomenon occurs, the pump does not discharge any liquid, and there is no noise or vibration. To prevent the occurrence of \"air locking,\" the vessel should be filled with liquid before starting. “\"Cavitation\": This is an erosion phenomenon that occurs when the suction height of a pump is too high, causing the pressure inside the pump to equal or fall below the saturated vapor pressure of the liquid being transported. As a result, the liquid vaporizes, bubbles form and burst, leading to cavitation. When cavitation occurs, the liquid generates noise and vibration due to the impacts, the flow rate decreases, and in severe cases, no liquid is discharged at all. To prevent the occurrence of \"cavitation\" ； The actual installation height of the pump should not be higher than the allowable suction lift.
During the startup of a centrifugal pump, if air is present inside the pump casing or the pump is not filled properly, the fluid within the casing does not generate enough negative pressure as it moves centrifugally along with the motor to draw liquid into the casing. The pump appears to be trapped by \"gas,\" a phenomenon known as gas locking in centrifugal pumps； When the liquid drawn into the pump casing vaporizes exactly at the pump’s inlet due to the reduced pressure, it creates a tremendous hydraulic shock on the inner wall of the casing, causing the wall to be corroded as if by \"gas.\" This phenomenon is known as cavitation ； The method to avoid or eliminate this is to fill the pump casing with the liquid to be transported before starting the pump, and to close the outlet valve at the time of startup.
”\"Air locking\": The phenomenon in which the pump cannot draw in liquid after being started, due to air present inside the pump, is known as \"air locking\". “After the \"air binding\" phenomenon occurs, the pump does not discharge any liquid, and there is no noise or vibration. To prevent the occurrence of \"air locking,\" the vessel should be filled with liquid before starting. “\"Cavitation\": This is an erosion phenomenon that occurs when the suction height of a pump is too high, causing the pressure inside the pump to equal or fall below the saturated vapor pressure of the liquid being transported. As a result, the liquid vaporizes, bubbles form and burst, leading to cavitation. When cavitation occurs, the liquid generates noise and vibration due to the impacts, the flow rate decreases, and in severe cases, no liquid is discharged at all. To prevent the occurrence of \"cavitation\" ； The actual installation height of the pump should not be higher than the allowable suction lift. Before starting, the centrifugal pump is not filled with the liquid to be pumped, or air gets into the pump during operation. Since the density of gas is lower than that of liquid, the centrifugal force generated is insufficient to expel the air. The fluid inside the pump casing moves in a centrifugal motion along with the motor, creating a negative pressure that is not enough to draw liquid into the pump casing. As a result, the pump appears to be trapped by the gas, losing its self-priming ability and thus being unable to pump liquid – this phenomenon is known as gas entrapment in centrifugal pumps. Before starting, fill the pump and ensure that the pump casing is filled with the liquid to be transported; close the outlet valve at the time of startup. To prevent the liquid pumped into the pump casing from flowing back into the sump due to gravity, a check valve (foot valve) is installed at the inlet of the pump’s suction line ； If the pump is located below the liquid level in the tank, there is no need to prime the pump at startup. Ensure proper sealing of the housing; the water filling valve must not leak, and it needs to have good sealing properties. When the liquid drawn into the pump casing vaporizes exactly at the pump’s suction inlet due to the reduced pressure, it exerts a tremendous hydraulic impact on the inner wall of the casing, causing the wall to be corroded as if by \"gas.\" This phenomenon is known as cavitation.
Cavitation occurs when, at a constant pressure on the blade surface of the tank, the pressure at the center of the impeller drops to equal the saturated vapor pressure of the liquid being transported at that temperature. This causes numerous bubbles to form in the liquid at the inlet of the impeller; these bubbles enter the high-pressure area and are then quickly crushed and condensed, creating a vacuum in their vicinity. The liquid particles surrounding them rush toward the center of these bubbles at high speeds, generating an instantaneous shock pressure that can rapidly damage parts of the impeller. This is accompanied by vibrations in the pump body and noise, as well as a significant decrease in the pump’s flow rate, head, and efficiency. This phenomenon is called cavitation. Solution: 1. Remove any debris from the inlet pipeline to ensure unobstructed flow, or increase the diameter of the pipe ； 2. Reduce the temperature of the conveying medium ； 3. Reducing the installation height: Air entrapment occurs when a centrifugal pump is started; if there is air inside the pump, its low density results in a weak centrifugal force generated during rotation. As a result, the low pressure created in the center 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 locking. Solution: Centrifugal pumps do not have self-priming capability; therefore, the pump chamber must be filled with liquid before starting.