Reasons for air binding and cavitation in centrifugal pumps? How to prevent measures?

　　Centrifugal pumps in the startup process and the work process if the operation is not appropriate or the liquid in the low-pressure region of the gasification, will cause the occurrence of air binding and cavitation phenomenon.

　　Cavitation and air binding phenomenon for centrifugal pumps can cause serious damage, so today to bring you a detailed understanding of the reasons for the occurrence of the two phenomena and the corresponding preventive measures, so as to try to avoid the occurrence of cavitation and air binding phenomenon in the work, to ensure that the centrifugal pumps normal and efficient operation.

　　Air binding: due to the pump memory gas, start the pump after the suction of liquid phenomenon, called “air binding” phenomenon. After the phenomenon of “air binding” occurs, the pump has no liquid discharge, no noise and vibration. In order to prevent the “air binding” phenomenon, start the pump before fully filling the pump, exhaust the pump gas.

　　Cavitation: due to the pump suction height is too high or the pump inlet medium temperature is too high, so that the pump pressure is equal to or lower than the temperature of the transported liquid saturated vapor pressure, liquid vaporization, bubble formation, rupture and other processes caused by the stripping phenomenon, known as “cavitation” phenomenon, “cavitation” occurs when the liquid due to impact, no noise, vibration. “When the occurrence of liquid due to the impact of noise, vibration, so that the flow is reduced, or even no liquid discharge. In order to prevent “cavitation” phenomenon, the actual installation height of the pump should not be higher than the allowable suction height, strict control of the pump inlet medium temperature.

　　I. Cavitation phenomenon

　　1、The reason of air binding

　　Centrifugal pumps in the start-up before not filled with the liquid being transported, or in the process of running the pump into the air, because the density of the gas is less than the density of the liquid, resulting in a small centrifugal force can not be thrown out of the air, the fluid inside the pump casing along with the motor for the centrifugal movement produces a negative pressure is not enough to suck the liquid to the pump casing, the pump like a “gas” bound as, losing its self-sufficiency, and then the pump will be “gas”, and then the pump will not be “gas”. The pump is like being bound by “gas”, losing the ability to self-priming and unable to transport liquid, called the centrifugal pump air binding phenomenon.

　　2、Air binding produces harmful situation

　　The pump can not hit the liquid, the unit produces violent vibration, accompanied by strong ear-piercing noise, motor idling, easy to burn the motor. Impact on the efficiency of the conveyed liquid and the normal work of the centrifugal pump.

　　3、Preventive measures

　　Before starting, fill the pump and make the pump casing full of liquid to be conveyed, and close the outlet valve when starting. In order to prevent the liquid in the pump casing from flowing into the low slot due to gravity, a non-return valve (bottom valve) is installed at the entrance of the pump suction line; if the pump position is lower than the liquid level in the slot, there is no need to irrigate the pump at startup. Do a good job of sealing the casing, filling the valve can not leak, sealing should be good.

　　Second, cavitation phenomenon

　　1、The reason of cavitation

　　When the pump casing suction of the liquid in the pump suction inlet due to reduced pressure just gasification, to the inner wall of the pump casing to bring a huge hydrodynamic impact, so that the casing wall like a “gas” corrosion, the phenomenon is known as the phenomenon of cavitation. The main reasons for cavitation:

　　1) the inlet pipeline resistance is too large or too thin pipeline; 1) the inlet pipeline resistance is too large or too thin pipeline

　　2) the transport medium temperature is too high; 3) the flow rate is too large.

　　3) the flow is too large, that is, the exit valve is open too large.

　　4) the installation height is too high, affecting the pump suction volume.

　　5) Selection issues, including the selection of pumps, pump material selection.

　　Liquid containing bubbles squeezed into the high-pressure area after the sharp condensation or rupture. Due to the disappearance of bubbles to produce a local vacuum, the surrounding liquid at a very high speed flow to the center of the bubble, instantly produced a great up to tens of thousands of kilopascals of high-speed impact, resulting in the impeller and the pump casing of the impact, so that the material is eroded and damaged.

　　From the causes of cavitation and air binding different reasons: air binding is the pump body air, generally occurring in the pump start time, mainly in the pump body air is not discharged; and cavitation is due to the liquid at a certain temperature to reach the saturation vapor pressure of the medium, the medium vaporization.

　　2, the location of cavitation

　　According to the different parts of the pump where cavitation occurs, cavitation can be divided into the following four categories:

　　1) foliar cavitation:

　　Foliar cavitation is occurring on the surface of the blade cavitation, mainly because the pump is installed too high, or flow deviation from the design flow is too large when the cavitation phenomenon. The formation and collapse of the vacuole occurs in the front and back of the blade or the inner surface of the front disk and the root of the blade.

　　2) gap cavitation:

　　Gap cavitation pump water flow through the suddenly narrowed gap, the speed increases, the local pressure drop, will also produce cavitation. Such as axial pump blade outer edge and the gap between the pump casing, centrifugal pump sealing ring and the outer edge of the impeller at the gap, due to the impeller inlet side and outlet side of the gland is very large, resulting in high-speed reflux, resulting in a localized pressure drop, causing interstitial cavitation.

　　3) Vortex band cavitation:

　　Vortex band cavitation due to the catch basin, inlet runners poor design or pumps working under non-design conditions, may also be generated below the impeller top-down band vortex (referred to as vortex band), when the vortex band center pressure is lower than the vaporization pressure, the vortex band that is, the vortex band has become a vapor corrosion zone