If the centrifugal pump is improperly operated during the start-up process and during the working process or if the liquid is vaporized in the low pressure zone, air binding and cavitation may occur.
Cavitation and air-binding will cause serious damage to the centrifugal pump. It’s important to learn the causes of the two phenomena and the preventive measures, so as to avoid cavitation and air-binding, and ensure the centrifugal pump in a stable condition.
Air binding
Centrifugal pump is not the filling of liquid to be delivered before starting, or is in the process of operation within the pump into the air, because the density of gas less than the density of liquid, the centrifugal force is small, can't take air out, the fluid inside the pump casing with motor for centrifugal movement is to produce negative pressure to suction fluid inside the pump casing, so it can not self-priming to transport the liquid, which is called air binding of a centrifugal pump.
The pump can't transfer liquid, the mechanism produces severe vibration, and it is accompanied by strong and harsh noise. The motor is idling and it is easy to burn the motor. Affect the efficiency of the transport liquid and the normal operation of the centrifugal pump.
Before starting, the pump should be filled and the pump casing filled with the liquid to be delivered, and the outlet valve is closed at the start. In order to prevent the liquid poured into the pump casing from flowing into the lower tank due to gravity, a check valve (bottom valve) is installed at the inlet of the pump suction pipe. If the pump position is lower than the liquid level in the tank, no water must be filled with when start the pump. Sealing is important for the pump. The valve that is filled with water should not leak water and the sealing performance should be good.
Cavitation
A. Causes of cavitation
When the liquid sucked in the pump casing is justified at the suction port of the pump due to the pressure reduction, it brings a huge hydraulic impact to the inner wall of the pump casing, so that the shell wall is corroded by "gas". This is called cavitation. phenomenon.
The main causes of cavitation are:
The bubble-containing liquid swells or ruptures after being squeezed into the high pressure zone. Due to the disappearance of the bubble, a partial vacuum is generated, and the surrounding liquid flows to the center of the bubble at a very high speed, which instantaneously generates a high-speed impact force of up to several tens of thousands of KPA, causing impact on the impeller and the pump casing, causing the material to be eroded and damage.
From the point of view of the cause of cavitation and air binding: the air binding is the air in the pump body, which usually occurs when the pump is started, mainly because the air in the pump body is not discharged; and the cavitation is due to the liquid reaching at a certain temperature. It has reached its vaporization pressure.
B. Location of cavitation
According to the different parts of the pump cavitation, the cavitation can be divided into the following four categories:
(1) Foliar erosive:
Foliar cavitation is cavitation that occurs on the surface of the blade, mainly because the pump is installed too high, or the flow is deviated from the cavitation when the design flow is too large. Its cavitation formation and collapse occur mostly at the front and back of the blade or at the inner surface of the front wheel and at the root of the blade.
(2) Clearance cavitation:
When the water in the gap cavitation pump passes through the suddenly narrowing gap, the speed increases, the partial pressure drops, and cavitation also occurs. For example, in the gap between the outer edge of the axial pump blade and the pump casing, the gap between the seal ring of the centrifugal pump and the outer edge of the impeller is caused by the large pressure of the inlet side and the water outlet side of the impeller, resulting in high-speed reflow and partial pressure drop causing gap cavitation.
(3) Vortex cavitation:
Vortex cavitation due to the sump, poor design of the inlet runner or the pump operating under non-design conditions, it is also possible to produce a top-down band vortex (referred to as vortex band) below the impeller, when the vortex center pressure is lower than When the pressure is vaporized, the vortex band becomes a cavitation zone.
(4) Rough cavitation:
Rough cavitation is when the water flows through the uneven inner wall surface and the over-current component in the pump, and the local negative pressure is easily generated downstream of the protrusion to cause cavitation, which is called rough cavitation.
C. The damage
(1) Deterioration of the performance of the pump, a large amount of cavitation will occur when cavitation occurs, and when the water contains a large number of cavitation, the normal law of the water flow is destroyed, the effective flow area of the groove is reduced, and the flow direction changes accordingly. The loss increases, causing a rapid drop in pump flow, head and efficiency, and even a break when cavitation is severe.
(2) Damage to the over-current components, the wall surface of the pump under the repeated action of high-intensity impact force, local deformation and hardening and brittleness of the metal surface, resulting in metal fatigue phenomenon, causing metal cracking and peeling. In addition to the mechanical action, it is also mixed with the chemical corrosion of the metal by the deep and active gas (such as oxygen) escaping from the water body and the electrochemical corrosion of the metal by the water body. Under the combined effect, the wall surface of the pump initially appears as a pitting, and then becomes a honeycomb shape. In severe cases, the wall surface will be hollowed out in a short period of time.
(3) Vibration and noise are generated. When the bubble collapses, the liquid particles collide with each other and also hit the metal surface to generate noise of various frequencies. In severe cases, the violent explosion sound in the pump can be heard and the unit vibration is caused. Under the repeated action of the huge impact of the impeller, there are spots and cracks on the surface, and even the sponge gradually falls off, which reduces the service life of the pump.
Therefore, noise and vibration are also one tip to see whether cavitation occurs and disappears.
D. Preventive measures
An effective measure to reduce cavitation is to prevent the generation of bubbles.
First, the surface moving in the liquid should be streamlined to avoid eddy currents in local areas, because the vortex zone has low pressure and is prone to bubble formation. In addition, the gas content in the liquid and the disturbance in the liquid flow should be reduced, and the formation of bubbles will also be limited.
Choosing the right material can improve the resistance to cavitation. Metal materials with high strength and toughness generally have good cavitation resistance, and improving the corrosion resistance of the material will also reduce cavitation damage.
The pressure at the inlet of the centrifugal pump should not be too low, but there should be a minimum allowable value. The corresponding NPSH is called the necessary NPSH, which is generally determined by the pump manufacturer through cavitation test and as the performance of the centrifugal pump. Listed in the pump product sample. When the pump is in normal operation, the actual NPSH must be greater than the necessary cavitation margin, which should be greater than 0.5m in China's standards.
At the same time, it is necessary to clean the foreign matter in the inlet pipe to make the inlet open, or increase the diameter of the pipe.
In addition, for the pump manufacturer, it is necessary to improve the anti-cavitation ability of the centrifugal pump itself, such as improving the structural design of the suction port to the vicinity of the impeller; using the front induction wheel to increase the flow pressure; increasing the inlet angle of the blade, minus Bending at the inlet of the small blade to increase the inlet area.
The air binding and cavitation of the centrifugal pump is very unfavorable for the centrifugal pump. Therefore, before using the centrifugal pump daily, it must be carried out according to the operating procedures to avoid the occurrence of air binding. At the same time, it is necessary to regularly inspect and maintain the inlet and outlet pipes and blades of the centrifugal pump to prevent cavitation.
