If the Centrifugal Pumps are improperly operated during the startup and operation process or the liquid is vaporized in the low-pressure area, it will cause gas binding and cavitation.
Cavitation and gas binding can cause serious damage to Centrifugal Pumps. Therefore, you need to understand the causes of the two phenomena and the corresponding preventive measures in detail, so as to avoid the occurrence of gas binding and cavitation during operation as much as possible, and ensure the normal and efficient operation of Centrifugal Pumps.
Causes of gas binding
Centrifugal Pumps are not filled with the liquid to be transported before starting, or air penetrates into the pump during operation. Because the density of gas is less than that of liquid, the centrifugal force generated is small and the air cannot be thrown out. The fluid in the pump casing generates negative pressure with the centrifugal motion of the motor, which is not enough to suck the liquid into the pump casing. The pump is like being bound by "gas", losing its self-priming ability and unable to transport liquid. This is called the gas binding phenomenon of Centrifugal Pumps.
Harmful situations
The pump cannot pump out liquid, the unit vibrates violently, accompanied by strong and harsh noise, the motor runs idle, and the motor is easy to burn out. Affect the efficiency of conveying liquid and the normal operation of Centrifugal Pumps.
Preventive measures
Prime the pump before starting and fill the pump casing with the liquid to be conveyed. Close the outlet valve when starting. In order to prevent the liquid poured into the pump casing from flowing into the low-level tank due to gravity, a check valve (bottom valve) is installed at the inlet of the pump suction pipe; if the pump is located below the liquid level in the tank, there is no need to prime the pump when starting. Do a good job of sealing the shell, the valve for filling water cannot leak, and the sealing should be good.
Causes of cavitation
When the liquid sucked into the pump casing is vaporized 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, making the shell wall corroded like "gas". This phenomenon is called cavitation.
The main causes of cavitation are:
1. The resistance of the inlet pipeline is too large or the pipeline is too thin;
2. The temperature of the conveying medium is too high;
3. The flow rate is too large, that is, the outlet valve is opened too large;
4. The installation height is too high, which affects the pump's suction volume;
5. Selection issues, including pump selection, pump material selection, etc.
The liquid containing bubbles condenses or ruptures rapidly after being squeezed into the high-pressure area. Due to the disappearance of the bubbles, a local vacuum is generated, and the surrounding liquid flows to the center of the bubble at a very high speed, instantly generating a huge high-speed impact force of up to tens of thousands of kpa, causing impact on the impeller and pump casing, causing erosion and damage to the material.
From the different causes of cavitation and gas binding: gas binding is air in the pump body, which usually occurs when the pump is started, mainly manifested in the air in the pump body is not completely discharged; while cavitation is caused by the liquid reaching its vaporization pressure at a certain temperature.
Location of cavitation
According to the different locations of cavitation in water pumps, cavitation can be divided into the following four categories:
Blade surface cavitation:
Blade surface cavitation is cavitation that occurs on the surface of blades. It is mainly caused by the installation of water pumps too high or the flow rate deviating from the design flow rate. The cavitation bubble formation and collapse mostly occur on the front and back of the blades or the inner surface of the front impeller and the root of the blades.
Gap cavitation:
Gap cavitation When the water flow in the pump passes through a suddenly narrowed gap, the speed increases and the local pressure drops, which will also cause cavitation. For example, in the gap between the outer edge of the axial flow pump blade and the pump casing, the gap between the Centrifugal Pumps seal ring and the outer edge of the impeller, the glands on the water inlet and water outlet sides of the impeller are large, resulting in high-speed backflow, causing local pressure drop and gap cavitation.
Vortex belt cavitation:
Due to the poor design of the water collection tank and the water inlet flow channel or the pump working under non-design conditions, a top-down belt-shaped vortex (abbreviated as vortex belt) may also be generated below the impeller. When the central pressure of the vortex belt is lower than the vaporization pressure, the vortex belt becomes a cavitation belt.
Rough cavitation:
Rough cavitation is when the water flows through the uneven inner wall and flow-through components of the pump, it is easy to generate local negative pressure downstream of the protrusion and cause cavitation. This cavitation is called rough cavitation.
Hazards caused
(1) Deterioration of pump performance. When cavitation occurs, a large number of cavitation bubbles will be generated. When there are a large number of cavitation bubbles in the water, the normal law of water flow is destroyed, the effective flow area of the blade groove is reduced, the flow direction changes accordingly, and the energy loss increases, thereby causing a rapid decrease in the flow rate, head and efficiency of the pump. When cavitation is severe, it may even cause flow interruption.
(2) Damage to flow-through components. Under the repeated high-intensity impact force, the metal surface of the pump wall undergoes local deformation, hardening and brittleness, resulting in metal fatigue, which causes the metal to crack and peel off. In addition to mechanical effects, there are also chemical corrosion of metals by active gases (such as oxygen) escaping from the water body and electrochemical corrosion of metals by water bodies. Under the combined effect, pitting appears on the pump wall at first, and then becomes honeycomb-shaped. In severe cases, the wall will be hollowed out in a short period of time.
Generate vibration and noise. When the bubbles collapse, the liquid particles collide with each other and also hit the metal surface, generating noise of various frequencies. In severe cases, a "crackling" explosion sound can be heard in the pump, causing vibration of the unit. Under the repeated huge impact, the impeller surface has spots and cracks, and even gradually falls off in a sponge-like state, reducing the service life of the pump.
Therefore, noise and vibration are also one of the main bases for judging whether cavitation occurs and disappears.
Preventive measures
An effective measure to reduce cavitation is to prevent the generation of bubbles.
First of all, the surface moving in the liquid should be streamlined to avoid eddies in local areas, because the pressure in the eddy area is low and bubbles are easily generated. In addition, the gas content in the liquid and the disturbance in the liquid flow should be reduced, which will also limit the formation of bubbles.
Selecting appropriate materials can improve the ability to resist cavitation. Generally, metal materials with high strength and toughness have good cavitation resistance. Improving the corrosion resistance of materials will also reduce cavitation damage.
The pressure at the inlet of Centrifugal Pumps cannot be too low, but should have a minimum allowable value. The corresponding cavitation margin is called the required cavitation margin, which is generally determined by the pump manufacturer through cavitation experiments and listed in the pump product sample as the performance of Centrifugal Pumps. When the pump is operating normally, the actual cavitation margin must be greater than the required cavitation margin. The Chinese standard stipulates that it should be greater than 0.5m.
At the same time, foreign objects in the inlet pipeline should be cleaned to make the inlet unobstructed, or the diameter of the pipe should be increased.
In addition, pump manufacturers need to improve the anti-cavitation ability of Centrifugal Pumps, such as improving the structural design from the suction port to the impeller; using a front inducer to increase the liquid flow pressure; increasing the blade inlet angle and reducing the bending at the blade inlet to increase the inlet area.
The air binding and cavitation of Centrifugal Pumps are very unfavorable to Centrifugal Pumps. Before using Centrifugal Pumps in daily life, you must follow the operating procedures to avoid the occurrence of air binding. At the same time, the inlet and outlet pipelines and blades of Centrifugal Pumps should be regularly inspected and maintained to prevent the occurrence of cavitation.
