Improper operation or vaporization of liquid in low-pressure areas during the start-up and operation of centrifugal pumps can cause gas entrapment and cavitation.
As cavitation and gas entrapment may cause serious damage to centrifugal pumps, it is necessary to have a detailed understanding of the causes cavitation and gas entrapment and adopt preventive measures to prevent them during operation and ensure the normal and efficient operation of centrifugal pumps.
Causes of air entrapment
Air entrapment will occur if the centrifugal pump is not filled with liquid before starting or if air is trapped the pump during operation. As the density of air is lower than that of liquid it is difficult to throw air out with centrifugal force. The fluid inside the pump generates negative pressure due to the centrifugal motion of the motor, which is not enough to suck the liquid into the pump. As a result, the pump loses its self-priming ability and cannot transport liquid as if it is bound by "air", which is called the air entrapment of centrifugal pump.
Generating hazardous situations
The pump cannot deliver liquid, causing severe vibration in the equipment and accompanied by strong piercing noise. The motor runs idle, which can easily burn out the motor. Affects the efficiency of liquid transportation and the normal operation of centrifugal pumps.
preventive measures
Before starting, the pump should be filled with the liquid and the outlet valve should be closed when starting. To prevent the liquid fed into the pump from flowing into the low-level tank due to gravity, a check valve (foot valve) should be installed at the inlet of the pump suction pipeline; If the position of the pump is lower than the liquid level in the tank, there is no need to pump during startup. Ensure proper sealing of the casing, and ensure that the inlet valve does not leak and has good sealing performance.
Reasons of cavitation
When the liquid sucked into the pump vaporizes at the suction port due to a decrease in pressure, it causes a huge hydraulic impact on the inner wall of the pump casing, making it appear to be corroded by "air". This phenomenon is called cavitation.
The main causes of cavitation are:
1. The resistance of the inlet pipeline is too high or the pipeline diameter is too small;
2. The temperature of the conveying medium is too high;
3. Excessive flow, which means the outlet valve is almost fully open;
4. The installation location is too high above liquid surface, which affects the suction capacity of the pump;
5. Improper selection, including pump selection and pump material selection, etc.
The liquid containing bubbles rapidly condenses or ruptures after being squeezed into the high-pressure zone. Due to the disappearance of bubbles, a vacuum is generated and the surrounding liquid flows towards the center of the bubble at extremely high speeds, instantly generating a high-speed impact force of up to tens of thousands of kPa, causing impact on the pump impeller and pump casing, resulting in material erosion and damage.
About the different causes of cavitation and air entrapment, air entrapment refers to the air trapped in the pump, which usually occurs when the pump is started, mainly manifested by the incomplete discharge of air out of the pump; And cavitation is caused by the liquid reaching its vaporization pressure at a certain temperature.
Location of cavitation occurrence
According to the different locations where cavitation occurs in water pumps, cavitation can be divided into the following four categories:
Impeller surface cavitation:
The reasons of Impeller surface cavitation are either caused by installation position of the pump too high above liquid level or the pump flow rate deviates too much from the designed flow rate. The formation and collapse of air bubbles often occur on the front and back of the impeller or on the inner surface or rear end of the impeller.
Clearance cavitation:
When water flows through the suddenly narrowing clearances between the impeller and pump body, flow speed increases while the local pressure decreases and cavitation also occurs. In the clearances between the outer edge of the axial flow pump impeller and the pump casing, as well as the clearances between the sealing ring of the centrifugal pump and the outer edge of the impeller, the large pressure covers on the inlet and outlet sides of the impeller cause rapid backflow, resulting in local pressure drop and cavitation in the clearances.
Vortex strip cavitation:
Because of poor design of the water collection tank and inlet channel or as the result of the pump working in quite different condition than originally designed, a vortex strip may also be generated below the impeller from top to bottom. When the center pressure of the vortex strip is lower than the vaporization pressure, it becomes a cavitation strip.
Rough cavitation:
Rough cavitation refers to the phenomenon where water flows through the uneven inner walls and wet components of a pump and local negative pressure is easily generated downstream of the protrusions, leading to cavitation. This type of cavitation is called rough cavitation.
Harm situation generated
(1) Pump performance get deteriorated and when cavitation occurs large quantity of air bubbles will be generated. When water contains a large quantity of air bubbles, it disrupts the normal flow pattern, reduces the effective flow surface area of the impeller passage, changes the flow direction and increases energy loss, causing a rapid decrease in the flow rate, head and efficiency of the pump. In severe cases of cavitation, even flow interruption may occur.
(2) Damaged wet components cause local deformation and hardening of the metal surface on the pump interior surface under repeated high-intensity impact forces, resulting in metal fatigue and causing metal fracture and peeling. In addition to mechanical effects, there are also chemical corrosion of metals by deeply active gases (such as oxygen) escaping from water, as well as electrochemical corrosion of metals by water. Under the comprehensive action, the interior surface of the pump initially appears as pockmarks, which then turn into honey comb like structures and in severe cases, the pump body may be eroded and broken in a short period of time.
Vibration and noise will generate. When bubbles collapse, liquid particles collide with each other and also with the metal surface, producing various frequencies of noise. In severe cases, a "cracking" explosion sound can be heard inside the pump, causing vibration of the unit.
So, noise and vibration are also one of the main criteria used to determine whether cavitation occurs and disappears.
preventive measures
An effective measure to reduce cavitation is to prevent the generation of bubbles.
Firstly, the surface moving in the liquid should be streamlined to avoid the occurrence of vortex in local areas, as the pressure in the vortex zone 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.
Choosing appropriate materials can improve the ability to resist cavitation. Metal materials with high strength and toughness usually have good resistance to cavitation and improving the material's corrosion resistance will also reduce cavitation damage.
The inlet pressure of a centrifugal pump should not be too low but should have a minimum allowable value. The corresponding cavitation allowance is called the necessary cavitation allowance, which is generally measured by the pump manufacturer through cavitation experiments and listed as the performance of the centrifugal pump in the pump product sample. When the pump is operating normally, the actual NPSHa must be greater than the NPSHr, which is specified in Chinese standards to be greater than 0.5m or more.
At the same time, foreign objects in the inlet pipeline should be cleaned to ensure smooth flow or the diameter of the pipe should be increased.
In addition, for pump manufacturers, it is necessary to improve the ability of centrifugal pumps to resist cavitation, such as improving the structural design from the suction port to the impeller; Adopting a front inducer to increase liquid flow pressure; Increase the impeller inlet angle and reduce the bending at the blade inlet to increase the inlet area of the pump to prevent the occurrence of cavitation.
Air entrapment and cavitation in centrifugal pumps has a very adverse effect on them. Before using centrifugal pumps it is necessary to follow the operating procedures to avoid the occurrence of air entrapment. At the same time, it is necessary to regularly inspect and maintain the inlet and outlet pipelines as well as the impeller of the centrifugal pump to prevent the occurrence of cavitation.