【Daily Question】Chemical Engineering Principles 147: Cavitation Phenomenon (August 20)

Answer: The suction power of a centrifugal oil pump is provided by the pressure difference between the pressure at the liquid surface and the low pressure created as the impeller discharges the liquid. As the pressure at the inlet of the impeller decreases, its suction capacity increases. However, if the pressure falls below the saturation vapor pressure, bubbles form, and the gases originally dissolved in the liquid escape as well. When these small bubbles reach the high-pressure area inside the impeller, they re-condense under the higher pressure of the surrounding liquid, resulting in a reduction in their volume; this creates a void. The surrounding liquid then rushes toward this void at extremely high speeds, generating very high local pressures that continuously strike the surface of the impeller. Due to these high-speed, high-pressure hydraulic impacts, the surface of the blades becomes eroded and develops pitting and a honeycomb-like structure as a result of fatigue. This phenomenon of vaporization, condensation, impaction, and erosion is known as cavitation. Methods that can be used to prevent cavitation in CYZ self-priming centrifugal oil pumps include: (1) Measures to improve the cavitation resistance of the centrifugal pump itself: These measures are mainly implemented by the design and manufacturing parties; for example, the inlet geometry of the impeller can be altered, a double-suction impeller can be used, or the inlet velocity of the impeller can be reduced along with an increase in the inlet diameter of the impeller. (2) Appropriately increasing the width of the blade inlet edge can also reduce the relative velocity at the impeller inlet. (3) The impeller is manufactured from cavitation-resistant material. (4) Increasing the limited net positive suction head of the unit, such as raising the pressure on the liquid level in the suction tank or determining the geometric installation height appropriately, can all enhance the effective net positive suction head of the pump. (5) Reducing the resistance loss in the suction pipeline and lowering the vaporization pressure of the liquid surface can both increase the effective net positive suction head.

Answer: When the liquid pressure at the inlet of the centrifugal pump impeller drops to the saturation vapor pressure at the temperature of the liquid being pumped, the liquid vaporizes； At the same time, gases dissolved in the liquid may escape from it, forming numerous small bubbles. When these small bubbles flow with the liquid into areas within the impeller channel where the pressure is above the critical value, since the pressure inside the bubbles is the vaporization pressure while the liquid pressure outside is higher than this vaporization pressure, the small bubbles collapse and dissolve again under the influence of the surrounding liquid pressure. Inside the impeller, when the small bubbles that are formed re-condense and collapse, a cavity is created. The liquid surrounding this cavity rushes toward it at extremely high speeds; the liquid particles collide with each other, resulting in localized hydraulic shock that can cause local pressures to reach several hundred atmospheres. The larger the bubble, the greater the local water hammer pressure generated when it condenses and collapses. If these bubbles collapse near the surface of the impeller metal, the liquid particles strike the metal surface continuously, like countless small projectiles. This type of hydraulic impact, with very high speeds and a frequency of up to 2500 times per second, causes the surface of the material to gradually become fatigued and damaged, leading to erosion of the metal surface and the formation of honeycomb-shaped cavities of various sizes. If the bubbles contain some active gases (such as oxygen), the heat released during the condensation of these bubbles can raise the local temperature to 200–300°C, leading to electrochemical corrosion of the metal and accelerating its degradation. The combined phenomenon of vaporization, condensation, impact, and erosion of metal by such gases is known as cavitation. The instability of the cavitation process causes vibrations and noise in the pump. Additionally, during cavitation, bubbles clog the impeller channels, resulting in a decrease in both flow rate and head, as well as a drop in efficiency. In severe cases, the pump fails to operate properly. Therefore, when operating centrifugal pumps, it is essential to prevent cavitation from occurring. Once cavitation occurs, the pump must be stopped immediately, the inlet valve closed, the pump completely drained, and then refilled. Make sure the pump is filled with liquid before starting it.

Cavitation occurs when the local pressure of the liquid in the pump’s flow channels drops to the critical pressure (usually close to the vaporization pressure), causing gas nuclei in the liquid to grow into bubbles; it is a term that refers to the overall process of the accumulation, movement, splitting, and collapse of these bubbles.

What is cavitation in centrifugal pumps? How to prevent it? When the liquid pressure at the inlet of the centrifugal pump impeller drops to the saturated vapor pressure at the temperature of the liquid being transported, the liquid vaporizes ； At the same time, gases dissolved in the liquid may escape from it, forming numerous small bubbles. When these small bubbles flow with the liquid into areas within the impeller channel where the pressure is above the critical value, since the pressure inside the bubbles is the vaporization pressure while the liquid pressure outside is higher than this vaporization pressure, the small bubbles collapse and dissolve again under the influence of the surrounding liquid pressure. Inside the impeller, when the small bubbles that are formed re-condense and collapse, a cavity is created. The liquid surrounding this cavity rushes toward it at extremely high speeds; the liquid particles collide with each other, resulting in localized hydraulic shock that can cause local pressures to reach several hundred atmospheres. The larger the bubble, the greater the local water hammer pressure generated when it condenses and collapses. If these bubbles collapse near the surface of the impeller metal, the liquid particles strike the metal surface continuously, like countless small projectiles. This type of hydraulic impact, with very high speeds and a frequency of up to 2500 times per second, causes the surface of the material to gradually become fatigued and damaged, leading to erosion of the metal surface and the formation of honeycomb-shaped cavities of various sizes. If the bubbles contain some active gases (such as oxygen), the heat released during the condensation of these bubbles can raise the local temperature to 200–300°C, leading to electrochemical corrosion of the metal and accelerating its degradation. The combined phenomenon of vaporization, condensation, impact, and erosion of metal by such gases is known as cavitation. The instability of the cavitation process causes vibrations and noise in the pump. Additionally, during cavitation, bubbles clog the impeller channels, resulting in a decrease in both flow rate and head, as well as a drop in efficiency. In severe cases, the pump fails to operate properly. Therefore, when operating centrifugal pumps, it is essential to prevent cavitation from occurring. Once cavitation occurs, the pump must be stopped immediately, the inlet valve closed, the pump completely drained, and then refilled. Make sure the pump is filled with liquid before starting it.

Cavitation: When the impeller of a centrifugal pump rotates at high speed, it generates a strong centrifugal force. Under the influence of this force, and due to hydrodynamic effects, a vacuum lower than atmospheric pressure is created at the pump’s inlet. When the pressure of the flowing liquid drops to the vaporization pressure of that liquid at that temperature, the liquid begins to vaporize and form bubbles. Also, when the pressure decreases, the gas dissolved in the liquid often escapes before vaporization, forming bubbles. In this way, the bubbles formed in the moving liquid flow along with the liquid. When the bubble reaches a region where the static pressure exceeds the saturated vapor pressure, the gas inside the bubble condenses suddenly, causing the bubble to burst. When the bubble bursts, the surrounding liquid moves at high speed toward the center of the bubble, resulting in high-frequency water hammer effects that strike the surface of the impeller, generating noise and vibration. The repeated formation and collapse of such bubbles cause damage to the impeller surface in this area, resulting in a decrease in pump flow rate, a drop in head, and reduced efficiency. This phenomenon is known as cavitation. Measures to prevent cavitation: 1. Reduce the geometric suction head or increase the geometric backflow head. 2. Minimize the hydraulic losses of the liquid being drawn in. 3. When the pump operates at high flow rates, the net positive suction head must be increased; since the effective net positive suction head decreases, it is necessary to ensure that there is a sufficient margin of this effective net positive suction head. Otherwise, long-term operation at high flow rates should be avoided. Sometimes it is because the pump head is set too high. In fact, the pump operates at high flow rates, making cavitation likely to occur; this should be taken into account when selecting a pump. 4. At the same speed and flow rate, double-suction pumps are less prone to cavitation. 5. When cavitation occurs in a pump, the flow rate should be reduced or the pump should operate at a lower speed. 6. Use materials resistant to cavitation.

According to the first volume of \"Principles of Chemical Engineering\" published by Tianjin University, cavitation is defined as follows: when the lowest pressure near the inlet of the blade is less than or equal to the saturated vapor pressure of the liquid at the operating temperature, the liquid vaporizes at that location and bubbles are formed; these bubbles then move with the liquid from low-pressure areas to high-pressure areas； Under high pressure, the bubbles rapidly condense or burst; at this point, the surrounding liquid rushes into the space previously occupied by the bubbles at extremely high speeds, generating very high shock pressures at the impact points, with a very high frequency of such impacts ； Due to the impact forces, the pump body vibrates and generates noise; moreover, the impeller and pump casing are subjected to repeated high impacts, which causes fatigue in the material surface. Starting from pitting, cracks form, and ultimately the impeller or pump casing is damaged – this phenomenon is known as cavitation. Prevention and control measures include: (1) The installation height of the pump body must be lower than the allowable suction vacuum level. ⑵ The effective NPSH provided by the pump installation is greater than the NPSH required by the pump. ⑶The operating temperature for transporting liquids should not be too high.

When the pressure at the lowest point inside the pump equals the saturated vapor pressure of the liquid being transported at the same temperature, the liquid will vaporize rapidly, generating numerous bubbles. This leads to cracks on the surface of the impeller or the inner casing of the pump, causing the pump to vibrate and produce noise, while its flow rate and head capacity decrease significantly. This phenomenon is known as cavitation. Determine the appropriate installation height to ensure that cavitation does not occur.

A phenomenon of cavernous corrosion and degradation on the metal surface in contact with a fluid, under conditions of high flow velocity and pressure changes; bubbles form near the solid surface when the pressure at the point where the liquid comes into contact with the solid surface is lower than its vapor pressure. Additionally, gases dissolved in the liquid may also precipitate to form bubbles. Subsequently, when the bubbles move to areas where the liquid pressure exceeds the bubble pressure, they burst, generating extremely high shock forces and high temperatures at the moment of bursting. When a solid surface is subjected to repeated impacts of this kind, the material becomes fatigued and breaks away, resulting in small pits on the surface which eventually develop into a spongy structure. There are many methods to prevent cavitation, such as installing bypass valves in centrifugal pumps, enhancing the pump’s own resistance to cavitation, and increasing the effective net positive suction head of the liquid inlet system

What is cavitation in centrifugal pumps? How to prevent it? Answer: The phenomenon where a centrifugal pump can only spin idly without transferring liquid. Preventive measure: The centrifugal pump should be filled with liquid before starting.

When there is air inside the centrifugal pump casing, the lower density of air compared to that of the liquid results in a smaller centrifugal force. As a result, the pressure difference between the surface of the liquid in the tank and the pump’s suction inlet is not sufficient to force the liquid in the tank into the pump; in other words, the centrifugal pump does not have self-priming capability, which prevents it from transporting liquid. This phenomenon is known as \"cavitation\". 1. The installation height of the pump must not exceed the allowable suction height. 2. When the liquid flow rate and resistance in the suction pipeline are too high, the installation height should be reduced.