Purification Technology Edition Daily Question (Short Answer) [2018-2-22]

When the absolute pressure at the pump inlet is lower than the saturated vapor pressure of the liquid, the liquid boils at the inlet, generating numerous bubbles that strike the impeller, pump casing, and pump body, causing vibrations and abnormal noises. This can even lead to the impeller losing its integrity, cracking, or being damaged. As a result, the pump’s flow rate, head, and efficiency all decline sharply. This phenomenon is known as pump cavitation. To prevent cavitation in the pump, it is necessary to take into account the pump’s installation height and the liquid temperature, so that during operation the pressure at the pump inlet is greater than the saturated vapor pressure of the liquid.

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. @_@Determine the installation height reasonably @_@ (1) During installation, the distance between the pump’s suction inlet and the liquid surface should be as low as possible to reduce suction pressure losses ； (2) Increase the diameter of the pump suction pipe to reduce the resistance loss in the suction line ； (3) Under the condition of meeting the requirements for head and flow rate, the lower the rotational speed, the better, as this reduces the vacuum level at the pump’s suction inlet ； (4) Use a double-suction pump or a centrifugal pump equipped with a pre-inducer wheel to improve the suction conditions ； (5) Where process conditions permit, avoid an increase in the temperature of the liquid being transported to prevent its vaporization

Answer: When the absolute pressure at the pump inlet is lower than the saturated vapor pressure of the liquid, the liquid boils at the inlet, generating numerous bubbles that strike the impeller, pump casing, and pump body, causing vibrations and abnormal noises. This can even lead to the detachment, cracking, and damage of the impeller. As a result, the pump’s flow rate, head, and efficiency all decline sharply. This phenomenon is known as pump cavitation. To prevent cavitation in the pump, it is necessary to take into account the pump’s installation height and the liquid temperature, so that during operation the pressure at the pump inlet is greater than the saturated vapor pressure of the liquid.

When the absolute pressure at the pump inlet is lower than the saturated vapor pressure of the liquid, the liquid boils at the inlet, generating numerous bubbles that strike the impeller, pump casing, and pump body, causing vibrations and abnormal noises. This can even lead to the impeller losing its integrity, cracking, or being damaged. As a result, the pump’s flow rate, head, and efficiency all decline sharply. This phenomenon is known as pump cavitation. To prevent cavitation in the pump, it is necessary to take into account the pump’s installation height and the liquid temperature, so that during operation the pressure at the pump inlet is greater than the saturated vapor pressure of the liquid.

The suction power of a centrifugal oil pump is provided by the pressure difference between the suction 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 towards 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. (1) Measures to improve the cavitation resistance of the centrifugal pump itself: These measures are mainly implemented by the design and manufacturing units; for example, the inlet geometry of the impeller can be changed, a double-suction impeller can be used, or a lower impeller inlet velocity along with an increased impeller inlet diameter can be adopted. (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 materials. (4) Increasing the limited net positive suction head of the unit, such as raising the pressure on the liquid surface of the suction tank or determining the geometric installation height appropriately, can all increase 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.

The suction power of a centrifugal pump is provided by the pressure difference between the surface of the liquid being sucked in 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 towards 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, impact, and erosion is known as cavitation. Methods to prevent cavitation in centrifugal pumps include: (1) Measures to improve the cavitation resistance of the centrifugal pump itself: These measures are mainly implemented by the design and manufacturing units; for example, the inlet geometry of the impeller can be altered, a double-suction impeller can be used, or a lower inlet velocity for the impeller along with an increased inlet diameter can be adopted. (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 materials. (4) Increasing the limited net positive suction head of the unit, such as raising the pressure on the liquid surface of the suction tank or determining the geometric installation height appropriately, can all increase 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.

When the absolute pressure at the pump inlet is lower than the saturated vapor pressure of the liquid, the liquid boils at the inlet, generating numerous bubbles that strike the impeller, pump casing, and pump body, causing vibrations and abnormal noises. This can even lead to the impeller losing its integrity, cracking, or being damaged. As a result, the pump’s flow rate, head, and efficiency all decline sharply. This phenomenon is known as pump cavitation. To prevent cavitation in the pump, it is necessary to take into account the pump’s installation height and the liquid temperature, so that during operation the pressure at the pump inlet is greater than the saturated vapor pressure of the liquid.

When the absolute pressure at the pump inlet is lower than the saturated vapor pressure of the liquid, the liquid vaporizes at the inlet, generating numerous bubbles that strike the impeller, pump casing, and pump body, causing vibration and abnormal noise; in severe cases, this can even lead to the impeller losing its integrity, cracking, or being damaged. This phenomenon, characterized by a sharp decline in the pump’s flow rate, head, and efficiency, is known as pump cavitation. To prevent pump cavitation, it is necessary to take into account the pump’s installation height and the temperature of the liquid, so that during operation the pressure at the pump inlet is greater than the saturated vapor pressure of the liquid

The suction power of a centrifugal oil pump is provided by the pressure difference between the suction 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 towards 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, impact, and erosion is known as cavitation. Methods to prevent pump cavitation include: (1) Measures to improve the cavitation resistance of the centrifugal pump itself: These measures are mainly implemented by the design and manufacturing units; for example, the inlet geometry of the impeller can be altered, a double-suction impeller can be used, or a lower impeller inlet velocity along with an increased impeller inlet diameter can be adopted. (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 materials. (4) Increasing the limited net positive suction head of the unit, such as raising the pressure on the liquid surface of the suction tank or determining the geometric installation height appropriately, can all increase 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.

When the absolute pressure at the pump inlet is lower than the saturated vapor pressure of the liquid, the liquid boils at the inlet, generating numerous bubbles that strike the impeller, pump casing, and pump body, causing vibrations and abnormal noises. This can even lead to the impeller losing its integrity, cracking, or being damaged. As a result, the pump’s flow rate, head, and efficiency all decline sharply. This phenomenon is known as pump cavitation. To prevent cavitation in the pump, it is necessary to take into account the pump’s installation height and the liquid temperature, so that during operation the pressure at the pump inlet is greater than the saturated vapor pressure of the liquid.