Understanding cavitation in centrifugal pumps
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Cavitation in centrifugal pumps is one of the most common phenomena that occur in these pumps. It arises when, at the corresponding saturated vapor pressure and with a high liquid temperature, pressure losses occur during flow, which leads to the formation of bubbles that burst, causing vibrations and a drop in performance. Today, I discussed this with an experienced equipment engineer; he said that the saturated vapor temperature for centrifugal pumps is 235 degrees, and the higher the temperature, the more likely cavitation will occur. Is that true??? If a pump has an inlet pressure of 3.5 MPA and a corresponding temperature of 235 degrees, will cavitation occur if the pump is started without venting?
It is true that the higher the temperature, the more likely cavitation will occur. The higher the temperature, the greater the corresponding saturated vapor pressure; and the greater the saturated vapor pressure, the easier it is for vaporization to take place, thereby increasing the likelihood of cavitation. Could gas entrapment occur independently of temperature? Gas entrapment is caused by evacuation, with leakage at the inlet being the main reason; it has nothing to do with temperature
It mainly depends on the difference between the net positive suction head available at the device and the required net positive suction head of the pump. Cavitation will not occur even at high temperatures if the inlet pressure is sufficient
Cavitation can be caused by various factors; the equipment manual will specify the cavitation margin of this pump, as well as the inlet pressure and temperature. 1. The higher the corresponding temperature, the more likely cavitation will occur, but this can be compensated for by increasing the static head, that is, the inlet pressure. 2. To reduce the occurrence of cavitation, the pump must be thoroughly vented and preheated before being started, so that the temperature of the pump itself approaches that of the inlet medium.
The explanation is really good; I’ve learned something!
Cavitation and gas entrapment are not the same thing. Cavitation occurs when, in the low-pressure area at the center of the impeller, the low temperature causes the vapor pressure to rise, resulting in the formation of bubbles. As the liquid flows toward the high-pressure areas of the pump, these pressurized gases suddenly liquefy, creating voids; the surrounding liquid then rushes in to fill these voids, thereby generating hammering effects. Gas trapping is caused by non-condensable gases being unable to escape from the low-pressure area within the pump.
The cavitation problem is mainly a concern for designers; it can be avoided as long as the properties of the process medium are fully taken into account during design, and an adequate pressure head at the pump inlet is ensured. Actually, for the installation site, this can be avoided as long as the height of the pump inlet tank and the pressure control of the tank are properly managed.
It can be understood in this way: the saturated vapor pressure is a function of temperature. What is the relationship between the gas phase and the liquid phase at which the saturated vapor pressure exists? It can be assumed that at a temperature of 25°C, a certain liquid consists of 99 grams of liquid and 1 gram of gas above it. If the temperature is increased, the molecular thermal motion speeds up, and some of the liquid will vaporize and join the gas phase. As the temperature rises, the corresponding saturated vapor pressure also increases, meaning more of the liquid turns into gas. As a result, the initial amount of liquid might change to 90 grams of liquid and 10 grams of gas above it. Due to the law of conservation of mass, if the diffusion rate remains constant, the volume of the gas won’t change much. Nevertheless, the saturated vapor pressure will increase. It can be understood in this way: as the temperature rises, some of the liquid vaporizes and joins the gas phase, until the gas reaches a certain pressure, which is the saturated vapor pressure.