Cavitation in centrifugal pumps
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Cavitation in centrifugal pumps is perhaps one of the most fundamental concepts in chemical engineering. Although people usually remember this concept, at the site yesterday, a centrifugal pump had an inlet water temperature of 100 degrees, and cavitation occurred suddenly. When I went to the site to check, the impeller made a loud noise, similar to the sound of firecrackers. However, I have a question for everyone: the normal operating pressure at the outlet of this centrifugal pump is 2.4 Mpa, but when cavitation occurs, the pressure fluctuates between 1.2 and 2.4 Mpa. The decrease in pressure when cavitation takes place is due to part of the liquid turning into gas. The pump’s power output decreases, which causes the outlet pressure to drop. But why does the pressure rise back to its previous operating level of 2.4 Mpa? I hope fellow sailors can give some guidance
Since the pressure inside the pump keeps changing, vaporization occurs at times while it does not occur or occurs to a very limited extent at other times; as a result, the pressure at the pump outlet can rise
Cavitation also varies depending on the stage at which it occurs? In the initial stage of cavitation, there are severe fluctuations in pressure, flow rate, and power; performance is good at times and poor at other times, with a high degree of randomness. By the later stages of cavitation, nothing can be improved anymore
We also have a stage, which is the bottom pump; normally, it handles wax oil. During shutdown, diesel is used, as water in the diesel causes cavitation in the pump.
Reason for the erosion phenomenon: During pump operation, the pressure inside the impeller varies; the pressure is lower at the inlet and higher at the outlet. The vaporization temperature of a liquid is related to pressure: the lower the pressure (or the higher it is), the lower the corresponding vaporization temperature (or the higher it is). If, for some reason, the absolute pressure of the liquid being pumped in a local area of the flow path – usually somewhere slightly downstream of the inlet to the impeller blades – drops to the vaporization pressure of the liquid at that temperature, the liquid begins to vaporize there, generating large amounts of steam and forming bubbles. When this liquid containing many bubbles moves forward through the high-pressure areas inside the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly until they burst. The process in which bubbles are formed and burst in a water pump, causing damage to the flowing components, is known as cavitation in water pumps. Explanation of pressure fluctuations: As for pressure fluctuations, in my opinion it’s similar to surging in a compressor – when cavitation occurs at the inlet, it means there isn’t enough flow rate there, so the outlet pressure drops. At that point, the outlet pressure flows back toward the inlet, which effectively increases the inlet pressure; this restores normal conditions and the outlet pressure returns to its normal level. After that, cavitation occurs again. Therefore, cavitation occurs periodically, and the operating conditions should be changed at such times
It has nothing to do with the outlet pressure; it is related to the inlet temperature and pressure. It is the fluctuations in these two data points that cause the problem you mentioned.