Issues regarding the corrosion of centrifugal pump impellers
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In October 2010, we installed a 1800 centrifugal pump produced by Harbin Qinggonglin in our workshop. After running for a year, the impeller suffered severe corrosion; it developed holes, and the mouth ring also cracked – the thickness of the mouth ring had been reduced to about half due to corrosion. If it’s cavitation, the mouth ring shouldn’t corrode, right? Our medium is ordinary circulating water. pH greater than 7, temperature between 18-30°. I would like to ask everyone what causes this type of corrosion! Thank you
The poster is sure that the damage is caused by corrosion rather than cavitation; cavitation can also damage the pump’s inlet ring, pump casing, and impeller. It should be easy to tell, as the damage resembles fatigue-induced peeling. It would be better if the poster could send a photo for a more accurate assessment. As for corrosion, it usually occurs, but it depends on the rate of corrosion. Chemical corrosion and electrochemical corrosion are the most common types, with electrochemical corrosion often resulting in a fast rate of corrosion. Electrolyte accumulation in small gaps can accelerate this process. However, if the pump keeps running, corrosion is less likely to occur. Has your pump been shut down for long periods of time?
The water quality of the cooling tower needs to be tested; pH alone is not sufficient. Our company adds a large amount of reagents to the cooling tower on a regular basis.
It might be caused by cavitation. The principle of cavitation damage in centrifugal pumps: Cavitation is a hydrodynamic phenomenon; its root cause lies in the local pressure drop that occurs during the flow of the liquid, resulting in the formation of low-pressure areas. When the pressure at the pump inlet drops to the saturated vapor pressure at that temperature, the liquid boils and vaporizes, resulting in the formation of numerous bubbles within the flowing liquid. These bubbles contain steam of the liquid being transported, as well as a small amount of air that was originally dissolved in the liquid and has now escaped. As the bubbles move with the liquid flow from a low-pressure area to a high-pressure area, they shrink and condense rapidly under the influence of the surrounding high-pressure liquid, resulting in their sudden collapse. Due to the very rapid and sudden occurrence of steam condensation, a local vacuum is created where the bubbles disappear. The fluid under pressure in the surrounding area rushes rapidly toward this vacuum space from all directions, resulting in severe water hammer and generating enormous force. Due to the extremely small size of the bubbles, this impact force is concentrated on the tiny surfaces of the components in contact with the bubbles, resulting in pressures of several hundred atmospheres or more. The shock frequency can reach 25,000 times per second. As a result, the material walls are subjected to repeated loads of high frequency and high pressure, which gradually leads to fatigue failure ； At the same time, if the bubbles formed also contain reactive gases (such as oxygen), the heat released during the condensation of these bubbles can cause chemical corrosion of the metal, resulting in pitting on the metal surface and ultimately leading to perforations. In severe cases, the metal grains become loose and flake off, forming a honeycomb pattern; the wall surface may even be corroded through. This process in which bubbles are continuously formed, grow, and burst, thereby damaging the material, is collectively referred to as cavitation.
There are many microorganisms in the circulating water, and they cause significant corrosion to the impellers. We have encountered this problem as well; we modified the filters, and now the results are satisfactory.
The information you provided is not sufficient to determine whether it is corrosion or cavitation; having photos would help make a more accurate judgment. However, there are differences between the two types of corrosion. In centrifugal pumps, there are specific areas where cavitation is likely to occur: it happens in those parts of the pump where the liquid pressure is lowest, that is, in areas with high flow rates but low pressure. The main areas are the back side of the impeller inlet, the turning point of the fluid flow inlet, and the sections where the flow channel is particularly narrow. The areas of centrifugal pumps that are prone to corrosion vary; it can occur on the front surface of the impeller blades as well as along the outer edge of the impeller. Due to the erosion caused by sediment and water flow, as well as corrosion from the medium, the impeller of the pump gradually wears out and gets corroded. Grooves or striations often form on the surface of the blade, or it may be damaged by cavitation, resulting in honeycomb-like holes or even perforations in the blade. If the blade contains pores, sand holes, or inclusions at the time of casting, pores, or even cracks may appear on its surface during operation.
This condition in the circulating water pump is likely caused by erosion corrosion; as for whether cavitation is the cause, it depends on whether the installation height is sufficient.
It’s been running the whole time. I’ll go back and find some photos before discussing it with everyone; my USB drive is broken
:Victory: So there are so many considerations related to cavitation after all; it seems that combining theory with practical experience is truly the best approach! ! !
I hope the original poster can post a photo for discussion; in my opinion, the circulating water should also be analyzed, as it’s not sufficient to rely solely on the pH value as an indicator.