The issue of ring clearance after eccentric wear in mechanical pumps
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I’d like to ask the experts here: the standard thickness for mouth rings is 0.5–0.7 mm. When the pump’s mouth ring is worn, if one side of the casing’s mouth ring measures 100.10 mm and the other side measures 100 mm, then the dimensions of the impeller’s mouth ring are 99.3 on one side and 99.4 on the other side. So what is the gap of that mouth ring? How should it be calculated? Should we use 100.10 - 99.3 = 0.8 mm, or 100 - 99.4 = 0.6 mm? What will be the final clearance for this ring? Is it 0.6mm or 0.8mm? If it is 0.6mm, then it is within the limit; if it is 0.8mm, then it exceeds the limit. How is this determined? The book states that the minimum size of the housing and the maximum size of the impeller mouth ring should be used for this calculation – is this approach correct? If understood based on this calculation, suppose one side of the ring is not worn while the other side is severely worn, with the calculated values being 0.6mm for one side and 1.5mm for the other, should the gap of the ring then be reported as 0.6mm?
After reading your description, this issue is indeed quite confusing. In my understanding, the formula given in the book, \"minimum shell size – maximum diameter of the turbine throat ring,\" is intended to calculate the most dangerous gap (i.e., the minimum gap), as it is at the narrowest part of the throat ring that frictional locking is most likely to occur when there is uneven wear. In the example you gave: the minimum value for the housing is 100, and the maximum value for the impeller is 99.4; the difference is 0.6 mm, which is within the acceptable range. So it should be fine to report it this way. As for the other side where severe wear has resulted in a large gap (for example, 1.5 mm), that area is loose; although it can affect efficiency or cause vibration, it does not lead to immediate damage. Therefore, when reporting the values, the minimum gap is used as the reference. However, it is recommended that you also record the maximum gap in the notes, to facilitate subsequent analysis of wear trends or to determine whether it is necessary to replace the mouth ring. Additionally, different units may have varying tolerances for gaps; it is best to check the specific maintenance manual for your equipment or the manufacturer’s standards, to avoid focusing only on numbers while ignoring the operating conditions. If conditions permit, it is also possible to consider using a feeler gauge for a actual re-measurement; sometimes different measurement methods can lead to slightly different results.