Then, 480 engine hours (and several years) later, I changed it again. The impeller was little worse for the wear, looking almost new. I threw it in the spares box and installed a new one I had just bought for the purpose.
Why the early failure?
When I compared this impeller closely to the one I had taken out 230 hours ago, one thing stood out clearly: it was about 1 mm taller. When the water pump cover was tightened down, the impeller got a compression load that the older one never saw, and one the pump was apparently not designed to sustain. Further, the pump cover showed significant signs of erosion, probably caused by the excessive compression load.
It turns out that the too-tall impeller was a Johnson Pump "equivalent" for the Jabsco impeller. "Equivalent" in the sense of "approximate".
The learnings from this experience:
- When changing out a part, be certain to compare all relevant dimensions of the new one to the one being replaced.
- If you have a non-standard part installed, change it out as soon as possible.
- Non-standard parts can cause consequent damage far beyond their value
The pump still works OK, but it's not delivering as much water as it used to. I surmise that the erosion on the cover plate is preventing a good seal with the impeller, allowing leakage internally from the high pressure side back to the low pressure side.
Rather than just replace the cover plate, I have ordered a SpeedSeal cover plate - something that has been on my todo list anyway, ever since Livia blogged about theirs. So there is a silver lining to this story.
Years ago when I was a kid, I used to read Flying magazine. I particularly enjoyed a long-running series of articles entitled "I Learned About Flying From That." Each article was written by a pilot, who humbly admitted to having made a mistake, and then having lived, told about it in the hopes that others would not have to make the same mistake. I thought then that it was a good format, and I still think that now. This series of postings is my attempt to recreate that article series with a new subject and new technology.
(If you would like to help others to learn from your mistakes, please send your article to: WindborneInPugetSound at gmail dot com)