Monday, March 7, 2011

I learned about sailing from that: A Tale of Two Impellers

Having no idea of its condition, not long after we took possession of Eolian, I changed out the raw water pump impeller.  It needed it.

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.

Today I pulled out that impeller, after 230 hours of operation.  Now, there's a lesson here for everybody.  The impeller was in poor shape - three of the vanes were cracked (but none missing, thankfully - I don't have to disassemble the heat exchanger to find lost vanes).  There were a few small chunks missing from the bottom surface, small enough to have passed thru the heat exchanger.  After less than half the operating time.

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)



Anonymous said...

Good to know. I've been tempted to buy the offbrand impellers, rather than shell out $85+ for the Jabsco.

Question: How are you measuring the raw water flow rate? I have a rough idea from looking at what comes out of the exhaust, but having a real measure would be great.

SV Estrellita 5.10b said...

I have no idea why but we had a somewhat opposite experience with off-brand versus brand name.

The Johnson impeller I pulled out lasted more than 500 hours until one blade cracked and the Volvo part I replaced it with only lasted 200 before it had several vanes cracked.

Now, to be fair, this is only two impellers so who knows. Also, I don't know how old the Volvo impeller was (it came with the boat in a stack of spares) and our Johnson is the (someone told me) heavier duty diesel-OK version of the same impeller (not the water only version). A lot of variables going on there.

bob said...

Nope, sorry. I'm just looking at the exhaust discharge.

One never knows how old that one hanging on the peg in the store is either... I am convinced my failure was due to incorrect dimensions tho, instead of poor or old material.

Anonymous said...

I learned the hard way - losing a blade on my impeller - which, by the way, I couldn't find even with taking apart the heat exchanger.

But it was three years old with 300+ hours on it. I knew I was pushing the limit that last year, but assured myself it would be okay.

Now I've gone to the other extreme, and change it every years. I have multiple backups though :-)

bob said...

Rick -

I change mine every couple hundred hours too. And if the one I pull out is OK, it goes in the spares box.


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