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Old 01-08-2012, 12:43   #1
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Impeller Not Holding Prime

I recently replaced the impeller in the Oberdorfer water pump">raw water pump on my Kubota D-905 generator. When following the path hunting for broken fin parts I removed the end caps on the heat exchanger and made sure all was clear from the through hull to the mixing elbow. I used Permatex grey gasket sealant on the endcaps of the heat exchanger and the bronze impeller plate, checked and refit the hoses. After the refit impeller was found not to be pumping water I opened the pencil zinc at the top of the heat exchanger and added water, finding that all the water I added dropped down to the waterline level just at the base of the Oberdorfer pump and when starting, no water is pumped. I've been using my generator by closing the through hull, filling the systen with water and opening the through hull just after starting the Kubota and all works well. If I close the through hull at the moment before shutting down I can restart with all operating, but once again, opening the through hull at the moment after starting. Is there someone in the crowd that would confirm that the gasket sealant is a bad choice? Does anyone see evidence of the location of the air entering the system that would cause the water level to drop? Wouldn't the water level only drop beneath the impeller if the air leak was at the impeller plate? I am cruising Maine and often without a wifi connection so I'll pick up your thoughts in a week or so. Thanks,-
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Old 01-08-2012, 13:54   #2
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Re: Impeller Not Holding Prime

Perhaps you inadvertently enlarged the gap between the impeller and the cover plate by using the sealant instead of just a thin paper gasket?? Other thought is perhaps you put the cover plate on back-to-front and that has also resulted in a larger gap. Increasing the gap even slightly can result in lower pump efficiency and allow reverse flow as you are experience. As they say in London "Mind the Gap"!! Of course I could be totally wrong - it did happen once.
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Old 01-08-2012, 16:22   #3
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Re: Impeller Not Holding Prime

I would look in the pump, after all the raw water circuit is not an airtight cct. Perhaps the new impeller is narrower than the original?
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Old 01-08-2012, 16:29   #4
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Re: Impeller Not Holding Prime

If you used a globe (blue neoprene) impeller it won't work. It looks the same size but won't prime. Well this is my experience a couple of times now after trying to use the Globe impeller.
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Old 03-08-2012, 16:32   #5
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Re: Impeller Not Holding Prime

Wrong sized impeller?
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Old 04-08-2012, 12:59   #6
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Re: Impeller Not Holding Prime

All of these excellent ideas support my own thoughts that there is a passage "gap" between the impeller blades and the cover plate. I did attempt to use a Permatex "shellac" gasket material that is very thin. There was no remnant of any gasket material on the plate when it was removed. I appreciate the thought that I my have reversed the cover plate, but I'm safe from that specific idiocy! My first attempted repalcement was a Globe impeller, but my second was the original for the Oberdorfer pump. My latest run required priming again, but I left the through hull open after running and I'll see if it works at restart. If not, I'll attempt the attachment of the machined bronze plate onto the pump without a gasket at all. I'm sure that those who hypothsize that there is a gap between the spinning blades and the cover plate are correct. Thanks for your ideas.
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Old 14-08-2012, 20:09   #7
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Re: Impeller Not Holding Prime

Hi
The pump impeller fit is one known air leakage point, the end plate sealing is another and the drive shaft seals are the final leakage point. All these need to be air tight. There should be no probs with the sealing compond that you have used.

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Old 16-08-2012, 09:35   #8
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Re: Impeller Not Holding Prime

Quote:
Originally Posted by Acushla View Post
.........the end plate sealing is another and the drive shaft seals are the final leakage point.............
Acushla
Thanks, I was discounting these two added possibilities because I assumed that I would see water leakage at the end plate or from the drive shaft seals when the generater is running. As long as I close the through hull seacock at stopping and open it at starting, I'm running well and pumping raw water cooling with no leaks. Is this proper for me to discount these two added possibilities?
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Old 16-08-2012, 18:32   #9
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Re: Impeller Not Holding Prime

The risk with shutting off the cooling water supply is that someone may forget to open it when starting the generator but this is for you to assess. In general these small pumps work by causing a small depression in the pump casing and suction pipework. Therefore if there is any leakage either at the pump or in the suction pipework at best they will loose prime when stopped or worse not be able to pull the required suction head on start up to prime and cool the engine.
See my post on page 4 on "Diesel Engine Overheating" for the repair requirements on my pump.
Suffice to say there should be no end clearance between the impeller end faces and the pump casing, the impeller blades should be in contact with outer dia of the casing, the shaft sealing has to be air tight and the suction pipe and filter cap has to be air tight sufficient to withstand the vacuum the pump develops on start up.

In my case the machining of the pump casing back face flat and restoring the impeller end face fit (0.001 ins interferance) fixed the problem.

Hope this helps
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Old 16-08-2012, 19:46   #10
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Re: Impeller Not Holding Prime

The raw water pump on my Yanmar will not self prime after I change the impeller. I lubricate the impeller blades with a bit of Dow Molykote 33 silicone grease (one tube will last several lifetimes) and then it never fails to prime. The Yanmar impellers come with a bit of glycerine for the same purpose.
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