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Old 22-07-2011, 21:52   #1
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Burning Through Genset Impellers . . . Need Lift Pump ?

Hi:

My 9.5 kW Onan genset runs about 120 hours on an impeller; it's situated about five feet above the water (catamaran). I'm on my 5th impeller in 500 hours ... yes, installed correctly, right circular motion, silicone grease on the blades ... the thing gets shredded over time, I think because it works so hard.

I wonder if I need to install an electric water pump to ease the demand on the impeller ... and if so, how straightforward is this project? How do I get the pump to start when and only when the genset is running/demanding water?

Many thanks for thoughts ...
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Old 22-07-2011, 22:08   #2
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Re: Burning Thru Genset Impellers ... Need Lift Pump?

It's not the impeller when working, it's when it isn't. The water drains from the pump and the impeller dries out. It then sticks to the pump housing and breaks the next time you run the genny. Same thing happens on engine impellers when a boat gets hauled out. When the boat splashes a few days later the impeller need to be replaced. I have a similar problem as my engine(s) impellers are above the water line (catamaran, shallow draft).

I don't know about your impeller face plate but I installed a small fitting that allows me to put a spoonful of oil into the pump before starting (if not run for a week). The fitting screws shut, seals so no water leaks out when running. It has saved me a fortune on impellers. The Globe "RUN-DRY" impellers also lasted longer. JMO... but you can ask at boat yards about impellers breaking when boat splash out of the yard. Very common problem.
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Old 22-07-2011, 22:15   #3
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Re: Burning Thru Genset Impellers ... Need Lift Pump?

Most likely its doing a dry start every time. It it was me I would put a loop in the inlet line with the top of the loop higher then the generator. That would keep water in the raw water system.


An inline electric pump would work too, but I worry about adding another piece of equipment that will break down at the wrong time. The 5 foot lift is the problem though.
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Old 22-07-2011, 22:54   #4
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Re: Burning Thru Genset Impellers ... Need Lift Pump?

I think you have a problem with the pump that should be fixed. You shouldn't need a booster pump. Examine the inside of the face plate. If you see scratches on it, replace it or flip it over to use the other side. Check the cam to see that there is a very smooth transition from the pump case to the cam. The cam should have a knife like edge where it meets the housing. If it is rounded or chipped it will tear up impellers. Make sure that the cam screw doesn't extend past the cam. There is always water between the impeller blades unless you remove the face plate so lubrication on start up should not be a problem.
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Old 24-07-2011, 19:52   #5
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Re: Burning Through Genset Impellers . . . Need Lift Pump ?

Thanks all -- very helpful, and will do the suggested diagnostics on the face plate, etc ... makes sense that the impeller blades should always hold water in between them even if water hose drains ...

Again, thanks all.

/jon
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Old 27-07-2011, 14:35   #6
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Re: Burning Through Genset Impellers . . . Need Lift Pump ?

Pumps are primarily made to push water they do not like to pull water. ( of course there are exceptions ) Chances are they suction line is very small along with the through hull fitting and valve. Increasing these will help. If your generator is raw water cooled I would tend to stick with its water pump and try the loop idea. Within reason it makes no difference how high the loop is as list will only be two the height of the inlet on the pump, something I learned from the fire service. If it is the exchange are cool you could use 120 V pump that would be connected to the generator output, most probably a centrifugal pump mounted below the water line. My two cents, Mike.
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