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Old 24-06-2010, 20:00   #1
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Burning Thru Impellers - Too High a Water Lift ?

Oh gurus:

I have an Onan 9.5 kW genset, and I'm burning thru impellers at the rate of 1/ 75 hours. I'm asking it to lift the water about 6 feet (catamaran), through a water strainer.

Too high? If so, is there a fix -- an intermediate pump of some sort? A bigger impeller?

I also note that when I install (correctly spiraled) and then re-open an hour later, one or two of the compressed blades are almost always pointed in the "wrong" direction ... I am sure I am installing these correctly, and the problem has occurred with marina installations as well ...

Hmmm ... help!

/jon
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Old 24-06-2010, 20:15   #2
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Sometimes when the engine stops it rotates back a little bit, making the vanes look like they are in the wrong direction they will staighten out as soon as you crank it back up again. Not a problem as far as I know.
Steve
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Old 24-06-2010, 22:50   #3
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check wear on cam plate some times when they get worn can increase wear on impellor
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Old 24-06-2010, 23:34   #4
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Cavitation!

Is the system sucking in air?

A static head of 6' should be OK for a rubber impeller.

Is it a factory installed pump? If so, I would call the dealer.
If not, maybe the RPM's are too high. Or, the inlet/outlet lines are too small.
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Old 25-06-2010, 07:55   #5
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Hold on to the inlet hose to the pump. If it is sucking air you will feel it surging. Ditto for the outlet. Could there be something between the sea strainer and the pump that is malfunctioning ( like a check valve) that is starving the pump. The most probable cause is something growing in your thru hull, partially blocking it.
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Old 25-06-2010, 08:55   #6
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A six foot head (at rest) is even more underway, even on a cat.
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Old 25-06-2010, 09:04   #7
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we have a 9 Kw gen set on our leopard 46 and it has a intermediate pump
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Old 25-06-2010, 09:12   #8
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Put a check valve in the line between the strainer and pump. This would help hold the prime when shutdown.
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Old 25-06-2010, 10:00   #9
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I've had the same problem with probably no more than a 4-5' lift. Pretty long intake line however of about 15'. She'll start pumping no prob but there is a delay. Perhaps there is a bit of loss of prime. I also have a strainer in the line. In any event, I'd like to know what the issue is, just like jglauds.

Stacy, what type of arrangement is your intermediate pump? I'm concerned a bit about over pressuring the regular impeller but not certain that is really something to worry about.
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Old 25-06-2010, 10:37   #10
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we have a small electric pump just after the raw water strainer. the boat came this way straight from the factory. I can't go to the boat to check the make and stats of the aux pump as the boat is on the east coast and we are currently on the west coast. Since it seems to be a standard set up you may be able to get the info from a leopard dealer.
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Old 25-06-2010, 20:27   #11
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Impeller failure is usually from running dry - so all the answers that address checking for air leaks and clogged or too small raw water supply hoses/strainers are your first place to investigate. Also too long a raw water supply hose imparts "drag" on the supply water to the pump. A larger hose should alleviate this possible problem.
- - A wild card is to see if you are using the wrong impeller in the pump. Metric impellers are very slightly different from English Imperial size impellers.
- - Any centrifugal water pump down by the strainer can help an undersized raw pump on the engine. Using a centrifugal impeller pump will not impede the water flow should the "pre-pump" cease functioning.
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Old 04-07-2010, 19:53   #12
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Thanks for Ideas ...

We're going into the marina in Taravao, Tahiti (unrelated issue with my Volvo saildrive) and I appreciate all the ideas and diagnostics; we're at 120 hours and counting on this one, but still not confident ...

Many thanks to all, and will report back when/if I make any changes.

/jon
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Old 10-07-2010, 06:57   #13
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FWIW: I had this problem with my old Westerbeke - It had a belt driven pump and would chew up impellers withint 50 hours. Eventually, I replaced the whole pump ($$$) and sent the old one out - It turned outthe pump itself was not cast correctly and was causing the failures!! New pump worked like a charm for years until I repowered.
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