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Old 29-09-2008, 18:53   #1
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Genset pump prime problem

Hello:

After three years, the water pump on my Nextgen 3.5 KW generator has started to consistently losing its prime when I start it up. In the previous three years, I have not had this problem.

The problem occurs if I do not immediately shut off the seacock to the generator when I shut it off. The next time I start it, I must manually put water in the hose leading to the pump. Because things work fine after that, it means that there is not an obstruction. The strainer is empty of debris. Lastly, the gasket between the impeller and the cover is good. There is a siphon break downstream of the pump.

The system has a seacock, some tubing, the strainer at the waterline, to the water pump. The total lift is about 2 feet above the waterline which is between the seacock and the strainer. The run between the strainer and the pump is almost level. No recent modifications have been made that might cause the problem to have arisen.

Since the problem has arisen, I have changed the impeller with a new Globe. The problem remains.

Any help would be appreciated.

Thanks.
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Old 29-09-2008, 20:12   #2
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Nextgen has, I believe a Kubota Engine....I would suspect that the lip seal in the raw water pump has failed....allowing air to enter when unit is shut off
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Old 29-09-2008, 20:41   #3
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Chief Engineer:

Yes, it uses a Kubota.

Could the lip seal be the problem even though the pump shaft is not leaking any water?
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Old 29-09-2008, 21:12   #4
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When the pump is running, there may be enough internal pressure to keep the seal tight on the shaft.

You might also want to check the siphon break downstream to see that it is working properly.
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Old 01-10-2008, 13:44   #5
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More bizarre information.

In additon to the suggestions offered here, I also contacted Next Gen. They advised me of the possibility that a downstream obstruction may be causing the problem. Before rebuilding the pump, they suggested that I run a series of tests.

I was skeptical of this because I had already determined that there was not a problem either downstream or upstream of the pump.

However, I subsequently removed and diverted the water hose from just after the pump, the heat exchanger, siphon break, and exhaust manifold. In each case, the pump pumped water into a separate bucket. I did not find any obstructions.

When I put the whole thing back together, the generator started working perfectly to my surprise.

So, even though the generator now works, I am at a loss as to why this malfunction started and persisted. This is bothersome, because it means that the problem could crop up again randomly.

Other than gremlins does anyone have an idea of what else might be happening.
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Old 01-10-2008, 16:00   #6
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How about a hole in the raw water intake hose that is very when you tightened everything back to gether the hole was sealed. I had this happen to me with a diesel pump. The hose clamp had cut a hole in the hose and when Itightened it it worked then when the engine ran for a couple of hours the hose wore its way thru. Just a thought.
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Old 01-10-2008, 18:56   #7
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It's funny how that stuff "clears up" sometimes...you don't know wheter to be happy or "on guard"
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Old 03-10-2008, 09:55   #8
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I have solved the genset pump problem.

With the pump off of the engine, I discovered that I could blow in one fitting and out the other without resistance. Under these conditions, there was no way that the pump could maintain a prime or a lift. This led me to believe that there was something wrong with the impeller.

This was quite confusing since I had just replaced the impeller.

It turns out that the Globe 1112 impeller that I used to replace the Jabsco impeller is not exactly the same. Although they are dimensionally the same with respect to diameter and width, the Globe impeller is more flexible than the Jabsco. This results in the Globe impeller not creating an air tight seal to the level as the Jabsco. When I replaced the Globe impeller with a Jabsco, I could no longer easily blow air through the pump.

With the Jabsco impeller, I started the genset and was satisfied to see the expected amount of water exiting the exhaust.

This is sort of sad because the Globe is advertised as being superior to normal impellers in terms of both life and the ability to run dry. These benefits are not acheived if the impeller doesn't work in the first place.

I suppose the moral of the story is better may not necessarily result in as good as before.
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Old 03-10-2008, 10:00   #9
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Nice work figuring out the problem.
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Old 03-10-2008, 18:49   #10
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Good Show

Thanks for the insight

Quote:
Originally Posted by The Mons View Post
I have solved the genset pump problem.

With the pump off of the engine, I discovered that I could blow in one fitting and out the other without resistance. Under these conditions, there was no way that the pump could maintain a prime or a lift. This led me to believe that there was something wrong with the impeller.

This was quite confusing since I had just replaced the impeller.

It turns out that the Globe 1112 impeller that I used to replace the Jabsco impeller is not exactly the same. Although they are dimensionally the same with respect to diameter and width, the Globe impeller is more flexible than the Jabsco. This results in the Globe impeller not creating an air tight seal to the level as the Jabsco. When I replaced the Globe impeller with a Jabsco, I could no longer easily blow air through the pump.

With the Jabsco impeller, I started the genset and was satisfied to see the expected amount of water exiting the exhaust.

This is sort of sad because the Globe is advertised as being superior to normal impellers in terms of both life and the ability to run dry. These benefits are not acheived if the impeller doesn't work in the first place.

I suppose the moral of the story is better may not necessarily result in as good as before.
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