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No return valve


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I want to install a non-return valve in the seawater pipe between though-hull valve and raw water pump/impeller on my genset. Purpose being to avoid pump/impeller running dry for first few seconds each time I start genset - which is reducing impeller lifespan.

Anyone else done this? And does it make any difference where in the pipe I install the non-return valve? High up close to pump or low down near through-hull? There’s a Vetus strainer in the system so my instinct is to install above that to avoid any leakage back through that (I know they are prone).

And finally what’s the best non-return valve to fit and where to buy?

Thanks

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Are you sure the water is draining back, the pump impeller is usually a pretty good seal? 

Air could be getting in before the pump letting it drain, otherwise should not be draining. 

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BOI is correct. These pumps won't allow the water to drain back normally. So the pump body maybe worn, the front plate maybe worn and that can also cause then to not draw in the water quickly or not at all. Or there maybe too much restriction somewhere causing the blades to be overloaded. Example is a blockage due to old blade fragments in heatexchanger. Too much back pressure will cause premature failure.
And when you say the impeller is wearing, how long a life do you expect to get? They should be replaced annually, needed or not. Although personally I never did. Even if "dry", the blades can handle a second or two running dry just fine, as there should be enough moisture inside the pump and line. It is then they are dry for sometime that the friction creates too much heat and will wreck the blades.
 

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My last impeller lasted about 30hrs.

I reckon the pump is at least 500mm above the waterline.

It’s a Cummins Onan genset which is generally well regarded but suffers from poor installation by many production boat factories. Other Jeanneau owners report same problem with their gensets chewing through impellers due to running dry for first few seconds.

And so many have made changes to installation such as:

1. Different brand impellers (Jabsco not Sherwood apparently last longest)

2. Add non-return valve

3. Add electric pump which activates during the 5-8s priming period when you press the start button before genset actually turns over. Which helps get the impeller wet in advance.

For sure I might have old impeller fragments causing issues which need removal. But I think there is also an installation issue that needs addressing and adding a non-return valve is reported to make a difference by other owners with same problem.

 

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extend the intake hose and create a "gooseneck" so as the line rises up above the Genset pump level and back down to the pump. Place a U bend anti siphon at the top of the bend. This allows some water to always remain in the pipe on the pump intake side. When the pump starts, the water is always there to lubricate the impeller.

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2 hours ago, wheels said:

extend the intake hose and create a "gooseneck" so as the line rises up above the Genset pump level and back down to the pump. Place a U bend anti siphon at the top of the bend. This allows some water to always remain in the pipe on the pump intake side. When the pump starts, the water is always there to lubricate the impeller.

This is a good idea. I would be suspicious a non-return valve would work consistently in a raw water environment. (Note, I've never tried one), but in municipal wastewater infrastructure can often get material affecting them and either jamming or not getting a tight seal, so leaking back over short periods of time. A siphon / goose neck and anti-siphon valve as wheels suggests would be a good way to go.

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I forgot one important point. The Ubend must not have the anti-siphon valve fitted, as it would just suck in air due to the the Pipe being under suction and not pressure. You might be able to do the bend with some flexi hose, but it must be able to withstand suction without collapsing.

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