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Old 19-03-2020, 05:36   #1
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Anyone used Run Dry impeller for Onan

I have read that the Onan impellers can disintegrate and shed bits into the heat exchanger
The options to avoid the problem include using an electric pump rather than the mechanical pump. So a pump as used for air conditioning for example

Another option is to fit a trap to collect the broken blades before they get to the heat exchanger

or



However the easiest solution that I have come across are impellers that are more robust that then ones made by Onan called Run Dry Impeller
https://www.globemarinedirect.com/Impellers-s/176.htm


I wonder if anyone has used them.

Also if someone has 5Kw MDKBH Ona could they measure their spare impeller as I could not find the exact one on the Run Dry site

Thanks
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Old 19-03-2020, 07:07   #2
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Re: Anyone used Run Dry impeller for Onan

This is only a partial response.

The problem with burning up an impeller is not the source of the power (mechanical vs. electric), it's no water in the pump. An electric pump would open up new possibilities for it to run when the engine is not running, with more opportunities to run dry and destroy itself.

The solution is making sure that the pump stays primed. If Onans have a particular problem there, a small electric pump as an auxillary could be a neat solution; prime the Onan pump with the electric pump before starting the engine.

That's not denying the utility of a run-dry pump impeller. I think I'd look at how they rate themselves in time to run dry before they, too, destroy themselves.

Ultimately, there's a completely different approach. A destroyed impeller means no pumping. Start the engine, then look over the side to make sure that cooling water is being discharged, and move on. Carry a spare impeller.

I really like those fragment catchers. Thanks for posting them.
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Old 19-03-2020, 07:16   #3
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Re: Anyone used Run Dry impeller for Onan

We had a disaster with run dry impellers, they worked fine the first run but when the engine was turned off, raw water slowly seeped back down the plumbing until the hot water got to the impellers and destroyed them. And our only spares on board were more run dry type.

That was 30 years ago when they first came out - - newer versions may have improved.
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Old 19-03-2020, 07:17   #4
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Re: Anyone used Run Dry impeller for Onan

Quote:
Originally Posted by tkeithlu View Post
This is only a partial response.

The problem with burning up an impeller is not the source of the power (mechanical vs. electric), it's no water in the pump. An electric pump would open up new possibilities for it to run when the engine is not running, with more opportunities to run dry and destroy itself.

The solution is making sure that the pump stays primed. If Onans have a particular problem there, a small electric pump as an auxillary could be a neat solution; prime the Onan pump with the electric pump before starting the engine.

That's not denying the utility of a run-dry pump impeller. I think I'd look at how they rate themselves in time to run dry before they, too, destroy themselves.

Ultimately, there's a completely different approach. A destroyed impeller means no pumping. Start the engine, then look over the side to make sure that cooling water is being discharged, and move on. Carry a spare impeller.

I really like those fragment catchers. Thanks for posting them.
An electric pump is a neat solution as it is powered directly from the generator. As soon as the generator starts, the pump gets power and water flows in. As soon as the gen stops, the pump stops. No chance of pumping water in when the gen is not running. There are quite a few other threads discussing this.

I have quite a small generator so I am not sure that I can get the trap into the system. I will measure when flights resume from London to Greece!

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Old 19-03-2020, 07:24   #5
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Re: Anyone used Run Dry impeller for Onan

Quote:
Originally Posted by YANDINA View Post
We had a disaster with run dry impellers, they worked fine the first run but when the engine was turned off, raw water slowly seeped back down the plumbing until the hot water got to the impellers and destroyed them. And our only spares on board were more run dry type.

That was 30 years ago when they first came out - - newer versions may have improved.
I do know that several yachts on the Oyster World Rally were shredding impellers on their 9Kw and 11Kw Onans. They moved to the Run Dry and stopped having a problems apparently

TS
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Old 19-03-2020, 08:06   #6
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Re: Anyone used Run Dry impeller for Onan

I have one of these Onan's. Every time this comes up there's talk of the Run Dry impellers but there are always some knowledgeable people who said they had a bad experience with them. As far as the Oyster Rally, if you put in any new impeller your problem will stop for a while.

Several years ago, there was a rumor that Onan was about to release an impeller with improved lifespan. Has anyone heard if this happened?

Those who switched to the AC water pump wired to the genset so it comes on automatically seem very happy.

But I've stuck with the Onan impellers. I always buy fresh ones from a major supplier - as I worry that one that's been sitting on a shelf for five years may have deteriorated.

And with solar panels installed I don't run my genset as much. I change the impeller at less than 100 hours and have not seen any broken vanes for years.

A sure way to go through impellers is to have a genset mounted above the waterline and the hose rising to the impeller so it drains out between runs. It's often possible to reroute the hose so as to keep the impeller wet between runs. And if there's a long run of hose, move most of it below the waterline and only have a short length above the waterline.
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Old 19-03-2020, 08:52   #7
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Re: Anyone used Run Dry impeller for Onan

Can you just fit a small water strainer so any chunks can't get to the exchanger? That's all I would do.
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Old 19-03-2020, 08:57   #8
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Re: Anyone used Run Dry impeller for Onan

Some people fit the strainer. Not hard to do. But the cap to the heat exchanger is facing you. Very easy to take it off and pull pieces out. I've never had one go deep into the exchanger as it's small.

The added pain is that the impeller is surpringly hard to change as the housing comes off at a weird angle. Lining everything back up when reassembling is frustrating. I've found the best thing is to remove the hoses from the pump so you aren't fighting them too.
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Old 19-03-2020, 09:22   #9
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Re: Anyone used Run Dry impeller for Onan

Consider blue Globe impellers.....good reviews.

I have used for years...they last a much longer time than black stock neoprene. No lost vanes after 3 years...

I am thinking about adding a filter after the pump to catch loose vanes if necessary.

If switching to Globe impellers be sure to check diameter for shaft. I needed to have different shafts made for my Sherwood E20 pump. So far so good...
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Old 19-03-2020, 09:30   #10
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Re: Anyone used Run Dry impeller for Onan

While not an Onan, I took my raw water pump off after it failed and fitted a March airconditioner pump a few years ago.
Some Catamaran factory installations have an electric pump feeding the impeller pump, to me that’s ignorant, but I can see how they may have to do that to protect the generator warranty.

Anyway if you wire the March pump directly to the electrical output, fused of course, then it won’t begin to pump water for a few seconds until after the generator starts, so the generator will run for a few short seconds with no water, this hurts nothing and you can hear the exhaust note change when the water begins flowing. The water flow also stops well before the generator winds down too, meaning the exhaust system is blown clean of water during shutdown, so even if your sailing in heavy weather the water in the exhaust system can’t flow back into a non running motor.

You cannot overcrank the generator and fill the motor with water, because there is not water flow unless there is electricity.
The March water pumps however must be below water line, they will not self prime at all, however their life span is usually in pretty much decades and tens of thousands of hours. Think of how many hours they run in boats sitting in a marina with the AC on 24/7 for years.

I have used a Globe dry run impeller in my main engine water pump, in my opinion they work fine, but it seems due to the greater hardness of the blades that they caused more water pump wear, that is an impression, I can’t verify it.
For a main engine you need a variable flow water pump as the engine varies in RPM so an electric pump would be tough to fit, so they may not be a good idea there.

But for a fixed RPM generator I can’t see any reason not to fit one, and they put an end to water pump maintenance, once installed you forget about it, there is never any need to change anything, of course carry a spare pump.

This is the strainer I installed to catch impeller vanes on my main engine, I once tried an after market budget impeller, and yes it soon came apart, so after having to disassemble the heat exchanger in a deserted anchorage, I fitted the strainer.
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Old 19-03-2020, 09:49   #11
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Re: Anyone used Run Dry impeller for Onan

Question—where is your raw water inlet located? Is it flush with the hull? The answer may be to put a nice sized clamshell over the opening to keep seawater pooled in the vertical while underway and even at anchor. It solved my problem with frequent impellor replacement.
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Old 19-03-2020, 09:55   #12
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Re: Anyone used Run Dry impeller for Onan

I have an Onan 9.5 and ”RunDry” did not work for me. I have now eliminated the use of impellers all together by installing an electric water pump. Should have done it 14 years ago. The pump always gives me solid high flow of water and keeps my motor running cooler than impellers ever did. A cool running motor will last longer. Pricey, but in the long run worth every dollar.
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Old 19-03-2020, 10:11   #13
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Re: Anyone used Run Dry impeller for Onan

Hm, my experience with the blue stuff is totally negative on yanmar ym30. Maybe they last longer in cold climate, but in the Philippines their duty cycle was inferior and I switched back to the originals.
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Old 19-03-2020, 11:39   #14
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Re: Anyone used Run Dry impeller for Onan

Quote:
Originally Posted by Cheechako View Post
Can you just fit a small water strainer so any chunks can't get to the exchanger? That's all I would do.
A friend with a 7Kw Onan could not fit a simple strainer so had to fit the stainless Y strainer that I put into my original post.

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Old 19-03-2020, 12:10   #15
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Re: Anyone used Run Dry impeller for Onan

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Originally Posted by Tudorsailor View Post
An electric pump is a neat solution as it is powered directly from the generator. As soon as the generator starts, the pump gets power and water flows in. As soon as the gen stops, the pump stops. No chance of pumping water in when the gen is not running. There are quite a few other threads discussing this.

I have quite a small generator so I am not sure that I can get the trap into the system. I will measure when flights resume from London to Greece!

TudorSailor
Best improvement we did.
No more blood, sweating and swearing trying to remove the pump for impeller change.
No more wondering if the seal is going to keep the seawater out of the oil.
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