ONAN Genset Raw Water Impeller Failure

[ozdownunder]

Hi Dirkdig,

I just used a cheap strainer, it has a 3/4" inlet /outlet, all I did was used a bit of extra rubber hose to seal it, the strainer is rusting a little, been there for 11 months so far.
I am concerned that it is a weak fitting $11.00) so have a back up strainer that is $64 , it has 1" inlet and outlet. So just leaving this one in till it fails and just see now it goes.

It was simple to fit, just cut the raw water outlet hose half way to the heat exchanger and fitted it as shown in photo.

Been working good, as the impeller starts to fail you can see bits in the strainer, change it all out in 27 mins.

Hope this helps

Regards Terry

[St. Elsewhere]

Does that Onan use a Sherwood G1006, G1008, or G1010? If so, the cam may be wearing the impeller out quickly. You may need to replace the whole pump to rectify it. I have ordered replacement wear part kits for these pumps, and found that they did not fit very well.

[s/v Jedi]

Here is my impeller piece catching strainer; the insert is made from thin wall 1-1/4" PVC which fit the housing, then drilled it with holes smaller than the channels in the heat exchanger. http://sv-jedi.smugmug.com/Projects/...IMG_1636-L.jpg

[Albro359]

I too have this problem of failing impellers on my 7.5kVA set...200 hours is about what I get and the impeller is completely destroyed.
Yet the Yanmar engine impellers seem to last forever.
It must be something to do with the actual pump housing that causes them to die so quickly.
Lots of people seem to have this issue yet ONAN claims they don't know anything about it, it seems.
Cheers
Alan

[svlamorocha]

Originally Posted by ozdownunder

I have a Sanya 2012 model with Onan 7.5 KVA.
(Snip)
I would like to know why they are not lasting 500 hrs like the book said?

My experience in delivery trips of new boats fitted with the Cummins Onan MDKBJ 6/7.5 kVA genset is consistent with yours. First failure tends to happen before 50 hours and from then on I am told that they last a bit longer but not more than 100 hours.

I can explain the difference because the original impeller is very old by the time you use it the first time. I cannot explain the 100-hr approximate MTBF thereafter because these boats have an electric water lift pump that keeps the thing well supplied with water. I suspect the genset pump's design is bad or the rotor design or materials are bad.

A nice trivia finding is that if you have a setup like on these boats, with another pump before the built-in one,you can run forever with a broken impeller provided the pieces do not get stuck somewhere and restrict water flow. Fortunately in many cases all the pieces get together in a little space before the exchanger and they do
not block anything, hence the genset runs fine until you do schedule maintenance and discover that the blades have disappeared.

Maybe we should do away with the impeller!

Charlie

[impi]

As a follow on to our earlier posts here ... We had removed the impeller and fitted a standard aircon type pump at the hull thru fitting - wired to the genset (220v) - has worked a charm all this time without any hassles whatsoever.

[Albro359]

I've bought a Sherwood replacement pump..it has a cast and machined pump house - no end plate, and a Global run-dry impeller, so we'll see how that goes and report back
Cheers
Alan

[St. Elsewhere]

Originally Posted by Albro359

I've bought a Sherwood replacement pump..it has a cast and machined pump house - no end plate, and a Global run-dry impeller, so we'll see how that goes and report back
Cheers
Alan

I believe that Sherwood has been the supplier to Onan for their raw water pumps for the past 16 or more years. In my case there have been three or four model number changes over that period of time for my 12 KVA Onan genset. As I recall three of the Sherwood P/N's have been G1006, G1008, and currently G1010. The pump bodies have been cast bronze. I am not familiar with a run dry impeller being an option. Mine have all been black rubber that fails promptly if run dry. I would like to hear more about the run dry possibility. And also, what does it have in place of an end plate? Do you mean that it is a flat plate, maybe stainless steel, instead of a bronze casting with a shaft support bearing?

[Albro359]

Originally Posted by St. Elsewhere

I believe that Sherwood has been the supplier to Onan for their raw water pumps for the past 16 or more years. In my case there have been three or four model number changes over that period of time for my 12 KVA Onan genset. As I recall three of the Sherwood P/N's have been G1006, G1008, and currently G1010. The pump bodies have been cast bronze. I am not familiar with a run dry impeller being an option. Mine have all been black rubber that fails promptly if run dry. I would like to hear more about the run dry possibility. And also, what does it have in place of an end plate? Do you mean that it is a flat plate, maybe stainless steel, instead of a bronze casting with a shaft support bearing?

The original ONAN pump on my MDKAV has an outer end plate, a housing and an inner end plate.
The Sherwood pump for my MDKAV is G702, it has a bronze cast pump housing that bolts directly to the drive coupling...no removable end plate.
see : Sherwood Pumps - G702 - Related Products


The run dry impellers are from Globe (sorry)
see : Photo Gallery

The Globe part no. for a Sherwood G702 is 1130...they make impellers to fit just about everything.

Hope this helps

Cheers
Alan

[DavefromNZ]

I would just like to add that I have the same problem on an Onan 11kva. The second impeller has had most of the blades shredded and both failures were within 150 hours. I am wondering if it would help if i run the prime for a short time before starting as that has the electric pump push water to the impeller.

[impi]

Originally Posted by DavefromNZ

I would just like to add that I have the same problem on an Onan 11kva. The second impeller has had most of the blades shredded and both failures were within 150 hours. I am wondering if it would help if i run the prime for a short time before starting as that has the electric pump push water to the impeller.

Dave ... I would be cautious of that. I was told by Onan agents that this could result in water ingress into the engine. We installed aircon type pump wired to the genset to provide current to the pump whilst cranking the engine over on start up. This works a charm !

[Albro359]

The run-dry impeller solves all these issues

[St. Elsewhere]

Originally Posted by Albro359

The run-dry impeller solves all these issues

Can you supply more information? Does Sherwood/Onan manufacture these impellers? What design features enable them to run dry? How is friction and wear eliminated?

[Captain Bill]

I had this problem for quite a while with my onan. I was getting an average of 50 hours per impeller. The pump in my boat is about 3 feet above the waterline. The installation manual says this is ok. The truth is that it's OK for about 50 hours per impeller. I solved the problem by putting a check valve in the raw water line. The water no longer drains to sea between runs. I've replaced only one impeller since putting in the check valve and that was with about 600 hrs on it. I only have about 300 hours on the current impeller so I don't have an average life yet, but it's a lot more than 50.

[azsailor12]

Another trick is to put Superlube(teflon grease) on the impeller when installing it. Much slicker than soapy water and lasts longer.