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Author Topic: OWBs and cavitation.  (Read 587 times)

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Offline Dave Shepard

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OWBs and cavitation.
« on: February 14, 2022, 10:55:07 PM »
My 6048 finally failed just after Christmas due to pinholes in the firebox. I believe the holes were caused by cavitation, but I'm not sure what caused that. It is a 2006 that I got used and installed in 2014. It was filled, and refilled as needed, with softened well water. I've cut out the failed area for inspection. The rust on the sample is from being outside. The steel looked like new when I cut it out, so I don't think it's a rust issue, but I am wondering if I had used the additive if it would prevented the cavitation. When the boiler was making heat I could hear it rumbling, which I think was the cavitation happening. This happened at any temperature once the fire really got going. After a couple of days rust appeared in the firebox near this hole, and if I scratched at it, I found more holes. I have to assess how many more holes there are to see if I will try to fix it, but I'd like to know the cause before I put any effort into it.




 
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Offline PoginyHill

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Re: OWBs and cavitation.
« Reply #1 on: February 15, 2022, 07:01:54 AM »
Cavitation should only impact the pump impellor and anything in the immediate area. I'm guessing the noise you hear is localized boiling on the hottest surfaces against the firebox. I would also guess that the pinholes are localized sites of corrosion/erosion where the localized boiling occurs. When the water is evaporated in that localized place, it will concentrate anything, particularly pH, that can accelerate corrosion. Once a pit develops on the surface, it will be the site of any further boiling and corrosion, until the pit turns into a pin hole.

Do these pin holes appear near where the fire is hottest?
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Offline hedgerow

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Re: OWBs and cavitation.
« Reply #2 on: February 15, 2022, 10:19:47 AM »
Dave Welcome to the wonderful world of open system wood heaters or as some will call them boilers. Even running treated water doesn't seem to keep it from happening. When the first weld on my Garn failed and it started leaking. I drained to to get inside and repair the weld I couldn't believe how much pitting it had that looked like you picture. I over laid a fair amount of the burning chamber to repair a lot of spots that were going to do like yours did and start leaking where the pitting was. A few years later I had a second weld in a different location start leaking and we over laid some steel on the blower box this time. I have talked to the builder the chemical guy and no one seems to have a answer to keep this from happening. The oxygen, heat of burning and the water seem to do it. I send a water sample in ever six months on mine. I would never own a open system again. The closed system don't have this pitting problem as they don't have any oxygen in the water after the fill and first heat up. All you can do it repair it and move on. 

Offline PoginyHill

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Re: OWBs and cavitation.
« Reply #3 on: February 15, 2022, 01:13:05 PM »
The closed system don't have this pitting problem as they don't have any oxygen in the water after the fill and first heat up. All you can do it repair it and move on. 


Probably less oxygen. The pressure will reduce the localized boiling effect also.
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Offline hedgerow

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Re: OWBs and cavitation.
« Reply #4 on: February 15, 2022, 04:19:58 PM »

Probably less oxygen. The pressure will reduce the localized boiling effect also.
That is for sure. If I would have known back in 2009 what I know today about open vs closed I wouldn't ever bought my Garn which is a open system and I would have bought a Switzer which is a closed system. 


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