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Old 03-19-2010, 03:48 AM   #1
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Default Keggle false bottom collapse!

I am posting this to see if anyone has gone through this and can shed some light as to why this happened: (sorry for the length)

I have a brutus 10 system using keggles. My mash tun has a full diameter false bottom made out of approx. 1/8" thick perforated stainless sheet. It has no hinges as I cut slots in the top of the keggle to get it in and out. It is supported at the bottom of the keg by the sides as the keg narrows at the bottom. It's fairly strong stuff. Last night I brewed my first 10 gal. batch on the system (24 lbs grain). I have done numerous 5 gal. batches with no problems. Half way through the mash, with the pump recirculating, I noticed the mash was getting pretty thick on the bottom 1/3 or so of the keg when i went to stir it. I mashed at 1.75 qts/lb of grain. I stirred it and mixed things up a bit, but about 10 mins after that I noticed grain in my lines. I knew something was very wrong and tried to clear the lines numerous times, but every time, more grain in the line. I finally had to physically move the mash into another keg and remedy the problem. The problem was that the false bottom collapsed on itself. It was crinkled in one corner, and I have a feeling the failure was due to a vacuum being created rather than sheer weight. As it is a direct fire mash tun, I had the pump on almost full blast to recirculate. After hammering the false bottom back to shape the best I could, I reassembled everything, and actually saved the brew. I was sure to stir the mash several times during the remainder, and knocked the pump down to about 1/2 or less.

Has this happened to any of you? Do you think if I keep the pump at a lower level I should be fine next time? I was afraid of scorching the wort, and trying to keep a uniform mash temp, so I had the pump on wide open. I really don't want to go through this again! I've seen others use a false bottom like this without issue. I also don't want to worry about constantly stirring the mash every couple of minutes.

Thanks.

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Old 03-19-2010, 04:46 AM   #2
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I really doubt that a wort pump could create enough suction to collapse 1/8" steel, if that is what you are suggesting.

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Old 03-19-2010, 05:31 AM   #3
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I really doubt that a wort pump could create enough suction to collapse 1/8" steel, if that is what you are suggesting.
I guess that is what I'm suggesting. The power of a vacuum is a powerful thing. My guess is that a vacuum was created when the sparge compacted, and the pump kept on. The weight from the grain/wort on top added some force as well. Perhaps someone who's a bit more up on their fluid dynamics / physics than myself will shed light.

The way the thing crumpled in one small place was strange, not a total uniform failure around the edges, or the center caving in, it's like the thing quickly snapped in on one small place - like an accordian fold. Didn't seem like something that happened from too much weight.
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Old 03-19-2010, 05:48 AM   #4
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I really doubt that a wort pump could create enough suction to collapse 1/8" steel, if that is what you are suggesting.
A 12" dia. circle is 113 sq in of area. Even one PSI of vacuum would be 113 lbs of force, plus another 24lbs of grain sitting on top of it. I don't think it's outside the realm of possibility- especially if the mash was so sticky that it was not flowing well. If it was basically sealing off the false bottom, I think it would be possible to create enough vacuum to do some damage.
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Old 03-19-2010, 01:45 PM   #5
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A 12" dia. circle is 113 sq in of area. Even one PSI of vacuum would be 113 lbs of force, plus another 24lbs of grain sitting on top of it. I don't think it's outside the realm of possibility- especially if the mash was so sticky that it was not flowing well. If it was basically sealing off the false bottom, I think it would be possible to create enough vacuum to do some damage.
While your math is good, 1 PSI of vacuum is on a 113 in^2 surface. Your pump hose is, what, 1/2" ID? that's an area of 0.2 in^2. To get 1 PSI of vacuum on the false bottom requires (113/0.2) 565 PSI vacuum inside the hose. I sincerely doubt the March pump can pull 565 PSI.

Edit: In fact, March's site (PDF) lists the 809HS can pull 5.26 PSI max. 5.26 PSI over 0.2 in^2 is 0.0093 PSI over 113 in^2, giving a max vacuum total pressure of ~1 lb on the false bottom.

Double edit: My thinking may be quite wrong here...feel free to flame...need more coffee.
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Old 03-19-2010, 01:56 PM   #6
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Edit: In fact, March's site (PDF) lists the 809HS can pull 5.26 PSI max.
No, it doesn't.


It says it can PUSH 5.26 PSI max. The way these pumps are designed, they handle head pressure a lot better than they handle intake restriction. I'd be surprised if they can pull more than 2 psi.
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Old 03-19-2010, 02:10 PM   #7
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I'm wondering if a couple of stainless nuts and bolts near the middle could keep this from happening again? Think tiny little legs

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Old 03-19-2010, 02:31 PM   #8
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This is an interesting problem. Clearly something happened that should not have happened.

Don't forget that he mentioned that the false bottom was 1/8" thick steel. That is some REALLY thick stuff, which is why I doubted that the pump would have been able to crumple it.

You mentioned that the MLT is direct fired and the false bottom rests against the curved bottom of the keg, right? I wonder how much of a factor heat played in this?

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Old 03-19-2010, 03:15 PM   #9
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The false bottom is just a smidge under 1/8" thick, still, it is quite strong considering all of the holes. I put all the force I could with my hand, and couldn't get it to budge. I don't know the math involved, but I still gotta think the vaccum idea holds some weight here (no pun intended).

In searching the web, I came across a brewery mash tun manufacturer website that said something like their mash tun being a closed system that eliminates the possibility for mash tun vacuum collapse. I'm sure that system has one hell of a pump, but that tun is several times as large as well. If you scale everything down, maybe these march pumps can pull enough do do what happened to me?

I guess I'll just make sure to stir the mash more often, and scale the pump down during the mash. I thought for sure someone had come across this before...

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Old 03-19-2010, 03:19 PM   #10
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I think beerthirty had the same thing happen to him once.

EDIT: Here's a post about it: http://www.homebrewtalk.com/f36/brew...41/#post784415

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