Fusible Plug Thoughts

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Steve Goodbody
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Re: Fusible Plug Thoughts

Post by Steve Goodbody »

Hi Steve,

My interpretation of Ben Nixon's design, based only on this drawing, is that the copper rivet performs two functions: firstly it provides a rapid heat conduction path into the solder core to promote melting and, secondly, it restricts the flowrate of steam into the firebox once the solder core melts. I think you're correct about the purpose of the saw cuts. Presumably the intention of restricting the flow is to put the fire out without potentially blowing burning coals through the firehole door or out of the ashpan.

Regarding sealing the pipe thread, others may have a different opinion but my view would be to use a small amount of teflon tape. The melting and decomposition temperatures of teflon are each above the melting point of soft solder, tin or lead.

Best regards
Steve
Marty_Knox
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Re: Fusible Plug Thoughts

Post by Marty_Knox »

I wouldn't use teflon in the firebox. Burning teflon gives off toxic fumes.
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Bill Shields
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Re: Fusible Plug Thoughts

Post by Bill Shields »

do not use PTFE in that location. Marty is absolutely right...while the amount of gas given off will be very small, the PTFE will just burn off in the heat and you can easily end of with a leak...

wipe the threads with soft solder and screw it in.

If things get hot enough to melt the solder...that is the least of your worries...
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Kimball McGinley
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Re: Fusible Plug Thoughts

Post by Kimball McGinley »

I figured Nixon's drawing was upside down, and the rivet is supposed to fall out?
SteveM
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Re: Fusible Plug Thoughts

Post by SteveM »

Kimball McGinley wrote: Tue Jun 05, 2018 6:02 pm I figured Nixon's drawing was upside down, and the rivet is supposed to fall out?
No. I thought that too. It's right side up.

The steam doesn't come out of the hole left by the rivet falling out. It comes around the rivet (implying that the hole for the rivet needs to be a loose fit, with the space take up by the solder).

The head of the rivet would seal the hole under the pressure, but there is am undercut under the head that allows the steam past the head of the rivet.

Steve
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Fender
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Re: Fusible Plug Thoughts

Post by Fender »

A number 30 drill is only 0.1285" in diameter, 0.0035" larger than the rivet. This doesn't seem to give much space for the steam to escape (?)
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Bill Shields
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Re: Fusible Plug Thoughts

Post by Bill Shields »

you can push a lot of steam through that space at 1100 feet / second....

it'll put the fire out real quick....
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John Hasler
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Re: Fusible Plug Thoughts

Post by John Hasler »

63/37 ("eutectic") tin/lead solder melts at 360F. Solders with higher or lower tin/lead ratios begin to soften at about that temperature but have a pasty range where they are mushy but not liquid.
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Greg_Lewis
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Re: Fusible Plug Thoughts

Post by Greg_Lewis »

SteveM wrote: Tue Jun 05, 2018 1:19 pm ...

What would you seal the pipe thread with? It's just a spiral, so it would leak.

Steve
I'd probably look at Key graphite paste. It's supposed to be good to 750°F. I've used it for other threads in other places and it works great. You can get it from McMaster, and one can is a lifetime supply.
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Steve Goodbody
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Re: Fusible Plug Thoughts

Post by Steve Goodbody »

Thanks to all for the notes about Teflon - you're absolutely correct of course - and for the recommended alternatives.
I've ordered a small tub of copper/graphite grease (marketed as Cop-Graf 11002) which is rated to 1800 F and should do the trick.

Best regards
Steve
Steve Goodbody
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Re: Fusible Plug Thoughts

Post by Steve Goodbody »

As an update, last night I removed the plug and checked things out. The plug turned out to be 1/4" BSP not NPT and the center was threaded 2BA as a key for the core rather than a tapered hole. Not having a 1/4" BSP die to hand I decided to re-use the plug - drilled out the remains of the core, retapped the 2BA thread, wetted it with solder and then filled the hole with tin. All went well - I'll make up some spare plugs when the BSP die arrives.

Thanks again to all for the advice.
Steve
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