Fusible Plug Thoughts

This forum is dedicated to the Live Steam Hobbyist Community.

Moderators: Harold_V, WJH, cbrew

Steve Goodbody
Posts: 124
Joined: Thu May 25, 2006 7:16 am

Re: Fusible Plug Thoughts

Post by Steve Goodbody » Tue Jun 05, 2018 2:07 pm

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

User avatar
Marty_Knox
Posts: 1318
Joined: Thu Jan 09, 2003 6:50 pm
Location: Michigan, USA

Re: Fusible Plug Thoughts

Post by Marty_Knox » Tue Jun 05, 2018 3:39 pm

I wouldn't use teflon in the firebox. Burning teflon gives off toxic fumes.

User avatar
Bill Shields
Posts: 5082
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Fusible Plug Thoughts

Post by Bill Shields » Tue Jun 05, 2018 5:49 pm

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...

Kimball McGinley
Posts: 688
Joined: Wed May 31, 2006 2:13 pm
Location: Laguna Niguel CA

Re: Fusible Plug Thoughts

Post by Kimball McGinley » Tue Jun 05, 2018 6:02 pm

I figured Nixon's drawing was upside down, and the rivet is supposed to fall out?

User avatar
SteveM
Posts: 6535
Joined: Mon Jun 27, 2005 6:18 pm
Location: Connecticut

Re: Fusible Plug Thoughts

Post by SteveM » Tue Jun 05, 2018 7:02 pm

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

User avatar
Fender
Posts: 2498
Joined: Wed May 02, 2007 8:33 pm
Location: Chattanooga TN

Re: Fusible Plug Thoughts

Post by Fender » Tue Jun 05, 2018 7:17 pm

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 (?)
Dan Watson

User avatar
Bill Shields
Posts: 5082
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Fusible Plug Thoughts

Post by Bill Shields » Tue Jun 05, 2018 8:38 pm

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

it'll put the fire out real quick....

John Hasler
Posts: 875
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: Fusible Plug Thoughts

Post by John Hasler » Tue Jun 05, 2018 9:47 pm

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.

User avatar
Greg_Lewis
Posts: 1544
Joined: Wed Jan 15, 2003 2:44 pm
Location: Fresno, CA

Re: Fusible Plug Thoughts

Post by Greg_Lewis » Wed Jun 06, 2018 9:58 am

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.
Greg Lewis, Prop.
Eyeball Engineering — Home of non-interchangeable parts.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

Steve Goodbody
Posts: 124
Joined: Thu May 25, 2006 7:16 am

Re: Fusible Plug Thoughts

Post by Steve Goodbody » Wed Jun 06, 2018 1:55 pm

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
Posts: 124
Joined: Thu May 25, 2006 7:16 am

Re: Fusible Plug Thoughts

Post by Steve Goodbody » Thu Jun 21, 2018 8:08 am

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

Post Reply