Fusible Plug Thoughts

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Steve Goodbody
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Fusible Plug Thoughts

Post by Steve Goodbody » Mon Jun 04, 2018 11:07 am

Hello all,

While washing out the traction engine boiler this weekend I noticed that the fusible plug was weeping, I'd planned to replace it this winter anyhow and so it's no big deal. I haven't removed it yet but it's a 1/4" NPT plug I believe.

I intend making the replacement (and some spares) using 660 bronze for the bodies, 63/37 tin/lead solder paste to prep the hole, and then filling the core with molten lead. I was intending to use a tapered hole for the lead plug with a 2 included taper or thereabouts.

If you have experience of making fusible plugs I'd appreciate your thoughts on the above please. Anything seem obviously out-of-whack?

Many thanks
Steve

tetramachine
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Re: Fusible Plug Thoughts

Post by tetramachine » Mon Jun 04, 2018 12:34 pm

What temp is the melting point of solder, 500+, Lead is 625 F. If the steam is at 100psi the temp is around 350, would something in the 400F range be more usable. There is a series of low melting point metals, made to melt at a specific temp. Often called fusing metals they are available from industrial suppliers.I have bought some on Ebay.

Brazing metals that hold you boiler together melt between 1000 and 1425F. if the temp at the steam dome gets to 625, I would guess the firebox area would be closer to that 1000 level than anyone would be comfortable with.
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Builder01
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Re: Fusible Plug Thoughts

Post by Builder01 » Mon Jun 04, 2018 12:58 pm

What causes the steam dome get to 625 degrees?

David

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Marty_Knox
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Re: Fusible Plug Thoughts

Post by Marty_Knox » Mon Jun 04, 2018 1:03 pm

ASME fusible plugs are filled with pure tin.
FRA rules require that a fusible plug be removed, cleaned, and inspected every time the boiler is washed, but not less frequently than 31 service days.
Here is a fusible plug I removed during a boiler wash. It was caught just in time.
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Steve Goodbody
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Re: Fusible Plug Thoughts

Post by Steve Goodbody » Mon Jun 04, 2018 1:15 pm

Thanks Marty.

I was assuming lead based on the UK Heritage Railway Association guidelines (see https://static1.squarespace.com/static/ ... +Plugs.pdf).

Unalloyed lead melts around 625 F, unalloyed tin around 450 F. I see no practical problem in making the core from either metal. From your experience, for a non-ASME boiler, do you see any pros or cons with either?

Best regards
Steve

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Marty_Knox
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Re: Fusible Plug Thoughts

Post by Marty_Knox » Mon Jun 04, 2018 1:32 pm

Ben Nixon used to make his own, he used 50/50 solder and put a copper rivet in the middle so it used less solder.
I think cleaning and inspecting it regularly is more important than the filler metal used.

Steve Goodbody
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Re: Fusible Plug Thoughts

Post by Steve Goodbody » Mon Jun 04, 2018 1:45 pm

Thanks Marty - good to know.
I think I'll make a few from lead and a few from tin and see how they perform over time.
Best regards
Steve

darjeeling
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Re: Fusible Plug Thoughts

Post by darjeeling » Mon Jun 04, 2018 2:57 pm

Hi Steve

I have used 99% pure tin that has been certified to a melting point of 320C and is easy to obtain. Pure tin is recommended by the Australian AMBSC codes. I have used 316 S/S hand made plugs and just used liquid style solder flux to bond the tin to the plug 6mm holed body. Note that these plugs are used in duplex boilers and bronze or steel may be more suitable for your situation. No problems to date!

Safe steaming, John Heald, New Zealand

Steve Goodbody
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Re: Fusible Plug Thoughts

Post by Steve Goodbody » Tue Jun 05, 2018 6:54 am

Many thanks John, that's good to know and is consistent with Marty's note about the ASME. I've got some tin on order and will fit a tin-cored plug first to see how that performs.

One of the things about a traction engine (versus a loco) is that the water inevitably sloshes about a lot more front-to-back and side-to-side when on the road making the level of water above the firebox crown much more instantaneously variable. I'll fit tin-cored plugs to both the loco and the traction engine and see how they compare in appearance after an equivalent time in steam.

Best regards
Steve

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dnevil
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Re: Fusible Plug Thoughts

Post by dnevil » Tue Jun 05, 2018 11:31 am

Here is a drawing by Ben Nixon for his fusible plug.

http://ibls.org/mediawiki/index.php?tit ... rnal_Links

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Steve Goodbody
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Re: Fusible Plug Thoughts

Post by Steve Goodbody » Tue Jun 05, 2018 12:10 pm

Many thanks Daris,
Steve

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SteveM
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Re: Fusible Plug Thoughts

Post by SteveM » Tue Jun 05, 2018 1:19 pm

Here's Ben Nixon's design:

Image

The rivet is in position where it would not fall out if the solder holding it in melted.

Is it that the steam / water vents thru the grooves cut into the top under the rivet head?

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

Steve

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