fusable plugs

[peterdanastas]

I have to make fusable plugs for my 5" Britannia. The bushes in the boiler have got a 3/8" unef threads in it . What size hole do I put in the bush for lead ect in plug and what composition of tin and lead do I put in plug peter

[SteveW]

Peter,

Perhaps a bit of an off-the-wall answer but the size of hole in the plug with depend on how much hot steam you want to deal with when the lead melts.

I once witnessed an entire smoke box outer wrapper along with most of the stays let go during a nude boiler steam test. It was inside and quite a spectacular.

The point of the plug is to make it clear things ain't good in there if it's just melted but you don't want too much hot coal getting blown around. When it goes you only really need to know its returning to a relatively safe state although still 'kin hot in there.

From a practical perspective the hole needs to be big enough, perhaps also with a lead-in, to make getting it filled with lead easy but not too big to compromise the integrity of the parent plug's thread form.

[johan]

Composition will depend on the pressure in your boiler. The temperature of wet steam is directly related to the pressure in the boiler. It should be at least a bit above that. There is solder of very different melting points available, take a look here: www.mbouk.co.uk/solderwire.htm

[chris vine]

I have seen reference to soft soldering (with suitable solder) a copper rivet into a loose fitting hole. Round head inside firebox (not at the top!!).
The copper head of the rivet protects the solder from the direct heat of the fire.

Not sure if this is recommended or frowned upon, but it does sound like a cunning plan.

You should not need a huge flow of steam through the hole. As long as the steam fills the firebox, then the fire will go out very quickly as the air is displaced. Also, you will see/hear it.

Chris.