Society for the Preservation of Steam Engines

Recently we inspected the fusible plug of a boiler that is still in use. It is the sad accident with a Case traction engine in the USA that made us wonder whether the plug was still in good order. We did put the plug in an industrial oven with the temperature control set to 250 C. Much to our surprise only a minor amount of alloy was melted, so the heat was increased, eventually up to 500 C and still the alloy did not melt ! We then took a closer look at the plug, just to find out that the alloy inside the plug was completely corroded over the years. All tin and plumb was oxidized and would certainly not have melt under 1000 C
loodprop.JPG (11984 bytes) This image shows the plug after the heating exercise. Most alarming is what you see on the right hand of the image. This is a small amount of intact alloy on the very bottom of the fusible plug. Every year we cleaned the plug gently with a steel brush and had the feeling that is was in good condition. 
In between is a piece of corroded alloy that came loose after the plug was heated up to 500 C


We have cleaned the plug and removed all corroded material. We then did poor in an alloy consisting of 50% tin and 50% plumb, something you plumber has available in great quantities. This boiler is made for 140 PSI.  A 60/40 alloy can stand even more heat probably sufficient for 200 PSI.

Nijkerk, January 2003