We have told you about a safety valve, we will now have something to say
of a safety plug. A safety, or fusible plug, is a hollow brass plug or
bolt, screwed into the top crown sheet. The hole through the plug being
filled with some soft metal that will fuse at a much less temperature
than is required to burn iron. The heat from the firebox will have no
effect on this fusible plug as long as the crown sheet is covered with
water, but the moment that the water level falls below the top of the
crown sheet, thereby exposing the plug, this soft metal is melted and
runs out, allows the steam to rush down through the opening in the lug,
putting out the fire and preventing any injury to the boiler. This all
sounds very nice, but I am free to confess that I am not an advocate of
a fusible plug. After telling you to never allow the water to get low,
and then to say there is something to even make this allowable, sounds
very much like the preacher who told his boy "never to go fishing on
Sunday, but if he did go, to be sure and bring home the fish." I would
have no objection to the safety plug if the engineer did not know it was
there. I am aware that some states require that all engines be fitted
with a fusible plug. I do not question their good intentions, but I do
question their good judgment. It seems to me the are granting a license
to carelessness. For instance, an engineer is running with a low gauge
of water, owing possibly to the tank being delayed longer than usual, he
knows the water is getting low, but he says to himself, "well, if the
water gets too low I will only blow out the plug," and so he continues
to run until the tank arrives. If the plug holds, he at once begins to
pump in cold water, and most likely does it on a very hot sheet, which
of itself, is something he never should do; and if the plug does blow
out he is delayed a couple of hours, at least, before he can put in a
new plug and get up steam again. Now suppose he had not had a soft plug
(as they are sometimes called). He would have stopped before he had low
water. He would not even have had a hot crown sheet, and would only
have lost the time he waited on the tank. This is not a fancied
circumstance by any means, for it happens every day. The engineer
running an engine with a safety plug seldom stops for a load of water
until he blows out the plug. It frequently happens that a fusible plug
becomes corroded to such an extent that it will stand a heat sufficient
to burn the iron. This is my greatest objection to it. The engineer
continues to rely on it for safety, the same as if it were in perfect
order, and the ultimate result is he burns or cracks his crown sheet. I
have already stated that I have no objection to the plug, if the
engineer did not know it was there, so if you must use one, attend to
it, and every time you clean your boiler scrape the upper or water end
of the plug with a knife, and be careful to remove any corrosive matter
that may have collected on it, and then treat your boiler exactly as
though there was no such a thing as a safety plug in it. A safety plug
was not designed to let you run with any lower gauge of water. It is
placed there to prevent injury to the boiler, in case of an accident or
when, by some means, you might be deceived in your gauge of water, or if
by mistake, a fire was started without any water in the boiler.
Should the plug melt out, it is necessary to replace it at once, or as
soon as the heat will permit you to do so. It might be a saving of time
to have an extra plug always ready, then all you have to do is to remove
the melted one by unscrewing it from the crown sheet and screwing the
extra one in. But if you have no extra plug you must remove the first
one and refill it with babbitt. You can do this by filling one end of
the plug with wet clay and pouring the metal into the other end, and
then pounding it down smooth to prevent any leaking. This done, you can
screw the plug back into its place.
If you should have two plugs, as soon as you have melted out one replace
it with the new one, and refill the other at your earliest convenience.
By the time you have replaced a fusible plug a few times in a hot boiler
you will conclude it is better to keep water over your crown sheet.