In some States the insertion of fusible plugs at the highest fire line
in boilers is compelled by law under a heavy penalty. Its design is
to give the most emphatic warning of low water, and at the same time
relieve the boiler of dangerous pressure.
[Illustration: Fig. 78.]
[Illustration: Fig. 79.]
Figs. 78 and 79 exhibit two of the forms most commonly used, and on
the succeeding page, in cut 80, is shown the device in operation where
the water has sunk to a dangerously low level. In the illustration
the device is shown in connection with a locomotive boiler, in the
common tubular boiler the plug is usually inserted in the rear head of
the boiler, so that in case of its operation it will not endanger the
fireman.
These devices are designed to be screwed into the boiler shell at the
safety line. The Figs. 78 & 79 exhibit their construction. The part
to be screwed into the boiler is called _the shell_ and is commonly
made of brass; the internal part is plug and is made of a soft metal
like banca tin or a compound consisting of lead, tin and bismuth. This
composition melts easily at the proper point to allow escape, where the
water has sunk to a dangerously low level.
There is considerable diversity in the make up of the material used for
filling the plug, which must not have its melting point at anything
less than the temperature of the steam lest it should “go off” at the
wrong time.
[Illustration: FUSIBLE SAFETY PLUG