Beat the nozzle out of
shape, to give room for the escape of the air displaced by the water.
The best form of filling cap has a self-seating ground plug, which, if
properly made, is steam-tight without any packing. If needed, asbestos
packing can easily be inserted between plug and cap.
Mud-holes.--All but the smallest boilers should have a mud-hole and plug
in the bottom at a point not directly exposed to the furnace. In Fig. 82 it
is situated at the bottom of the barrel. In Figs. 86 and 87 there should be
a mud-hole in one end of each of the three drums, A, B, and C. The plug may
be bored at the centre for a blow-off cock, through which the boiler
should be emptied after use, while steam is up, and after the fire has been
"drawn." Emptying in this way is much quicker than when there is no
pressure, and it assists to keep the boiler free from sediment.
[Illustration: FIG. 92.--Steam cock.]
Steam Cocks.-The screw-down type (Fig. 92) is very preferable to the "plug"
type, which is apt to leak and stick.
Testing Boilers.--The tightness of the joints of a boiler is best tested
in the first instance by means of compressed air.
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