The pressure shown by a steam gauge is that above atmosphere,[69] and not that above vacuum.
[69] See [page 367], Vol. II., for remarks on total pressure and pressure by gauge.
The stop valve of a marine boiler is a valve that is opened to let the steam into the main steam pipe.
A blow off cock is a cock employed to blow off, or let all, or a part of, the water out of a boiler. There are generally two, one on the bottom of the boiler, and the other at the ship’s side, so that if the pipe was to break or get damaged, the cock at the vessel’s side can be closed to keep the sea water out, while that on the boiler may be closed to keep the water and steam in the boiler. These two ends cannot obviously be obtained if one blow off cock only was used.
Blow off cocks are opened and closed by a spanner or key that is removable from the cock, and to prevent the possibility of taking off the spanner or key, before the blow off cock is closed, a spanner guard is employed.
A spanner guard is a cap having a lug or tongue, which projects into the hole in the spanner guard, through which the spanner or key must pass before it can fit on the head of the blow off cock, and the key or spanner has a corresponding recess, so that the spanner or key can only be put on or taken off when the cock is closed.
Blowing off a boiler is emptying it entirely, as for examining the whole interior of the boiler.
Blowing down a boiler is letting out a portion of the water, so as to carry off the loose scale, mud, or sludge that may accumulate on the bottom of the boiler. The mud or sludge would form into scale if allowed to remain.
A scum cock is a cock employed to blow off a portion of the surface water in a boiler, and thus remove the scum, salt, and impurities which float or are thrown up to the surface.
Two scum cocks are employed, one on the side of the boiler, and one on the side of the ship. These two cocks are connected by a pipe. That on the boiler is placed a little below the working level, which is supposed to be (and is kept as nearly as possible) about 9 inches above the top row of tubes.