A wet up take is one which passes through the boiler, and therefore has fire on one side and steam on the other. It is therefore under a collapsing pressure.
The furnace of a marine boiler extends from the fire door to the combustion chamber (i. e., the box in which the heat of the furnace passes before returning through the tubes).
The superheater of a marine boiler is a cylindrical vessel receiving the steam from the boiler, and delivering it to the main steam pipe, whence the steam is delivered to the engines, etc. When it has no connection with the up take, it may, however, be more properly termed a steam driver, since it serves to separate the steam from entrained water, and does not superheat the steam.
In some cases, however, the superheater takes the form of a spherical ended cylinder standing in the up take.
The receiver of a marine boiler is a drum or cylinder that receives the steam from the boiler and from which the steam passes through the steam pipe to the engine. The receiver is by some called the steam chest of the boiler.
The fittings essential for a marine boiler are: The safety valves; the test cocks (or gauge cocks, as they are sometimes termed); the water gauge glass; the stop valves; the check valve for the boiler feed pipe, and the valves for letting on steam to the main engine and such other engine or engines as may take steam from the main boiler; the scum cocks; the blow off cocks; and a small cock to enable the drawing of water from the boiler to test its degree of saltness.
There are two kinds of safety valves, the dead weight and the spring loaded.
A dead weight safety valve is one in which the valve is held to its seat by dead weight, the objection to which is, that when the vessel rolls the effect of the weight or weights upon the valve is diminished; hence under heavy rolling the steam may blow off at a less pressure than the valve is set for.
A lock up safety valve is a dead weight safety valve, the top of whose spindle is provided with a cast iron cap or bonnet with two handles on. This cap is keyed to the spindle, and the keyway is so disposed that no extra weight can be added to the valve, while at the same time the valve can be lifted from its seat and turned around.
A spring loaded safety valve is one in which the valve is held down by the pressure of a spiral spring, and this pressure will obviously not vary, no matter how much the ship rolls.