For all first class work, and work where high pressures are to be used, the common material for boiler construction is mild steel, made by the open hearth process, and having a tenacity of about 28 tons per square inch. Copper is often used in the fire boxes of locomotives, but seldom in any other description of boiler. Brass boiler tubes are sometimes seen, and on account of its better conducting qualities, brass is to be preferred to iron, but its costliness prevents it superseding iron in the great majority of cases.
Every boiler has, to render it complete and workable, a number of fittings or mountings, of which the following are the principal: A glass gauge to show the level of the water inside the boiler, and gauge cocks for the same purpose; a gauge to show the pressure of the steam; a valve for admitting water; a cock at the bottom for emptying or blow ing off; a valve for the discharge of the steam; one or two safety valves, weighted so that when the pressure of steam in the boiler reaches a certain height, they open and allow the steam to rush into the air; a door by which a man can get in to clean the boiler, etc.
The Cornish boiler has often two in ternal flues or tubes, which is a much more advantageous construction. In the Galloway boiler, there are two furnaces. but these join together in one chamber just behind the bridges, and the gases are made to pass through a space con siderably narrowed by side pockets pro jecting inward in order that they may be well mixed. From this point to the
back of the boiler there is just one flue, made oval in section, and crossed by a considerable number of vertical taper tubes, which form a direct communica tion between the water beneath and that above the flue. These tubes both pro mote circulation and strengthen the flue. A later development of the boiler is the water tube boiler, which is extensively used. In this type one or more drums of water legs connected by tubes of corn paratively small diameter are used. These are filled with water and the gases from the furnace pass around and between the tubes, heating the water con tained in them. An advantage of the water tube boiler is the possibility of making it light, powerful, and compact. It is especially well adapted for forced draught, and steam can be raised quickly. These boilers are the safest against ex plosions. For these and other reasons they are largely used on war vessels and especially torpedo boats.
The most frequent cause of boiler ex plosions is the inability of the boiler to resist the regular working pressure. A boiler may be weak through various rea sons, and old boilers are likely to be weakened by rust and general decay. See STEAM.