Boiler Accessories Furnaces

All mechanical stokers in which the movable parts are inside the furnaces, are likely to get out of order because of the heat and dirt.

Fusible Plugs. Fusible plugs are usually inserted in the top sheet or crown sheet of boilers, as a safeguard against collapse of the furnace crown-should the water in any way be drawn out of the boiler while the fire is burning. These plugs consist of acore composed of an alloy of tin, lead, and bismuth, with a covering of brass or cast iron. The United States inspection law requires at least one fusible plug to be put in every marine boiler, with the exception of watertube boilers, the plug to be made of a bronze casing filled with goodquality "Banca" tin from end to end. While this plug is kept at a comparatively low temperature by water on one side, the fire on the other side will not melt it; but when the water-level becomes low enough to leave one end of the plug uncovered, the alloy core of the plug, having a comparatively low melting point, will fuse, thus running out of its casing, relieving the pressure in the boiler, and allowing the excess of steam to extinguish the fire, which otherwise would be likely to destroy the crown sheet.

Fusible plugs are frequently unreliable. Sometimes they will blow out when there is no apparent cause, and sometimes remain intact when the plates have become overheated. If a coating of hard scale is allowed to accumulate over the plug, it may stand considerable pressure, even after the core has become melted. To provide against this, the plug should be replaced frequently. If allowed to remain in the boiler for any length of time, the composition of the alloy is likely to change, the plug thus becoming more or less unreliable.

Figs. 18 and 19 illustrate the ordinary plug. It should be so made that, when screwed into the crown sheet, it will project 1-1/2 or 2 inches above the plates, so that when the alloy melts there will be a sufficient depth of water over the crown sheet to prevent injury from heat.

Sometimes the core is covered with a thin copper cap, as shown in Fig. 18, which protects the alloy from contact with the water, thus preventing a chemical change and the formation of scale. It does not necessarily follow that a hole inch or inch in diameter will liberate steam fast enough to prevent excess of pressure. If a small quantity of steam is introduced into the firebox, it may have the effect of brightening the fire and increasing the heat of combustion, owing to the formation of water gas as the steam mingles with the burning coal. The steam, moreover, might have the effect of inducing additional draft. If, however, the quantity of escaping steam and water is considerable, combustion will be retarded, and the fire will be partially extinguished. It will operate to warn the fireman of what has happened; and if the escape of steam is not too rapid, he may throw on wet ashes and deaden the fire.