Sometimes also two safety-valves are provided, one loaded a little heavier than the other. The escape of steam from the lighter valve in this case gives notice to the engine-man of the growing increase of pressure, and warns him to check the production of steam. The lever by which the safety-valve is held down is sometimes acted on by a spiral spring, capable of being so adjusted as to produce any required pressure on the valve. This arrangement is adopted in locomotive engines, where steam of very high pressure is used; and in such cases also there are always provided two such valves, one of which cannot be increased in its pressure.
The pipe by which the boiler is fed with water will [Pg280] necessarily act as a safety-valve, for when the pressure of the steam increases in an undue degree, it will press the water in the boiler up through the feed-pipe, so as to discharge it into the feed-cistern, a circumstance which would immediately give notice of the internal state of the boiler. The steam-gauge, already described ([fig. 79.]), would also act as a safety-valve; for if the pressure of steam in the boiler should be so augmented as to blow the mercury out of the steam-gauge, the steam would then issue through the gauge, and the pressure of the boiler be reduced, provided that the magnitude of the tube forming the steam-gauge were sufficient for this purpose.
(164.)
Although fusible plugs may be used, in addition to other means of insuring safety, they ought not to be exclusively relied on at the ordinary working pressure of the boiler. The fusible plug ought to be capable of more than resisting the pressure; but if it be so, its point of fusion would be one at which the steam would have a pressure of at least two atmospheres above its working pressure. The plug would therefore be capable of being fused only as soon as the steam would acquire a pressure of 30 lbs. per inch above its regular working pressure.
When a boiler ceases to be worked, and the furnace has been extinguished, the space within it appropriated to steam [Pg281] will be left a vacuum by the condensation of the steam with which it was previously filled. The external pressure of the atmosphere acting on the boiler would, under such circumstances, have a tendency to crush it inwards. To prevent this, a safety-valve is provided, opening inwards, and balanced by a weight sufficient to keep it closed until it be relieved from the pressure of the steam below.
A large aperture closed by a flange secured with screws, represented at O in [fig. 71.], called the man-hole, is provided to admit persons into the boiler for the purpose of cleaning or repairing its interior.
(165.)
In order to vary the production of steam in proportion to the demands of the engine, it is necessary to stimulate or mitigate the furnace, as the evaporation is to be augmented or diminished.
The activity of the furnace must depend on the current of air which is drawn through the grate-bars, and this will depend on the magnitude of the space afforded for the passage of that current through the flues. A plate called a damper is accordingly placed with its plane at right angles to the flue, so that by raising and lowering it in the same manner as the sash of a window is raised or lowered, the space allowed for the passage of air through the flue may be regulated. This plate might be regulated by the hand, so that by raising or lowering it the draught might be increased or diminished, and a corresponding effect produced on the [Pg282] evaporation in the boiler: but the force of the fire is rendered uniformly proportional to the rate of evaporation by the following arrangement, without the intervention of the engineer. The column of water sustained in the feed-pipe (figs. 71, 72.) represents by its weight the difference between the pressure of steam within the boiler and that of the atmosphere. If the engine consumes steam faster than the boiler produces it, the steam contained in the boiler acquires a diminished pressure, and consequently the column of water in the feed-pipe will fall. If, on the other hand, the boiler produce steam faster than the engine consumes it, the accumulation of steam in the boiler will cause an increased pressure on the water it contains, and thereby increase the height of the column of water sustained in the feed-pipe. This column therefore necessarily rises and falls with every variation in the rate of evaporation in the boiler. A hollow float P is placed upon the surface of the water of this column; a chain connected with this float is carried upwards, and passed over two pulleys, after which it is carried downwards through an aperture leading to the flue which passes beside the boiler: to this chain is attached the damper. By such an arrangement it is evident that the damper will rise when the float P falls, and will fall when the float P rises, since the weight of the damper is so adjusted, that it will only balance the float P when the latter rests on the surface of the water.
Whenever the evaporation of the boiler is insufficient, it is evident from what has been stated, that the float P will fall and the damper will rise, and will afford a greater passage for air through the flue. This will stimulate the furnace, will augment its heating power, and will therefore increase the rate of evaporation in the boiler. If, on the other hand, the production of steam in the boiler be more than is requisite for the supply of the engine, the float will be raised and the damper let down, so as to contract the flue, to diminish the draught, to mitigate the fire, and therefore to check the evaporation. In this way the excess, or defect, of evaporation in the boiler is made to act upon the fire, so as to render the heat proceeding from the combustion as nearly as possible proportional to the wants of the engine. [Pg283]