As soon as the piston reaches the bottom of the cylinder, the valves r and t must be shut, and u and q opened; when the steam, being no longer able to g -t through r, would pass down the pipe p, and enter the lower part of the cylinder through q. Mean time u being opened to the condenser by the pipe v, would cause the necessary vacuum above the piston to permit its ascent, which being completed, the valves must be again put into the position shown in the figure, to produce its descent, and so on. It will be sufficient to state that those valves are operated upon either by levers, passing in a steam-tight manner through the side pipes, or that nine-times the spindles of the valves are made to act one through the other, in stuffing, as in the present instance, when they are worked by extern; / applications.
By this most ingenious contrivance no waste of steam arises, excepting in the small aperture between the valves; and the friction is obviously much less than in either slides, cocks, or perhaps any other kind of valve; the only resistance to their motion being the pressure upon the upper side by the steam, when in their seats. Their cost, compared to slide-valves, is much greater; but as they are not liable to material wear, and work with great accuracy, the extra ex pense docs not prevent their very general adoption in large engines.
Having thus briefly noticed a very important class of valves, we proceed to describe another kind, which have even stronger claims upon our attention, as will be immediately acknowledged by naming them, a represents an aperture in the upper part of a boiler; over this aperture is fitted a short tube b, turned true at the top with a round edge, so that a steel plate c, flat and smooth on its under side, may touch at every part; this steel plate is suspended by a joint, to a curved lever d, whose fulcrum is at e, and which is loaded at the other end with a weight of 10 pounds. Now, as the lever has a power of five, (as shown by the five equal dotted spaces,) the plate c is pressed down upon the edge of b, with a foece of 50 pounds; but when the lever and weight are raised by the pressure of the steam, into the position shown by "he lots. the force acting against the steam is reduced to 40 pounds; and, in proportion to the force of the rush of the steam, by which the lever would be raised higher and higher, would the resistance be reduced to 30, 20, etc. as marked.
This valve might be enclosed as usual, in a box, with a pipe to conduct off the waste steam.
Safety-valves; these are well-fitted covers or stops to apertures made in the upper part of a boiler, and loaded to such a degree only as the steam will overcome when it exceeds the required pressure. The contrivance, in nearly its usual form, (the steelyard,) was invented by Dr. Papin, in 1684, as an appendage to his apparatus for dissolving bones by steam at high pressure; but the first application of it to the steam-engine was by Savery. It received some improvement by Beighton in 1718, since whose time the same form continues to be used, as will be recognised in numerous steam apparatuses in various parts of this work. Mr. Tredgold, in his able work on the steam-engine, observes, that it would be a great improvement upon safety-valves, if they could be so constructed as to be relieved of a part of their load, when raised from their seat. With the view of effecting this object in the simplest possible way, we suggested many years ago, (see Register of Arts, etc. for January, 1829) the employment of a bent lever, instead of the straight one in common use, the action of which will be understood by reference to the subjoined diagram, wherein is also represented some other modifications of the safety-valve, which it is presumed are worthy the consideration of the practical man.
The subjoined diagram is explanatory of another mode of producing a similar result, but by different means; a is the aperture in the boiler, on which is fixed a gun-metal plate or valve seat; b is a steel cup-valve, turned rounding at the edges, resting on the seat, and suspended to a straight lever c, whose fulcrum is at d. At e, is the weight suspended to the axle of a little wheel, which is made to traverse freely the upper side of the lever c, but whereon its range may be limited by means of a sliding stop f, provided with a set screw. g is the valve box, and h a pipe to carry off the waste steam. It will now be obvious that when the steam lifts the valve, the load on the lever will move towards the fulcrum to any extent desired, and thus the boiler may be relieved in proportion to the exigency of the case. It scarcely need be remarked, that our reason for making the valves with edges pressing upon flat surfaces, was to prevent the possibility of their sticking in their seats, which, with the conical plug-valves, is a common occurrence, and one that has been productive of serious accidents.
The safety-valves employed by Woolf, are calculated to prevent adhesion to their seats, and are of great simplicity; their form is represented in the margin, Fig. 1 being a plan, and Fig. 2 a vertical section. The shape may be considered at first as a solid cylinder with a circular plate at top; three large longitudinal grooves, as shown at a a a, reduce the cylinder to the figure represented. The plug thus made, fits easily into the aperture of the boiler, and the steam which fills the grooves, pressing against the under surface of the head, raises the plug and escapes. The plug is loaded either by a weight, suspended to it inside the boiler, by weights laid directly upon the top, or by the agency of a loaded lever.
In a letter to the editor of the Leeds Mercury, Mr. Benjamin Hicks, of the steam-engine manufactory at Bolton, in Lancashire, says, "I am induced, in order to prevent the accidents occasioned by the bursting of steam-boilers, which are of such frequent occurrence, and generally so dreadful in their consequences, to send to you the drawing and description of a self-acting safety-valve, of my invention, (or rather application to a new purpose; a similar valve having been used as a clack for a pump, upwards of a hundred years ago.) You will readily perceive, from the several advantages it possesses, that wherever its adoption shall take place, it would scarcely be possible for an accident of this nature to arise.