The Young Mechanic / Containing directions for the use of all kinds of tools, and for the construction of steam engines and mechanical models, including the art of turning in wood and metal

by James Lukin

THE SAFETY-VALVE.

To find out what pressure is exerted by the safety-valve, it must be clearly understood upon what principle it acts. I have in a previous chapter told you that the atmospheric pressure equals 15 lbs. on each square inch, so that if the surface of the valve which is exposed to the air is 1 inch in area or surface, it is pressed down with a force of 15 lbs. The steam, therefore, inside the boiler will not raise it until its elasticity exceeds this atmospheric pressure. If, therefore, we desire to have only just 15 lbs. per square inch pressing against the inside of the boiler (i.e., a pressure of “one atmosphere,” as it is called), we have only to load the valve so that, inclusive of its own weight, it shall equal 15 lbs. But it is plain that we must not load it at all in reality; for a flat plate, 1 inch square, of no weight, is all that is needed, the atmosphere itself being the load. Suppose, then, that we do load it with 15 lbs. in addition to the 15 lbs. with which nature has loaded it, we shall not find the steam escape until it presses with a force of 30 lbs. on the square inch, or two atmospheres (which, however, is not 30 lbs. of useful pressure upon one side of the piston, if the cylinder is open as in an atmospheric engine, but only 15 lbs.) This is not the strain which the boiler has to stand, because the atmosphere is pressing upon it and counteracting it up to the 15 lbs., so that this strain tending to burst it is but 15 lbs. The number of pounds, therefore, which is straining the boiler can readily be seen; being always that with which the safety-valve is loaded, and this is also the useful pressure for doing any required work. Unfortunately, however, even in the best constructed engines, a pressure of 15 lbs. upon the boiler by no means represents that in the cylinder. Now it would be inconvenient to place weights upon the safety-valve itself, and therefore a lever is added, as seen in the sketch, with a weight hung at one end of it. This is shown at B, Fig. 68, where a section of the valve is given with its stem passing through a guide to insure the correct motion of the valve. The lever is hinged at one end; and the rule of the pressure or weight which is brought to bear upon the valve is, that it is multiplied by the distance at which the weight hangs from the valve, compared with its distance from the hinge or fulcrum. If a weight of 7 lbs. is hung at 1, i.e., at a distance as far on that side of the valve as the fulcrum is on the other side of it, 7 lbs. will be the actual power exerted; at 2, where it is twice the distance, it will be doubled, and, as shown in the drawing, a pressure of 14 lbs. will be brought to bear upon the valve; while, if the weight is hung at 3, it will exercise a force of 21 lbs. This is very easy to understand and to remember. Sometimes (always in locomotives) the weight is removed and a spring balance is attached at the long end. Upon this is marked the actual pressure exerted; there being a nut to screw down, and thus bring any desired strain upon the spring. Mind, however, in case you should try this in any of your models, that the scale marked on the balance when you buy it must be multiplied, as before, according to the length of your lever. Thus, if I attach such a balance at 3 of the drawing, a real weight of 5 lbs. shown by the balance will be 3 × 5, or 15 lbs. upon the valve, and a balance made for such engine would be marked 15 lbs., to prevent the possibility of dangerous error.