II. EXPLOSIONS CAUSED BY DEFECTIVE AND OVER-LOADED SAFETY-VALVES. The percentage of explosions from these causes is high, and goes to show that what are invariably named "safety"valves should frequently be considered danger-valves. The design and construction of safety-valves are often such as to render the keeping of them in order a matter of great difficulty, and they are sometimes in such positions that the shutting of a tap or steam-valve will cut off their connection with the boilers for which they are intended to act. In addition to these drawbacks, it is notorious that there are no mountings so much abused as safety-valves. They are found loaded and overloaded by all kinds of weights, and frequently inoperative by wedging, corrosion, or neglect. Such conduct betokens either great ignorance or a total disregard of the serious risks incurred, and in any case those who are guilty of it cannot be too strongly condemned, or too quickly dispensed with. Lever Safety-Valves are most affected by such treatment, for they are easily overloaded, owing to the multiplying power of the lever, by which a slight addition to the weight may increase the load on the valve to a serious extent. They are usually fitted with close-top lever guards, which have the appearance of being specially designed to facilitate the wedging-down process, and, in addition to the chances of corrosion at spindles and valves, the fulcrum joints are liable to become furred up to such an extent as to make the levers immovable. Notwithstanding these objections to their use, there are few boilers that are not fitted with one or more lever safetyvalves, and as many mechanics and boiler-attendants are not conversant with the rules for determining the weights. and dimensions of these valves, the following examples may be of service : : Fig. 1 represents an ordinary lever valve loaded by means of one weight fixed at the extremity of the lever. L F = the length of lever in inches from the fulcrum to the point at which the ball or weight is suspended. = the length of lever in inches from fulcrum to centre of valve. G = the length of lever in inches from fulcrum to centre of gravity. A = area of valve in square inches, square of diameter x 7854. W weight of ball in lbs. = P = pressure in lbs. per square inch at which the valve should blow. The lengths L, F, and G should be taken as accurately as possible, and the weights W, wl, and wv should be taken separately. The lever should be balanced on a knife-edge to find centre of gravity, and having obtained the diameter of the valve, refer to the table on page 24 for its area. The formula for determining the pressure at which a valve should begin to blow, is as follows: = (3 inches diameter), W = 565 lbs., wl 8.5 lbs., and wv = 2 lbs. If the valve is working freely, a pressure of 60 lbs. per square inch would be obtained when L 27 inches, and the load on safety-valve at any distance less than 27 inches will bear the same proportion to that of 60 lbs. per square inch, as the distance to which the weight has been moved will. bear to 27 inches-thus, when L is reduced to 22 inches, or one-sixth less than 27 inches, the load on valve will be 50 lbs. per square inch, or one-sixth less than 60 lbs., and so on. When it is required to know at what point the weight or ball should be set to obtain a certain pressure, the formula is A x P x F-[(wl × G) + (wv × F)] L = W and taking the pressure obtained (60 lbs.), with the figures given in the first example, the formula is worked out thusAX PX F=8·29 × 60 × 3.25=1616.55 from which deduct (wl × G)+(wv× F) = (8·5 × 10)+(2 × 3·25) = = 91.5 Divide the remainder by W = 56.5 To find the weight of ball required to give a certain pressure, the formula is the same as in the preceding example, with the exception that L becomes the divisor thus, W = A x P x F-[(wl × G) + (wv × F)] L When balanced levers are employed, the terms wl and uv in the preceding formulæ are not taken into account. Fig. 2 represents a balanced lever, the weight D at fulcrum end being equal to the combined weights of lever, valve, and connections; and it follows that W, which is 565 lbs. in the foregoing example, must be increased by the amount of the effective weight due to lever, valve, and connections to obtain the same pressure-thus, W = A x P x F = 8.29 × 60 × 3·25 = 59.27 lbs. It will be seen from these examples of lever-valves, that the pressure at which they will blow may be seriously affected by even a slight alteration of their dimensions or weights; and it should also be understood that unless the levers, valves, and spindles are quite free in their action, it is impossible to arrive at a correct idea of the steam-pressure necessary to lift them. Lever-valves loaded by "Salter's" spring balances are usually proportioned as illustrated by Fig. 3. The length of the fulcrum (F) is equal to the diameter of the valve, and the total length of the lever (L) is equal to the diameter multiplied by the area (A), the index on the balance being graduated in lbs. By arranging the proportions in this way, a pressure of 1 lb. per square inch on the valve is obtained by each 1 lb. at the end of the lever, and it follows that the number to which the index-finger on the balance points, represents the pressure per square inch on the valve. The formula for calculating the pressure is the same as already explained, the "Salter's" balance being |