SAFETY-VALVES.] 



APPLIED MECHANICS. 



853 



turn part of it into steam, exerting unlimited pressure, 

 there would be constant risk of explosion, unless some 

 measures were taken for limiting the force of the steam, 

 and preventing its pressure from exceeding that which 

 the boiler could safely sustain. Every boiler is, there- 

 fore, fitted with one or more safety-valves, which 

 constitute the most important of the boiler fittings. 



The principle of the safety-valve is exceedingly simple. 

 As fluids, and therefore steam, press equally in all 

 directions, any part of the boiler-casing, such as 1 square 

 inch, is subjected to the pressure of the steam. If, then, 

 we make a hole in the upper part of a boiler 1 square 

 inch in area, and cover it with a lid, laying on this lid a 

 weight such as 50 Ibs. , we can apply heat and generate 

 steam, which, as soon as its pressure exceeds 50 Ibs. per 

 square inch, will lift the loaded lid, and permit a portion 

 to escape. Should the generating power of the boiler be 

 moderate, the raising of the lid, and escape of a portion 

 of steam, would prevent the pressure from ever exceed- 

 ing that due to the weight on the lid ; but should the 

 steam be generated more rapidly than it can escape 

 through the hole, the pressure must go on accumulating, 

 until the strain to which it subjects the boiler exceeds 

 the strength of the material of which it is made, and an 

 explosive rupture is the consequence. It is, therefore, 

 important to provide a safety-valve, with an opening of 

 sutiicient size to permit the escape of steam as rapidly 

 as it can ever be generated, and to load it with a weight 

 not greater than the pressure which the boiler can safely 

 bear. Boilers are generally tested before use, under a 

 pressure very much greater than that with which they 

 are to be used. For condensing engines, the pressure 

 seldom exceeds 20 Ibs. per square inch ; for ordinary 

 non-condensing engines, 50 Ibs. or 60 Ibs. per square 

 inch ; and for locomotives, it is as high as 120 Ibs. and 

 150 Ibs. per square inch above that of the atmosphere. 

 The area of the safety-valve should not be less than Jth 

 square inch per horse-power. 



Fig. 100 represents a safety-valve of the ordinary 

 construction. A is a box fixed over a hole B in 

 the upper surface of the boiler, having a truly-faced 

 seating on which the valve C can rest. The stem of the 

 valve passes through the cover of the box, where there 

 is a gland or stuffing-box to prevent the escape of steam 

 round the stem. A pipe D conducts the steam that 

 passes the valve, when it is lifted, to the chimney or 

 elsewhere. The valve is kept down by a lever E, which 

 works on a pin, or fulcrum, at F, and has a sliding 

 weight suspended from it at any point such as G. The 

 arm of the lever is graduated so that the weight can be 

 placed to give such pressure on the valve as may be re- 

 quired. If we suppose, for example, that the area of 

 the valve-opening is 1 square inch, that the length from 

 the centre of the valve-stem to that of the pin F is 2 

 inches, and that a weight of 10 Ibs. hangs at G, 16 inches 

 from F ; then the effect of the weight to press down the 

 stem of the valve is as its weight multiplied by the 

 length of lever at which it acts, divided by the length of 



Kg. 160. 



of tho valve is 1 square inch, this weight is capable of 

 resisting a steam-pressure of 80 Ibs. per square inch 

 within the boiler. If the lever be graduated by divisions 

 each 2 inches in length, each of these will correspond to 

 a pressure of 10 Ibs. on the valve ; that is to say, the 

 weight at 



16 inches gives a pressure of 80 Ibs. 



>i '" ,, 



12 ,, ,, ,, 60 ,, and so on. 



We have not reckoner! the effect of the weight of the 

 valve and lever, which should generally be weighed, so 

 that the pressure due to them, exclusive of the weight, 

 may be estimated before graduating the lever. As a 

 practical example, we will suppose that it is required to 

 make a safety-valve of 4 inches diameter, and load it by 

 a weight and lever graduated to steam-pressures varying 

 from 20 Ibs. to 50 Ibs. per square inch above atmospheric 

 pressure. We will suppose that the weight of the valve 

 and stem is 5 Ibs., that a convenient leverage for the 

 valve is 3 inches, and that the whole lever from F to the 

 end is 30 inches, the lever itself being of uniform depth 

 and thickness, and weighing 9 Ibs. 



The area of the valve (a circle 4 inches in diameter) is 

 12i square inches ; the effect of the weight of the lever 

 is the same as if it were collected at its middle point H, 

 15 inches from F, and its 



Pressure on the valve is therefore 



9 Ibs. X 1 5 in. 



-- : 



3 111. 



= 45 Ibs. 



To which we add the weight of valve and stem 



Making a total constant weiglit on the valve = . 50 ,, 

 And as the area of the valve is 12 J Ibs., this gives a con- 

 stant pressure of - - = 41bs. per square inch. For a 



1J f 



pressure of 50 Ibs. per square inch, or a load of 59 X 12 |j 

 = 625 Ibs. on the valve, the additional load must be 

 575 Ibs. at a leverage of 30 inches against that of the 

 valve at 3 inches ; and therefore a weight of 67lbs. at 

 the end of the lever gives the required pressure ; Ibecause 



57$ Ibs. x 30 

 3 



' 575 Ibs. on the valve. The same weight, 



lover at which the valve acts ; that is to say, the pressure 



10 Ibs. X 16 inches _., . ,, 



the valve is - _ = 80 Ibs. As the area 



2 inches 



on 



to give a pressure of 40 Ibs. per square inch, should act 

 on the valve with a force of 12i X 40 50 = 450 Ibs ; and 

 its distance from F must be about 23-48 inches, because 



57iX 23-48 inches 



= 4uO. Now the difference between 

 o 



30 inches, the leverage for 50 Ibs., and 23-48 inches, the 

 leverage for 40 Ibs., is 6 '52 inches a division that may 

 be repeated along the lever for 30 and 20 respectively. 



We might attain the same result by another process, 

 thus : Since the constant pressure due to the weiglit 

 and valve is 4 Ibs. per square inch, the additional pressure 

 to be derived from the weight of 57^ Ibs., to make a 

 total of 10 Ibs. per square inch, would be 6 Ibs. per 

 square inch, or 6 X 12 J = 75 Ibs. in 

 all. The leverage of the weight to pro- 

 duce this load would be found from the 

 simple proportion : 



Weight. ? 



57ilbs. : 75 : 



Repeating the same process for 50 Ibs. 

 pressure per square inch, we should find 

 Ba the leverage of the weight to bo 30 

 inches. The difference of 30 inches 

 and 3-913 inches, viz., 26 087 inches, 

 being divided into 40 equal parts, eacii 

 0'652 inches because 40 is the dif- 

 ference between 50 Ibs. and ]01bs. 

 would mark the lever for each Ib. of 

 pressure. Every 10 Ibs. would thus be graduated by in- 

 tervals of 10 X '652= 6 "52 inches as before. 



In locomotives and boilers where a weight sliding along 

 a lever ^would be inconvenient, the lever is affixed to a 



Leverage of Leverage of 

 valve. weight. 



3 in. : 3 913 in. 



