SAFETY VALVE. 



This apparatus will evidently act in the same manner and on the 

 same principles as that already described. 



11. In different applications of the engine, steam of different 

 pressure is required. The pressure of steam is usually expressed 

 by stating the number of pounds weight upon each square inch of 

 surface which would exactly resist or balance it. All boilers are 

 provided with a valve which opens outwards, and which is loaded 

 with a certain limited and regulated weight. When the bursting 

 pressure with which the steam urges this valve exceeds the weight 

 with which it is loaded, the valve yields, is opened, and the steam 

 escapes through it, and thus continues to escape until the quantity 

 pent up in the boiler is so diminished, that its pressure upon the 

 valve no longer exceeds the weight with which the valve is 

 loaded. When this happens, the valve will remain closed, but 

 will be ready to yield and to open upon the least increase of the 

 pressure of the steam. 



Such a valve is called a " safety valve" for the obvious 

 reason that it prevents the pressure of the steam in the boiler 

 from ever attaining such a force as Avould endanger the boiler. 



It sometimes happens that it is necessary to vary from time to 

 time the pressure of the steam according to the work to which the 

 engine is applied, and consequently to vary the weight upon the 

 safety valve. In such cases it is usual to provide two safety 

 valves, one of which shall be regulated by the engineer, and the 

 other placed out of his power. The latter in that case is loaded 

 with the greatest pressure which the boiler can bear without 

 danger ; so that even though the engineer should indiscreetly load 

 the valve left at his disposition beyond the limit of safety, the 

 other valve would yield the moment the steam attained a dangerous 

 pressure. 



Safety valves are of numerous forms. They consist usually of 

 a circular aperture cut in the boiler, with conical edges inclining 

 from within outwards. In this is placed a circular plate or 

 stopper of corresponding size, with corresponding conical edges, so 

 that it shall exactly fit the aperture ; and when pressed upon it, the 

 conical edges shall be in steam-tight contact. This circular plate 

 is attached at its centre to an iron rod, which rises perpendicular 

 to it. Upon this rod sliding weights are placed so as to press 

 down the valve with a greater or less force, according as their 

 number is increased or diminished. 



In the general view of a boiler of the form called waggon boiler, 

 shown in fig. 7, the safety valve is shown at IT. It is provided 

 with a handle, by means of which the engineman can raise it 

 when necessary. 



12. It is necessary to provide a ready method of indicating at all 



9 



