STEAM BOILERS 113 



The maximum weight of combustion is thus fixed by the 

 height of the chimney; the minimum rate may be anything 

 less. 



EXAMPLE. Under ordinary conditions, what is the maxi- 

 mum rate of combustion of anthracite coal if the chimney is 

 120 ft. high ? 



SOLUTION. By formula 2, 



F = 1.5>/120- 1 = 15| Ib. per sq. ft. per hr. 



BOILER FITTINGS 



Types of Safety Valve. The safety valve is a device 

 attached to the boiler to prevent the steam pressure from rising 

 above a certain point. When steam is made more rapidly than 

 it is used, its pressure must necessarily rise; and if no means of 

 escape is provided for it, the result must be an explosion. 

 Briefly described, the safety valve consists of a plate, or disk, 

 fitting over a hole in the boiler shell and held to its place by a 

 dead weight, by a weight on a lever, or by a spring. The 

 weight or the spring is so adjusted that when the steam reaches 

 the desired pressure the disk is raised from its seat, and the 

 surplus steam escapes through the opening in the shell. 



FIG. 1 



Weight of Ball for Lever Safety Valve. A simple diagram 

 of a lever safety valve is shown in Fig. 1. The valve stem and 

 the bail are attached to the lever at C and B, respectively, 

 and the fulcrum is at F. 



