STEAM BOILERS 



337 



FIG. 235 BALL AND LEVER SAFETY VALVE 



the least expensive, also the less reliable. It is 

 generally used upon stationary boilers. To increase the 

 pressure in the boiler before it blows off, the ball must 

 be moved farther out on the lever, and inversely to de- 

 crease the pressure. The ball should be set at the proper 

 point to blow off at the desired pressure, and then the 

 lever marked so that the point can be seen distinctly. 



Spring safety valves are generally used on traction en- 

 gines and the better class of boilers. They are more re- 

 liable and also act 

 much more quickly. If 

 properly constructed 

 they will allow the 

 pressure to fall about 

 5 pounds before clos- 

 ing, while the ball and 

 lever type only falls to 

 a trifle less than the 

 blow-off pressure. By 

 referring to Fig. 236 F1G 236 _ POP VALVE 



