BOILERS AND GENERATION OF STEAM 311 



The manhole, through which it is necessary to enter to 

 inspect the inside of the boiler, is cut in the top or in one 

 of the heads, and is made steam-tight by a rubber gasket. 

 Hand-hole plugs are located in the bottom of the front and 

 rear heads for the purpose of permitting the boiler to be 

 cleaned. The blow-off is connected, at the bottom of the shell 

 at the rear end, with a valve on the pipe outside of the brick- 

 work called the blow-off valve. This valve is designed to 

 empty the boiler and should be used every morning, so that 

 the sediment that has settled at the bottom of the boiler over- 

 night may be blown out. The boiler should be emptied and 

 washed out at least once a month. 



352. Safety Valves. As the cylinder of the boiler is made 

 to stand a certain pressure, any excess may cause it to burst. 

 Therefore it is essential that the fireman should know when 

 that pressure is exceeded. Various devices have been de- 

 signed to give the fireman warning. Among these are the 

 safety valve, the pressure gauge, and the water gauge. 



A boiler usually has two safety valves, a water gauge, and 

 a pressure gauge. The function of a safety valve is to relieve 

 the boiler of all p ressure in excess of that allowed . The valve is 

 placed at the top of the boiler and piped outside. As it often- 

 times becomes corroded and sticks, it should be tried every day. 



The size of the safety valve is a very important matter, 

 and is determined by the area of the grate, the weight of fuel 

 burned, and the steam pressure. The amount of steam gen- 

 erated in a given time will depend upon the weight of coal 

 burned, while the velocity of escape through the valve will 

 depend upon the pressure. Low pressure safety will not run 

 higher than 30 Ibs. The figure stamped on the lever shows 

 the limit. 



