STEAM BOILERS 119 



that the accumulation of water in it will cause an extra pressure 

 to be shown. 



Bottom Blow-Off. For the double purpose of emptying 

 the boiler when necessary and of discharging the loose mud and 

 sediment that collect from the feedwater, each boiler is pro- 

 vided with a pipe that enters the boiler at its lowest point. 

 This pipe, which is provided with a valve or a cock, is com- 

 monly known as the bottom blow-off. The position of the blow- 

 off pipe varies with the design of the boiler; in ordinary return- 

 tubular boilers, it is usually led from the bottom of the rear end 

 of the shell through the rear wall. Where the boiler is fitted 

 with a mud-drum, the blow-off is attached to the drum. 



Blow-Off Cocks and Valves. While in many boiler plants 

 globe valves are used on the blow-off pipe, their use is objection- 

 able, because the valve may be kept from closing properly by a 

 chip of incrustation or similar matter getting between the valve 

 and its seat. As a result, the water may leak out of the boiler 

 unnoticed. 



Plug cocks packed with asbestos are widely used, the asbestos 

 packing obviating the objectionable features of the ordinary 

 plug cock. Gate valves are also used to some extent, but are 

 open to the same objection as globe valves. In the best modern 

 practice, the blow-off pipe is fitted with two shut-off devices. 

 The one shut-off may be an asbestos-packed cock and the other 

 some form of valve, or both may be cocks or valves, the idea 

 underlying this practice being that leakage past the shut-off 

 nearest the boiler will be arrested by the other. 



Protection of Blow-Off Pipe. When exposed to the gases of 

 combustion, the bottom blow-off pipe should always be pro- 

 tected by a sleeve made of pipe, by being bricked in, or by a 

 coil of plaited asbestos packing. If this precaution is neglected, 

 the sediment and mud collecting in the pipe, in which there is 

 no circulation, will rapidly become solid. The blow-off pipe 

 should lead to some convenient place entirely removed from 

 the boiler house and at a lower level than the boiler. Some- 

 times it may be connected to the nearest sewer. In many 

 localities, however, ordinances prohibiting this practice are in 

 force; the blow-off is then connected to a cooling tank, whence 

 the water may be discharged into the sewer. 



