BOILERS AND GENERATION OF STEAM 305 



plicated, as a general thing, than some of the forms of fire- 

 tubular boilers and under the. best conditions for each type 

 have not shown any particular increase in economy. This 

 type is claimed, however, to be the safer of the two because it 

 contains a less amount of water. When an explosion occurs 

 the tubes simply blow out. The cause is generally defective 

 welds or the thinning of the tubes from corrosion. 



The common type of water-tube boiler is made of lap- 

 welded wrought iron tubes placed in an inclined position, 

 connected with each other and with a steam-and-water drum 

 on the top of the tubes by a vertical passage at each end. A 

 mud drum is connected to the rear and lowest point of the 

 boiler. The steam-and-water drums are made of sheets of 

 iron or steel of the desired thickness to withstand the pressure. 

 The plates are double-riveted. The mud drum is made of 

 cast iron, as this is the best material to withstand corrosion. 



The tubes are fitted by an expander into drilled holes ac- 

 curately sized and tapering at'the end connections. These con- 

 nections are in one piece for each vertical row of tubes. The 

 tubes are arranged so that each row comes over the space in 

 the previous row. 



346. Boiler Building Boilers 14 ft. or less in length are 

 constructed of two plates, each forming the entire circum- 

 ference. Above 14 ft. in length the shell is constructed in 

 three parts, i.e., three plates are required to make the length 

 of the boiler shell. These steel plates are J^, ^g, J^, or -$ 

 of an inch thick and range from 45,000 to 85,000 Ibs. per 

 square inch tensile strength. They are ordered by the boiler- 

 maker from the steelmill usually }/% in. larger than the 

 finished size required and they come to the shop perfectly 

 flat. Here they are first weighed to find out if they are up to 



