STEAM BOILERS 171 



of the plate around the hole; fixing the boiler too rigidly in 

 its setting, thus causing it to be fractured on account of unequal 

 expansion; defective water circulation in a boiler, which may 

 lead to excessive incrustation and thus indirectly to 'explosion; 

 and a poorly designed feed apparatus or safety valve. Defects 

 in workmanship and material may include the use of faulty 

 material containing blisters, lamination, etc. ; careless punching 

 and shearing of plates; burning and breaking of rivets; burning 

 or otherwise injuring the plates in flanging, bending or weld- 

 ing; scoring of the plates along the joints by sharp calking 

 tools; and injury of the plates by the use of the drift pin. 

 Old boilers may, while being patched with new plates, be injured 

 by the operation of removing the old rivets and putting in 

 new ones, and also by the greater expansion and contraction 

 of the new plate as compared with the old plate. The strength 

 of the shell may be weakened by corrosion, pitting, and groov- 

 ing. In some exploded boilers, the plates have been found to 

 have wasted to little more than the thickness of wrapping 

 paper. Fractures that ultimately end in explosion may be 

 produced by letting the cold feedwater come directly into con- 

 tact with the hot plates. 



If a boiler fractures while undergoing the hydrostatic test, 

 the water escapes through the rent in the plate and no explo- 

 sion takes place, because the cold water has little or no stored 

 energy. But when a boiler filled with steam and water at a 

 high temperature fractures, a violent explosion generally 

 follows. The steam escaping through the opening diminishes 

 the pressure, and, consequently, a new body of steam is formed 

 from the water, which, by escaping, lowers the pressure still 

 more, allowing the formation of another new body of steam 

 at a lower pressure, and this operation is continued until the 

 pressure reaches that of the atmosphere. The formation of 

 several successive large bodies of steam in this way, which 

 occurs almost instantaneously, produces a 'Disastrous explosion. 

 Generally speaking, the larger the body of the contained 

 water, the more disastrous is the result. For this reason 

 water-tube boilers, which contain a relatively small amount of 

 water, are generally considered to be much safer than fire- 

 tube boilers. 



