132 



STEAM BOILERS 



tubes be found badly corroded but not yet leaking from that 

 cause, the tubes should by all means be removed and replaced. 

 Lamination. Sometimes what is called lamination, or the 

 splitting of a plate into thin layers, is revealed by the action of 

 the fire in causing a bag or blister to appear. Laminations 

 due to slag and other impurities in the metal, which become 



FIG. 3 



flattened out when the plates are rolled, are shown at a, Fig. 3. 

 Under the action of the heat the part exposed to the fire will 

 form a blister, which may finally open at the point b or c. If 

 the laminated portion of the plate is small, it may be cut out 

 and a patch put in its place. If there are a number of lamina- 

 tions in the same plate, it is advisable to put in a new plate. 



Overheating. The heating of a plate beyond its normal 

 temperature is called overheating, and may be caused by low 

 water or by incrustation. When the plate is covered by a 

 heavy scale, the plate becomes overheated, so that it yields to 

 the steam pressure, forming a pocket, as shown in Fig. 4, 

 which represents the shell sheet, or the sheet of a horizontal 

 return-tubular boiler directly over the fire. If the pocket is 



FIG. 4 



not discovered in time for the plate to be repaired, it stretches 

 until finally the material becomes too thin to withstand the 

 steam pressure; the pocket then bursts with more or less lia- 

 bility of an explosion. The vegetable or animal oils carried 

 into the boiler from a surface condenser are particularly liable 

 to cause the formation of pockets. 



