130 



STEAM BOILERS 



Removal of Scale by Chipping. A hard scale, when once 

 formed, is generally removed by chipping it off with scaling 

 hammers and scaling bars; soft scale can be largely removed 

 during running by a periodic use of the bottom and surface 

 blow-offs, and the remainder can usually be washed out and 

 raked out when the boiler is blown down and opened. In 

 order to prevent the scale-forming substances deposited on 

 the metal from baking hard, it is advisable to let the boiler 

 cool down slowly until entirely cold preparatory to blowing 

 off, whenever circumstances permit this to be done. This 

 cooling process will generally take from 24 to 36 hr. 



Removal of Mud. Mud and earthy matter by itself will 

 not form any hard scale, but will often do so when carbonate 

 of lime and sulphate of lime are present. An accumulation 

 of such matter can be prevented, and most of it can be 

 removed, by a periodic use of the bottom blow-off, removing 

 the remainder whenever the boiler is opened. 



Internal Corrosion. Corrosion of boiler plates may be defined 

 as the eating away or wasting of the plates due to the chemical 

 action of water. Corrosion may be internal and external. 



FIG. 1 



FIG. 2 



Internal corrosion may present itself as uniform corrosion, 

 pitting or honeycombing, and grooving. 



Uniform Corrosion. In cases of uniform corrosion large areas 

 of plate are attacked and eaten away. There is no sharp line 

 of division between the corroded part and the sound plate. 



