STEAM BOILERS 357 



the flues, it will be safe to pump in more, but if the water 

 is low, let the boiler cool down somewhat before filling. 



A boiler is more likely to foam with a high-water level 

 than with a low. It is also more likely to foam with low 

 pressure than high. A sudden strain on an engine will 

 sometimes cause the boiler to foam. If a boiler is likely 

 to foam, it is advisable to carry low water and high pres- 

 sure. Then if it still persists in foaming, shut down and 

 pump in a quantity of water and allow some to run out. 

 This will change the water. If this does not remedy it, 

 the boiler must be cleaned. 



495. Low water. Should the water happen to get be- 

 low the danger line in a boiler, immediately cover the fire 

 with ashes, dirt, or even fresh coal, and as soon as it can 

 be drawn without increasing the heat do so. But never 

 draw the fire until it is in this condition. Do not start 

 the feed pump, or start or stop the engine, or open the 

 safety valve. Simply let it cool down. After it has be- 

 come cool, then examine it for injuries. 



If a failure of the injector or pump has caused the 

 water to become low and there is still an inch over the 

 flues or crown sheet, the engine should be shut down and 

 attention given to the feed supply. When the water has 

 become so low as this, do not try to repair the injector 

 or pump with the engine still running, as it will run the 

 water below the crown sheet before it is anticipated and 

 thus make the boiler more dangerous. 



496. Corrosion and incrustation. It is practically im- 

 possible for an engineer to get for his boilers water which 

 does not have some detrimental ingredients. Nearly all 

 hard waters will form some sort of scale. While soft 

 waters do not do this, they do contain acids which act 

 on the boiler and fittings in a harmful manner. 



The general impurities to contend with are the car- 



