134 STEAM BOILERS 



fresh water can be prevented by giving a protective coating to 

 the metal, which may be a thick red -lead paint made up with 

 boiled linseed oil, or a thin coating of scale. Sometimes organic 

 substances containing tannic acid, such as oak bark, hemlock, 

 or sumac, are used to loosen or prevent scale. They are liable 

 to injure the plates by corrosion and hence should not be 

 used. The preceding table gives a list of troublesome scale- 

 forming substances and the means of preventing or neutralizing 

 them. 



Use of Zinc in Boilers. Zinc is much used in marine boilers 

 for the prevention of both incrustation and corrosion. The 

 scale may acquire thickness and hardness, but can easily be 

 removed from the plates. The zinc is distributed through the 

 boiler in the form of slabs. About 1 sq. in. of zinc surface 

 should be supplied for every 50 Ib. of water in the boiler. 



TESTING OF FEEDWATER 



Testing for Corrosiveness. It is a good plan to test the 

 feedwater and also the water in the boiler occasionally for cor- 

 rosiveness. This may be done by placing a small quantity in 

 a glass and adding a few drops of methyl orange. If the sam- 

 ple of water is acid, and hence corrosive, it will turn pink. If 

 it is alkaline, and hence harmless, it will be yellow. The 

 acidity may also be tested by dipping a strip of blue litmus 

 paper in the water. If it turns red, the water is acid. This 

 method is not so sensitive as the previous one, which should be 

 used in preference. If litmus paper is kept in stock, it should 

 be kept in a bottle with a glass stopper, as exposure to the 

 atmosphere will cause the paper to deteriorate. If the water 

 in the boilers has become corrosive and corrosion has set in, 

 the water in the gauge glass will show red or even black. As 

 soon as the color is beyond a dirty gray or straw color, it is 

 advisable to introduce lime or soda to neutralize the acid. 



Testing for Carbonate of Lime. Pour some of the water 

 to be tested into an ordinary tumbler. Add a little ammonia 

 and ammonium oxalate, and then heat to the boiling point. 

 If carbonate of lime is present, a precipitate will be formed. 



Testing for Sulphate of Lime. Pour some of the feedwater 

 into a tumbler and add a few drops of hydrochloric acid. 





