Boilers are tested in two ways: (1) by hydraulic pressure, and (2) by the hammer test. The hydrostatic test consists in filling the boiler with water and then exerting by means of a boiler test-pump (Fig. 163) one-half more pressure than the boiler is expected to carry. For instance, if it is expected to carry 100 lbs. pressure, it is tested up to 150 lbs. The hammer test is made by going over the boiler and tapping it with a small hammer. An experienced ear can tell by the sound of a blow and by the feel of the iron whether a weakness has developed. Corrosion and strains from expansion and contraction are liable to cause a decrease in the strength of a steam boiler. Corrosion, which may be either internal or external, is the wasting away of the material of the boiler by pitting, grooving, etc. Internal corrosion is mainly caused by the action of oxygen, minerals, or acids in the water. External corrosion takes place generally through rusting and from the action of sulphur in the fuel. Under certain conditions this sulphur attacks the metal when the boiler is "starting up" or "cooling down," a time at which the gases are much reduced in temperature.
Boilers should be fed with hot water. Cold water tends to reduce the temperature of the water already in the boiler, particularly in the parts near the opening of the feed pipe, causing these parts to contract. This contraction strains the seams and the plates more or less severely, according to the temperature and volume of the water introduced. Draughts of cold air have the same effect, often resulting in leaky tubes and seams.
Fig. 163. - Boiler Test-Pump.