When a boiler is under steam of say eighty pounds per square inch, the body of water in it will have a temperature of about 324 degrees Fahr., and the shell plates will necessarily be somewhat hotter, especially on the bottom (just how much hotter will depend entirely upon the quantity of scale or sediment present).  Now introduce a large volume of cold water through an opening in the bottom, and what becomes of it?  Does it rise at once, and become mixed with the large body of water in the boiler?  By no means.  It cannot rise until it has become heated, for there is a great difference between the specific gravity of water at 60 deg., or even 212 deg.  Fahr., and water at 324 deg..  Consequently, it “hugs” the bottom of the boiler, and flows toward the front end, or hottest portion of the shell.  Now let us examine the effect which it produces.

We know that wrought iron expands or contracts about 1 part in 150,000 for each degree that its temperature is raised or lowered.  This is equivalent to a stress of one ton per square inch of section for every 15 degrees.  That is, suppose we fix a piece of iron, a strip of boilerplate, for instance, 1/4 of an inch thick and 4 inches wide, at a temperature of 92 degrees Fahr., between a pair of immovable clamps.  Then, if we reduce the temperature of the bar under experiment to that of melting ice, we put a stress of four tons upon it, or one ton for each inch of its width.

[Illustration:  FIG. 1]

Now this is precisely what happens when cold water is fed into the bottom of a boiler.  We have the plates of the shell at a temperature of not less, probably, than 350 deg.  Fahr.  A large quantity of cold water, often at a temperature as low as 50 deg.  Fahr., is introduced through an opening in the bottom, and flows along over these heated plates.  If it could produce its full effect at once, the contraction caused thereby would bring a stress of 300 / 15 = 20 tons per square inch upon the bottom plates of the shell.  But fortunately it cannot exert its full effect at once, but it can act to such an extent that we have known it to rupture the plates of a new boiler through the seams on the bottom no less than three times in less than six weeks after the boilers were started up.

The effect in such cases will always be the most marked, especially if the plant is furnished with a heater, when the engine is not running, for then, as no steam is being drawn from the boilers, there is comparatively little circulation going on in the water in the boiler, and the water pumped in, colder than usual from the fact that the heater is not in operation, spreads out in a thin layer on the lowest point of the shell, and stays there, and keeps the temperature of the shell down, owing to the fires being banked or the draught shut, while the larger body of water above, at a temperature of from 300 to 325 degrees, keeps the upper portion of the shell at its higher