Safety--The most important requirement of a steam boiler is that it
shall be safe in so far as danger from explosion is concerned. If the
energy in a large shell boiler under pressure is considered, the thought
of the destruction possible in the case of an explosion is appalling.
The late Dr. Robert H. Thurston, Dean of Sibley College, Cornell
University, and past president of the American Society of Mechanical
Engineers, estimated that there is sufficient energy stored in a plain
cylinder boiler under 100 pounds steam pressure to project it in case of
an explosion to a height of over 3½ miles; a locomotive boiler at 125
pounds pressure from one-half to one-third of a mile; and a 60
horse-power return tubular boiler under 75 pounds pressure somewhat over
a mile. To quote: "A cubic foot of heated water under a pressure of from
60 to 70 pounds per square inch has about the same energy as one pound
of gunpowder." From such a consideration, it may be readily appreciated
how the advent of high pressure steam was one of the strongest factors
in forcing the adoption of water-tube boilers. A consideration of the
thickness of material necessary for cylinders of various diameters under
a steam pressure of 200 pounds and assuming an allowable stress of
12,000 pounds per square inch, will perhaps best illustrate this point.
Table 1 gives such thicknesses for various diameters of cylinders not
taking into consideration the weakening effect of any joints which may
be necessary. The rapidity with which the plate thickness increases with
the diameter is apparent and in practice, due to the fact that riveted
joints must be used, the thicknesses as given in the table, with the
exception of the first, must be increased from 30 to 40 per cent.
In a water-tube boiler the drums seldom exceed 48 inches in diameter and
the thickness of plate required, therefore, is never excessive. The
thinner metal can be rolled to a more uniform quality, the seams admit
of better proportioning, and the joints can be more easily and perfectly
fitted than is the case where thicker plates are necessary. All of these
points contribute toward making the drums of water-tube boilers better
able to withstand the stress which they will be called upon to endure.
The essential constructive difference between water-tube and fire-tube
boilers lies in the fact that the former is composed of parts of
relatively small diameter as against the large diameters necessary in
the latter.
The factor of safety of the boiler parts which come in contact with the
most intense heat in water-tube boilers can be made much higher than
would be practicable in a shell boiler. Under the assumptions considered
above in connection with the thickness of plates required, a number 10
gauge tube (0.134 inch), which is standard in Babcock & Wilcox boilers
for pressures up to 210 pounds under the same allowable stress as was
used in computing Table 1, the safe working pressure for the tubes is
870 pounds per square inch, indicating the very large margin of safety
of such tubes as compared with that possible with the shell of a boiler.