The Locomotive - Lighthouse Survival Blog

44 THE LOCOMOTIVE. [Makch,
should have to apply a tensile strain of 37,700 pounds per square inch before Ave could
pull it back to its original length, provided the steel stretched always at the same rate
as it does when small forces are applied. This means that under the conditions we
have imagined the feed-water would jM-oduce in the boiler shell a ten.sile strain of
37,700 pounds jier square inch; and this would be sufRcient to tear the girth joints
apart at once.
Now, as a matter of fact, we know that although feed-water does a great deaJ,of
damage when discharged against boiler plates in the way we have described, we know
equally well that feed-pipes are often put in as though such an action were specially
desired and invited ; and we know that although seams are usually started and other
damage done, imder these conditions, yet the strains that are set up do not actually
pull the joints suddenly apart, as our calculation indicates. We shall therefore have to
consider why the full calculated strain is not realized in practice. One of the principal
reasons is, that although the chilled spot is in large measure prevented from contracting
by the surrounding parts of the plate, it is not altogether prevented. There is
undoubtedly an elastic yielding of the metal for a considerable distance around the place
where the feed is discharged ; for we have to remember that the whole boiler is elastic
in the same degree as the strip we have been considering. As soon as the chilling
occurs and the plate tries to contract locally, the pull is transmitted through the shell
to a considerable distance,— let us say for thirty inches in every direction— diminishing
in intensity in proportion as we go away from the center of disturbance. If a strip of
metal 10 inches long be chilled as we have supposed, and be held at each end by strips
30 inches long whose remote extremities are held rigidly, and if, further, we suppose the
strain to be uniform throughout the 10-inch central part and to diminish uniformly
as we pass outward along the retaining strips until it becomes nothing at their outer
ends, then it is easy to show, by a little mathematics, that the stretch of the two 30-inch
strips will relieve the strain on the central part so as to bring it from 37,700 pounds per
square inch down to 9,470 pounds per square inch. This state of things does not hold
strictly true in the boiler shell, because there the strains radiate from a center in all
directions ; but it is probabh' apj^roximately true because those strains which run girth-
wise ])robably relieve themselves by causing the shell to straighten out a little for the
moment. This view is also sustained by the fact that cold feed-water rarely affects the
longitudinal seams, although its cflFects are very marked on the girth seams.
The principal objection to the process above considered seems to be that we cannot
say with certainty at what distance from the feed-pipe the strains- cease to be felt.
However, we may feel pretty confident that in the case of the boiler we have been
considering, a longitudinal strain of .somewhere in the neighborhood of 8,000 or 10,000
pounds per squa^-e inch may l)e produced by the feed-water striking directly upon the
plates ; and this, in addition to the normal strain produced by the steam pressure, is
quite enough to tax the girth seams beyond their elastic limit, if the feed-pipe discharges
anywhere near them. Hence it is not surpri.sing that the girth seams develop leaks
and cracks in 99 cases out of every hundred in which the feed discharges directly upon
the fire-sheets. In some cases the local strains maj' not be felt as far away from the feed-
pipe as 30 inches, and then we should expect them to be greater in amount and
correspondingly more severe in their effects.
The reality and the magnitude of the temperature strains in bodies will be readily
conceded by those who have had experience with .steel hardened by plunging it in
water while hot. It is well known that such treatment is apt to warp the steel out of