[10] Annales Industrielles.
The tubes of tubular boilers must, for different reasons, be taken out
when the generator has worked for a certain length of time. Such a
necessity presents itself when the extremities of the tubes are worn
out and can no longer be fastened with sufficient tightness into the
plate, or when the portion of the tube in contact with water is so
incrusted that there results a notable diminution in the production of
steam, or when the tubes exhibit local injuries, or, finally, when the
interior of the boiler must be examined.
[Illustration: REPAIR OF BOILER TUBES.]
This latter contingency arises for every boiler after a period of
from 6 to 8 years, and it requires the removal of all the tubes. It
furnishes an occasion to remedy the other defects, that would have of
themselves required the renewal of only a certain number of the tubes.
In the interval between these thorough inspections defects may present
themselves which require the removal of a certain number of tubes. The
frequency of such repairs depends upon the nature of the feed water,
upon the quality of the fuel, upon the pressure at which the generator
operates, upon the state of repair in which the boiler is kept, and
naturally also upon the quality of the metal of which the tubes
themselves are composed.
Selenitic water deposits in the long run a very hard and adhesive
incrustation, which acts as an obstacle to the transmission of heat.
The more calcareous waters fill the intervals between the tubes with
deposits which can be but partially removed by the washing of the
boiler, and which often form a calcareous mass such as to prevent all
circulation of water around the tubes.
In both cases the tubes are heated beyond measure, elongate, detach
themselves from the tube-plates, and burn in places, or lose enough of
their resistance to allow them to become flattened by the pressure of
the steam.
The loosening of the tubes likewise acts injuriously upon the plates,
which the pressure causes to bend outwardly. The result is that the
tubes may become completely detached.
Sulphurous fuel corrodes the extremities of the tubes near the fire-box
and also notably attacks the hind extremities, in the interior, against
the tube-plate. It likewise renders brittle those tubes whose metal is
bad, so that they split either of themselves or at the least effort
made to tighten them up in the tube-plate.
In tubes made of poor metal these brittle places are not only found
near the plates, but also in other parts.
The tubes likewise have to undergo too lively a combustion when the
boilers are driven. Leakages from the tubes often proceed from the fact
that an expansion of the boiler lengthwise is prevented, or from a
cooling of the tubes by a current of air which passes, without becoming
heated, through a badly covered grate. Leakages may also occur if a
boiler that has just been emptied is filled too soon.
It will be seen that the causes of the deterioration of tubes are
numerous; and the repairs that they give rise to in railway shops are
therefore very important, and are generally known as a whole. Yet they
differ in some points of detail according to the shop in which they are
made, so that it may not be without utility to pass them in review,
in order to compare the results of the practice of several persons
pursuing the same object.
The author of this article has had, during a long experience, occasion
to make such comparisons: several of the methods that he describes were
derived by him in shops that he directed, and have been applied upon a
large scale; and numerous visits to other shops have permitted him to
see different processes and to judge of results.
The different repairs to be made in boiler tubes may be classified as
follows: