-1/4 inches and, as a consequence, nearly the
entire tube surface is inaccessible. When scale forms upon such tubes it
is impossible to remove it completely from the inside of the boiler and
if it is removed by a turbine hammer, there is no way of knowing how
thorough a job has been done. With the formation of such scale there is
danger through overheating and frequent tube renewals are necessary.

[Illustration: Portion of 29,000 Horse-power Installation of Babcock &
Wilcox Boilers in the L Street Station of the Edison Electric
Illuminating Co. of Boston, Mass. This Company Operates in its Various
Stations a Total of 39,000 Horse Power of Babcock & Wilcox Boilers]

In Scotch marine boilers, even with the engines operating condensing,
complete tube renewals at intervals of six or seven years are required,
while large replacements are often necessary in less than one year. In
return tubular boilers operated with bad feed water, complete tube
renewals annually are not uncommon. In this type of boiler much sediment
falls on the bottom sheets where the intense heat to which they are
subjected bakes it to such an excessive hardness that the only method of
removing it is to chisel it out. This can be done only by omitting tubes
enough to leave a space into which a man can crawl and the discomforts
under which he must work are apparent. Unless such a deposit is removed,
a burned and buckled plate will invariably result, and if neglected too
long an explosion will follow.

In vertical fire-tube boilers using a water leg construction, a deposit
of mud in such legs is an active agent in causing corrosion and the
difficulty of removing such deposit through handholes is well known. A
complete removal is practically impossible and as a last resort to
obviate corrosion in certain designs, the bottom of the water legs in
some cases have been made of copper. A thick layer of mud and scale is
also liable to accumulate on the crown sheet of such boilers and may
cause the sheet to crack and lead to an explosion.

The soot and fine coal swept along with the gases by the draft will
settle in fire tubes and unless removed promptly, must be cut out with a
special form of scraper. It is not unusual where soft coal is used to
find tubes half filled with soot, which renders useless a large portion
of the heating surface and so restricts the draft as to make it
difficult to burn sufficient coal to develop the required power from
such heating s