THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CVIII.

FRIDAY, NOVEMBER 20, 1914.

No. 2812.

Internal Corrosion of Colliery Boilers.

By EDWARD INGHAM, A.M.I.Mech.E.

In an article on “ Steam Boilers and Brickwork,”
which appeared in the Colliery Guardian for July 23,
1909, the conditions which usually lead to external cor-
rosion of the boiler plates were considered, and it was
pointed out that large sums of money may be spent
annually in repairs to the boilers at a large colliery if
this question of corrosion be neglected. It is now pro-
posed to supplement the article referred to by one deal-
ing with internal corrosion, i.e., wasting of the internal
surfaces of the plates.

External corrosion, it was shown, can, in the majority
of cases, be easily prevented, all that is necessary to
prevent it being to keep the external surfaces of the
plates free from damp and moisture. Internal corro-
sion, however, is not so easily prevented. Whilst in
many cases the remedy is simple, the causes of deterior-
ation are sometimes so complex that prevention is a
difficult problem, and cases are not wanting to show that
every known remedy has been tried, but without
success.

Generally speaking, internal corrosion is due to acidity
of the feed ‘water. Where good water is available, as a
rule little trouble is experienced, but in some colliery
districts, where the water pumped up from the pits
finds its way into the feed supply to the boilers, the
results are often very disastrous. The highly-coloured
water known as “ ochrey ” water, for instance, is highly
corrosive, due to the presence of free sulphuric acid,
and boilers fed with such water may be ruined in the
course of three or four years. In a case which resulted
in litigation some years ago, in the Wigan district, a
large colliery company sued another company for some
thousands of pounds damages for polluting the streams
from which their boilers had to be supplied. Boilers
had, from time to time, to be renewed, and large sums
of money had been spent in the course of two or three
years in renewing wasted plates, etc.

Internal corrosion takes a number of different forms,
of which the most common is that known as “ pitting.”
(See fig. 1.) This consists of a number of hollows or
dish-shaped holes eaten into the boiler plates to vary-
ing depths. The holes usually vary in size from |in. to
about IJin. diameter. If they occur scattered
over the plates, the pitting is said to be
“ isolated,” but if the “ pits ” are crowded closely
together, as in fig. 2, the defect is termed “ close
pitting” or “honeycombing.” Isolated pitting is
not a serious defect, because, comparatively speaking,
only a small proportion of plate is affected, and the
strength to resist the pressure is therefore not reduced
to any appreciable extent. Even should a “ pit ” be
eaten completely through the plate, no great harm would
be done, as the steam would - merely rush through the
perforated part.

Honeycombing is obviously rather more serious, but
still not of any great importance, unless the wasting
generally is very deep. The worst form of pitting is
that known as “ confluent ” pitting. (See fig. 3.) This
is brought about by the “ pits ” which have formed
originally gradually merging into one another until the
plate is generally, but irregularly, wasted. The original
surface of plate has now almost completely disappeared,
and as a large surface of plate is affected, the defect is
a serious one, particularly if the wasting is deep. If
it is suspected that the wasting is deep, it is a wise pre-
caution to drill a small hole through what appears the
thinnest part of the plate, in order that the remaining
thickness may be correctly ascertained.

A more serious form of internal corrosion than any
of those already considered is “ smooth wasting,” which
is illustrated by fig. 4. This is perhaps the worst, with
one exception, of all the internal defects from which
steam boilers suffer. As will be seen from, the illustra-

tion, the wasting consists of a uniform reduction of the
metal in both plates and rivet heads. Not only is the
whole of the plate affected, but owing to the smooth
character of the wasting, the defect is extremely liable
to escape detection. It can only be discovered by
making careful examination of the overlaps of the seams
and the rivet heads. Drilling should in all cases be
resorted to, unless it is known with certainty that the
wasting is not deep.

To detect readily such defects as confluent pitting and
smooth wasting requires an experienced person, such as
an inspector of a leading boiler insurance company. It
is a mistake for firms to allow their boilers, to be
inspected only by their engineer. The engineer may,
of course, be a thoroughly trained and practical man,
but unless he has had special experience in the work of
boiler inspection, he is liable to overlook serious defects,
which may ultimately lead to disaster. According to
the Coal Mines Act of 1911, “ every steam boiler used
for generating steam in or about a mine must, whether

separate or one of a range, be examined thoroughly by
a competent person at least once in every 14 months.”
An engineer who has had no special training in boiler
inspection cannot, strictly speaking, be regarded as a
competent person, and hence the advisability of entrust-
ing the work to a thoroughly trained boiler inspector.

Another form of internal corrosion is “ grooving,”
which is also a very dangerous defect. It is usually
caused in the first instance by alternations of pressure
and temperature within the boiler, i.e., it is brought
about by mechanical action rather than by chemical
action. The action is somewhat similar to that which
takes place when a hoop iron is repeatedly bent to and
fro : the repeated bending eventually breaks the skin
of the metal. In a steam boiler, various parts are sub-
jected to repeated bending backwards and forwards, due
to the expansion and contraction brought about by
changes of pressure and temperature within the boiler.
Take, for instance, the Lancashire boiler, now so gener-
ally used at collieries. The usual length of this type

of boiler is 30 ft. Now, it is a simple matter to show
by calculation that a boiler of this length expands
approximately | in. when the fires are lighted and steam
raised. The furnace and flue tubes probably expand a

good deal more than in. It is obvious that the greater
expansion of the tubes must be accommodated to a large
extent by the end plates, which are in consequence sub-
jected to a “ drum head ” action. This “ drum head ”
action naturally causes straining at the attachments of
the tubes to the end plates, and in time breaks the skin
of the metal, forming a fine “ groove.” (See fig. 5.)
If the feed water is corrosive, the defect becomes
aggravated by chemical action, because once the
skin of the metal is broken, the corrosive action
of the water becomes concentrated at the affected
part. The form of the groove is then altered from that
shown in fig. 5 to that shown in fig. 6, which is the
more general form.

It will be obvious that if, in the design of the boiler,
suitable provision be made for taking up the expansive
movements, the tendency to grooving will be minimised.
If, for example, the end plates be made comparatively
thin, and a certain amount of “ breathing space ” be
left between the rivets round the flanges of the tubes
and the bottom or “ toe ” rivets of the gusset stays,

Fig. 7.

tlren?' a' certain amount of flexibility is provided, and
the expansion of the furnace and flue tubes can be
accommodated without causing undue straining at the
attachments. Generally speaking, the defect, grooving,
can be largely prevented by so designing the boiler that
the various parts are sufficiently flexible to accommo-
date expansive movements in different parts of the
structure. An example of grooving in the front end
plate of a Lancashire boiler round the flange of the fur-
nace tube is shown in fig. 7.

Many instances of severe grooving at this particular
part have been cured simply by removing or cutting out
the lowest rivets in the angle irons of the gusset stays.
By so doing, a greater amount of “ breathing space ” is
provided, and the expansive movements of the furnace
and flue tubes can then be taken up without unduly
straining the tube and end plate attachments.

Grooving in the end plates of Lancashire boilers is so
common that special screwed and nutted toe screws have
been designed by certain boiler insurance companies to