Transactions A.S.M.E.

DISCUSSION OF ATTACK ON STEEL IN HIGH-CAPACITY BOILERS 717
one product of the reaction, quite literally gets in the way and
slows down the reaction to such an extent that it is scarcely noticeable.
Anyone can observe the progress of this reaction, however,
by allowing a quantity of very finely divided metallic iron to
stand in distilled water in a small bottle on his desk. On shaking
this bottle once a day, small bubbles of gas will be observed to
rise from the powder. This will continue day after day; a test
after a sufficiently long time will demonstrate that the gas is
hydrogen.
This same oxidation of steel by water, or reduction of water by
steel, goes on continuously in every boiler, but it is only when the
oxide resulting from it fails to retard the reaction that the boiler
operator has a problem on his hands.
The protective oxide film can be at least partially destroyed or
rendered more permeable by very high concentrations of sodium
hydroxide. Because it is brittle and differs in thermal expansion
from steel, it may also be cracked by repeated temperature
changes. Anything which causes excessive local concentration
of a boiler water containing some caustic soda or repeated overheating
and quenching of a tube surface is thus potentially
dangerous. To the authors, steam blanketing seems inevitably
to tend to produce one or the other or both of these effects.
The factor of caustic attack has not been questioned in the
discussion, but Mr. Place has gently indicated his disbelief in the
validity of thermocouple measurements or of changes in the
microstructure of steel as indications of excessive temperature
in the top of a steam-blanketed tube. Base-metal thermocouples
admittedly do fail all too rapidly when exposed to the conditions
in a boiler furnace, yet recent studies at the Bureau of Standards
show a maximum error of only 21 F, and this low rather
than high, when chromel-alumel couples were heated for long
periods of time in air. This maximum error was found after a
couple had been exposed for 200 hr at 2200 F .T Failure occurred
before 300 hr.
While the authors attach more significance than does Mr.
Place to the temperature measurements mentioned in the paper,
they feel that the microstructure of the steel is a still more certain
criterion of overheating. This is demonstrated particularly
well by the photomicrographs presented by Mr. Hankison correlating
the change in structure in the tube wall with the zone of
damage along the internal surface. Perhaps the sample which
inspired Mr. Place’s lack of faith in metallographic evidence actually
had been overheated at some time in some unrecorded
manner.
Mr. Place has argued that, if a tube were seriously overheated,
it would deform to a greater extent than has been observed in
many cases where grooving along the ceiling produced failure.
In a tube of 3.5 in. outside diam with a wall thickness of 0.5 in.
subjected to an internal pressure of 1400 psi, the nominal stress
tending to rupture the tube is, however, only 3500 psi for metal
in the longitudinal section of the wall. That creep of a lowcarbon
steel subjected to this stress at a temperature of 1000 F
is slight has been shown by White, Clark, and Wilson.* They
observed that SAE 1015 steel held at 1000 F for 16,000 hr under a
load of 4000 psi showed an average rate of creep of only 0.043
per cent per 1000 hr.
It must be remembered also that a narrow band of steam
along the ceiling of a tube would produce only a narrow band of
overheated metal and a narrow groove, as in Fig. 15 of the paper.
7 “Stability of Base-Metal Thermocouples in Air From 800 Degrees
to 2200 Degrees Fahrenheit,” by A. I. Dahl, Research Paper
R P 1278, N ational Bureau of Standards, Journal of Research, vol. 24,
1940, pp. 205-224.
8 “ Influence of Time at 1000 F on the Characteristics of Carbon
Steel,” by A. E. W hite, C. L. Clark, and R. L. Wilson, Proceedings of
the American Society for Testing M aterials, vol. 36,1936, p art 2, pp.
139-156, see Fig. 1, p. 141.
Deformation in such a case might be limited to a strip not more
than 1 in. wide. Where the steam blankets more of the tube
ceiling, as in the cross section shown by Mr. Hankison, there,
usually is definite stretching of the tube wall prior to failure
resulting in longitudinal cracks in the internal and external layers
of oxide.
Overheating of a tube is, of course, not a new or unique occurrence.
While we have thought for years in terms of the overheating
due to deposits of solid on a heat-transfer surface, much
less consideration has been given to the insulating effect of
a blanket of steam, which was the starting point of the paper.
The experiences at Springdale and at Waterside described
in the paper and the discussion by Mr. Hanlon bring us back to
the fact that sludge settling out on the bottom of a tube can interfere
with the transfer of heat from metal to water just as seriously
as steam along its ceiling. The sludge which caused the
failures at Waterside, reported by Mr. Hanlon, like that which
led to the failures along the bottom of the cored tubes at Springdale
was not, however, the relatively light calcium phosphate
sludge which results from phosphate conditioning but, instead, a
heavy sludge of magnetic iron oxide and metallic copper. Unless
powdered metallic iron or ferrous hydroxide is being introduced
intentionally into the boiler feed, such a sludge can result only
from undesirable attack of water on steel in some region within
the boiler, not necessarily the region where the sludge is found.
The initial settling out of the sludge on the bottom of the tubes
described by Mr. Hanlon may have been favored by the presence
of restrictors at the inlet ends of the lower tubes, as it was apparently
by the cores in the tubes at Springdale. Overheating of
the portion of the tube covered by the sludge could then have
led to progressive oxidation of the tube wall.
Attempts to increase flow in one particular region of a boiler
by specifically retarding it in another region seem, in a number
of cases, to have transferred trouble rather than to have prevented
it. Of the various expedients which have been tried to
minimize steam blanketing in tubes through which there was
positive circulation, the spirals introduced into the screen tubes at
Logan, described by Mr. Webb, seem the most promising.
The suggestion of Mr. Kreisinger that tubes in high-pressure
boilers are too thick-walled for their own good merits further
study. Reduction in wall thickness will not, however, change
the heat input to a tube or minimize the formation of a steam
blanket within it. While cracking of tubes subjected to repeated
overheating and quenching may be slower with thin than
with thick walls, as suggested by Mr. Kreisinger, the paper records
extensive damage to the thin-walled tubes of the low-pressure
boilers in the Beacon Street Heating Plant in 8 years. The
authors believe that grooving along the top of a tube might occur
as rapidly in a thin-walled as in a thick-walled tube.
It was first demonstrated at Port Washington that grooving
of the ceilings of inclined tubes could be minimized without
mechanical changes by eliminating caustic alkalinity from the
boiler water. This expedient was adopted at Logan, but the
slope of the screen tubes affected was also increased, the spirals
were inserted to throw water against the tops of the tubes, and
the burners were tilted upward to reduce flame impingement.
At Springdale, no change was made in water conditioning, but
the difficulties first encountered were eliminated solely by mechanical
changes. An appropriate conclusion to the question of
the relative importance of the chemical and the mechanical factors
has been written into the record by the announcement that,
after operation for 2Vs years with no caustic alkalinity in the
boiler water, extensive damage of the original type was recently
discovered in the screen tubes at Port Washington.*
9 “P o rt W ashington Sustains Its Economy,” Combustion, vol. 11,
no. 7, 1940, pp. 33-34.