PLATE THICKNESS REQUIRED
FOR VARIOUS CYLINDER
DIAMETERS
ALLOWABLE STRESS,
12000 POUNDS PER
SQUARE INCH,
200 POUNDS GAUGE
PRESSURE, NO JOINTS
+---------+-----------+
|Diameter | Thickness |
|Inches | Inches |
+---------+-----------+
| 4 | 0.033 |
| 36 | 0.300 |
| 48 | 0.400 |
| 60 | 0.500 |
| 72 | 0.600 |
| 108 | 0.900 |
| 120 | 1.000 |
| 144 | 1.200 |
+---------+-----------+
A further advantage in the water-tube boiler as a class is the
elimination of all compressive stresses. Cylinders subjected to external
pressures, such as fire tubes or the internally fired furnaces of
certain types of boilers, will collapse under a pressure much lower than
that which they could withstand if it were applied internally. This is
due to the fact that if there exists any initial distortion from its
true shape, the external pressure will tend to increase such distortion
and collapse the cylinder, while an internal pressure tends to restore
the cylinder to its original shape.
Stresses due to unequal expansion have been a fruitful source of trouble
in fire-tube boilers.
In boilers of the shell type, the riveted joints of the shell, with
their consequent double thickness of metal exposed to the fire, gives
rise to serious difficulties. Upon these points are concentrated all
strains of unequal expansion, giving rise to frequent leaks and
oftentimes to actual ruptures. Moreover, in the case of such rupture,
the whole body of contained water is liberated instantaneously and a
disastrous and usually fatal explosion results.
Further, unequal strains result in shell or fire-tube boilers due to the
difference in temperature of the various parts. This difference in
temperature results from the lack of positive well defined circulation.
While such a circulation does not necessarily accompany all water-tube
designs, in general, the circulation in water-tube boilers is much more
defined than in fire-tube or shell boilers.
A positive and efficient circulation assures that all portions of the
pressure parts will be at approximately the same temperature and in this
way strains resulting from unequal temperatures are obviated.
If a shell or fire-tubular boiler explodes, the apparatus as a whole is
destroyed. In the case of water-tube boilers, the drums are ordinarily
so located that they are protected from intense heat and any rupture is
usually in the case of a tube. Tube failures, resulting from blisters or
burning, are not serious in their nature. Where a tube ruptures because
of a flaw in the metal, the result may be more severe, but there cannot
be the disastrous explosion such as would occur in the case of the
explosion of a shell boiler.
To quote Dr. Thurston, relative to the greater safety of the water-tube
boiler: "The stored available energy is usually less than that of any of
the other stationary boilers and not very far from the amount stored,
pound for pound, in the plain tubular boiler. It is evident that their
admitted safety from destructive explosion does not come from this
relation, however, but from the division of the contents into small
portions and especially from those details of construction which make it
tolerably certain that any rupture shall be local. A violent explosion
can only come from the general disruption of a boiler and the liberation
at once of large masses of steam and water."
Economy--The requirement probably next in importance to safety in a
steam boiler is economy in the use of fuel. To fulfill such a
requirement, the three items, of proper grate for the class of fuel to
be burned, a combustion chamber permitting complete combustion of gases
before their escape to the stack, and the heating surface of such a
character and arrangement that the maximum amount of available heat may
be extracted, must be co-ordinated.
Fire-tube boilers from the nature of their design do not permit the
variety of combinations of grate surface, heating surface, and
combustion space possible in practically any water-tube boiler.
In securing the best results in fuel economy, the draft area in a boiler
is an important consideration. In fire-tube boilers this area is limited
to the cross sectional area of the fire tubes, a condition further
aggravated in a horizontal boiler by the tendency of the hot gases to
pass through the upper rows of tubes instead of through all of the tubes
alike. In water-tube boilers the draft area is that of the space outside
of the tubes and is hence much greater than the cross sectional area of
the tubes.
Capacity--Due to the generally more efficient circulation found in
water-tube than in fire-tube boilers, rates of evaporation are possible
with water-tube boilers that cannot be approached where fire-tube
boilers are employed.
Quick Steaming--Another important result of the better circulation
ordinarily found in water-tube boilers is in their ability to raise
steam rapidly in starting and to meet the sudden demands that may be
thrown on them.
In a properly designed water-tube boiler steam may be raised from a cold
boiler to 200 pounds pressure in less than one-half hour.
For the sake of comparison with the figure above, it may be stated that
in the U. S. Government Service the shortest time allowed for getting up
steam in Scotch marine boilers is 6 hours and the time ordinarily
allowed is 12 hours. In large double-ended Scotch boilers, such as are
generally used in Trans-Atlantic service, the fires are usually started
24 hours before the time set for getting under way. This length of time
is necessary for such boilers in order to eliminate as far as possible
excessive strains resulting from the sudden application of heat to the
surfaces.
Accessibility--In the "Requirements of a Perfect Steam Boiler", as
stated by Mr. Babcock, he demonstrates the necessity for complete
accessibility to all portions of the boiler for cleaning, inspection and
repair.
Cleaning--When the great difference is realized in performance, both as
to economy and capacity of a clean boiler and one in which the heating
surfaces have been allowed to become fouled, it may be appreciated that
the ability to keep heating surfaces clean internally and externally is
a factor of the highest importance.
Such results can be accomplished only by the use of a design in boiler
construction which gives complete accessibility to all portions. In
fire-tube boilers the tubes are frequently nested together with a space
between them often less than 1¼ 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 surface as is not covered by soot.
Water-tube boilers in general are from the nature of their design more
readily accessible for cleaning than are fire-tube boilers.
Inspection--The objections given above in the consideration of the
inability to properly clean fire-tube boilers hold as well for the
inspection of such boilers.
Repairs--The lack of accessibility in fire-tube boilers further leads to
difficulties where repairs are required.
In fire-tube boilers tube renewals are a serious undertaking. The
accumulation of hard deposit on the exterior of the surfaces so enlarges
the tubes that it is oftentimes difficult, if not impossible, to draw
them through the tube sheets and it is usually necessary to cut out such
tubes as will allow access to the one which has failed and remove them
through the manhole.
When a tube sheet blisters, the defective part must be cut out by
hand-tapped holes drilled by ratchets and as it is frequently impossible
to get space in which to drive rivets, a "soft patch" is necessary. This
is but a makeshift at best and usually results in either a reduction of
the safe working pressure or in the necessity for a new plate. If the
latter course is followed, the old plate must be cut out, a new one
scribed to place to locate rivet holes and in order to obtain room for
driving rivets, the boiler will have to be re-tubed.
The setting must, of course, be at least partially torn out and
replaced.
In case of repairs, of such nature in fire-tube boilers, the working
pressure of such repaired boilers will frequently be lowered by the
insurance companies when the boiler is again placed in service.
In the case of a rupture in a water-tube boiler, the loss will
ordinarily be limited to one or two tubes which can be readily replaced.
The fire-tube boiler will be so completely demolished that the question
of repairs will be shifted from the boiler to the surrounding property,
the damage to which will usually exceed many times the cost of a boiler
of a type which would have eliminated the possibility of a disastrous
explosion. In considering the proper repair cost of the two types of
boilers, the fact should not be overlooked that it is poor economy to
invest large sums in equipment that, through a possible accident to the
boiler may be wholly destroyed or so damaged that the cost of repairs,
together with the loss of time while such repairs are being made, would
purchase boilers of absolute safety and leave a large margin beside. The
possibility of loss of human life should also be considered, though this
may seem a far cry from the question of repair costs.
Space Occupied--The space required for the boilers in a plant often
exceeds the requirements for the remainder of the plant equipment. Any
saving of space in a boiler room will be a large factor in reducing the
cost of real estate and of the building. Even when the boiler plant is
comparatively small, the saving in space frequently will amount to a
considerable percentage of the cost of the boilers. Table 2 shows the
difference in floor space occupied by fire-tube boilers and Babcock &
Wilcox boilers of the same capacity, the latter being taken as
representing the water-tube class. This saving in space will increase
with the size of the plant for the reason that large size boiler units
while common in water-tube practice are impracticable in fire-tube
practice.