Part 6 (1/2)

Safety--The most important requirement of a steaer froe shell boiler under pressure is considered, the thought of the destruction possible in the case of an explosion is appalling

The late Dr Robert H Thurston, Dean of Sibley College, Cornell University, and past president of the Aineers, estiy stored in a plain cylinder boiler under 100 pounds steaht of over 3 miles; a locomotive boiler at 125 pounds pressure from one-half to one-third of a mile; and a 60 horse-power return tubular boiler under 75 pounds pressure somewhat over a mile To quote: ”A cubic foot of heated water under a pressure of froy as one pound of gunpowder” From such a consideration, it h pressure stea the adoption of water-tube boilers A consideration of the thickness of material necessary for cylinders of various dia an allowable stress of 12,000 pounds per square inch, will perhaps best illustrate this point

Table 1 gives such thicknesses for various dia into consideration the weakening effect of any joints which may be necessary The rapidity hich the plate thickness increases with the diameter is apparent and in practice, due to the fact that riveted joints iven in the table, with the exception of the first, must be increased from 30 to 40 per cent

In a water-tube boiler the drums seldom exceed 48 inches in diameter and the thickness of plate required, therefore, is never excessive The thinner metal can be rolled to a , and the joints can be more easily and perfectly fitted than is the case where thicker plates are necessary All of these points contribute towardthe drums of water-tube boilers better able to withstand the stress which they will be called upon to endure

The essential constructive difference betater-tube and fire-tube boilers lies in the fact that the forainst the large diameters necessary in the latter

The factor of safety of the boiler parts which come in contact with the her than would be practicable in a shell boiler Under the assumptions considered above in connection with the thickness of plates required, a nue tube (0134 inch), which is standard in Babcock & Wilcox boilers for pressures up to 210 pounds under the sa Table 1, the safe working pressure for the tubes is 870 pounds per square inch, indicating the very large in of safety of such tubes as compared with that possible with the shell of a boiler

TABLE 1

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 | 0033 | | 36 | 0300 | | 48 | 0400 | | 60 | 0500 | | 72 | 0600 | | 108 | 0900 | | 120 | 1000 | | 144 | 1200 | +---------+-----------+

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 ives 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 accoeneral, 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 approxi 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 fro, 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 coeneral 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 requirerate for the class of fuel to be burned, a coases 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 fron do not per 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 reater 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 ater-tube boilers that cannot be approached where fire-tube boilers are e--Another important result of the better circulation ordinarily found in water-tube boilers is in their ability to raise stea and to meet the sudden dened 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 coure above, it may be stated that in the U S Govern up steam in Scotch marine boilers is 6 hours and the tie double-ended Scotch boilers, such as are generally used in Trans-Atlantic service, the fires are usually started 24 hours before the tith of time is necessary for such boilers in order to eli 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 econo surfaces have been allowed to become fouled, itsurfaces clean internally and externally is a factor of the highest importance