1254
THEORY AND PRACTICE OF ENGINEERING.
Book II.
iron or copper ; cast-iron as well as wrought-iron plates are commonly applied ; whore fuelis abundant, cast-iron boilers, with proper care, have been extensively and advantageouslyemployed. Copper, however, is decidedly the best of all substances for this purpose, costbeing the only objection; one copper boiler will, however, probably endure as long asfive or six iron ones, the uniformity of its texture at the same time insuring greatersecurity against bursting, though its strength to resist flexure at high temperatures isnot equal to that of iron ; sheet-iron is liable to vary materially in its structure, althoughmanufactured by the same process. The pressure which occasions the bursting of a boileris proportional to the load on its safety-valve; that to crush it together is equal to the powerof the atmosphere : the boiler should always be of such a strength that when in aworking state, it will bear three times the pressure of what is on the valve; in low-pressure this may be too much, but in high-pressure boilers it is insufficient.
Copper loses 5 per cent, of its strength between the temperature of freezing and boilingwater; at 550° a quarter of its strength; at 850° it loses one-half; and at 1330° itis entirely lost, becoming an incohesive substance, although it does not totally melt tillit reaches 2000°. Iron plate, on the contrary, increases in strength, for at a temperatureof 550° it is 16 per cent, stronger than when cold: after this point its strength rapidlydiminishes, so that its maximum of strength does not certainly exceed 570°. The specificgravity of iron boiler plates is said to be about 7*7344, and by repeated piling and weldinga great increase of strength may be obtained: the greatest practical strength is only tobe taken at one-sixth of the absolute cohesion; and to prevent explosion, we ought to havefour times the strength that any boiler is ordinarily worked at; 2500 pounds of extensionon each square inch of cohesive action is the safe working strain for those made of iron.
To ascertain the thickness of the boiler plates made of any of the ductile metals thefollowing rules are given: for the upper plates, when of iron, multiply the load in poundsper circular inch on the safety-valve, by the greatest diagonal of the section in inches, anddivide the product by 120 times the cubic contents of the boiler per horse-power ; theresult is the thickness in inches : when the plates are copper, divide by 72 instead of 120;it is usual to increase the thickness of the bottom plates to about one and a half timesthose of the upper. For spherical boilers, of malleable iron, multiply the diameter ininches by the pressure on the valve in pounds per circular inch, and divide by 240 timesthe cubic contents of the boiler for each horse-power. For copper, divide by 144, insteadof 240. "When cast-iron is used, its unequal expansion must be taken into account; and forthe strength of tubes exceeding 8 inches in diameter, multiply the square of the diameterby the pressure on the safety-valve in pounds on a circular inch, and divide the productbv 150 times the cubic feet of space in the boiler per horse power, multiplied by thedifference between the diameter and 7*4 inches ; the result is the thickness in inches thatshould be given, together with some increase for wear and tear : thus, with a tube the in-ternal diameter of which is 10 inches, the cubic feet of the boiler per horse-power being 10,and the load on the safety-valve 36 pounds on the circular inch, its thickness will be foundto be *92 inches : when the thickness is greater, the risk from unequal expansion increases ;and when less, failure may result from the effect of pressure and inequalities in the iron itself.
The Piston Gauge is a small tube 2 inches in diameter ; the cylinder has a solid plugturned and ground, to set loosely in it, the pressure of thesteam bearing up the piston on the level, one end of which,attached to the spring indicator, gives the pressure of thepiston.
Mercurial Gauges of various kinds are applied to the crowns
PISTON GAUGE.
^v
Fig. 1979. MERCURIAL GAUGE.
Fig.1980
DYNAMOMETER.
of boilers, which, by means of a graduated scale above, mark off the pressure at all timesexerted by the steam.
The Dynamometer , employed to measure the force of steam in the boiler, is a simple benttube, of glass or iron, in the form of the letter U, the two ends of which are curved; oneis applied to the boiler, and placed in communication with the steam : half the tube isfilled with mercury, which rises and falls according to the pressure exerted on it.