2Ö
MODERN STEAM PRACTICE.
the length is measured between the end plates of the boiler. Inthe fiues as proposed, between the T-iron ribs, the dimensions givenare for a collapsing pressure of 450 lbs. per square inch; the safeworking pressure should be 75 lbs. per square inch.
Diameter ofFlues in inches.
Thickness of Plates.
10 Feet Long.
20 Feet Long.
30 Feet Long.
12
'291
•399
•480
18
•350
•480
•578
24
'399
•548
•659
30
450 lbs.
•442
•607
•730
36
*480
•659
794
42
■516
•707
•851
48
•548
752
•905
The above are founded on the supposition that the 20-feet and30-feet long flues have T-iron or angle-iron hoops at the necessary
joints, the hoops to be placed 10 feetapart. Some makers prefer placing theT-iron hoops at each joint, the platesbutting on one another, and at thelongitudinal joints likewise. 1 When the joints are planed, andthe butt Strips properly fitted, the strain is entirely taken off therivets, the compressive strain being taken on the ends of the platesdirectly.
In the cylindrical boiler, with round flues, the forces are divergingfrom the central axis as regards the outer shell, and convergingas applied to every separate flue which the boiler contains. Toshow the amount of strain upon a high-pressure boiler 30 feetlong and 6 feet in diameter, having two centre flues, each 2 feet3 inches diameter, working at a pressure of 50 lbs. on the squareinch, we have only to multiply the number of square feet of sur-face—1030 exposed to pressure—by 3'2i, and we have the forceof 3306 tons which a boiler of these dimensions has to sustain. Wemention this to show that the statistics of pressure, when workedout, are not only curious in themselves, but instructive as regards aknowledge of the retaining powers of vessels so extensively used.