Boiler tubes are listed and described from the outside diameter. This should be noted, as gas-pipe is described from the inside diameter. Thus a 1-inch gas-pipe is nearly 11⁄4 outside diameter while a 1-inch boiler tube is exactly one inch. Another difference between the two consists in the fact that the outside of boiler tubes is rolled smooth and even; gas-pipe is left comparatively rough and uneven.
When the boiler tubes are new and properly expanded there is a large reserve or surplus of holding power for that part of the tube sheet supported by them, this has been proved by experiment made by chief engineer W. H. Stock, U. S. N., as shown in the following
Table of Holding Power of Boiler Tubes.
| Outside diameter of end of tube where fracture took place. | Area of cross section of body of tube. | Thickness of tube plate. | Strain in pounds. Mean result. | Method of Fastening. |
|---|---|---|---|---|
| Inches. | Sq. ins. | Inches. | Pounds. | |
| 25⁄8 | .981 | 7⁄16 | 22650 | Expanded by Dudgeon tool, end riveted over. |
| 25⁄8 | .981 | 7⁄16 | 22150 | Expanded by Dudgeon tool, end partly riveted over. |
| 23⁄8 | .981 | 3⁄8 | 25525 | Expanded by Dudgeon tool, end riveted over. |
| 23⁄8 | .981 | 3⁄8 | 29675 | Expanded by Dudgeon tool, ferruled, not riveted over. |
| 23⁄8 | .981 | 3⁄8 | 13050 | Simply expanded by Dudgeon tool. |
Mr. C. B. Richards, consulting engineer at Colt’s Armory at Hartford, Conn., made some experiments as to the holding power of tubes in steam boilers, with the following results: The tubes were 3 inches in external diameter, and 0.109 of an inch thick, simply expanded into a sheet 3⁄8 of an inch thick by a Dudgeon expander. The greatest stress without the tubes yielding in the plate was 4,500 pounds, and at 5,000 pounds was drawn from the sheet. These experiments were repeated with the ends of the tubes which projected through the sheet three-sixteenths of an inch, being flared so that the external diameter in the sheet was expanded to 3.1 inches. The greatest stress without yielding was 18,500 pounds; at 19,000 pounds yielding was observed; and at 19,500 pounds it was drawn from the sheet. The force was applied parallel to the axis of the tube, and the sheet surfaces were held at right angles to the tube axis.
Note.
When the tube sheet and tube ends near the sheet become coated with scale or the tubes become overheated, the holding power of the tubes becomes largely reduced, and caution must be used in having the tube ends re-expanded and accumulated scale removed.
Note 2.—In considering the stress or strain upon the expanded or riveted over ends of a set of boiler tubes, it may be remembered that the strain to be provided against is only that coming upon tube plate, exposed to pressure, between the tube ends—the space occupied by the tubes has no strain upon it.