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.
The gauge to be employed by inspectors to determine the thickness of
boiler plates will be any standard American gauge furnished by the
Treasury Department.
All samples intended to be tested on the Riehle, Fairbanks, Olson, or
other reliable testing machine, must be prepared in form according to
the following diagram, viz.: eight inches in length, two inches in
width, cut out their centres as indicated.
[Illustration: Fig. E.]