steam will be discharged up the stack. For this reason,
tubes l-inch-outside diameter (OD) up to and
including 2 inches OD are expanded by "boilerman's
feet" as only a small amount of expansion is required to
hold the tubes firmly in place. With tubes 3 inches OD
and larger and all external downcomers, special
precautions must be taken to ensure the tubes are
properly expanded in the tube seat. Through a series of
tests, the point of maximum holding power for various
sizes of tubes has been found and is expressed in terms
of standard diameters that should be measured after the
tube has been expanded in place. In new construction
or replacement of tubes where the tube and hole
measurements can be obtained, the correct amount of
expansion can be found by using the following
For tubes in drums: Diameter of tube hole minus
OD of tube, plus 0.012 inch per inch OD of tube.
For tubes in headers for boiler design pressure
under 500 psi: Diameter of tube hole minus OD of tube,
plus 0.015 inch per inch OD of tube.
For tubes in headers for boiler design pressure over
500 psi: Diameter of tube hole minus OD of tube, plus
0.020 inch per inch OD of tube.
The figure arrived at by using the above formulas
should be added to the OD of the tube as measured to
give the required OD of the tube after rolling.
If it is impossible to reach the outside of the tubes
in drums to gauge them, the inside diameter (ID) of the
tube must be mea sured. Since the plastic deformation
of the tube wall varies with tube wall thickness, the ID
of the tube for different wall thickness will vary.
Where the outside of the tube is inaccessible, the
following formula is used in the expansion of a tube:
The ID of the tube, plus the tube hole diameter
minus the OD of the tube plus the expansion increase
Boiler tubes should be expanded with the
expanding equipment furnished to the shop. Select
expanders of the proper size for the tube size and the
seat thickness and expanders proper for the operation
to be perform ed.
There are two types of expanders: roller-type and
ball-drift type. Roller-type expanders are furnished for
use by the shop labor force. Roller-type expanders are
shown in figure 2-9. A series of adapters are furnished
for use if tube holes are not readily accessible. Some of
these adapters are shown in figure 2-10.
Figure 2-9.Roller-type tube expanders.
Figure 2-10.Adapters for tube expanders.
Tube expanders are operated by air motors. The
air, at about 100 psi, enters through a controlling
handle and goes into the motor housing where it drives
an air turbine. The turbine is attached to the shaft of the
motor. The controlling handle can be turned clockwise
or counterclockwise. A chuck with a tapered shank
engages the shaft of the air motor, thus transmitting the
power of the motor to the rollers used for expanding
tubes into the tube sheet.
Both the air motors and the chucks are available in
various sizes. The large sizes of motors and chucks are
used for expanding the larger sizes of tubes. Figure
2-11 shows a tube expander in use.
Figure 2-11.Expanding a tube.