Tube expanders must be used carefully to avoid
damage to the expanders and to prevent injury to
personnel. The centrifugal force developed by the air
turbine is great, so the air motor must be gripped firmly
with both hands. If the roller-mandrel combination
should bind, the force of the air motor could break the
mandrel and quite possibly cause injury to the
operator. Always have a person stationed nearby to
give immediate assistance if necessary. If you run into
any trouble, your safety person may be able to crimp
the hose quickly and thus keep the mandrel from
Most tubes are expanded and belled. However,
Figure 2-12.Belling tool.
check the drawing to determine if any specific
instructions are shown. Some tubes in 1,200 psi boilers
are lightly expanded or belled before welding; some
are directly butt-welded to the studs. A roller-type or a
drift-type belling tool is used. The drift-type tool is
shown in figure 2-12.
When belling a tube, be careful not to overdo the
operation. Tubes up to, but not including 2 inches OD
should be belled at least 1-15 inch but no more than 1/8
inch. Tubes 2 inches OD and larger should be belled at
least 1/8 inch, but no more than 3/16 inch. The increase
is to be measured over the outside tube diameter at the
end of the tube. Figure 2-13 shows the process of
belling a tube.
Some expanders are fitted with belling rolls, as
shown in figure 2-9. When these expanders are used,
the tubes are expanded and belled at the same time;
thus, there is no need for a separate belling job.
RENEWING WELDED TUBES
In some boilers of recent design, the superheater
tubes and the economizer tubes are welded after they
have been expanded. The renewal of these tubes is
more complicated than the renewal of ordinary tubes.
Procedures for renewing welding tubes are given in the
appropriate manufacturers technical manual.
PLUGGING BOILER TUBES
Any tube that is plugged must have a hole
drilled in it to prevent pressure buildup in
the tube when the boiler is steamed.
As an emergency measure, it is sometimes
necessary to plug defective boiler tubes until they can
Figure 2-13.Belling a tube.
Various sizes of tube plugs are carried in the
supply system. The plugs are tapered to the required
shape and are usually drilled and threaded at the larger
end, so they may be removed with a tube plug
Tubes must be plugged at each end. Before driving
a tube plug into position, be sure the plug and the inside
of the tube are absolutely clean, so the plug makes
good metal-to-metal contact with the tube. Drive the
plug far enough in to ensure it will hold, but do not
drive it so far in that it damages the tube sheet.
In plugging superheater tubes, use an offset driver
to drive in the plugs when the tube holes do not fall in
line with the handhole opening. When a superheater
tube is plugged, it will eventually burn away after a
period of service. When tubes have burned away (or
when they have been removed) so much that they leave
a gas lane more than three tube rows wide through the
entire superheater tube bank, plug the gas lane with a