Page 240 - Boiler_Operators_Handbook,_Second_Edition

Maintenance 225

Stringers
Additional passes that cover the hot pass can con-
sist of several “strings” of weld to fill in the weld until
nearly flush with the surface. Careful cleaning of the
slag from each of these passes is necessary to ensure it
isn’t left in the edges which dramatically weakens the
joint. The stringer in Figure 6-20 is as wide as they are
typically made with additional ones running alongside
each other.

Figure 6-21. Cover pass of completed weld

Figure 6-22. Check for excess external reinforcement
Figure 6-20. Stringers in progression of a pipe weld

Cover Pass pressure steam” or “it’s only low pressure gas” and I
This is normally called a “weave” pass because it don’t buy them. Joints like those shown can fail dramati-
provides a smoother look to the welded joint as shown cally. Low pressure steam in an 8 inch pipe produces an
in the photograph in Figure 6-21. On completion it’s axial force of 750 pounds which is more than enough to
checked with the gage to measure the external reinforce- do some serious damage if it gets loose. In the typical
ment as shown in Figure 6-22. The external reinforce- high pressure boiler plant the force in the same 8 inch
ment shouldn’t exceed 3/32 of an inch (less as design pipe is over three tons and in the typical utility plant at
pressure and temperature increase). Also, any groves the throttle inlet it’s thirty tons. I trust that explains why
in the external joint shouldn’t exceed 1/32 of an inch we have Codes and why you shouldn’t allow welders in
which is also called incomplete penetration. your plant to be cavalier about the welds they make.
Why did you have to go through all that? Because
lately I’ve seen several jobs where the weld section
looked like the section in Figure 6-23 and a few where
failed joints, when sectioned, look like the one shown
in the photograph of Figure 6-24. Even though a weld
can look good on the outside like Figure 6-25a—when
you can, take a look at the inside because it might look
like Figure 6-25b. When someone makes a weld like this
in your plant he or she is showing no concern for your
life. I’ve heard all the welder’s excuses like “It’s just low Figure 6-23. Incomplete weld section