Page 239 - Boiler_Operators_Handbook,_Second_Edition

224 Boiler Operator’s Handbook

downstream of the joint where indicated. Normally
internal reinforcement cannot exceed 1/16 inch but
that decreases with higher pressures and temperatures.
On the other hand incomplete penetration as shown in
Figure 6-19 isn’t permitted in excess of 1/32 inch. The
root and following passes should have a surface texture
similar to what’s difficult to see in Figure 6-20, it is pro-
duced by the welder swinging the rod in overlapping
circles. It’s difficult to see because the photo is one of a
filler pass, commonly called a stringer, using low hydro-
gen electrodes. It’s also hard to see in the photo of the
inside of a root pass in Figure 6-16. A reasonably smooth
Figure 6-15b. Checking fit up with gage root weld inside the pipe and within the parameters de-
scribed here is always a sign of a good welder.

Figure 6-18. Excess internal weld reinforcement

Figure 6-16. View inside of root weld

Figure 6-19. Incomplete penetration of a weld

Hot Pass
The hot pass follows the root pass and is commonly
checked for cracking. Once the hot pass is complete, the
stresses associated with the cold pipe metal and hot weld
during fit-up are reduced by even heating. When welding
carbon steel pipe most welders use a procedure calling for
low hydrogen electrodes for everything except the root
Figure 6-17. Weld cross section pass. Low hydrogen electrodes have to be kept hot after
the container is opened to prevent the absorption of water
and special ovens are usually used for that purpose. If
Root Pass you see those electrodes laying around and getting cold
The root pass indicated in Figure 6-17 is the first then used in a weld, that weld will have porosity (holes)
full circumferential weld and the inspector should check in it. Low hydrogen electrodes produce a weld that flows
it in stages to ensure there is no excess internal reinforce- and fills well with a slag (molten minerals that coat the
ment as shown in Figure 6-18. Welders normally refer electrodes that melt and form an airtight barrier over the
to this as “grapes” and it can produce damaging eddy weld to prevent oxygen from the air combining with the
currents in the flowing fluid that will erode the piping molten metal) that’s readily removed.