Page 223 - Boiler_Operators_Handbook,_Second_Edition

208 Boiler Operator’s Handbook

may be seeing them but modern boilers with mostly wa-
ter cooled walls will have very few refractory problems.
The one difficulty with modern boilers, especially
the ‘A’ and ‘O’ type package boilers is retention of the
refractory seal where tangent or finned tubes are offset
or lacking fins next to the boiler drums. Those sections
consist of very small pieces of refractory with very little
to hold them in place and, for those particular boilers,
the grip has to overcome gravity so their weight is a fac-
tor. The best way to repair those is to completely remove
a section and replace it. You’ll find that new material
doesn’t bond to old refractory at all. As the new material
cures and dries it shrinks and simply pulls away from
the old material.
Any refractory repair that isn’t just for a short term
should consist of complete replacement of a section with
adequate provisions for expansion. That repair will last. Figure 6-6. Refractory anchor
Patches are exactly that and they don’t last. Don’t be
afraid to improve on an installation either. If a repair is der that’s mixed with water to form a very dense soupy
made because a furnace wall buckled into the furnace mixture that can be poured into spaces surrounded by
you should improve the anchoring as well as provide for forms. Small areas, less than sixteen inches in diameter
thermal expansion. Either lack of anchoring or buckling should be “keyed in” to the existing material. That’s
due to thermal expansion was the cause of the failure so accomplished by undercutting the face of the existing
take measures to counter both problems. material (Figure 6-5) so the patch is wedged between the
Any temporary patch has to be anchored or it will edges of the existing material and the casing insulation.
be more temporary than you intended; falling out as Larger patches should be anchored by installing a
soon as the boiler heats up. Since the repair material refractory anchor (Figure 6-6) secured to the casing or
will shrink a little as it dries. It doesn’t matter how hard brick setting so the patch is secured and will not tend to
you hammer on the wet plastic refractory material (or crack and buckle out as it’s heated. Refractory anchors
how thick any slurry of castable refractory is) it has to should be installed within 18 to 24 inches of each other
be anchored somehow. Castable, by the way, is a pow- if you don’t have a successful wall to compare to.
Almost any refractory repair requires a “dry-out”
as described in the chapter on new start-ups. If the
repair consists of brick or tile laid up dry, a common
arrangement for sealing the furnace access opening on
many boilers, then there’s no need for a dry out because
there is no moisture imbedded in the refractory. Any-
thing else will have to be dried out.
When the patch is made with plastic refractory the
dry out will be accelerated if you provide vents in the
material. You provide vents by poking the material with
a small welding rod to produce small round holes about
two-thirds of the thickness of the wet material on three
to four inch centers. Steam forming in the material will
then have an escape route. If the repair is due to vana-
dium pentoxide damage the venting isn’t recommended
because it will provide places for the oxide to soak into
the refractory.
Some refractory materials are labeled as air drying,
some are heat drying but most are combination air and
Figure 6-5. Undercut for refractory patch heat drying. A heat drying material reacts to a small de-