Page 222 - Boiler_Operators_Handbook,_Second_Edition

Maintenance 207

boilers, also pulverized coal burners. The throat mate- again you might give serious consideration to rebuild-
rial is usually rated for very high temperatures because ing the entire thing to get that flexibility. Burner throat
the throat is closest to the fire and will be the hottest repair and replacement is best left to the experts, men
refractory in the furnace. Those of you firing gas know and women skilled in installing the materials because
that the throat is glowing cherry red when the boiler is it isn’t easy to properly position throat tile so you get a
in operation. Actually it’s always red hot, regardless of perfect circle or shape a refractory throat in perfect form
the fuel, you just can’t see the glow with pulverized coal along the sweep.
or oil fires because the bright fire lights up the furnace. Sweep? That’s a special tool used to shape a burner
Throats are either made up of pieces of a pre-fired throat out of plastic refractory. Normally it’s a piece of
refractory material we call “tiles” or a plastic material. flat steel plate welded to a pipe that fits into the oil burn-
When we use the word “plastic” in discussions of refrac- er guide pipe and cut to produce the form of the burner
tory we mean a material that can be molded and shaped throat. (Figure 6-4) I had one on one ship that consisted
as desired until it is dried. Plastic refractory has the con- of several pieces which, when assembled, formed the
sistency of stiff clay and looks and feels like mud with burner cone completely with four scraper bars and it
lots of sand and fine gravel in it. was designed to spin into the packed plastic to produce
Either of the throat materials will expand consid- a finished throat. I can also remember that a refractory
erably during boiler operation so there should always crew in a foreign shipyard thought they didn’t need that
be some form of expansion joint around the throat. I’ve sweep to form the throats and I ended up going back
seen many installations of plastic refractory where the into the boiler to replace their work shortly thereafter
throat and burner wall were monolithic (all one big because they produced a completely different shape. If
piece) and they do manage to stay intact for quite a you have plastic throats make certain the installers use
while despite the differences in temperature; I just pre- that throat sweep and use it properly.
fer separating them because a prepared joint provides a If anyone tries to sell you a refractory “mainte-
perimeter for expansion and eventually, a repair. nance coating” kick them out of your plant. I may incur
A problem we used to have, and one that I’m cer- the wrath and ire of some manufacturers and salesmen
tain is still possible, is sagging of a plastic refractory wall that believe they’re providing a valuable service but
which bears down on the burner throats to distort them. I don’t care. So called maintenance coatings don’t do
I still insist on a “bull ring,” a circle of special pre-fired squat as far as I’m concerned and I’ve never seen them
arch brick or tile around the burner throat that supports do anything good, they’re usually quite harmful. Those
the wall and prevents it’s weight bearing down on the materials are, in some instances, nothing more than mud
throat tile. The bull ring should be designed to provide somebody dug up. Higher quality materials are seldom
a half inch gap between the inside diameter of the bull matched to the refractory in your boiler so their thermal
ring and the throat tile which, today, would be packed expansion rates are matched. The result is that much of
lightly with ceramic fiber. the spalling I’ve seen is just the maintenance coating
If you find yourself repairing your burner throat breaking away. It also fills the small cracks that provided
for expansion to create stress on the face of the refrac-
tory.
Another regular problem with those materials is
they are applied carelessly. In many of the situations
where I’ve been asked to help with problems with fir-
ing gas I’ve found the openings in the gas ring partially
blocked with that so-called maintenance coating. In-
stead of spending money on that junk put it in the bank
to pay for a complete replacement of the refractory some
years in the future. If your refractory is suitable for the
application there will not be any serious degradation
unless you create it.
You shouldn’t encounter all the problems I had
with refractory because the materials and installation
methods have improved considerably in the past forty
Figure 6-4. Throat sweep years. If you do have a forty year or older boiler you