Page 298 - Wastewater Solids Incineration Systems

Incinerator Maintenance 259

1.4.2 Hearth Sagging
All hearths are constructed in the shape of an arch to be self-supporting. The circular
arch is restrained by the incinerator shell and the first row of “skew-back” bricks. The
hearth assembly is critical. The temporary supports placed under each hearth during
construction must have the correct arch angle. Each ring of bricks must be installed
tightly and the amount of mortar on the adjacent brick surfaces must be kept to a
minimum thickness.
As an incinerator heats up, the shape of the arch can change for several reasons:
expansion of skew-back, hearth brick expansion, loads on top of the hearth, slag
buildup, deterioration of the refractory, and bulging of the shell. The correct arch of
each hearth must be maintained at all times. If the arch flattens out, the self-support
will be lost and the hearth will collapse onto the hearth below. If a second hearth is
overloaded, a domino effect can occur, and additional hearths will collapse.
The operators should periodically check the arch of each hearth within the incin-
erator. Observations from three different locations should be taken because it is pos-
sible for a portion of a hearth to sag while the remainder keeps the original arch (see
Figure 11.2).
1.4.3 Rabble Arms and Teeth
Online inspection of rabble arms and teeth can alert operators to several issues. The
sagging arm may be the result of excessive heat, a crack in the arm, or a center shaft
rabble arm socket failure. By observing the shape of the arm and the clearance
between teeth and hearth, an operator can tell if the arm is sagging.
Some sagging is tolerable, but excessive sagging can allow teeth to drag on the
hearth, which will result in chattering of the center shaft and potentially center shaft
drive failure. This should be corrected as soon as possible.
The center shaft must never be stopped where a rabble arm is located directly in
front of an active burner. This will warp the arm regardless of whether it is insulated.
The burner flame can also be deflected into the hearth refractory located above or
below the burner and cause severe refractory damage by impingement of the burner
flame on the refractory.
Refractory insulation failures on the arms can cause problems. Small cracks in the
refractory surface are normal. Large cracks or missing sections of refractory that
expose the metal surface of the arm are cause for concern.
Cracked rabble arm refractory can also plug drop holes. Damage or worn refrac-
tory on rabble arms should also be repaired or replaced as soon as possible.