Page 307 - Wastewater Solids Incineration Systems

268 Wastewater Solids Incineration Systems

1.8.8 Rabble Arm Replacement
Rabble arms may need to be replaced for several reasons: excessive arm sagging,
cracked arm (broken off), or rabble teeth that no longer dovetail in the arm because
the exposed metal surfaces of the dovetail have eroded over a long period of time.
Removal of the arm may require removing the teeth. The arm with teeth attached
will not pass through a standard access door. Replacing the standard door with a
larger one permits the arm to be removed with the teeth intact.
Teeth used for a long time are typically difficult to remove from the dovetail slot.
Use of pneumatic hammers is recommended. The pin that anchors the arm in the
central shaft socket may also be hard to remove; a pneumatic hammer can be used or
it can be drilled out.
A new rabble arm must be insulated. It can be insulated either before installation
or afterwards in the incinerator with a castable refractory. If preinsulated, care must
be taken not to damage the arm while transporting and installing it. After the arm is
in place, the socket-hub area should be insulated with castable refractory. No bare
metal surfaces should be exposed to high temperatures.
1.8.9 Rabble Teeth Replacement
Rabble teeth should be replaced when they become deformed, broken, or missing.
Teeth are held on the rabble arm in a dovetail slot. A pin holds the last (outermost)
tooth in place, preventing the others from sliding out during use. To remove the teeth
from an arm, the pin on the outer tooth needs to be pulled and the teeth can slide out
of the dovetail.
Typically, teeth used for an extended period of time are difficult to slide out. Oper-
ators can use a pneumatic hammer to assist removal. Before installing new teeth, the
tooth assembly drawing provided by the incinerator supplier should be consulted. The
correct tooth must be installed in the correct location in the correct arm.
New teeth should be laid out on an assembly area to duplicate the assembly
drawing. Each tooth needs to be checked for angle, size, and proper location.
Spacer blocks should be inserted where required. High-temperature material con-
taining 24 to 28% chromium and 18 to 22% nickel (ASTM A-297, Grade HK, Class
25-20) is typically used for the rabble teeth. However, a few incinerator installations
use lower heat resistant material for rabble teeth located in the ash cooling hearths.
When inserting the new teeth to the rabble arm, workers should start with the
inner-most tooth and work toward the outside. The first arm should be completed
before proceeding to the second, and so on. A small (6 mm [0.25 in]) gap between the