The ball bearings, on which the damper shafts were mounted, showed clear signs of overheating, some with burn marks. All the grease had melted and burnt away.
Failed dampers sagged 2-3 inches near the center due to their self-weight and as a result of exposure to the flue gases at 650°F. There was a 3"-thick refractory lining on the walls of the flue gas duct. Once the dampers sagged, their ends rubbed against the lining, preventing free movement. Rub marks were visible on the lining, confirming this observation.
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Old Design of Dampers
Figure 42.2 Old Design: Bearing and Pulley Arrangement
The bearings, which were mounted outside the duct, became very hot during operation. The damper shafts were at the flue gas temperature. The solid shaft ends conducted heat continuously to the bearings. We reviewed the design and came to the conclusion that the bearings had to be kept cool and that damper sagging should somehow be avoided.
We asked the process technologist and operations staff how leak-tight the dampers should be when closed. They said that a small gap of, say, 3" along all four edges of each of the dampers would be acceptable.
In order to reduce the weight of the dampers, we selected a box frame section. We planned to use 1/8" thick 18/8 stainless steel (AISI 304) sheet metal to fabricate an open box design. Two sheets would form the top and bottom of the box. So as to provide some stiffness, we decided to provide a few cor-
Figure 42.3 Perspective View of Old and New Design of Dampers
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Figure 42.4 shows longitudinal and cross sections of the new damper design.
Figure 42.4 New Design of Dampers
rugations on the sheets, along the length. The box section had open sides and ends. The two sheets would be bolted together 4" apart, using machined pipe spacers. We planned to use a 2" stainless steel pipe as the axle on which the new damper would be mounted. The pressure inside the duct was considerably lower than outside the duct, due to the powerful suction effect of the chimney. A few 1/4" holes drilled through the wall of this pipe-axle would allow outside air to leak into the duct from the open pipe ends. The air flow would help cool the bearings. Roller bearings would replace the existing ball bearings, as they were more suited to the small and infrequent movements of the dampers.
The new dampers were expected to weigh less than 120 lbs. As their section was quite light and stiff, it was unlikely that they would sag. Figure 42.3 shows perspective views of the old and new designs.
In this design, the bearing is mounted on a small step machined on the pipe, while the pulley is mounted on a further step on the pipe, as can be seen in Figure 42.5.
Figure 42.5 New Design: Bearing and Pulley Arrangement