This invention relates to a sealing assembly, and more particularly, to an assembly for sealing against the egress of air from a pressurized ball mill pulverizer assembly.
In the operation of any type of plant using pulverized coal for fuel, it is common practice to supply raw coal to a ball mill pulverizer to reduce the size of the coal and mix it with air. The main component of a typical ball mill is a large drum with hollow trunnions at each end forming an extension of the drum and providing space for introduction of the raw coal to the mill. The trunnions also define a bearing surface for permitting rotation of the drum and the drum is filled with a charge of forged steel balls to a level just below the bottom of the trunnions. As the drum is rotated, the balls are continuously lifted and tumbled by gravity onto the coal to pulverize the coal. Classifiers are usually located at each end of the mill and serve as separators, so as the pulverized coal leaves the grinding zone, oversized particles are rejected by gravity separation in a low velocity central section of the classifier and by centrifugal separation as the coal air stream passes around the baffles. The rejected material is mixed with the incoming raw coal feed and returned through the trunnion by screw conveyors for further grinding.
The mill is operated under positive pressure to avoid the abrasive wear and outages experienced with the exhausters on suction mills. Simple seals, usually consisting of a steel ring, are attached to the mill trunnion and a flexible gasket is attached to the classifier to prevent coal from blowing out through the clearance between the mill and the classifier trunnions, with the actual medium that seals the pulverizer being seal air introduced from a separate control source. In order for this seal air to be effective, it must be enclosed and prevented from escaping into the boiler house.
However, the relatively simple steel ring/flexible gasket seal assembly is not without problems. For example, due to axial expansion, the cold set position at start-up must be such that the seals are closed. Thus, as the pulverizer undergoes thermal expansion, the ring pushes on the fabric-like material causing increased pressure by deflection of the circular spring and resultant wear of the material. Also, coal dust that often infiltrates this area under various conditions acts like an abrasive and accelerates wear over and above the increased pressure caused by the deflected spring. The wearing of the material causes leakage over an extended period of time and resultant failure of the seal.