FIG. 60
The dummy rings of a turbine, namely, those rings which prevent excessive leakage past the balancing pistons at the high-pressure end, should have especial attention before a test. A diagrammatic sketch of a turbine cylinder and spindle is shown in Fig. [60], for the benefit of those unfamiliar with the subject. In this A is the cylinder or casing, B the spindle or rotor, and C the blades. The balancing pistons, D, E, and F, the pressure upon which counterbalances the axial thrust upon the three-bladed stages, are grooved, the brass dummy rings G G in the cylinder being alined within a few thousandths of an inch of the grooved walls, as indicated. After these rings have been turned (the turning being done after the rings have been calked in the cylinder), it is necessary to insure that each ring is perfectly bedded to its respective grooved wall so that when running the several small clearances between the groove walls and rings are equal. A capital method of thus bedding the dummy rings is to grind them down with a flour of emery or carborundum, while the turbine spindle is slowly revolving under steam. Under these conditions the operation is performed under a high temperature, and any slight permanent warp the rings may take is thus accounted for. The turbine thrust-block, which maintains the spindle in correct position relatively to the spindle, may also be ground with advantage in a similar manner.
The dummy rings are shown on a large scale in Fig. [61], and their preliminary inspection may be made in the following manner:
The spindle has been set and the dummy rings C are consequently within a few thousandths of an inch of the walls d of the spindle dummy grooves D. The clearances allowed can be gaged by a feeler placed between a ring and the groove wall. Before a test the spindle should be turned slowly around, the feelers being kept in position. By this means any mechanical flaws or irregularities in the groove walls may be detected.
FIG. 61
It has sometimes been found that the groove walls, under the combined action of superheated steam and friction, in cases where actual running contact has occurred, have worn very considerably, the wear taking the form of a rapid crumbling away. It is possible, however, that such deterioration may be due solely to the quality of the steel from which the spindle is forged. Good low-percentage carbon-annealed steel ought to withstand considerable friction; at all events the wear under any conditions should be uniform. If the surfaces of both rings and grooves be found in bad condition, they should be re-ground, if not sufficiently worn to warrant skimming up with a tool.