Steam Turbines / A Book of Instruction for the Adjustment and Operation of the Principal Types of this Class of Prime Movers

by Hubert E. Collins

Packing Glands

Where the shaft passes through the casing at either end it issues from a chamber in which there exists a vacuum. It is necessary to pack the shaft at these points, therefore, against the atmospheric pressure, and this is done by means of a water-gland packing W W (Fig. [34]). Upon the shaft in Fig. [35], just in front of the dummy pistons, will be seen a runner of this packing gland, which runner is shown upon a larger scale and from a different direction in Fig. [43]. To get into the casing the air would have to enter the guard at A (Fig. [44]), pass over the projecting rings B, the function of which is to throw off any water which may be creeping along the shaft by centrifugal force into the surrounding space C, whence it escapes by the drip pipe D, hence over the five rings of the labyrinth packing E and thence over the top of the revolving blade wheel, it being apparent from Fig. [43] that there is no way for the air to pass by without going up over the top of the blades; but water is admitted to the centrally grooved space through the pipe shown, and is revolved with the wheel at such velocity that the pressure due to centrifugal force exceeds that of the atmosphere, so that it is impossible for the air to force the water aside and leak in over the tips of the blades, while the action of the runner in throwing the water out would relieve the pressure at the shafts and avoid the tendency of the water to leak outward through the labyrinth packing either into the vacuum or the atmosphere.

FIG. 43

FIG. 44

The water should come to the glands under a head of about 10 feet, or a pressure of about 5 pounds, and be connected in such a way that this pressure may be uninterruptedly maintained. Its temperature must be lower than the temperature due to the vacuum within the turbine, or it will evaporate readily and find its way into the turbine in the form of steam.

FIG. 45

In any case a small amount of the steaming water will pass by the gland collars into the turbine, so that if the condensed steam is to be returned to the boilers the water used in the glands must be of such character that it may be safely used for feed water. But whether the water so used is to be returned to the boilers or not it should never contain an excessive amount of lime or solid matter, as a certain amount of evaporation is continually going on in the glands which will result in the deposit of scale and require frequent taking apart for cleaning.

FIG. 46

When there is an ample supply of good, clean water the glands may be packed as in Fig. [45], the standpipe supplying the necessary head and the supply valve being opened sufficiently to maintain a small stream at the overflow. When water is expensive and the overflow must be avoided, a small float may be used as in Fig. [46], the ordinary tank used by plumbers for closets, etc., serving the purpose admirably.

When the same water that is supplied to the glands is used for the oil-cooling coils, which will be described in detail later, the coils may be attached to either of the above arrangements as shown in Fig. [47].

FIG. 47

When the only available supply of pure water is that for the boiler feed, and the condensed steam is pumped directly back to the boiler, as shown in Fig. [48], the delivery from the condensed-water pumps may be carried to an elevation 10 feet above the axis of the glands, where a tank should be provided of sufficient capacity that the water may have time to cool considerably before being used. In most of these cases, if so desired, the oil-cooling water may come from the circulating pumps of the condenser, provided there is sufficient pressure to produce circulation, as is also shown in Fig. [48].

FIG. 48

When the turbine is required to exhaust against a back pressure of one or two pounds a slightly different arrangement of piping must be made. The water in this case must be allowed to circulate through the glands in order to keep the temperature below 212 degrees Fahrenheit. If this is not done the water in the glands will absorb heat from the main castings of the machine and will evaporate. This evaporation will make the glands appear as though they were leaking badly. In reality it is nothing more than the water in the glands boiling, but it is nevertheless equally objectionable. This may be overcome by the arrangement shown in Fig. [49], where two connections and valves are furnished at M and N, which drain away to any suitable tank or sewer. These valves are open just enough to keep sufficient circulation so that there is no evaporation going on, which is evidenced by steam coming out as though the glands were leaking. These circulating valves may be used with any of the arrangements above described.

FIG. 49