7.1.1 Labyrinth glands
The labyrinth gland has superseded the carbon ring gland on large steam turbines because it can withstand higher steam conditions. The labyrinth gland consists of a ring with a series of machined fins that form a number of fine annular restrictions, every restriction being followed by an expansion chamber. A simple form of labyrinth seal is shown in Fig 2.69 (a).
As steam enters the restriction, the velocity increases and kinetic energy is developed at the expense of pressure energy; ideally, when the steam enters the expansion chamber, the kinetic energy is converted by turbulence into heat with no recovery of pressure energy. In practice, as the steam is throttled at successive restrictions at approximately constant enthalpy the pressure is progressively reduced.
To ensure that the maximum kinetic energy is converted in the expansion chambers, the finned ring and the shaft are usually stepped as shown in Fig 2.69 (b). This type of gland can only be used where the axial differential expansions between the rotor and casing are small. Stepped labyrinth glands may have an alternative arrangement with two or more large diameter fins as shown in Fig 2.69 (c). This arrangement is able to accept a larger change in relative axial dimensions since, under all conditions, at least two of the three fins per pitch form effective restrictions.
This principle can be extended to a greater number of large diameter fins, but the number of effective restrictions per unit of axial length becomes progressively less and it becomes preferable to adopt a simpler form of seal, such as those shown in Fig 2.69 (a), where the larger number of restrictions compensate for the decreased efficiency compared with the stepped gland.
Another design of gland that is independent of differential expansion is the vernier gland shown in Fig 2.69 (d). Both the shaft and seal ring are finned, the pitch of the fins being slightly different on the two seal components. This has the advantage that some of the fins will always be directly opposite, providing a greater restriction.
Figure 2.70 (a) shows a form of labyrinth gland with axial as well as radial fins which increases the number of restrictions in a given length of gland.
The tip thickness of labyrinth glands is made as thin as practicable so that if an accidental rub occurs between the shaft and the gland, the fins will rub away with little heating of the shaft. A heavy rub would quickly generate so much heat that the shaft would bend and become unbalanced.
The radial clearance and diameter of the labyrinth gland is made as small as possible, since the leakage flow through the gland is directly proportional to the leakage area. In practice, the minimum radial clearance adopted is approximately 0.5 mm. To minimise the effects of a 'rub' with close-clearance glands, the gland rings are often spring-loaded as shown in Fig 2.70 (b). The gland rings are usually made up of four or more segments.
The flow through a labyrinth gland is a function of the initial pressure and temperature, the final pressure and the clearance area under the restrictions. As the mass flow through a series of restrictions is constant, the velocity of the steam through the successive throt-tlings must increase as the steam is expanding; the velocity through the final restriction cannot exceed the sonic velocity. The pressure ratio across the last restriction is then equal to the critical value and if the back pressure is further reduced, no increase in mass flow will occur. Thus for a gland with a given number of restrictions, there is an associated pressure ratio that produces the maximum leakage through the gland.
The gland sealing system is designed to supply steam to seal the turbine shaft glands at all operating conditions and to extract leak-off steam from the glands.