Principle of the Labyrinth Gland

he leakage of steam is reduced by the use of labyrinths, these provide a tortuous path for the steam to follow to exit the turbine reducing the pressure across a series of fine clearances to a level that can easily be managed by the gland steam system.
Within the cavity where the flow is turbulent, the velocity of the steam is increased with an associated drop in pressure. The kinetic energy is then dissipated by the change in direction, turbulence and eddy currents.fugal action. Very small heads can deal with large pressure drops

Materials

A typical clearance between the rotor and the fixed gland is about 0.25 to 0.38mm, hence with very little rotor distortion the possibility of rub occurs. This has led to the use of soft, self lubricating materials for the gland segments. The simplest form of gland consists of carbon rings held on to the shaft by the use of garter springs. Carbon Dioxide is formed with contact with superheated steam making this material only suitable for low temperature requirements.
Brass and Copper led alloys have been found suitable with an alloy of Lead, Copper and Nickel being suitable up to 520^oC.

Shaft Rub

Should the rotor bend, say due to carry over the area of rub on the gland will be over a small arc. With successive revolutions the heat generated will increase bend. This increases the area of contact and magnifies the condition by the increased generation of heat. Plastic flow occurs when the material yields under compression to reduce the stress and on cooling a permanent set occurs.
One side of the gland, typically the stationary part is made up of thinned sections thereby reducing the contact area to a minimum. Spring loaded glands prevent this from happening by giving under contact with the rotor , limiting the heat generated and giving time for the rotor to recover its shape.

Spring backed gland

The minimum clearance for a spring backed gland ranges from 0.3 mm fot eh HP inlet to 0.63 mm for the Astern turbine. For fixed glands the minimum is 0.5 mm. With the smaller clearances there is an increase in efficiency

Hydrostatic Gland

A wheel forged on the rotor ends runs in a water bath. This water is flung out by centrifugal action. The gland only needs to be small as large pressure drops require little head. The system cannot be used on reversible sets as the seal effect is lost at reduced revolutions. It is more likely to be seen on turbo-alternators.