The two functions of the turbine glands and seals are:1
To prevent or reduce steam leakage between the rotating and stationary components of the turbines if the steam pressure is higher than atmospheric.
To prevent or reduce air ingress between the rotating and stationary components of the turbines if the steam pressure is less than atmospheric. The last few stages in the low-pressure (LP) turbines are normally under vacuum.
The leakage of steam or air could occur where the shaft is extended through the turbine endwalls to atmosphere. A power loss is associated with steam leakage or air ingress. Thus, the design of glands and seals is optimized to reduce any leakage.
Modern steam turbines use labyrinth glands to restrict steam and air leakage. However, the carbon ring gland is still used on some older turbines.
The labyrinth gland can withstand higher steam conditions than the carbon ring gland. Figure 9.1 ( a) illustrates a simple form of a labyrinth seal. It consists of a ring having a series of machined fins. The fins form a number of fine annular restrictions. An expansion chamber follows each restriction. When the steam enters a restriction, the velocity increases and the pressure decreases (conversion of pressure energy into kinetic energy the first law of thermodynamics). When the steam enters the expansion chamber, the kinetic energy is converted by turbulence into heat. The pressure is not recovered. The pressure is progressively reduced when the steam goes...
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