The Critical Role of Steam Turbine Gland Seals

A steam turbine consists of two major components: the stator and the rotor. During operation, the rotor spins at high speed while the stator remains stationary; consequently, a specific clearance must be maintained between the rotor and the stator to prevent mutual friction. As steam flows through the various stages of the turbine to perform work, its pressure and temperature decrease progressively, resulting in a pressure differential across the sides of each diaphragm. Furthermore, in stages where the moving blades possess a degree of reaction, a pressure differential also exists across the upstream and downstream sides of the moving blades themselves. While the vast majority of the steam flows through the passages formed by the stationary and moving blades to perform work, a small fraction inevitably leaks through various clearances without contributing to work output; this constitutes a loss that ultimately reduces the overall efficiency of the turbine unit.

Additionally, the rotor shaft must pass through the turbine casing to be supported by bearings; naturally, clearances must also be provided at these points. At the ends of the high-pressure casing and the inlet end of the intermediate-pressure casing-where the internal steam pressure exceeds the external atmospheric pressure-steam will inevitably leak outward. This leakage not only diminishes the unit's efficiency but also results in a loss of condensate.

Conversely, at the exhaust end of the intermediate-pressure casing and at both ends of the low-pressure casing-where the internal steam pressure falls below the external atmospheric pressure-ambient air will inevitably leak inward through the clearances where the main shaft penetrates the casing. Since air cannot condense within the condenser, its ingress reduces the vacuum level within the system, thereby diminishing the steam's capacity to perform work.

To minimize leakage through the aforementioned clearances while simultaneously ensuring the safe and reliable operation of the steam turbine, various types of steam seals are specifically installed. These seals can be broadly categorized into three main groups: seals for the steam flow path, diaphragm seals, and shaft-end seals. In terms of their fundamental operating principles, all three categories of seals are classified as labyrinth seals.