7.1.2 System layout
A typical gland sealing system is shown in Fig. 2.71. In order to accommodate the range of temperatures experienced throughout the turbine, the system is usually divided into two parts: one part supplies steam to the HP and IP turbine glands and the other to the LP turbine glands.
Two modes of operation are used: one employs a supply of steam at superheater outlet conditions, which is referred to as 'live steam' and is used during start-up, shutdown and low load operation. In the other mode, steam leaked off from the HP and IP turbine is employed to seal the LP glands when the turbine is operating on-load. The use of HP/IP glands leak-off steam produces a useful thermal gain over the permanent use of live steam. The changeover from one source of sealing steam to the other is entirely automatic.
To ensure that the steam is supplied to the glands at a suitable temperature, it is cooled by desuper-heaters. An HP desuperheater regulates the temperature of the steam of the HP/IP glands and an LP desuperheater regulates the temperature of the steam to the LP glands. Some systems also employ a third desuperheater to cool the steam which is bled to an LP heater during on-load operation.
In order to reduce the loss of energy in external glands at the ends of the HP and IP cylinders, and to promote a gradual temperature gradient along a shaft, it is usual to divide such glands into sections, leading steam back to an appropriate stage in the turbine or to a feedheater after each section. As a result, heat is returned to the cycle.
The arrangement of the final glands section for an HP cylinder is shown in Fig 2.72 (a).
The HP leak-off belt is usually connected to the IP/LP crossover pipes and the pressure is thereby maintained at the IP exhaust pressure. The pressure at the packing/leak-off point is usually stabilised slightly above atmospheric by the pressure control system. The steam bled from this leak-off is normally used to seal the glands of the LP cylinders; the LP cylinder gland arrangement is shown in Fig 2.72 (b). The outflowing steam prevents the ingress of air into the cylinder and condenser. The final belt in all of the glands is connected to the gland steam condenser, which maintains the pressure at the belts slightly below atmospheric. This prevents steam leaking into the turbine hall by maintaining an inward flow of air through the outboard gland section.
At low loads, live steam enters the system through a motorised isolating valve and a pressure reducing valve to the HP desuperheater. Here the steam is cooled to a temperature suitable for the HP/IP glands. It then passes through a motorised isolating valve to the HP/IP glands or, via a separate motorised valve, to the LP desuperheater which further cools the steam to a temperature suitable for the LP glands.
At higher loads, when the HP/IP glands are self-sealing, excess steam from these glands flows to the LP desuperheater and is used to seal the LP glands. The steam flowing from the HP/IP glands tends to increase the pressure in the system. This is sensed by the control system (see below) which closes the pressure reducing valve in the live steam supply line. If more steam is available from the HP/IP glands than is required to seal the LP glands, the control system opens the dump valve which allows the excess steam to pass to an appropriate LP heater.
To prevent any foreign matter from entering the turbine shaft glands, two strainers are provided, one for the HP/LP system and the other for the LP system, with each strainer positioned after the appropriate desuperheater.