Gas Turbine Cooling and Sealing Air System

The cooling and sealing air system provides the necessary air flow from the gas turbine compressor to other parts of the gas turbine rotor and stator to prevent excessive temperature buildup in these parts during normal operation and for sealing of the turbine bearings. Air from two centrifugal type blowers is used to cool the turbine exhaust frame. These two motor fans are part of a cooling system located on a base, near the lower part of the gas turbine exhaust system duct.

88 TK-1 and 88 TK-2 motors are equipped with heaters 23 TK-1 and 23 TK-2 to prevent condensation when the motors are not operating.

Cooling and sealing functions provided by the system are as follows:

• Sealing of the turbine bearings.

• Cooling of internal turbine parts subjected to high temperature.

• Cooling of the turbine outer shell and exhaust frame.

• Providing an operating air supply for air operated valves.

The cooling and sealing air system consists of specially designed air passages in the turbine casing, turbine nozzles and rotating wheels, piping for the compressor extraction air and associated components. Associated components used in the system include:

• Turbine exhaust frame cooling blowers (as mentioned here above).

• Air filter (with poro-stone element).

• Pressure gauge.

• Dirt separator.

Functional Description
General
Air from the axial flow compressor, extracted from several points, is used for sealing the bearings, cooling turbine internal parts and to provide a clean air supply for air operated control valves. Compressor extraction air is also used for pulsation protection of the compressor during turbine start-up and shut-down.

Bearing sealing air is extracted from the fifth stage of the compressor. Internal cooling air is extracted from the discharge of the compressor including the internal flow of cooling air through the turbine rotating and stationary parts. Air used in cooling the turbine external casing is ambient air supplied by motor driven blowers. The schematic flow diagram, Figure CS-1, shows both the internal and external flow of cooling and sealing air.


Bearing cooling and sealing
Cooling and sealing air is provided from two connections on the compressor casing at the fifth stage and is piped externally to each of the three turbine bearings. Orifices in the air lines to the turbine bearings limit the flow of air and the pressure to the proper value. The centrifugal dirt separator located in the fifth-stage piping removes any particles of dirt or foreign matter that might be injurious to the bearings.

This pressurized air cools and seals the bearings by containing any lubricating fluid within the bearing housing that otherwise might seep past the mechanical seals. Air is directed to both ends of each bearing housing providing a pressure barrier to the lubricating fluid. After performing this function, the air is vented via the oil drain passage from the N° 1 and N° 3 bearings while air from the N° 2 bearing is vented to atmosphere. Limit switch 33 BQ is mounted on the valve and is used to monitor the valve position. If the valve VA 14 sticks and does not transfer bearing sealing air from eleventh-stage air, an alarm is annunciated on the control panel.

Exhaust frame and turbine shell cooling
Cooling of the exhaust frame and turbine shell is accomplished by two electric motor-driven, centrifugal blowers, 88 TK-1 and 88 TK-2, which are mounted external to the turbine. An inlet screen is provided with each blower and the discharge of each passes through a back-draft damper (check valve), VCK7-1 or VCK7-2 before entering openings in the exhaust frame outer sidewall cavity. The cooling air flow splits, with part of the air passing along and cooling the turbine shell and the other portion flowing through the exhaust frame strut passages. The air flow through the struts divides, with a portion directed through passages to cool the third-stage turbine aft wheelspace and the remainder flowing into the load shaft tunnel where it discharges through a duct to atmosphere. See Figure CS-1.

Air for cooling the exhaust frame and turbine shell is normally provided by the two blowers operating simultaneously in parallel. Each blower has a pressure switch, 63 TK-1 or 63 TK-2, to sense blower discharge pressure. If one of the blowers should fail, the loss of blower discharge pressure will cause contacts of the respective 63 TK-1 pressure switch to close and an alarm will be annunciated. The turbine will continue to run with the automatic change over starting the blower. If three blowers should fail, the turbine will continue to run with the other blower providing not in operation cooling air at a reduced flow rates. If all blowers should fail the turbine will be shutdown in a normal shutdown sequence.

NOTE: If one blower fails, it should be repaired or replaced as soon as possible to preclude the possibility of shutting down the turbine by failure of the remaining blower.

Pulsation protection
The pressure, speed and flow characteristics of the gas turbine compressor are such that air must be extracted from the 11th-stage and vented to atmosphere to prevent pulsation of the compressor during the acceleration period of the turbine starting sequence and during deceleration of the turbine at shut-down.

Pneumatically operated 11th stage air extraction valves, controlled by a three-way solenoid valve, are used to accomplish the pulsation protection function.


Pressurized air supply
Compressor discharge air is also used as a source of air for operating various air operated valves in other systems. Air for this purpose is taken at the discharge of the compressor and is then piped to the various air operated valves. In addition, compressor discharge pressure is monitored by redondant pressure transducers 96 CD-1A,-1B,-1C for use in control of the gas turbine.

NOTE: For turbine buckets and nozzle partitions cooling, refer to 2.5. in the actual part. For components descriptions, see the "Gas turbine subcontractor’s literature chapter".


Water wash provisions
When water washing the gas turbine compressor or turbine section, it is important to keep water out of the components that are actuated by compressor discharge air and out of the turbine bearings. To prevent water from entering these components and the bearings, isolation valves are provided in each compressor discharge extraction line and in the sealing lines to the n° 1, n° 2, n° 3 bearings and in 20 CB-1 and 96 CD feed lines.

During normal operation of the gas turbine, all isolation valves are to be open. Before initiating water wash, the isolation valves must be closed. At the conclusion of the water wash, the isolation valves must be reopened to allow normal operation of the turbine.