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2.4. COMBUSTION SECTION

2.4.1. GENERAL

Description

The combustion system is of the reverse-flow type with 14 combustion chambers arranged around the periphery of the compressor discharge casing. This system also includes fuel nozzles, spark plug ignition system, flame detectors, and crossfire tubes. Hot gases, generated from burning fuel in the combustion chambers, are used to drive the turbine. High pressure air from the compressor discharge is directed around the transition pieces and into the combustion chambers liners. This air enters the combustion zone through metering holes for proper fuel combustion and through slots to cool the combustion liner. Fuel is supplied to each combustion chamber through a nozzle designed to disperse and mix the fuel with the proper amount of combustion air.

Orientation of the combustion chambers around the periphery of the compressor is shown on figure next page. Combustion chambers are numbered counter-clockwise when viewed looking down-stream and starting from the top of the machine. Spark plugs and flame detectors locations are also shown.

Combustion Chamber Arrangement

Elevation View

(View looking downstream)

Combustion Chamber Arrangement

To Note:

  • Flame detectors (1,2,3, and 14)
  • Spark plugs (11,12)

2.4.2. COMBUSTION WRAPPER, COMBUSTION CHAMBERS AND CROSSFIRE TUBES

Combustion wrapper :

The combustion wrapper forms a plenum in which the compressor discharge air flow is directed to the combustion chambers. Its secondary purpose is to act as a support for the combustion chamber assemblies. In turn, the wrapper is supported by the compressor discharge casing and the turbine shell.

Combustion chambers :

Discharge air from the axial flow compressor flows into each combustion flow sleeve from the combustion wrapper (see figure). The air flows up-stream along the outside of the combustion liner toward the liner cap. This air enters the combustion chamber reaction zone through the fuel nozzle swirl tip, through metering holes in both the cap and liner and through combustion holes in the forward half of the liner.

The hot combustion gases from the reaction zone pass through a thermal soaking zone and then into a dilution zone where additional air is mixed with the combustion gases. Metering holes in the dilution zone allow the correct amount of air to enter and cool the gases to the desired temperature. Along the length of the combustion liner and in the liner cap are openings whose function is to provide a film of air for cooling the walls of the liner and cap as shown in figure. Transition pieces direct the hot gases from the liners to the turbine nozzles. All fourteen combustion liners, flow sleeves and transition pieces are identical.

Crossfire tubes :

All fourteen combustion chambers are interconnected by means of crossfire tubes. These tubes enable flame from the fired chambers to propagate to the unfired chambers.

Combustion Chamber Details and Flow Diagram

Combustion Chamber Details

2.4.3. SPARK PLUGS AND FLAME DETECTORS

Spark plugs :

Combustion is initiated by means of the discharge from two high-voltage, non-retractable spark plugs bolted to flanges on the combustion chambers and mounted in a primary zone cup in adjacent combustors (N° 11 and 12).

These spark plugs receive their energy from ignition transformers. At the time of firing, a spark at one or both of these plugs ignites the gases in the primary zone of the chamber ; the remaining chambers are ignited by crossfire through the tubes that interconnect the reaction zones of the remaining chambers.

Spark Plug

Spark Plug

Flame detectors :

During the starting sequence, it is essential that an indication of the presence or absence of flame be transmitted to the control system. For this reason, a flame monitoring system is used consisting of eight sensors, each pair installed on four combustion chambers (n° 4 and 5, 10 and 11 primary and secondary zone) and an electronic amplifier which is mounted in the turbine control panel.

The ultraviolet flame sensor consists of a flame sensor containing a gas filled detector. The gas within this flame sensor detector is sensitive to the presence of ultraviolet radiation which is emitted by a hydrocarbon flame. A DC voltage, supplied by the amplifier, is impressed across the detector terminals. If flame is present, the ionization of the gas in the detector allows conduction in the circuit which activates the electronics to give an output defining flame. Conversely, the absence of flame will generate an opposite output defining "no flame". After the establishment of flame, if voltage is reestablished to the sensors defining the loss (or lack) of flame a signal is sent to a relay panel in the turbine electronic control circuitry where auxiliary relays in the turbine firing trip circuit, starting means circuit, etc... shut down the turbine. The FAILURE TO FIRE or LOSS OF FLAME is also indicated on the annunciator. If a loss of flame is sensed by only one flame detector sensor, the control circuitry will cause an annunciation only of this condition.

Flame detectors are water cooled.


2.4.4. FUEL NOZZLES (GAS)

Description :

Each combustion chamber is equipped with a fuel nozzle that emits the metered amount of the required fuel into the combustion liner. Fuel nozzles are used in gas turbines burning gas. The fuel nozzle functions to distribute the gas fuel into the reaction zone of the combustion liner, in a manner which promotes uniform, rapid and complete combustion. Gas fuel enters the fuel nozzle assembly through the fuel gas connection flange and is routed through nozzle internal passages to orifices located in the gas tip.

Fuel Gas Nozzles Flanges

2.4.5. TRANSITION PIECES

Description :

Transition pieces direct the hot gases from the liners to the turbine first stage nozzle. Thus, the first nozzle area is divided into 14 equal areas receiving the hot gas flow. The transition pieces are sealed to both the outer and inner sidewalls on the entrance side of the nozzle, so minimizing leakage of compressor discharge air into the nozzle.

Transition Piece

Transistion Piece

Transition Piece Arrangement (Typical)

Transistion Piece Arrangement Typical

2.4.6. FALSE START DRAIN

False start drain valves shown on the washing diagram are opened for washing purpose. Air pressure from the discharge unit's axial-flow compressor is used to actuate these valve. Valves drain water from the combustion chambers.

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