Combustion Gas Turbines - Turbine System Details PDF Print E-mail
Written by Norrie   
Wednesday, 13 January 2010 10:12
Article Index
Combustion Gas Turbines
Principles and Operation
The Air Compressor
The Combustion Chamber
The Compressor (HP) Turbine
The Variable-Angle Nozzles Load Turbine
Turbine/Compressor Lube Control Oil System
Turbine Hydraulic Oil Trip System
Turbine Overspeed Trip Mechanisms
Turbine System Details
All Pages

TURBINE SYSTEM DETAILS

This explanation has been simplified as far as possible in order to make understanding of the system easier.

STARTING EQUIPMENT
The gas turbine is cranked for starting by an 'EXPANSION GAS TURBINE'. The starting turbine is connected to the gas turbine through the 'ACCESSORY GEAR' and a 'JAW CLUTCH’ The drive shaft of the starter turbine is splined to receive the clutch hub. The driven hub of the clutch assembly is installed on one shaft of the accessory gear.


CONTROL SYSTEM
The gas turbine control system controls a complete power plant. The system utilises electrical and hydraulic devices which regulate the flow of fuel gas to the combustion chambers and adjust the position of the variable angle second stage nozzles. This in turn regulates the POWER and SPEED of the gas turbine. Various alarms and trips are installed to give alarm and/or shutdown should undesirable operating conditions arise.

THE FUEL GAS REGULATOR
The FUEL GAS REGULATOR control system consists of THREE major inputs; Start-up, Speed & Temperature. The outputs from these are fed into the Fuel Gas Regulator, where the input signal requiring the LEAST fuel takes control of the system. The OUTPUT of the Fuel Gas Regulator is a Constant Control Oil ('CCO') which is then converted to Variable Control Oil ('VCO') which will determine the fuel gas flow. CCO is also converted to another variable oil called Nozzle Control Oil ('NCO') that controls the 2nd stage variable nozzles through the Nozzle Regulator and the Nozzle Control Cylinder.

START-UP LOOP
See Figure. 27 below:
SIMPLIFIED TURBINE CONTROL LOOP

Figure. 27

The FIRST input, is the START-UP LOOP.
While the Speed & Temperature inputs are CLOSED loops, the start-up loop is OPEN. Two major inputs to the Fuel Regulator are used during Start-up.
1. MANUAL SELECTOR SWITCH POSITION
(Off, Crank, Fire, Accelerate and Run)
2. START-UP / TEMPERATURE PROGRAMME.
The Manual Selector Switch is used to select pre-determined values of VCO which, in turn, operate the gas control valve and controls the turbine start-up.


The start-up temperature control device, first suppresses the VCO to a limit which controls the EXHAUST temperature for a warm-up of one minute, then, slowly removes this temperature suppression to accelerate the limit under a controlled condition. Speed is controlled by an electric governor.

TEMPERATURE CONTROL LOOP
This control loop consists of the following components: -

  1. Twelve (12) exhaust thermocouples.
  2. Thermocouple averaging cabinet.
  3. MV/I (Millivolt to Milliamp) converter.
  4. Milliamp (MA) to PSI transducer. (I/P).
  5. Fuel Regulator Temperature Bellows.

The 12 thermocouples detect the turbine exhaust temperature and produce an average Millivolt signal. The Millivolt (MV) signal is converted to a Milliamp (MA) signal by the MV/I converter. This, in turn, is converted to a pneumatic (air) signal by the MA to PSI (I/P) transducer. This air signal then goes to the temperature bellows of the fuel regulator where it over-rides the normal turbine control signal and reduces the VCO to the gas control valve. This decreases the fuel and thereby decreases the exhaust temperature.

SPEED CONTROL
At 95% speed, the speed controller takes over and reduces fuel to ' Full-speed, No-load ' value. The variable, 2nd stage nozzles are opened to high flow position.

LOADING
In order to pick up load on the LP turbine, more fuel is admitted into the combustion chambers by a signal into the fuel regulator speed governor circuit. This in turn increases the VCO and opens the fuel gas control valve. At the same time, the 2nd stage nozzles close down to control the speed of the HP turbine.

NORMAL SHUTDOWN
The machine loading is slowly decreased and the speed control brought down to minimum governor. The machine can then be shut down by operating the manual trip lever or push button on the control panel. Check that auxiliary lube oil system is operating.

EMERGENCY SHUTDOWN
This is carried out by operating the manual shutdowns as above. Again, check auxiliary lube oil system.

PROTECTION SYSTEMS
The gas turbine / driven machine systems are protected by a number of alarms and trips which will activate in the event of an undesirable condition arising. The alarms will give warning of impending problems. Mechanical trips are the LP & HP turbine overspeed trips and the Hydraulic Manual Trip Valve. Electrical trips will operate the solenoid dump valve and consist of : -

  • High vibration trip
  • Low lube oil pressure
  • High lube oil temperature
  • High bearing temperature
  • Cooling water failure or high temperature
  • Power failure
  • Fuel gas failure ... etc

If any of the mechanical or electrical trips operate, the hydraulic oil will dump and cause the fuel gas stop valve to close. At the same time, the NCO will dump to return the 2nd stage nozzles to zero setting. Refer to the notes and diagrams on the lube, control and hydraulic oil systems.

 

 

About the Author

Norrie is a retired professional who has been working in Oil and Gas and LNG production in Marsa-el-Brega, Libya for 30 years.

Norrie used to be in the Training Dept. and prepared Programmes for Libyan Trainees.

 



Last Updated on Wednesday, 24 February 2010 20:06