Gas Turbine Introduction

Led by : Damian Haworth

Invensys Triconex Singapore

Training Course AgendaWhat Is A Gas Turbine?Gas Turbine Main ComponentsGas Turbine Auxiliary EquipmentControl System RequirementsDiscussion and Q & A Session

What Is A Gas Turbine?Machinery used to convert fuel gas energy into useful electrical or mechanical workMechanical drives include compressors, pumps, fansElectrical drives comprise generators connected to various systems such as utilities, local isolated plantFuel typically divided into two types:Gas Fuel (Natural Gas)Liquid Fuel (Distillate, Heavy Fuel Oil)

Gas TurbinesCan be split into two main areas,IndustrialSingle ShaftTwo ShaftTend to be larger, less efficient but less maintenance-intensive. More robust.Aero-derivativesMulti-shaft (up to three)Typically very high speedHighly efficientHigh maintenance

Differences Between Turbine TypesSingle-Shaft Heavy-DutyTypically used for generator applications, fixed speedTwo-Shaft Heavy-DutyVariable speed HP and LP shafts, used for both mechanical drives and electrical drivesAero-derivativeBased on aircraft engine technology, highly efficient but typically less robust than Industrial turbines. Variety of shaft configurations (some have 3 shafts!). May incorporate multiple variable stator vanes for increased efficiency over a wide speed range. Used for both mechanical and electrical applications.

GE90 Aircraft Engine

ABB GT10 Gas Turbine

GE 9001H Gas Turbine

Single Shaft Gas Turbine

Main Turbine ComponentsInlet Air Ducting and FilteringProvides clean, filtered and possibly heated or cooled air into the compressor inlet stageCompressor SectionProvides high pressure air into combustion zone for both combustion and coolingCombustion ZoneContains fuel nozzles and can/transition piece for directing hot gases at high velocity into turbine sectionHigher temperature units have Thermal Barrier Coating (TBC) on combustion components to limit stresses

Main Turbine Components, contd.Turbine SectionConverts high pressure, high velocity combustion gases into mechanical work to rotate turbine compressor and provide mechanical / electrical loadHigher temperature 1st and 2nd stage nozzles and turbine blades may be TBC coated to reduce thermal stressExhaust SectionDirects hot exhaust gases, typically 450 ~ 600C either to atmosphere for simple cycle turbines, or through a HRSG for combined cycle / cogeneration unitsSection components are protected from excessive temperatures by Exhaust Temperature control algorithm

Turbine Axial CompressorThis section takes the fresh air drawn in through the turbine inlet, and compresses it for injection into the combustion section, where it is mixed with the fuel at the correct ratio for efficient combustion. The majority of this air is actually used for cooling, only a small fraction is used for combustion.

Combustion ChamberThis section injects the fuel (gas or liquid) at the rate determined by the Control System, mixes it with the air from the compressor, and passes the resultant combustion gas (approx. 1200 degC) into the turbine section, where it is converted into mechanical work.

Nozzles and Turbine StagesThis section takes the high pressure, high temperature gases produced within the combustion section, and extracts the energy contained within the gas, converting it into mechanical work which drives the turbine compressor and produces work output.This output is converted into energy in the form of the mechanical driving of a pump, or into electrical energy via a generator.These sections of the turbine are often coated in TBC (Thermal Barrier Coating) in order to protect the metal components from overtemperature.

Nozzles and Turbine Stages

Brayton CycleThis describes the chemical-mechanical energy conversion carried out by a gas turbine.The chemical energy in the fuel is converted into mechanical work by the compressor/combustion/turbine sections

Industrial Single Shaft GT

Two Shaft Gas TurbineFuel Control ValveTrip ValveCombustorCompressorGas Generator TurbineBasketPower Turbine

Aeroderivative Gas Gen Rotor

Aero-Derivative Turbine

CombustorFuel NozzleLinerTransition DuctHousing

Combustor Basket / Fuel Nozzle

Typical Gas Turbine Start-Up Profile

Gas Turbine Sub-Components

Control ElementsFuel ValvesModulate fuel flow to turbine fuel nozzles to control required parameter, eg acceleration, speed, load, temperature, etc. Variable Turbine NozzlesNot the same as fuel nozzles. These nozzles act to re-direct first stage turbine exhaust gas onto second and third stage nozzles to change power distribution between the turbine stages. Typically utilised on two-shaft mechanical drive turbines

Control Elements, contd.Inlet Guide VanesUsed to control mass air flow into the turbine to prevent surge during part-speed or startup conditionsCan be two-position (open-closed) or modulating (servo-actuator)Modulating controls provide efficient part-speed operation, and can be used to maximise exhaust temperatures for combined cycle and cogen applications

Control Elements, contd.Bleed ValvesUsed to maintain the turbine compressor in a safe operating region during part-speed or startup operating conditionsFixed stage axial compressors are designed to run at rated speed, and can easily surge during part-speed operation if the bleed valves are not operated

Control InputsSpeed: Magnetic PickupsPassiveActiveProvide speed feedback to the control system for startup, speed control and on-line controlExhaust Temperature thermocouplesProvide control and protection for the turbine to prevent the turbine internals from being too stressed and possible failingCompressor Discharge Pressure and TemperatureUsed typically to calculate the operating condition of the turbine, and to provide a reference for the exhaust temp control limit

Main Gas Turbine Control FeaturesStartup Control (Cranking, Purging, Firing, Accelerating)Speed ControlAuto / Manual SynchronisingInitial LoadingLoaded, On-Line operation (Droop / Isochronous)Temperature Control

What Is A Governor?A device to provide accurate speed control for rotating machineryOlder governors utilized rotating fly-balls with manual adjustments for frequency controlDigital governors utilize electrical speed feedback devices such as magnetic speed pickupsThe actual speed is compared to a speed setpoint to provide tight speed controlOnce synchronized, a digital governor will provide accurate speed / load control over the full range of turbine operations

Startup ControlMain Features:Bringing the machine to a minimum firing speedPurging the compressor / turbine / exhaust plenum to ensure no fuel (liquid or gas) remains from the previous shutdownInjecting minimum fuel and igniting (Firing). Flame is self-preserving from this point onwards. Possibly utilize specific ignition fuel (eg Propane bottles) if primary fuel is difficult to light (eg some liquids)Fuel limiting to prevent excessive internal turbine temperatures (Warming up)Bringing turbine to minimum operating speed in preparation for synchronizing (connecting to the grid)

Speed ControlMain Features:Run turbine shafts (HP and LP) such that minimum operating speed is maintained when off-lineAdjust turbine speed to match system frequency for Automatic SynchronisationSpeed adjustment for Overspeed Test utility

On-Line ControlControl split into three main types:DroopIsochronousMW PID Control

Droop ControlMost common mode of on-line control for Industrial Gas Turbines when paralleled with a utilityProvides assistance to grid in the event of upset conditions that cause the system frequency to either increase or decrease (droop)Digital governors provide easily adjustable droop regulationTypical droop regulation is 4 ~ 6 %For a 4% droop regulated system, a decrease in system frequency of 4% will cause the governor to increase load by 100%The operator typically adjusts the droop speed setpoint to adjust the steady state load on the turbine, or adjusts a load setpoint, and the governor automatically adjusts the speed setpoint to attain the desired load

Droop Control (contd.)With similar machines paralleled, each unit will adjust its load in an amount proportional to its rated load in the event of a system frequency disturbance.Eg Consider 2 machines with 4% droop regulation, one rated at 100MW (GT1), the other at 20MW (GT2)Steady state conditions (Governor free operation):GT1 = 50MW, GT2 = 10MWSystem upset causes system frequency to fall by 1%GT1 loads to 75MW, GT2 to 15MW, ie 25% of rated loadAs the system frequency is restored to 50Hz by the grid operators, each machine returns to its original load

Droop Control Philosophy50 HZSpeedSetpointAdjustLoad(based on Valve position)

Isochronous ControlThis mode means Constant SpeedIn this mode, the governor will attempt to keep the turbine frequency at the speed setpoint (typically Synchronous Speed)Except in special circumstances, it is not possible to run more than one machine in Isochronous when paralleled, otherwise one machine will pick up all the load, while the other unloads completelyThis mode is normally used by Isolated machines in order to keep a plant frequency steady.Load changes do not result in frequency changes, other than the transient speed changes when the load is picked up / dropped offThe isochronous controller will adjust the governor output to return the system frequency to the frequency setpoint

Isochronous Control Philosophy50 HZSpeedSetpointAdjustLoad

Megawatt PID ControlProvides megawatt control utilizing standard Proportional - Integral Derivative control blocksAdjusts the governor output until the desired turbine load is achievedDoes not respond to system frequency changesNormally not suitable for utility-connected turbines due to regulatory requirements for droop responseTypically used for isolated plants, where specific turbines are desired to run at a particular load, and other machines respond in either droop or isochronous in order to maintain system frequency and provide transient response in the case of system upsets

Auxiliary SystemsLubrication oilControl oilCooling AirCooling WaterSealing AirFuel (Gas and Liquid)NOx abatement (Water, Steam or Dry Low NOx technologies)Water WashFire and Gas Detection

Auxiliary Systems (contd.)Inlet air filteringInlet air cooling (refrigerant, evaporative cooling)Inlet air heating (NOx mode transfer setpoint, anti-icing)Generator cooling (air, hydrogen, water)Starting means (electric, diesel, VFD)

Turbine Instrumentation

Magnetic PickupsTypically at least duplicated on even simplex controllersUsually triplicated, sometimes 6 are present, 3 for control and 3 for over-speed protectionMagnetic speed-wheel needs to be added to the turbine shaft in the case of mechanical governor retrofits (replacing mechanical linkages)Can be active or passiveAlso used on liquid fuel flow dividers to measure fuel flow

LVDT - Linear Variable Differential TransformerSupply position feedback indication for modulating controllers (gas valves, IGVs etc.)Very often not used on older turbines, signal out often assumed to place the valve in the correct positionUseful for precise position control, and tracking alarms and shutdowns

SwitchesCan be used to alarm and trip the turbine in the event of a measured parameter exceeding allowable limitsCritical examples are low lube oil pressure, high lube oil temperature, low control oil pressureCritical switches are typically triplicated in critical turbo-machinery applications, non-critical usually simplexModern retrofits often replace old switches with more reliable transmitters, improving reliability and parameter monitoring

TransmittersProvide accurate feedback on a multitude of turbine parameters (pressure, temperature, level, etc.)Modern transmitters are more reliable than switches, having a lower PFDMultiple transmitters (2oo3) provide ideal replacements for unreliable existing simplex instrumentation

Safety InputsSpeed pickupsAutomatically trip the turbine on over-speed conditionsMay be wired to a dedicated over-speed device, subject to end-user and regulatory requirements and standardsFlame DetectorsProvide loss of flame indication, prevent explosive atmospheres from forming from excessive unburnt fuel in the turbineOverspeed Mechanical BoltTypically used as a backup device in the event of a Primary (Electric) over-speed trip failure to operate

Safety Inputs (contd.)Vibration ProbesUsed to prevent turbine damage from misalignment, imbalance, etc.Can be Seismic (magnitude only, little or no diagnostic value) or Proximity (typically installed in 90 degree-apart pairs, provide excellent diagnostic analysis when coupled with powerful software, eg Bently System 1)Oil PressurePrevents bearing and journal damage from lack of lubricationExhaust Temperature ThermocouplesThermocouples used to provide over-temperature and temperature spread protection

Safety Functions and ConsequencesOverspeed (HP or LP shaft) Potential Catastrophic Destruction of TurbineFlame Failure Detection Explosion Risk, Severe Damage to Rotor and Stationary componentsLoss of Lube Oil Pressure Damaged Bearings and Rotor JournalHigh Lube Oil Temperature Loss of oil film in journal bearings, reduced lubrication. Potentially severe bearing / journal damageExhaust Overtemperature Stress on Hot Gas Path Components and potential shortened life cycle. In severe cases, immediate loss of turbine blades and extensive turbine damage

Safety Functions (contd.)Exhaust Temperature Spread Hot / Cold Spots in combustion area, Hot Gas Path component damageVibration High Potentially catastrophic turbine damageFire Detection Caused by gas leaks, liquid spills, etc. Potentially catastrophic damage to turbine and auxiliary equipment

Safety ElementsTrip ValveImmediately shuts off the fuel flow in the event of ANY trip situation arisingFuel Control ValveShuts immediately on any trip condition, typically not gas tight, designed primarily for accurate fuel flow modulation into the turbineVent ValveBleeds off trapped gas in between the stop and control valves in a trip or shutdown conditionPrevents gas from leaking through control valve into the turbine prior to the next turbine start