Gas Turbine Training

Gas Turbine Introduction

• Led by :

• Damian Haworth

Invensys Triconex Singapore

Training Course Agenda 1.

What Is A Gas Turbine?

2.

Gas Turbine Main Components

3.

Gas Turb rbiine Auxiliary Equipm pmeent

4.

Control System Requirements

5.

Discussion and Q & A Session

What Is A Gas Turbine? •

Machinery used to convert fuel gas energy into useful electrical or mechanical work

•

Mechanical drives include compressors, pumps, fans

•

Electrical drives comprise generators connected to various systems such as utilities, local isolated plant

•

Fuel typically divided into two types: 1. Ga Gass Fue Fuell (N (Nat atur ural al Gas as)) 2. Li Liqu quid id Fuel Fuel (D (Dist istil illa late te,, Hea Heavy vy Fuel Fuel Oi Oil) l)

Gas Turbines • Can be split into two main areas, • Industrial  –  Single Shaft  –  Two Shaft  –  Tend to be larger, less efficient but less maintenanceintensive. More robust. • Aero-derivatives  –  Multi-shaft (up to three)  –  Typically very high speed  –  Highly efficient  –  High maintenance

Differences Between Turbine Types •

Single-Shaft SingleShaft ‘Heavy‘Heavy-Duty’ •

•

Two-Shaft TwoShaft ‘Heavy‘Heavy-Duty’ •

•

Typically used for generator applications, fixed speed Variable speed HP and LP shafts, used for both mechanical drives and electrical drives

Aero-derivative •

Based 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 bo th mechanical and electrical applications.

GE90 Aircraft Engine

ABB GT10 Gas Turbine

GE 9001H Gas Turbine

Single Shaft Gas Turbine

Main Turbine Components •

Inlet Air Ducting and Filtering  –  Provides clean, filtered and possibly heated or cooled air into the compressor inlet stage

•

Compressor Section  –  Provides high pressure air into combustion zone for both combustion and cooling

•

Combustion Zone  –  Contains fuel nozzles and can/transition piece for directing hot gases at high velocity into turbine section  –  Higher temperature units have Thermal Barrier Coating (TBC) on combustion components to limit stresses

Main Turbine Components, contd. •

Turbine Section  –  Converts high pressure, high velocity combustion gases into mechanical work to rotate turbine compressor and provide mechanical / electrical load  –  Higher temperature 1st and 2nd stage nozzles and turbine blades may be TBC coated to reduce thermal stress

•

Exhaust Section  –  Directs hot exhaust gases, typically 450 ~ 600C either to atmosphere for simple cycle turbines, or through a HRSG for combined cycle / cogeneration units  –  Section components are protected from excessive temperatures by Exhaust Temperature control algorithm

Turbine Axial Compressor This 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 Chamber This 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 Stages • This 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 Cycle This 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 Turbine

Compressor

Power Turbine Fuel Control Valve Trip Valve

Gas Generator Turbine Basket

Combustor

Aeroderivative Gas Gen Rotor

Aero-Derivative Turbine

Combustor

Housing

Fuel Nozzle

Liner

Transition Duct

Combustor Basket / Fuel Nozzle

Fuel

Combustion Air 

Typical Gas Turbine Start-Up Profile 1200

6000

1000

5000

 tT_XD_1 800

4000

 tT_XD_3  tT_XD_4  tT_XD_5

600

3000

 tT_XD_6  tT_XD_7  tT_XD_8

400

2000

 tT_XD_9  tT_XD_10  tT_XD_12

200

1000

 tT_XDT_1  tT_XDT_2 Spread  rSPEED

0 33:12.889

0 34:39.289

36:05.689

37:32.089

38:58.489

40:24.889

41:51.289

Gas Turbine Sub-Components

Control Elements • Fuel Valves  –  Modulate fuel flow to turbine fuel nozzles to control required parameter, eg acceleration, speed, load, temperature, etc. • Variable Turbine Nozzles  –  Not the same as fuel nozzles. These nozzles act to redirect first stage turbine exhaust gas onto second and third stage nozzles to change power distribution  between the turbine stages. Typically utilised on twoshaft mechanical drive turbines

Control Elements, contd. • Inlet Guide Vanes  –  Used to control mass air flow into the turbine to prevent surge during part-speed or startup conditions  –  Can 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 Valves  –  Used to maintain the turbine compressor in a safe operating region during part-speed or startup operating conditions  –  Fixed 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 Inputs •

Speed: Magnetic Pickups  –  Passive  –  Active  –  Provide speed feedback to the control system for startup, speed control and on-line control

•

Exhaust Temperature –  thermocouples  –  Provide control and protection for the turbine to prevent the turbine internals from being too stressed and possible failing

•

Compressor Discharge Pressure and Temperature  –  Used typically to calculate the operating condition of the turbine, and to provide a reference for the exhaust temp control limit

Main Gas Turbine Control Features • Startup Control (Cranking, Purging, Firing, Accelerating) • Speed Control • Auto / Manual Synchronising

• Initial Loading • Loaded, ‘On-Line’ operation (Droop / Isochronous) • Temperature Control

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

Startup Control •

Main Features: 1.

Bringing the machine to a minimum firing speed

2.

Purging the compressor / turbine / exhaust plenum to ensure no fuel (liquid or gas) remains from the previous shutdown

3.

Injecting 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)

4.

Fuel limiting to prevent excessive internal turbine temperatures (‘Warming up’)

5.

Bringing turbine to minimum operating speed in preparation for synchronizing (connecting to the grid)

Speed Control •

Main Features: 1. Run turbine shafts (HP and LP) such that minimum operating speed is maintained when off-line 2. Adjust turbine speed to match system frequency for Automatic Synchronisation 3. Speed adjustment for Overspeed Test utility

On-Line Control •

Control split into three main types:

1. Droop 2. Isochronous 3. MW PID Control

Droop Control •

Most common mode of on-line control for Industrial Gas Turbines when paralleled with a utility

•

Provides ‘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 regulation

•

Typical 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 = 10MW

•

System upset causes system frequency to fall by 1%

•

GT1 loads to 75MW, GT2 to 15MW, ie 25% of rated load

•

As the system frequency is restored to 50Hz by the grid operators, each machine returns to its original load

Droop Control Philosophy Speed Setpoint  Adjust 50 HZ

Load (based on Valve position)

Isochronous Control •

This mode means ‘Constant Speed’

•

In 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 completely

•

This 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 off

•

The isochronous controller will adjust the governor output to return the system frequency to the frequency setpoint

Isochronous Control Philosophy

Speed Setpoint  Adjust

50 HZ

Load

Megawatt PID Control •

Provides megawatt control utilizing standard Proportional - Integral –  Derivative control blocks

•

Adjusts the governor output until the desired turbine load is achieved

•

Does not respond to system frequency changes

•  Normally not suitable for utility-connected turbines due to regulatory requirements for droop response •

Typically 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 Systems • • • • • • •

Lubrication oil Control oil Cooling Air Cooling Water Sealing Air Fuel (Gas and Liquid)  NOx abatement (Water, Steam or Dry Low NOx technologies) • Water Wash • Fire and Gas Detection

Auxiliary Systems (contd.) • Inlet air filtering • Inlet 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 Pickups • Typically at least duplicated on even simplex controllers • Usually triplicated, sometimes 6 are present, 3 for control and 3 for over-speed protection • Magnetic ‘speed-wheel’ needs to be added to the turbine shaft in the case of mechanical governor retrofits (replacing mechanical linkages) • Can be active or passive • Also used on liquid fuel flow dividers to measure fuel flow

LVDT - Linear Variable Differential Transformer • Supply 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 position • Useful for precise position control, and tracking alarms and shutdowns

Switches • Can be used to alarm and trip the turbine in the event of a measured parameter exceeding allowable limits • Critical examples are low lube oil pressure, high lube oil temperature, low control oil pressure • Critical switches are typically triplicated in critical turbomachinery applications, non-critical usually simplex • Modern retrofits often replace old switches with more reliable transmitters, improving reliability and parameter monitoring

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

Safety Inputs •

Speed pickups  –  Automatically trip the turbine on over-speed conditions  –  May be wired to a dedicated over-speed device, subject to end-user and regulatory requirements and standards

•

Flame Detectors  –  Provide loss of flame indication, prevent explosive atmospheres from forming from excessive unburnt fuel in the turbine

•

Overspeed Mechanical Bolt  –  Typically used as a backup device in the event of a Primary (Electric) over-speed trip failure to operate

Safety Inputs (contd.) •

Vibration Probes  –  Used 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 Pressure  –  Prevents bearing and journal damage from lack of lubrication

•

Exhaust Temperature Thermocouples  –  Thermocouples used to provide over-temperature and temperature spread protection

Safety Functions and Consequences •

Overspeed (HP or LP shaft) –  Potential Catastrophic Destruction of Turbine

•

Flame Failure Detection –  Explosion Risk, Severe Damage to Rotor and Stationary components

•

Loss of Lube Oil Pressure –  Damaged Bearings and Rotor Journal

•

High Lube Oil Temperature –  Loss of oil film in journal bearings, reduced lubrication. Potentially severe bearing / journal damage

•

Exhaust 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 damage • Vibration High –  Potentially catastrophic turbine damage • Fire Detection –  Caused by gas leaks, liquid spills, etc. Potentially catastrophic damage to turbine and auxiliary equipment

Safety Elements •

Trip Valve  –  Immediately shuts off the fuel flow in the event of ANY trip situation arising

•

Fuel Control Valve  –  Shuts immediately on any trip condition, typically not gas tight, designed primarily for accurate fuel flow modulation into the turbine

•

Vent Valve  –  Bleeds off trapped gas in between the stop and control valves in a trip or shutdown condition  –  Prevents gas from leaking through control valve into the turbine  prior to the next turbine start