Gas Turbines

References

Required•Principles of Naval Engineering (pp 106-115)

Optional•Introduction to Naval Engineering (Ch 12).

Objectives

A. Comprehend the thermodynamic processes occurring in a gas turbine. B. Comprehend the basic operation, key components, and safety considerations of gas turbine engines (single and split shaft) and propulsion plants, including support systems. C. Know the features of shipboard gas turbine plant arrangements, including main propulsion and auxiliary machinery configuration on the CG-47 class ships. D. Know the features of intake and exhaust duct systems in a typical gas turbine plant. E. Know the propulsion plant lineup variations for a 4 engine, 2 shaft gas turbine plant.

http://en.wikipedia.org/wiki/Image:USS_Arleigh_Burke_Mediterranean.jpg

http://upload.wikimedia.org/wikipedia/commons/8/8b/GE_H_series_Gas_Turbine.jpg

Background

• Aircraft turbojet/turbofan engines are precursors to gas turbines

• Installed for propulsion in:– FFG’s– DD’s– DDG’s– CG’s– M-1 tanks

• Also used for electrical generation & auxiliary applications

Background• Advantages of GTE• High power to weight ratio• Smoothness of operation• Less radiated noise compared to Diesel Engine• Fuel economy is comparable to Diesel Engines• Efficient at high speeds

• Disadvantages• Large quantities of air necessary (contaminated air can cause

damage)• Loud high pitched noises can cause hearing loss• High exhaust heat makes the susceptible to anti-ship missiles• Major casualties are not easily repaired on board.• Maintenance is complicated and costly if performed incorrectly

(a pencil mark on a blade or a finger print in the wrong place can cause failure)

Brayton Cycle• Unlike diesels, operate on STEADY-FLOW cycle• Open cycle, unheated engine1. Engine Nacelle

2. Fan3. Low pressure

compressor4. High Pressure

Compressor5. Combustion

Chamber6. High Pressure

Turbine7. Low Pressure

turbine8. Exhaust cone

Basic Components

Basic Components

Basic Components• CompressorCompressor– Draws in air & compresses it.

• Combustion Chamber– Fuel pumped in and ignited to burn with compressed

air• Turbine– Hot gases converted to work– Can drive compressor & external load

Basic Components• Compressor– Draws in air & compresses it

• Combustion ChamberCombustion Chamber– Fuel pumped in and ignited to burn with compressed

air• Turbine– Hot gases converted to work– Can drive compressor & external load

Basic Components• Compressor– Draws in air & compresses it

• Combustion Chamber– Fuel pumped in and ignited to burn with compressed

air• TurbineTurbine– Hot gases converted to work– Can drive compressor & external load

Compressor

• Supplies high pressure air for combustion process

• Compressor types– Radial/centrifugal flow compressor– Axial flow compressor

Compressor

• Radial/centrifugal flow– Adv: simple design, good

for low compression ratios (5:1)

– Disadv: Difficult to stage, less efficient

• Axial flow – Good for high

compression ratios (20:1)– Most commonly used

Compressor

• Controlling Load on Compressor– To ensure maximum efficiency and allow for

flexibility, compressor can be split into HP & LP sections

– Vane control: inlet vanes/nozzle angles can be varied to control air flow

• Compressor Stall– Interruption of air flow due to turbulence

Use of Compressed Air

• Primary Air (30%)– Passes directly to combustor for combustion

process• Secondary Air (65%)– Passes through holes in perforated inner shell &

mixes with combustion gases• Film Cooling Air (5%)– Insulates/cools turbine blades

Combustion Chambers

• Where air & fuel are mixed, ignited, and burned

• Spark plugs used to initially ignite fuel• Types– Can: for small, centrifugal compressors– Annular: for larger, axial compressors (LM 2500)– Can-annular: for really large turbines

Combustion Chambers

Annular Can-Annular

Turbines• Consists of one or more stages designed to

develop rotational energy• Uses sets of nozzles & blades• Single shaft– Power coupling on same shaft as turbine– Same shaft drives rotor of compressor and

power components

Gas Turbine Systems

• Fuel System– Uses either DFM or JP-5– Regulates the speed of the engine during steady

state and transient conditions. Controls acceleration.

• Lubrication System– Supply bearings and gears with oil– Provides source of hydraulic power for controls

Gas Turbine Systems

• Air System– Air intakes are located high up & multiple filters– Exhaust discharged out stacks

Air Intake & Exhaust

• Must minimize space and weight

• Must keep air inlet losses to a minimum to ensure maximum performance

• Intake has screens/filters to ensure clean, filtered air at all times

Air Intake & Exhaust• Exhaust generates thermal and acoustic

problems– Possible damage to personnel & equipment– Increased detection & weapons guidance

from heat (IR signature)

• Silencers and eductor nozzles used to silence and cool exhaust

Air Intake & Exhaust

Air Intake & Exhaust

Gas Turbine Systems

• Starting System– To get compressor initially rotated, HP air used

(can use electrical also)– Once at certain RPM, fuel injected and spark

ignited• Power Transmission System– Reduction gears used to transfer torque– With split shaft, turbines can run @ different

speeds

Split Shaft Design• Split Shaft– Gas generator turbine drives compressor– Power turbine separate from gas generator turbine– Power turbine driven by exhaust from gas generator

turbine– Power turbine drives power coupling

LM 2500

• In DDG’s and CG’s, have 4 engines• In FFG’s, have 2 engines• Engines are shock mounted to minimize noise

and allow for protection• Advantages of LM 2500– Compact & light– Easy to maintain & repair– Quick start time (~ 1 min)

LM 2500

LM 2500 Components• Starter– Pneumatic - driven by pressurized air

• Compressor– 16-stage, axial flow (17:1 compression ratio)– Has some controllable pitch vanes to

provide proper air flow and prevent stall

LM 2500 Components

• Combustion Chamber– Annular design – 30 fuel nozzles

LM 2500 Characteristics

• Stage efficiency = 92.5%• R&D: 30,000+ hrs of op-testing• Two versions available:– LM 2500-20 (22,500 shp)– LM 2500-30 (30,000 shp) – USN warships

LM 2500 Engine Control

• Speed Governor– Used to prevent power turbine from exceeding

speed limit (104%)– Reduces fuel to gas generator section which

reduces gases to power turbine

• Overspeed Trip– If governor fails, trip secures fuel to LM 2500 to

shut it down (108%)

CRP Propeller & Propulsion Shafting

• Shaft is hollow to provide flow of oil to propellers

• LM 2500 cannot operate at < 5,000 RPM (corresponds to ~11 kts for DDG)

• Pitch of blades controlled hydraulically through pistons and gears

• Pitch must be adjusted to go slower than 11 kts

CRP Propeller & Propulsion Shafting

• In order to go faster than 11 kts, shaft RPM increased

• In order to go astern, pitch varied to reverse flow

• Overall purpose– Controllable pitch to improve efficiency– Reversible to allow for ahead/astern flow with

single direction rotation of shaft

Plant Lineups

• Disadvantage of gas turbine– VERYVERY poor partial load fuel economy

• LM 2500’s connected to reduction gears via pneumatic clutch

• Three possible lineups– Full Power– Split Plant– Trail Shaft

Reduction Gears

Reduction Gears

13

24

Clutch

ClutchClutch

STBDShaft

PORTShaft

Full Power Line Up

Reduction Gears

Reduction Gears

13

24

Clutch

ClutchClutch

STBDShaft

PORTShaft

Split Plant Line Up

Reduction Gears

Reduction Gears

13

24

Clutch

ClutchClutch

STBDShaft

PORTShaft

Trail Shaft Line Up

Plant Lineups

• Full PowerFull Power Lineup– 2 turbines/shaft with 2 shafts (4 turbines)– Max speed > 30+ kts

• Split PlantSplit Plant Lineup– 1 turbine/shaft with 2 shafts (2 turbines)– Max speed = 30 kts

• Trail ShaftTrail Shaft Lineup– 1 turbine/shaft with 1 shaft (1 turbine)– Other shaft windmilling– Max speed = 19 kts

Take Away• Give the advantages and disadvantages to GT

propulsion• Draw a one-line diagram of a gas turbine and

explain the Brayton cycle• Describe purpose/advantage of split shaft vs.

single shaft GTE• Describe the various methods of power

transmission and speed control for GTE• List and describe the various plant lineups

used with GT propulsion plants

Questions?