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.

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 Disadvantages 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 engine Engine Nacelle Fan Low pressure compressor High Pressure Compressor Combustion Chamber High Pressure Turbine Low Pressure turbine Exhaust cone

Basic Components

Basic Components

Basic Components Compressor Combustion Chamber Turbine 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 Combustion Chamber Turbine 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 Combustion Chamber Turbine 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

Compressor Supplies high pressure air for combustion process Compressor types Radial/centrifugal flow compressor Axial flow compressor

Compressor Radial/centrifugal flow Axial 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 Compressor Stall 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%) Secondary Air (65%) 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 Lubrication 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 Power Transmission 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 Compressor 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 Overspeed Trip 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 VERY poor partial load fuel economy LM 2500’s connected to reduction gears via pneumatic clutch Three possible lineups Full Power Split Plant Trail Shaft

Full Power Line Up 4 2 Reduction Gears Reduction Gears 3 1 PORT Shaft Clutch Clutch Clutch Reduction Gears 3 1 PORT Shaft STBD Shaft

Split Plant Line Up 4 2 Reduction Gears Reduction Gears 3 1 PORT Shaft Clutch Clutch Clutch Reduction Gears 3 1 PORT Shaft STBD Shaft

Trail Shaft Line Up 4 2 Reduction Gears Reduction Gears 3 1 PORT Shaft Clutch Clutch Clutch Reduction Gears 3 1 PORT Shaft STBD Shaft

Plant Lineups Full Power Lineup Split Plant Lineup Trail Shaft Lineup 2 turbines/shaft with 2 shafts (4 turbines) Max speed > 30+ kts Split Plant Lineup 1 turbine/shaft with 2 shafts (2 turbines) Max speed = 30 kts Trail 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?