Engineering Training

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ASSIGNMENT SHEET

Assignment Sheet 60B-104

INTRODUCTION

LESSON TOPIC LEARNING OBJECTIVES

Terminal Objective:

Enabling Objectives:

STUDY ASSIGNMENT

1. Read Information Sheet 60B-104.
1. Outline Information Sheet 60B-104 using the enabling objectives for lesson 60B-104 as a guide.
1. Answer study scenarios.

After looking over the message traffic, you notice that one of the FFGs in your battle group is not around. Curious, you ask the OOD if she knows where they went and she tells you that they had to pull into Bahrain to replace and engine because of a bad combustor.

INFORMATION SHEET

BASIC GAS TURBINE ENGINES

Information Sheet 64B-104I

INTRODUCTION

REFERENCES

INFORMATION

1. Lesson Overview: The gas turbine engineering plant represents an innovative concept for shipboard power plants. US Naval vessels use aircraft derivative gas turbine engines for both main propulsion and ship's service electrical power. A high degree of plant automation is achieved with an integrated system of control and monitoring consoles.
2. Advantages: The advantages of a gas turbine plant as compared to a steam plant of comparable horsepower include:
1. Weight reduction of 70%
   
2. Simplicity (fewer propulsion auxiliaries)
   
3. Reduced manning due to automated propulsion plant control
   
4. Quicker response time
   
5. Faster acceleration/deceleration
   
3. Gas Turbine Principles:
   
1. Components of the basic gas turbine engine include:
   
1. Compressor
   
2. Combustor
   
3. Turbine
   
2. Operating Cycle: In a gas turbine engine the compression, combustion and expansion take place continuously in different chambers. Gas turbine engines operate on the Brayton Cycle (open engine cycle).
   



Fig 1: Brayton Cycle


1. Intake phase: Outside air is drawn into the engine by the action of the compressor. Pressure, temperature and volume remain the same through the intake phase.
   
2. Compression phase: Intake air is mechanically compressed. Pressure and temperature increase with a corresponding decrease in volume. Mechanical energy driving the compressor is converted to kinetic energy in the form of compressed air.
   
3. Combustion phase: Fuel is sprayed into the combustor and burned converting the chemical energy to thermal energy in the form of a hot expanding gas. Volume and temperature greatly increase while pressure remains constant through the combustor.
   
4. Expansion phase: Thermal energy is converted to mechanical energy as the hot expanding gases from the combustor turn the turbine rotor. Pressure and temperature decrease while volume increases through the expansion phase.
   
5. Exhaust phase: Hot exhaust gases are ducted through ships uptake to the atmosphere. Pressure, temperature and volume remain the same through the exhaust phase.
   
4. Gas Turbine Components:
   
1. Compressors: There are two basic types of gas turbine compressors.
   
1. Centrifugal compressor: This compressor uses a spinning impeller to draw in intake air and accelerates it outward by means of centrifugal force into a diffuser. It is used in small gas turbines and is best suited for low pressure ratios where the overall engine diameter is not important.
   



Fig 2: Centrifugal Compressor


2. Axial flow compressor: Consists of rotating blades and stationary vanes. Air is compressed as it flows axially along the shaft. This allows greater efficiency and higher pressure ratios by multi-stage construction. A stage of compression consists of one row of rotating blades followed by a row of stationary vanes. This is the most common type of compressor used in marine gas turbine engines.
   



Fig 3: Axial Flow Compressor


3. Compressor stall: A stall or surge is defined as an interruption of air flow through the compressor. A stall on an operating engine can cause extensive damage to the engine from excessive vibrations and overheating the combustor section. To prevent compressor stalls, engines are equipped with compressor bleed off valves or variable geometry compressor blades. The bleed off valves vent the compressor during starting and the variable compressor blades adjust the air flow to avoid turbulence, thus preventing compressor stalls.
   
2. Combustors: The combustor mixes the compressed air with fuel and burns the mixture to provide a hot expanding gas. There are three basic types of combustors.
   
1. Can: Individual burner cans are mounted around the periphery of the engine. Each can is an individual combustor and liner receiving its own fuel supply.
   
1. Advantage: Easy replacement
   
2. Disadvantages - Inefficient, structurally weaker
   



Fig 4: Can Type Combustor


2. Annular: One large combustor within the engine case. Multiple fuel nozzles form a solid "ring of fire". This type is used on the LM2500.
   
1. Advantages: Most efficient, strongest, frame member of engine.
   
2. Disadvantage: A repair or replacement requires complete engine disassembly.
   



Fig 5: Annular Combustor


3. Can-annular: This hybrid type employs a number of individual cans with separate fuel nozzles that receive air from a common annular housing (Allison 501-K17).
   
1. Advantages: Strong, ease of replacement.
   
2. Disadvantage: Less efficient than annular combustor.
   



Fig 6: Can Annular Combustor


3. Turbine:
   
1. Energy: Thermal energy from the combustor's hot expanding gasses are converted to mechanical energy by turning a turbine wheel.
   
2. Construction: Consist of stationary vanes (nozzles) and rotating blades. A stage of turbine is one row of nozzles and one row of blades.
   
4. Accessory drive assembly: The accessory drive assembly is driven by the compressor through bevel gears. The accessory drive is used to drive components to make the engine self sufficient. Common accessories include such components as engine lube oil and fuel oil pumps.
   
5. Engines:
   
1. Two basic types used in the US Navy:
   
1. Single shaft engine: The single shaft engines have one shaft which extends throughout the engine. All rotating parts of the engine are mounted on this shaft. An extension of this same shaft, the power take-off assembly, drives the load. The primary use of this type of engine is where constant speed is required such as electrical power generation. The Allison 501-K17 engine is used for this purpose.
   

Fig 7: Turbine Rotor


2. Split shaft engine: The engine is split into two major sections, the gas generator, and the power turbine section. The gas generator section consists of a compressor, combustor, and high pressure (HP) turbine. The gas generator's purpose is to produce a hot expanding gas for use in the power turbine. The power turbine is aerodynamically coupled to the gas generator but the two shafts are not mechanically connected. The power turbine converts the thermal energy from the gas generator to mechanical energy to drive the load.
   
1. The output speed is varied by controlling the speed of the gas generator which determines the amount of exhaust gases sent to the power turbine.
   
2. Split shaft gas turbine engines such as the LM2500 are suitable for main propulsion applications. The advantages in this application are:
   
1. The gas generator is more responsive to load demands because the compressor is not restricted in operation by the load on the power turbine.
   
2. The gas generator section and power turbine section operate near their most efficient speeds throughout a range of load demands.
   
6. Gas turbine air intake system:
   
1. High hat assembly:
   
1. Construction: An external structure which supports the moisture separators and houses the blow-in doors.
   
2. Moisture separators (louvers and mesh screens): The moisture separators remove water droplets and dirt from the intake air to prevent erosion of compressor components. Electric strip heaters prevent ice formation on the louvers.
   
3. Blow-in doors: Blow-in doors are installed to prevent engine air starvation when the moisture separators become dirty.
   
1. These doors operate automatically upon an increase in differential air pressure across the moisture separators.
   
2. When open, inlet air bypasses clogged moisture separators and provides unfiltered air to the engine to prevent engine air starvation.
   



Fig 8: High Hat Assembly


2. Intake duct:
   
1. Purpose: The intake duct provides combustion air for the engine and cooling air for the module.
   
2. Module Cooling System: The module cooling system routes a portion of the intake air to the engine enclosure for module ventilation and external engine cooling. The module cooling air swirls about the engine removing heat and ventilating the module before exiting via a small air gap around the aft end of the power turbine. The running engine’s exhaust causes an eductor effect pulling the module cooling air into the exhaust ducting.
   



Fig 9: GTM Intake Ducting


3. Anti-icing manifold:
   
1. Purpose: The anti-icing manifold is to inject hot bleed air into the intake trunk, below the module cooling air duct, to prevent ice formation.
   
2. Icing: Icing can occur in the intake duct when outside air temperature drops to 38^o F. The icing alarm will illuminate at 41^o F with 70% humidity to alert the operator prior to formation of ice in the intake.
   
3. Effects: Icing at the compressor inlet can restrict airflow causing a stall and also presents a serious foreign object damage (FOD) hazard to the engine.
   
4. Sensors: An ice detector sensor, located in the inlet plenum, generates an alarm warning the operator of the possibility of ice formation in the air intake.
   
5. Control: The anti-icing air system is activated manually by watch standers and monitored to prevent ice formation.
   
4. Silencers:
   
1. Location: Intake silencers are located halfway down the intake duct to reduce airborne noise.
   
2. Construction: The silencers consist of vertical vanes of sound deadening material encased in perforated stainless steel sheets.
   
3. The module cooling air duct: The module cooling air duct contains a single bullet shaped silencer to silence the noise created by the cooling air.
   
5. Expansion joint: The expansion joint is a rubber boot connecting the intake duct to the module inlet plenum. This prevents the noise of the module from being transmitted to the hull of the ship.
   
7. LM2500 gas turbine base enclosure assembly (module):
   
1. Description: The base enclosure assembly consists of an enclosure module (26'x8'x9') on a shock mounted base.
   
1. Module Base: The base is an I-beam fabricated steel frame with mounts to secure the engine.
   
2. Penetrations: Service connections penetrate the base for all engine services such as electrical, air, oil, fuel, CO₂ or Halon.
   
3. Protection: The enclosure is thermally and acoustically insulated to provide the engine a controlled environment.
1. Inlet plenum: The forward portion of the module is separated from the engine enclosure by a barrier wall. The inlet plenum is considered the clean section of the module. The gas turbine inlet FOD screen is mounted in this area on the front of the engine to prevent large foreign objects from being ingested into the compressor.
   



Fig 10: GTM Module Assembly


2. Engine Enclosure: The enclosure contains the actual engine and exhaust elbow and receives air from the module cooling duct. Access to the engine is provided by a side door and a top hatch.
   
2. The fire detection and extinguishing system: The fire detection and extinguishing system provides automatic fire protection to the gas turbine engine and module.
   



Fig 11: Module Base Assembly


1. Fire detection system components include:
   
1. Ultraviolet flame detectors- which look for flames in the combustor area.
   
2. Temperature sensors- which are set at 400 ^o F to sense fires outside the UV detectors viewing range.
   
3. Manual "FIRE" pushbutton- which can be used by the watchstander to activate the fire system.
   
2. Fire extinguishing system components include:
   
1. Bank of primary CO₂ bottles to rapidly flood the module.
   
2. Bank of secondary CO₂ to maintain an inert atmosphere in the module if required.
   
3. A CO₂ release inhibit switch located at the control consoles. This switch allows the operator to stop the automatic release of primary CO₂ into a module in case of a false alarm or personnel in the module.
   
4. An electronic fire stop signal used to stop the engine when a fire is detected by the ultraviolet flame detectors, the temperature switches, or the manually operated fire alarm push button. This signal will activate the fire stop sequence. Fire stop initiates the following actions:
   
1. "FIRE" alarms at the control consoles.
   
2. Secures fuel to the engine.
   
3. Stops the module cooling fan and closes the vent damper.
   
4. Releases CO₂ after a 20 second time delay.
   

Safety note: When entering the module, ensure the fire extinguishing system is disabled and signs are posted on the module and control consoles warning that personnel are in the module.

Note: FFGs equipped with Halon systems.


8. Exhaust duct system:
   
1. Function: Routes engine exhaust gases to the atmosphere while reducing both the heat and noise of the exhaust.
   
2. Exhaust Collector: The exhaust elbow directs exhaust gases into the exhaust uptake duct. A gap between the exhaust elbow and ship's uptake causes an eductor effect drawing module cooling air into the exhaust uptake.
   
3. Uptake ducting: Exhaust uptake ducting is insulated to control heat and noise as the exhaust is passed to the atmosphere.
   
4. Silencing: A vane type silencer is located in the center of the duct. These silencers are the same as those in the intake ducting, but are permanently mounted.
   
5. Exhaust eductors: Exhaust eductors are located at the uppermost end of the exhaust ducting. Exhaust eductors cool the exhaust gases by mixing with cool ambient air to reduce the infrared signature of the ship.
   
6. Boundary layer infrared suppression system (BLISS): Bliss caps are installed on the top of each mixing tube to further cool the exhaust air by mixing it with layers of ambient air. This is accomplished by use of several louvers that are angled to create an eductor effect. This allows cool ambient air to mix with the hot exhaust gases.
   



Fig 12: GTM Exhaust System


9. Water wash system:
   
1. Purpose: Used to remove dirt and salt buildup on the compressor blades.
   
2. Components: Consists of a 40 gallon tank and permanently installed piping to direct water wash solution into the inlet of the compressor.
   
3. Procedure: In accordance with PMS the compressor must be washed to maintain efficiency and prevent compressor stalls.
   



Fig 13: Water Wash System


10. Bleed Air:
    
1. Sources: Customer bleed air is extracted from the last stage of the compressor on the gas turbine generators (GTG) and gas turbine mains (GTM)
   
2. Bleed air users:(SPAM):
   
1. Starting or motoring of other gas turbines.
   
2. Prairie air to mask propeller noises.
   
3. Anti-icing air for prevention of intake icing.
   
4. Masker air to mask the main propulsion hull noises.
   



Fig 14: LM2500 Major Rotating Parts


11. LM2500 Gas Turbine Engine Assembly:
    
1. Gas generator components:
   
1. Compressor section: The LM2500 has a 16 stage axial flow compressor made up of the following components:
   
1. Compressor rotor: 16 stages of moving blades driven by the high pressure turbine.
   
2. Compressor stator: Compressor casing containing one stage of Inlet Guide Vanes (IGV), six stages of Variable Stator Vanes (VSV) and 10 stages of stationary stator vanes.
   
1. The IGVs and stator vanes 1-6 are variable, meaning they are variable geometry. The angle of attack of the blades can be changed to prevent compressor stall.
   
2. Bleed air is extracted from the compressor for use in the ship's bleed air system and for internal use in the engine.
   
2. Combustor:
   
1. The combustor is an annular type with 30 fuel nozzles and 2 spark ignitors.
   
2. Of the air from the compressor approximately 30% is mixed with fuel to support combustion. The other 70% is used to cool and center the flame within the combustion liner.
   
3. The ignition system produces a high intensity spark to ignite the fuel/air mixture during the start sequence. Once the engine is started the ignitors are no longer needed and will be de-energized.
   
3. High pressure turbine section:
   
1. The HP turbine extracts enough energy from the hot expanding gasses to drive the compressor and accessory drive.
   
2. The HP turbine is a two stage axial flow type which is mechanically coupled to the compressor rotor.
   
3. The HP turbine uses approximately 65% of the thermal energy from the combustor to drive the compressor and engine mounted accessories.
   
4. Accessory drive assembly:
   
1. Driven through the compressor rotor shaft via the inlet gearbox, radial drive shaft, and transfer gearbox.
   
2. The Accessory gearbox provides mounting for the fuel pump, lube oil pump, air/oil separator, and pneumatic starter.
   
2. Power turbine:
   
1. Construction: The power turbine is a six stage axial flow type turbine. The power turbine extracts the remaining 35% of useable energy and uses this to drive the main reduction gear. The power turbine drives the reduction gear through a high speed flexible coupling shaft and clutch assembly. The high speed flexible coupling absorbs the radial and axial misalignment between the GTM and the main reduction gear.
   

Fig 15: LM2500 Component View