gas turbines Flashcards
what is a working fluid
Any gas or liquid that can do work by expansion or flow, or can be compressed or made to flow
what is a turbine
A bladed spool designed to rotate when acted on by a working fluid
what is a compressor
A bladed spool designed to do work on a fluid when it is rotated
what is the purpose or function of a gas turbine
Gas turbines convert heat energy into rotary motion for ships propulsion
what are the different types of gas turbines
IEP integrated electric propulsion
CODLAG -combination of diesel electric and gas
COGAG - combination of gas and gas
COGOG - combination of gas or gas (big and small gas turbines)
what are the advantages of gas turbines
- Allow the ship to get underway quickly
- Ease of operation and emphasis on remote control
- High Power to Weight Ratio
- Minimum ship-borne maintenance
- Lower Man-Power requirement
explain the 3 stage filter
Air is drawn in from the upper deck via the 3-stage filter, removing particle and moisture contamination. There is also a manometric leg to ensure no unfiltered air enters the intake system.
what happens in the stage 1 of the air intake filter
SPRAY ELIMINATOR (encourages the air to let go of the water) Aluminium fins change direction of air, catching the spray and causing the majority to drop off and mainly dry air passes through.
what happens in stage 2 of the air intake filter
- Stage 2 – KNITMESH FILTER (Aluminium frame supporting woven material) Removes particulate matter
what happens in stage 3 of the air intake filter
- Stage 3 – WATER SEPARATION (Manometric Leg) (U-bend principle) Water drains into a water trough, which ensures a water seal across the bottom of the filters
describe the splitter silencer
The splitter silencer splits up the air flow stream which reduces air noise and buffeting.
Air passes over aerofoil shaped vanes which “straightens” the air flow and reduces resistance to air flow.
describe the stone guard
this prevents large course particles from entering the gas turbine (around 1 inch square)
what is the transition duct
Provides a smooth change from square cross section air flow to circular cross section air flow, in preparation for entering the engines air intake casing.
describe the cascade bend
Directs the air smoothly through a 90-degree bend without causing any turbulence to the already smoothed out air flow.
Turbulence of the air entering the engine will affect the engines performance.
what is the engine intake flare
Seals the engine intake from the rest of the module and houses the “water wash ring” (windscreen washer jets) used for cleaning the engine.
what is module ventilation
The function of the module vent flaps is to isolate the module in the event of an emergency e.g. a fire. When the engine is running the module vent flaps are to be open to allow airflow to cool the module and expel any exhaust fumes.
how is the module ventilated
- Bernoulli’s Theorem
- The Venturi Effect
- Vent by Volute (V x V)
step by step of module ventilation
- Air is drawn into the module because of the depression formed at the other end
- Air circulates around the module keeping it cool
- A depression of air is formed by the rushing exhaust gasses (refer to Bernoulli’s Theorem)
- Vent flaps are fitted to seal the module should a fire establish within. The position of the flaps must be known at all times. When the engine is running the flaps must be open (flaps shut – no cooling)
what is the gas turbine cycle
- Induction
- Compression
- Combustion
- Power
- Exhaust
describe low pressure and high pressure compressors
- LP & HP Compressors are AXIAL FLOW COMPRESSORS
- Made up of rotors and stators, air is constantly compressed as it passes through each compressor (section)
what is the air compression ratio
An air compression ratio of around 19:1 can be obtained
what is primary used for in the combustion process
Primary Air – 15% - helps to atomise the fuel and provides air for the initial combustion.
what is secondary air used for
10% - is used to ensure complete combustion of the fuel and provides a layer of air between the fuel and the flame tube preventing flame impingement.
what is tertiary air
75% - after cooling the combustion can the tertiary air is heated by the combustion process causing it rise in pressure and becomes the “working fluid” to drive the turbines
what do the high pressure and low pressure turbines do
The LP and HP Turbines act like water wheels, but use the “working fluid” produced in combustion to drive the corresponding compressors. The HP turbine drive shaft connects to the HP compressor and is hollow. The LP turbine drive shaft passes through the HP shaft and drives the LP compressor.
Approximately 1/3 of the energy from the “working fluid” is used by the LP and HP turbines.
explain the power turbine
The power turbine is driven in the same way as the LP & HP turbines and drives a shaft into the main gearbox/generator. The remaining 2/3 of energy from the working fluid is used up by the power turbine.
what is the exhaust
The remaining hot exhaust gasses exit to atmosphere via the funnel.
what lubrication us used
OX22 lub oil is fed by a gear pump, driven off the GT through a closed loop system.
what are mag chips
Magnetic chips, fitted for Early Failure Detection, are located in the oil return (Scavenge) leg. They collect ferrous debris and are inspected every 24 running hours, the debris measured and recorded – never wiped off!
what safety devices are set in place
Hand Trip
fire detectors
power turbine entry temperature
partial overspeed
LP/HP governors
vibration monitors
PT overspeed
safety interlock
what is a hand trip
isolates the fuel to the engine by closing the HPSOC (high pressure shut of cock)
fire detectors
give an indication of fire or high temperature on the LCP or the SCC/MCR
what is the power turbine entry temperature (PTET)
if the temperature of the gas entering the turbine exceeds the maximum limits the engine will shut down
explain partial overspeed
If the propeller comes out of the water this will reduce the amount of fuel to the engine to prevent over-speeding
explain LP/HP governors
controls and monitors the speed of the HP/LP compressors
vibration monitors
if vibration levels exceed limits it could lead to component or total engine failure (alarm sounded)
PT overspeed
if the power turbine exceeds 110% of maximum speed the engine will shut down.
safety interlocks
if certain parameters are not met e.g. certain electrical supplies not sensed, the engine will not start in remote mode.
what are the hazards of gas turbines
Igniters – operate up to 10000 volts – must be isolated for over 5 mins to allow static charge to dissipate.
Noise – The engine module can be entered with the engine running but only at idle speed, permission is required and suitable ear defenders must be worn.
what kind of fire extinguishers can you use on a GT
A BCF or BTM drench system is fitted to the gas turbine modules.
Prior to starting the engine the system must be armed.
Prior to entering the engine module the system must be inhibited.
BCF – Bromochlorodifluoromethane
BTM – Bromotrifluoromethane
when watch keeping what checks need to be carried out every hour
- Pressures and temperatures
- Oil Levels
- Any excessive/unusual noises
- Any leaks
what is a wet motor cycle
A wet motor cycle is when the engine is spun over using the starter motor, with the fuel ON, but the igniters isolated and fuses removed. This is done from the local control position only.
why is a wet motor cycle done
A wet motor will be done to:
* Prove the fuel system after maintenance
* Bleed any air from/prime the fuel system
what is a dry motor cycle
A dry motor cycle is when the engine is spun over using the starter motor, with the fuel OFF and the igniters isolated and fuses removed. This is done from the local control position only.
why is a dry motor cycle done
A dry motor will be done to purge fuel and explosive gasses from the combustion chambers:
* After a failed or aborted start
* After a wet motor cycle
* To enable water washing
what is water washing
Water washing is carried out to prevent the build up of debris and FOULING on the compressor blades. Distilled water and Compressor Cleaning Fluid (CCF) is sprayed into the intake flare when carrying out a Dry Motor Cycle.
how does water washing differ
It is done at different frequencies depending on the engine:
* Spey – 100 run hours or 2 monthly
* Olympus – 100 run hours or 2 monthly
* Tyne – 150 run hours
what PPE should you wear when water washing
- Apron
- Gauntlet Gloves
- Face Visor
- Goggles
