[Rotating Equipment] Chapter 6 - Gas Turbine
43 flashcards · Other · by Ahmed Adil
What is the principle operation of gas turbine
Gas turbines operates on the principle of Brayton cycle. Gas turbine has a compressor coupled to a turbine with a combustion chamber in between. First the air is drawn by the compressor and then compressed and sent to the combustion chamber. Then air is burnt by injecting fuel. The resulting hot gas moves to the turbine which rotates the the turbine blades to preform work. In case of land based GT, the turbine is coupled to a generator to generate electricity.In case of aero-derivative GT, the…
Brayton cycle 2 - Compression 3 - Combustion 4 - Expansion
The Otto and Diesel cycles are not continuous combustion but intermittent and working fluid is an air and fuel mixture. The Rankine cycle for a steam turbine, is continuous, but uses a boiler to heat water to make the working fluid in this case steam, and then condenses the steam back to water and so forth. The Brayton cycle is continuous combustion and the working fluid is air and fuel mixture.
Gas turbine cycle Air enters air compressor and is compressed to a pressure of 1100 to 3000 kPa and then passes into the combustor The combustor adds fuel to some of the air from the compressor to achieve continuous combustion (increases air temperature to about 2000°C which is the flame temperature) The remaining air mixes with hot combustion gases to lower the temperature to 920°C (it can’t be too hot for turbine section or will damage parts/materials) The air/hot gases then enter the turbine…
Advantages of land based gas turbine
Produce large amounts of power (up to 250 MW or more) Use a wide variety of fuels (liquid and gas) Start/stop rapidly (important for ‘backup power’ generation) Remote operation capability (computer workstation) with minimal man-power requirements Able to use all types of fuels Flexible layout More efficient than aero-derivative type Stronger and longer lasting than aero-derivative type
Disadvantages of land based gas turbine
Slow to respond to load changes Less suitable for low-power applications (at partial throttle conditions, efficiency decreases) Requires inter-coolers, regenerators, re-heaters to reach efficiencies comparable to current gasoline engines…more equipment to maintain and costs are higher Efficiency less than 50% Not portable More expensive than aero-derivative type (more equipment) Requires large and complex lubrication systems
List 3 Applications for land based gas turbine
For electrical power generation (using electrical generators) To drive equipment (pumps, compressors, chillers) For steam production (using a waste-heat boiler)
List some fuels used in land based gas turbine
1. Natural gas 2. LPG (Liquified Petrol Gas) 3. Alcohol 4. Kerosene 5. Gasoline/petrol 6. Diesel 7. New Bio-Fuel
Land based gas turbine set up Oil Storage Air Intake Compressor Turbine Exhaust Generator Combustion chamber
Advantages of aero-derivative gas turbine
Low weight (suitable for offshore platforms, ships, etc…) Easily maintained and replaced Fast starting/shutdown, work better with load changes (good for ‘backup power’ generation) More efficient than reciprocating (piston) engines with the same power rating Can be multi-staged (compressor and/or turbine sections) Cheaper in cost than land-based units Portable
disadvantages of aero-derivative gas turbine
Only able to use either natural gas or good quality/clean liquid fuels Do not last as long as land-based units Not as efficient as land-based units
Turbojet Turboprop Turbofan Propfan
Air compressor Fuel injector combustion chamber Gas turbine Exhaust nozzle
“Land - based” gas turbine major components (5)
‘Compressor’ section ‘Combustor’ section ‘Turbine’ section Lubrication systems Wash system for compressor blades
Multistage compressors are used in gas turbine for higher pressure and temperature
What are the 3 techniques used in wash system for compressor
Spraying a cleaning fluid on the blades while the compressor is running (slow speed) Then spraying water to rinse off dirt and cleaning fluid Spraying a sealing fluid/high temperature paint (protects blades)
‘Combustor’ section: - 20% of compressed air is “primary air” and is introduced with the fuel and ignited/for combustion - 80% of the remaining air is “secondary air” and acts as cooling air for turbine blades and other parts (casings and bearings)
Combustor designs Annular Can Can annular
Combustor ‘Annular’: Consists of a single flame tube/burner - Advantages: - There are no cans, so there is a 25% reduction in weight from the other types - More uniform combustion…so more efficient than the other two - Less parts than the other 2 types so easier to maintain
Combustor ‘Annular’: Disadvantages: - Because it is one complete unit, ______________________ - _____________________. In the other two types, you can test just each individual can
if anything break down…..it is completely out of service You must test the whole unit
Combustor ‘Can’: - Cans/chambers are arranged around the engine with air being delivered into ducts in each chamber - Each "can" has its own fuel injector, igniter, liner, and casing - All of the tubes are inter connected which allows each tube to operate at the same pressure (Note: pressure drop in this type is increased which makes it less efficient)
‘Can’ Combustor Advantages: - You can remove one faulty can - It can run with a couple of individual cans plugged or missing Disadvantages: - More weight that the annular type - More parts to maintain and break down - Pressure drop in this type is increased which makes it less efficient than the annular type
Combustor ‘Can - annular’: A combination of the can and the annular designs Advantages: - No need for an igniter in each can….once one igniter lights one can, the others light off of that one - The exhaust is more uniform/steady than the can type - Like the can type, it can run with cans plugged or missing Disadvantages: - Same as the can type on previous page…..heavier than annular, more parts to maintain and as not as efficient as the annular (pressure drop)
Turbine section: The blades in this section get very hot because of the heat/flames of combustion Hot gases are expanded into the blades and turn ‘heat’ energy into ‘mechanical’ energy - Shaft arrangement turns mechanical energy into work (pump or other process equipment or electrical generator for electricity) - Blades introduced to very high pressures and high temperatures
Blade cooling for turbine section: - Gas turbines use either: a) Air b) A special coolant liquid/glycol mixture c) A special high temperature coating/paint to reduce metal temperatures and increase blade life - Blades are hollow in design to allow for air or fluid flow through them - A blades is damaged by overheating
Lube oil systems: Almost all gas turbines have a lube oil system that lubricates the bearings that support the rotor or shafts ‘Aero-derivative’ (jet engine) gas turbines normally use ‘antifriction’ bearings (ball, roller) which require only a small lube oil system split into two parts (one for supplying the oil and a scavenge system to return the oil to the reservoir). A chip detector (magnet) is installed to attract metal particles and notify the operator (alarm).
“Heavy-duty” or “land based” gas turbines use 'friction' bearings (journal, thrust, radial) because of a larger lube oil system. The reservoir tank is usually larger than the aero-derivative type. Oil temperatures are not as high in this gas turbine, so mineral oil is used
lube oil inlet Thin film NA housing lube oil shaft
What is the name of this bearing
thrust bearing
All “land - based” turbine oil systems consist of these 7 basic components: a) Oil reservoir: to ensure adequate supply b) Oil filter: to ensure clean oil c) Pump: to provide the pressure needed d) Oil cooler: to ensure target temperatures e) Protection devices: (emergency trips, etc…) f) Monitoring devices: (temperatures, pressures, levels and alarms) g) Control devices: (cooling, pressures, temperatures, etc…)
oil tank strainer pump filter oil cooler bearing
6 maintenance checks for oil systems:
1. Check system for levels, dirt/color, water (daily) 2. Check system for leaks (daily) - 3. Monitor pressures and temperatures (daily) - 4. Check chip detectors when they alarm and clean - 5. Change oil filters (manual recommendations) - 6. Take oil samples (preventive maintenance)
open closed
The ‘closed’ cycle type plant: Makes use of the exhaust gases as a working medium Advantages: - Higher pressure can be reached - Exhaust gases is not wasted so more efficient than the ‘open’ cycle type (saves money!) Disadvantages: - A supply of cooling water is required to condense the steam (more equipment needed) - The complexity, size, and initial cost of the system is higher
a safety device/switch which has to be satisfied in order for the gas turbine to be allowed to start. It protects the equipment, operator and surrounding area.
permissives
gas turbine ‘permissives’:
2 fuel gauges and one valve: temperature, pressure. fuel valve. 3 lube oil gauges: temperature, pressure, level one extra valve: emergency stop valve.
What is hogging and bowing
during shut down due to temp difference there is probability of shaft or rotor to bend upward (hogging) or downward (bowing)
How to prevent hogging and bowing
by using Turing gear
Why do we need emergency shut down
in case of abnormal conditions
‘Automatic’ gas turbine trips: (8)
1 shaft related: shaft/rotor over speed 2 exhaust related: b) Exhaust gas over temperature c) Exhaust gas pressure outside of targets 2 fuel related: d) Fuel pressure outside of targets e) Fuel temperature outside of targets 3 lube oil related: f) Lube oil pressure outside targets g) Lube oil temperature outside targets h) Lube oil levels too low
how to control the speed and power output of GT
by using speed governor
How speed governor is controlled
by PLC and CPU
4 methods to maximize efficiency of gas turbines:
Cool the air entering the compressor/use compressor intercooler with heat exchanger Increase air temperature entering the combustor (regeneration) Turbine reheat heat recovery