Gas Turbines Flashcards
Describe a gas turbine
It is it in internal combustion constant flow engine designed to convert the energy of fuel into useful power
Describe the basic flow through a gas turbine
Gas flows through an axial compressor then into a combustion chamber then to a turbine for power and exhaust
What does efficiency depend on in a gas turbine
Efficiency depends primarily on the temperature to which the air can be raised. To maximize the efficiency you must operate gas turbines at maximum possible temperatures without harming the machine.
What is the purpose of an external combustion chamber?
The external combustion chamber allows the use of a wider range of fuels.
What is regeneration?
Regeneration is when some of the exhaust heat is used to heat the air in the compressor for combustion
What is cogeneration
It is when the exhaust heat is used to generate steam for power generation of other plant processes
Name the three shaft types
Single shaft, split shaft, twin spool shaft
Define a single shaft and the split shaft
Single shaft has one shaft connected directly to the compressor rotor.
Split shaft has two shafts one connected to the compressor and the gas generation turbine, while the other shaft is attached to the power turbine in the load or speed reducer
Describe a twin spool shaft
Twin spool shafts have one shaft running inside another there are two separate compressors and two separate turbines.
How much of the power that created is used for output and how much for gas generation to run the compressor
Approximately 65 to 75% of the power and is created is for gas generation
Describe an open cycle
The air is drawn in from the atmosphere and used in the turbine then return to the atmosphere after use. There is no provision to capture any of the wasted heat from the exhaust gases
Describe the semiclosed cycle
The semi-close cycle turbine has either a heat exchanger or regenerators attached to it in order to use the wasted exhaust heat
What is the biggest disadvantage of a gas turbine?
It’s low thermal efficiency
Where does the waste heat go in regeneration?
It is added to the air after the compressor but before the combustion chamber. It adds approximately 3% increase in efficiency
What is one of the greatest advantages of the gas turbine?
It has a very high power to weight ratio. Compared to other engines the gas turbine is smaller and lighter for the same power output
What is the difference in start up between a steam turbine, and a gas turbine?
Gas turbines can be started very quickly with no harmful affects to the engine and can be shut down in a reasonably short time. This is the opposite for steam turbines
What are some disadvantages of gas turbines
Low thermal efficiency approximately 30 to 40%, high air intake. If they have a loss of air in the intake system surging can result
Where is the highest pressure in the gas turbine
In the diffuser. It must always be higher than in the combustion chamber, to keep the flame from coming back into the diffuser
What causes surge?
Restrictions at the inlet, if the pressure in the combustion chamber becomes higher than the diffuser
What is the purpose of the inlet guide vanes?
They straighten the airflow and direct it into the first stage of the compressor rotor
What are the two basic styles of compressors used for gas turbines?
Radial flow axial flow
Where is the air at the highest pressure on the gas turbine?
Directly after the diffuser before the combustion chamber
Which are more efficient axial or radial flow compressors?
Axial flow are more efficient
How many stages are typically in the axial turbine compressor section?
8 to 16 stages
What type of rotors are used on most present day high-speed turbines?
Disc type rotors
How are the blades usually fitted to the rotor?
They are fit by a bulb or Firtree type root The blades are locked in place by setscrews peening, lock wire, pins or keys
What is the purpose of variable pitch stater blades?
They help the turbine engine maintain maximum compressor efficiency over the operating speed of the engine
What controls the blades position on variable pitch stater blades
The fuel control system
What are the three zones of the combustion chamber?
Primary zone, intermediate zone, dilution zone
Describe what happens at each of the zones in the combustion chamber.
Primary zone- The first part of the chamber liner where fuel injection and ignition begins
Intermediate zone- where combustion continues and begins to mix with secondary air
Dilution zone- combustion is complete at this stage more secondary air is mixed with the burnt gases
What is the difference between primary air and secondary air
Primary air enters the combustion liner to be mixed with fuel and burnt. Secondary air has no part in the combustion process it moves around the outside of the liner to control the temperature. In the intermediate zone it enters the liner through small holes in the dilution zone the air comes completely mixed with the burned gases through larger holes
What is flame propagation?
Once the fuel in the combustion chamber is ignited burning is continuous as long as there is an uninterrupted supply of fuel
What is a transition piece?
It connects the combustion chamber to the turbine and is a duct or passage long enough to make sure the hot air is thoroughly mixed so there can be no remaining hot gas pockets
What are the three types of combustion chambers?
Can type, annular type, and can-annular type
Where is the can type chamber normally used?
On engines with centrifugal compressors. Each can must have its own igniter
Where is an annular type chamber used?
Used with axial flow compressors. Fuel nozzles are evenly spaced around the liner. They are very efficient and light, but repairs require extensive engine disassembly
Describe a can annular chamber design
It is a combination of can, and annular type. The cans are interconnected by crossfire tubes, which allow flames to pass to each can during start up
What can cause serious damage to the rotor blades?
Unburned liquid fuels. It pits and wears down the blades
The turbine rotor assembly consists of:
The blades which are mounted on the discs, which are mounted on the shaft.
What are labyrinth seals assisted by for sealing?
Jets of air routed from the compressor section
Where does a water seal get its water from?
It uses condensate to help seal
Why do larger gas turbines have at least two oil control systems?
One for active use, one for backup
How are governor speeds sensed?
Centrifugal weights are used to sense speed
Where is the highest temperature in a gas turbine?
The point when the heated air leaves the combustion chamber
How is vibration usually monitored on the turbine?
Usually with onboard vibration proximity sensors
What are temp sensors called, and where would they be located?
They are called pyrometers and they are located around the combustion section
How do you check for excessive interference and rubbing of air seals or blades?
Manually rotate the engine slowly without the fuel to check
A 1% loss in intake pressure results in how big of a loss in engine power?
2.5%
How is the start up procedure performed?
The procedures in place are to be performed by the plant operators
