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Fourth Year Millwright > Gas Turbines > Flashcards

Gas Turbines Flashcards

1
Q

Describe a gas turbine

A

It is it in internal combustion constant flow engine designed to convert the energy of fuel into useful power

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2
Q

Describe the basic flow through a gas turbine

A

Gas flows through an axial compressor then into a combustion chamber then to a turbine for power and exhaust

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3
Q

What does efficiency depend on in a gas turbine

A

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.

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4
Q

What is the purpose of an external combustion chamber?

A

The external combustion chamber allows the use of a wider range of fuels.

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5
Q

What is regeneration?

A

Regeneration is when some of the exhaust heat is used to heat the air in the compressor for combustion

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6
Q

What is cogeneration

A

It is when the exhaust heat is used to generate steam for power generation of other plant processes

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7
Q

Name the three shaft types

A

Single shaft, split shaft, twin spool shaft

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8
Q

Define a single shaft and the split shaft

A

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

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9
Q

Describe a twin spool shaft

A

Twin spool shafts have one shaft running inside another there are two separate compressors and two separate turbines.

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10
Q

How much of the power that created is used for output and how much for gas generation to run the compressor

A

Approximately 65 to 75% of the power and is created is for gas generation

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11
Q

Describe an open cycle

A

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

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12
Q

Describe the semiclosed cycle

A

The semi-close cycle turbine has either a heat exchanger or regenerators attached to it in order to use the wasted exhaust heat

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13
Q

What is the biggest disadvantage of a gas turbine?

A

It’s low thermal efficiency

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14
Q

Where does the waste heat go in regeneration?

A

It is added to the air after the compressor but before the combustion chamber. It adds approximately 3% increase in efficiency

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15
Q

What is one of the greatest advantages of the gas turbine?

A

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

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16
Q

What is the difference in start up between a steam turbine, and a gas turbine?

A

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

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17
Q

What are some disadvantages of gas turbines

A

Low thermal efficiency approximately 30 to 40%, high air intake. If they have a loss of air in the intake system surging can result

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18
Q

Where is the highest pressure in the gas turbine

A

In the diffuser. It must always be higher than in the combustion chamber, to keep the flame from coming back into the diffuser

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19
Q

What causes surge?

A

Restrictions at the inlet, if the pressure in the combustion chamber becomes higher than the diffuser

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20
Q

What is the purpose of the inlet guide vanes?

A

They straighten the airflow and direct it into the first stage of the compressor rotor

21
Q

What are the two basic styles of compressors used for gas turbines?

A

Radial flow axial flow

22
Q

Where is the air at the highest pressure on the gas turbine?

A

Directly after the diffuser before the combustion chamber

23
Q

Which are more efficient axial or radial flow compressors?

A

Axial flow are more efficient

24
Q

How many stages are typically in the axial turbine compressor section?

A

8 to 16 stages

25
Q

What type of rotors are used on most present day high-speed turbines?

A

Disc type rotors

26
Q

How are the blades usually fitted to the rotor?

A

They are fit by a bulb or Firtree type root The blades are locked in place by setscrews peening, lock wire, pins or keys

27
Q

What is the purpose of variable pitch stater blades?

A

They help the turbine engine maintain maximum compressor efficiency over the operating speed of the engine

28
Q

What controls the blades position on variable pitch stater blades

A

The fuel control system

29
Q

What are the three zones of the combustion chamber?

A

Primary zone, intermediate zone, dilution zone

30
Q

Describe what happens at each of the zones in the combustion chamber.

A

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

31
Q

What is the difference between primary air and secondary air

A

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

32
Q

What is flame propagation?

A

Once the fuel in the combustion chamber is ignited burning is continuous as long as there is an uninterrupted supply of fuel

33
Q

What is a transition piece?

A

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

34
Q

What are the three types of combustion chambers?

A

Can type, annular type, and can-annular type

35
Q

Where is the can type chamber normally used?

A

On engines with centrifugal compressors. Each can must have its own igniter

36
Q

Where is an annular type chamber used?

A

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

37
Q

Describe a can annular chamber design

A

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

38
Q

What can cause serious damage to the rotor blades?

A

Unburned liquid fuels. It pits and wears down the blades

39
Q

The turbine rotor assembly consists of:

A

The blades which are mounted on the discs, which are mounted on the shaft.

40
Q

What are labyrinth seals assisted by for sealing?

A

Jets of air routed from the compressor section

41
Q

Where does a water seal get its water from?

A

It uses condensate to help seal

42
Q

Why do larger gas turbines have at least two oil control systems?

A

One for active use, one for backup

43
Q

How are governor speeds sensed?

A

Centrifugal weights are used to sense speed

44
Q

Where is the highest temperature in a gas turbine?

A

The point when the heated air leaves the combustion chamber

45
Q

How is vibration usually monitored on the turbine?

A

Usually with onboard vibration proximity sensors

46
Q

What are temp sensors called, and where would they be located?

A

They are called pyrometers and they are located around the combustion section

47
Q

How do you check for excessive interference and rubbing of air seals or blades?

A

Manually rotate the engine slowly without the fuel to check

48
Q

A 1% loss in intake pressure results in how big of a loss in engine power?

A

2.5%

49
Q

How is the start up procedure performed?

A

The procedures in place are to be performed by the plant operators

Fourth Year Millwright (8 decks)

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