Gas Turbine Interview Questions and Answers

Study the top 100 Gas Turbine Interview Questions and Answers useful to review the basics and prepare for common technical questions on Gas Turbine.

Gas Turbine Questions

What is Gas turbine?

It is a thermic machine running according to the admission-compression-combustion-exhaust 4 step cycle.

How many stages are there in gas turbine?

3 stages.

How many stages are there in the compressor?

17 stages.

What type of compressor is there in the gas turbine?

Axial flow compressor.

How many bleed valves are there in the compressor?

4 nos. of bleed valve’s.

At what stage of the compressor, air will be extracted through bleed valves?

11th stage of the compressor.

Why air bleeding required through bleed valves?

Pulsation protection during start-up

When these compressor bleed valves will be closed?

Speed relay (14HS) corresponding to 95 % speed.

What are the IGV minimum and maximum position in gas turbines?

Minimum – 34 DGA and Maximum – 85 DGA

When IGV will start opening?

Speed relay(14HS) corresponding to 95% speed.

How many combustion chambers are there in frame # 9 & # 6 ?

Frame #9 – 14 nos. and Frame #6 – 10 nos . of combustion chambers.

How many spark Plugs?

2 nos. of spark plugs.

What is the function of crossfire tube?

All combustion chambers are interconnected by means of crossfire tube. This tube enables flame from the fired chambers to propagate to the unfired chambers.

How spark plugs will retract after igniting combustion chambers?

After the turbine rotor approximates operating speed, combustion chamber pressure causes the spark plugs to retract to remove their electrodes from the hot flame zone.

Where exactly transition pieces are located?

Transition pieces are attached to the aft end of the combustion chamber liners.

Explain the stages of the Gas turbine?

Each stage consists of a row of fixed nozzles followed by a row of rotatable turbine buckets. In each nozzle row, the kinetic energy of the jet is increased, with an associated pressure drop, and in each following row of moving buckets, a portion of the kinetic energy of the jet is absorbed as useful work on the turbine rotor.

Where exactly turning vanes are located?

Exhaust hood and diffuser contains turning vanes located in the exhaust path.

What is the function of turning vanes?

Turning vanes turn the gases from the axial direction to a radial direction to minimize exhaust hood losses.

What is the function of turbine legs?

Turbine legs assist in maintaining alignment between the turbine and the generator. Support legs maintain the axial and vertical positions of the turbine.

Why turbine legs cooling is required?

Cooling water is circulated through the jackets to minimize the thermal expansion of the supports legs and assist in maintaining alignment between the turbine and generator.

How many types of blades are there in the compressor?

2 types of blades. Rotor & Stator blades.

What is the function of rotor blades in the compressor?

Rotor blades supply the force needed to compress the air in each stage.

What is the function of stator blades in the compressor?

Stator blades guide the air so that it enters the following rotor stage at the proper angle.

Where does compressor air will be used?

Turbine cooling, bearing sealing, pulsation control, purging.

Describe the compressor rotor

Compressor rotor is an assembly of 15 individual wheels, 2 stud shafts, each with an integral wheel, a speed ring, tie bolts and the compressor rotor blades.

Describe the compressor stator?

The stator (casing) area of the compressor section is composed of inlet casing, forward compressor casing, aft compressor casing, compressor discharge casing.

Where the compressor inlet casing is located?

Compressor inlet casing is located at the forward end of the gas turbine.

What is the function of the compressor inlet casing?

Its prime function is to uniformly direct air into the compressor. It also supports no.1 bearing housing.

Where IGV is located?

Inlet guide vanes are located at the aft end of the compressor inlet casing.

What does the compressor forward casing contain?

Forward compressor casing contains the first 4 compressor stator stages.

What does aft compressor casing contains?

Aft compressor casing contains the 5th & 11th stages of the compressor

Where does the extracted air used for 5th & 11th stages of compressor?

This air is used for cooling & sealing functions and also used for starting and shutdown pulsation control.

Explain the functions of the compressor discharge casing?

The functions of the compressor discharge casings are to contain the final 7 compressor stages, to form both the inner and outer walls of the compressor diffuser and to join the compressor and turbine stators.

They also provide support for no.2 bearing, the forward end of the combustion wrapper, and the inner support of the first stage turbine nozzle.

What type of combustion system is there in gas turbine?

Reverse flow type.

Where does spark plugs located?

Combustion chamber no.13 & 14.

Where does flame detectors are located?

Combustion chamber no.4, 5, 10 &11.

What is the function of combustion wrapper?

The combustion wrappers form a plenum in which the compressor discharge air flow is directed to the combustion chambers. Its secondary purpose is to act as a support for the combustion chamber assemblies.

In turn, the wrapper is supported by the compressor discharge casing and the turbine shell.

Why flame detectors are required?

During the starting sequence, it is essential that an indication of the presence or absence of flame be transmitted to the control system.

How does flame detectors work?

The ultraviolet flame sensor consists of a lame sensor containing a gas-filled detector. The gas within this flame sensor detector is sensitive to the presence of ultraviolet radiation which is emitted by a hydrocarbon flame. D.C. voltage, supplied by the amplifier, is impressed across the detector terminals.

If flame is present, the ionization of the gas in the detector allows conduction in the circuit which activates the electronic to give an output defining flame. Conversely, the absence of flame will generate an opposite output defining “on flame”.

What is the function of the fuel nozzle?

The fuel nozzle functions to distribute the liquid or gas fuel into the reaction zone of the combustion liner, in a manner that promotes uniform rapid and complete combustion.

How does atomizing air is utilized?

Atomizing air is utilized with liquid fuel to assist in the formation of a finely divided spray.

What is the function of transition pieces?

Transition pieces direct the hot gases from the liners to the turbine first stage nozzle. Thus, the first nozzle area is divided into 14 equal areas receiving the hot gas flow.

Which components are included in the combustion section?

The combustion system includes fuel nozzles, spark plugs ignition system, flame detectors, and crossfire tubes.

Which components are included in the turbine section?

The turbine section components include the turbine rotor, a turbine rotor, a turbine shell, nozzles, shrouds, exhaust frame, and exhaust diffuser.

Which components are included in the turbine rotor assembly?

The turbine rotor assembly consists of 2 wheel shafts, the 1st, 2nd, and 3rd stage turbine wheels with buckets and 2 turbine spacers.

What is the function of diaphragms?

Diaphragms prevent air leakage past the inner sidewall of the nozzles and turbine rotor. Attached to the inside diameters of both the second and third stage nozzle segments are the nozzle diaphragms.

What is shroud?

Unlike the compressor blading, the turbine bucket tips don’t run directly against an integral machined surface of the casing but against annular curved segments called turbine shrouds.

What are the functions of shrouds?

The primary function of the shrouds is to provide a cylindrical surface for minimizing tip clearance leakage. The secondary function is to provide a high thermal resistance between the hot gases and the comparatively cool shell.

By accomplishing this function, the shell cooling load is drastically reduced, the shell diameter is controlled, the shell roundness is maintained, and important turbine clearances are assured.

How many bearing are there is in gas turbine?

3 bearings.

Which types of bearing are those?

Journal bearings, elliptical type.

Where these 3 bearings does located?

• No.1 bearing-center of the inlet casing assembly.(Compressor inlet)
• No.2 bearing-pressurized space between the compressor and the turbine.
• No.3 Bearing-aft end of the turbine shaft in the center of the exhaust frame assembly.

What is the capacity of the lube oil tank?

12540 liters.

What is function coupling?

Couplings are used to transmit starting torque from the accessory gear to the gas turbine axial compressor and to transmit shaft horsepower from the turbine to the generator.

Where exactly accessory drive gear is located?

The accessory drive gear, located at the compressor end of the gas turbine.

What is the function of the accessory gear drive?

Its function is to drive each gas turbine accessory at its proper speed, in addition, it contains the system’s main lube oil pump and the turbine over-speed bolt trip mechanism.

Name auxiliaries driven by accessory drive gear?

Main lube oil pump, main hydraulic pump, liquid fuel pump, main atomizing air compressor.

How gas turbine will trip on mechanical over speed?

A high-pressure turbine over speed trip capable of mechanically dumping the oil in the trip circuits, the over speed bolt which actuates the trip upon over speed is installed in the main shaft.

At what speed gas turbine will trip on normal over speed?

107% TNH or 3210 RPM.

At what speed gas turbine will trip on electrical over speed?

110% TNH or 3300 RPM

What is pour point?

Pour point is the lowest temperature at which a fluid is observed to flow.

What is flashpoint?

Flashpoint is the temperature at which the fluid contained in a test cup and heated at a constant rate will flash but not burn when a flame is passed over the cup.

What is fire point?

Fire point is the temperature at which the fluid contained in a test cup and heated at a constant rate will burn for at least 5 seconds when a flame is passed over the cup.

Explain coded designation for GE gas turbine?

• 12 will be used for the over speed mechanism.
• 20 for solenoid valve
• 23 for the heating device
• 26 for the temperature switch
• 33 for the limit switch
• 39 for the vibration detector
• 43 for the manual switches
• 45 for the fire detector
• 49 for the over load protection
• 63 for the pressure switches
• 65 for the servo valve
• 71 for the level detecting system
• 77 for the speed sensors
• 88 for the electrical motor
• 90 for the modulating valve
• 96 for the pressure transmitter

Why gas turbine should come on turning gear following a shutdown?

Immediately following a shutdown, after the turbine has been in the fired mode, the rotor should be turned to provide uniforms cooling.

Uniform cooling of the turbine rotor prevents rotor bowing and resultant, rubbing and imbalance, and related damage that might otherwise occur when subsequent starts are attempted without cool down.

How many hours turbines will remain on turning gear following a shutdown?

At least 14 hours as per cooldown timer.

Why the turbine compartment fan (SSBT) will cut off soon after off?

To avoid the fast cooling of the turbine.

When turbine compartment fan (SSBT) will remain in service after flame off?

With flame off, it will cut off and again it will start after 1 hour.

How long turbine compartment fan (SSBT) will remain in service after flame off?

Till wheel space temperature reaches 148 degrees.

What will happen if the accessories are stopped when the unit is not yet cool down?

The shaft line will continue to cool but it will bow and important vibration may occur at the next start up, with possible damage to the bearings.

Name particular parameters to be noticed or to be adjusted during unit synchronization with the system grid?

Machine speed, machine voltage system voltage and power factor.

How many extractions are used from air compressor?

3 extractions (5th / 11th/ 17th)

Where air will be used from the 5th, 11th, and 17th stages?

• From 5th stage air will be used for cooling & sealing of bearings
• From 11th stage air will be used for pulsation protections
• From 17th stage air will be used for combustion chamber.

How hot gas will flow from the combustion chamber towards exhaust plenum?

The hot gases from the combustion chamber expand into the 14th separate transition pieces attach to the aft end of the combustion chamber liners and flow towards the 3 stage turbine section of the machine.

Each stage consists of a row of fixed nozzles followed by a row of rotatable buckets, in each nozzle row, the kinetic energy of the jet is increased, with an associated pressure drop, and in each following row of moving buckets, a portion of the kinetic energy jet is absorbed as useful work on the turbine rotor.

After passing through the 3rd stage buckets, the exhaust gases directed into the exhaust hood and diffuser. Then, the gases pass into exhaust plenum.

What types of spark plugs are provided in the gas turbine?

High voltage, retractable electrodes, spring injected & pressure-retractable types of spark plugs are provided.

What is the function of the transition piece?

The transition piece directs the hot gases from the liners to the first stage nozzles.

Turbine buckets increase in size from the 1st to the 3rd stage. Why?

Because of the pressure reduction resulting from energy conversion in each stage, an increased annulus area is required to accommodate the gas flow, thus necessitating increasing the size of the buckets.

What’s the definition of wheel space?

Space between the turbine wheel with the bucket and the stator, into the main gas stream area, is called the wheel space.

How wheel space is being cooled by air?

The first stage forward wheel space is cooled by air that passes through the high pressure passing seal at the aft end compressor rotor.

The second stage forward wheel space is cooled by the compressor discharge air that passes through the stage 1 shroud and then secondary inward through stage 2 nozzles vanes, the 3rd aft wheel space is cooled by cooling air exit from the exhaust frame cooling circuit.

What is the cooling provision provided fro turbine shell, why it is required?

3 exhaust frame cooling fans( 88TK ) are provided for turbine shell cooling.

The turbine shell controls the axial & radial positions of the shrouds and nozzles. It determines turbine clearances and the relative position of the nozzles to the turbine buckets. The position is critical to gas turbine performance.

What is the function of turbine nozzles?

Turbine nozzles direct the high-velocity flow of the expanded hot combustion gas against the turbine buckets, causing the rotor to rotate.

How many nozzle segments consisted of 1st, 2nd & 3rd stage nozzle?

18, 16, 16 segments respectively.

How many bearings are contained in bearing # 1 assembly?

Bearing # 1 contained 3 nos. of bearings.

Active thrust bearing, inactive thrust bearing & journal bearing.

What type of load coupling provided between turbine shaft and generator?

Hollow coupling.

What do you mean by the axial flow compressor?

Axial flow means that air flow general direction is axial.

How does pressure & velocity behave in compressor?

The pressure is increased from stage to stage by reducing in the stator blade row the air velocity obtained from the rotation of the rotor blade row, that’s why, on the fixed blade of the stator, whirled air velocity decreases and air pressure increases.

How many percentages of the air used for the thermal insulation as well as for the combustion?

70 to 80% of compressed air is used for thermal insulation, the remaining 20 to 30% air is used for combustion.

How much percentage of power consumed or absorbed by the axial compressor?

53% of the total machine output.

Which factors are affecting the performance of the gas turbine?

Load, ambient pressure, ambient temperature rate of humidity, steam injection.

How generator output will increase by decreasing the ambient temperature?

Inlet air density increases at lower temperatures, so the compressor supplies a higher mass flow into the combustor, so the generator output will increase.

How generator output will increase by increasing the atmospheric pressure?

As pressure increases, air density increases, and the compressor supplies a greater mass flow. Flow into the combustor, so the generator output will increase.

How generator output can be measured?

The generator output is measured by the help of 2 wattmeters, which are connected to the appropriate sockets on the generator control panel.

Which are the standard conditions to get maximum output from the machine?

• Ambient temperature – 15 degree C
• Atmospheric pressure – 1.0132 bar
• Relative humidity – 60%

Which factors can affect generator output?

• Pressure loss in the inlet duct.
• Pressure loss in the exhaust duct,
• Ambient temperature.
• Atmospheric pressure.

Name the location we can carry out the baroscopic test?

• Compressor stages no 4, 12 & 17
• 1st,2nd & 3rd stage nozzle trailing edge
• 2nd & 3rd stage nozzle leading edge
• 1st,2nd & 3 rd stage bucket leading edge
• 1st,2nd stage bucket trailing edge
• Liners and transition pieces.

What are the interlocks for offline water wash?

• The machine must be down
• The temperature difference between water wash & wheel space temperature must not be greater than 67 degrees C.
• The maximum wheel space temperature must not be greater than 149 degrees C.

What are the interlocks for online water wash?

• IGV must be in fully open condition
• Water wash tank temperature should be @ 70 degrees C
• The turbine must be running at maximum load and not in the process of shutdown
• The ambient temperature must be superior to 10 degrees C

Which factors are affecting the heat rate in gas turbine?

• Ambient temperature
• Pressure loss in the inlet duct
• Pressure loss in the exhaust duct.

Name the composition of natural gas

• CH4 – 89.55 % (methane)
• C2H6 – 5.68 % (ethane)
• CO2 – 3.94 % (carbon dioxide)
• C3H8 – 0.14 %
• C4H10- 0.11 %
• C6H14- 0.06 %
• C5H12- 0.03 %
• N2 – 0.50 % (nitrogen)

How we can determine the firing temperature in the combustion chamber?

Firing temperature can be determined as a function of exhaust temperature and pressure ration across the turbine, it is also determined from the measured CPD & FSR.

Which factors can cause high wheel space temperature?

High wheel space temperature may be caused by any of the following reasons.

• Restriction in cooling air liner
• Wear in turbine seals
• Excessive distortion of the turbine stator
• Improper positioning of the thermocouples
• Malfunctioning of the combustion system
• Leakage in external piping’s
• Excessive distortion of exhaust inner diffuser

Which circuit or system needs to be isolated for offline water wash?

• Atomizing air circuit
• Cooling air circuit
• Sealing air circuit.

What is firing temperature?

The temperature control system is designed to measure and control turbine exhaust temperature because it is impractical to measure temperature in the combustion chamber or at the turbine inlet directly, this indirect control of the turbine inlet temperature called firing temperature.

What is the direction of gas turbine?

Anti-clockwise ( viewing from compressor end towards generator )

What is the direction of gas turbine during ratcheting (frame#6)?

Clockwise (reverse compare to normal because of the load gear)

At what speed A.O.P will cut off during start up of gas turbine?

95 % TNH (FSNL)

At what speed starting motor will cut off during start up of gas turbine?

60 % TNH

Why on 60% TNH starting motor will cut off?

At 60% TNH, machine will have a self-sustaining speed so further there is no need for the starting motor as shaft RPM will increase itself up to FSNL.

How does humidity affect the machine output power?

The rate of humidity affects the compressibility of atmospheric air. The energy absorbed by the compressor decreases and the output power slightly decreases.

How does steam injection improves the machine output?

If we inject the steam into the combustion chamber, the massic flow expanded through the turbine is higher, the expansion work provided by the turbine is higher while the compression energy required keeps constant, thus the machine output improves slightly.

What is the definition of the nominal speed of the compressor?

An axial compressor is designed and optimized of a given speed which is called nominal speed, at nominal speed, the compressor is able to pressurized or constant air flow with the operating range of pressure rations.

At what turbine speed compressor capability will be very low?

The starting engine drives the turbine, from 50% of nominal speed, the pressure inside the combustion zone will rapidly increase du toe temperature increases. AT this intermediate speed the capability of the compressor will be vey low.

Why ambient temperature is being measured under the shade, why not directly in the inlet side?

Ambient temperature is measured in the shade near the filter inlet because the thermometer should not be positioned directly in the inlet air flow, as its speed provokes pressure drops that affect the temperature.

After synchronizing the machine which criteria you will consider as machine has been warmed up uniformly?

When wheel space temperature does not increase any more than 3 degrees C within 15 minutes, we can assume that the machine has been warmed up uniformly. Machine should not be loaded until it is not warmed up.

On which atmospheric conditions, the performance of gas turbine strongly depends?

• Atmospheric pressure
• Ambient temperature
• Relative humidity.

How does pressure drops in the inlet duct, how could we measure the total loss of inlet pressure?

The pressure drop in the inlet duct is a result of line losses, the drop in the pressure in the air filter and in the silencers.

To measure the total loss of inlet pressure, a water-filled U tube is connected to the appropriate checkpoint on the inlet duct. The pressure difference to atmospheric pressure will be read in mmh2o.

On which factors exhaust temperature depends, how?

• Ambient temperature
• Pressure losses in inlet duct
• Pressure losses in exhaust duct.
• Exhaust temperature increases with rising ambient temperature as well as pressure losses.

When exhaust over-temperature damage can occur. Why so?

The particularly critical period for over-temperature damage to occur is during the start-up phase before the turbine reaches governing speed. At this time air flow is low and the turbine is unable to accelerate away from the excess fuel.

Name the accessories being examined during combustion inspection?

• Fuel nozzles
• Combustion liners and cross fire tube
• Transition pieces
• Firing system and flame detectors
• Possibility to achieve a boroscopic inspection.

Which types of inspection include in hot gas path inspection?

The inspection includes the combustion inspection, in addition, a detailed inspection of the following parts which can be achieved after removal of the upper shell of the turbine.

• 1st 2nd and 3rd stage nozzles
• 1st 2nd and 3rd stage buckets of the turbine wheels
• Condition of thermocouples
• Condition of inlet and exhaust plenum
• Appearance of compressor inlet blades
• Condition of couplings and auxiliaries gear teeth.

Which types of inspection are involved in major overhauling?

This inspection involves the inspection of all the major flange to flange components of the gas turbine. It is the most complete of all, After a major inspection, the turbine is considered in new condition.

It required all operations required for combustion inspection and HGPI and in addition.

• Complete rotor inspection
• Compressor fixed blades examinations
• Bearing and seals inspection
• Control and safety system
• On base auxiliaries
• Off base auxiliaries
• Generator and auxiliaries.

What is FSR

FSR – Fuel stroke reference is the electrical signal that determines the amount of fuel delivered to the turbine combustion system.

What’s the duration being followed for C.I /HGPI/Major overhaul?

• Combustion inspection – 8000 fire hours (1 year)
• HGPI – 24000 fired hours(3 years)
• Major overhaul – 48000 fired hours(6 years)