Study the top 100 Gas Turbine Interview Questions and Answers useful to review the basics and prepare for common technical questions on Gas Turbine.
It is a thermic machine running according to the admission-compression-combustion-exhaust 4 step cycle.
3 stages.
17 stages.
Axial flow compressor.
4 nos. of bleed valve’s.
11th stage of the compressor.
Pulsation protection during start-up
Speed relay (14HS) corresponding to 95 % speed.
Minimum – 34 DGA and Maximum – 85 DGA
Speed relay(14HS) corresponding to 95% speed.
Frame #9 – 14 nos. and Frame #6 – 10 nos . of combustion chambers.
2 nos. of spark plugs.
All combustion chambers are interconnected by means of crossfire tube. This tube enables flame from the fired chambers to propagate to the unfired 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.
Transition pieces are attached to the aft end of the combustion chamber liners.
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.
Exhaust hood and diffuser contains turning vanes located in the exhaust path.
Turning vanes turn the gases from the axial direction to a radial direction to minimize exhaust hood losses.
Turbine legs assist in maintaining alignment between the turbine and the generator. Support legs maintain the axial and vertical positions of the turbine.
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.
2 types of blades. Rotor & Stator blades.
Rotor blades supply the force needed to compress the air in each stage.
Stator blades guide the air so that it enters the following rotor stage at the proper angle.
Turbine cooling, bearing sealing, pulsation control, purging.
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.
The stator (casing) area of the compressor section is composed of inlet casing, forward compressor casing, aft compressor casing, compressor discharge casing.
Compressor inlet casing is located at the forward end of the gas turbine.
Its prime function is to uniformly direct air into the compressor. It also supports no.1 bearing housing.
Inlet guide vanes are located at the aft end of the compressor inlet casing.
Forward compressor casing contains the first 4 compressor stator stages.
Aft compressor casing contains the 5th & 11th stages of the compressor
This air is used for cooling & sealing functions and also used for starting and shutdown pulsation control.
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.
Reverse flow type.
Combustion chamber no.13 & 14.
Combustion chamber no.4, 5, 10 &11.
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.
During the starting sequence, it is essential that an indication of the presence or absence of flame be transmitted to the control system.
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”.
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.
Atomizing air is utilized with liquid fuel to assist in the formation of a finely divided spray.
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.
The combustion system includes fuel nozzles, spark plugs ignition system, flame detectors, and crossfire tubes.
The turbine section components include the turbine rotor, a turbine rotor, a turbine shell, nozzles, shrouds, exhaust frame, and exhaust diffuser.
The turbine rotor assembly consists of 2 wheel shafts, the 1st, 2nd, and 3rd stage turbine wheels with buckets and 2 turbine spacers.
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.
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.
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.
3 bearings.
Journal bearings, elliptical type.
12540 liters.
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.
The accessory drive gear, located at the compressor end of the gas turbine.
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.
Main lube oil pump, main hydraulic pump, liquid fuel pump, main atomizing air compressor.
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.
107% TNH or 3210 RPM.
110% TNH or 3300 RPM
Pour point is the lowest temperature at which a fluid is observed to flow.
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.
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.
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.
At least 14 hours as per cooldown timer.
To avoid the fast cooling of the turbine.
With flame off, it will cut off and again it will start after 1 hour.
Till wheel space temperature reaches 148 degrees.
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.
Machine speed, machine voltage system voltage and power factor.
3 extractions (5th / 11th/ 17th)
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.
High voltage, retractable electrodes, spring injected & pressure-retractable types of spark plugs are provided.
The transition piece directs the hot gases from the liners to the first stage nozzles.
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.
Space between the turbine wheel with the bucket and the stator, into the main gas stream area, is called the wheel space.
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.
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.
Turbine nozzles direct the high-velocity flow of the expanded hot combustion gas against the turbine buckets, causing the rotor to rotate.
18, 16, 16 segments respectively.
Bearing # 1 contained 3 nos. of bearings.
Active thrust bearing, inactive thrust bearing & journal bearing.
Hollow coupling.
Axial flow means that air flow general direction is axial.
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.
70 to 80% of compressed air is used for thermal insulation, the remaining 20 to 30% air is used for combustion.
53% of the total machine output.
Load, ambient pressure, ambient temperature rate of humidity, steam injection.
Inlet air density increases at lower temperatures, so the compressor supplies a higher mass flow into the combustor, so the generator output will increase.
As pressure increases, air density increases, and the compressor supplies a greater mass flow. Flow into the combustor, so the generator output will increase.
The generator output is measured by the help of 2 wattmeters, which are connected to the appropriate sockets on the generator control panel.
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.
High wheel space temperature may be caused by any of the following reasons.
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.
Anti-clockwise ( viewing from compressor end towards generator )
Clockwise (reverse compare to normal because of the load gear)
95 % TNH (FSNL)
60 % TNH
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.
The rate of humidity affects the compressibility of atmospheric air. The energy absorbed by the compressor decreases and the output power slightly decreases.
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.
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.
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.
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.
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.
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.
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?
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.
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.
FSR – Fuel stroke reference is the electrical signal that determines the amount of fuel delivered to the turbine combustion system.
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Great help to understand the basics for all levels
Young fresh engineering trainees will learn-good
Great help to understand the basics for all levels
Very nice Gas turbine related all questions coverd
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Very helpful material related to gas turbine.