Gas Turbine Compressor Section

General Description
The axial-flow compressor section consists of the compressor rotor and the inclosing casing. Included within the compressor casing are the inlet guide vanes, the 17 stages of rotor and stator blading and the exit guide vanes.

In the compressor, air is confined to the space between the rotor and stator blading where it is compressed in stages by a series of alternate rotating (rotor) and stationary (stator) airfoil shaped blades. The rotor blades supply the force needed to compress the air in each stage and the stator blades guide the air so that it enters in the following rotor stage at the proper angle. The compressed air exits through the compressor discharge casing to the combustion chambers. Air is extracted from the compressor for turbine cooling, for bearing sealing, and during startup for pulsation control.

Since minimum clearance between rotor and stator provides best performance in a compressor, parts have to be made and assembled very accurately.

Compressor Rotor

The compressor rotor is an assembly of 15 individual wheels, two stub-shafts, each with an integral wheel, a speed ring, tie bolts, and the compressor rotor blades. Each wheel and the wheel portion of each stub-shaft has slots broached around its periphery. The rotor blades and spacers are inserted into these slots and are held in axial position by staking at each end of the slot. The wheels and stub-shafts are assembled to each other with mating rabbets for concentricity control and are held together with tie bolts. Selective positioning of the wheels is made during assembly to reduce balance correction. After assembly, the rotor is dynamically balanced to a fine limit.

The forward stub-shaft is machined to provide the forward and aft thrust faces and the journal for the n° 1 bearing, as well as the sealing surfaces for the nø 1 bearing oil seals and the compressor low pressure air seals.

Compressor Stator
The stator (casing) area of the compressor section is composed of four major sections :
  • Inlet casing forward compressor casing
  • Aft compressor casing
  • Compressor discharge casing
These sections, in conjunction with the turbine shell and exhaust frame form the primary structure of the gas turbine. They support the rotor at the bearing points and constitute the outer wall of the gas path annulus.

The casing bore is maintained to close tolerances with respect to the rotor blade tips for maximum efficiency.

Gas Turbine Combustion Section

The combustion system is of the reverse-flow type with 14 combustion chambers arranged around the periphery of the compressor discharge casing. This system also includes fuel nozzles spark plug ignition system, flame detectors, and crossfire tubes. Hot gases, generated from burning fuel in the combustion chambers, are used to drive the turbine. High pressure air from the compressor discharge is directed around the transition pieces and

into the combustion chambers liners. This air enters the combustion zone through metering holes for proper fuel combustion and through slots to cool the combustion liner. Fuel is supplied to each combustion chamber through a nozzle designed to disperse and mix the fuel with the proper amount of combustion air.

Orientation of the combustion chambers around the periphery of the compressor is shown on figure next page. Combustion chambers are numbered counter-clockwise when viewed looking down-stream and starting from the top of the machine. Spark plugs and flame detectors locations are also shown.

Combustion Chamber and Crossfire Tubes

 Combustion wrapper
The combustion wrapper forms 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.

Combustion chambers
Discharge air from the axial flow compressor flows into each combustion flow sleeve from the combustion wrapper (see figure). The air flows up-stream along the outside of the combustion liner toward the liner cap. This air enters the combustion chamber reaction zone through the fuel nozzle swirl tip, through metering holes in both the cap and liner and through combustion holes in the forward half of the liner.

The hot combustion gases from the reaction zone pass through a thermal soaking zone and then into a dilution zone where additional air is mixed with the combustion gases. Metering holes in the dilution zone allow the correct amount of air to enter and cool the gases to the desired temperature. Along the length of the combustion liner and in the liner cap are openings whose function is to provide a film of air for cooling the walls of the liner and cap as shown in figure. Transition pieces direct the hot gases from the liners to the turbine nozzles. All fourteen combustion liners, flow sleeves and transition pieces are identical.

Crossfire tubes
All fourteen combustion chambers are interconnected by means of crossfire tubes. These tubes enable flame from the fired chambers to propagate to the unfired chambers.

Gas Turbine Spark Plugs and Flame Detectors

Spark plugs
Combustion is initiated by means of the discharge from two high-voltages, non-retractable spark plugs bolted to flanges on the combustion chambers and mounted in a primary zone cup in adjacent combustors (N° 11 and 12).

These spark plugs receive their energy from ignition transformers. At the time of firing, a spark at one or both of these plugs ignites the gases in the primary zone of the chamber; the remaining chambers are ignited by crossfire through the tubes that interconnect the reaction Zones of the remaining chambers.

Spark Plug

Flame detectors
During the starting sequence, it is essential that an indication of the presence or absence of flame be transmitted to the control system. For this reason, a flame monitoring system is used consisting of eight sensors, each pair installed on four combustion chambers (n° 4 and 5, 10 and 11 primary and secondary zone) and an electronic amplifier which is mounted in the turbine control panel.

The ultraviolet flame sensor consists of a flame 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. A 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 electronics to give an output defining flame. Conversely, the absence of flame will generate an opposite output defining "no flame". After the establishment of flame, if voltage is reestablished to the sensors defining the loss (or lack) of flame a signal is sent to a relay panel in the turbine electronic control circuitry where

auxiliary relays in the turbine firing trip circuit, starting means circuit, etc... Shut down the turbine. The FAILURE TO FIRE or LOSS OF FLAME is also indicated on the annunciator. If a loss of flame is sensed by only one flame detector sensor, the control circuitry will cause an annunciation only of this condition. Flame detectors are water cooled.

For more information about the flame detectors, see "Gas turbine subcontractor’s literature chapter" for G.T. control and protection system).

Gas Turbine Fuel Nozzles (Gas)

Each combustion chamber is equipped with a fuel nozzle that emits the metered amount of the required fuel into the combustion liner. Fuel nozzles are used in gas turbines burning gas. The fuel nozzle functions to distribute the gas fuel into the reaction zone of the combustion liner, in a manner which promotes uniform, rapid and complete combustion.

Gas fuel enters the fuel nozzle assembly through the fuel gas connection flange and is routed through nozzle internal passages to orifices located in the gas tip.

Gas Turbine Enclosures

Gas turbine enclosures, referred to as compartments, are those partitioned area in which specific components of the overall power plant are contained. These compartments are built for all weather conditions and designed for accessibility when performing maintenance. They are provided with thermal and acoustical insulation and lighted for convenience.

Compartment construction includes removable panels, hinged doors, and a thermally insulated roof section with welded frame structuring providing the support for these parts. The panels are thermally insulated and held in place with bolts. Doors are kept tightly closed by sturdy latches. Gaskets between panels and framing maintain a weather-tight condition. Inspection and maintenance are facilitated as the door panels allow easy access for station personnel and the removable panels provide greater accessibility for major inspections and servicing. There is an inlet plenum between the accessory and the turbine compartments, and an exhaust plenum between the turbine and generator compartments.

Thus, in the compact integrated gas turbine-generator packaged design for a power generating station, there is an in-line sequence of lagged compartments, the sequence being broken by the inlet plenum and the exhaust plenum. These compartments enclose the turbine control compartment, the turbine accessory compartment, the gas turbine unit, the load gear, the driven generator.

Gas Turbine Bearings

The MS 9001 E gas turbine unit contains three main journal bearings used to support the gas turbine rotor. The unit also includes thrust bearings to maintain the rotor-to-stator axial position. These bearing assemblies are located in three housings: one at the inlet, one in the compressor discharge casing and one in the exhaust frame. All bearings are pressure lubricated by oil supplied from the main lubricating oil system. The oil flows through branch lines to an inlet in each bearing housing.

Bearing N*. Class Type

1 -Journal Elliptical

2 -Journal Elliptical

3 -Journal Elliptical

1 -Loaded thrust Self-aligned (equalized)

1 -Unloaded thrust Tilting pad

The three main turbine bearings are pressure-lubricated with oil supplied by the 12540 liters capacity lubricating oil reservoir. Oil feed piping, where practical, is run within the lube oil reservoir drain line, or drain channels, as a protective measure. This procedure is referred to as double piping and its rationale is that in the event of a pipe-line leak, oil will not be lost or sprayed on nearby equipment, thus eliminating a potential safety hazard.

When the oil enters the bearing housing inlet, it flows into an annulus around the bearing liner. From the annulus the oil flows through machined slots in the liner to the bearing surface. The oil is prevented from escaping along the turbine shaft by labyrinth seals.

Oil seals
Oil on the surface of the turbine shaft is prevented from being spun along the shaft by oil seals in each of the three bearing housings. These labyrinth packings and oil deflectors (teeth type) are assembled on both sides of the bearing assemblies where oil control is required. A smooth surface is machined on the shaft and the seals are assembled so that only a small clearance exists between the oil and seal deflector and the shaft. The oil seals are designed with two rows of packing and an annular space between them. Pressurized sealing air is admitted into this space and prevents lubricating oil from spreading along the shaft. Some of this air returns with the oil to the main lubricating oil reservoir and is vented through a lube oil vent.

Gas Turbine Bearing Assembly

 Gas Turbine Load Thrust Bearing

Gas Turbine Unload Thrust Bearing

Gas Turbine Exhaust Frame and Diffuser

The exhaust frame assembly (figure here after) consists of the exhaust frame and the exhaust diffuser. The exhaust frame is bolted to the aft flange of the turbine shell.

Structurally, the frame consists of an outer cylinder and inner cylinder interconnected by ten radial struts. On the inner gas path surfaces of the two cylinders are attached the inner and outer diffusers. The no.3 bearing is supported from the inner cylinder.

The exhaust diffuser, located at the extreme aft end of the gas turbine, bolts to, and is supported by, the exhaust frame. The exhaust frame is a fabricated assembly consisting of an Inner cylinder and an outer divergent cylinder that flairs at the exit end at a right angle to the turbine centerline. At the exit end of the diffuser between the two cylinders are five turning vanes mounted at the bend. Gases exhausted from the third turbine stage enter the diffuser where velocity is reduced by diffusion and pressure is recovered. At the exit of the diffuser, turning vanes direct the gases into the exhaust plenum.

Exhaust frame radial struts cross the exhaust gas stream. These struts position the inner cylinder and no.3 bearing in relation to the outer casing of the gas turbine. The struts must be maintained at a uniform temperature in order to control the center position of the rotor in relation to the stator. This temperature stabilization is accomplished by protecting the struts from exhaust gases with a metal fairing fabricated into the diffuser and then forcing cooling air into this space around the struts.

Turbine shell cooling air enters the space between the exhaust frame and the diffuser and flows in two directions. The air flows in one direction into the turbine shell cooling annulus and also down through the space between the struts and the airfoil fairings surrounding the struts and subsequently into the load shaft tunnel and turbine third-stage aft wheelspace.

Exhaust frame assembly 

Gas Turbine Inlet and Exhaust Sections

It is necessary to treat incoming atmospheric air before it enters the turbine in order to adapt to the environment and realize the desired machine performance. Specially designed equipment is installed to modify the quality of the incoming air to make it suitable for use in the unit. It is necessary also to attenuate the high frequency noise in the air inlet, caused by the rotating compressor blading. At the exhaust end of the gas turbine, gases produced as the result of combustion in the turbine require specific equipment according to their exhaust to atmosphere or towards heat recovery boilers.

Air Inlet System

The air inlet system, down-stream of the air filtering installation, is not described in details in this paragraph as it is not a part of the gas turbine assembly itself. It consists of an air duct, followed by sections of parallel baffles silencers, then a screen system located in an inlet elbow, and an expansion joint after which air will reach the gas turbine air inlet plenum. The gas turbine inlet plenum contains the compressor inlet casing.

The silencers are of baffle-type construction to attenuate the high frequency noise in the air inlet, caused by the rotating compressor blading. More details are given in the "Auxiliary plants and systems" volume, especially about the filtering installation.

Exhaust system
In the exhaust section, the gases which have been used to power the turbine wheels are redirected to be either released to atmosphere, or towards a heat recovery boiler when it is the case.

After leaving the exhaust frame, the hot gases reach the diffuser, located in the exhaust plenum. On the exhaust plenum wall facing the exhaust diffuser, a circular arrangement of thermocouples permits exhaust gas temperature measurement. The thermocouples send their signals to the gas turbine temperature control and protection system. The exhaust plenum configuration is that of a box open at the top and welded to an extension of the turbine base.

Insulation in the plenum fabrication provides thermal and acoustical protection. A flow path from the exhaust plenum open side to a duct is provided by an extension plenum and an expansion joint.

Two silencers are installed in series in the duct (the first one for the low frequency noises, the second one for the high frequency noises), after which there is another expansion joint, before exhaust either to atmosphere, upwards, or to a recovery boiler.

The exhaust system, down-stream of the exhaust plenum, is not described in detail here as it is not a part of the gas turbine assembly itself. More details are given in the "Auxiliary plants and systems".

Gas Turbine Turbine Section

General Description
The three stage turbine section is the area in which energy in the form of high energy pressured gas, produced by the compressor and combustion sections, is converted to mechanical energy.

Each turbine stage is comprised of a nozzle and the corresponding wheel with its buckets. Turbine section components include the turbine rotor, turbine shell, nozzles, shrouds, exhaust frame and exhaust diffuser.

Turbine Section

Turbine Rotor

The turbine rotor assembly consists of two wheel shafts; the first, second, and third-stage turbine wheels with buckets ; and two turbine spacers. Concentricity control is achieved with mating rabbets on the turbine wheels, wheel shafts, and spacers. The wheels are held together with through bolts. Selective positioning of rotor members is performed to minimize balance corrections.

The forward wheel shaft extends from the first-stage turbine wheel to the aft flange of the compressor rotor assembly. The journal for the n° 2 bearing is a part of the wheel shaft. The aft wheel shaft connects from the third-stage turbine wheel to the load coupling. It includes the n° 3 bearing journal.

Spacers between the first and second, and between the second and third-stage turbine wheels determine the axial position of the individual wheels. These spacers carry the diaphragm sealing bands. The spacer forward face includes radial slots for cooling air passages. The 1-2 spacer also has radial slots for cooling air passages on the aft face.

Turbine Rotor Location (in black)

The turbine buckets increase in size from the first to the third-stage. 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. The first-stage buckets are the first rotating surfaces encountered by the extremely hot gases leaving the first-stage nozzle. Each first-stage bucket contains a series of longitudinal air passages for bucket cooling. Air is introduced into each first-stage bucket through a plenum at the base of the bucket dovetail. It flows through cooling holes extending the length of the bucket and exits at the recessed bucket tip. The holes are Spaced and sized to obtain optimum cooling of the airfoil with minimum compressor extraction air.

Like the first-stage buckets, the second-stage buckets are cooled by spanwise air passages the length of the airfoil. Since the lower temperatures surrounding the bucket shanks do not require shank cooling, the second-stage cooling holes are fed by a plenum cast into the bucket shank. Spanwise holes provide cooling air to the airfoil at a higher pressure than a design with shank holes. This increases the cooling effectiveness in the airfoil so airfoil cooling Is accomplished with minimum penalty to the thermodynamic cycle.

The third-stage buckets are not internally air cooled; the tips of these buckets, like the second-stage buckets, are enclosed by a shroud which is a part of the tip seal. The shrouds Interlock from bucket to bucket to provide vibration damping.

Turbine buckets for each stage are attached to their wheels by straight, axial entry, multiple tang dovetails that fit into matching cutouts in the turbine wheel rims. Bucket vanes are connected to their dovetails by means of shanks. These shanks locate the bucket-to-wheel attachment at a significant distance from the hot gases, reducing the temperature at the dovetail. The turbine rotor assembly is arranged so that the buckets can be replaced without unstacking the wheels, spacers, and wheel shaft assemblies.

Turbine rotor cooling
The turbine rotor must be cooled to maintain reasonable operating temperatures and, therefore, assure a longer turbine service life.

Cooling is accomplished by means of a positive flow of cool air radially outward through a space between the turbine wheel with buckets and the stator, into the main gas stream. This area is called the wheel space.

The turbine rotor is cooled by means of a positive flow of relatively cool (relative to hot gas path air) air extracted from the compressor. Air extracted through the rotor, ahead of the compressor 17th stage, is used for cooling the 1st and 2nd stage buckets and the 2nd stage Aft and 3rd stage forward rotor wheel spaces. This air also maintains the turbine wheels, turbine spacers and wheel shaft at approximately compressor discharge temperature to assure low steady state thermal gradients thus ensuring long wheel life.

The first stage forward wheel space is cooled by air that passes through the high pressure packing seal at the aft end compressor rotor. The 1st stage aft and 2nd stage forward wheel spaces are cooled by compressor discharge air that passes through the stage 1 shrouds and then radially inward through the stage 2 nozzle vanes. The 3rd aft wheel space is cooled by cooling air that exits from the exhaust frame cooling circuit.

Turbine stator
The turbine shell and the exhaust frame constitute the major portion of the gas turbine stator structure. The turbine nozzles, shrouds, n° 3 bearing and turbine exhaust diffuser are internally supported from these components.

Turbine Stator (in black)

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

Hot gases contained by the turbine shell are a source of heat flow into the shell. To control the shell diameter, it is important that the shell design reduces the heat flow into the shell and limits its temperature. Heat flow limitations incorporate insulation, cooling, and multilayered structures. The external surface of the shell incorporates cooling air passages. Flow through these passages is generated by an off base cooling fan.

Structurally, the shell forward flange is bolted to flanges at the aft end of the compressor discharge casing and combustion wrapper. The shell aft flange is bolted to the forward flange of the exhaust frame. Trunnions cast onto the sides of the shell are used with similar trunnions on the forward compressor casing to lift the gas turbine when it is separated from its base.

Turbine nozzles
In the turbine section, there are three stages of stationary nozzles which direct the high velocity flow of the expanded hot combustion gas against the turbine buckets, causing the rotor to rotate. Because of the high pressure drop across these nozzles, there are seals at both the inside diameters and the outside diameters to prevent loss of system energy by leakage. Since these nozzles operate in the hot combustion gas flow, they are subjected to thermal stresses in addition to gas pressure loadings.

First stage nozzle
The first stage nozzle receives the hot combustion gases from the combustion system via the transition pieces. The transition pieces are sealed to both the outer and inner sidewalls on the entrance side of the nozzle, so minimizing leakage of compressor discharge air into the nozzles. The 18 cast nozzle segments, each with two partitions (or airfoils) are contained by a horizontally split retaining ring which is center-line supported to the turbine shell on lugs at the sides and guided by pins at the top and bottom vertical center-lines. This permits radial growth of the retaining ring, resulting from changes in temperature while the ring remains centered in the shell.

The Aft outer diameter of the retaining ring is loaded against the forward face of the first stage turbine shroud and acts as the air seal to prevent leakage of compressor discharge air between the nozzle and shell. On the inner sidewall, the nozzle is sealed by direct bearing of the nozzle inner load rail against the first-stage nozzle support ring bolted to the compressor discharge casing. The nozzle is prevented from moving forward by four lugs welded to the aft

outside diameter of the retaining ring at 45 degrees from vertical and horizontal centerlines. These lugs fit in a groove machined in the turbine shell just forward of the first stage shroud Thook. By moving the horizontal joint support block and the bottom centerline guide pine, the lower half of the nozzle can be rolled out with the turbine rotor in place.

Second stage nozzle
Combustion gas exiting from the first stage buckets is again expanded and redirected against the second stage turbine buckets by the second stage nozzle.

The second stage nozzle is made of 16 cast segments, each with three partitions (or airfoils).

The male hooks on the entrance and exit sides of the sidewall fit into female grooves on the Aft side of the first stage shrouds and on the forward side of the second stage shrouds to maintain the nozzle concentric with the turbine shell and rotor. This close fitting tongue and groove fit between nozzle and shrouds acts as an outside diameter air seal.

The nozzle segments are held in a circumferential position by radial pins from the shell into axial slots in the nozzle outer sidewall.

The second stage nozzle partitions are cooled with compressor discharge air.

Third stage nozzle
The third stage nozzle receives the hot gas as it leaves the second stage buckets, increases its velocity by pressure drop and directs this flow against the third stage buckets.

The nozzle consists of 16 cast segments, each with four partitions (or airfoils). It is held at the outer sidewall forward and aft sides in grooves in the turbine shrouds in a manner identical to that used on the second stage nozzle. The third stage nozzle is circumferentially positioned by radial pins from the shell.

Attached to the inside diameters of both the second and third stage nozzle segments are the nozzle diaphragms (figure here after).

These diaphragms prevent air leakage past the inner sidewall of the nozzles and the turbine rotor. The high/low, labyrinth-type seal teeth are machined into the inside diameter of the diaphragm. They mate with opposing sealing lands on the turbine rotor. Minimal radial clearance between stationary parts (diaphragm and nozzles) and the moving rotor are essential for maintaining low interstage leakage; this results in higher turbine efficiency.

Unlike the compressor blading, the turbine bucket tips do not 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.

The shroud segments are maintained in the circumferential position by radial pins from the shell. Joints between shroud segments are sealed by interconnecting tongues and grooves.

Gas Turbine Accessory Drive

The accessory drive gear, located at the compressor end of the gas turbine, is a gearing assembly coupled directly through a flexible coupling to the turbine rotor. Its function is to drive each gas turbine accessory at its proper speed. In addition, it contains the system main lube oil pump.

Contained within the gear casing are the gear trains which provide the proper gear reductions to drive the accessory devices at the required speed, with the correct torque values. Accessories driven by the gear include : the main lube oil pump and the main hydraulic supply pump. Lubrication of the gear is from the turbine's pressurized bearing header supply.

For ease of maintenance and inspection, the gear casing is split at the horizontal plane into an upper and lower section. Interconnected shafts are arranged in a parallel axis in the lower casing. Three of the shafts are located on the same horizontal plane as the casing joint. The gear consists of four parallel axis, interconnected shafts arranged in a casing which provides mounting pads for the various driven accessories. With the exception of the lube oil

Pump and hydraulic supply pump shaft, all the shaft center lines are located on the horizontal joint of the accessory drive casing. Numbers are assigned to the various shafts in the "Gas turbine equipment publications chapter" according to their function and therefore their speed. The gear casing is made of cast iron and split at the horizontal joint to facilitate assembly. The lower half casing has a closed bottom with opening for lube oil pump suction and discharge lines and casing drain line.

All of the shafts are connected together by single helical gears which are shrunk to the shafts after the teeth are cut. It is possible, in some instances to remove individual gears which may have been damaged in service, and to replace them with new gears.

This operation, however, should be performed at the factory so that the required precision may be maintained. All of the shafts located on the horizontal joint are contained in babbitt-lined steel-backed journal bearings with integral thrust faces which are split on the horizontal joint of the casing.

The thrust faces of the bearings maintain the shafts in their proper axial location and the necessary thrust clearance is preset at the factory. The shafts which are not on the horizontal joint are contained in babbitt-lined, steel-backed, non-split bushings with integral thrust faces. Their thrust clearance is likewise preset at the factory.

The main lubricating oil pump is located on the inboard wall of the lower half casing of the accessory drive gear. 
Accessory Drive

Gas Turbine Starting System

Turbine compressor strings require a large electric motor and variable frequency drive (VFD) capable of starting the string (Starter Mode), adding torque at any operating speed point (Helper Mode), and returning excess GT torque available under specific conditions as electrical power to the QGII electrical distribution system (Generator Mode).

Furthermore, the motor shall drive the GT in order to perform Normal Turning at 450 rpm (or Emergency Turning at 300 rpm) and Off-line water wash at crank speed (300 rpm).

In order to manage this function, specific logics and regulations have been implemented in Mark VI.

This paragraph describes these logics and regulators.

Two major analog signals are generated into MARK VI: torque request to VFD and Power available to Power Management System.

From a regulation point of view, a split range as a secondary cascade of a maximum selection scheme is implemented.

As shown in the following scheme there, are several regulators that are maximum selected:

The first 50% of TSP_MO1X is effected as a decrease of power available for electric generation and the second 50% is effected as a torque request to the helper motor. Both actions perform a decrease of the turbine total load without interacting with the plant request.

TSP_MO1X= 50% or THEORETIC POWER AVAILABLE FOR GENERATION =0 is the crossing condition from generator to motor way of functioning.

Before the end of the start sequence, the minimum value of regulators is 50%, so the Output is activated as a torque request; in this phase the TSP_TNH, speed regulator via motor torque, is the winner of the selection, because the other regulators are not active. During a shutdown, the TSP_MO1X signal is forced to 50%, by a ramp if we have a normal shutdown or by a step if we have an emergency trip.

Before the end of the start sequence, the minimum value of regulators is 50%, so the output is activated as a torque request; in this phase the TSP_TNH, speed regulator via motor torque, is the winner of the selection, because the other regulators are not active. During a shutdown, the TSP_MO1X signal is forced to 50%, by a ramp if we have a normal shutdown or by a step if we have an emergency trip.

Note: As far as Low NOX operation is concerned, in case of power decrease in Premix mode, MKVI will provide an APPROACHING PREMIX TO LEAN_LEAN signal for QGII operation to indicate nearness to “Lean-Lean” mode operation. This indication allows the operator to increase the VFD generator absorbed power, since during this mode NOX emission limits are not guaranteed.

TSP_P, TSP_FQRG, TSP_SRV regulators perform a decreasing of power available or a  torque request that are generated by the fuel governor in order to avoid:

TSP P : low fuel gas intervalve pressure (96FG-2A/2B/2C);

TSP_FQRG : fuel gas control valve (VGC-1) max opening condition;

TSP_SRV : fuel gas ratio valve (VSR-1) max opening condition;

TSP_LTA : high load tunnel temperatures (TT-IB1/2/3).

This is the regulator active during the start up sequence and during the cooldown. Depending on the unit status (Crank, Firing, WarmUp, Acceleration, End of Sequence, Cooldown, Water Wash), the MarkVI will select the appropriate speed set point and the appropriate speed rate of change in order for the GT start up to follow the predefined schedule. During startup, the minimum value of TSP_TNH is forced to 50%, therefore only

the TSP_TNH regulator output is selected by the Max Selector and this signal will develop the torque demand sent to The VFD. After completing the sequence, the minimum value of TSP_TNH drops to zero and the other regulators are enabled.

This regulator performs a torque request when the unit is very close to temperature via fuel control, which means that helper motor torque is needed to achieve further load request. Obviously in this condition the THEORETIC POWER AVAILABLE FOR GENERATION shall be zero, so a generation-to-helper commutation command is sent to the VFD, and the regulator output is immediately activated as torque request.

Under normal operation, when power can be generated by the VFD, the TSP_FSR regulator is outputting 0% and the Power Available multiplier is 1. If there is a process load change or the PMS makes an additional power demand on the VFD, the Power Available calculation will be controlling the Power Available Signal to the PMS/VFD, and will prevent the GT from hitting the high temperature limit. If the Power Available calculation is not able to

prevent the GT from hitting the high temperature limit, then the TSP_FSR regulator will reduce the multiplier for the Power available signal from 1 toward 0, if this is not enough to overcome the disturbance, the VFD will enter helper motor mode. It is possible that other override regulators

may be selected by the Max Selector, which will reduce the Power Available signalor enable the helper motor.

In the event that the VFD is in the generation mode and it does not respond to a reduction in the Power Available signal, then the Mark VI will change the Generation Mode Available bit L83GA from 1 to 0, when Power Available becomes 0. The Generation Mode Available bit (false) L83GA is sent directly from Mark VI to the VFD, that will leave the Generation Mode.
This signal is a manual value to the maximum selector. As soon as MANUAL_ON command is sent from HMI, the TSP_MO1X value is copied to TSP_MAN variable. From now on, the TSP_MAN value can be changed by the operator from HMI until MANUAL_OFF command is sent. Analog signals are ramp actuated.

Alarms and Trip
The following alarms and trips have been implemented:

SPEED DOESN'T FOLLOW SETPOINT = Excessive (Speed set-Actual speed) difference during start sequence – hold speed set point ramp

TORQUE NOT FOLLOWING TRIP = Excessive (Torque request-Actual torque) difference during start sequence –Trip

TORQUE NOT FOLLOWING ALARM = Excessive (Torque request-Actual torque) difference
after start sequence –Alarm

Air Intake Filter

The GDX system is a single stage self-cleaning air filter utilizing conical and cylindrical filter cartridges sequentially cleaned by a reverse flow pulse of compressed air. Combustion air is aspirated by the turbine through the filter. The air intake is located at the front side of the filter. Entrance into the inlet plenum is protected by the inlet hoods.

These inlet hoods and plenum protect the cartridges against foreign object damage, excessive ingestion of rain, sunshine. Inside these inlet hoods, a trace heating cable is installed to prevent possible icing damage inside the inlet hoods. A quantity of high efficiency filter cartridges composed of a combination of a conical and cylindrical filter element is attached to the vertical module plate (tube sheet).During normal operation, ambient air flows downwards between the elements, through the filter cartridges into the clean air plenum. Blowpipes are installed in the clean air plenum; one in front of each filter element set.Cleaning is accomplished by intermittently injecting compressed air through the blowpipes. Each pulse of air from the blowpipe into the filter cartridge provides a shock wave inside the filter cartridge and a momentary reverse flow.

These actions together provide the necessary cleaning energy. Many of the dust particles agglomerate on the media to form a thin cake.

When pulsed, they come off in larger masses than when they were deposited.

The possibility that these larger particles will be re-entrained when the normal flow is re-established, is reduced. The clean air plenum pressure is monitored and cleaning is initiated when the differential pressure becomes higher than a pre-set value. Cleaning continues until the pressure decreases to a lower pre-set value. The dust which is pulsed-off from the cartridges drops down into the dust evacuation hoppers. Each of these hoppers is fitted with an exhaust fan. Since these fans are running at the same time as the cleaning cycle is in operation, the dust falling down into the hopper is automatically blown away into the atmosphere.

The single stage filtration of the filter is provided by conical and cylindrical filter elements Inertial separators and pre-filters, generally used as first or second stages of filtration in traditional "3-stage" turbine filter systems, are not required in the GDX design. These extra stages were required to reduce premature plugging of the rectangular filter cells used in these systems. In the GDX, the high efficiency filter elements are automatically cleaned during the turbine operation and they do not require these extra stages thanks to the automatic cleaning feature of the filter.

The cartridge filter medium has been used for several years in filtering combustion air for turbine engines.

Particle count efficiency on a clean element is remarkably high, even on particles of 0.5 μm. The capture efficiency is practically total on the particles of 5μm and virtually total on particles of 1 μm after an initial loading period of a few hundred grams.
These efficiencies, provided by the filter elements, assure proper protection of the machines against both dust and alkaline salts.

Both the airflow pattern and the specified flow rates have been calculated to improve filtration and pulse cleaning of the filters.

During pulse cleaning, normal flow through two element sets is interrupted for about 0.1 second.

The effect of this interruption on the nominal flow of the turbine is negligible.

The self-cleaning filter pressure drop begins at a lower value: about 300 Pa.

The dust accumulation on the filter elements increases the pressure drop gradually.

The module clean air plenum pressure (vacuum) is monitored and cleaning is initiated when this pressure drop reaches a pre-set level with reference to ambient.

Cleaning continues until the Δp decreases to a lower pre-set limit.

After some time, depending upon the environmental conditions, the lower set value may not be reached anymore and the filter house will pulse continuously.

The pressure drop will then become stabilized approximately between 750 and 1000 Pa depending on the environment and the dust type to be filtered.

Strong recommendation
The average pressure drop of the filter left continuously in the automatic cleaning mode could be reduced if, at each shut-down of the engine, the manual cleaning mode feature is used.

Ventilation System

The turbine and load compartments are equipped with a ventilation system. Both compartments are fitted with thermally insulated side panels and roofs.

Dampers are used in the system to automatically provide sealing when the water mist fire fighting system is activated.

The turbine and load compartments are pressurized and cooled by ventilation fans (88BA-1, 2) installed in the pressurized and cooled ventilation ducting after the inlet filter compartment.

The ventilation system consists of two separate fans driven by their respective motors; one fan provides ventilating air during normal turbine operation. The other operates as a stand-by fan and starts when:

  •  Pressure inside the turbine compartment reaches the set point of the differential pressure transmitter 96SV-1 or, if
  • A turbine or load compartment temperature high alarm is on, detected by voted temperature transmitters TT-BA-2A/B/C or TT-BA-3A/B/C.
  • Enclosure gas high level alarm, detected by gas detectors 45HD-2A/B/C, 45HD-3A/B/C, 45HD- 4A/B/C, 45HD-5A/B/C. The ventilation system permissive to start is given when all the enclosure doors are closed and all dampers are open.

Gas Turbine Fuel System

The gas fuel system is designed to deliver gas fuel to the turbine combustion chambers at the proper pressure and flow rates to meet all of the starting, acceleration and loading requirements of gas turbine operation. A schematic diagram of the gas fuel system is provided in Figure GF-1.

The major components of a gas fuel system are the gas stop/ratio and gas control valves located in the gas fuel module. Associated with the gas valves are the necessary inlet piping and strainer, fuel vent valve, control servo valves and the distribution piping to the 14 combustion fuel nozzles.

The fuel gas stop ratio valve and the gas control valves are independent valves, located side by side in the gas fuel piping of the module. The gas fuel flows through the gas stop ratio valve and then into the gas control valves on its way to the gas manifold and individual combustion chambers. The position of each valve is servo controlled by electrical signals from the gas turbine SPEEDTRONIC control system. Both the gas stop ratio valve and the gas control valves are actuated by single-acting, hydraulic cylinders.

The following major components comprise the gas fuel system:
  • Filters
  • Gas fuel stop solenoid valves 20 FG’s
  • Fuel gas supply pressure alarm switch
  • Gas stop ratio valve VSR
  • Gas control valve VGC’s
  • Stop ratio LVDT 96 SR
  • Gas control valve LVDT 96 GC
  • Stop ratio valve-control servovalve 90 SR-1
  • Gas control valve-control servovalve 65 GC
  • Gas fuel trip valves VH 5
  • Gas fuel vent solenoid valves 20 VG-1
  • Pressure gauges
  • Lines to the 14 combustion chambers

Part Nomenclature

20 VG-1 Gas fuel (purge) vent solenoid valve.

20 FGC-1,-2,-3 Trip solenoid valve for gas control valve VGC-1,-2,-3.

20 FGS-1 Fuel gas stop valve solenoid valve.

33 VG-11 Limit switch on solenoid valve 20 VG-1.

65 GC-1,-2,-3 Gas control valve (servo-valve).

90 SR-1 Stop/speed (pressure) ratio valve servo-valve.

96 FG-2A,-2B,-2C Fuel gas inter-valve pressure transmitter.

96 FG-4A,-4B,-4C Fuel gas inter-valve pressure transmitter.

96 FG-5A,-5B,-5C Fuel gas inter-valve pressure transmitter.

96 FG-6A,-6B,-6C Fuel gas inter-valve pressure transmitter.

96 FG-1 Fuel gas supply pressure transmitter.

96 GC-1,-2 Gas control valve L.V.D.T.

96 SR-1,-2 Stop/speed (pressure) ratio valve L.V.D.T.

AHI-3,-4 Control oil hydraulic accumulator.

FH 7-1 Gas fuel Stop Valve (VSR-1). Servo Hydraulic Oil Supply Filter.

FH 8-1,-2,-3 Gas fuel Control Valve (VGC-1,-2,-3). Servo Hydraulic Oil Supply Filter.

FT_GI-1A,-1B Fuel gas temperature sensor.

FT_GI-2A,-2B Fuel gas temperature sensor.

MG1-1 Gas fuel nozzle - primary.

MG1-2 Gas fuel nozzle - secondary.

MG1-3 Gas fuel nozzle - transfer.

VGC-1 Gas control valve - Primary.

VGC-2 Gas control valve - Secondary.

VGC-3 Gas control valve - Transfer.

VH 5-1 VSR-1 security discharge valve.

VH 5-2,-3,-4 VGC-1,-2,-3 trip valve.

VSR-1 Fuel gas stop/ratio valve.

Gas Fuel System Schematic

Functional description of the gas fuel system

The gas control valves and the gas stop ratio valve, although similar, each perform separate functions. The fuel gas control valves meters fuel for use by the combustion chambers. It is activated by a SPEEDTRONIC control signal to admit the proper amount of fuel required by the turbine for a given load or speed. The fuel gas stop ratio valve is a dual function valve. It serves as a stop valve to shut off fuel flow to the turbine whenever

required during either normal operation or in an emergency shut-down situation. The stop ratio valve also serves as a pressure regulating valve to hold a known fuel gas pressure ahead of the gas control valve and enable the gas control valves to control fuel flow over the wide range required under turbine starting and operating conditions. Because of these dual functions the valve is sometimes called a stop/speed ratio valve.
Gas strainer
A gas strainer is installed upstream of the turbine base fuel inlet connection point, to facilitate site maintenance requirements. Connection of the fuel gas supply is made at the purchaser’s connection in the supply line ahead of the gas strainer. Foreign particles that may be in the incoming fuel gas are removed by the strainer.

Gas stop/ratio and gas control valves
The gas control valves VGC’s regulates the required control valve area and utilizes an hydraulic cylinder controlled by an electrohydraulic servo valve. The gas control valve provides a fuel gas metering function to the turbine in accordance with its speed and load requirements. The position of the gas control valve (hence fuel gas flow to the turbine) is a linear function of a Fuel stroke reference voltage (FSR) generated by the SPEEDTRONIC control.

A dump valves VH 5-2,-3,-4 are operated by trip oil acting on the piston end of a spool. An hydraulic trip solenoid valves, 20 FGC-1,-2,-3 are located in the trip oil line to the dump valves. When the trip oil pressure is normal and the 20 FGC-1,-2,-3 solenoid valves are energized to reset, the spool of the dump valves are held in a position that allows hydraulic Oil to flow between the control servo valves and the hydraulic cylinders. In this position, normal control of the gas control valve valve is allowed.

The control voltage generated acts to shift the electrohydraulic servo valve to admit oil to, or release it from, the hydraulic cylinder to position the gas control valve so that the fuel gas flow is that which is required for a given turbine speed and load situation.

The gas control valves also provides a shut-off of the fuel gas flow when required by either normal operation or emergency conditions. The hydraulic trip relays (dump valves) VH 5-2,- 3,-4 are located between the electrohydraulic servo valves 65 GC-1,-2,-3 and the hydraulic cylinders. The operation of this dump valves is the same as the trip relays (dump valves) VH 5-2,-3,-4.

The plugs in the VGC’s are contoured to provide the proper flow area in relation to valve stroke. The VGC’s use a skirted valve plug and venturi seat to obtain adequate pressure recovery. High pressure recovery occurs at valve pressure ratios substantially less than the critical pressure ratio. The result is that the flow through the VGC’s are independent of the pressure drop across the valves and is a function of valves inlet pressure, P2 temperature and valve area only.

The combined position of the control valves is intended to be proportional to FSR, which represents called-for fuel – required by the control system to maintain either speed, load, or another set point. FSR2 is the percentage of maximum fuel flow required from the Gas Fuel System. FSR2 is further divided so that a command is sent via the servo valves (65 GC-1,- 2,-3) on VGC-1,-2,-3 so that the required split of gas fuel is achieved. Dual redundant

Linear Variable Differential Transformers (LVDT’s) – 96 GC-1,-2,-3,-4,-5,-6 are used for Control valves for position sensing.

The VSR 1 and VGC's are equipped with hydraulically actuated spring return dump valves (VH 5-1,-2,-3,-4). The dump valves are held in their normal operating state by a supply of hydraulic oil referred to as trip oil. The trip oil system is triple redundant to ensure that no single device failure can disturb the operation of the power generating unit. The gas stop/ratio valve VSR-1 is similar to the gas control valves VGC’s. The ratio function of the stop ratio/valve provides a regulated inlet pressure for the control valve as a function of turbine speed. The SPEEDTRONIC pressure control loop generates a position signal to position the stop ratio valve by means of a servo valve controlled hydraulic cylinder to provide required inter valve pressure.

The gas stop ratio valve VSR-1 functions as a stop valve in the fuel gas system to provide a positive fuel shut off when required by either normal or emergency conditions. Any emergency trip or normal shut-down will trip the valve to its closed position. This is done either by dumping hydraulic oil from the valve’s hydraulic cylinder or driving the position control closed electrically. A dump valve VH 5-1 is operated by trip oil acting on the piston

end of a spool. An hydraulic trip solenoid valve, 20 FGS-1, is located in the trip oil line to the dump valve. When the trip oil pressure is normal and the 20 FGS-1 solenoid valve is energized to reset, the spool of the dump valve is held in a position that allows hydraulic oil to flow between the control servo valve and the hydraulic cylinder. In this position, normal control of the stop ratio valve is allowed.

In event of a drop in trip oil below a predetermined limit, a spring in the dump valve shifts the spool to interrupt the flow path of oil between the control servo valve and the hydraulic cylinder. Hydraulic oil is dumped and the ratio valve closes, shutting off gas fuel flow to the turbine.

Gas fuel system protective devices

Gas pressure transmitter
A low gas pressure alarm transmitter (96 FG-1) is installed in the gas piping ahead of the gas stop/ratio valve assembly. This transmitter initiates a gas fuel pressure low alarm when gas supply pressure drops below the switch setting. It also initiates a transfer to liquid fuel if gas supply pressure drops below its set point.

Gas fuel vent valve

Solenoid-operated valve 20 VG-1 is installed in the vent piping between the gas stop/speed ratio and gas control valve. When the turbine is shut down, any gas fuel that might accumulate in the compartment between the stop/speed ratio and gas control valves, vents to atmosphere through the piping. It also ensures that no gas fuel will leak past the closed gas control valve to collect in the combustors or exhaust.

Limit switch 33 VG-11 controls the full opening of solenoid valve 20 VG-1, if not open, it activates an alarm.

Pressure transmitters
Pressure transmitters, 96 FG-2A,-2B,-2C, 96 FG-4A,-4B,-4C, 96 FG-5A,-5B,-5C and 96 FG-6A,-6B-,-6C are installed in the fuel system on the gas fuel discharge side of the stop/speed ratio valve and the gas control valves, to provide the operational pressure feedback signal to the SPEEDTRONIC control system. The DC voltage output signal is the median of the 3 transmitters readings.

Servo-hydraulic supply filters
A filter FH 7-1 and FH 8-1,-2,-3 are installed in the hydraulic supply to the speed ratio and gas control valve servo-valves 90 SR-1 and 65 GC-1,-2,-3 to provide 15 microns filtration. A high filter differential pressure indicator is included.

Fuel manifolds and nozzles
Fuel from the control valve is distributed through the manifold to the fuel nozzle Assemblies mounted in each combustion chamber. Fuel from the nozzles is mixed with air in the combustion liner where combustion takes place.

Final gas filters
The final gas filters are mounted in the lines to the primary, secondary and transfer manifolds. They are part of the final gas conditioning and are to be definitely removed after 100 hours of operation. The filters are mounted vertically on lines. The filter cartridge is of cleanable filter element type.

Temperature sensors
The K-type thermocouples sensors (FT_G1-1A,-1B; FT_G1-2A,-2B) are installed in the gas fuel supply line downstream of the SRV to confirm the gas fuel inlet temperature is within specified requirements at startup and throughout turbine operation. A median select, mismatch-alarm methodology is utilized by the control to interpret the three temperature signals. High levels will result in an indication to the operator, whereas High-high levels result in lockout of premix mode. Excessively high temperature indications result in turbine trip. Low levels result in lockout of premix mode.

In event of a drop in trip oil below a predetermined limit, a spring in the dump valve shifts the spool to interrupt the flow path of oil between the control servo valve and the hydraulic cylinder. Hydraulic oil is dumped and the ratio valve closes, shutting off gas fuel flow to the turbine.

Gas purging system

Gas fuel purge

The fuel transfer manifold must be purged.

A schematic diagram of the fuel purging system is provided in Figure GP-1.

Air is the purge medium, supply by compressor discharge air for the gas side.

Gas backflow to compressor discharge must be prevented. The gas fuel system purge valves air actuated VA 13 must be closed tightly. If it does not happen, protective measures are to be taken.

On the contrary, during fuel gas operation of the unit, solenoid valves 20 PG-3, -4 are de-energized, so shutting off the pneumatic actuation of VA 13-3 and VA 13-4.

  • There are redundant VA 13 purge valves.
  • The 20 VG-3 valve vents the line to atmosphere between the purge valves. Pressure switch 63 PG-2 will alarm if excessive pressure builds up between the valves, indicating the presence of too much gas.
  • 33 PG limit switches are used to indicate the position of the VA 13 valve: open or closed. The transfer gas purge operates the same except the VA 13 valves are normally open as the transfer system is usually not flowing gas fuel.

Gas Purging System Schematic