58232385 Gas Turbin Mark VI

By

GAS TURBINE Gas turbine is a device to convert heat energy produced by combustion of fuel gas into mechanical (kinetic) energy. These thermal drivers function by converting heat energy from a fuel into mechanical energy which can be used to run the different rotary equipment's, such as compressor. In a gas turbine, the natural gas is mixed with compressed air in a suitable ratio to give good combustion. The resultant hot flue gases are directly impinged on the gas turbine blades to rotate the turbine shaft. Gas turbine has been selected as a driver because of the following advantages: 1. Less auxiliaries 2. nominal maintenance cost 3. No support facilities or laboratory required 4. Less manpower required for operation and maintenance 5. Clean Service 6. No fuel storage required.

DISADVANTAGES OF GAS TURBINE 1. Complicated control 2. Low efficiency i.e. 20-25 % Axial compressor 75%. 3. Higher noise level. Essentially, gas turbine comprises of five basic components: 1. The axial air compressor 2. Combustion section 3. Transition Piece 4. The Impulse & Reaction stage 5. Wheel space

THE BRAYTON CYCLE The thermodynamic cycle upon which all gas turbines operate is called the Bray ton cycle. Classical pressure volume (PV) and temp-entropy (TS)diagrams for this cycle. Path 1 to 2 represents the compression , path 2 to 3 represents addition of heat in the combustion ,path 3 to 4 showing the pressure reduction in turbine, path from 4 back to 1 cooling process. This cooling is done by the atmosphere, which provides fresh cool air at point 1 on a continuous basis in exchange for the hot gases exhaust to the atmosphere at point 4. The actual cycle is an open rather than closed cycle, as indicated.The pressure ratio of the cycle is the pressure at point 2 divided by the pressure at point. In an ideal cycle, this pressure ratio is also equal to the pressure at point 3 divided by the pressure at point 4. However, in an actual cycle there is some slight pressure loss in the combustion system and, hence, the pressure at point 3 is slightly less than at point 2.

PARAMETERS AFFECTING EFFICIENCY AMBIENT TEMPERATURE: The power efficiency reduces rapidly with the increase in ambient temp. for each increase of 18 °F, the power decreases by about 8% and efficiency by 2%. ATMOSPHERIC PRESSURE: The power of turbine decreases with drop in atm. Press., while efficiency remains constant, but air mass flow decreases. TURBINE INLET TEMPERATURE: The power rises rapidly with increase in turbine inlet temperature. Turbine inlet temperature means the exhaust temp. of the combustion chamber for each 75 °F rise, the power increases about 10% and efficiency by 1 to 1.5%. LOSSES IN AIR INTAKE: Compressor discharge pressure is directly Affected by high-pressure drop at inlet air filters. For every 100 mm WC loss in air intake, power decreases by 2% and efficiency by approximately 1% EXHAUST SYSTEM LOSES: Back pressure is produced on the turbine by exhaust system and waste heat boilers. for each 100 mm H2O of back pressure, power and efficiency reduces by about 1% each.

AXIAL AIR COMPRESSOR SECTION In the compressor, air is confined to the space between the rotor and stator where it is compressed in stages by an alternate series of rotating (rotor) and stationary (stator) air-foil shaped blades. These blades are airfoil shaped and are designed to compress air efficiently at high blade tip velocities. Rotor blades supply the force needed to compress the air in each stage and the stator blades guide the air so that it enters the following rotor stage at the proper angle. The compressed air exits thru the compressor discharge casing to the combustion chambers. Axial Compressors are very sensitive to erosion or deposits on their blades. Similarly gas turbine fuel nozzles, 1st / 2nd stage nozzles and buckets are to be kept clean and prevented from erosion. For the best performance of a gas turbine, it is essential that air entering the compressor should be clean and free from dust and foreign particles.

Any material in the incoming air that has a tendency to erode or adhere to the Internals of the gas turbine and the compressor must be removed by use of filters to maintain its high efficiency. Dynawane inertial dust particle separators followed by high velocity crinkle wire mesh (CWM) panel filters coated with an adhesive oil have been provided to ensure a clean dust free air to the axial flow compressor suction. Air for combustion enters axial flow compressor through a large, rectangular duct which is equipped with a mesh screen to prevent large foreign objects from entering. Inertial Separates Dynawane filters are provided prior to the mesh screen to prevents smaller objects from entering and fouling the blades of the machine. Dirty air enters the large opening of the wedge-shaped cell of the inertial separator. As the air carrying dust flows down the cell most of the air passes through the vanes in an “S” curve and the dirt particles get separated.

COMBUSTION SECTION The combustion section consists often combustion chambers, fuel nozzles, cross fire tubes and transition pieces. Air for combustion is supplied directly from the axial-flow compressor to the combustion chambers. This arrangement is called a reverse flow system since the compressor discharge air flows forward around the liners and then enters and flows back toward the turbine. Fuel is fed into the chambers through fuel nozzles. The air flow through the combustion chambers has three functions: oxidize the fuel, cool the metal parts, and adjust the extremely hot combustion products to the desired turbine inlet temperature. The high pressure air flow from the compressor discharges into the annular. space created by the aft end of the discharge casing and the forward section of the turbine shell. Up to that point, the air flow is in an aft direction, then the air flow reverses. The air enters the combustion chambers and flows forward, entering the liner through holes and louvers in the liner wall. A portion of the air reaches the head end of the combustion chamber and enters the liner through the cap where an axial swirler creates a vortex within the liner.

Combustion chambers are designed to minimize exhaust emissions during The operation of the gas turbine after the start sequence. Lean primary combustion followed by a “thermal soaking” assures that soot is burned during combustion. Dilution of the combustion products to provide an adjusted turbine inlet temperature is delayed to allow consumption of any soot that was not burned in the combustion zone. The burning of this soot is accomplished by providing adequate residence time before air is introduced to dilute the hot gases an adjust the temperature pattern. The combustion chamber liners and casings may not all be identical in design nor interchangeable on different model series of gas turbines. The combustion chamber outer casings have machined pads for mounting the spark plugs and flame detectors. The combustion liners have holes through which the spark plugs and flame detector body projects.

TRANSITION PIECES The transition pieces are the hot gas path link between the combustion liners and the first stage nozzle. They are bolted to the forward side of the nozzle assembly. The nozzle assembly is sealed at both its outer and inner periphery to prevent leakage of hot gases. The transition piece assembly and support arrangement hold the assembly in proper alignment in the gas path and the floating seals make allowances for the effects of thermal expansion. Before the compressor discharge air flows into the combustion chamber, it must first pass around the transition pieces. This encounter affords an exchange of heat, cooling the transition pieces and preheating the combustion air.

TURBINE SECTION The first-stage or high pressure wheel, and the second-stage, or low pressure wheel, are Bolted together to make up a single unit through which the first and second stage Nozzles direct the flow of combustion gases. First-stage nozzle consists of airfoil Shaped partitions between an inner and Outer sidewall. The nozzle assembly is divided into segments, The nozzle ring and partitions are cooled by compressor discharge air which is bled from the combustion chamber transition space. The nozzle partitions are hollow with bleed holes drilled in the trailing edge for cooling. Both stages consist of arrow of fixed nozzles followed by a row of rotating turbine buckets. In each nozzle row, the kinetic energy of the jet is increased, with an associated pressure drop. In the following row of moving buckets, a portion of the kinetic energy of the jet is absorbed as useful work on the turbine rotor. After passing through the 2nd-stage buckets, the gases are directed into the exhaust hood and diffuser.

Hood and diffuser contain a series of turning vanes to turn the gases from an axial direction to a radial direction, to minimize exhaust hood loses. The gases then pass into the exhaust plenum. Resultant shaft rotation is used either to turn a generator rotor for electrical Power production, or to drive a compressor in industrial process applications. EXHAUST PLENUM The exhaust plenum is the beginning of the exhaust duct, receiving the gas flow from the exhaust diffuser. It consists of a box, open at each side and at the top, which is welded to an extension of the turbine base. The exhaust plenum is connected to the exhaust frame assembly with flex-plate expansion joints. The open sides and top of the plenum are covered with a wrapper whose purpose is to enlarge the plenum volume and force the exhaust gases to one side and into the exhaust ducting. The wrapper is supported by the foundation mounted pedestals.

COOLING / SEALING AND CONTROL AIR SYSTEM When the turbine is operating, air is extracted from appropriate stages of the gas turbine’s axial-flow compressor and is used to cool the turbine parts, to seal the turbine bearings and to provide an operating air supply for the control system of the gas turbine. COOLING AIR In addition to supplying combustion air, the axial flow compressor supplies cooling And sealing air for various parts of the turbine unit. 4TH STAGE AIR Air from the fourth stage of the compressor through two taps; on the upper left hand side of the unit and other on the lower right. It is piped externally to the turbine Compartment ventilation educators and to the turbine shell. The cooling air passes through holes in the exhaust frame and is directed to the aft (back) side of the second stage turbine wheel space and exhausts to the exhaust plenum.

10TH STAGE AIR Air from the 10th stage of the compressor through two taps, one on the upper left hand side of the unit and other on the lower right side. This air is piped externally and is used for sealing air on the turbine bearings. Tenth stage cooling air is also mixed with forth stage cooling air at the aft side of the second stage turbine, after cooling exhausts to the exhaust plenum. 16TH STAGE AIR A slipstream of the sixteenth stage discharge passes through the opening formed by the clearance between the axial-flow compressor discharge casing and the (back) side of the compressor rotor ahead of the guide vanes. The air then passes into the first stage turbine wheel space. The air provides a seal between the axial-flow compressor discharge and the turbine and cools the forward face of the first stage turbine wheel. Another stream of the sixteenth stage routed to the forward side of 2nd stage nozzles and turbine wheel, cools the 2nd stage turbine nozzle and blades.

FUEL GAS SYSTEM The fuel gas system is designed to deliver fuel gas to the turbine combustion Chambers at the proper pressure and flow rates in order to meet all starting, acceleration and loading requirements for gas turbine operation. The major component of a fuel gas system is the gas stop/ratio and control valve assembly, located in the accessory area. Associated with this gas valve are a vent valve, control servo valves, pressure gauges and the distribution piping to the combustion fuel nozzles. See the schematic piping diagram. The fuel gas system comprises the following major components: a. 1st , 2nd Shut Off valve (20-FG-1 or FCV-103, 20FG-2 or FCV-104). b. Fuel gas pressure gauges(PT-102), pressure transmitters (96FG-4A/BCV/C) c. low pressure alarm /switch (PAL/PALL-105) d. Fuel gas vent valve (20VG-1 or FCV-102). e. Gas metering control valve (GCV-1 or FCV-105).

Flame Detection Mark V supplies the 335 V dc excitation monitors the pulses of current being generated to determine if carbon buildup or other contaminates on scanner window are causing reduced light detection. The ultraviolet flame sensor in Mark VI consists of a flame sensor containing a gasfilled detector. The gas within this flame sensor detector is sensitive to the presence of ultraviolet radiation which is emitted by a hydrocarbon flame. A DC 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 both sensors indicate the loss (or lack) of flame, a signal is sent to the turbine control circuitry where the appropriate circuit shuts down the turbine. The FAILURE TO FIRE or LOSS OF FLAME is also indicated on the anunciator. If a loss of flame is sensed by only one flame detector sensor, the control circuitry will cause an annunciation only of this condition.

HYDRAULIC TRIP SUBSYSTEM The low pressure trip circuit control oil is supplied by the lube oil system. The lube oil system connects to the low pressure trip circuit through an orifice, supplying oil to the manual emergency and turbine over speed trip devices. From this point trip oil is supplied to the turbine fuel systems through a parallel check valve/orifice piping system and 02 solenoid stop valves. Operation of the solenoid valves permits use of the fuel systems for turbine operation and turbine trip action when required. The system also provides a direct connection to the over speed alarm device for turbine shutdown whenever an over speed condition occurs.

HYDRAULIC TRIP SUBSYSTEM The low pressure trip circuit control oil (OLT) is supplied by the lube oil system. The lube oil system connects to the low pressure trip circuit through an orifice, supplying (OLT) oil to the manual emergency and turbine overspeed trip devices. From this point, (OLT) is supplied to the turbine fuel systems through a parallel check valve/orifice piping system and solenoid stop valves. Operation of the solenoid valves permits use of the fuel systems for turbine operation and turbine trip action when required. The system also provides a direct connection to the overspeed alarm device for turbine shutdown whenever an overspeed condition occurs.

The lubrication system is designed to provide an ample supply of filtered lubricant at the proper temperature and pressure for operation of the turbine and its associated equipment. The lubrication system including all major components is shown in the system schematic diagram. Major system components include: 1. Lube reservoir in the accessory base. 2. Main lube pump 9-P-12603X (shaft driven from the accessory gear). 3. Auxiliary lube pump 9-P-12604X. 4. Emergency lube pump 9-P-12605X. 5. Pressure relief valve VR-1 in the main pump discharge. 6. Lube oil coolers (9-E-12602X) 7. Lube oil filters (9-FL-12604XA) 8. Bearing header pressure regulator VPR-2. 9. Trip oil supply filters 10. Oil vapour separator (9-FL-12605X).

CGT-102 STARTUP PRE-STARTUP CHECKS Ensure that CGT-102 suction filter house of dust, clean CWM’S are installed, Axial air compressor suction screens, dynawane filters are cleaned and filter house is properly boxed-up. Ensure C-102 suction filters are cleaned and installed properly. Ensure Fuel gas line to CGT-102 is commissioned. Check that lube oil console oil level is maximum. Check lube oil temperature is above 80 oF Ensure Inter stage knock out drums of C-102 are boxed up. Ensure ignitors and flame detectors are installed & checked. Ensure Lube oil filters properly boxed up, one of them is in service. Both lube oil pumps (Emergency & Auxiliary) are ready to start.

Starting System of Steam Turbine Start permissive:Once the gt is ready to crank is given (L3ARC) the control oil solinoid valve (20HD) is energised

MARK-VI STARTUP SEQUENCE The start sequence will check shut down system & trip sequence. Gas turbine speed zero signal L14 HR is active The (redundancy 1 out of 3)R, S, T cores/Core engine are active. Gas turbine auxiliary motor driven lube oil/ hydraulic oil pump permissive signal will initiate (L3ARS) Press MASTER RESET on HMI of Mark-VI. Select START command L3PRS and press OK,, the CRT/MMI will show SEQUENCE IN PROGRESS From the main display, select CRANK & press OK. The HMI (Human Machine interface) will change to READY TO START. Then the HMI will change to STARTING. Ensure that T&T valve hydraulic mechanism is “Reset” in field & open T&T valve.

AUXILIARIES STARTING Master selector if on remote then sequence will be run on Auto otherwise manual selection of Crank, Fire, Manual , Auto commands will be executed. Turbine auxiliaries will start with L1X signal start the ventilation system & then auxiliary lube oil pump activates, when lube oil pressure is ok then process start(L3PSP) passed & auxiliaries start passed (L3ASP). Master Protective signal enabled L4=1 signal will be shown.

CRANKING MODE The turbine shaft will start rotating & zero speed signal 14 HR signal will be displayed at MARK-VI HMI. Shut off & vent valve position is ok & fuel blcck v/v 20-FB-1 SOV in open position due to which 20 FG valve will be closed.

When Auxiliaries are ready to crank L3ARC signal will be shown. When gas turbine reaches to 10 Rpm HMI display will change to CRANKING status, 20 HD hydraulic dump solenoid and steam stop valve 20 SC (99 SD) will energized to close hydraulic dump & steam helper turbine inlet T& T drain closes after proper heating & helper/gas turbine shaft starts rolling. The speed starts increasing, control RPM at 1200 to removes stresses of machine for 5 minute. PURGING MODE The gas turbine will proceed for purge mode, however turbine can be kept on cranking manually. When the turbine reaches at 20% speed purging display 14HM on HMI, purge timer of 30 sec. will start & 20 FV-1 will open. After purging Fuel gas line vent 20FV-1 will close.CGT-102 will automatically go to 1800 RPM at a rate of 500 rpm per minute. Purpose of purging is to purge gas turbine if any combustible fuel mixture is present.

FIRING MODE After purging timer Aux. ready to fire L3ARF signal will initiate when Core engine will be ready to fire ignition trnsformer signal L2TVX initiates & spark generates in spark plug & then Open fuel gas shut off valves VS4-1/2 & 20 VG vent on fuel will open to provide minimum firing to avoid high exhaust temperature flame will produce .FSR. Fuel stroke reference gives electronic signal determining amount of fuel to gas turbine, if flame detectors sense flame in 60 seconds L28FX signal initites, FSR will open metering valve temp. at exhaust shoots up rapidly, otherwise Loss of flame indication appears & FSR will be reset to firing value, at this time operator will attempt to fire again or shut down & select CRANK & OK command again. The ignitors are turned off after flame detection occurs. The status bar will change to STARTUP/ FIRING sequence, FSR limits fuel to avoid any excessive mechanical & thermal stresses.

WARM UP MODE The purpose of warm up is to remove thermal stresses during initial part of startup. When entering warm up the following events will occur. The warm up timer logic is started & STARTUP/ WARM UP sequence will be displayed on status bar. During firing fuel gas SOV vent 20VG will remain open .The fuel stop solenoid (20 FG) is energized and warm up timer will turned on for 60 sec. Maximum assistance from the starting turbine during warm up minimizes firing temperature as much as possible. If firing does not occurs master selector will disable ignition transformer & close fuel gas valves & go to crank mode again ACCELERATION MODE The start up control FSR is ramped to the maximum start up value. This setting is called “ACCELERATION LIMIT ” and the

Gas turbine will increase in speed at approx. 50% speed on the HMI. Steam helper turbine regulator VR-5 will be active. The status bar will display START UP/ ACCELERATION sequence. 20VG will close when acceleration starts. FSR maximum will open GCV at maximum opening. When gas turbine reaches about 60 % speed. The HMI will indicate the change in status from “START UP CONTROL “ to “ SPEED CONTROL “.When the turbine reaches operating speed (95%), the operating speed signal, the starter steam turbine steam valve 20 SC output will be ramped to zero. RUN MODE When the turbine reaches operating speed (95%), The status bar will display RUN sequence. When Gas turbine reaches operating speed, the HMI will display FULL SPEED NO LOAD (FSNL)

Helper St. Turbine

Gas Turbine

LP Casing

CGT-102

C-102

3800 (min) 5100 (max) RPM

Gear Box 2.43

5100 RPM (MCS)

HP Casing C-102 12400 RPM (MCS)