Gas Turbine Principles What is the operating

principal of gas turbine

Now the idea of gas turbine is clear: 1- The left hand fan represents the COMPRESSOR 2- The right hand fan represents the TURBINE

3- The flame represents the COMBUSTION

CHAMBER 4- The electric motor represents the START UP UNIT of the gas turbine. 5- Another applied load could be an electric generator, pump, or thrust propulsion as in aircraft .etc. 6- And there is additional part of the gas turbine which is the AIR FILTER to insure clean air entrance.

How can land base gasturbines classified? a-Open cycle gas turbines: 1-Single shaft gas turbine

It's the simplest form of land base gas turbines where

compressor & turbine are connected via the same shaft yet they have the same speed of rotation.

Land Base GE Gas Turbine

b-Twin spool gas turbine. More complex configuration of

gas turbine, in this engine there are two concentric shafts the first shaft is low pressure shaft & the other is high pressure shaft & both shafts are rotating with different speeds, the main advantages of this configuration is that the star up torque required to turn the machine is minimized compare to single shaft with the same load since only high pressure shaft needed to be turned, also compressor surge is minimized in this configuration, also its is shorter smaller & lighter than single shaft engine & has less number of blow off lines , the main disadvantage of this configuration is additional complexity to the design & added cost.

Gas Turbine Priciple A simple gas turbine is comprised of three main

sections a compressor, a combustor, and a power turbine. The gas-turbine operates on the principle of the Brayton cycle, where compressed air is mixed with fuel, and burned under constant pressure conditions. The resulting hot gas is allowed to expand through a turbine to perform work. In a 33% efficient gas-turbine approximately two / thirds of this work is spent compressing the air, the rest is available for other work i.e. (mechanical drive, electrical generation)

Gas turbines usually operate on an open cycle, as shown in

Figure 1. Fresh air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high-pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. (2) The resulting high-temperature gases then enter the turbine, where they expand to the atmospheric pressure through a row of nozzle vanes. This expansion causes the turbine blade to spin, which then turns a shaft inside a magnetic coil. When the shaft is rotating inside the magnetic coil, electrical current is produced. The exhaust gases leaving the turbine in the open cycle are not re-circulated.

Gas turbines are ideal for this application as they can

be started and stopped quickly enabling them to be brought into service as required to meet energy demand peaks. However, their previously small unit sizes and their low thermal efficiency restricted the opportunities for their wider use for electricity generation.

The open gas-turbine cycle can be modeled as a

closed cycle as shown in Figure 2 by utilizing the airstandard assumptions. Here the compression and expansion process remain the same, but a constant-pressure heat-rejection process to the ambient air replaces the combustion process. The ideal cycle that the working fluid undergoes in this closed loop is the Brayton cycle, which is made up of four internally reversible processes: 1-2 Isentropic compression (in a compressor) 2-3 Constant pressure heat addition 3-4 Isentropic expansion (in a turbine) 4-1 Constant pressure heat rejection (2)

To improve the overall efficiency and the output of the gas turbine by: Regeneration Turbine reheat Compressor intercooling Steam or water injection

Regeneration of waste heat.

Regeneration is the internal exchange of heat within

the cycle. In the gas turbine cycle, the gases leaving the turbine are at a relatively high temperature. This temperature is higher than the outlet-compressor temperature. Therefore, regenerator (a surface-type heat exchanger) is used to preheat the compressed gas by exhaust gases. See figure 3. The consequence of this action is reduction of the amount of fuel which is injected into the combustor. The exchange of heat between the two flow take place in the regeneator. the regenerator effectiveness, is defined as:

Fig (3) Simple Gas Turbine with Regenerator

The regenerator gas turbine cycles are more efficient

than simple gas turbine cycles. They reduce fuel consumption by 30 percent or more. It should be mentioned that using a regenerator reduce somewhat the specific power because of added pressure losses in the regenerator

Compressor intercooling Another method of increasing the overall efficiency of a gas turbine is to decrease the work input to the compression process. The effect is to increase the net work output. This could be achieved by cooling the passing gas through the compressor.

Cooling or to have much heat transfer through the compressor casing is not possible or practical. Compression intercooling is effective when a

comparatively large pressure change is desired. The compression with intercooling could be also performed in a multistage compression process.

The overall result of the compression intercooling is a

lowering of the net work input required for a given pressure ratio. Intercoolers can be air-cooled heat exchangers but are

more commonly water-cooled.

Fig (5) Gas Turbine cycle with Intercooler

Reheat CycleAnother method of increasing the overall efficiency is eeping the gas temperature in the turbine as high kas possible.

See figure 6 and 7 for one stage turbine reheat. In this case,

the gases are allowed to expand partially before they are returned to combustion chamber, where heat is added at constant pressure until the limiting temperature is reached. The use of reheat increases the turbine work output without changing the compressor work or the maximum limiting temperature. The final turbine exhaust temperature T9 shown in the figure 8 is somewhat above the outlet turbine temperature without reheat, Ty. Consequently, reheating is quite effective when used in conjunction with regeneration.

Fig (6) Gas Turbine Cycle with Reheat

Figure (7) T-s diagram of a gas turbine cycle with reheat

From practical consideration, a gas turbine power

cycle is improved most when a combination of intercooling and reheating is employed with regeneration. It should be mentioned that the intercooling with reheating effect with regeneration is always to decrease the thermal efficiency. The reason is that intercooling and reheating used alone decrease the average temperature of the heat supply and increase the average temperature of heat rejection. The major purpose in employing intercooling and reheating is to increase the effective use of a regenerator.

Water or steam injection Steam or water injection is a method by which the

output power of a gas turbine cycle can be increased. These methods not only increase the maneuverability of the gas turbine during part load operation, but also decrease the exhaust emissions of CO and unburned hydrocarbons by at least half. Steam injection which can be done in the form of saturated or superheat steam, increase the overall efficiency of the gas turbine. The introduced steam mostly injected into the combustion chamber. Water injection It is more common to inject water at compressor outlet in order to increase the mass flow rate. Temperature reduction can be compensated in a regenerator.

Figure (8) A gas Turbine cycle with steam injection

Combined Cycle Plants There are many concepts of the combined cycle, these cycles range from the single pressure cycle, in which the steam for the turbine is generated at only one pressure, to the triple pressure cycles where the steam generated for the steam turbine is at three different levels. Figure (9) shows the distribution of the entering energy into its useful component and the energy losses. In most combined cycle applications the gas turbine is the topping cycle and the steam turbine is the bottoming cycle. The major components that make up a combined cycle are the gas turbine, the HRSG and the steam turbine as shown in the Figure 12 a typical combined cycle power plant with a single pressure HRSG

Fig (9) Combined Cycle Power Plant

Two Shaft Combined Cycle

Gas Turbine Components

Gas Turbine Intake System

Air Intake system in gas turbines: Gas turbines consume large mass of air (a 125 MW

machine will take in 438 kilograms of air per second for 50C ambient temperature), Careful design of the intake system is needed to ensure that frictional losses are a minimum The noise of the air entering the machine is kept to an acceptable limit. The Figure Air of an intake filter house from inside showing air filter elements before& after installation& the hole behind each filter is for the aim of purging (pulseair system)

Air intake filter house from inside showing air filter elements before & after installation& the hole behind each filter is for the aim of purging (pulse air system)

Air intake main parts: a-Air intake filter house Filter house normally designed to have large shape so to

make sure that the pressure drop across it will minimized (each 10 mbar pressure drop will reduce turbine power by 1%) It contains air filters, air entrance guide vanes & the implosion doors. b-Filter element : Air entering compressor must be filtered from any dust or residues that may enter compressor & cause fouling which reduce compressor efficiency, The use of series of filters in stead of one big filter is more because design & erection complexity are reduced if pressure drop in air intake increases, a set filters can easily be removed in stead of removing the hall big filter. The Figure shows Air intake filter house

Air intake filter house

Compressed air lines for filters purging & pulse air outlet can be seen Pulse air header pressure us 7.5 bar & each pulse has a pressure of 3 bar

The above figure illustrates the methodology employed in determining the Fault Indices of engine components.

Photographs were taken of the outlet and inlet pipes from a compressor In the photos you will see the deposit coating the interior surface of the outlet pipe, and how clean the inlet pipe is. (The inlet pipe interior is dark brown in color, most likely from the entrained iron oxide in the chilled water system.)

1-As air intake pressure drop increases, the air

pressure after silencer decreases, this will create a local vacuum area. 2-Due to the vacuum inside air intake filter house, the implosion doors (4 to 6 doors) will be sucked in 3-There are limit switch on each door so for any door reach full open position, this mean that the limit switch will send open signal which will initiate gas turbine trip to protect gas turbine from operating with air intake filter house is blocked. Implosion doors are designed to make sure that the pressure inside air filter house is equalized to the atmospheric pressure to avoid air intake damage.

If the gas turbine on load, normally pulse air system

will activated only due to pressure drop signal

Pressure sensor s used in air intake gas turbine

Axial Flow Compressor

Axial-flow compressor The axial flow compressor compresses its working

fluid by first accelerating the fluid and then diffusing it to obtain a pressure increase. The fluid is accelerated by a row of rotating blades called the rotor, and then diffused in a row of stationary blades (the stator). The diffusion in the stator converts the velocity increase gained in the rotor to a pressure increase.

Stages of an Axial-flow Compressor

A compressor consists of several stages. One rotor and

one stator make up a stage in a compressor. One additional row of fixed blades (inlet guide vanes) is frequently used at the compressor inlet to ensure that the air enters the first stage rotor at the desired angle. In addition to the stators, another diffuser at the exit of the compressor further diffuses the fluid and controls its velocity entering the combustors. In an axial compressor air passes from one stage to the next, each stage raising the pressure slightly. The use of multiple stages permits overall pressure increases of up to 40:1

Schematic representation of an axial flow compressor

It is easy to design a turbine that will work. It requires a considerable skill to design a compressor that will work

Fig (10) Multistage High Pressure Axial Compressor

Specifications of An Axial Compressor There are several different parameters that can specify a

particular compressor. The first set of input parameters are based on the running conditions for the machine. These involve mass flow, pressure ratio , rotational speed and the number of stages. Stage degree of reaction : For controlling the distribution of the load between the rotor and the stator. If this is not of importance, the outlet flow angle for the each stage must be set instead.

Axial Compressor Pressure and speed Curve

a- Moving blades accelerate air. b- Fixed blades slow down air & hence change kinetic

energy of air into pressure energy. c- Discharge velocity should equal suction velocity.

The design of compressor blades are different than

turbine blades. compressor blades divergent profile which works as diffuser to increase air pressure. while turbine blades have convergent profile which is works as nozzle since turbine is reducing air pressure by changing its pressure energy into kinetic energy

Compressor Cleaning The aim of compressor cleaning process is to remove

deposits from blades that have caused a decline in output and efficiency of Gas Turbines. *-Pollution of the ambient air cannot be completely removed by the air intake filter system. *-compressor washing are mainly two types online & off line washing. *- Compressor washing is carried out by using detergent solution and demineralized water for the final flushing. Both fluids are sprayed onto the compressor blades through two types of nozzles:

a- Jett nozzles which are consisting of two jet nozzles

which inject water at high speed. Jet nozzles are used for off load washing only since them inject high speed water jet which will be harmful to the blades of compressor which are rotating at synchronizing speed-3000 rpm. The effect of one droplet of water will become like a bullet. b- Spray nozzles which are uniformly spaced around the circumference of compressor inlet, upstream of the variable compressor inlet guide vanes, each producing a gentle jet of water.

Alternative methods for compressor cleaning

include: a- Dissemble the compressor partially to clean the blades of the rotor, this method gives excellent result but it required long time outage. b- Injection of crushed walnut shells, Injection of rice & Injection of concentrated carbon. The second method should not be used in modern gas turbines because: a- Fire hazard. b- Oil system contamination & blockage. c- Result in blade cooling system fouling.

Stators The stators may be carried in a separate inner casing or

may be carried by the outer, center section of the main casing. Because of leakage at the mounting bushings in the stator liner or earner, the single-case unit has some sealing problems, which are inherently taken care of in the double-case construction. Casing Connections The casing inlet and outlet are flanged, for connection to the users piping system, with the exception of some rectangular inlet connections to provide more axial clearance

Compressor Design Casings The casings on axial compressors are somewhat unusual, because of the disproportionately large inlet and outlet nozzles. This makes the compressors appear to be only nozzles connected by a long tube. In some designs, the casing is an outer shell containing an inner shell, which acts as the stator vane carrier. In other designs, the stators are directly carried on the casing, which are of one-part construction.

Rotor Rotor construction tends to vary from vendor to

vendor. The blades are attached to the outer surface of the rotor. The rotor may be of basic disc or drum type construction, with the disc type having some variations. The two most common disc construction modes are shrunk discs on a shaft and stacked discs, normally through-bolted together. A final method is the solid rotor construction. For smaller compressors, where the speed is relatively high and space is limited, a solid rotor construction is used

Shaft Shafting takes on several forms to match the various

rotor construction methods. Obviously, for the solid rotor, the shaft is a part of the overall rotor. For the shrunk-on discs, the shaft is a continuous member, carrying the discs in the center section. Concentricity of all turns and good control on the roundness of the shaft are critical, if a balanced, smooth running compressor is to result.

Bearings The bearings used in axial compressors are the same

journal and thrust type used in the centrifugal compressor. For axial compressors, the journal bearings are of the plain sleeve type for the larger, slower speed compressors. They are of the tilting pad type for the smaller, higher speed machines. The sleeve bearing is normally housed in a spherically seated carrier. The bearings require pressure lubrication as do most of the other compressors. The thrust bearing is generally the tilting-pad type bearing. Most vendors apply the recommendation that the thrust bearing be of the symmetrical design with leveling links. Axial compressors have a high inherent thrust load, so the thrust bearing is quite important in the overall reliability of the compressor.

Sleeve type journal bearing

Tilting bad type

Balance Piston The axial compressor is inherently always a reaction

type of machine. In regards to axial thrust, this means the rotor is subjected to a differential pressure across each rotating blade row. The differential pressures convert to an axial force at each rotor row that totals to a rather high value when taken over the normal number of stages. A thrust bearing would be prohibitive in size to carry the generated thrust. Fortunately, the geometry of the axial provides space for a large balance piston at the discharge end of the compressor.

Degree of reaction The degree of reaction in an axial flow compressor is

defined as the ratio of the change of static head in the rotor to the head generated in the stage. The symmetrical stage (50% reaction) is widely used, since an adverse pressure rise on either the rotor or stator blade surfaces is minimized for a given stage pressure rise. when designing a compressor with this type of blading, the first stage must be preceded by inlet guide vanes to provide prewhirl and the correct velocity entrance angle to the first stage rotor. The serious disadvantage of the symmetrical stage is the high exit loss resulting from the high axial velocity component.

Fig (12 )Variation of velocity, and pressure through an axial flow compressors

Compressor surge A compressor is in surge when the main flow through the

compressor reverses its direction and flows from the exit to the inlet for short time intervals. If allowed to persist, this unsteady process may result in irreparable damage to the machine. A condition known as "choke" or "stone walling" is indicated on the map, showing the maximum flow rate possible through the compressor at that speed. Compressor surge is a phenomenon of considerable

interest, yet it is not fully understood. It is a form of unstable operation and should be avoided in both design and operation. Surge has been traditionally defined as the lower limit of stable operation in a compressor and involves the reversal of flow.

Figure 11 shows a typical performance map of a

centrifugal compressor. Total pressure ratio can be seen to change with flow and speed. Compressors are usually operated at a working line separated by some safety margin from the surge line. Surge is often symptomized by excessive vibration

and an audible sound, however, there have been cases in which surge problems that were not audible have caused failures

Figure (11 ) Typical compressor performance map

External causes and effects of surge The following are some of the usual causes of surge

that are not related to machine design: 1. Restriction in suction or discharge of a system. 2. Process changes in pressure, temperature, or gas composition. 3. Internal plugging of flow passages of compressor (fouling). 4. Inadvertent loss of speed. 5. Instrument or control valve malfunction. 6. Malfunction of hardware such as variable inlet guide vanes.

7. Operator error8. Maldistribution of load in parallel operation of two

or more compressors. 9. Improper assembly of a compressor, such as a mispositioned rotor The effects of surge can range from a simple lack of performance to serious damage to the machine or to the connected system. Internal damage to labyrinths,, the thrust bearing, and the rotor can be experienced.

Combustors, construction, types

Reciprocating Engine Vs Jet Engine

How to make it More reliable Resource.

Laws of Combustion A Special Device is required to hold the flame,

called Burner. The maximum quantity of fuel that can be handled by a single burner is limited. The maximum allowable air to establish efficient combustion of a given amount of fuel is limited. The maximum temperature due to efficient combustion is always higher than the maximum safe temperature: limited by turbine blades. Infra structure should be provided to facilitate these conditions.

Requirements of A Reliable CombustionMATtr Theory Mixing: Fuel preparation systems. Air: Draught systems. T : Preheating of fuel. t : Dimensions of combustion chamber. r: Turbulence generation systems.

Combustion Section The combustion section contains the combustion

chambers, igniter plugs, and fuel nozzle or fuel injectors.

Designed to burn a fuel-air mixture and to deliver

combusted gases to the turbine at a temperature not exceeding the allowable limit at the turbine inlet. its air by volume to the combustion chamber.

Theoretically, the compressor delivers 100 percent of The fuel-air mixture suitable for efficient combustion

has a ratio of l5 parts air to 1 part fuel by weight.

Combustion Section Contd. Approximately 25 30 percent of total compressor air is used to attain the desired fuel-air ratio. The remaining 70 -- 75 percent is used to form an air blanket around the burning gases and to dilute the temperature. The diluted temperature may reach as high as 1500 C, by approximately one-half of the flame temperature. This ensures that the turbine section will not be destroyed by excessive heat.

Air Distribution in A CombustorCompressor end Turbine end

Functions The air used for burning is known as primary air; Remaining for cording is secondary air. Secondary air is controlled and directed by holes and

louvers in the combustion chamber liner. Igniter plugs function during starting only; they are shut off manually or automatically. Combustion is continuous and self-supporting. After engine shutdown or failure to start, a pressure actuated valve automatically drains any remaining unburned fuel from the combustion chamber. The primary function of the combustion section is, of course, to bum the fuel-air mixture, thereby adding heat energy to the air.

To do this efficiently, the combustion chamber must Provide the means for mixing the fuel and air to ensure

good combustion.

Bum this mixture efficiently. Cool the hot combustion products to a temperature

which the turbine blades can withstand under operating conditions.

Deliver the hot gases to the turbine section.

The location of the combustion section is directly

between the compressor and turbine sections.

.

The combustion chambers are always arranged coaxially with the compressor and turbine, regardless of type, since the chambers must be in a through-flow position to function efficiently.

All combustion chambers contain the same basic elements: A casing A perforated inner liner. A fuel injection system. Some means for initial ignition. A fuel drainage system to drain off unburned fuel after engine shutdown.

Basic Anatomy of A CombustorCasing

Swirler

Liner

Liner holes

Classification of Combustors Basis for this classification: A burner handles finite amount of fuel. Arrangement of multiple burners. There are currently three basic types of Burner

Arrangements The multiple-chamber or can type. The annular or basket type. The can-annular type.

A can or tubular combustor. Each can has both a liner

and a casing, and the cans are arranged around the central shaft An annular combustor with the liner sitting inside the outer casing. Many modern burners have an annular design. A can-annular design, in which the casing is annular and the liner is can-shaped. The advantage to the canannular design is that the individual cans are more easily designed, tested, and serviced.

Components All combustion chambers contain the same basic

elements as shown in Figure : A casing A perforated inner liner. A fuel injection system. Some means for initial ignition. A fuel drainage system to drain off unburned fuel after engine shutdown.

Can annular combustor

Annular Combustion Chamber

Combustion Liner Materials / Repair A critical repair operation for combustion liners is air-

flow testing. Flow testing assures that each combustion liner in the set is flowing the same amount of air, thus minimizing temperature variance. When a liner is tested, it is placed on the flow check machine and a vacuum is drawn through the liner. The test machine calculates the effective air flow area. A determination is then made as to whether corrections are necessary

Fuel Nozzle In most gas turbines, liquid fuel is atomized and injected

into the combustors in the form of a fine spray. A typical low pressure fuel atomization nozzle is shown in the Figure . The fuel spray entrains air however, this process a low pressure region inside the spray cone that causes it to converge downstream of the nozzle. This low pressure region is counteracted by upstream axial flow of combustion products, preventing convergence in the combustion chamber. In simple pressure atomizing fuel nozzle the flow rate varies as the square root of the pressure. Ignition is usually obtained from an ignitor interfaced with a high energy capacitive discharge ignition system.

Low Pressure atomizer

Ignitors An ignitor plug is shown in the Figure. This plug is a

surface discharge plug, thus energy does not have to jump an air gap. The plug end is covered by a semiconductive material and is formed by a pellet, permitting an electrical leakage from the centeral high tension electrode to the body. The discharge takes the form the electrode to the body. Flow testing of fuel nozzle tips and passageways is of critical importance, especially for multi-nozzle systems. It is vital that the same amount of fuel flows through each nozzle in a set.

Equalized fuel flow minimizes temperature spreads as

measured by the exhaust thermocouples and can-tocan pressure differences between the combustors. Computerized flow-test equipment is used to meter and measure flows. Air-flow testing is used for gas and air passages and water-flow testing is used for liquid passages. The flow-test units use a computer-controlled system that calculates the percent variation of each to that of the set tips. The final assembly is typically limited to a variance of less than 2% between assemblies. All accumulated fuel-flow data is saved for permanent records

Firing concepts and emission control There are two distinct categories of exhaust emissions

from a stationary gas turbine . The major species (CO2, N2, H2O, and O2) are present in percent concentrations. The minor species (or pollutants) such as CO, NOx, SOx, and particulates are present in parts per million concentrations.

Gas turbine exhaust emissions burning conventional fuels

Nitrogen Oxides Nitrogen oxides (NOx = NO + NO2) must be divided into two

classes according to their mechanism of formation. Nitrogen oxides formed from the oxidation of the free nitrogen in the combustion air or fuel are called thermal NOx. The following is the relationship between combustor operating conditions and thermal NOx production: _ NOx increases strongly with fuel-to-air ratio or with firing temperature _ NOx increases exponentially with combustor inlet temperature. _ NOx increases with the square root of the combustor inlet pressure _ NOx increases with increasing residence time in the flame zone NOx decreases exponentially with increasing water or steam injection or increasing specific humidity

The gas turbine is controlled to approximate constant

firing temperature and the products of combustion for different fuels affect the reported NOx correction factors.

Gas Turbine Inspection

Inspection Schedules Maintenance inspection types may be broadly classified as standby, running and disassembly inspections. The standby inspection is performed during off-peak periods when the unit is not operating and includes routine servicing of accessory systems and device calibration. The running inspection is performed by observing key operating parameters while the turbine is running. The disassembly inspection requires opening the turbine for inspection of internal components and is performed in varying degrees

Standby Inspections This inspection includes routinely servicing the

battery system, changing filters, checking oil and water levels, cleaning relays and checking device calibrations. Servicing can be performed in off-peak periods without interrupting the availability of the turbine. A periodic startup test run is an essential part of the standby inspection

The Operations and Maintenance Manual, as well as

the Service Manual Instruction Books, contain information and drawings necessary to perform these periodic checks. Among the most useful drawings in the Service Manual Instruction Books for standby maintenance are the control specifications, piping schematic and electrical elementaries. These drawings provide the calibrations, operating limits, operating characteristics and sequencing of all control devices. This information should be used regularly by operating and maintenance personnel.

Running Inspections Running inspections consist of the general and

continued observations made while a unit is operating Data should be taken to establish normal equipment start-up parameters as well as key steady state operating parameters. Steady state is defined as conditions at which no more than a 3C change in wheelspace temperature occurs over a 15-minute time period. Data must be taken at regular intervals and should be recorded to permit an evaluation of the turbine performance and maintenance requirements as a function of operating time

Operating inspection data parameters

Combustion Inspection The combustion inspection is a relatively short

disassembly shutdown inspection of fuel nozzles, liners, transition pieces, crossfire tubes and retainers, spark plug assemblies, flame detectors and combustor flow sleeves. This inspection concentrates on the combustion liners, transition pieces, fuel nozzles and end caps which are recognized as being the first to require replacement and repair in a good maintenance program.

Tubo-annular or can-annular combustor for a heavy-duty gas turbine. (Courtesy of General Electric Company,)

The combustion liners, transition pieces and fuel nozzle

assemblies should be removed and replaced with new or repaired components to minimize downtime. Typical combustion inspection requirements for MS6001B/7001EA/9001E machines are: Inspect and identify combustion chamber components. Inspect and identify each crossfire tube, retainer and combustion liner. Inspect combustion liner for TBC spallation, wear and cracks. Inspect combustion system and discharge casing for debris and foreign objects. Inspect flow sleeve welds for cracking. Inspect transition piece for wear and cracks. Inspect fuel nozzles for plugging at tips, erosion of tip holes and safety lock of tips.

Inspect all fluid, air, and gas passages in nozzle

assembly for plugging, erosion, burning, etc. Inspect spark plug assembly for freedom from binding; check condition of electrodes and insulators. Replace all consumables and normal wear-and tear items such as seals, nuts, bolts, gaskets, etc. Perform visual inspection of first-stage turbine nozzle partitions and borescope inspect turbine buckets to mark the progress of wear and deterioration of these parts. This inspection will help establish the schedule for the hot-gaspath inspection.

Perform borescope inspection of compressor. Enter the combustion wrapper and observe the

condition of blading in the aft end of axial-flow compressor with a borescope. Visually inspect the compressor inlet and turbine exhaust areas, checking condition of IGVs, IGV bushings, last-stage buckets and exhaust system components. Verify proper operation of purge and check valves. Confirm proper setting and calibration of the combustion controls

Hot Gas Path Inspection The purpose of a hot gas path inspection is to examine

those parts exposed to high temperatures from the hot gases discharged from the combustion process. The hot gas path inspection outlined in the Figure includes the full scope of the combustion inspection and, in addition, a detailed inspection of the turbine nozzles, stator shrouds and turbine buckets.

Typical hot gas-path inspection requirements for

all machines are: Inspect and record condition of first-, secondand third-stage buckets. If it is determined that the turbine buckets should be removed, follow bucket removal and condition recording instructions. Buckets with protective coating should be evaluated for remaining coating life. Inspect and record condition of first-, second and third-stage nozzles. Inspect and record condition of later-stage nozzle diaphragm packings. Check seals for rubs and deterioration of clearance.

Record the bucket tip clearances. Inspect bucket shank seals for clearance, rubs and

deterioration. Check the turbine stationary shrouds for clearance, cracking, erosion, oxidation, rubbing and build-up. Check and replace any faulty wheel space thermocouples. Enter compressor inlet plenum and observe the condition of the forward section of the compressor. Pay specific attention to IGVs, looking for corrosion, bushing wear evidenced by excessive clearance and vane cracking. Enter the combustion wrapper and, with a borescope, observe the condition of the blading in the aft end of the axial flow compressor.

Visually inspect the turbine exhaust area for any signs

of cracking or deterioration. The first-stage turbine nozzle assembly is exposed to the direct hot-gas discharge from the combustion process and is subjected to the highest gas temperatures in the turbine section. Such conditions frequently cause nozzle cracking and oxidation and, in fact, this is expected. The second- and third-stage nozzles are exposed to high gas bending loads, which in combination with the operating temperatures, can lead to downstream deflection and closure of critical axial clearances.

Major Inspection The purpose of the major inspection is to examine all

of the internal rotating and stationary components from the inlet of the machine through the exhaust. A major inspection should be scheduled in accordance with the recommendations in the owners Operations and Maintenance Manual or as modified by the results of previous borescope and hot gas path inspection.

Typical major inspection requirements for all

machines are: All radial and axial clearances are checked against their original values (opening and closing). Casings, shells and frames/diffusers are inspected for cracks and erosion. Compressor inlet and compressor flow-path are inspected for fouling, erosion, corrosion and leakage. The IGVs are inspected, looking for corrosion, bushing wear and vane cracking. Rotor and stator compressor blades are checked for tip clearance, rubs, impact damage, corrosion pitting, bowing and cracking.

Turbine stationary shrouds are checked for clearance,

erosion, rubbing, cracking, and build-up. Seals and hook fits of turbine nozzles and diaphragms are inspected for rubs, erosion, fretting or thermal deterioration. Turbine buckets are removed and a nondestructive check of buckets and wheel dovetails is performed (first stage bucket protective coating should be evaluated for remaining coating life). Buckets that were not recoated at the hot gas path inspection should be replaced. Wheel dovetail fillets, pressure faces, edges, and intersecting features must be closely examined for conditions of wear, galling, cracking or fretting. Rotor inspections recommended in the maintenance and inspection manual or by Technical Information Letters should be performed.

Bearing liners and seals are inspected for clearance

and wear. Inlet systems are inspected for corrosion, cracked silencers and loose parts. Exhaust systems are inspected for cracks, broken silencer panels or insulation panels. Check alignment gas turbine to generator/gas turbine to accessory gear.

Safety Precautions Make sure the generating facility is well ventilated

when using cleaning solvents. The following requirements must be met when the engine room is entered. (1) The gas turbine shall be shut down or limited to idle power. (2) The enclosure door shall be kept open. If the gas turbine is operating, station an observer at the enclosure door. 3) Do not touch any part of an operating engine, as the engine becomes extremely hot. Wear insulated gloves as necessary. (4) Wear approved ear protection if the engine is operating

(5) Do not remain in the room or enclosure, or in the

plane of rotation, when starting or monitoring the engine. (6) Attach an approved safety clearance tag such as DA Form 4324 to the starting control when work is being done. (7) Make sure the engine, generator, and related equipment are clean. Keep oil-soaked rags out of the generating facility to avoid a fire hazard.