Microsoft Word - Maintenance and Overhaul of Steam Turbines WGP42_05_.doc

MAINTENANCE PROGRAMS FOR GAS AND STEAM TURBINES

GAS TURBINES

Maintenance costs and availability are two of the most important concerns to a gas turbine equipment owner. Therefore, a well thought out maintenance program that optimizes the owners costs and maximizes equipment availability should be instituted. Parts unique to a gas turbine requiring the most careful attention are those associated with the combustion process, together with those exposed to the hot gases discharged from the combustion system. These are called the combustion section and hot gas path parts, and they include combustion liners, end caps, fuel nozzle assemblies, crossfire tubes, transition pieces, turbine nozzles, turbine stationary shrouds and turbine buckets. Additional areas for consideration and planning, though longer term concerns, are the lives of the compressor rotor, turbine rotor, casings and exhaust diffuser. The basic design and recommended maintenance of gas turbines are oriented toward: Maximum periods of operation between inspections and overhauls In-place, on-site inspection and maintenance Use of local trade skills to disassemble, inspect and re-assemble gas turbine componentsIn addition to maintenance of the basic gas turbine, the control devices, fuel-metering equipment, gas turbine auxiliaries, load package, and other station auxiliaries also require periodic servicing. The primary maintenance effort involves five basic systems: controls and accessories, combustion, turbine, generator and balance-of-plant. Controls and accessories are typically serviced in outages of short duration, whereas the other four systems are maintained through less frequent outages of longer duration.

INSPECTIONAs with any power equipment, gas turbines require a program of planned inspections with repair or replacement of damaged components. A properly designed and conducted inspection and preventive maintenance program can do much to increase the availability of gas turbines and reduce unscheduled maintenance. Inspections and preventive maintenance can be expensive, but not as costly as forced shutdowns. Nearly all manufacturers emphasize and describe preventive maintenance procedures to ensure the reliability of their machinery, and any maintenance program should be based on manufacturer's recommendations. Inspection and preventive maintenance procedures can be tailored to individual equipment application with references such as the manufacturer's instruction book, the operator's manual, and the preventive maintenance checklist.

Inspections range from daily checks made while the unit is operating to major inspections that require almost total disassembly of the gas turbine. Daily inspections should include (but are not limited to) the following checks:1. Lubrication oil level2. Oil leakage around the engine3. Loose fasteners, pipe and tube fittings, and electrical connections4. Inlet filters5. Exhaust system6. Control and monitoring system indicator lightsThe daily inspection should require less than an hour to perform properly and can be made by the operator. The interval between more thorough inspections will depend on the operating conditions of the gas turbine. Manufacturers generally provide guidelines for determining inspection intervals based on exhaust gas temperatures, type/quality of fuel utilized, and number of starts.

Minor inspections should be performed after about 3000-6000 hours of operation, or after approximately 200 starts, whichever comes first. This inspection requires a shutdown for two to five days, depending on availability of parts and extent of repair work to be done. During this inspection, the combustion system and turbine should be checked. The first minor inspection or overhaul of a turbine forms the most important datum point in its maintenance history, and it should always be made under the supervision of an experienced engineer. All data should be carefully taken and compared with the turbine erection information to ascertain if any setting changes, misalignment, or excessive wear have occurred during operation. Subsequent inspections are also of great importance, since they verify manufacturers' recommendations or help to establish maintenance trends for particular operating conditions.

When the established time for major maintenance approaches, a meeting should be arranged between the operating department and the manufacturer's engineer to discuss and arrange for the date of turbine outage. A short time before taking the turbine out of service a complete operational test should be made at zero, one-half, and normal maximum loads, preferably in the presence of the manufacturer's engineer. These tests are for reference temperatures and pressures, which will serve as a means of comparison with identical tests that should be made immediately after the unit is overhauled. The operational tests should end with an over-speed trip test to indicate whether attention should be given to the governor or tripping mechanism during the shutdown. These specific data will also serve together with the logged operational data or case history (which should be reviewed with the manufacturer's engineer) to determine the focal point or items requiring special attention or investigation. 1. Increase or change in vibration2. Decrease in air compressor discharge pressure3. Change in lube oil temperatures or pressure4. Air or combustion gases blowing out at the shaft seals5. Incorrectly reading thermocouples6. Change in wheel space temperatures7. Fuel oil or gas leakage8. Fuel control valves operate satisfactorily9. Hydraulic control oil pressures changed10. The turbine governor hunts''11. Change in sound level of gear boxes12. Overspeed devices operate satisfactorily13. Babbitt or other material found on lubricating oil screens14. Lube oil analysis shows corrosion factor increase15. Change in pressure drop across heat exchangers16. Turbogenerator reaches rated load at design ambient and exhaust temperature conditions.

The major maintenance program carried out on a Gas turbine includes:Borescope InspectionBorescope inspection is carried out because of the following benefits it can provide in the maintenance program:1. Internal on-site visual checks without disassembly2. External periods between scheduled inspections3. Allows accurate planning and scheduling of maintenance actions4. Monitors condition of internal components5. Increased ability to predict required parts, special tools, and skilled manpowerA borescope inspection is done every year that is every 8,000 hours of operation as part of the annual inspection. It is a way of examining the internals of the gas turbine by snaking a fiber-optic cable inside the turbine casing without going through the trouble of opening up the turbine and looking at its internals directly. Besides doing the borescope inspection, the OEM maintenance technician during this annual inspection also checks every alarm and shut down device to see if it is working properly. He also checks all major fluid levels and filters. In general, an annual or semiannual borescope inspection should use all the available access points to verify the safe and uncompromised condition of the static and rotating hardware. This should include, but is not limited to, signs of excessive gas path fouling, symptoms of surface degradation (such as erosion, corrosion, or spalling), displaced components, deformation or impact damage, material loss, nicks, dents, cracking, indications of contact or rubbing, or other anomalous conditions.

BorescopeGas and DistillateFuel OilAt Combustion Inspection or Annually, Whichever Occurs First

Heavy Fuel OilAt Combustion Inspection or Semiannually, Whichever Occurs First

Borescope inspection programming

During BIs and similar inspections, the condition of the upstream components should be verified, including all systems from the filter house to the compressor inlet. The application of a borescope monitoring program will assist with the scheduling of outages and preplanning of parts requirements, resulting in outage preparedness, lower maintenance costs and higher availability and reliability of the gas turbine.

CompressorThe compressor is upstream from the combustor and, as such is not subject to hot combustion gases, but only to the ambient air drawn in to be compressed before it flows into the turbines combustor. Essentially, the only maintenance needed for the compressor is that done every 32,000 hours (about 4 years), as part of a total shop overhaul and rebuilding of the entire turbine.

Hotsection inspectionHot section inspection is an examination of those parts of a turbine that are exposed to the hot gases created when compressed intake air is mixed with natural gas or other fuel inside the combustor and ignited by lighters. In other word, the hot sections are mainly the combustor (i.e. the combustor chamber) and the turbine section and any other components exposed directly to flame or to hot combustion gases. This hot section inspection is done every 16,000 hours (about every two years). The inspection involves opening up the combustor and turbine section; carefully examining walls, linings, turbine blades, and vanes etc; replacing any worn or damaged components; then reassembling and starting up the system. The hot section inspection is usually thorough than the borescope inspection since it involves opening up the turbine and examining it directly. Both the service technician and the turbine owner are able to see the internals of the turbine directly and what its actual condition is. During a hot section inspection, the turbine technician replaces damaged turbine blades, vanes and any other components showing wear. Upon completion of the inspection and of any needed repairs, there is a guarantee that the turbine will be good for another 16,000 hours of operation.Typical hot gas path inspection requirements for most gas turbines are:

Inspect and record condition of first, second and 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.

Perform inspections on cutter teeth of tip-shrouded buckets. Consider refurbishment of buckets with worn cutter teeth, particularly if concurrently refurbishing the honeycomb of the corresponding stationary shrouds.

Check the turbine stationary shrouds for clearance, cracking, erosion, oxidation, rubbing and build-up.

Check and replace any faulty wheelspace thermocouples.

Enter compressor inlet plenum and observe the condition of the forward section of the compressor.

Visually inspect the compressor inlet, checking the condition of the IGVs, IGV bushings, and first stage rotating blades.

Check the condition of IGV actuators and rack-and-pinion gearing.

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 compressor discharge case struts for signs of cracking.

Visually inspect compressor discharge case inner barrel if accessible.

Visually inspect the turbine shell shroud hooks for sign of cracking.

Visually inspect the exhaust diffuser for any cracks in flow path surfaces. Inspect insulated surfaces for loose or missing insulation and/or attachment hardware in internal and external locations. In E-class machines, inspect the insulation on the radial diffuser and inside the exhaust plenum as well.

Inspect exhaust frame flex seals, L-seals, and horizontal joint gaskets for any signs of wear or damage.

Combustion InspectionThe 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. Proper inspection, maintenance and repair of these items will contribute to a longer life of the downstream parts, such as turbine nozzles and buckets. In a gas turbine, the combustion liners, transition pieces and fuel nozzle assemblies should be removed and replaced with new or repaired components to minimize downtime. The removed liners, transition pieces and fuel nozzles can then be cleaned and repaired after the unit is returned to operation and be available for the next combustion inspection interval. Typical combustion inspection requirements for gas turbines are:

Inspect and identify combustion chamber components.

Inspect and identify each crossfire tube, retainer and combustion liner.

Inspect combustion liner for TBC spalling, 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, lockplates, 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 gas path 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, checking the condition of the IGVs, IGV bushings, and first stage rotating blades.

Check the condition of IGV actuators and rack-and-pinion gearing.

Visually inspect compressor discharge case struts for signs of cracking.

Visually inspect compressor discharge case inner barrel if accessible.

Visually inspect the last-stage buckets and shrouds.

Visually inspect the exhaust diffuser for any cracks in flow path surfaces. Inspect insulated surfaces for loose or missing insulation and/or attachment hardware in internal and external locations. In E-class machines, inspect the insulation on the radial diffuser and inside the exhaust plenum as well. Inspect exhaust frame flex seals, L-seals, and horizontal joint gaskets for any signs of wear or damage.

Verify proper operation of purge and check valves. Confirm proper setting and calibration of the combustion controls.

After the combustion inspection is complete and the unit is returned to service, the removed combustion hardware can be inspected by a qualified field service representative and, if necessary, sent to a qualified Service Center for repairs. The removed fuel nozzles can be cleaned on-site and flow tested on-site, if suitable test facilities are available.

The CombustorThe Combustor is the heart of the gas turbine system. Control system keeps the temperature of the combustor stages within normal operating limits, a measure that extends the life of the hot section inspection components. The combustor is essentially a trouble free component of the turbine.

Gear box unitPredictive maintenance is widely used in the electric-power generation field. Maintenance technicians collect baseline performance data on the gearbox unit and on the electric generator- recording periodically such things as vibration levels, temperatures and pressures. As equipment ages, there are changes in these variables; they tend to drift away from the baseline that was established when the equipment was new. An increase in the vibration levels of the generator or gearbox for instance very likely indicates a problem with the bearings.

Maintenance technicians will also analyze oil circulating through the bearings in the gearbox, in the generator and in the turbine. They are looking for metal particles suspended in the oil and signs that the oil have been breaking down due to overheating an indication that something is wrong with the bearings. This is all part of predictive maintenance.

Lubricating OilChanging the lubricating oil depends on how hot the oil get during service and how often lubricating oil filters are changed. Synthetic oils are used in gas turbine systems, and that can last for years.

Shaft bearingsShaft bearings will typically last between six and eight years, the need to be replaced. With periodic bearing replacement, an electric generator can typically last for 20 or 30 years. The bearings are the most vulnerable component of a generator. Actual bearing lifetime will depend on how cool the oil circulating through them is kept. Some bearings are lubricated by a splash system, with oil in a reservoir splashing up to keep metal surfaces coated. In a better system, oil is actually pumped through the bearings, cooled, filtered, and then re-circulated through the bearings in a continuing cycle.

HEPA FiltersHigh-efficiency particulate air or HEPA filter is a type of air filter used in gas turbines that must be changed periodically. The HEPA filter keeps the turbine cleaner, resulting in higher power output. Changing the HEPA filters is one of the most important thing an operator must do to maintain the gas turbine system. The overall lifetime of a gas turbine system is greatly affected by the physical environment it is placed in. Without an effective air-filtering system, particles in the air over time would reduce the turbine efficiency and potentially damage the gas turbine system. Poor air filtration could cause foreign object damage (FOD). Good air filtration system prevents damage from occurring thereby increasing the lifetime of the equipment.

Major InspectionThe 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. The work scope involves inspection of all of the major flange-to-flange components of the gas turbine, which are subject to deterioration during normal turbine operation. This inspection includes previous elements of the combustion and hot gas path inspections, in addition to laying open the complete flange-to-flange gas turbine to the horizontal joints.

Removal of all of the upper casings allows access to the compressor rotor and stationary compressor blading, as well as to the bearing assemblies. Prior to removing casings, shells and frames, the unit must be properly supported. Proper centerline support using mechanical jacks and jacking sequence procedures are necessary to assure proper alignment of rotor to stator, obtain accurate half shell clearances and to prevent twisting of the casings while on the half shell. 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.

Visually inspect the compressor inlet, checking the condition of the IGVs, IGV bushings, and first stage rotating blades.

Check the condition of IGV actuators and rack-and-pinion gearing.

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 should be performed.

Bearing liners and seals are inspected for clearance and wear.

Inlet systems are inspected for corrosion, cracked silencers and loose parts.

Visually inspect compressor and compressor discharge case hooks for signs of wear.

Visually inspect compressor discharge case struts for signs of cracking.

Visually inspect compressor discharge case inner barrel if accessible.

Visually inspect the turbine shell shroud hooks for sign of cracking.

STEAM TURBINES

Steam turbines are utilized in numerous industries to drive boiler fans, boiler feed and water pumps, process and chiller compressors, blast furnace blowers, paper mill line shafts, sugar mill grinders, and generators in a variety of industries and applications. Consequently, steam turbines can range from being small and simple in design/construction to large, highly complex designs/arrangements consisting of multiple sections and multiple shafts.

Specifying the desired maintenance and overhaul intervals for steam turbines, therefore, has to take into account the design/construction of the turbine as well as the industry and application utilizing the turbine. Besides the configuration and industry associated with the steam turbine, the infrastructure for monitoring, operations and maintenance including specific practices, and steam quality can have a major effect on the reliability of steam turbines regardless of the industry or application. Regardless of the size, number of casings, steam conditions, and arrangements, it is essential that steam turbines have effective monitoring, operating and maintenance procedures/practices.

STEAM TURBINE MONITORING

Equipment MonitoringTo effectively manage the health and performance of steam turbines, there are a number of turbine parameters which should be measured, monitored and/or displayed on a continuous basis. How much information is monitored is a function of the steam turbine design and application, but with todays modern steam turbines, the following parameters should be monitored:Speed (RPM) and load (kW/MW, or shaft horsepower (SHP))Steam turbine inlet pressure and temperatureSteam turbine 1st stage pressure and temperature (these are the conditions downstream of the first/large impulse stage before remaining HP section blading, as applicable)HP turbine outlet (or cold reheat), IP turbine inlet (or hot reheat), and IP turbine outlet/LP turbine inlet (or crossover) pressures and temperatures for reheat and multiple shell turbines onlySteam turbine rotor/shell differential expansions (as applicable for large turbines)Steam turbine shell and steam chest temperatures/differentials (lower and upper half thermocouples installed in HP and IP turbine sections for large turbines)Admission and extraction pressures and temperatures (as applicable)Extraction line thermocouples to detect water induction (as applicable)Water and steam purity at the main steam inlet and condensate pump dischargeSealing steam and exhauster pressures (as applicable)Steam turbine exhaust pressure and temperatureLube oil and hydraulic fluid supply pressures and temperatures Cooling water supply pressures and temperatures for the lube oil and hydraulic fluid systemsJournal bearing and thrust bearing metal temperatures (or drain temperatures, if applicable) for the turbine and gearbox (as applicable)Bearing vibration seismic, shaft rider, or shaft x-and-y proximity probes measurements for all turbine and gearbox (pinion) bearing locations (as applicable)

Monitoring of these and other parameters is typically done in conjunction with todays modern turbine digital controls and plant control room systems. These systems will also handle the starting sequence, synchronizing, loading, speed governing, alarms, and trip logic for the turbine, gearbox (if present), generator, and any supporting systems. These systems also provide the electronic portion of the protection (i.e., turbine overspeed) for critical turbine and generator parameters. For older units there may be an analog control system which provides limited protection along with mechanical/electrical devices on the unit. There usually is a limited display of monitoring parameters. For even older units, all operation will be manual with only a gage panel to monitor a few turbine parameters. Vibration monitoring is done periodically using hand-held instrumentation. These older units are dependent solely on the knowledge of the operating staff, the presence and use of written operating procedures, and the mechanical/electrical devices on the unit for protection. All of these issues are important for every unit but the consequence is higher with older, outdated units. Because the amount of equipment monitoring may depend on the complexity of the steam turbine, the minimum acceptable turbine parameters that should be monitored by turbine type/size are indicated below:

Recommended Steam Turbine Monitoring Parameters by Turbine Size/Type

Steam Turbine Parametersto be Monitored ContinuouslySmall Single Stage Units0.5-2MWMediumSize Multi- stage Units1.5-10MWAdmission/ Extraction and Non- Reheat Units