Maintenance procedure of major component of gas turbine power plant
● Maintenance costs and machine availahility are two of the most important concerns to a gas turbine equipment. Therefore, a well thought out maintenance program that reduces the costs while increasing equipment availability should be instituted.
● For this maintenance program to be effective, it should develop a general understanding of the relationship between the operating plans and priorities for the plant the skill level of operating and maintenance personnel, and all equipment manufacturer's recommendations regarding the number and types of inspections, spare parts planning, and other major factors affecting component life and proper operation of the equipment.
● The gas turbine is designed to withstand severe duty and to be maintained on-site, with off-site repair required only on certain combustion components, hot gas path parts, and rotor assemblies needs specialized shop service. The following features are designed into gas turbines to facilitate on-site maintenance
● All casings, shells and frames are split on machine horizontal centerline. Upper halves may be lifted individually for access to internal parts.
● Key inspection and maintenance areas for casings are bolt holes, shroud pin and bore scope boles in the turbine shell (case), Compressor stator hooks, Turbine shell shroud hooks, Compressor discharge case struts, Inner barrel and inner barrel bolts. Inlet case bearing surfaces and hooks, Inlet case and exhaust frame gibs and trunions, Extraction manifolds for foreign objects
Compressor part:
● With upper half compressor casings removed, all stationary vanes can be slid circumferentially out of the casings for inspection or replacement without rotor removal
● Compressor Condition and Performance Maintenance and operating costs are also influenced by the quality of the air that the turbine consumes. In addition to the negative effects of airborne contaminants on hot gas path components, contaminants such as dust, salt, and oil can cause compressor blade erosion, corrosion, and fouling
● Fouling can be caused by submicron dirt particles entering the compressor as well as from ingestion of oil vapor, smoke, and industrial vapors. Corrosion of compressor blading causes pitting of the blade surface, which, in addition to increasing the surface roughness. also serves as potential sites for fatigue crack initiation.
● These surface roughness and blade contour changes will decrease compressor airflow and efficiency, which in turn reduces the gas turbine output and overall thermal efficiency. Generally, axial flow compressor deterioration is the major cause of loss in gas turbine output and efficiency
● Recoverable losses, attributable to compressor blade fouling. typically account for 70-85% of the performance losses seen. Compressor fouling to the extent that airflow is reduced by 5%, will reduce output by up to 8% and increase heat rate by up to 3%
● Fortunately, much can be done through proper operation and maintenance procedures both to minimize fouling type losses and to limit the deposit of corrosive elements
● Compressor wash systems are available to maintain compressor efficiency by washing the compressor while at load, before significant fouling has occurred. Off-line compressor wash systems are used to clean heavily fouled compressors. Other procedures include maintaining the inlet filtration system, inlet
● Casing Parts gas turbines have inlet, compressor, compressor discharge, and turbine cases in addition to exhaust frames. Inner barrels are typically attached to the compressor discharge case. These cases provide the primary support for the bearings, rotor, and gas path.
● The exterior of all casings should be visually inspected for cracking, loose hardware, and casing slippage at cach combustion, hot gas path, and major outage. The interior of all casings should be inspected whenever possible.
● The level of the outage determines which casing interiors are accessible for visual inspection. Borescope inspections are recommended for the compressor cases, and inlet cases, compressor discharge casesduring gas path borescope inspections.
● All interior case surfaces should be visually inspected during a major outage
Turbine part:
● With the upper-half of the turbine shell lifted, cach half of the first stage nozzle assembly can be removed for inspection, repair, or replacement without rotor removal.
● On some units, upper-half, later-stage nozzle ussemblies are lifted with the turbine shell, also allowing inspection and/or removal of the turbine buckets.
● All turbine buckets are moment weighed and computer charted in sets for rotor spool assembly so that they may be replaced without the need to remove or rebalance the rotor assembly
● All bearing housings and liners are split on the horizontal centerline so that they may be inspected and replaced when necessary. The lower half of the bearing liner can be removed without removing the rotor.
● All seals and shaft packings are separate from the main bearing housings and casing structures and may be readily removed and replaced
● On most designs, fuel nozzles, combustion liners and flow sleeves can be removed for inspection, maintenance, or replacement without lifting any casings. All major accessories, including filters and coolers, are separate assemblies that are readily accessible for inspection or maintenance. They may also be individually replaced as necessary.
● Casings can be inspected during any outage or any shutdown when the unit enclosure is cool enough for safe entry. The exterior of the inlet, compressor case, compressor discharge case, turbine case, and exhaust frame can be inspected during any outage or period when the enclosure is accessible,
● The interior surfaces of these cases can be inspected to various degrees depending on the type of outage performed. All interior surfaces can be inspected during a major outage when the rotor has been removed.
● Exhaust diffusers can be inspected during any outage by entering the diffuser through the stack access doors. The flow path surfaces, flex seals, and other flow path hardware can be visually inspected with or without the use of a borescope
● Diffusers can be weld-repaired without the need to remove the exhaust frame upper half. Inlets can be inspected during any outage or shut down.
Rotor Parts :
● Disassembly and inspection of all rotor components is required when the accumulated rotor starts or hours reach the inspection limit. Rotors that are cold when the startup commences experience transient thermal stresses as the turbine is brought in operation.
● After a shutdown, turning of the warm rotor is essential to avoid bow, or bend, in the rotor Initiating a start with the rotor in a bowed condition could lead to high vibrations and excessive rubs.
