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Repair and maintenance of gas turbine units

On the photo: Medium repair. Orsha CHPP. (inspection of the hot path)

At the moment, in conditions of limited and high cost of energy carriers it is necessary to constantly modernize and reconstruct the existing generating capacities. No one doubts the correctness of gas turbine technologies introduction, as the efficiency of power units significantly increases with the introduction of gas turbine units (GTU).

After GTU commissioning the owner of the equipment faces the issue of gas turbine maintenance and repair. Companies-manufacturers impose strict requirements to the operating organization on observance of gas turbine and GTU maintenance regulations. This is done in order to guarantee reliable operation. Despite the fact that today in the power system of the Russian Federation power GTUs of different manufacturers (General Electric, Alstom, Mitsubishi, Siemens, Ansaldo); of different generations, classes, with different designs of combustion chambers, burners, turbine rotor and stator protection systems against overheating are operated – the average period between alternating inspections is 8000-12000 EOH (equivalent operating hours). Thus, at normal operation of GTU once in a year and a half for any model of GTU an inspection should be carried out

GTU diagnostics by periodicity

Inspections (repairs) are divided by periodicity and scope of work into the following types:

A-Inspection

Figure 1. Inspection of the GT13E2 combustion chamber with a boroscope

A inspection – inspection of the combustion chamber parts (CI – Inspection). It includes assessment of the general condition of the combustion chamber parts, hot gas path, inlet compressor and compressor blades without disassembly of the turbine using special equipment – boroscope (in this connection A inspection is called boroscopic (BI – Boroscope Inspection)). It is necessary to determine the actual scope of work for the subsequent B or C inspection. It includes minimal recalibrations and adjustments if necessary. Outcome: A report on the condition of the engine, which includes recommendations for further operation and maintenance.

With boroscopic inspection, there is no need to disassemble the block (CI+TP removal), (TP – Transition Piece). Visual access to the blade apparatus is through boroscope hatches in the turbine casing, and for the inlet guide vanes and turbine final stages the access is usually through hatches in the air intake and exhaust diffuser casing respectively.

B-inspection

Figure 2. Measurement of compressor 1st stage clearances through the VNA. Alstom GT13E2

B-inspection – inspection of hot gas path and combustion chamber (HGP – Hot Gas Path Inspection) – for different GTU models includes different scope. It can include the same type of inspections as A-inspection, but in a slightly larger volume (i.e. it is possible to do only with a boroscope). For other models, some parts of the hot path and combustion chamber may be scheduled for replacement according to the regulations.

In any case, the scope of work and the need to replace parts is specified during the inspection.

C-inspection

Fig. 3. Alstom GT13E2 GTU rotor installation

C-inspection – Major Inspection (MI – Major Inspection) – complete disassembly and revision of the turbine, including the compressor. It includes inspections of all internal components, protective and control equipment, gas turbine unit monitoring equipment. The main parts after disassembly undergo visual and, if necessary, non-destructive testing. Planned works on replacement and restoration of components are carried out (some of which are determined in the process of visual inspections of type A and B, some of which are determined according to the regulations).

The downtime for inspection C may be longer than planned, but this risk can be mitigated with proper planning.

It should be noted that for GTU the previously mentioned rigidity of manufacturers’ requirements to compliance with routine maintenance is due to the fact that the exact resource of trouble-free operation of the most critical components (such as parts of combustion chambers, GTU turbine blades, parts of thermal barrier elements and coatings) is currently not calculable. (All mathematical models used in the design of these GTU units have a very large number of tolerances).

Therefore, if the manufacturer requires replacement of the 1st stage blade after 40000 hours, it means that it should be replaced even in the absence of critical defects, because development of existing damages in it and material behavior during further operation is not subject to modeling and can be allowed only with the owner’s consent.

The majority of GTUs operated in the Russian Federation, to varying degrees, lack sufficient technological documentation and special tooling necessary to perform the entire typical scope of GTU inspection work without recourse to the manufacturing plants. It is in the interests of operating organizations not to be highly dependent on manufacturers in such an important issue as maintenance and repair of the main equipment. In this connection it is recommended to demand from GTU suppliers to provide all necessary set of documentation and special tooling as buyers’ rights. This will give an opportunity to choose a service company independently. Today the majority of GTU owners have considerably limited choice.

GTU repair and maintenance

The main task is to provide an organization operating GTU with an opportunity to make an independent decision how to maintain and repair the equipment. Supply of components, spare parts, consumables, necessary tools, and, most importantly, provision of highly qualified personnel for service – the list of opportunities that the operating organization gets for realization of current service tasks. The company carries out maintenance and repair of gas turbines in strict accordance with the regulations of the manufacturer. It provides both the full range of works on long-term maintenance and repair of gas turbines and GTUs and its separate parts: provision of chief engineers, field personnel, tools, consumables, spare parts, etc.

A complete set of repair documentation for GTU should, among other things, include:

  • detailed description of the technological sequence and requirements for performance of necessary works during scheduled inspections, stipulated by the manufacturer’s regulations, with indication of tools and devices, necessary for specific stages of repair works and measurements (technical process, TI);
  • criteria for evaluation of resource of all GTU units and parts, as well as their condition and suitability for operation (defect inspection criteria, TU);
  • nodal and assembly drawings of GTU units and parts, as well as complex assembled fixtures for repair;
  • full set of measurement forms to be performed during scheduled routine inspections of GTU (A, B, C, etc.) with description of all technical requirements of the manufacturer to the measured parameters;
  • lists of necessary tools, devices, consumables required for performance of scheduled maintenance and repair works, which should be detailed (without using the concept “kit”, or with detailed descriptions of these “kits”). They should contain information allowing to unambiguously identify the items specified in the lists (market name, brief technical characteristics, company-manufacturer of the tool or material);
  • lists of assemblies and parts to be replaced routinely during maintenance and repair, as well as lists of spare parts recommended for permanent availability during operation.

 

Repair of damages of gas turbine unit parts

Returning to possible defects of GTU parts and their elimination, it should be noted that the main reasons of defects occurrence in GTU parts are as follows: high-temperature corrosion, destruction due to vibration, thermal deformations, mechanical and erosion wear, non-compliance with fuel preparation requirements. The nature of damages caused by the above mentioned causes, in most cases allows to detect them only by nondestructive testing methods, and to eliminate them by welding. More details about some possible damages of GTU parts:

Repair of a crack in the turbine nozzle guide apparatus

Fig. 4. Crack in the turbine nozzle guide apparatus

Crack in turbine nozzle guide apparatus (Fig. 4) – cracking is caused by thermal cycles “heating-cooling”. The crack size exceeds the allowable limits and this segment should be replaced as soon as possible, regardless of the scheduled inspection dates. This defect can be repaired by crack extraction, subsequent welding, mechanical and thermal treatment, and a final thermal barrier coating. The repaired segment can then be used to replace a similarly damaged segment;

Repair of compressor blade outlet edge

Figure 5. Compressor blade outlet lip damage

Damage of the compressor blade outlet edge (Fig. 5) is often encountered on the working blades of the first stages of GTU compressors. The reason of appearance of such “nicks” is penetration of foreign objects into the flowing part. Specific damages do not exceed permissible limits and can be eliminated at the next scheduled inspection by manual mechanical treatment;

Repair of blades of the 1st stage of a gas turbine

Fig. 6. Damage of the 1st stage turbine blades (erosive wear of the inlet edge)

Damage of blades of the 1st stage of the turbine (Fig. 6) – the condition of working blades on the picture is typical for the majority of working blades of the 1st stages of the GTU hot gas path, worked out before the next C-inspection and subject to replacement. This condition corresponds to normal wear and is caused by the cumulative effect of the above mentioned causes;

Repair of flame tubes

Figure 7. Damage to flame tubes. Wear and deformation

Damage of flame tubes (Fig. 7) – the damage (melting, deformation) of flame tube sockets in the area of connection with cross-firing tubes indicates that the permissible exhaust gas temperatures in this area have been exceeded. The causes may be different, but if such defects are detected, they must be identified and eliminated as soon as possible. The specific damage is within the permissible limits and can be remedied;

Repair of burner devices

Fig. 8. Contamination and erosion of fuel distributing holes of burner units

Contamination and erosion of burner devices (Fig. 8) – the condition on the picture is typical for the majority of GTU burner devices, worked out before the next B-inspection. Contamination of fuel-dispensing orifices is connected with poor quality of fuel gas preparation. Contamination leads to non-uniformity of the temperature field at the turbine outlet and, as a consequence, to damage of turbine blades.

Contamination and erosion of 1st stage compressor blades

Figure 9. Contamination and erosion of 1st stage compressor blades

Contamination and erosion of the 1st stage compressor blades (Fig. 9) – the condition of working blades on the image is typical for the majority of working blades of the 1st stages of GTU compressors worked out before the next C-inspection. Specific contamination is assessed as significant, removed during manual inspection with non-abrasive material. Such level of contamination indicates unsatisfactory operation of the systems of the Complex Air Purification Unit.

Maintenance and repair of gas turbine plants today

Today the power system incurs significant foreign exchange costs due to maintenance and repair of gas turbines and GTUs using services of foreign companies. Thus, the cost of services of specialists of manufacturing plants and large foreign service companies is a large amount in US dollars per day of work. At the same time, it should be noted that the cost of repair service of gas turbines and gas turbine equipment consists of two main components: the cost of spare parts, consumables and payment for the services of repair technicians.

The cost of spare parts is about 80-90% of the cost of all repair and maintenance costs. At the same time, to ensure reliable operation of high-tech equipment, which is a gas turbine, and guaranteed compliance with all its characteristics declared by the manufacturer at delivery, it is required to purchase only original spare parts from the manufacturers of this equipment.

A constraining factor in the development of maintenance and repair of gas turbines and GTUs is the lack of a unified policy in the industry aimed at achieving the above-mentioned common goal. There is no unified position with regard to the issue of organization of GTU maintenance and repair. Each owner of equipment solves the issue of maintenance and repair independently, sometimes in ways that allow to fulfill only short-term tasks to the detriment of long-term benefit.

For any organization, operating GTU, the choice of a company, which will carry out repairs, is of paramount importance.