Mon, 01 February, 2021
TWI Services Sdn. Bhd. (TWI) was recently contracted by an independent power utility company to carry out outage inspection work on the company’s heat recovery steam generator (HRSG) and unfired pressure vessels. Fitness-for-service assessment, inspection and life assessment for natural and fuel gas pipeline, as well as inspection supports for failure cases have all been part of the extended services offered to the client since the year 2011.
The client company is a Combined Cycle Gas Turbine (CCGT) power plant with a capacity of about 287 Megawatts (MW). The company is currently undertaking inspection as part of their pre-start-up preparation for continuous operation in service. For the last 10 years, the HRSGs have been mothballed and TWI was asked to provide total solution services. During the inspection, some cracks were found at different locations such as headers and tubes.
An on-site failure investigation was conducted on the tubes through a combination of analysis processes including:
- Interpretation of failure location and service data
- Examination of the parts to determine the failure mechanism
- Engineering root cause, including design, fabrication, transportation, installation and service
- Establishment of a non-destructive evaluation strategy for remaining components
The findings suggested
that the cracking is considered to be a form of stress corrosion cracking (see Figure 1).
Fitness-for-Service and Integrity assessment
Finite element analysis and fracture assessment as per BS 7910 were performed on the affected headers and tubes using the TWI software CRACKWISE (see Figure 2). In summary, the results show that the components are fit for continued service.
Risk-Based Inspection (RBI)
TWI also performed the risk Based inspection (RBI) on the HRSG components. The results of the assessment in terms of RLI (Run-length Indexes - TWI's safe future operating period based on risk) were presented. This is the basis for setting up an inspection interval and evaluate future inspection planning for safe continued service.
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