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Case Study: Condition and Life Assessment of Boiler

at a Power Station

Overview

Quest Integrity Group performed a complete condition and life assessment of a boiler at a thermal power plant in Southeast Asia. The boiler examined was a Riley Stoker design, constructed by Mitsui Engineering in 1982. The boil-er forms part of a 300MW unit capable of being fueled by fuel oil or natural gas. The unit had completed approxi-mately 120,000 hours of operation at the time of this project. The objective of this project was to assess the general and metallurgical condition and remaining life of the super-heater tubes, piping and pressure vessels of the boiler. The scope of work included the following:

A summary of the sections of the boiler that were tested and the tests performed are presented in the table below:

CS-Boiler1-P

The sectional drawing of the boiler, indicating the various examined locations, is shown above.

Ultrasonic thickness measurement (USTM)

Magnetic particle in-spection (MPI)

In-situ metallographic examination

In-situ hardness testing

Fiberscope inspection

Laboratory examination and documentation

Evaluation and reporting

Boiler Section Test(s) Performed

High Temperature Reheater Tubes

USTM

High Temperature Superheater Tubes

USTM

Intermediate Temperature Su-perheater Tubes

USTM

Feedwater Pipe USTM and MPI

Boiler Steam Drum USTM and MPI

Deaerator and Storage Tank USTM

Boiler Blowdown Vessel USTM and MPI

Boiler Blowdown Flash Vessel USTM

HP Clean Drain Collection Vessel

USTM and MPI

Main Steam Pipe USTM, MPI, surface metal-lography and hardness test

Hot and Cold Reheater Pipe USTM, MPI, surface metal-lography and hardness test

Superheater Cross-over Pipe Fiberscopic inspection

+64 (0)4 978 6630 www.QuestIntegrity.com Sales@QuestIntegrity.com

Case Study: Condition and Life Assessment of Boiler

at a Power Station

Results

The results of this assessment indicated that the boiler and steam pipework were in generally good condition, and should be capable of operation well beyond their design life. There were indications that sections of the high tempera-ture superheater have been operated at temperatures in excess of the design values. Consequently, a greater than expected fraction of the life of associated components has been expended. In the case of the final (high temperature) superheater outlet header, it was recommended that accelerated post-exposure creep tests be considered. This would involve removal of small samples from these critical pressure parts, preferably by a core drilling method. The data derived from such tests would enable the remaining life of these compo-nents to be predicted with an otherwise unachievably high level of confidence. The results would allow optimum capital replacement scheduling. Evidence was found that flow assisted corrosion (FAC) had taken place within the deaerator and feedwater pipework leading to the boiler feed pumps. The water temperature between the deaerator and HP heaters was such that mag-netite solubility could increase to a peak level rapidly with a small drop in pH. Consequently, the feedwater pipework in this area was considered to be at a higher risk of failure due to FAC. Failures in other parts of the world have led to several fatalities. It was therefore recommended that consideration be given to a more complete and detailed survey of this area at a later date. A significant amount of oil-ash fouling was discovered on the external surfaces of the boiler tubes in the (front) high-temperature section of the boiler. These deposits were found to have a composition that is associated with severe cor-rosion of Cr-50 material as used for tube support lugs. The latter were extensively damaged, particularly in the area of the intermediate and high temperature superheater. It was recommended that these supports be repaired to prevent

consequential mechanical damage to nearby areas due to fatigue. The deposits were found to be fused to the external surfaces of the intermediate and high temperature superheater and were difficult to remove. Other plants used fuel additives that can reduce or remove these deposits within a rela-tively short period of operation (2000 hours to 4000 hours), and thereby improve heat transfer characteristics and over-all efficiency. It was suggested that consideration be given to trials of such additives on Units 1 and 2 if the boilers are to be fueled by oil for a significant part of their future operational campaign.

CS-Boiler 1-P

Quest Integrity Group

Quest Integrity Group is focused on asset integrity and reliability management solutions for our clients. Our solutions are comprised of technology-enabled advanced inspection and engineering assessment services and products that help companies in the refining and chemical, pipeline, syngas and power industries increase profitability, reduce operational and safety risks and improve opera-tional planning.

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