API 579-1/ ASME FFS-1 Fitness-For-Service evaluation was performed on a steam drum in a wood-fired boiler. The 30,000 pound boiler (120 psi) was overheated due to a loss of makeup water and a failure of a low-water cutoff control.
The API 579-1 standard contains very specific methodologies for evaluating pressure vessels that have been damaged or compromised due to operational or environmental factors such as fire, overpressure, corrosion, fatigue, and mechanical damage. Typical damage resulting from these factors includes pitting/loss of metal, embrittlement, residual stress, cracks, and reduced material properties. The net result is often reduced vessel integrity, shortened operating life, or even required repair/replacement. The methodology presented in Section 11 of API 579-1 / ASME FFS-1 “Assessment of Fire Damage” was primarily used.
A few preliminary observations were made.
a. The initial site inspection revealed that the drum did not show any distortion from the overheating incident. However, random hardness testing revealed results slightly lower than expected for typical boiler pressure plate steel (e.g.; ASTM SA-516 Grade 70). At the time, the actual material used for fabrication of the steam drum was unknown. It was decided to perform replication to assess grain size and potential creep damage or creep precursors.
b. Replication was performed on different areas of the steam drum (inside and outside surfaces.) Examination showed which location had the most grain growth and other changes indicating elevated temperature but not critical phase change. It was decided to utilize destructive testing at the area of maximum exposure to elevated temperature (based on the metallographic results).
c. The Fitness-for-Service evaluation must include a fatigue life analysis. However, the wood-fired boiler was installed around 1979; it thus has approximately 18 years of initial service for which the cyclic loading is unknown. The most effective method for evaluating fatigue damage was therefore a comprehensive crack inspection utilizing magnetic particle and dye penetrant techniques. This inspection included all of the tube holes in the steam drum (especially the ligaments between holes), all nozzles attached to the steam drum (including manholes) and non-pressure component attachments to the steam drum.
Metallurgical, material properties and NDT tests were performed. The NDT testing included Ultrasonic thickness testing (UT) of the steam drum and Magnetic Particle testing (MT) for cracks at the tube ligaments of all three drums (steam drum and two mud drums).
We concluded that the steam drum material was not compromised by the overheating incident as evidenced by metallography, hardness testing, tensile testing and NDE inspection. We also determined:
• ASME B&PV Code calculations show that the steam drum meets minimum thickness requirements for a MAWP of 160 psig using the Allowable Stress Values derived from the tensile testing of the actual steam drum material.
• The Safety Relief Valves on the Boiler are set at 140 psig (North) and 135 psig (South), respectively. Therefore, the Boiler should never operate at the original 200 psig MAWP, and the recommended 160 psig MAWP (derating) will have no effect on plant production requirements.
• Periodic internal inspections of the steam drum should be conducted in accordance with ASME recommendations.
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