Tools for Preventing Boiler Tube Failure: Part 2
Tube Sample Analysis
04 April 2017
This blog is the second in a series discussing boiler tube condition assessments. While these basics might be familiar to many, this second installment will serve as a refresher to the experienced and an introduction to novices. Tube failures, which range from pinhole leaks to ruptures, require shutting down a boiler to repair. Once the failed section has been cut out and replaced and the boiler restarted, the first question which should come to mind is, "Why did the tube fail?" The most effective method of determining the failure mechanism is to perform a metallurgical analysis.
What should you expect from this analysis? The results are partly under your control, i.e., sample preparation. Here are a few things which you can do to ensure successful analysis:
- Cut the tube so that the sample is long enough to include the failure location and material away from the failure. Use a dry saw cut to avoid damaging or contaminating the sample.
- Once cut, cap or tape the sample ends so that the inner surfaces are left undisturbed.
- Ship with sufficient packing material to keep the sample undisturbed.
- The sample location should be identified by the wall and tube number. The flow direction should be indicated on the sample.
- Component sectional side view drawings may be used to indicate the failure elevation.
- Data including operating pressure, operating hours, etc., should be provided.
Typically, the laboratory will utilize the following techniques:
- Visual examination – photographic documentation, examination for wastage, deposits, discoloration, distortion, crack size, etc.
- Dimensional measurements – wall thickness and inside diameter measurements will document wall thinning, localized swelling or bulging.
- Alloy analysis –to determine if the tube meets the design alloy specifications and if the as-designed alloy was used.
- Hardness testing – may indicate higher than design temperatures (softening) or quenching (hardening).
- Deposit analysis – deposits, if any, may be analyzed for composition.
- Metallographic examination – tube cross-sections will be prepared, mounted in plastic, ground and etched for examination under a light microscope. Deposit or scale thickness, porosity, pitting and microstructure are some of the attributes that require use of a microscope. The microstructure may indicate exposure to excessive temperatures.
Additional or alternative techniques may be used on an as-needed basis. The analysis will determine the failure mechanism, but will seldom identify the root cause. Additional measures, typically involving instrumentation monitoring or operating history review, are required to determine the condition(s) responsible for the failure.
The report should provide the failure mechanism, potential root cause(s), and recommendations for future actions to determine root cause(s) and prevent failures.
Once the report is received and reviewed, it's important to not just put the report away and move onto the next project. The results should be input into the plant data management software, such as Intertek's AWARETM software. If the plant does not have this type of software, consider installing it. Historical records are an important part of moving from a reactive maintenance program (i.e., replacing tube failures as they occur) to a preventative or proactive maintenance program (using examination techniques such as TubeAlert to determine the current condition and future examination intervals), and allows for maintenance or replacement activities during planned as opposed to forced outages.
For additional information on tube samples, contact the author or Henry Vaillancourt at email@example.com (408) 636-5382.
Clayton Q. Lee, Director, Thermal Engineering, Intertek Asset Integrity Management (AIM). The author has over thirty years of experience consulting to the power industry in the areas of heat transfer, fluid mechanics, stress analysis and fracture mechanics. He has authored several sponsored applied research reports used by the power industry.