Failure Analysis-Q9

“Microstructural examinations were conducted on boiler tube from local petrochemical plant for assessment on the reusability of the tube. Tube diameter was measured along the tube and it was shows that there is no bulges on the as received tube. The boiler tube was found bending at fire side area (Figure 1). The structure has equated grains consisting of pearlite and ferrite. Microstructure examination revealed that the grain size at the bend was slightly bigger as compared to grains at far from than the bend side area. There is no change in structure of grain at both areas; it means that the temperature applied to the tube was not higher than the limit of permissible temperature of the tube. According to the different in grain size, it means that temperature at the bent area was also different with the straight area of the tube. Temperature gradient should be existed along the tube due to the different in temperature. The bending in the tube was caused by the temperature gradient.”

Figure 1 Schematic diagram shows the condition of bending and metallographic spot on the tube surface marked as R1, R2, R3 & R4

From the statement above, plan your investigation procedures to implement failure analysis on the boiler tube. Give your reason for each procedures

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1) Obtaining background data

• To investigate the basic information that possible to be a primary clues of the root cause analysis.
• These importance information are including its location, manufacturer, function of component, operational condition, maintenance and cleaning history as well as opinions of related personnel

2) Visual examination followed by closed-up photographs by digital camera.

• Since many of the subsequent steps in the evaluation will involve destructive examinations, the as-received photographs provide an archival record of the original condition of the failed component prior to cutting and sectioning
• The as-received photographs also provide an effective place to indicate the locations from which the various samples used in the destructive examination were obtained

3) Chemical composition analysis by using XRF or spark emission spectrometer

• Analysis of the chemical composition of the component is a routine part of a failure analysis
• The analysis is used either to determine if the material meets the specified composition limits (in this case, boiler tube is carbon steel) or to identify an unknown material

4) Dimension measurement by vernier calliper

• The operational condition of boiler tube normally involves a high temperature stress. Thus as-received boiler tubes should have to document their dimension along the tube to investigate the occurrence of bulging on the tubes resulted by its operational condition. However there’re no bulging occur on the sample

5) The straightness of the tubes is carried by rolling the tube on the flat & smooth surface.

• Any bending or uneven on the tube can be identified when unstable rolling is observed resulted by high temperature operational condition that probably hits the one side of the tubes.

6) Determination of failure mechanism & recommendations

• Without identification of the failure mechanism, it is usually impossible to determine the root cause of the failure, which forms the basis for recommendations for immediate action whether the components should be replaced or not and to eliminate or reduce the frequency of future failures.
• Determination of the failure mechanism is considered a critical step in the failure investigation. In this particular case, the failure was caused by the occurrence of temperature gradient on the tube.