By Carlos Molina
Hi friends. I hope you are all doing well.
Today we are going to review 3 relevant damage mechanisms you should know in order to pass the API 653 certification exam. All of them appear in the BOK for the March 2015 exam. Download this pdf and you´ll find all of the 52 questions, based in API 571 (2 more questions than other pdfs, because I was so into it)
These are Brittle Fracture, Mechanical Fatigue and Atmospheric Corrosion.
1. BRITTLE FRACTURE
Definition of fracture: Fracture is the separation of an object into pieces due to stress, at a temperature lower than the melting point. In pipes, pressure vessels and tanks, fracture can be ductile or brittle. Both are bad, but Brittle fracture is very bad, given that cracks in the stressed material travel so fast that there´s usually no chance to react.
Brittle fracture*. Brittle fracture shows no plastic deformation and rather happens if temperature is low, there are tensile stresses applied and if there are stress concentrators as gouges or flaws in the material. Failure is usually catastrophic.
Brittle fracture is more common shortly after erection during hydrostatic testing or on the first filling in cold weather, or in operation, after a change to lower temperature service, or after a repair/alteration. Special care should be taken to not cause overfilling of an old tank. If an old tank has sharp corners or inserts, it could be wise to remove these stress concentrators.
Brittle fracture risk in tanks is minimal if the tank´s shell thickness is no greater than 0.5in. Besides, brittle fracture is very rare if shell metal temperature is over 60°F (Section 5). Older thanks are more susceptible, because manufacturing techniques were defficient.
Sharp cracks and large defects both lower the fracture strength of the material. If a brittle fracture should occur, cracks run almost perpendicular to the applied stress, and with little plastic deformation.
2. Mechanical fatigue.
Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength (strength of a material defined as a measure of its ability to resist deformation) of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.
Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands, and grain interfaces. Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets will therefore increase the fatigue strength of the structure.
In tank inspections, you should be wary of Low Cycle Fatigue, as this can affect shell-to-bottom welds, specially when changing from a low temperature service to a higher temperature service (184.108.40.206). All indications in welds close to settlement areas shall be evaluated for fatigue failure (B.3.4.1/2)
3. Atmospheric Corrosion.
Atmospheric corrosion refers to the corrosive action that occurs on the surface of a metal in an atmospheric environment.
The most important factor in atmospheric corrosion is moisture, in the form of rain, dew, condensation and high relativity humidity. Contaminants also create atmospheric corrosion, although the effect is greater when there is water present. The less severe environment for metals is one dry atmosphere, that has a corrosion rate lower than 1 mils per year.
In tanks, atmospheric corrosion is the driving force of loss of material of the exterior of tanks. Ultrasonic thickness measurement can be used as a means of determining the rate of uniform
general corrosion while the tank is in service, and can provide an indication of the integrity of the shell. Formulas for assesment of shell material for continued service can be foun in API 653.
I had to put a lot of effort in this investigation. Yet, the set of questions in the pdf is taken only from API571. Questions are formulated in a way easy to remember. Of course, all of the information for the exam is within the boundaries of the BOK, but is not a bad idea to have this collection you can study forever.
Thanks for your attention and see you next week. Dont forget to make suggestions in the comment section.
More pages on the API 653 questions series
*En español, el término es “fractura frágil”. No obstante, en inglés existen los términos “fragile” y “brittle”, que tienen diferencias de significado.