Part 4 of API 579 addresses “General” metal loss and part 5 addresses “Local” metal loss. But which part is appropriate to use in your case? The standard introduces 3 criteria to select the assessment method, including the type of thickness measurement (point thickness readings or thickness profiles), the characteristics of the metal loss and the degree of conservatism acceptable for the assessment. Furthermore, API 579 standard provides an integrated procedure to evaluate metal loss using part 4 and part 5. In this procedure, the ultimate selection of assessment method is left to the expert.
On the other hand, based on API RP 581, part2, table 4.4, there are two important criteria for recognize thinning type; corroded area and corroded thickness. general corrosion is defined as affecting more than 10 % of the surface area and the wall thickness variation is less than 1.27 mm. Localized corrosion is defined as affecting less than 10 % of the surface area or a wall thickness variation greater than 1.27 mm.
So it is recommended that you first determine the type of metal loss using the API RP 581 or other methods. Then go to the relevant part (general or local metal loss). What is your opinion? Do you have any experience in this field? Have you had any challenges in selecting the type of metal loss?