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Inspectioneering Journal

FFS Forum: Thickness Grids & Critical Thickness Profiles

By Greg Garic, Managing Principal at Stress Engineering Services, Inc. This article appears in the November/December 2019 issue of Inspectioneering Journal.
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Introduction

The corrosion-related Parts of the API 579-1/ASME FFS-1, Fitness-for-Service (API 579) standard are certainly the most frequently used. This is because corrosion is an enormous and pervasive damage mechanism that affects countless aspects of industry. You’ve probably heard of the much-quoted NACE study which concludes that the “direct cost” of corrosion in 1998 was $276 billion, or 3.1% of the US gross domestic product (GDP). A recent extrapolation by G2MT Labs, accounting for the inflation-adjusted, direct-plus-indirect cost, put the total 2016 cost at $1.1 trillion; over 6% of the GDP. Suffice it to say that corrosion assessment deserves some attention.

A cornerstone of the Level 1 and Level 2 metal loss assessment methods in API 579 is the concept of measuring thickness on a grid and constructing a “critical thickness profile.” These concepts are fundamental to Level 1 & 2 assessments in both Parts 4 and 5 (general and local metal loss).

The Grid

Most fundamentally, the inspection grid is just a regular series of (typically parallel) longitudinal and circumferential lines to help organize evenly spaced thickness measurements.

Typical Inspection Grid on a Cylindrical Shell
Figure 1: Typical Inspection Grid on a Cylindrical Shell

Figure 1 shows a typical inspection grid encompassing a thin area on a cylindrical shell. In setting up this grid, there are several points to consider.

Grid Spacing

The grid spacing should be adequate to capture the variation in the thickness. Figure 2 shows an example of the effect of grid spacing. In Figure 2(a), the spacing is too large to capture the highs and lows of the contour. In Figure 2(b), the tighter spacing captures an adequate representation. That said, it is not necessary to have spacing so refined as to ensure that all maximums and minimums are represented. This is essentially a statistical sample that requires only a good representation of the contour.

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Comments and Discussion

Posted by Hervandil Santanna on January 27, 2020
Hello, Congratulations! Another excellent... Log in or register to read the rest of this comment.

Posted by Lyle Smith on February 17, 2020
Which "in service" NDE technologies are... Log in or register to read the rest of this comment.

Posted by FERNANDO VICENTE on February 18, 2020
Great article, very useful. The major structural... Log in or register to read the rest of this comment.

Posted by Nasheed Ur Rahim on February 18, 2020
Good Article, Very useful Log in or register to read the rest of this comment.

Posted by Greg Garic on February 18, 2020
Lyle...Highly irregular surface do present a... Log in or register to read the rest of this comment.

Posted by Greg Garic on February 18, 2020
Fernando...I agree. There is some confusion over... Log in or register to read the rest of this comment.

Posted by FERNANDO VICENTE on February 19, 2020
Yes Greg, that small article about MSD was very... Log in or register to read the rest of this comment.

Posted by Julio Naranjo Fonseca on March 26, 2020
My question is similar to that of Lyle Smith.... Log in or register to read the rest of this comment.

Posted by Greg Garic on March 31, 2020
Julio…You can always do something more... Log in or register to read the rest of this comment.

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