8.4 Evaluation of Pitting, Localized Thin Areas (LTA) & Critical Length (L)

8.4 Evaluation of Pitting, Localized Thin Areas (LTA) & Critical Length (L)

API 653 Core Principle: When a tank shell suffers corrosion, the degradation is rarely uniform across an entire plate. Requiring an entire shell course to meet nominal thickness when metal loss is restricted to small depressions would force unnecessary, costly repairs. API 653 provides quantitative engineering criteria for widely scattered pitting and Localized Thin Areas (LTAs), utilizing the critical vertical length parameter ($L$) to evaluate stress redistribution and ensure structural safety.

In structural mechanics, a pressurized cylindrical shell possesses substantial redundant strength. When a localized depression occurs, internal circumferential hoop stress redistributes around the depression into adjacent, thicker sound metal through membrane arching and vertical shell bending (beam action). API 653 establishes rigorous mathematical rules to determine whether a localized flaw can be safely left in service, derated, or repaired.

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1. Widely Scattered Pitting Criteria (API 653 Section 4.3.2.2)

Pitting corrosion creates small, steep-sided cavities in the plate. Rather than requiring complex profiling for every minor pit, API 653 Section 4.3.2.2 permits widely scattered pitting to be accepted without engineering calculation, provided all three of the following conditions are met:

                    WIDELY SCATTERED PITTING EVALUATION

            | <----------------- 8 inches -----------------> |
            +-------------------------------------------------+
            |         (.) d1                  (.) d2          |
            |                                                 |
            |    Vertical Inspection Window                   |
            |    Sum of Pit Lengths: (d1 + d2 + d3) <= 2.0"   |
            |                                                 |
            |                     (.) d3                      |
            +-------------------------------------------------+

            CRITERIA:
            1. Remaining Shell Under ANY Pit >= 0.5 * t_min
            2. Cumulative Pit Length Along ANY 8" Line <= 2.0 inches
            3. Pitting Must Not Cross a Weld Seam (Unless meeting weld criteria)

Condition 1: Minimum Remaining Thickness Threshold

No individual pit depth shall result in a remaining shell thickness less than half the minimum acceptable thickness:

$t_{\text{remaining}} \ge 0.50 \times t_{\text{min}}$

If the nominal or surrounding thickness is $t$, and the measured pit depth is $d_{\text{pit}}$, the remaining steel ligament at the root of the pit is $t - d_{\text{pit}}$. If $t - d_{\text{pit}} < 0.50 \times t_{\text{min}}$, the pit must be repaired (e.g., via weld overlay or patch plate) regardless of its diameter.

Condition 2: The 8-Inch Vertical Line Rule

Along any vertical line of length 8 inches (200 mm) drawn through the pitted region, the cumulative sum of the pit dimensions (pit diameters/lengths intersected by the line) shall not exceed 2 inches (50 mm):

$\sum_{i=1}^{n} d_i \le 2.0\text{ inches (in any 8-inch vertical span)}$

This rule guarantees that at least 75% of the vertical ligament ($6\text{ inches}$ out of $8\text{ inches}$) consists of full-thickness, un-pitted steel capable of carrying membrane hoop tension without plastic necking.

Condition 3: Proximity to Weld Seams

Pits cannot coalesce into continuous grooves across weld seams. If widely scattered pitting crosses or borders a vertical or horizontal weld joint, the remaining plate and weld thickness must satisfy the weld joint criteria.

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2. Localized Thin Areas (LTA) Mechanics & Critical Length ($L$)

When metal loss extends beyond isolated pits over a broader zone (such as product-level washouts, acid attack, or abrasive erosion), the area is classified as a Localized Thin Area (LTA). An LTA is a region of reduced thickness surrounded by thicker, sound shell plate.

                      LOCALIZED THIN AREA (LTA) GEOMETRY

      +---------------------------------------------------------------+
      | Sound Shell Plate (Full Thickness t)                          |
      |                                                               |
      |                +-----------------------------+                |
      |                | LTA Depression              |                |
      |                |     Least Thickness t_2     |                |
      |      |         |     (Exclusive of Pits)     |         |      |
      |      v                                                 v      |
      |    <------------------- Critical Length L ---------------->   |
      |    L = 3.7 * sqrt( D * t_2 )   [Ceiling: L <= Course Height]  |
      +---------------------------------------------------------------+

The Critical Length Formula ($L$)

Under API 653 Section 4.3.3.1, localized thinning is assessed by profiling thickness along a vertical line spanning the critical length ($L$):

$L = 3.7 \sqrt{D \cdot t_2}$

where:

• $L$ = critical vertical length in inches over which thickness readings are averaged
• $D$ = nominal tank diameter in feet
• $t_2$ = least thickness within the LTA exclusive of pits, in inches

Maximum Ceiling Rule: The critical length $L$ shall not exceed the height of the shell course under consideration ($L \le h_{\text{course}}$).

Physical Significance of Critical Length

In shell elasticity theory, $3.7 \sqrt{r t}$ represents the characteristic longitudinal half-wavelength of localized axisymmetric deformation in a pressurized cylinder. Thinning that extends over a vertical length less than $L$ allows hoop loads to bridge across the depression into the stiffer adjacent steel. If thinning exceeds length $L$, stress redistribution cannot bridge the span, and the plate behaves as a globally thinned cylinder.

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3. Profile Measurement Procedure & Averaging Protocol

To evaluate an LTA in accordance with API 653 Section 4.3.3.1, the inspector and NDE technician must execute a systematic profiling survey:

                      LTA VERTICAL PROFILING PROTOCOL

      Elevation
          ^             UT Inspection Points (Equally Spaced)
          |                    
      +L/2|----- (1) o [t_1 = 0.380"]
          |      (2) o [t_2 = 0.360"]
          |      (3) o [t_3 = 0.330"]
          |      (4) o [t_4 = 0.270"]
       0  |----- (5) * [t_least = 0.220"]  <--- Absolute Lowest Point (t_2)
          |      (6) o [t_6 = 0.280"]
          |      (7) o [t_7 = 0.340"]
          |      (8) o [t_8 = 0.370"]
      -L/2|----- (9) o [t_9 = 0.390"]
          +---------------------------------------------> Thickness (t)
          <----------------- Length L ------------------>

          Average Thickness: t_avg = (1/n) * SUM( t_i )

Step-by-Step Profiling Procedure:

1. Locate Minimum Thickness ($t_2$): Scan the LTA with ultrasonic thickness gauges to pinpoint the absolute lowest thickness reading exclusive of pits ($t_2$).
2. Calculate Critical Length ($L$): Solve $L = 3.7 \sqrt{D \cdot t_2}$. Confirm that $L$ does not exceed the course height.
3. Establish Vertical Profile Line: Establish a straight vertical inspection line of length $L$ passing directly through the lowest thickness point, centered such that $L/2$ extends above $t_2$ and $L/2$ extends below $t_2$.
4. Record Ultrasonic Readings: Divide length $L$ into equal increments (typically 5 to 10 equal divisions spaced 1 to 2 inches apart). Take accurate UT thickness measurements at each division point ($t_1, t_2, \dots, t_n$).
5. Calculate Average Thickness ($t_{\text{avg}}$): Compute the arithmetic mean of all thickness readings along the critical length:

$t_{\text{avg}} = \frac{1}{n} \sum_{i=1}^{n} t_i$

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4. Dual Acceptance Criteria for LTAs

For a Localized Thin Area to be declared fit for continued service without repair or derating, it must simultaneously satisfy two independent acceptance criteria:

Criterion 1: Average Thickness Criterion

$t_{\text{avg}} \ge t_{\text{min}}$ The average thickness along the critical length $L$ must equal or exceed the minimum acceptable shell thickness ($t_{\text{min}}$) calculated in accordance with Section 4.3.3.

Criterion 2: Local Minimum Thickness Criterion (The 60% Rule)

$t_2 \ge 0.60 \times t_{\text{min}}$ The absolute lowest thickness reading at any point within the LTA (exclusive of pits) must equal or exceed 60% of $t_{\text{min}}$.

Failure Response: If either $t_{\text{avg}} < t_{\text{min}}$ OR $t_2 < 0.60 \times t_{\text{min}}$, the LTA is unacceptable. The owner must:

1. Derate the maximum fill height ($H_{\text{max}}$) so that recalculated $t_{\text{min}}$ satisfies both criteria; OR
2. Repair the area via weld overlay (API 653 Section 9.7); OR
3. Install a flush insert plate (Section 9.2) or lap patch plate (Section 9.3).

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5. Proximity to Weld Seams & Joint Efficiency Relaxation

A pivotal engineering rule governing LTA evaluation is the physical distance between the LTA boundary and adjacent weld seams:

+-------------------------------------------------------------------------+
|                    LTA PROXIMITY TO WELD SEAMS                          |
|                                                                         |
|   VERTICAL WELD PROXIMITY:                                              |
|   - LTA Boundary >= 1.0 inch from Vertical Weld:                        |
|     Use JOINT EFFICIENCY E = 1.00 (Solid Plate Rule)!                   |
|   - LTA Boundary < 1.0 inch from Vertical Weld:                         |
|     Must use the JOINT EFFICIENCY E of that Vertical Weld!              |
|                                                                         |
|   HORIZONTAL WELD PROXIMITY:                                            |
|   - LTA must not extend closer than 1.0 inch OR 3 * t (whichever is     |
|     greater) from a horizontal girth weld without special evaluation.   |
+-------------------------------------------------------------------------+

The Solid Plate Rule for Vertical Seams

API 653 4.3.3.1 states the rule precisely: $E = 1.0$ when evaluating the retirement thickness in a corroded plate, when away from welds or joints by at least the greater of 1 in. or twice the plate thickness. Both halves of that clearance test must be applied:

• Clearance $\ge \max(1\text{ in.},; 2t)$: the localized metal loss lies entirely in sound base plate, so $E = 1.00$ may be used in the $t_{\text{min}}$ formula for that LTA, even where the vertical seams in that course carry $E = 0.70$ or $0.85$.
• Clearance $< \max(1\text{ in.},; 2t)$: the stress field interacts with potential weld flaws, and the joint efficiency $E$ of that specific weld seam must be used.
• Worked check: on a 0.625-in. plate the qualifying clearance is $\max(1.0,; 1.25) = 1.25$ in., not 1 in. On a 0.375-in. plate it is $\max(1.0,; 0.75) = 1.0$ in. The $2t$ term governs on plate thicker than 1/2 in.

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6. Complete Worked Numerical Example: LTA Assessment

An API 653 out-of-service inspection evaluates Course 1 of an atmospheric storage tank:

• Nominal Diameter $D = 80\text{ ft}$, Total liquid height $H = 36\text{ ft}$
• Specific Gravity $G = 1.00$, Course height $h_{\text{course}} = 96\text{ in.}$ (8 ft)
• Shell steel: ASTM A283 Grade C ($S = 23,600\text{ psi}$, bottom-and-second-course column of API 653 Table 4.1)
• Vertical seams: Un-radiographed butt welds ($E = 0.70$)
• An LTA is discovered in Course 1. The edge of the LTA is 6 inches away from the nearest vertical weld seam.
• Within the LTA, the absolute minimum thickness measured is $t_2 = 0.220\text{ in.}$

Step 1: Determine Governing Joint Efficiency ($E$)

The qualifying clearance is $\max(1.0\text{ in.},; 2 \times 0.220) = 1.0\text{ in.}$ Because the LTA boundary is 6 inches from the nearest vertical seam, the Solid Plate Rule of API 653 4.3.3.1 applies: $E = 1.00$ for evaluating this LTA.

Step 2: Calculate $t_{\text{min}}$ for the LTA

$t_{\text{min}} = \frac{2.6 \cdot D \cdot (H - 1) \cdot G}{S \cdot E} = \frac{2.6 \times 80 \times (36 - 1) \times 1.00}{23,600 \times 1.00} = \frac{208 \times 35}{23,600} = \frac{7,280}{23,600} = 0.3085\text{ in. } \approx 0.309\text{ in.}$

Step 3: Check Criterion 2 (The 60% Rule for $t_2$)

$t_{\text{limit}} = 0.60 \times t_{\text{min}} = 0.60 \times 0.3085 = 0.1851\text{ in.}$ Actual minimum thickness is $t_2 = 0.220\text{ in.}$ $\text{Check: } 0.220\text{ in.} \ge 0.1851\text{ in.} \implies \mathbf{PASS}$ Criterion 2 is satisfied.

Step 4: Calculate Critical Length ($L$)

$L = 3.7 \sqrt{D \cdot t_2} = 3.7 \sqrt{80 \times 0.220} = 3.7 \sqrt{17.6} = 3.7 \times 4.1952 = 15.52\text{ inches}$ Check the ceiling: API 653 4.3.2.1 caps $L$ at 40 in., and $15.52\text{ in.} \le 40\text{ in.} \implies \mathbf{PASS}$. Use a vertical profile length $L = 15.5\text{ inches}$.

Step 5: Profile Thickness Measurements & Average ($t_{\text{avg}}$)

A vertical line of 16 inches centered on $t_2$ is laid out. Ultrasonic thickness measurements are recorded at 2-inch increments (9 points total):

Sum of thickness readings = $0.390 + 0.370 + 0.340 + 0.280 + 0.220 + 0.270 + 0.330 + 0.360 + 0.380 = 2.740\text{ inches}$.

$t_{\text{avg}} = \frac{2.740}{9} = 0.3044\text{ inches}$

Step 6: Check Criterion 1 (Average Thickness Rule)

$\text{Check: } t_{\text{avg}} = 0.3044\text{ in.} \ge t_{\text{min}} = 0.3085\text{ in.} \implies \mathbf{FAIL!}$ Result: Even though $t_2$ passed the 60% rule, $t_{\text{avg}}$ is less than $t_{\text{min}}$. The LTA fails acceptance criteria for full 36-foot head.

Step 7: Derate Maximum Fill Height ($H_{\text{max}}$)

To keep the tank in operation without executing a physical plate repair, determine $H_{\text{max}}$ based on $t_{\text{avg}} = 0.3044\text{ in.}$:

$H_{\text{max}} = \frac{S \cdot E \cdot t_{\text{avg}}}{2.6 \cdot D \cdot G} + 1 = \frac{23,600 \times 1.00 \times 0.3044}{2.6 \times 80 \times 1.00} + 1 = \frac{7,183.84}{208} + 1 = 34.54 + 1 = 35.54\text{ ft}$

Engineering Decision: The tank is derated to a maximum liquid level of 35.5 feet, or repaired via weld overlay to raise $t_{\text{avg}}$ above $0.309\text{ in.}$