12.4 B31.3 Materials, Design Conditions & Pressure Design of Components

12.4 B31.3 Materials, Design Conditions & Pressure Design of Components

ASME B31.3 supplies 16 of the 60 items on the endorsement, and the AWS test specifications reserve a Materials and Design slice of at least 5% across all three codes. The B31.3 half of that slice is Chapter II Part 2 (pressure design) and Chapter III (materials). As with B31.1, the questions are lookup and nomenclature questions — but the nomenclature is different enough from B31.1's that mixing the two is the most reliable way to lose points.

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1. Design Conditions and Allowable Stresses

Para 301 sets design conditions; Para 302.3 establishes allowable stresses and their source. Three sources feed every pressure calculation:

$W$ has no counterpart in the B31.1 straight-pipe equation and is the single most distinctive feature of B31.3's formula. It penalizes longitudinal and spiral (helical) seam welds at elevated temperature, because the creep strength of a seam weld falls faster than that of the base metal:

[!IMPORTANT] $W$ applies to the longitudinal seam, not the girth weld. A seamless pipe has no longitudinal seam and therefore no $W$ penalty. The CSEF subcritical-PWHT row at $W = 0.5$ is the most dramatic entry in Table 302.3.5 and a favourite exam target: it means a welded Grade 91 seam is credited with half the strength of the base metal at temperature.

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2. Straight Pipe Under Internal Pressure: Para 304.1

The Two Equations

Para 304.1.1(a) starts with the assembly equation:

$t_m = t + c \qquad \text{(Eq. 2)}$

and requires that the minimum thickness $T$ for the pipe selected, considering manufacturer's minus tolerance, shall be not less than $t_m$.

Para 304.1.2(a) then supplies the pressure design thickness $t$, valid for $t < D/6$:

$t = \frac{P D}{2\left(SEW + P Y\right)} \qquad \text{(Eq. 3a)}$

with an inside-diameter alternative:

$t = \frac{P\left(d + 2c\right)}{2\left[SEW - P\left(1 - Y\right)\right]} \qquad \text{(Eq. 3b)}$

The Nomenclature (Para 304.1.1(b))

For $t \ge D/6$, $Y$ is not read from the table but computed as

$Y = \frac{d + 2c}{D + d + 2c}$

The Validity Limit

Para 304.1.2(b) is the boundary condition candidates forget: for $t \ge D/6$ or for $P/SE > 0.385$, calculating the pressure design thickness for straight pipe requires special consideration of factors such as theory of failure, effects of fatigue, and thermal stress. The thin-wall equation simply does not apply to heavy-wall high-pressure geometry.

External Pressure

Para 304.1.3 sends external-pressure design out of B31.3 entirely: wall thickness and stiffening for straight pipe under external pressure follow the procedure of ASME BPVC Section VIII, Division 1, UG-28 through UG-30, using the running centerline length between stiffening points as the design length $L$.

+-----------------------------------------------------------------------------------------+
|                      B31.1 vs. B31.3 STRAIGHT-PIPE NOMENCLATURE                         |
+-----------------------------------------------------------------------------------------+
|                         ASME B31.1 Eq. (3)        |        ASME B31.3 Eqs. (2) + (3a)   |
|  Formula      t_m = PD / (2(SE + Py)) + A         |  t = PD / (2(SEW + PY));  t_m = t+c |
|  Allowance    A, INSIDE the expression            |  c, added OUTSIDE via Eq. (2)       |
|  Coefficient  y, Table 104.1.2(A)                 |  Y, Table 304.1.1                   |
|  Seam factor  E folded into the Appendix A SE     |  E from Table A-1A/A-1B, separate   |
|  Creep factor (none in the equation)              |  W, Table 302.3.5                   |
|  Mill tol.    Added to t_m before ordering        |  T of the pipe selected must be     |
|               (Para 104.1.2(A.1.1))               |  >= t_m after minus tolerance       |
+-----------------------------------------------------------------------------------------+

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3. Other Pressure Design Rules in Para 304

Para 306.5.1 governs fabricated branch connections generally, and Para 306.5.2 adds requirements for branch connections in severe cyclic conditions — the same detail figures (328.5.4E, 328.5.4F, 328.5.5) that reappear in the Category M and severe cyclic examination rules.

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4. Materials: Chapter III (Para 323) and Components (Table 326.1)

Para 323 governs material selection and limitations, and its structure is worth memorizing because each sub-paragraph answers a different examinable question:

• 323.1 General Requirements: Distinguishes listed materials (those in Table A-1 or a listed specification) from unlisted materials, which may be used only when they conform to a published specification covering chemistry, physical and mechanical properties, method and process of manufacture, heat treatment, and quality control, and when allowable stresses are determined per the applicable Para 302.3 criteria.
• 323.2 Temperature Limitations: Sets upper and lower temperature limits, including the low-temperature toughness requirements and exemption curves.
• 323.3 Impact Testing: Specifies when Charpy V-notch testing is required and the acceptance criteria. When it is invoked, every Section IX supplementary essential variable becomes an essential variable — the connection back to Chapter 4.2 of this guide.
• 323.4 Fluid Service Requirements for Materials: Adds restrictions for specific material classes in specific fluid services.

Table 326.1 lists the component standards — ASME B16.5 flanges, B16.9 buttwelding fittings, B16.11 forged fittings, B16.25 buttwelding ends, B36.10M and B36.19M dimensions, and the rest — that are acceptable for B31.3 construction. Components built to a listed standard carry that standard's pressure-temperature rating; anything else is an unlisted component requiring qualification under Para 304.7.2.

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5. Worked Example and Exam Traps

Worked Example: A Welded-Seam Line at Temperature

An NPS 10 ($D = 10.75\text{ in.}$) longitudinally welded Grade 91 line operates at $P = 400\text{ psig}$ and 1,100°F. Suppose Table A-1 gives $S = 8{,}000\text{ psi}$ at temperature, Table A-1B gives $E = 1.00$ for the seam, Table 302.3.5 gives $W = 0.5$ because the seam received a subcritical PWHT, Table 304.1.1 gives $Y = 0.4$, and the specification sets $c = 0.0625\text{ in.}$

$t = \frac{400 \times 10.75}{2\left(8{,}000 \times 1.00 \times 0.5 + 400 \times 0.4\right)} = \frac{4{,}300}{2\left(4{,}000 + 160\right)} = \frac{4{,}300}{8{,}320} = 0.517\text{ in.}$

$t_m = t + c = 0.517 + 0.0625 = 0.579\text{ in.}$

Check the validity limit: $D/6 = 10.75/6 = 1.79\text{ in.}$, and $t = 0.517\text{ in.} < 1.79\text{ in.}$, so Eq. (3a) applies. Then confirm that the selected pipe's minimum wall, after the manufacturer's minus tolerance, is at least 0.579 in.

Now notice what $W$ did. Had the seam not been penalized ($W = 1.0$), the denominator would be $2(8{,}000 + 160) = 16{,}320$ and $t$ would be 0.263 in. The weld joint strength reduction factor doubled the required wall. That is the practical reason inspectors care about a design factor.

Common Exam Traps to Avoid

• Dropping $W$: B31.1's equation has no $W$. B31.3's does. Omitting it from Eq. (3a) under-sizes the wall wherever a longitudinal or spiral seam runs at elevated temperature.
• Putting $c$ Inside the Fraction: B31.3 computes $t$ first and then adds $c$ through Eq. (2). B31.1 adds $A$ inside the same expression. Both give a minimum required thickness, but the algebra differs.
• $c$ Is Not Just Corrosion: It is mechanical allowances (thread or groove depth) plus corrosion and erosion. For threaded components use the nominal thread depth of ASME B1.20.1, and for unspecified machining tolerances add 0.02 in. (0.5 mm) to the specified cut depth.
• Ignoring the $D/6$ and $P/SE > 0.385$ Limits: Beyond either limit, Para 304.1.2(b) requires special consideration, not the thin-wall formula.
• Using $Y$ From the Table Outside Its Range: Table 304.1.1 values are valid for $t < D/6$; above that, $Y$ is computed from $(d + 2c)/(D + d + 2c)$.
• External Pressure: B31.3 does not have its own external-pressure equation — Para 304.1.3 sends you to Section VIII, Division 1, UG-28 through UG-30.
• Listed vs. Unlisted: A component built to a Table 326.1 standard carries that standard's rating. An unlisted component must be qualified under Para 304.7.2 by analysis, proof test, or documented experience.