What is Barlow's formula?

Barlow's formula calculates minimum wall thickness for pipes under internal pressure: t = P × D / (2 × S × E), where P = internal pressure, D = outside diameter, S = allowable stress, E = weld joint efficiency factor.

What allowable stress should I use?

Allowable stress depends on material and temperature. For carbon steel (A106 Gr.B) at ambient: ~138 MPa (20,000 psi). For stainless 316 at ambient: ~138 MPa. Always refer to ASME B31.3 Table A-1 for design values.

What is the weld joint efficiency factor?

Weld efficiency (E) accounts for joint quality: E = 1.0 for seamless pipe, E = 0.85 for ERW (electric resistance welded), E = 0.60 for furnace butt welded. Higher E requires more inspection (radiography).

Do I need to add a corrosion allowance?

Yes. Add corrosion/erosion allowance (typically 1.5–3 mm for carbon steel in process service) to the calculated minimum wall thickness. Also consider manufacturing tolerance (typically -12.5% for seamless pipe).

What is the difference between Barlow and ASME B31.3?

Barlow's formula is a simplified version. ASME B31.3 uses t = PD/(2(SE+PY)), where Y is a temperature coefficient. For most cases at moderate temperatures, Y ≈ 0.4. The ASME formula gives slightly thinner walls.

免費工程工具 — #154

管壁厚度計算器 — Barlow’s 公式

使用 Barlow’s 公式計算管道在內部壓力下的最小壁厚：t = P×D/(2×S×E)。包含腐蝕裕量與公差。

t = PD/(2SE)ASME B31.3腐蝕裕量

結果

最小必要壁厚（含腐蝕裕量與公差）

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計算 t (Barlow)

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+ 腐蝕裕量

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內徑

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Barlow’s 公式

含腐蝕裕量與公差

P — 設計壓力
D — 管道外徑
S — 設計溫度下的允許應力
E — 焊縫係數（1.0 無縫管，0.85 ERW，0.60 FBW）
c — 腐蝕/侵蝕裕量
tol — 製造公差（通常為 12.5%）

範例 — DN100 於 10 MPa

已知： OD = 114.3 mm, P = 10 MPa, S = 138 MPa, E = 1.0

t = 10 × 114.3 / (2 × 138 × 1.0) = 4.14 mm

+ 1.5 mm 腐蝕裕量 = 5.64 mm

÷ (1 − 0.125) = 6.45 mm 最小值

為此應用選擇 Sch 80（壁厚 8.56 mm）。

⚠️ 注意： 此為 Barlow’s 簡化公式。如需符合規範，請使用包含 Y 係數的 ASME B31.3 公式 3a。請務必選擇高於計算最小值的下一個標準管壁等級。