ASME B31.3 Hydrostatic & Pneumatic Test Pressure Calculations

Leaving the $S_t/S$ temperature correction out of an ASME B31.3 hydrotest pressure calculation under-pressurizes hot lines tested cold — hydro and pneumatic alike. Compute $P_t$ in the live Hydro/Pneumatic Test Pressure calculator before writing the test pack.

Quick Summary (TL;DR)

• Takeaway: Minimum $P_t = k\cdot P\cdot(S_t/S)$ with $k = 1.5$ (hydrostatic) or $1.1$ (pneumatic); FEK caps $S_t/S$ at 6.5 and stores $P$ in MPa (metric) or psi (imperial) — not bar.
• Governing relation: $P_t = 1.5\,P\,(S_t/S)$ hydro · $P_t = 1.1\,P\,(S_t/S)$ pneumatic (FEK screening per ASME B31.3 Para. 345.4.2).
• Code / basis: ASME B31.3 Para. 345.4 / 345.5 · Table A-1 allowables · site $\sigma_h \le 0.90\,S_y$ (callout only — not solved in-app).

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ASME B31.3 hydrotest pressure calculation in the field

Hydrostatic leak tests prove joints with water; pneumatic tests use gas and store far more energy at the same pressure. When design metal temperature exceeds test temperature, both paths need the temperature stress ratio $S_t/S$.

Use Hydro/Pneumatic Test Pressure for $P_t$, $S_t/S$, and the NPS holding-time guide. Pair wall selection with ASME B31.3 Pipe Thickness and class limits with Flange Pressure-Temperature Rating. For gas tests, also screen exclusion distance with Pneumatic Test Safety Distance.

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Core formulas & parameter definitions

Hydrostatic and pneumatic $P_t$ (FEK engine)

$$P_t = 1.5\,P\,\frac{S_t}{S}\quad\text{(hydrostatic)}$$ $$P_t = 1.1\,P\,\frac{S_t}{S}\quad\text{(pneumatic)}$$ $$\frac{S_t}{S} \le 6.5\quad\text{(FEK field screening cap)}$$

Unit note: Metric FEK inputs store $P$, $S$, and $S_t$ in MPa (same scale as allowables). Convert gauge bar with $P_{\mathrm{MPa}} = P_{\mathrm{bar}}/10$. Entering 20 instead of 2.0 overstates $P_t$ by 10×.

Difference between hydrostatic 1.5× and pneumatic 1.1×

Field sheets that use $1.1P$ without $S_t/S$ understate pneumatic $P_t$ on hot lines tested cold (see worked example: 22.0 vs 23.81 bar g).

Maximum allowable test pressure vs yield (site check)

ASME B31.3 expects test stress not to exceed yield at test temperature. A common screen is:

$$\sigma_h = \frac{P_t\,D}{2\,t_{\mathrm{act}}} \le 0.90\,S_y$$

with $t_{\mathrm{act}}$ often $0.875\times t_{\mathrm{nom}}$ for 12.5% mill under-tolerance. FEK shows a yield-limit callout; it does not solve $\sigma_h$.

Parameter definitions

FEK NPS holding-time guide

Code minimum hold is ≥ 10 min; FEK bands are a stricter field guide.

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How to calculate ASME B31.3 hydrostatic test pressure with temperature stress ratio

High-temperature process line — worked example

ASTM A106 Grade B NPS 6 Sch 40 line. Design $P = 20.0\text{ bar g}$ → enter $2.0\text{ MPa}$ in FEK, $T_{\mathrm{des}} = 300^\circ\mathrm{C}$, hydrostatic water at $T_{\mathrm{test}} = 20^\circ\mathrm{C}$.

Step 1 — Stress ratio $S_t/S$

$$\frac{S_t}{S} = \frac{138.0}{127.5} = 1.08235\quad(\le 6.5)$$

Step 2 — Minimum hydrostatic $P_t$

$$P_t = 1.5 \times 2.0 \times 1.08235 = \mathbf{3.247\text{ MPa}} = \mathbf{32.47\text{ bar g}}$$

FEK hero displays 3.25 MPa for this duty.

Step 3 — Alternative pneumatic $P_t$ (same $S_t/S$)

$$P_{t,\mathrm{pneu}} = 1.1 \times 2.0 \times 1.08235 = \mathbf{2.381\text{ MPa}} = \mathbf{23.81\text{ bar g}}$$

Step 4 — Holding time and site yield screen

NPS 6 → FEK guide 60 minutes at $P_t$, then examine per procedure.

Optional site hoop check (not an in-app output), using ASME B36.10M NPS 6 Sch 40 $D = 168.28\text{ mm}$, $t_{\mathrm{nom}} = 7.11\text{ mm}$, $t_{\mathrm{act}} = 0.875\times 7.11 = 6.22\text{ mm}$:

$$\sigma_h = \frac{3.247\times 168.28}{2\times 6.22} \approx 44\text{ MPa} \le 0.90\times 240 = 216\text{ MPa}$$

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Interactive tool CTA

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Frequently Asked Questions (FAQ)

Q1. How to calculate ASME B31.3 hydrostatic test pressure with temperature stress ratio?

Enter design pressure $P$ in MPa (metric FEK), set $S_t$ and $S$ from Table A-1 (or enter St/S directly in Advanced 1.2), and compute $P_t = 1.5\,P\,(S_t/S)$ with the ratio capped at 6.5. Reproduce the worked duty via the tip link above.

Q2. What is the difference between hydrostatic leak test 1.5× and pneumatic test 1.1× per B31.3 Para. 345?

Hydrostatic uses multiplier 1.5; pneumatic uses 1.1. In FEK both multiply by $S_t/S$. Pneumatic stores much more energy — treat it as a controlled hazardous operation with written authorization and barriers, not a “lighter hydro.”

Q3. What is the maximum allowable hydrostatic test pressure avoiding yield stress failure?

B31.3 expects test stress not to exceed yield at test temperature. A common screen is $\sigma_h = P_t D/(2 t_{\mathrm{act}}) \le 0.90\,S_y$. FEK warns on yield but does not compute $\sigma_h$ — confirm with actual $D$ and $t_{\mathrm{act}}$ on the isometric / mill cert.

Q4. Why does FEK ask for MPa when my isometric shows bar?

Because the hydro calculator’s metric unit is MPa (consistent with stress inputs). Convert with $P_{\mathrm{MPa}} = P_{\mathrm{bar}}/10$. Entering 20 as if it were MPa would overstate $P_t$ by 10×.