ASME B16.5 Flange Pressure-Temperature Rating Guide

Operating pressure alone is never enough to select a flange. ASME B16.5 flange pressure-temperature rating tables set the temperature-adjusted maximum allowable working pressure (MAWP) for NPS ½–24 flanges by material group and class.

Quick Summary (TL;DR)

• Takeaway: Flange MAWP falls as metal temperature rises — Class is a designator, not a fixed psi/bar rating.
• Governing relation: $P = P_1 + \frac{(T - T_1)}{(T_2 - T_1)}(P_2 - P_1)$ between Table 2 nodes; $P_{\mathrm{hydro}} = 1.5 \times P_{\mathrm{ambient}}$.
• Code / basis: ASME B16.5 Table 2 (Groups 1.1–3.19); FEK Phase-1 screening covers Groups 1.1 and 2.2.

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Why ASME B16.5 flange pressure-temperature rating matters

A Class 150 mark on the hub does not mean “always 285 psi.” At elevated temperature the allowable working pressure from ASME B16.5 Table 2 drops with material strength. Specifying class from ambient line pressure alone is a common FEED error: the joint may be overstressed (or under-rated) once design metal temperature is applied.

For geometry, stud count, and mated-pair weight after the rating check, continue with the Blind Flange Thickness Calculator or Pipe Schedule & Dimension Lookup.

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Core equations and methodology

Linear interpolation between Table 2 nodes

When design temperature $T$ falls between published temperatures $T_1$ and $T_2$ with ratings $P_1$ and $P_2$:

$$P = P_1 + \frac{(T - T_1)}{(T_2 - T_1)} \times (P_2 - P_1)$$

FEK evaluates the interpolation in °C / bar on the Phase-1 extract, then displays dual units.

Hydrostatic shell test pressure

Per ASME B16.5 practice for shell hydrostatic test at ambient conditions:

$$P_{\mathrm{hydro}} = 1.5 \times P_{\mathrm{ambient}}$$

where $P_{\mathrm{ambient}}$ is the Table 2 rating for the −29…38 °C band — not $1.5\times$ the derated elevated-temperature MAWP.

Parameter definitions

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Flange rating curve — Group 1.1 carbon steel scan

The flange rating curve (P vs $T$) for Group 1.1 (A105 / A106 Gr.B / A350 LF2) shows how working pressure ASME B16.5 derates with temperature. Values below match the FEK Phase-1 engine (data/piping/b16_5_pt_ratings.json).

Group 2.2 (A182 F316 / F316L) ambient Class 150 is 19.0 bar — slightly below carbon steel for the same class — and holds more capacity at high temperature than Group 1.1 Class 150 (e.g. 9.8 bar at 538 °C vs 1.4 bar).

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Worked example: maximum allowable working pressure flange check @ 200 °C

Field scenario

Refinery hydrotreater feed flange:

Table 2-1.1 (Phase-1): at 200 °C, Class 150 → $P_2 = 14.0$ bar (exact node — no interpolation required).

Step 1 — Read MAWP

$$P_{\mathrm{MAWP}} = 14.0\ \mathrm{bar}$$

Step 2 — Compare to operating pressure

• Operating $P = 12.5$ bar ≤ $14.0$ bar → PASS
• Margin $= 14.0 - 12.5 =$ 1.5 bar
• Hydro shell test $= 1.5 \times 19.6 =$ 29.4 bar

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Interactive engineering tool

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Frequently Asked Questions (FAQ)

Why does Class 150 flange pressure rating temperature derating look so steep above 200 °C?

Class 150 flanges are geometrically lighter (thinner rings, lower bolt loads). ASME B16.5 therefore applies aggressive Table 2 derating as temperature rises — Group 1.1 Class 150 falls from 19.6 bar ambient to 14.0 bar at 200 °C and 1.4 bar at 538 °C. That is intentional code behavior, not a calculator bug.

How should dual certified flange rating (316/316L) be handled?

Dual-certified 316/316L forgings must follow the governing B16.5 material group on the MTR and design basis. FEK Phase-1 Group 2.2 covers typical A182 F316 / F316L screening. If the project assigns a different group (e.g. low-carbon high-temperature notes), use that group’s Table 2 — do not assume ambient Class 150 carbon-steel numbers apply to stainless.

Is Class the same as maximum allowable working pressure?

No. Class is a dimensionless designator. MAWP comes from ASME B16.5 Table 2 by material group and temperature. Ambient Group 1.1 Class 150 ≈ 19.6 bar (285 psi customary); the same flange at 200 °C is only 14.0 bar.

Does this guide cover ASME B16.47 large-diameter flanges?

No. ASME B16.5 covers NPS ½–24. Large-diameter steel flanges follow ASME B16.47 Series A/B with separate P-T tables. Do not extrapolate B16.5 ratings to B16.47 geometries.

Why might customary psi values differ slightly from bar × 14.5038?

ASME publishes metric and US customary Table 2 columns. FEK Phase-1 stores the metric (bar) extract and converts for display; published customary cells (e.g. 285 / 740 / 275 psi at ambient for the cases above) can differ by a psi or two from a pure conversion. Stamp design from the unit system required by the project specification.