About the author

Ma Yajiada, Chief Engineer at Hebei Yajiada Import and Export Co., LTD. 20 years experience in flange metallurgy (Cangzhou, Hebei since 2006). Specializes in ASME B16.5 / B16.47 / EN 1092-1 standards and PED 2014/68/EU compliance.
B.Sc. Materials Engineering (Yanshan University); PED 2014/68/EU Module H Notified Body auditor; 20 years flange metallurgy specialization.

Editorial review: Ma Yajiada, PE. Article published 2026-08-03. Last updated 2026-07-28.

6 frequently asked questions

1. What is the difference between ASME Class 150 and Class 300?

Class 150 is rated 285 PSI (19.6 bar) at 38°C in carbon steel; Class 300 is rated 740 PSI (51.1 bar). Class 300 flanges are thicker, larger diameter, and use more (or larger) bolts. Typical uses: Class 150 for water, air, HVAC; Class 300 for general process and low-pressure oil & gas.

2. Why can't I order ASME B16.5 NPS 28?

ASME B16.5 only covers NPS 1/2 to NPS 24 (DN 15 to DN 600). Sizes NPS 26 to NPS 60 are governed by ASME B16.47 (Series A from MSS-SP44 or Series B from API 605). For larger sizes, you must specify B16.47 Series A or B — not B16.5.

3. Can I use A105 at low temperature?

No — ASTM A105 is not impact-tested for sub-zero service. ASME B16.5 limits A105 (Group 1.1) to -29°C minimum. For -29°C to -46°C, use A350 LF2 CL1. For -46°C to -60°C, use A350 LF6 CL1/CL2. For LNG service (-100°C), use A350 LF787 or austenitic stainless F304/F316L.

4. What is the maximum NPS for Class 2500?

ASME B16.5 Class 2500 covers NPS 1/2 to NPS 12 maximum. NPS 14 and above at Class 2500 do not exist in B16.5 — you would need B16.47 or custom design. In practice, Class 2500 NPS 10-12 are extremely rare; most projects cap at NPS 8.

5. What stud bolt material should I use for ASME B16.5 flanges?

For carbon steel flanges: B7 studs with 2H nuts (ASTM A193 / A194). For stainless steel flanges: B8 (AISI 304) or B8M (AISI 316) studs. Never use B7 with stainless flanges — galvanic corrosion occurs. For sour service (NACE MR0175), use B7M, B8M, or B16 depending on the alloy and chloride level.

6. How do I verify a flange is truly ASME B16.5 compliant?

Check three documents: (1) Mill Test Certificate EN 10204 3.1 or 3.2 with material chemistry and mechanical results; (2) Dimensional Inspection Report confirming OD, BC, bore, thickness, facing dimensions per B16.5 tolerances; (3) Hard stamp or label on the flange showing standard, class, NPS, material, manufacturer name or trademark. Our shipments include all three.

ASME B16.5 Complete Specification Guide: Classes, Sizes, Materials & Tolerances