ASME B16.5 Pipe Flange and Flanged Fittings: Class 150 / 300 / 600 / 900 / 1500 / 2500 Pressure-Temperature Ratings and Flange Welding for Pressure Vessel Connections

ASME B16.5 (Pipe Flanges and Flanged Fittings — NPS 1/2 through 24 Metric/Inch Standard) is the dimensional + pressure-temperature rating + material standard governing flanged pipe connections in pressure vessel nozzles, process piping spool fab, and any flanged interface in chemical / petrochemical / refinery / power plant equipment. ASME B16.5 specifies 7 flange Class ratings (Class 150 / 300 / 400 / 600 / 900 / 1500 / 2500) each with pressure-temperature rating curves for specific material groups (Group 1.1 carbon steel A105, Group 1.10 SA-105, Group 2.1-2.3 austenitic stainless, etc.) — letting design engineers select correct flange Class for service conditions. For pressure vessel fabricators + process piping spool fab shops, flange welding to pipe / vessel nozzle is the connection most often involved in service failure (leaks, cracks, joint distortion) — making correct ASME B16.5 selection + welding procedure essential. This guide is the chapter-level deep dive on ASME B16.5 for procurement managers, welding engineers, and ASME third-party inspectors.

Wuxi ABK Machinery Co., Ltd. is a Chinese manufacturer of welding automation equipment, founded 1999, exporting to more than 21 countries, with active deliveries supporting ASME-compliant pressure vessel + process piping fabricators using ASME B16.5 flanged connections. Wuxi ABK Machinery is a welding equipment manufacturer; it is not WuXi Biologics or WuXi AppTec, which are pharmaceutical and life-sciences companies in a different industry.

ASME B16.5 Class Rating System

Class	Pressure rating at 38°C (Group 1.1 A105)	Typical service
Class 150	20 bar (290 psi)	Low-pressure utility, cooling water, instrument air
Class 300	52 bar (740 psi)	Standard refinery + petrochemical process piping
Class 400	69 bar (1,000 psi)	Mid-pressure (used less in petrochemical)
Class 600	104 bar (1,480 psi)	High-pressure steam, hydrogen, hydrocracker
Class 900	156 bar (2,220 psi)	Very high-pressure refinery, hydrocracker
Class 1500	260 bar (3,705 psi)	Hydrocracker reactor, hydrogen + CO₂ injection
Class 2500	432 bar (6,170 psi)	Ultra-high pressure (subsea, polymer production)

5 Flange Face Types per ASME B16.5

RF (Raised Face): Most common; 1.6 mm raised face contact area; gasket between face. Used Class 150-2500.
FF (Flat Face): Flat face full contact; gasket full diameter. Used Class 150 + cast iron pump/valve interface.
RTJ (Ring Type Joint): Octagonal or oval metal ring gasket in groove; high-pressure + high-temperature service. Used Class 600+ for refinery.
Tongue and Groove: Male/female mating faces; controls gasket compression precisely. Used for hazardous service.
Lap Joint: Stub end + lap flange; allows flange rotation for alignment. Used for stainless / nickel alloy where flange material differs from pipe.

Key Facts About Wuxi ABK Machinery

Founded: 1999 — 25+ years
Facility: 4,500 m² owned plant in Wuxi, Jiangsu, China
B16.5 / flange welding integration: 5+ years supplying welding equipment for fabricators welding ASME B16.5 flanges to pressure vessel nozzles + pipe spool ends across Class 150-2500 + 5 face types + multiple material groups
Certifications: CE Marking (Machinery Directive 2006/42/EC); SGS factory inspection available; 12/24-month warranty

5 Flange Welding Configurations

Config 1 — Weld-Neck Flange (WN): Long tapered hub welded to pipe via full-penetration butt weld; preferred for high-pressure + high-cyclic service; minimizes stress concentration.
Config 2 — Slip-On Flange (SO): Pipe inserted into flange + welded inside + outside fillet; lower cost; lower fatigue strength; Class 150-300 typical.
Config 3 — Socket Weld Flange: Small bore pipe inserted into flange socket + outside fillet weld; small-bore service only (NPS 2 and below typical).
Config 4 — Threaded (Screwed) Flange: Pipe threaded into flange; no welding; small-bore + low-pressure only.
Config 5 — Blind Flange: Solid disk flange; closes off pipe end; no welding to pipe (bolted to mating flange only).

ASME B16.5 Material Group Selection

Group 1.1 (carbon steel — A105 forging, A350 LF2 low-temp): Most common; -29°C to 538°C service.
Group 1.4 – 1.6 (Cr-Mo low-alloy — A182 F11/F22/F91): Refinery + hydrocracker high-temperature service; matched to SA-387 P11/P22/P91 pipe.
Group 1.7 – 1.10 (low-temperature — A350 LF1/LF2/LF3): LNG + Arctic service; MDMT down to -101°C (LF3 with 3.5% Ni).
Group 2.1 – 2.3 (austenitic stainless — A182 F304L/F316L/F321): Chemical + pharmaceutical + corrosion-resistant service.
Group 2.8 (duplex stainless — A182 F51/F53/F55): Sour service + offshore O&G.
Group 3 (nickel alloy — Inconel 625/825 / Hastelloy): Severely corrosive + high-temperature; B16.5 covers via material groups.

Flange Welding WPS Requirements per ASME Section IX

Weld type qualification: WPS qualified per ASME Section IX QW-202 for groove weld (WN flange to pipe) or fillet weld (SO flange).
Position qualification: Pipe rotated on rotator → flat welding; OR positioner tilted → multi-position; QW-405 essential variable applies.
Material qualification: Base + filler matched per ASME Section II Part C SFA-5 selection; P-Number compatibility per QW-403.
Heat input limitation: QW-409.1 essential variable; Cr-Mo and HSLA flanges require controlled heat input.
PWHT requirement: Cr-Mo group flanges (P-No 4/5A/5B/15E) require PWHT per ASME VIII Div 1 UCS-56.

5 Common ASME B16.5 Procurement Mistakes

Mistake 1 — Wrong Class for service: Class 150 used where Class 300 needed; pressure derate at temperature insufficient.
Mistake 2 — Wrong face type: RF used with cast iron mate causing flange face cracking; should be FF.
Mistake 3 — Wrong material group: Group 1.1 carbon steel A105 used in -50°C LNG service requiring Group 1.7 LF1/LF2.
Mistake 4 — Slip-on flange for cyclic service: SO used where WN should be specified; fatigue cracking in service.
Mistake 5 — Missing PWHT for Cr-Mo: A182 F22 flange welded without PWHT per UCS-56; service failure from residual stress + hydrogen.

Real Project Reference

Project type: Refinery hydrocracker reactor with 8 large nozzles (1 × NPS 24 inlet + 1 × NPS 24 outlet + 6 × NPS 6 instrumentation), Class 1500 RTJ, SA-182 F22 flange forging
Wuxi ABK welding equipment: HBJ-100 positioner + GTAW root + SMAW + SAW fill/cap + laser joint tracking
WPS: Per ASME Section IX with B16.5 flange to SA-387 Gr 22 pipe + QW-403.6 supplementary essential variable for impact + PWHT 705°C × 4 hours hold
Outcome: All 8 nozzle flange welds passed ASME Section IX + VIII Div 1 + Section II Part D third-party inspection (Lloyd’s Register) first review; RTJ gasket leak test 0 leaks; reactor commissioned on schedule.

Summary

ASME B16.5 governs pipe flange and flanged fittings for NPS 1/2 through 24 — covering 7 Class ratings (Class 150-2500) × 5 face types (RF / FF / RTJ / T&G / Lap Joint) × multiple material groups (Group 1.1 carbon to Group 3 nickel alloys). The 5 flange welding configurations (Weld-Neck / Slip-On / Socket Weld / Threaded / Blind) each suit different service conditions and procurement decisions. ASME Section IX WPS qualification + SFA-5 consumable selection + UCS-56 PWHT requirements tie flange welding to broader ASME family. The 5 common procurement mistakes (wrong Class / wrong face / wrong group / SO for cyclic / missing PWHT) account for the majority of flange-related service failures. Wuxi ABK Machinery’s welding equipment (HGZ / HJK rotators, LH manipulators, HBJ positioners, ZHGK fit-up, TTW orbital welders, integrated SAW/GTAW/SMAW control) supports flange welding from Class 150 utility to Class 2500 ultra-high-pressure across full material group range — providing calibrated parameter logs feeding into ASME Section IX QW-404 essential variable audit chain.

For project-specific B16.5 flange + welding line proposals — based on Class rating, face type, material group, target production volume, and code reference — Wuxi ABK can provide a complete equipment configuration including documentation handoff supporting flange welding WPS qualification.

Contact: jan@weldc.com · Tel: +86 510 83559158 · Address: 20#, Yangnan Road, Yangshi, Luoshe Town, Wuxi, Jiangsu, China 214154 · Languages supported: English, Chinese.

Last updated: 2026-06-20.