Flange Rating Standards | Industrial Engineering Studio

Flange Standards Overview

Pipe flanges provide a removable, pressure-tight joint between piping components. The primary standard is ASME B16.5 (formerly ANSI B16.5) for NPS ½" through 24", and ASME B16.47 for larger diameters (26"–60").

This guide covers B16.5, the most common standard in oil/gas, chemical, and power industries worldwide.

Pressure Classes

Flanges are designated by "class" (formerly called "pound rating"):

Class	Approximate Working Pressure (psi) at Ambient Temp
150#	290 psi (~20 bar)
300#	740 psi (~50 bar)
400#	1000 psi (~69 bar)
600#	1480 psi (~100 bar)
900#	2220 psi (~153 bar)
1500#	3705 psi (~255 bar)
2500#	6170 psi (~425 bar)

Ratings Are Temperature-Dependent

These are NOT "pressure at temperature" values! Ratings decrease significantly as temperature increases. A Class 150 A105 flange is rated 290 psi at 38°C but only 100 psi at 425°C. Always consult the B16.5 pressure-temperature tables for your design conditions.

Pressure-Temperature Ratings for A-105 (Carbon Steel) Flanges

Temp (°C)	Class 150 (bar)	Class 300 (bar)	Class 600 (bar)	Class 900 (bar)
38	19.6	51.1	102.1	153.2
100	17.7	46.6	93.2	139.8
200	13.8	43.8	87.6	131.3
300	9.3	40.2	80.3	120.5
400	6.9	33.9	67.7	101.6
425	5.9	27.7	55.5	83.2

Note how Class 150 drops to just 6.9 bar at 400°C — this surprises many engineers.

Material Groups

Ratings vary by material (Group 1.1 through 1.18). Carbon steel (A-105) is Group 1.1; stainless (304/316) is Group 2.2; chrome-moly is different groups. Each material group has its own pressure-temperature table.

Flange Facing Types

The flange face determines gasket type and sealing reliability.

Facing Type	Description	Application
Raised Face (RF)	1.6mm (150-300#) or 6.4mm (400-2500#) raised lip; serrated finish	Most common; general process service
Flat Face (FF)	Full face flat; no raised lip	Cast iron flanges, low-pressure; matches CI valves
Ring Type Joint (RTJ)	Groove cut into face holds metal ring gasket	High pressure/temperature; critical service
Male-Female / Tongue-Groove	Interlocking faces; gasket confined	Heat exchangers, special applications

Never Mix RF and FF Flanges!

Connecting a raised-face flange to a flat-face flange concentrates the bolt load on the raised ring area and can crack cast iron or create leaks. Use matching facing types on both sides.

Flange Face Finishes

The surface roughness affects gasket sealing:

Serrated (phonographic): Standard; 125-500 µin Ra; holds soft gaskets
Smooth (cold water): 63-125 µin Ra; for spiral-wound and RTJ gaskets
RTJ groove: Machined to very fine finish for metal ring gasket

Common Flange Types

Type	Description
Weld Neck (WN)	Long tapered hub, butt-welded to pipe; best for high pressure, cyclic service; full penetration weld
Slip-On (SO)	Slips over pipe, fillet-welded inside and out; cheaper; two welds
Socket Weld (SW)	Pipe inserts into socket, fillet-welded; small sizes only (<4"); high pressure
Threaded (NPT)	Screws onto pipe; no welding; for small sizes, low pressure, galvanized
Blind (BL)	Blank flange for closing ends; no bore
Lap Joint (LJ)	Loose flange with stub end; for easy alignment; expensive alloys (use carbon steel flange)
Orifice	WN or SO with extra tappings for flow measurement

Gasket Selection

The gasket fills flange face imperfections to create a seal. Selection depends on facing type, pressure, temperature, and fluid.

Gasket Types

Gasket Type	Face Type	Pressure Class	Temp Range	Best For
Compressed non-asbestos (CNA)	RF	150-300#	-50 to 400°C	General water/oil service
Spiral-wound (SWG)	RF	All classes	-200 to 600°C	Process piping, steam, chemical
Ring Type Joint (RTJ)	RTJ	600#+	to 700°C+	High pressure/temp, critical service
Double-jacketed (DJ)	RF	All classes	to 650°C	Heat exchangers
PTFE envelope	RF/FF	Low-moderate	to 200°C	Chemical, food/pharma
Rubber (EPDM, Buna-N)	FF	150# only	-40 to 120°C	Water, low temp

Asbestos Gaskets Are Banned

Compressed asbestos gaskets were standard for decades but are now banned in most countries due to health hazards. Use compressed non-asbestos (CNA) with aramid fiber or other modern substitutes.

Spiral-Wound Gasket Details

Most common in process piping (300# and above):

V-shaped metal strip wound with filler (graphite, PTFE, or mica)
Color-coded outer ring: grey (CS), yellow (304), green (316)
Inner ring required on RTJ and for high vacuum
Requires controlled bolt torque

Bolt Torque

Proper bolt tightening is critical for leak-free flange joints:

Use correct bolt material (B7 studs most common; B8 for stainless, B16 for high temp)
Use lubricant (anti-seize) on bolt threads
Tighten in crisscross (star) pattern in 3 passes: 30%, 60%, 100% of target torque
Target gasket stress depends on gasket type (see ASME PCC-1 guidelines)
Consider hot-bolting for high-temperature services after heat-up

T = K × D × F / 1000 (N·m) — where K=nut factor (~0.2), D=bolt diameter (mm), F=bolt force (kN)

ASME PCC-1 is the Standard

ASME PCC-1 "Guidelines for Pressure Boundary Bolted Flange Joint Assembly" provides comprehensive procedures for flange assembly, bolt torque, and quality. It is the industry standard for achieving leak-free joints.

Flange Standards Worldwide

Standard	Region	Equivalent Class
ASME B16.5	US/international	150#, 300#, 600#, etc.
EN 1092-1 (PN)	Europe	PN16, PN40, PN64, PN100
JIS B2220	Japan	10K, 20K, 30K
BS 1560	UK (now withdrawn)	Same as ASME

PN ratings are in bar at ambient temperature: PN16 = 16 bar, PN40 = 40 bar.

Summary

ASME B16.5 flange classes (150# through 2500#) define pressure-temperature ratings that decrease significantly at elevated temperature. Raised face with spiral-wound gaskets is the most common process piping configuration for 300# and above. RTJ gaskets are used for 600# and high-temperature service. Proper bolt torque following ASME PCC-1 crisscross patterns is essential for leak-free service. Always verify pressure-temperature ratings from B16.5 tables at your actual design temperature.

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