ASME B16.5 · Pipe flanges

ASME B16.5 flange pressure classes, explained

What Class 150 through 2500 really mean, how material groups and temperature set the allowable pressure, and how to read the rating tables — with a worked example.

Need the rating tables? Get ASME B16.5 from ASME →

What the class number is — and isn't

Idea	What it means	Field takeaway
Class designations	150, 300, 400, 600, 900, 1500, 2500	A dimensionless rating family — not a psi number
Allowable pressure depends on temperature	Every class has a pressure rating that drops as temperature rises	A flange rated for a line at ambient may be under-rated at 600 °F
Allowable pressure depends on material group	Flange material is assigned to a group; each group has its own rating table	Carbon steel and stainless of the same class are not rated the same
Higher class = heavier flange	Thicker flange, bigger or more bolts, different bolt circle	Class 150 and 300 flanges do not bolt together
Size range	B16.5 covers NPS 1/2 through NPS 24	Larger sizes are in B16.47 (Series A and B, which differ)
Facing	Raised face, flat face, ring-type joint	Match the facing and gasket; don't bolt raised face to a flat-face cast-iron flange

Our own summary, not the ASME B16.5 text. Get pressure-temperature ratings from ASME B16.5 (asme.org), follow the piping specification and line class, and verify against the edition your contract references. Not engineering advice.

How to read a B16.5 pressure-temperature rating table, step by step

Step	What you do	Watch out for
1	Identify the flange material spec and grade (e.g. forged carbon steel, forged 316 stainless)	Use the flange material, not the pipe material
2	Find its material group in the B16.5 material list	Grades of the same alloy can land in different groups
3	Go to the rating table for that group	Tables are split by group number
4	Pick the row for the design temperature	Interpolate between rows; don't round temperature down
5	Read across to the class column to get the maximum working pressure	The notes can restrict some materials at high or low temperatures
6	Compare with the line's design pressure; the flange class must equal or exceed it at design temperature	The line class in the piping spec usually already made this choice

Worked example — Class 150 carbon steel flange (Group 1.1) at ambient temperature

Input / step	Value	Source
Flange material	Forged carbon steel, Group 1.1	Piping spec / MTR
Class	150	Line class
Maximum working pressure at ambient (−20 to 100 °F)	285 psig	B16.5 rating table for Group 1.1
Manufacturer's shell test	1.5 × 285 = 427.5 → 450 psig	Test is 1.5 × the ambient rating, rounded up to the next 25 psi
What happens at higher temperature	The allowable pressure is lower — read the table at design temperature	Rating drops as temperature rises
Check against system hydrotest	The system test pressure must not overstress the flange	See the B31 test pressure guide

Worked example values only (one rating value and the derived shell test). Get the full rating tables from ASME B16.5.

B36.10M vs B36.19M — which pipe standard?

Standard	Covers	Schedule style	Watch out for
ASME B36.10M	Welded and seamless wrought steel pipe (carbon and alloy)	Numbered schedules (10, 40, 80, …) plus STD, XS, XXS	STD and Sch 40 match only up to a certain size
ASME B36.19M	Welded and seamless stainless steel pipe	"S" schedules (5S, 10S, 40S, 80S)	40S/80S match Sch 40/80 in smaller sizes but diverge in larger sizes — check the dimension table

For walls and ODs, see our steel pipe schedule chart and the standards themselves. Our own summary, not the ASME B16.5 text. Get pressure-temperature ratings from ASME B16.5 (asme.org), follow the piping specification and line class, and verify against the edition your contract references. Not engineering advice.

Common field mistakes

Mistake	What to do instead
Reading "Class 150" as "150 psi"	The allowable pressure depends on material group and temperature — look it up
Using the carbon steel column for a stainless flange	Find the flange material's group first
Mixing classes at a connection	Class 150 and 300 flanges have different bolt circles and won't mate
Assuming Sch 40S = Sch 40 in all sizes	They match only in the smaller sizes — check B36.19M for larger pipe
Using B16.5 flange data for NPS 30 pipe	Above NPS 24, use B16.47 and confirm Series A vs Series B
Hydrotesting a system above what a flange class can take	The weakest component limits the test pressure

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Flange classes, in plain language

Get the actual ratings from ASME. ASME B16.5 (Pipe Flanges and Flanged Fittings) is sold by ASME. This page explains how the rating system works; it doesn’t reproduce the tables.

A class is a family, not a pressure. B16.5 defines seven classes. Each has a pressure-temperature rating that depends on which material group the flange is made from, and the allowable pressure falls as temperature rises. So the same Class 300 flange in carbon steel and in stainless can have different ratings, and a flange that’s fine at ambient may not be at design temperature.

Worked example. A forged carbon steel flange in Group 1.1, Class 150, is rated 285 psig at ambient temperature. The manufacturer’s shell test is 1.5 times that, rounded up to the next 25 psi: 427.5 → 450 psig. At higher temperatures the allowable pressure is lower — read the table at the design temperature.

Pipe standards. Carbon and alloy pipe dimensions come from B36.10M; stainless pipe from B36.19M, with its “S” schedules. See the steel pipe schedule chart, the NPT thread chart for threaded connections, and our B31 piping codes & test pressures guide for system testing and flange bolt-up. Field PM keeps MTRs, test packages, and QC records with each job.

FAQ

What does flange Class 150 or Class 300 mean?+

It is a pressure-rating family, not a pressure. Each class has an allowable working pressure that depends on the flange material group and the temperature. For example, a forged carbon steel (Group 1.1) Class 150 flange is rated 285 psig at ambient temperature, and less as the temperature rises. Get the full tables from ASME B16.5.

What is a flange material group?+

B16.5 sorts flange materials into groups with similar strength behavior — carbon and low-alloy steels, austenitic stainless steels, and nickel alloys, each split further. Each group has its own pressure-temperature table, so you must identify the group before reading a rating.

Can I bolt a Class 150 flange to a Class 300 flange?+

No. The classes have different outside diameters, bolt circles, and bolt counts or sizes, so they don't match. The connection has to be one class throughout.

What is the difference between B36.10M and B36.19M?+

B36.10M covers carbon and alloy steel pipe with numbered schedules and STD/XS/XXS weights. B36.19M covers stainless steel pipe with "S" schedules. The S schedules line up with the numbered schedules in smaller sizes but differ in some larger sizes.

Does B16.5 cover large-diameter flanges?+

B16.5 covers NPS 1/2 through NPS 24. Larger flanges are covered by ASME B16.47, which has two series (A and B) that are not interchangeable.

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