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Fitting and Flange Marking Chart

What the stamped markings on steel pipe fittings, flanges and valves mean, following MSS SP-25, and how to use them to confirm the right part.

Ref PV-0059MSS SP-25

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Steel fittings, flanges, valves and unions are stamped or printed with markings that identify what they are. MSS SP-25 is the standard marking system. Product standards such as ASME B16.5, B16.9 and B16.11 point to it and add their own rules. Markings let you confirm the material, rating and size, and trace the part to its mill test report through the heat number.

How to read it

Find the markings on the hub, rim or body. On butt-weld fittings they are often on the outside near one end.
Read each item and match it to the chart.
Compare the markings to the order and the piping spec.
Match the heat number to the mill test report.

Common markings

Marking	Example	What it means
Manufacturer name or trademark	Maker's logo	Who made the part
Material specification and grade	A105, A234 WPB, A182 F316L, A403 WP316L	Material and grade
Class or rating	150, 300, 600 for flanges; 3000, 6000 for forged fittings	Pressure class designation
Size	4, 2 x 1	Nominal size; reducing parts show both sizes
Schedule or wall	S40, SCH 80, STD, XS	Wall to match the pipe
Facing	RF, FF, RTJ	Flange face type
Ring groove number	R-37	Ring gasket size for RTJ flanges
Heat number or code	Letters and numbers	Links to the mill test report
Product standard	B16.5, B16.9, B16.11	Dimensional standard
Country of origin	Country name or code	Where it was made

Not every part shows every item. Small parts may use a short form allowed by the standard.

Example

A flange stamped "4 150 RF A105 S40 B16.5" and a heat number is an NPS 4, Class 150, raised face flange in A105 forged carbon steel, bored for Schedule 40 pipe, made to ASME B16.5.

Key points

"WP" in a grade, as in WPB or WP316L, marks a wrought butt-weld fitting material.
Forged threaded and socket-weld fittings to B16.11 carry a class of 2000, 3000 or 6000. That is a class designation, not a psi value.
Dual-certified materials may be stamped with both grades, such as 316/316L.
A missing or unreadable heat number is a traceability problem. Hold the part until it is resolved.

Common mistakes

Reading a forged fitting class of 3000 as 3000 psi.
Missing the schedule on a weld-neck flange bore and mismatching the pipe wall.
Accepting parts whose heat numbers do not match the paperwork.
Confusing the material stamp of the part with the stamp on a separate ring or nut.

Governing standards: MSS SP-25 (standard marking system); ASME B16.5, B16.9 and B16.11 (product marking rules).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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