PVF Reference Library · Measure & Identify

How to Read Fitting Markings

Pipe fittings, flanges and valves are marked with the maker, material, size, rating and heat code. This sheet explains how to read those stamps under the MSS SP-25 marking system.

Ref PV-0070MSS SP-25

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

What it is

MSS SP-25 is the standard marking system for valves, fittings, flanges and unions. Product standards such as ASME B16.9, B16.11 and B16.5 point to it. Marks may be cast, forged, stamped, stenciled or on a tag, depending on the product and size.

Typical marks include:

Maker's name or trademark
Material designation
Size
Pressure class or rating designation
Schedule or wall thickness, where it applies
Heat number or heat code
The product standard, where required

How to read it

Clean the fitting and find all marks. They may be on the body, the hub or the flange rim.
Read the material. Butt weld fittings show a grade such as WPB (carbon steel to ASTM A234) or WP316L (stainless to ASTM A403). Forged fittings show grades such as A105 or F316L. Cast valves may show WCB or CF8M.
Read the size. Reducing fittings show both sizes, such as 2 × 1.
Read the class. Forged socket-weld and threaded fittings show classes such as 2000, 3000 or 6000. Flanges show 150, 300, 600 and up. Malleable iron fittings may show 150 or 300.
Read the schedule or wall on butt weld fittings, such as SCH 40 or STD.
Read the heat code. It links the fitting to its material test report.

Common marks

Mark	Example	Meaning
Maker	Name or logo	Who made it
Material grade	WPB, A105, CF8M, WP316L	Material and specification grade
Size	2, or 2 × 1 for reducers	Nominal pipe size
Class	3000, 150	Pressure class designation, not psi
Schedule or wall	SCH 80, XS	Matching pipe wall
Standard	B16.9, B16.11	Dimensional standard
Heat code	Letters and numbers	Links to the MTR
Country of origin	Country name or code	Where it was made

Key points

The class number is a designation. It is not the working pressure. Ratings depend on material and temperature and are given in the product standard's tables.
A threaded fitting and a socket-weld fitting of the same class have different dimensions.
Dual-grade marks such as "304/304L" mean the material meets both grades.
See Fitting and Flange Marking Chart and Valve Trim Numbers and Material Codes.

Common mistakes

Reading class 3000 as 3000 psi.
Missing a reducing size on one end.
Mixing stainless grades that look alike without reading the mark.
Grinding off marks during fit-up.

Governing standards: MSS SP-25 (standard marking system for valves, fittings, flanges and unions); ASME B16.5, B16.9 and B16.11; ASTM A105, A234 and A403.

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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