Got the markings on the flange figured out? If not, come and brush up!

The current flange standards include national standards, chemical industry standards, machinery standards, and petrochemical standards, among others. The sheer number of standards can be overwhelming, and each specifies requirements for the markings on flanges. Here, we will take the chemical industry standard (PN series) as an example to introduce flange markings.

In the standard designation diagram, “HG20592-2009” represents the standard code implemented by this flange. Here, HG/T 20592-2009 corresponds to the “PN series” standard within the chemical industry standards, whereas the “Class series” standard is HG/T 20615-2009.

Given the numerous flange standards, for easier identification, you can remember: HG represents chemical industry standards, GB represents national standards, JB represents mechanical standards, SH represents petrochemical standards, and CB represents marine standards. For specifics, refer to this article: What are the current domestic [Flange Standards]? What are their characteristics? They will not be listed here again. As for international standards, ANSI represents American standards, DIN represents German standards, EN represents European standards, and JIS represents Japanese standards. Are there any other commonly used standards? Feel free to add.

II. Nominal Size
In the diagram, “DN25” represents the nominal size of the flange, which may be followed by the outer diameter series of the applicable steel pipe.

According to current chemical industry standards, the marking for the applicable steel pipe outer diameter series may be omitted for integral flanges, flange covers, lined flange covers, and threaded flanges. For flanges compatible with the A-series steel pipes specified in this standard, the marking for the applicable steel pipe outer diameter series may also be omitted. For flanges compatible with the B-series steel pipes specified in this standard, the marking should be indicated as “DNXXX(B)”.

Additional note: The chemical industry standards cover two series of applicable steel pipe outer diameters—Series A and Series B. Series A is the internationally adopted series (commonly known as the imperial pipe series), while Series B is the domestically used series (commonly known as the metric pipe series).

Nominal Pressure

“PN16” indicates the nominal pressure rating of the flange. Other common pressure ratings include PN2.5, PN6, PN10, PN25, PN40, and others.

Flange Type Designation

“PL” refers to the flange type designation—Plate Flat Welding Flange. Additionally, there are:
SO (Slip-On Welding Flange),
WN (Weld Neck Flange),
IF (Integral Flange),
SW (Socket Welding Flange),
Th (Threaded Flange),
BL (Blind Flange),
and PJ/SE (Loose Flange with Welding Neck Collar).

Note: In the image, the “100-16” following “PL” also indicates the applicable nominal size and nominal pressure of the pipeline.

Sealing Face Type Designation

“RF” refers to the sealing face type of the flange, where “RF” stands for Raised Face. Besides this, there are also: FF (Flat Face), MFM (Male and Female Face), TG (Tongue and Groove Face), and RJ (Ring Joint Face).

Material Grade

In the marking “S30408II”, “S30408” refers to the material grade, while “II” indicates the forging grade. In China, forgings are classified into four grades according to national standards, and the “II” here denotes a Grade II forging. Regarding material grades, the topic can be quite complex, and my understanding is limited. What I do know is that they include various categories such as stainless steel (e.g., 304, 316), carbon steel (e.g., Q235A, Q235B), and alloy steel, among others. Additionally, 304 can also be represented as 06Cr19Ni10, and 304L as 022Cr19Ni10. Here, “06” and “022” indicate carbon content levels of 0.06% and 0.022%, respectively, “Cr19” signifies a chromium content of approximately 19%, and “Ni10” denotes a nickel content of around 10%. Other material grades follow similar corresponding representation formats.

Regarding the marking of steel pipe wall thickness on flanges, not all flanges require such identification. According to chemical industry standards, only weld neck flanges and welding neck collars (loose flanges) need to indicate the steel pipe wall thickness.

All the above explanations are based on the current PN series standards in the chemical industry flange specifications. The Class series may have slight differences. As for why national standards were not used as the example, on one hand, chemical industry standards are also comprehensive and widely applied within the flange system; on the other hand, suitable images were not available to illustrate the national standards. This introduction may not be exhaustive, and any omissions or inaccuracies are welcome to be supplemented or corrected. For those who wish to delve deeper into flange markings, you may refer directly to the corresponding applicable standards, where detailed explanations are provided.