If you spend enough time managing plant inventory or working on piping retrofits, you will inevitably find one: a flange sitting on a pallet or welded to an old spool with absolutely no stamps, no heat numbers, and no manufacturer identification.

On forums like Reddit’s r/AskEngineers, the question "Can I use this unmarked flange?" pops up constantly. The short answer for any critical pressure boundary is no. ASME B16.5 and EN 1092-1 standards strictly require permanent markings. If they are missing, you are holding a liability, not a pipe fitting.

However, if you are working on a non-critical, low-pressure system (like atmospheric water drainage), or if you simply need to identify an old spool piece to order a correct replacement, you have to know how to reverse-engineer its specs. Here is the practical, step-by-step method to identify an unmarked flange.

Step 1: Measure the Bolt Circle Diameter (BCD)

You cannot guess a flange’s pressure rating just by looking at its thickness. The most reliable physical indicator of a flange's standard (ASME, DIN, JIS) and pressure class is the Bolt Circle Diameter (also known as Pitch Circle Diameter or PCD).

Grab a calibrated vernier caliper. You need to measure the distance from the center of one bolt hole to the center of the bolt hole directly opposite it.

• Quick Workshop Trick: Instead of guessing where the exact center of the hole is, measure from the inside edge of one hole to the outside edge of the opposite hole. This gives you the exact BCD.

• Count the number of bolt holes and measure the hole diameter.

• Cross-reference these three data points (BCD, hole count, hole size) against standard flange dimension charts. A 4-inch Class 150 flange will have a different BCD (7.50 inches) than a 4-inch Class 300 flange (7.88 inches).

Step 2: Measure the O.D. and Flange Thickness

Once you have the BCD, confirm your findings by measuring the Outside Diameter (O.D.) and the thickness of the flange.

Be careful here: if the flange has a Raised Face (RF), do not include the raised face height (typically 1/16" for Class 150/300 and 1/4" for higher classes) in your base flange thickness measurement unless your specific reference chart dictates otherwise.

Step 3: The Material Problem (Why Calipers Aren't Enough)

You have successfully determined it is a 6-inch ASME 150 RF flange. But what is it made of?

This is where unmarked flanges become dangerous. An A105 carbon steel flange looks identical to a low-temp A350 LF2 flange under warehouse lighting. A 304 stainless flange looks just like a 316L stainless flange.

Without a stamped Heat Number, you have zero material traceability. If you absolutely must verify the material, your only valid engineering option is Positive Material Identification (PMI) using X-Ray Fluorescence (XRF) or Optical Emission Spectroscopy (OES). A PMI gun will give you the exact chemical breakdown (Chrome, Nickel, Molybdenum percentages), allowing you to match it to an ASTM material grade.

The Reality Check for Procurement

Identifying an unmarked flange is a useful diagnostic skill, but it is a terrible procurement strategy.

Reverse-engineering dimensions and paying for 3rd-party PMI testing costs more in labor and delays than simply buying a fully certified, traceable flange from a reputable manufacturer. In modern piping systems, traceability is non-negotiable. If a vendor tries to ship you a batch of flanges with missing, shallow, or unreadable stamps, reject them at the receiving dock.

When you source components, demand absolute clarity: clear physical stampings (Manufacturer, Size, Class, Material Grade, Heat Number) backed by matching, verifiable Material Test Reports (MTRs). Anything less is just expensive scrap metal.