Pipe Standards

Yes! depending on the location, access and condition of the flange you can tell the size, rating and material of a flange just by reading the markings stamped on the outside diameter of a flange.

You might need a flashlight and/or a mirror. You might also need a wire brush.

If you are in a refinery or other facility subject to flammable hydrocarbons you need to make sure the flashlight is intrincicly safe, the wire of the brush is "non-sparking" and both are checked and approved by the plant safety group.

Is it right to differentiate a class 150 or class 300 flange by its face thickness or the no of PCD holes?

Not always.

You must first eliminate any other reason for a specific flange thickness.

Take ALL of the dimensions for both flange classes and do a process of elimination.

Even then you are only guessing. The best way still remains to look for and varify the flange markings.

Another point.

Flanges are not designated as "150psi or 300psi as you indicated. The proper designation is Class 150 or Class 300, etc. To determine how much pressure a specific flange class is rated for you must consider the maximum operating temperature.

Please see the following chart:

Pressure - Temperature Ratings

For Steel Pipe Flanges and Flanged FittingsAmerican National Standard ANSI B16.5 - 1988

Ratings apply to materials.

SA-105^1.2 SA-515-70² SA-516-70² SA-181-70^1.2 SA-350-LF2 SA-537-C1.1³ SA-216-WCB²

NOTES: 1. For service temperatures above 850 F it is recommended that killed steels containing not less than 0.10% residual silicon be used. 2. Upon prolonged exposure to temperatures above 800 F, the carbide phase of carbon steel may be converted to graphite. 3. The material shall not be used in thickness above 2-1/2 in.

Flanges of ANSI B16.5 shall not be used for higher ratings except where it is justified by the design methods of the Code.

Ratings are maximum allowable non-shock working pressures expressed as gage pressure, at the tabulated temperatures and may be interpolated between temperatures shown.

Temperatures are those on the inside of the pressure-containing shell of the flange. In general, it is the same as that of the contained material.

THIS INFORMATION IF PROVIDED FOR GENERAL INFORMATION PURPOSES ONLY. BEFORE USING ANY OF THIS DATA FOR ANY PURPOSE OTHER THAN FOR GENERAL INFORMATION , IT SHOULD BE REVIEWED AND CERTIFIED BY A QUALIFIED ENGINEER.

Not sure about your question, may be the flange marking is not visible (Dirty, Painted, oxidation......). If this is the problem. you only have to see how many holes have the flange ( Not in the center of the flange, there is only one, holes for nuts and bolts)

Regards