How To Measure Flange Size, Thickness & Bolt Circle Diameter

How to Measure Flange Size, Thickness & Bolt Circle Diameter

When selecting or inspecting flanges, it's essential to know how to accurately measure their size, thickness, and bolt circle diameter. Proper measurements ensure that the flange fits the piping system correctly and meets design requirements. Here's how to measure these critical dimensions:

1. Measuring Flange Size

Flange size refers to the nominal diameter of the flange, which corresponds to the size of the pipe it will be attached to.

Nominal Pipe Size (NPS): This is the general size for the flange and is defined by the pipe's nominal diameter. Commonly referred to as the "flange size," it typically matches the pipe's nominal size.

How to measure flange size:

Measure the outer diameter (OD) of the flange. This measurement corresponds to the nominal pipe size. For example, a 6" flange will typically have an outer diameter close to 6 inches.

Check the flange's standard (e.g., ASME, DIN, JIS) to confirm the exact size specifications, as flange dimensions can vary slightly depending on the standard.

2. Measuring Flange Thickness

The flange thickness is the distance between the flange's two surfaces (the raised face and the back).

Flange thickness is important because it ensures the flange can handle the pressure and stress requirements of the system. Thicker flanges generally provide greater strength and are needed for higher pressure applications.

How to measure flange thickness:

Use a caliper or micrometer to measure the thickness of the flange body from the raised face (or sealing surface) to the back of the flange.

Check flange standard specifications (such as ASME B16.5 or EN 1092-1) for the thickness range based on the size and pressure class of the flange.

3. Measuring Bolt Circle Diameter (BCD)

The bolt circle diameter (BCD) is the diameter of the circle that passes through the center of all the bolt holes in the flange.

This is one of the most critical measurements for ensuring proper bolt alignment when connecting flanges.

How to measure the bolt circle diameter:

Measure the distance from the center of one bolt hole to the center of the opposite bolt hole (across the center of the flange).

Alternatively, measure the radius from the center of the flange to the center of one bolt hole and multiply this value by 2 to get the diameter of the bolt circle.

Use a caliper or measuring tape to measure the diameter at several points around the flange to ensure consistency and accuracy.

4. Measuring Bolt Hole Diameter and Number of Holes

Besides the bolt circle diameter, it's also necessary to measure the bolt hole diameter and the number of holes to ensure compatibility with the bolts.

Bolt hole diameter is typically standardized (e.g., 0.5" for a 6" flange), but it should always be measured for accuracy.

How to measure bolt hole diameter:

Use a caliper to measure the internal diameter of the bolt holes.

5. Pressure Class and Rating

Flanges are available in various pressure classes (e.g., 150, 300, 600), which are tied to their ability to withstand certain pressures. These are typically identified by the flange's pressure rating stamped on the flange.

Ensure that the flange's pressure class is suitable for the intended application.

Example of Flange Measurements (ASME B16.5 Standard for a 6" Flange)

Conclusion

Accurate measurements of flange size, thickness, and bolt circle diameter are crucial for selecting the correct flange for your piping system. Make sure to use proper measuring tools like calipers and micrometers to ensure precision. Additionally, always refer to flange standards (ASME, DIN, JIS, etc.) for exact dimension guidelines based on the flange material, pressure class, and intended application.