A Step-by-Step Guide for Engineers and Industrial Buyers

Flanges are essential components in piping systems, acting as mechanical connectors that ensure secure, leak-proof, and serviceable joints between pipes, valves, pumps, and equipment. But with so many standards, pressure ratings, and sizes available, one key question arises — how to know the correct flange size

In this article, sasaflange explains how to identify the correct flange size for your application, including the key dimensions, measurement techniques, and reference standards. Whether you are sourcing parts for a new project or replacing an existing flange, this guide will help you avoid errors and ensure proper system performance.

---

Why Choosing the Right Flange Size Matters

Selecting the wrong flange size can lead to misalignment, pressure failures, increased leakage risk, and costly rework. Getting the flange size right is critical to

• Ensuring system compatibility

• Meeting pressure and temperature requirements

• Achieving proper sealing with gaskets

• Simplifying installation and maintenance

• Avoiding costly downtime or safety issues

At sasaflange, we support customers in choosing flange dimensions that comply with global standards such as ASME, EN, DIN, and JIS.

---

What Are the Key Flange Size Parameters

To identify or confirm the correct flange size, you need to understand and measure several key dimensions

1. Nominal Pipe Size (NPS) or DN

This refers to the size of the pipe the flange is intended to connect. In North America, NPS (in inches) is used. In Europe and other regions, DN (diameter nominal in mm) applies.

2. Pressure Rating (Class or PN)

Defines the maximum pressure a flange can withstand. Common ASME classes are 150, 300, 600, 900, and 1500. For EN/DIN standards, pressure ratings are shown as PN10, PN16, PN40, etc.

3. Outside Diameter (OD)

The total diameter of the flange face, important for proper alignment with mating components.

4. Bolt Circle Diameter (BCD or PCD)

The diameter of the circle passing through the center of each bolt hole.

5. Number and Size of Bolt Holes

Bolt quantity and diameter must match exactly to ensure tight, leak-free assembly.

6. Flange Thickness

Measured from face to back, thickness affects strength and gasket compression.

7. Bore or Inner Diameter

Should match the outside diameter of the connecting pipe to maintain flow continuity.

---

How to Determine the Correct Flange Size

Follow these steps to ensure you choose the correct flange size

Step 1: Identify the Pipe Size

Check the nominal pipe size (NPS or DN). If you’re replacing an existing flange, measure the inside diameter and wall thickness of the pipe it connects to.

Step 2: Confirm the Pressure Rating

Refer to the piping system design or operating conditions. If the system operates under high pressure or temperature, a higher class (e.g., Class 300 or PN40) is required.

Step 3: Measure the Flange Dimensions

Use calipers or a measuring tape to determine

• Outside diameter

• Bolt circle diameter

• Number and size of bolt holes

• Bore and thickness

Compare your measurements with a standard flange dimension chart (such as ASME B16.5 or EN 1092-1) to verify the size.

Step 4: Check the Flange Type and Face

Is it a weld neck, slip-on, blind, socket weld, or threaded flange Is it raised face, flat face, or ring-type joint This affects both installation and gasket compatibility.

Step 5: Confirm Standards and Material

Ensure the flange material (e.g., stainless steel, carbon steel, alloy steel) and manufacturing standard match your system requirements.

---

Flange Size Chart Example (ASME B16.5, Class 150, NPS 4)