How to Specify Flange Ratings for Critical Systems

Why Flange Ratings Are Easy to Get Wrong

Flange design is not about picking the biggest bolt or the thickest wall. It is about matching the mechanical class to the operating conditions. A flange rated for 150 psi at room temperature will not hold at 150 psi and 800°F. The class changes. The standard changes. The gasket changes.

The most common failure in critical systems is not a bad weld. It is a flange specified for the wrong class. The buyer sees a “150” and assumes it covers the process. The engineer sees the same “150” and knows it covers a specific temperature range. When those two interpretations diverge, the joint leaks.

This guide walks through the prerequisites, a six-step specification process, common mistakes, and a final verification step. It is written for buyers and engineers who need to put a flange on a drawing without ambiguity.

What You Need Before You Start

Before you write a flange specification, gather four things.

First, the operating pressure. Not the design pressure. Not the test pressure. The maximum continuous operating pressure in the system. If the system has a pressure relief valve set at 300 psi, the operating pressure may be 250 psi. The flange must be rated for 250 psi at the operating temperature, not 300 psi.

Second, the operating temperature. Flange ratings drop as temperature rises. A Class 150 flange holds more pressure at 70°F than at 700°F. The rating table is not a flat line. It bends.

Third, the standard. ANSI/ASME B16.5, EN 1092, or another standard. The class names may look the same, but the dimensions and ratings differ. A Class 150 ANSI flange is not interchangeable with a Class 150 EN flange.

Fourth, the material. Carbon steel, stainless steel, alloy steel, and ductile iron have different allowable stress values. The rating table changes when the material changes.

If you do not have these four values, stop. Do not guess. A flange specified without the operating temperature is a flange specified for the wrong class.

How to Match the Class to Pressure and Temperature

The class is the first number in the specification. Class 150, Class 300, Class 600, Class 900, Class 1500, Class 2500. These numbers are not pressures. They are approximate ratings in pounds per square inch at 100°F. That is the reference point.

The actual allowable pressure depends on temperature. The rating table for a given standard and material gives the maximum allowable pressure at each temperature. You find the class where the operating pressure falls below the allowable pressure at the operating temperature.

Example. A process runs at 150 psi and 350°F. The buyer picks a Class 150 flange because the pressure is below 150 psi. That is wrong. At 350°F, the allowable pressure for a Class 150 flange in carbon steel is lower than 150 psi. The correct class may be Class 300. The class number is not the pressure limit. It is a label. The rating table is the limit.

Use the rating table from the applicable standard. Do not use a memory of the number. Do not use a rule of thumb. The table gives the pressure in psi or bar at each temperature step. Find your operating point. Check the class.

Step-by-Step Specification Process

Follow these steps in order. Each step has a reason. Skipping one creates a gap that the vendor cannot fill.

1. Confirm the operating pressure and temperature. Write both on the drawing or specification. If the temperature varies, use the maximum. If the pressure varies, use the maximum continuous value. The flange must hold the worst case, not the average case.

2. Select the standard. ANSI/ASME B16.5 for North American systems. EN 1092 for European systems. Do not mix standards in one joint. The bolt hole pattern, face finish, and rating table must match on both flanges.

3. Find the minimum class. Open the rating table for your standard, material, and joint type. Locate the operating temperature. Find the pressure column. Choose the first class where the allowable pressure exceeds the operating pressure. This is the minimum. Do not round up to the next class unless required by a separate code rule.

4. Check the material allowance. The rating table for carbon steel is different from the table for stainless steel. If the flange is stainless, use the stainless table. If the flange is alloy, use the alloy table. The same class number means different allowable pressures for different materials.

5. Specify the gasket. The gasket type changes the joint. Spiral wound gaskets, ring gaskets, and flat gaskets have different allowable stress values. A Class 150 flange with a spiral wound gasket holds a different pressure than a Class 150 flange with a flat gasket. State the gasket type in the specification.

6. Write the full specification line. Do not write “Class 150 flange.” Write “ANSI/ASME B16.5 Class 300 flange, carbon steel, raised face, 6-inch nominal size, spiral wound gasket, operating pressure 150 psi, operating temperature 350°F.” This line removes ambiguity. The vendor sees the standard, class, material, face, size, gasket, and operating conditions.

Common Mistakes That Cause Leaks

The first mistake is using the class number as the pressure limit. Class 150 does not mean 150 psi at all temperatures. It means approximately 150 psi at 100°F. At 700°F, the allowable pressure is much lower. The buyer must check the table.

The second mistake is mixing standards. A joint with one ANSI flange and one EN flange will not bolt together correctly. The bolt circle diameter, the number of holes, and the face finish are different. The specification must call out one standard for both flanges.

The third mistake is ignoring the gasket. A flat gasket is cheaper and easier to install. A spiral wound gasket holds higher pressure and temperature. If the specification does not state the gasket type, the vendor chooses. The vendor may choose the flat gasket to save money. The joint leaks.

The fourth mistake is using the wrong material. Carbon steel and stainless steel have different allowable stress values. A Class 300 flange in carbon steel holds less pressure than a Class 300 flange in 316 stainless. The specification must state the material. “Flange” is not a material.

Final Verification Step

Before release, check the specification against three documents.

First, the process data sheet. Confirm the operating pressure and temperature. Confirm that the maximum temperature is the one used in the class selection.

Second, the piping drawing. Confirm that the nominal size matches the flange size. Confirm that the standard matches the drawing title block. Confirm that the material matches the spec sheet.

Third, the vendor data sheet. Ask the vendor to confirm the rating. The vendor should be able to show the allowable pressure for the selected class, material, gasket, and temperature. If the vendor cannot show it, the specification is incomplete.

Verification is not a formality. It is the point where the buyer and vendor agree on the same number. If the buyer says “Class 300” and the vendor says “Class 300,” but the buyer means 150 psi at 350°F and the vendor means 300 psi at 100°F, the joint leaks. The verification step closes that gap.

Quick Reference Table

The table below shows typical allowable pressures for ANSI/ASME B16.5 flanges in carbon steel. The values are approximate and depend on the joint type, gasket, and specific standard revision. Use the official rating table for final selection.

Class	100°F	350°F	550°F	700°F
150	150	110	60	35
300	300	220	120	65
600	600	440	240	130
900	900	660	360	195