Difference Between Class (#) and Rating in ASME Flanges
When working with piping systems, flanges, and fittings, engineers often encounter the terms “Class” and “Rating.” These terms are defined in ASME B16.5 (Pipe Flanges and Flanged Fittings) and ASME B16.47 (Large Diameter Steel Flanges). Understanding the distinction is critical for safe design, compliance, and avoiding costly mistakes.
🔹 What Does “Class” Mean?
- Class is a designation used to identify the pressure‑temperature rating of a flange or fitting.
- It is expressed as a dimensionless number, typically written as:
- Class 150
- Class 300
- Class 600
- Class 900
- Class 1500, Class 2500, etc.
- Historically, these classes were referred to as “pound ratings.” For example, Class 150 was often called a “150‑lb flange.”
👉 However, this does not mean that a Class 150 flange is rated for 150 psi at all temperatures. The actual allowable pressure depends on both temperature and material, which is why ASME provides detailed Pressure‑Temperature Tables.
🔹 What Does “Rating” Mean?
- Rating is the qualifying parameter that specifies the maximum allowable working pressure at a given temperature for a particular material and class.
- In simple terms:
- Class = the name or designation
- Rating = the actual pressure‑temperature capability
For example:
- A Class 150 carbon steel flange has a rating of 285 psi at 100°F.
- A Class 300 carbon steel flange has a rating of 740 psi at 100°F.
These values are defined in ASME B16.5 tables and vary with temperature and material type.
🔹 Why the Distinction Matters
It is a common misconception to assume that:
- Class 150 = 150 psi
- Class 300 = 300 psi
This is incorrect. The rating changes with temperature and material composition. For example:
- At 400°F, a Class 150 flange may no longer be sufficient for 100 psi design pressure.
- In that case, you would need a Class 300 flange to safely handle the load.
This highlights why engineers must always consult the ASME Pressure‑Temperature Tables rather than relying on assumptions.
🔹 Practical Example
Suppose you are designing a system with:
- Material: Carbon Steel A‑105 flange
- Design Pressure: 100 psig
- Design Temperature: 400°F
At this condition, a Class 150 flange is not adequate.
You must select a Class 300 flange to meet the rating requirements.
This example shows how Class is simply a label, while the Rating is the actual performance limit.
🔹 Key Takeaways
- Class is a designation (a name).
- Rating is the actual allowable pressure‑temperature performance.
- Always check the ASME B16.5 or B16.47 tables for accurate ratings.
- Ratings vary with temperature and material type.
- Never assume that Class numbers directly equal pressure values.