The Difference Between #150 and #300 Pound Flanges

Pressure class definitions for flanges can be a little confusing at first, and we’re not going to lie and say they’re easy to understand the first time you look at the differences. 

Primary Rating Standards

Flanged valves and fittings fall into class/rating categories developed by the American Society of Mechanical Engineers, ASME B16 Committee and approved as American National Standards. Here’s a summary of those class/ratings categories.

ASME / ANSI Standard (ASME B16.5 & ASME B16.34)

The ASME Designations include Class 150, Class 300, Class 400, Class 600, Class 900, Class 1500, and Class 2500. The class number is a nominal rating identifier, not an absolute pressure cap. For instance, a Class 150 carbon steel valve is rated up to 285 psi at room temperature, but that rating drops as the operating temperature rises.

European / DIN Standard (EN 1092-1)

These designations are for PN (i.e., nominal pressure) in bars at 20°C: (e.g., PN 6, PN 10, PN 16, PN 25, PN 40, PN 63, PN 100). Here are the approximate ASME / ANSI equivalents: Class 600 ≅ PN 100, Class 900 ≅ PN 160, Class 1500 ≅ PN 250, and Class 2500 ≅ PN 400. 

ASME Pressure Class Summary

The maximum ambient pressure limits below assume Group 1.1 Carbon Steel (e.g., ASTM A105 / A216 WCB). Higher alloy grades or higher temperatures alter these numbers significantly.

*Class 400 exists in standard pressure tables but is rarely specified in modern piping design, as engineering specs typically bump up directly to Class 600

Part of the confusion in this rating system stems from the interchangeability of the pound symbol #. This symbol usually serves as the abbreviation for lb or lbs, as in pound or pounds. While all of these designate the pressure rating, you’ll notice that none of them directly indicate the amount of pressure the flange can actually handle. For example, a #150 pound flange rating does not correlate with 150 psig, as many people mistakenly assume.

Critical Factors Affecting Flange Ratings

Four main things affect flange ratings: temperature derating, material groupings, flange facing styles, and the flange pressure class.

Material Groupings

ASME B16.5 organizes materials into specific groups based on their chemical composition and material properties. The table below shows an abbreviated summary of what those groupings look like.

ASME B16.5 Material Groupings

*This table is a simplified summary. The full ASME B16.5 standard (e.g., Table 1A) contains the exhaustive list of all permitted material specifications, including plates, and numerous explanatory notes regarding material forms, heat treatments, and application restrictions.

You’ll notice the material groupings include subgroups for the nominal designation along with a representative forging material and a representative casting material, except for cast non-ferrous alloys.

Each one of these material groups is assigned a specific set of pressure-temperature rating tables, which we’ll talk about next. This allows engineers to quickly determine the maximum working pressure for a flange made of that material at a given operating temperature. 

Temperature Derating

This leads us into another aspect of flanges: temperature derating. It is the progressive reduction of a flange or valve's Maximum Allowable Working Pressure (MAWP) as operating temperatures increase. Because metals lose yield strength, tensile strength, and resistance to creep at elevated temperatures, standard pressure ratings (e.g., Class 150, Class 300) don’t represent a fixed pressure cap. Instead, a pressure class represents a baseline from which allowable pressure degrades as process temperatures rise.

Below is a temperature derating chart for Group 1.2 Materials, including low-alloy/carbon steels such as ASTM A350 LF2, ASTM A352 LCB/LCC, and C-Mn-Si steels. The units of °F and psig. (Source: Engineering Toolbox, ANSI B16.5 - Steel Pipe Flanges - Pressure and Temperature Ratings - Group 1.2). Most valve manufacturers conform to ANSI regulations and provide pressure rating charts for their products, eliminating any iffy guesswork.