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The Difference Between #150, #300, and #600 Pound Flanges

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Pressure class definitions for flanges can be a little confusing at first, but they're actually quite simple.  Part of the confusion in this rating system stems from the interchangeability of the pound symbol--#--the abbreviation, lb or lbs, and pound or pounds.  While all of these designate the pressure rating of the  flange, 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 150psig, as many people mistakenly assume.

Flange Classes/Ratings

Flanged valves and fittings fall into class/rating categories established by the B16 Committees of ASME (American Society of Mechanical Engineers), and are also accredited by ANSI (American National Standards Institute).  These flanges are typically forged and machined, and correlate with Nominal Pipe Sizes (NPS).  The pressure ratings of these flanges run between #150 and #2500, and are based on two major factors:

  1. The material from which the flange is made
  2. The temperature at which the flange will safely operate

In example, a #150 pound flange, made of ductile iron, carbon steel, or stainless steel, may have a maximum pressure rating of around 270 psi at an ambient temperature.  But this number is subject to change if the temperature changes. At 400°F this same #150 flange will likely only be able to handle a pressure of 180 psig. When the temperature goes up; the pressure rating goes down.  This is true of all flanges.

Telling the difference between flange classes is probably best left to the identifying stamp on their side, and/or the expert guidance of an industrial  valve distributor.  To the trained eye, however, there are tell-tale differences between the #150, #300, and #600 pound flanges (say for a 2" pipe), that can be spotted right away: the outer diameter, the number of bolt holes, and the thickness of the flange.  For instance, a #600 pound flange for a 2" pipe will be twice as thick than a #150 pound flange for the same size pipe.

Rating Charts

When in doubt, refer to the charts.  Most valve manufacturers conform to ANSI regulations, and provide pressure rating charts for their products, eliminating any iffy guesswork.  Or ask the pros at valve distributor like ValveMan.com, knowing valve engineering is their passion.  It's why they are in business, and they have been a valve store for over 51 years.