David W. Spitzer, P.E.

The concept of flange ratings seems relatively straightforward. A Class 300 flange can handle more pressure than a Class 150 flange. Likewise, Class 600, Class 900, and Class 1,500 flanges are constructed with progressively more metal and can withstand progressively more pressure. Flowmeters are typically specified with connecting flanges that correspond to the piping specifications. However, the details involved with pipe flange ratings can also be misunderstood because there are a number of factors that can impact the pressure capability of a flange. That said, approximately how much pressure can a Class 150 flange withstand?
A. 270 PSIG;
B. 180 PSIG;
C. 150 PSIG;
D. 75 PSIG

The obvious answer might be that a Class 150 flange can withstand 150 PSIG. After all, why would it be called a Class 150 flange if it were not capable of withstanding Class 150. Well, it is not necessarily that straightforward.

Flanges can withstand different pressures at different temperatures. As temperature increases, the pressure rating of the flange decreases. For example, a Class 150 flange is rated to approximately 270 PSIG at ambient conditions (Answer A), 180 PSIG at approximately 400 F (Answer B), 150 PSIG at approximately 600 F (Answer C), and 75 PSIG at approximately 800 F (Answer D). In other words, all of the answers could be correct, depending upon the operating temperature of the fluid. Similar information is available for flanges with other ratings.

Notwithstanding the above, how are flange ratings selected? Often they are copied from similar designs, but sometimes they must be developed independently. In one such application, the maximum pressure of a fluid at ambient temperature was 220 PSIG. Using the information above, it can be seen that this fluid could be contained using Class 150 flanges. Nonetheless, the process engineer specified and installed Class 300 flanges in all of the piping. This may seem to be over-design, because Class 150 flanges are rated for higher pressures. However, the relatively small cost differential associated with purchasing and installing the Class 300 flanges in this new installation probably paid for itself over time by reducing the maintenance cost of the piping. It also reduced fugitive emissions because fewer leaks likely occurred with the Class 300 flanges that had a higher pressure rating than the Class 150 flanges.

Additional Complicating Factors
Complicating the pressure ratings presented above is that flanges can be constructed from different materials, such as carbon steel, stainless steel, cast iron, ductile iron, and others. Flanges constructed from these materials have different pressure ratings. In some applications, the nature of certain fluids may dictate the use of certain flange ratings or types.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting. He can be reached 845 623-1830.