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| David W. Spitzer, P.E.
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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
Commentary
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.
www.spitzerandboyes.com
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