There are multiple questions regarding how flanges should
be considered in computer-aided analyses. First, it is important to understand
that flange failures do not result in pipe ruptures, rather a leak is
often considered a flange "failure," especially if the contents
are hazardous. A B31.1 Interpretation in the late 1990's sought to clarify
this, and while there was general agreement to a statement that would
suggest that flanges don't normally break, the Interpretation failed to
be approved for publication because the B31.1 Code did not already say
this in some manner. [In general, ASME Interpretations are intended to
only address how a code is currently written, not how the committees wish
it were written.]
Further, the ASME B31 Codes, through the early 2000's, have done little
to suggest how flanges should be considered. This, to an extent, is an
indication that flanges are not much of a problem because ASME Codes are
safety codes and provide rules or cautionary notes to guard against practices
known to be unsafe.
Contributing to the problem is that the ASME Codes, particularly the non-Nuclear
codes, are currently written for hand computational (i.e., simplified),
not computer-aided, methodologies and are not likely to change soon. An
acknowledged motive for simplified rules is the desire for the widest
possible acceptance and appropriate application of the rules, whereas
the more complicated the rules, the more likely misunderstandings and
misapplications will arise. And determining the acceptability of a flange
is a relatively complicated problem, which based on experience may not
be a significant safety issue, except perhaps in the case that it is desirable
to avoid leakage (such as in B31.3 Category M fluid service where the
contents are considered very hazardous).
Another significant problem is how ASME flange acceptance data is currently
provided. A less than careful reading of B16.5 (Pipe Flanges and Flanged
Fittings, NPS 1/2 through NPS 24) would lead one to believe that merely
knowing the pressure and temperature (and material) of a flange would
be sufficient to select the proper Flange Class, e.g., Class 150, Class
300, Class 600, etc. Notice the word Class is used here to identify the
flange, and not 150 lb, 300 lb, 600 lb, etc. These latter "identifiers"
are often used to reference flanges, but they can lead to some confusion.
While there is somewhat of a correlation between working pressure and
Class at about 825 degF for more common materials, this is not a consistent
correlation for all materials and flange Classes (notably Class 150 flanges).
But from a careful reading of B16.5 - 2001, Para. 2.4, one can discover
that in addition to pressures and temperatures the "forces and moments
developed in the connected piping" must be taken into consideration.
Another authoritative set of flange rules exist in the ASME BPV Section
VIII, Div. 1, Appendix 2 which also requires the external forces and moments
on the flange to be considered.
The ASME Nuclear Codes do provide some conservative guidance in paras.
NB/NC/ND-3658 which have been incorporated into Caepipe and other piping
analysis programs. In the current nuclear codes, equations for an "effective"
flange design pressure are provided converting external moments into an
equivalent pressure which is then added to the system design pressure.
However, it must be pointed out that flanges which can accommodate this
"effective" flange design pressure may still leak under some
coincident operating and environmental conditions. What can be said of
flanges which can accommodate the "effective" nuclear flange
design pressure, if properly manufactured and installed and excepting
the gasket, that all components of the flange (the flange itself and appropriate
bolting) should not exceed code allowable stresses for base material and
bolting.
There are at present no published analytic methods that will assure that
flange leakage will not occur. B31.1 (see Table 112) and SC VIII, Div.
1, Appendix 2 have some guidance that will help the designer with materials,
flange types, bolting, and gaskets but the assurance that a selected flange
configuration will not leak is beyond these sources. A major source of
uncertainty is the flange gasketing. Gasket manufacturers often have guidance
on their product's proper installation and may have performance data,
based on testing, as well. If the assurance that no leakage will occur
is required, then, short of the designer performing testing, working with
a reputable gasket manufacturer will be necessary.
How to interpret CAEPIPE Flange reports - Read next month's tip (given
below).
Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (www.ppea.net) is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.
Tip for the Month (May 2003)Continued from Part 1 above
With regard to the Flange Report output in Caepipe, the program input
includes the piping system design pressure and temperature, B16.5 class,
and flange gasket diameter. Using the conservative nuclear code formulations,
an "effective" flange design pressure is calculated and compared
to the B16.5 flange pressure-temperature rating for the flange material.
Where the ratio of the "effective" design pressure divided by
the B16.5 pressure-temperature is greater than 1.0, the flange is "flagged."
(Caepipe, in addition to determining an equivalent pressure from external
moments, also includes in the equivalent pressure calculation an axial
force term based on similar terms.)
When a flange is "flagged," the designer should review the piping
system design pressure and temperature against the B16.5 pressure-temperature
ratings. If the B16.5 ratings are exceeded then a heavier flange is required.
If the B16.5 ratings are not exceeded, then the external moments and source
of those moments should be reviewed. Additional supports or heavier flanges
may be necessary if the moment stresses exceed more than about 1/3 of
the allowable stress. Additional supports or heavier flanges may not be
necessary or higher moment stresses may be permitted if the contents of
the pipe are benign or pressures and temperatures are low or the source
of the moment is an infrequent event.
Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (www.ppea.net) is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.
Tip for the Month (Jun 2003)This is another handy feature that increases your productivity.
For rows that repeat themselves (element, offsets, material/section/load,
data item), this is a time saver. In the example below, each row between
12 through 16 is a copy of the previous row. So, starting on row 12, simply
press Ctrl+Enter on each new row.
This feature is useful in modeling pipe racks or straight runs of pipe.
Simply input all data in a row and press Ctrl+Enter on the new empty row
to duplicate the previous row's information. The node number is automatically
incremented.
A Location/Comment/Hydrotest row cannot be duplicated. Neither can an
inserted row between two existing rows. This command works only on new
empty rows at the end of the model. If you press Ctrl+Enter right after
a Location/Comment/Hydrotest row. CAEPIPE will duplicate the row before
the Location/Comment/Hydrotest row.
So, how many fewer keystrokes did we need? Too many to count!
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