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COMPRESS reports the rated MDMT for each
component in the Pressure Summary
report. The rated MDMT is determined
using the applicable rules of UG-20 and
UCS-66. If the rated MDMT is warmer
than the value specified in the
component's MDMT input, COMPRESS reports
a deficiency but does not automatically
specify that impact testing be
performed. Depending on the
circumstances, MDMT may be reduced by a
number of variables such as the
application of PWHT, impact testing the
material and even changing the applied
stress ratio. It is the designer's
responsibility to arrive at the desired
MDMT by taking action that is
appropriate to the circumstances.
To find the rated MDMT, COMPRESS first
calculates the governing thickness per
the rules of UCS-66(1), (2) and (3).
COMPRESS then determines if the
component is exempt from impact testing
by the rules of UG-20(f) or UCS-66(d),
or if the governing thickness is exempt
based on the curves of Figure UCS-66. If
a material is exempt from impact testing
by UG-20, the rated MDMT is reported as
the lowest design value in UG-20, which
is -20° F.
If a material is not impact test exempt
by these paragraphs, COMPRESS rates the
material according to the rules of
UCS-66 paragraphs (a) and (b). Figure
UCS-66.2 provides rules for determining
the lowest MDMT permissible without
impact testing.
If the material is not exempt from
impact testing (at the specified MDMT)
under any of the above, COMPRESS returns
a deficiency when the reports are
generated. The designer can remedy MDMT
deficiencies in one of several ways by
returning to the component design
dialog. One way is to specify the
material as impact tested. Of course,
this may not be the most economical
method of achieving the desired MDMT.
Another important consideration is that
COMPRESS rates MDMT coincident with the
vessel maximum allowable working
pressure (MAWP). This is in accordance
with general note 2 of Figure UCS-66.2.
The COMPRESS option "Maximize MDMT/No
MAWP" can be used to lower the rated
MDMT by specifying that design pressure
be used as the coincident pressure
instead. Using the COMPRESS option
"Maximize MDMT/No MAWP" will
usually result in a lower rated MDMT but
by definition limits the MAWP to be
equal to the specified design pressure
(P) .
The COMPRESS option "Find coincident
pres. for MDMT" determines the
pressure at which the MDMT input in the
dialog is exempt from impact testing.
To use it, check the option and advance
to the next dialog and return to the
previous dialog. The design pressure
input is set to the maximum pressure
that can be specified without having to
impact test the material and the switch
is turned off.
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