Hydrocarbon Processing - April 2022 - 28

Maintenance and Reliability
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designed for fully corroded conditions.
Note: The factor of safety (FS) considered
in the code for new materials is less vs. the
FS for existing older equipment materials.
This is primarily due to enhancements in
materials and welding technology. This
difference in FS is noticeable in the allowable
stress value. Therefore, before starting
calculations, it is important to change
the allowable stress values in the design
software used for analysis. Otherwise,
by default, software uses allowable stress
values as per the current FS. This can lead
to a misinterpretation that the required
thickness is sufficient for the rerate conditions,
which otherwise would be failing
when used with the earlier FS.
The full-scale model of the equipment
must be created for the analysis being inputted
into the rerated design conditions.
A separate file must be created for the
base case that indicates the existing design
conditions, as well. This will provide
leverage to understand how the rerated
design case is different from the base case.
Rerate report. The rerate study should
be properly documented to elaborate the
findings. This report should be in an explicit
format that is easy to understand for
client/owner approval. A comprehensive
rerate report should include the following:
* The base case (old) design
conditions and rerated (new)
design conditions should be
compared and evaluated.
* The code used for rerating the
equipment. Allowable stress values
and joint efficiencies should be
tabulated. Typically, when the
rerated design pressure is less than
the maximum allowable working
pressure (MAWP) of the pressure
vessel, and when the pressure vessel
has been hydrotested to a pressure
based on the MAWP, then the
rerated equipment will
be acceptable in most cases.
* The minimum required
thicknesses vs. the existing
thicknesses for each component
affected by the rerate should
be tabulated. Each value
should be clearly indicated.
For identification, components
that are failing must be indicated
in red or in different fonts.
Additionally, for further
identification of the components
28 APRIL 2022 | HydrocarbonProcessing.com
that are failing, footnotes can be
added below the tables.
* Rerating shell-and-tube heat
exchangers will be more complicated
than vessels and columns, as there
are some components in heat
exchangers that are in contact with
both the shell side and the tube
side, and with the floating head and
tubesheet. Therefore, when rerating
shell-and-tube heat exchangers,
personnel must check how the shellside
and tube-side conditions are
affecting these components.
* The remaining life of each
component and the required
inspection interval determined by
the rerate study should be tabulated.
Instead of concluding that the
equipment must be replaced when
its corrosion allowance is consumed,
it can be downrated to allow its
continued use in a less-severe design
condition. Such recommendations
can be highlighted in the report for
owner approval.
* Components requiring any
physical modifications should also
be clearly identified. If a hydro
test must be carried out, test
pressure and temperature must
be mentioned. All the relevant
attachments to the report-such
as rerated design calculations,
original manufacturer's calculations,
drawings, the manufacturer's data
reports and equipment inspection
reports-should be included. Most
importantly, revised and/or markedup
drawings, as well as new drawings
for replacement components
and rerated nameplates, must be
included in the report.
Takeaway. Based on the rerating study
and further meetings with the owner/
stakeholders, one of the following recommendations
can be concluded for equipment:
use " as is, " modify or replace. Modification
or replacement can be initiated
with a new requisition.
SUTHAN RUPESH is a Consulting
Mechanical Engineer based in
Kerala, India. He has more than
19 yr of experience in the field of
static equipment. Mr. Rupesh is
also a member of the Institution
of Engineers (India), and holds
a BEng degree in mechanical engineering from
Lukhdhirji Engineering College in Morbi, India.
HPI
MARKET
DATA
2022
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Hydrocarbon Processing - April 2022

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