Hydrocarbon Processing - December 2020 - GP-36

HEAT EXCHANGE

exchanger has been exposed to a number
of cycles above this dT/dt guideline.
Repeat thermal fatigue will eventually
result in leaks. No reliable way exists to directly measure fatigue damage before it results in a leak. This is mainly due to complexities involved with the causes, such
as frequency of temperature fluctuations,
the duration of each cycle and the extent
of temperature variations. Nevertheless,
the majority of BAHEs that experience
thermal stress cracking, either from highcycle fatigue or from low-cycle thermal
shock events, exhibit small leaks that can
be detected and repaired in the field.5
As per API 572,6 " It is important for
the owner-user and the inspector to understand that fatigue cracking is likely to
cause vessel failure before detection with
any non-destructive methods. Of the fatigue cycles required to produce failure,
the vast majority are required to initiate
cracking, and relatively few cycles are required to propagate the crack to failure. "
After the failure of the BAHE at the
Pascagoula facility, the CSB calculated
historical temperature fluctuations for the

inlet and outlet streams.6,7,8 The data is
plotted in FIG. 7. Meanwhile, FIG. 8 shows
the time period for high-temperature
fluctuations per unit time. Temperature
fluctuations (dT/dt) are categorized into
five temperature ranges, split between
two graphs, to illustrate the magnitude of
stream temperature changes.
Note: The Y axis scale is different for
the graphs in FIG. 7 and FIG. 8. The CSB
compiled the data without filtering the
startup, shutdown or upset conditions. It
is also possible that these numbers were
very high, as the full data is not available.4
The plot in FIG. 8 demonstrates that
high dT/dt lasted a long time-sometimes even several days. This happened
every year from 1999-2006, which did
not result in immediate leakage; however, accumulated thermal fatigue during this time resulted in leakages in
2007, 2011, 2014 and 2015. Although
these leaks were repaired, the localized
repairs could not reverse the thermal fatigue accumulated on other parts of the
exchanger that had not yet resulted in a
leak. Literature suggests that " there is no

way to reverse damage accumulation due
to thermal fatigue. " 5
It appears that operators were more
vigilant in reducing the temperature variations after the first leak in 2007. However, the accumulated thermal stresses
had already damaged various sections of
the BAHE. Even small variations in temperature or pressure swings resulted in
regular leaks, which resulted in the major
accident in 2016. GP
1

LITERATURE CITED
" The Standards Of The Brazed Aluminum PlateFin Heat Exchanger Manufacturers' Association
(ALPEMA), " ALPEMA, 3rd Ed., May 2012.

Complete Literature Cited available online at
www.HydrocarbonProcessing.com.
IRFAN HUSSAIN is Lead Process
Engineer at OQ Oman. He is a
Chartered Engineer (CEng), a
member of MIChemE-UK and a
Chartered Professional Engineer
(CPEng) of MIEAust-Australia.
Mr. Hussain has been working is
the upstream and downstream oil and gas industries
for 19 yr. He has worked with various operating
and design engineering companies in the UK,
Oman, Australia and Pakistan.

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NOVEMBER/DECEMBER 2020 | GasProcessingNews.com

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Hydrocarbon Processing - December 2020

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