Chemical Engineering October 2015 - 74

possible; most of the movement
and loads will be absorbed at the
thermal-expansion loop). However,
the gap was likely initiated due to
some excessive external force (for
instance, an external bending moment)
from the molecular seal drain
line. A close review of the maintenance
history revealed that there
had been a modification of the drain
line - the original thermal-expansion
loop was removed because of
frequent plugging. Taking everything
into account, we concluded that the
root cause of the crack was excessive
stress due to the removal of the
thermal-expansion loops.
Since this gap had allowed for
continuous air ingress to the molecular
seal drum, that helped to
explain the longterm internal burning
that had led to coke buildup during
operation. Because there had
been frequent plugging due to refractory
debris and ice buildup, a
decision was made to remove the
90-deg elbow-type thermal-expansion
loop, which is the critical point
of debris collection and also critical
for water collection.
A new thermal-expansion loop
must be provided. To help reduce
the risk of plugging or icing, and
any such debris must be cleared as
soon as it is detected, regardless
of the complicated design requirements.
If the drain line is straight
all along the flare stack, then it can
be easy for maintenance personnel
to clean the pipe, even if the line
is plugged or frozen. But we must
consider the thermal-expansion of
the drain line, which is steam-traced
and insulated.
It is generally agreed today that, to
FIGURE 8. The ability to eliminate air ingress
through proper engineering design and ongoing
maintenance and troubleshooting is an important
factor in optimizing the reliability and safety of
flare systems
improve the reliability of a flare system,
special considerations are required,
in addition to the installation
of an insulation ring with a sloped
thermal-expansion loop (rather than
a potentially troublesome 90-deg
elbow) on the molecular-seal drain
line. One way to solve this problem
is to design the thermal-expansion
loop with a 45-deg elbow instead of
90-deg elbow, to reduce the opporGemco
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Circle 35 on p. 102 or go to adlinks.chemengonline.com/56202-35
74
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Chemical Engineering October 2015

Table of Contents for the Digital Edition of Chemical Engineering October 2015