Chemical Engineering December 2013 - 47

Manways
never
opened
Manhole
Trays
never
cleaned
!
!
Trays
accessed
and cleaned
Figure 3. Improper cleaning of the LPG debutanizer
caused a buildup of rust and dirt
ing the rest of the
refinery running at
a low capacity and
temporarily flaring
LPG as there was
no spare LPG plant.
The debutanizer column
was open, after
water washing and
air purging. The inspector's
first reaction
was surprise.
Everything seemed
fine and the column
seemed clean.
Mystery solved
The debutanizer column was
equipped with two-pass
trays, with side and central
downcomers. On
every other tray, the central
downcomer forms a
wall in the column center,
effectively blocking access
from one side of the tray
deck to the other. It is for this
reason that two-pass trays should
Figure 4. A modern design
of a two-pass valve tray includes
fast opening manways on both sides
treme vapor load. In this case, the
vapor rate could not cause any entrainment
because the reboiler duty
was almost none. One suspicion was
that one of the tray downcomers
was blocked with a piece of mineral
wool insulation or other debris.
Plant management was not very
pleased with this explanation,
commenting that " The column has
been just cleaned and inspected as
being in perfect operating condition,
but you claim the major malfunction
is caused by some debris?
Should the LPG plant be shut
down again for an inspection and
cleaning that was just performed? "
Management needed a justification
due to the expense involved
with another shutdown. The troubleshooter
proved that the problem
was real and it could not be fixed
without a unit shutdown.
The short-term shutdown of the
LPG unit was carried out, keephave
two manways - one for each
segment - to ensure access to both
column sections.
In the case of this particular
LPG debutanizer, the trays were
of an old design, with separate Ibeam
support and bolted decks.
The maintenance crew " did not
waste time with opening both
manways because trays had been
always clean in the past. " Only one
manway had been opened in every
tray for inspection and cleaning.
On trays with central downcomers,
only one half of each tray had
been cleaned as a consequence.
This incorrect cleaning procedure
has been applied for 20 years, since
the column first started up. The
sections " behind downcomer " had
never been inspected or cleaned.
When both manways were opened,
every other tray was found to be
covered with a 1-in. layer of rust
and dirt. Valves were stuck and
some downcomers were blocked. In
the past, cleaning was performed
by flushing with water, so washed
out dirt settled in the non-accessed
tray sections until they became
completely blocked (Figure 3).
Defining the limit as standard
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Chemical Engineering December 2013

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