Hydrocarbon Processing - February 2022 - 66

Plant Turnarounds and Project Management
exchangers (usually kettle type) with
some special design requirements. Medium-pressure
or low-pressure steam is generated
on the shell side, while liquid sulfur
flows on the tube side as sulfur vapors are
cooled below the dewpoint.
Both the steam side and channel side
of the condenser are provided with a mist
eliminator (demister). The shell side is
clean service (water and steam); therefore,
the shell-side demister does not
need frequent cleaning.
The channel-side wire mesh
type
demister is kept hot by a steam coil surrounding
the mesh pad to avoid sulfur
condensation. In one design, it was found
that the mesh pad was blocked from all
sides. Therefore, without modification, it
was impossible to remove the demister.
FIG. 5 provides details of the modified
demister box.
Shrouds and louvers. The main reaction
furnace is a vital piece of equipment
in a sulfur plant. The high temperatures
and operating conditions in the burner
and furnace require protection of the carbon-steel
body metal via a thermal shroud
and, internally, via an external and multilayered
refractory lining.
The system is designed to maintain the
temperature between
inner-side metal
150°C and 300°C to avoid both excessive
acid and sulfide corrosion. The shroud
also handles external temperature variations
during winter and rains, as it maintains
an air gap around the shell. The gap
is designed to control air flow around the
equipment to maintain the temperature
of the metal. For this purpose, variable
openings are provided at the top of the
shroud with louvers and hats (FIG. 6). The
louver openings can be manually or automatically
adjusted to control convective
air flow. The opening size and operability
of louvers must be checked and inspected
by a field engineer before plant commissioning.
Sometimes, louver openings differ
from drawings and are inoperable.
FIG. 6. Shroud and louvers for a furnace.
Overheating of the thermal oxidizer.
All tail gases are burned in an incinerator
at the end of a process.
A case was encountered where, during
summer days, the operator complained
of very high temperatures outside and
around the incinerator. It was not possible
to get closer to the equipment for operation
and maintenance, and there was also
the chance of damaging delicate instruments
and cables near the equipment.
Rechecking the design and thermal
calculations, it was noticed that:
* The original design was for a unit
in a colder country.
* The insulating castable refractory
used was below specification,
resulting in an increase in the
vessel's body temperature.
The challenges were resolved by replacing
the old refractory with one of superior
quality.
Wrong location of a guide support
for a steel stack. The final effluent gas
from the process is incinerated in an incinerator.
In a specific case, it was a smalldiameter,
12-in. stainless-steel stack, with
a height of around 25 m and a guide support
at two levels. A supporting structure
was used. One support was in the middle
and another was near the top. The guides
were a fin-type support (FIG. 7) that passed
through the structure's frame opening.
In this incident, everything went well
during startup, commissioning and operation.
However, when a shutdown was
taken, it was noticed that the top guide
structure had been slightly pulled down
by the stack.
After an investigation, it was discovered
that the guide fin lengths were not
placed at the right elevation to handle
the stainless-steel pipe expansion. During
operations, the stack expanded upward,
and the guide supports went above
the structure. It was resting on a beam
structure during operation and, due to
the wind load pressure on one side, got
stuck. During shutdown, the stack cooled
and shrunk, causing the guide fins to pull
down the structure.
PRAMOD DIXIT works a freelance
SME-Mechanical Consultant in
Baroda, India. He provides training
and troubleshooting services to
plant design engineering companies
and process plant companies. He
has more than 40 yr experience in
FIG. 7. Guide support for a vent stack.
66 FEBRUARY 2022 | HydrocarbonProcessing.com
design engineering, procurement and construction
companies, with specialization in heat and mass
transfer and static equipment. He earned a BTech
degree in mechanical engineering from the Indian
Institute of Technology, Roorkee, India.
http://www.HydrocarbonProcessing.com
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