Hydrocarbon Processing - November 2022 - 62

Plant Safety and Environment
a few of these cases, likely because the
potential dangers of oxygen enrichment
are relatively well known and safeguarding
procedures are well implemented.
Incident investigations for thermal
excursions in vessels outside of burner
chambers are aided by the relative prevalence
of accurate thermocouples in these
areas. Once the starting location of the
thermal excursion is determined, then
the operating history can be reviewed to
determine which of the potential causes
(auto-ignition, pyrophoric FeS ignition
or TGU catalyst self-heating) was most
likely the cause.
Incident investigations within burner
chambers (reaction furnace, fired heaters
and incinerators) can be made more
difficult by the fact that temperature
measurement devices in these areas are
more likely to be inaccurate or nonexistent,
and that peak flame temperatures
are often much greater than the average
or local temperatures measured by the
devices that do exist. In these cases, good
simulations of flame temperatures based
on metered or assumed flowrates are an
important part of the investigation.
Regarding prevention of thermal excursions,
the following recommendations
are associated with the most common
root causes:
* Auto-ignition of sulfur
vapor/liquid:
° Ensure that the process design
does not permit air ingress
into the SRU when the plant
is hot and contains sulfur. The
most common ways that this
unwanted air ingress occurs is
when reaction furnace nozzle
purges are done with air (air
purges should be automatically
switched to nitrogen during a
trip) or when purges of burner
management systems (BMSs)
use air as the purge medium
(again, nitrogen or some other
inert gas should be used as the
burner purge medium when the
plant is hot and full of sulfur).
° Be aware of the auto-ignition
characteristics of sulfur and
understand that operating
procedures prevent manual
introduction of air when the
plant is hot and full of sulfur.
° Use portable oxygen analyzers
whenever sulfur plants are
62 NOVEMBER 2022 | HydrocarbonProcessing.com
switched from acid gas firing
to fuel gas firing (i.e., for hot
standbys and shutdowns) to
confirm proper stoichiometry.
Even very brief periods of excess
stoichiometry at this point can
result in significant thermal
excursions.
° Set alarm points on all process
thermocouples that would be
consistent with a sulfur fire.
° Ensure that proper fire response
procedures are in place; eliminate
the excess air first and then use a
suitable fire suppression medium.
* Pyrophoric ignition of sulfur
at any temperature:
° Ensure that plant personnel
are aware of the properties of
pyrophoric FeS, and that fires can
occur at any temperature when
excess air is first introduced to
the sulfur plant. One of the most
common causes of fire is during
the startup process, when frozen
sulfur, left during a turnaround,
melts and exposes the underlying
pyrophoric material.
° Procedures for the introduction
of excess air during shutdowns/
turnarounds should involve
very gradual increases in excess
oxygen levels, and should allow
for careful monitoring and rapid
removal of the air if a fire is
discovered.
° Ensure proper insulation and/
or heating of appropriate sulfur
plant areas to minimize the risk of
pyrophoric FeS formation.
° Ensure that the sulfur plant
design and construction
eliminate low points where
liquid sulfur can collect in
process piping and vessels.
These unexpected sulfur pools
are the primary locations for
pyrophoric sulfur fires.
* Self-heating TGU catalyst:
° Plant personnel should be
aware of the self-heating nature
of this catalyst and ensure
that design and operating
procedures minimize the risk
of air introduction to hot and
activated TGU catalyst.
° To confirm proper stoichiometry,
portable oxygen analyzers should
be used whenever TGU burners
are first restarted on fuel gas.
Even very brief periods of excess
stoichiometry can result in
significant thermal excursions.
° TGU catalyst should either be
passivated during shutdown
(involving the gradual
introduction of increasing excess
oxygen levels) or kept under
an inert gas blanket during
turnaround and catalyst loading/
unloading.
* Overheating of burner chambers
on fuel gas firing [refractory
damage above 1,538°C (2,800°F)]:
° Stoichiometric natural gas firing
should always include a sufficient
volume of cooling gas, such as
nitrogen or dry steam. A 4:1
volume ratio of inert cooling
medium to fuel gas is usually
sufficient to result in acceptable
peak flame temperatures.
° Burner chamber design should
include the most accurate
modern combination of
thermocouples and pyrometers
for measuring process gas
temperatures and refractory
temperatures. Regardless of the
accuracy of the temperature
measurement, the addition
of a cooling medium is still
required during stoichiometric
firing-otherwise, peak flame
temperatures will exceed the
refractory melting temperature,
regardless of the reported
temperature.
° For burner chambers with
multiple temperature zones,
a temperature measurement
(or other suitable safeguards)
should exist for each zone.
* Overheating of burner chambers
on acid gas firing [refractory
damage above 1,538°C (2,800°F)]:
° Redundant safeguards should
be employed whenever oxygen
enrichment is used (e.g.,
temperature alarms and trips,
oxygen limits, and simulated
temperatures).
° Additional safeguards should
be used whenever multiple
temperature-increasing
operational modes are used (e.g.,
oxygen plus split flow, or split
flow plus fuel gas co-firing).
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