Hydrocarbon Processing - April 2022 - 72

Environment and Safety
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or equipment maintenance. Alarms with a suppression requirement
should be identified and documented to make the unit
Feed heater
AI
AT
Feed + hydrogen
Hydrotreater
Sulfur absorber
FIG. 3. Sulfur slippage from hydrodesulfurization reactors.
Mode-dependent alarms (alarms during normal operation,
startup or shutdown). Mode dependency is another
vital aspect to be defined during the alarm management study.
The client's operations team should advise to clearly define the
alarms that are mode dependent (i.e., to categorize based upon
startup, shutdown and normal operation). This provides a deep
insight to operators to understand the applicability of the alarm
during a particular phase of operation of a process unit.
Alarm verification. It is not uncommon to have spurious
alarms in a refinery. It is always useful if the alarm can be verified
with the help of another independent instrument reading. It is
advisable to consider a realistic alarm verification. For example,
a pressure alarm in a process stream should be verified against
another independent pressure instrument (such as a pressure
gauge), and an indirect verification (e.g., flowrate) should be
avoided. However, there may be certain instances where an indirect
verification cannot be avoided. The following example from
a hydrogen production unit explains the difference between a
direct and an indirect verification.
The hydrodesulfurization section of a hydrogen production
unit typically consists of a hydrotreater and a sulfur absorber.
The mixture of the hydrocarbon feed and hydrogen is heated to
the desired inlet temperature before entering the hydrotreater.
The sulfur compounds present in the hydrocarbon feed are converted
to hydrogen sulfide (H2
S) in the hydrotreater and are subsequently
absorbed in the sulfur absorber. Inefficient operation
of the hydrodesulfurization section may cause sulfur slippage,
which can damage the steam reformer catalyst located downstream.
For this purpose, an online analyzer with a high sulfur
alarm is usually provided at the outlet of the sulfur absorber to
alert the operator. A manual sampling station provided at the
same location is considered a reliable direct verification for the
high sulfur alarm; however, lab analysis may take some time before
the results are made available. Since a low hydrotreater inlet
temperature causes sulfur slippage due to poor conversion, the
low-temperature alarm available at the inlet of the hydrotreater
may be considered a credible indirect verification that can be
verified instantly (FIG. 3).
Suppression of alarms. Provision for alarm suppression
may be required for some alarms, such as during unit startup
72 APRIL 2022 | HydrocarbonProcessing.com
Clearly defined operator actions. The corrective actions
to be performed by an operator in case of the activation of an
alarm should be clear, specific and realistic in nature. The actions
for panel operators and for field operators should be segregated
and documented. If possible, actions may be outlined
in a sequential manner in order of importance. The response
time available for the operators is usually minimal; therefore, it
is important to avoid assigning any unnecessary and irrelevant
actions. Clearly defined actions provide a clear path forward
for the operators, allowing them to revive the process to avoid
a plant upset or trip. Obtaining a clear understanding and agreement
from operators is essential.
Takeaway. Alarms play an important role in normal plant operations,
as well as during startup and shutdown scenarios. Proper
configuration and definition of alarms are of prime importance.
A prime concept that is often neglected involves considering the
inputs from the end users of these alarms, i.e., the plant operators.
During the design phase for alarm configurations, it is important
to consider the operators' perspectives and provide the best parameters
for configuring them as defined in this article. This also
ensures that the proper priorities and corrective actions are considered
against each alarm. Therefore, it is important to configure
alarms by taking into consideration the operators' perspectives
for a safe, continuous and optimum operation of the facility.
NOTE
The conclusions presented in this article are solely those of the authors and cannot
be ascribed to Fluor Corporation and/or any of its subsidiaries.
DEBOPAM CHAUDHURI is a Senior Process Engineer with Fluor
New Delhi. He has more than 20 yr of experience in petroleum
refining, petrochemicals and upstream projects. He earned Bch
degrees in chemistry and chemical engineering from the
University of Calcutta, India.
PRANAY VEER SINGH is a Process Engineer with Fluor New
Delhi. He has more than 12 yr of experience in petroleum
refining, petrochemicals and fertilizer projects. He earned an MS
degree in chemical engineering from the Indian Institute of
Technology in Kharagpur, India.
VENKATA SRAVAN KUMAR is a Process Engineer with Fluor
New Delhi. He has more than 14 yr of experience in petroleum
refining, petrochemicals and fertilizer projects. He earned a Bch
degree in chemical engineering from Jawaharlal Nehru
Technological University in India.
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operator aware of the suppression aspect of a process alarm.
The automation contractor should devise a methodology to
suppress alarms that will go off during a particular mode of operation.
For example, in a typical process unit, when the feed is
cut in, the low-level alarms will go off in most of the vessels in
the unit, especially in the feed knockout drum, leading to confusion
during the startup. Such alarms are suppressed until a
steady-state operation is achieved. The operator can change the
status of such alarms from a suppressed state to a normal state
in the distributed control system once the unit is stabilized and
normal liquid levels are achieved.
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