Hydrocarbon Processing - April 2022 - 71

Environment and Safety
Avoidance of nuisance alarms. During the FEED stage,
the team develops the alarm documentation, considering every
single possibility where process alarms are needed to monitor
the behavior of a particular process unit. During the detailed
engineering phase, the client's operations team may recommend
that nuisance alarms be removed from the list of process
alarms where they may be redundant or not significant for the
operator to monitor during plant operations. A typical example
would be a pressure alarm on the potable water line inside the
unit. Operations typically advises that the interconnecting
units should have alarms on their headers to avoid multiple
monitoring within the process unit, as well. The rationalization
of alarms should take place under the guidance of the client
operations team and streamlined to keep only those process
alarms that are vital for proper plant operation.
Prioritizing process alarms. Prioritizing a process alarm
means assessing a risk associated with a process alarm and advising
the operator about the magnitude of impact on plant
operations-especially with respect to process safety-if the
alarm goes unattended. An emphasis should be placed on defining
the priority for an alarm. There can be cases where some
process alarms are initially labeled as high-priority alarms, but,
after a detailed analysis and discussion, the priority is lessened.
The same is true for low-priority alarms that can be upgraded
to high-priority levels. Too much conservatism should be
avoided in defining the priorities for alarms. The number of
high-priority alarms should be optimized to ensure that the operator
is not overburdened in case of an emergency.
Alarms are typically categorized into high, medium and low
priority, although the exact nomenclature may vary as defined
by the specific design practice applicable for that project. The
Battery limit
Pressure signal
Pressure signal
To SRU train 1
Pressure signal
To SRU - other trains
PT
PC
Acid gas
From ARU
H
L
Acid gas
To SRU train 1
Acid gas
To SRU - other trains
FIG. 1. Pressure alarm for acid gas to SRU.
Cooling water
From supply header
PT
PI
L
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FIG. 2. Low-pressure alarm for cooling-water header.
Hydrocarbon Processing | APRIL 2022 71
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following example of a typical low-pressure alarm on a coolingwater
main header in a process unit is used for explanation of
alarm priority (FIG. 2). Alarm priority for a low-pressure alarm
on a cooling-water header should be defined as high priority.
In the event this alarm goes off, it is very important for the operator
to acknowledge it and to take corrective action immediately
by identifying the cause of the alarm. Low pressure would
result in a low flow, leading to starvation on the cooling-water
exchangers and to high temperatures inside the process units,
subsequently leading to an upset of the process unit. Therefore,
it is defined as a high-priority alarm that can notify the operator
about the severity of the upset.
Time for the operator to act on an alarm. This is an important
aspect that needs significant attention. The primary objective
of a process alarm is to alert the operator about a possible deviation
from the normal operating range. The alarm provides the operator
a chance to take predefined corrective actions to mitigate a
potential trip scenario. The emergency shutdown of a process unit
can lead to production losses and negatively impact the plant life
due to the sudden fluctuation in the operating conditions caused
by an abrupt shutdown. Importance should be given during the
design phase to ensure that sufficient response time is available
for the operator should an alarm go off. Alarms with very little
operator response time are seldom useful and contribute to the
overall alarm count without adding much value. Consideration
should be given to categorize such alarms with a lower priority.
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