Hydrocarbon Processing - December 2020 - 67

Environment and Safety

How much protection is enough? A
refinery or major petrochemical plant
will have safety strategies and systems arranged to provide multiple layers of protection (FIG. 2), each designed to trigger
a minimally disruptive response to a developing incident. These systems include:
1. The distributed control system
(DCS), which keeps the process
on an even keel and controls
upsets
2. Safety instrumented systems
(SISs), which respond to
problems that the DCS cannot
handle to maintain product
containment-shutting
production down, if necessary
3. Gas detectors, which identify
when a release of flammable
products has occurred
4. Flame detectors, which identify
when an actual fire has started,
and, consequently, may trigger
suppression systems.
Note: In this list above, 1 and 2 are preventive, whereas 3 and 4 mitigate an incident
in progress, and each level has a progressively disruptive response connected to it.
Designing protection. Determining
what these systems should look like is
covered under a variety of standards. For
example, SISs are covered by the International Electrotechnical Commission
(IEC) under IEC 61508 and IEC 61511.
Moving into the realm of mitigation efforts, the picture changes to the National
Fire Protection Association (NFPA), since
fire is the primary concern. The NFPA has
standards applicable to virtually any type
of residential, commercial or industrial
building or facility, so it can be a challenge
to figure out what applies in each situation.
The two standards used most frequently for refineries and process plants
are NFPA 70: National Electrical Code

and NFPA 72: National Fire Alarm and
Signaling Code. These are enormous in
scope, as they apply to commercial and
industrial facilities.
NFPA 70 Chapter 5 is the origin of the
hazardous location classification system
used extensively in process plants, which
most will recognize as Class 1, Division 1,
etc. If more detail is needed, there is also
NFPA 497: Recommended Practice for
the Classification of Flammable Liquids,
Gases or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. The
purpose of this system is to avoid putting
electrical equipment capable of creating
arcs and heat in areas where there might
be flammable fumes present, thus denying a potential fire an ignition source.
NFPA 72 is more specialized, and concentrates on fire detection and alarming.
Again, it covers any conceivable application, so that much of it is irrelevant to a
refinery. It expands on NFPA 70 in that
it discusses the specialized wiring necessary to support fire detectors and alarms.
It also cross-references with other NFPA
standards, including those already mentioned, as well as the following:
* NFPA 15: Standard for Water Spray
Fixed Systems for Fire Protection
* NFPA 30: Flammable and
Combustible Liquids Code
* NFPA 59A: Standard for the
Production, Storage, and Handling
of LNG.
For a company trying to decide if it has

Historian

Control
layer
Fire detectors
Detection and notiﬁcation

other stakeholders also weigh in.
Local authorities want to make sure
that the plant has taken appropriate precautions to avoid causing environmental
damage or harm to workers and the local
populace. They enforce this through the
permitting process and inspections, and
they can force a shutdown, if necessary.
Moreover, the insurance carrier protecting the company and facility wants to ensure that it is not taking undue risks with
an irresponsible client.

Gas detectors
Manual ﬁre
stations
Smoke and heat
detectors

deployed adequate systems necessary to
protect a facility and its people, the standard is important but is often not adequate enough. Some specific areas are
well detailed, while others seem particularly vague. NFPA 72 Chapter 17 (Initiating Devices) offers a case in point when
comparing the discussions related to gas
detectors and flame detectors.
Gas detectors. Combustible gas detectors are installed to recognize when flammable hydrocarbons have escaped their
containment and might be drifting with
enough concentration to burn if they find
an ignition source. These can be gases
(e.g., methane and propane) or liquid vapors. Some designs can detect a presence
of gases or vapors when the concentration
is below the lower explosive level (LEL),
allowing responders to locate the source
before a fire is possible.
Gas detectors can use three technologies (FIG. 3), which include:
* Acoustic detectors, which listen
for the characteristic noise of a
compressed gas leak
* Point detectors, which use either
catalytic bead or infrared sensors
* Open-path detectors, which detect
a target gas moving through
a beam of light.
NFPA 72 Chapter 17.10 covers gas detectors, and it is frustratingly brief. It offers
several statements, including one saying
that detectors shall be listed by ANSI/
UL, and that " the selection and placement
Humanmachine
interface

Engineering
stations

Safety
instrumented
system

Fire and gas
system
Activate
protection

Emergency
shutdown

Distributed
control system
Instrumentation

Fire water pump
control

Valves

Signal to HVAC
system

Process assets

Deluge/suppression
activation

Horns, beacons,
alarms, paging

FIG. 2. A loss of containment means that an incident has broken through multiple protective
layers. Source: Kenexis Fire and Gas Systems Engineering Training Ltd.
Hydrocarbon Processing | DECEMBER 2020

Hydrocarbon Processing - December 2020

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