Hydrocarbon Processing - December 2020 - 68

Environment and Safety

FIG. 4. Flame detectors respond to specific
wavelengths produced by burning hydrogenor carbon-based fuels.

CO2 peak
Visible
IR
Solar radiation reaching earth's surface

Intensity

H2O peak

.2μ .29μ .4μ

FIG. 3. Combustible gas detectors use
different approaches, including (a) acoustica,
(b) point sourceb and (c) open pathc.

of the gas detectors shall be based on an
engineering evaluation. "
Flame detectors. Once a fire has started, detecting it as quickly as possible is
critical for avoiding plant damage and
possible harm to personnel. Several technologies have been used, most of which
respond to heat or smoke. The problem
with these is that they require the fire to
burn long enough to reach the measurement threshold. This works adequately in
an enclosed space, but is not ideal for the
open construction of refineries and chemical plants. The fire can easily grow and
spread before enough heat or smoke has
accumulated to be detected.
A newer technologyd (FIG. 4) detects a
fire in progress by " seeing " specific wavelengths of ultraviolet (UV) or infrared (IR)
radiation characteristic of hydrocarbon-,
hydrogen- and carbon-based fuel (FIG. 5).
As soon as a fire breaks out, the sensor can
recognize the change in a matter of seconds
and trigger the desired response. Since the
68

DECEMBER 2020 | HydrocarbonProcessing.com

.8μ
2.7μ
Wavelength

4.3μ

FIG. 5. Combustion of fuels containing
hydrocarbons, hydrogen and/or carbon
creates intensity peaks in detectable
wavelengths.

range of possible fuels in each situation is
limited, it is possible to select a unit able to
detect the relevant wavelengths, thereby
minimizing the potential for a false alarm
from an unexpected source, such as sunlight, arc welding or incandescent lighting.
NFPA 72 Chapter 17.8 (Radiant-Energy-Sensing Fire Detectors) details how
these should be selected and deployed.
Section 17.8.2 details the specific wavelengths involved and how the radiant energy is propagated. It discusses how smoke
from burning fuels affects radiation and
how to minimize interference. Section
17.8.3 provides an extensive discussion of
spacing, location and coverage (very helpful to someone designing an installation),
and even includes maintenance tips.
Uneven prescriptiveness. Given the

uneven treatment of various topics, what
is the purpose and value of a standard? It
is an authority, but not a design guide. A
technician could refer to it to determine
a critical detail (such as how to terminate

the cable of an acoustic gas detector correctly), but it will not say where to place
the gas detector. The person reviewing
plans for a new fire suppression system
will undoubtedly use it to verify many
specific points, but it will not help much
when plotting the initial strategy.
Standards also change over time.
NFPA updates its standards every 3 yr,
and there are changes for each version. A
facility that has been operating for many
years (e.g., refineries and chemical plants)
likely has items installed under many editions of the relevant standard.
It is rare when a standard is revised
drastically enough to call for something
installed under a previous version to be
changed. The more common situation is
where an old installation does not take advantage of new technologies that were not
available in previous years. For example,
a flame detector using a slow-acting heat
sensor could be updated to a newer flame
detector to provide a quicker response
and a higher level of protection.
The NFPA's National Fire Alarm and
Signaling Code Handbook (an expanded
version of NFPA 72, with extensive commentary and resources) advises:
Whenever a system is modified
or updated, it is vital that the
system designer have a thorough
understanding of the existing
equipment, including its capabilities
and the system's wiring (i.e., circuit
class, type, and configuration). Where
applicable, the software and firmware
of existing systems need to be
examined to verify compatibility with
the new equipment. Often, the existing
equipment is too old to interface easily
with the newer technology used in the
planned additional equipment. The
existing equipment may or may not
be able to be modified to conform to
current Code requirements. In some
cases, the most prudent choice may
be to install a new fire alarm system
or other emergency signaling system
where used.
A standard is most important to the people evaluating or inspecting an installation. This could be the state or local
permit-granting body or an insurance
company evaluating the risk at a client's
facility. The plant needs to know the standard well enough to defend its actions

http://www.HydrocarbonProcessing.com

Hydrocarbon Processing - December 2020

Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2020