Chemical Engineering October 2021 - 26

Cover Story
Monitoring Ammonia and Other
Toxic Gas Hazards
There are many technologies used in industry to protect workers from toxic gases.
Here are some guidelines for selecting one that will match your application
Sarah Rajasekera
MSA Safety, North America
T
or
he dual threat of toxicity and
combustibility applies to a
number of industrial gas hazards.
When such gases go
undetected, they can lead to catastrophic
accidents with the potential
to injure or kill employees and damage
destroy
plant equipment.
Ammonia is one of those potentially
double-trouble hazards, especially in
its most common usages as a refrigerant
or fertilizer product. It can be
found as a chiller or refrigerant in the
food and beverage industry, pharmaceutical
production, in air-conditioning
equipment, in electric power
generation plants and, of course, fertilizer
production. Understanding the
proper precautions to take with ammonia
provides a highly useful model
or example of what is required when
protecting people, equipment and
plants against the threats of many
other toxic and combustible gases.
In general, those responsible for plant
processes and safety need to understand
the health threat and symptoms
of exposure to any toxic gas, the nature
of any toxic or combustible gas
(how it behaves in a plant setting) and
the gas detection sensing technologies
available. The article explains the
advantages and limitations of gas detection
technologies, as well as their
maintenance requirements.
Understanding the gas
According to the U.S. Occupational
Safety & Health Administration
(OSHA; Washington, D.C.; www.
osha.gov): " Ammonia is considered
a high health hazard because it is
corrosive to the skin, eyes and lungs.
Exposure to 300 parts per million
(ppm) is immediately dangerous to
life and health. Ammonia is also flammable
at concentrations of approximately
15 to 28% by volume in air. "
OSHA also correctly points out that
when ammonia is mixed with lubricating
oils, its flammable concentration
range increases. It can explode if
released in an enclosed space with a
source of ignition present, or if a vessel
containing anhydrous ammonia
is exposed to fire. Fortunately, ammonia
has a low odor threshold of 5
ppm, so most people will seek relief
at much lower concentrations.
How gas behaves in the plant
Toxic gases, including ammonia, can
be difficult to detect, depending on
the specific gas and the plant layout.
For example, fertilizer plants are typically
crowded with equipment where
ammonia can be present. The same
can be said for many other materialsprocessing
or other industrial plants
where ammonia is present as a refrigerant
gas for chilling or refrigeration.
Layered, redundant monitoring
gas-detection sensing technologies
provide an added
degree of safety
against toxic and
combustible gases,
such as ammonia.
Leaking ammonia
FIGURE 1. Optical infrared sensors offer a high
specificity for ammonia without degradation when
exposed to high concentrations
26
also tends to form
clouds outside plant
buildings that are affected
by temperature
and weather.
These clouds also
have the potential
to travel beyond the
plant perimeter, poFIGURE
2. Photoacoustic infrared sensors can
continuously detect ammonia at very low concentrations
without the need for a reference point
tentially endangering other nearby
facilities or the community at large.
For these reasons, plant safety teams
typically rely on a mix of portable
detectors worn by employees and
plant fixed-gas detection systems,
including perimeter monitoring.
Gas detection and sensing
There are several types of fixedgas
detectors
that
are appropriate
for protecting plants from toxic
gases. Point detectors relying on
infrared (IR), electrochemical cell or
other contact sensing technologies
are typically placed throughout the
plant near equipment in production
areas, on pumps, valves, pipelines
and tanks for loading and unloading.
Along plant exterior boundary
fence lines, perimeter detectors
are placed. These detectors rely on
open-path detection technologies.
Many types of fixed-gas sensors
FIGURE 3. Enhanced laser diode spectroscopy can be used to detect specific
gases without false alarms from interference gases or water vapor
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OCTOBER 2021
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Chemical Engineering October 2021

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