Chemical Engineering April 2020 - 55

FIGURE 3. Every combustible gas burns differently, producing a signature flame. Compared to hydrocarbon
flames, depicted on the right and left, a hydrogen flame (center) emits little visible light and infrared
radiant heat, and is therefore more difficult to detect
FIGURE 2. Materials rated a " 4 " in terms of flammability
in NFPA 704 are those that will readily
burn at room temperature, including acetylene,
propane and hydrogen gas
a fire or explosion.
Two common technologies for
detecting combustible gases are IR
and catalytic bead (Pellistor) detectors.
Today's IR-based combustiblegas
detectors cannot detect H2. This
makes catalytic bead-type detectors
the appropriate technology choice
between the two for detecting H2 in
combustible concentrations.
A catalytic bead sensor detects a
combustible gas that combines with
oxygen to produce heat. This sensor
usually consists of a matched
pair of platinum wire-wound resistors,
one of which is encased in a
ceramic bead. The active catalytic
bead is coated with a catalyst, while
the reference catalytic bead remains
untreated. The resistors are then enclosed
behind a flame-proof sinter or
porous filter.
When a combustible gas comes
in contact with the active catalytic
bead surface, the gas is oxidized
and heat is released, which changes
the resistance of the wire. The reference
(or passive) bead maintains the
same electrical resistance in clean
air as the active bead, but does not
catalyze combustible gas. Combustible
gas concentrations are then
determined by comparing the difference
between the active and passive
bead circuits.
On the downside, catalytic bead
detectors may not have the ability
to signal a fault when they fail. They
are also susceptible to poisoning,
which can cause them to fail when
exposed to silicones and other common
chemicals in industrial environments.
In these cases, the porous
filter clogs, causing the active bead
to behave in the same manner as the
reference bead. If the active bead in
a catalytic detector cannot sense
gas, plant personnel have no way of
knowing. Therefore, periodic bump
or proof testing with calibration gas
is required to ensure proper sensor
operation.
Plant personnel or others placing
catalytic bead-type gas detectors
should keep in mind that light H2
gas quickly floats upward and disperses.
Therefore, detectors should
be located close to and above spots
where a leak might occur - just
above a valve, for example.
Flame detection options
If a H2 leak does ignite, F&G safety
systems include detectors that can
quickly sense a H2 flame. One option
for H2-flame detection is a thermal
heat detector, which will not sound
an alarm until the temperature of the
monitored area exceeds the detector's
alarm set point. For best results,
these detectors should be placed directly
above the sites of a potential
H2 flame.
Even in this position, the source of a
H2 leak may produce a flame directed
away from the detector, delaying the
response time. Another concern for
users of thermal detectors is that a
H2 flame's low IR radiation may not
be enough to trigger an alarm.
An alternative to thermal detection
is an optical detector that can
" see " a hydrogen flame. Compared
to hydrocarbon flames, however, hydrogen
flames emit little visible light
or IR radiant heat. One potential option
is to use an optical detector that
senses the ultraviolet (UV) radiation
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emitted by hydrogen flames.
UV flame detectors use anode/
cathode
Geiger-Mueller-type
vacuum
tubes to sense UV radiation.
When this radiation enters the vacuum
tube and strikes the cathode,
energy from the UV photon releases
a
photoelectron
and
creates
an
electrical impulse as it travels to the
anode. UV flame detectors excel at
fast detection of H2 flames.
On the other hand, UV flame detectors
are sensitive to arcs, sparks,
welding, lightning and other UV
emitting non-flame sources. These
sources can cause the detectors
to alarm, which may result in costly
production downtime.
Therefore, UV flame detectors are
best suited for enclosed rooms and
other locations isolated from potential
sources of false alarms. Even in
enclosed rooms, it may not be possible
to eliminate all sources of nuisance
alarms from UV sources.
Superior false-alarm rejection is
one of the main reasons multi-spectrum
infrared (MIR) flame detection
has become the preferred choice
of many operators for detecting H2
flames in most industrial settings
(Figure 4). With a unique set of IR
sensor filters, some MIR detectors
are designed specifically to detect
the IR radiation from H2 flames.
MIR flame detectors rely on a
combination of IR filters and software
analysis to detect flames and
reduce the potential for false alarms.
Unlike UV flame detectors, MIR detectors
are designed not to enter an
alarm condition when exposed to
arcs, sparks, welding and lightning.
In H2-detection applications, MIR
detectors offer good response time
and range. Equipped with an opti55

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Chemical Engineering April 2020

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