Chemical Engineering September 2018 - 54

rently available. In order to choose
from among them, plant personnel
should consider the capabilities, advantages
and disadvantages of each
technology and compare these to
the characteristics and requirements
of the application.
The technologies discussed in this
article are incorporated into what
are known as fixed-detection devices,
which are permanently placed
in a location where gas leaks might
occur. Fixed-detection devices are
part of systems that protect people
in a given area from harm caused
by toxic and combustible gases.
Besides performing their basic
functions, advanced versions of
fixed-detection instruments and
systems offer onboard digital intelligence
that allows diagnostic functions,
historical data logging, digital
communications and additional
microprocessor-based functionality.
The following sections detail the
main types of fixed-detection devices
offered by manufacturers of
gas-detection devices.
Point gas detectors
Point-type gas detectors monitor
a specific area or point in a facility.
These detectors are used to indicate
the presence of combustible or
toxic gas. The gas must come into
contact with the detector for sensing
to occur. Point detectors require
calibration for the gas type to
be detected and must be regularly
inspected to ensure that they are
capable of performing as expected.
Point detectors that are SIL 2 capable
have maintenance procedures
defined in their safety manual and
may, in some cases, only require an
annual bump test rather than quarterly
inspection. (Figure 6.)
Catalytic. The small catalytic gas
sensor (CGS) is the one most frequently
installed to detect combustible
gas. Operation of a CGS detector
is based on heat created by the
catalyzed reaction between oxygen
in the air and a combustible gas. A
CGS detector must be in the gas
cloud for detection to occur.
Of all available gas sensors, CGS
offers the greatest range in detection
of combustible vapors. Those detected
include hydrocarbons, hydrogen
and acetylene. Catalytic sensors
also offer good repeatability and accuracy,
as well as fast response time
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However, a rapid increase of highconcentration
combustible gas in
an environment can quickly move
ambient air out of the sensor so
that there is insufficient oxygen to
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In
addition,
catalytic
sensors
fail
without signaling plant personnel,
so they require routine bump testing
and calibration, typically every
three months. Catalytic sensors are
also susceptible to poisoning from a
variety of substances, including silicones,
halogens, acid, vapors from
polyvinyl chloride (PVC) and other
corrosive materials.
Infrared. IR point-gas detection is
based on the principle that hydrocarbon
combustible gases absorb specific
wavelengths of IR light. Detectors
using this technology include an
IR light source and a sensor to measure
light intensity at IR wavelengths.
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Chemical Engineering September 2018

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