Technology
History
Open Path Gas Detectors (OPGD) utilizing
NDIR detection techniques have been in use since the late 1980’s
and are widely accepted for many Oil & Gas, Petrochemical
and other industry’s combustible gas detection applications.
OPGD systems are currently used to monitor hydrocarbon gas leaks
at offshore and onshore oil and gas production facilities, refineries,
petrochemical plants, gas transmission stations and many other
industrial facilities.
The earliest NDIR OPGD systems were plagued
with environmental interferences and failed to provide consistent
performance when exposed to direct sunlight or weather conditions
such as rain, snow and fog. In order to cope with these environmental
challenges, designers of OPGDs pushed NDIR technology to its
limits, but this has left little room for further improvement.
Consequently, there are many demanding flammable gas detection
requirements which cannot be adequately met using NDIR-based
equipment. HVAC ducts and turbine acoustic enclosures are examples
of applications where NDIR-based systems cannot meet the needs
of many customers.
In
order to address the most demanding flammable gas detection
requirements and to provide
the first
reliable
open path toxic gas detector, Senscient developed Enhanced
Laser Diode Spectroscopy (ELDS™).
Enhanced Laser Diode Spectroscopy
(ELDS)
Enhanced Laser Diode Spectroscopy is a revolutionary
gas detection technology, specifically developed for safety related
applications in industry. ELDS provides the following unique
benefits and advantages:
- ELDS based OPGD systems provide reliable,
sensitive detection of both flammable and toxic gases at low
ppm concentrations. Toxic gases of primary interest include
hydrogen sulfide, hydrogen fluoride and ammonia.
- ELDS-based OPGD systems offer three orders
of magnitude in increased sensitivity for hydrocarbons, greatly
increasing the probability of detecting a flammable gas leak
before it reaches catastrophic proportions. Current NDIR methodology
fails to provide warnings early enough, or reliably enough
to facilitate any significant remedial action.
- ELDS based OPGD units can be produced
for combinations of toxic and / or flammable gas hazards, significantly
reducing the cost for a comprehensive gas detection system.
- Unique Simu-Gas™ feature
provides the long sought-after ability to accomplish remote,
on command, electronic functional testing of open path gas
detectors either locally or from the control room.
Theory of Operation Concepts:
Using a separate transmitter / receiver
configuration, ELDS systems detect and measure gas concentrations
at specific target gas absorption wavelengths over distances
of up to 200 meters. The detector measures absorbance changes
along the line-of-sight path when a combustible or toxic gas
passes through the beam. Enhanced Laser Diode Spectroscopy
(ELDS) utilizes highly reliable, solid-state
laser diode sources similar to those used in demanding telecommunications
applications. Innovative signal processing methods significantly
increase sensitivity, enabling reliable detection down to fractions
of a % LEL meter of combustible gases, and low ppm meter levels
of toxic gases. ELDS addresses problems experienced by traditional
laser diode systems including laser Relative Intensity Noise
(RIN), absorption by atmospheric gases, and coherence / fringe
effects. ELDS uses a combination of techniques which significantly
enhance the ability of an OPGD to detect small fractional absorbances
with an extremely low false alarm rate.
ELDS techniques allow our customers to finally
meet stringent regulatory and safety integrity requirements with
a false-alarm free system for low level combustible and toxic
gas detection.
Multiple Modulation Frequencies:
To successfully address system noise and
the associated unacceptable false alarm rates, the ELDS technique
employs Multiple Modulation Frequencies. The laser diode is driven
by a current as shown in Figure 1, comprising two components,
a bias component and a sinusoidal wavelength modulation component.
The bias component is chosen to operate the laser diode at a
wavelength close to a chosen optical absorption line of the target
gas. The sinusoidal component alternates between two, non-harmonically
related electrical frequencies f and f’. At each of the
chosen frequencies, the laser’s wavelength is alternately
scanned across the chosen absorption line for a designated time
interval.
When there is no gas present in the measurement
path, the combined Fourier transform of the detector signal will
look like Figure 2, with just two frequency components f and
f’.
When there is a substantial quantity of
target gas in the monitored space, the combined Fourier transform
of the detector signal will look like Figure 3, with sets of
harmonics of both f and f’. The probability of both measurements
simultaneously suffering noise induced deviations above the alarm
threshold is extremely small, lower than the targeted 1 in 100
years false alarm rate probability.
The benefits of modulation and measurement
at multiple, non-harmonically related electrical frequencies
are not limited to reducing the impact of inherent system noise.
The use of modulation at a number of non-harmonically related
frequencies also reduces the likelihood that electromagnetic
interference and/or thermal noise will affect all measurement
frequencies simultaneously, again enabling false alarm rates
to be significantly reduced.
Harmonic Fingerprints:
Although conventional Laser Diode Spectroscopy
(LDS) methods of measuring gases have been in use for several
years in process control and environmental monitoring applications,
such systems have not been popular in safety related applications
due to their high false alarm rates when detecting low levels
of hazardous gases. Conventional LDS systems suffer from the
combined effects of system noise, absorption by atmospheric gases
and coherent interference effects, all of which can produce spurious
readings and false alarms.
ELDS overcomes the false alarm problems
experienced by conventional LDS systems by the use of Harmonic
Fingerprints™. Using a small retained sample of
target gas inside the transmitter, the temperature and wavelength
modulation currents applied to the transmitter’s laser
diodes are actively controlled to lock the lasers such that absorption
by target gas produces specific Harmonic Fingerprints. The relative
amplitudes and phases of the harmonic components in a Harmonic
Fingerprint are so specific that only absorption by target
gas produces a signal with the desired Harmonic Fingerprint.
Noise, absorption by atmospheric gases and coherent interference
effects never produce signals with the Harmonic Fingerprint,
enabling an ELDS-based gas detector to effectively eliminate
false alarms from these causes.
Multiple Measurement Wavelengths:
It is widely understood that measuring gases
with a single wavelength can be limiting to the sensitivity and
accuracy of the measurement. Multiple wavelength measurement
and detection, employed by scanning spectroscopy systems, has
long been the technique of choice for analyzers providing low
level, selective analyses of gas concentrations in the lab and
on the process floor. But for ambient gas detection systems,
multiple-wavelength scanning has been too complicated and too
expensive to employ in the majority of applications. The Vertical
Cavity Surface Emitting Lasers (VCSELs) employed in Senscient’s
ELDS-based gas detectors provide economic scanning of wavelength
ranges of 2-3nm, enabling multiple wavelength or multiple species
measurements.
Multiple Laser Diodes:
Even though it is not possible to completely
eliminate coherence / fringe effects from a laser diode system
(especially one operating along an open measurement path) it
is possible to reduce the rate of false alarms arising from such
effects by using a system configuration as shown in Figure 5.
The ELDS system shown in Figure 5 contains two laser diodes,
operating at two different wavelengths corresponding to two different
absorption lines of one or more target gases. The outputs from
the lasers are collimated by a common optical element, aligned
such that optical radiation from both laser diodes reaches the
receiver after passing through the monitored path. The optical
radiation received from both laser diodes is concentrated onto
a detector, at which point the optical signals are combined into
a single electrical signal.
The use of multiple laser diodes and
multiple measurement wavelengths is an effective way of addressing
problems with coherent interference / fringe effects, a common
problem experienced by conventional LDS systems. Two and even
three VCSEL laser diodes can easily be mounted on a common
temperature stabilized mount, sharing all of the associated
optics and detectors; whilst keeping system costs reasonable.
The electrical signal from at the detector
contains two sets of independent frequency components proportional
to the amount of target gas present in the measurement path.
These effectively independent measurements of the quantity of
target gas in the monitored space can then be compared and used
to confidently determine the quantity of target gas present in
the monitored space, if any.
Multi-Gas Capability:
The use of two lasers, scanning different
wavelengths at different electrical frequencies makes it possible
to treat each measurement as being completely independent of
the other. Consequently, ELDS provides the opportunity to monitor
two or more different target gases simultaneously in the same
transmitter/receiver system (see Figure 6). Combinations of gases
that are likely to be of interest include methane + hydrogen
sulfide (solution
gas), butadiene + hydrogen fluoride (alkylation) and methane
+ methanol (methanol injection).
Increased system reliability, virtually
false-alarm free low level gas detection, and even simultaneous
multiple gas detection capability is now possible.
Simu-Gas:
Senscient introduces the concept of Simu-Gas™ for
the simplest and most reliable gas detector functionality test
available.
In an ELDS system with Simu-Gas as shown
in Figure 7, the transmitter’s microprocessor has direct
control of the synthesis of the laser diode drive waveforms,
and access to the Harmonic Fingerprints being produced by absorption
of laser diode radiation by the retained sample of target gases.
Upon receiving a command instruction from
an operator or control system, the transmitter’s microprocessor
adds Harmonic Fingerprint components to the laser diode drive
waveforms to simulate the presence of a given quantity of target
gas in the monitored space. The optical radiation leaving the
transmitter then faithfully simulates the presence of target
gas in the monitored path.
When the receiver processes the signal that
it is receiving from the transmitter, it sees the Harmonic Fingerprint
components and calculates and outputs the corresponding quantity
of target gas. By simply comparing the gas reading output by
the receiver to the quantity of target gas that the transmitter
was instructed to simulate, it is possible to verify the correct
operation of the gas detector.
Compared to the conventional techniques
currently used to test gas detectors, Simu-Gas testing has the
following advantages:
- Functional testing can be performed remotely,
without operators needing to gain access to difficult-to-reach
gas detectors. No more scaffolding or abseiling!
- Gas detectors can be functionally tested
much more frequently, providing greater safety integrity. Simu-Gas
enables SIL 2 or SIL 3 gas detection systems to be easily realised.
- There is no need for operators to carry
cylinders of hazardous gases around facilities in order to
test gas detectors.
- The results of detector functionality
testing can be gathered and logged automatically.
- The operation and maintenance costs for
a gas detection system are greatly reduced.
Cost of Ownership reduces to almost zero
with this innovative technique that permits gas detector functionality
to be confirmed on command, locally or remotely under any condition.
_
Home | About
Us | Products | Technology | Contact
Us | Support Center | Press |
