GAS / FLAME DETECTION
GAS / DUST MONITORING
YOUR ENVIRONMENT,YOUR SAFETY, LET ’S MASTER THEM TOGETHER
Gas Detection Objectives
• Measure gas before hazardous
concentration is present
• Provide outputs for
Emergency Action
• Suitable for extreme
environments (arctic, tropic,
etc...)
• Complement to Flame
Detection equipment
• Cost-effective protection
The
risks incurred:
•The explosion (EX)
•The poisoning (Tox)
•The suffocation (O2)
EXPLOSION
RISKS
3 parameters for an EXPLOSION
• Requirement for an explosion: the ignition
triangle
1)
2)
Fuel
Ignition Source
Flammable gases & vapours
(eg Methane)
Combustible dusts
(eg Coal dust)
1)
2)
3)
4)
Oxidiser
(Oxygen: approximately 21% of air)
Always present in the air
Electric arcs or sparks
Frictional heating & sparking
Hot surfaces
Static Electricity
IGNITION TEMPERATURE...
• It is the temperature from which a product
starts spontaneously its combustion without
the intervention of an outside source (spark).
• H2 : 560°C
• CH4 : 550°C
• acetone : 465°C
• ethyl ether : 160°C
• carbon sulphide : 102°C
FLASH POINT…(FLP)
• It is the minimal temperature from
which a mixing of vapours, issued from
liquids and air, may inflame in normal
conditions of pressure.
• Ethyl ether : - 45°C
• essence (oi 100) : - 37°C
• acetone : - 17°C
• gas oil : + 55°C
About explosive atmosphere...
• For each combustible gas or vapour in
usual conditions of given temperature and
pressure, there are:
• a LOW EXPLOSIVE LIMIT :
LEL
• an UPPER EXPLOSIVE LIMIT :
UEL
Combustible gases
• LEL :
lower
explosive
limit
• The minimum
concentration of gas
or vapor mixed with
air that will cause an
explosion when its
comes in contact with
an ignition source.
Combustible gases
• UEL :
upper
explosive
limit
• The maximum
concentration of gas
or vapor mixed with
air that will cause an
explosion when it
comes in contact with
a ignition source.
AIR/GAS CONCENTRATIONS
100% air
LEL5% volume
Alarm at 20% of LEL
lean
0%
explosive
100% of LEL
UEL14.7% volume
(catharometre: % gas)
rich
a NEW directive for EUROPE !
Potentially
EXplosive
ATmospheres
ATEX
DIRECTIVES
• 94/9/CE ( manufacturers’ directive) =
European instruments using
• 1999/92/CE ( workers’ directive) =
employees protection:
• annexe I:types of zones
• annexe II:categories of instruments for each zone.
NEW DIRECTIVES ATEX...
• Since July 2003, it has
been mandatory that all
equipment with a potential
source of ignition which is
being sold on the european
market for the first time is
accompagnied by a
declaration of conformity
to the ATEX product
Directive 94/9/EC.
• Since july 2003, it has been
mandatory that all new
installations (and
modifications to existing
installations) should meet
the requirements of the
ATEX User directive
1999/92/EC.
NEW DIRECTIVES ATEX...
• By July 2006, all plants within Europe
should be able to demonstrate full
compliance with the requirements of
1999/92/EC as adopted in each member
state.
• In the UK this is « the dangerous
substances and explosive atmospheres
regulations » : DSEAR.
NEW DIRECTIVE ATEX...
Or: europa.eu.int/comm/enterprise/atex/index.htm
The new directive ATEX
(01/07/2003)
• Requires in short:
• a certification delivered by a notified laboratory
• CE marking to be affixed to all electrical and electronic
instruments marketed in EUROPEAN UNION
MEMBER STATES(The CE marking guarantees that the
instrument conforms to applicable electromagnetic
radiation and immunity requirements).
• Compliance is mandatory,which means you must ensure
that your products meet all of the directive’s requirements.
List of European Standard
relevant to Performance Requirements ( additional standard to
Explosion Protection i, d, e , m, etc ) and mandatory for safety
relevant devices according article 1.5 de l’Annex II de la
Directive Atex 94/9/CE
EN 61779-1 ( replace EN 50054 ). Performance
requirements of Electrical apparatus for the detection and
measurement of flammable gases - Part 1: General
requirements and test methods
List of European Standard
EN 61779-2 ( replace EN 50055 ) :
Performance requirements for
group I apparatus indicating a volume fraction up to 5 % methane in air .
- EN 61779-3 ( replace EN 50056) : Performance requirements for
group I apparatus indicating a volume fraction up to 100 % methane in air
.
EN 61779-4 ( replace EN 50057) :
Performance requirements for
group II apparatus indicating a volume fraction up to 100% lower
explosive limit. .
- EN 61779-5 ( replace EN 50058) : Performance requirements for
group II apparatus indicating a volume fraction up to 100% volume. .
List of European Standard
EN 50104: Electrical apparatus for the detection and measurement of
oxygen – Performance requirements and test methods.
EN 50270:
Electromagnetic compatibility - Electrical apparatus for
the detection and measurement of combustible gases, toxic gases or oxygen
EN 50271:
Electrical apparatus for the detection and measurement
of combustible gases, toxic gases or oxygen - Requirements and tests for
apparatus using software and/or digital technologies.
List of European Standard relevant to Performance Requirements
not mandatory, eg not yet in the Atex list for harmonised standard
EN 45544-1 : Workplace atmospheres - Electrical apparatus used
for the direct detection and direct concentration measurement of toxic
gases and vapours -- Part 1: General requirements and test methods
EN 45544-2 :Workplace atmospheres - Electrical apparatus used for
the direct detection and direct concentration measurement of toxic gases
and vapours -- Part 2: Performance requirements for apparatus used for
measuring concentrations in the region of limit
List of European Standard relevant to Performance Requirements
not mandatory, eg not yet in the Atex list for harmonised standard
EN 45544-3 : Workplace atmospheres - Electrical apparatus used for
the direct detection and direct concentration measurement of toxic gases and
vapours -- Part 3: Performance requirements for apparatus used for measuring
concentrations well above limit values
EN 45544-4 :Workplace atmospheres - Electrical apparatus used for the
direct detection and direct concentration measurement of toxic gases and
vapours -- Part 4: Guide for selection, installation, use and maintenance
The new directive ATEX
(01/07/2003):continued...
• Safety systems fail safe application.
• GAS or DUST hazardous areas indication
(marking)
• production quality assurance
• product verification
• conformity to type tests carried out under
the responsability of notified organisme.
ATEX / CENELEC: marking
• Until July 2003, a new marking:
PROTECTION MODE
GAS GROUP
CENELEC
INSTRUMENT Group
europ Directives.
organism
explo
category
gas
T° Class
cenelec
protection
Class gas
T° Class
CE MARKING example
• The certified equipment must carry a plate indicating :
-the name and address of the manufacturer
-designation of the type of equipment
-CE marking followed by the identification number of the organism where
such body is involved in the production control stage:CE xxxx
-the serial number of the equipment
- year of construction
-specific marking followed by group II for group II,2 for category 2 and G
for gas
- complementary marking:EEx d IIB T4
INERIS xxATEXxxxx X
What is an
HAZARDOUS
AREA as per ATEX
1999/92/CE:annexe I ?
ATEX 1999/92/CE
• New directive ATEX concerning the
instruments in hazardous areas and mines
“group I”:
• the whole concentrations: M1
• above a limit value: M2
ATEX 1999/92/CE:ZONES
• New directive ATEX concerning the instruments in hazardous
areas and surface industries “group II” distinguishes the next
groups of zones:
• Zone 0 G (gas) and zone 20 D (dusts)
• Zone 1 G (gas) and zone 21 D (dusts)
• Zone 2 G (gas) and zone 22 D (dusts)
ZONE 0G /20 D
ATEX probability = very high
• Where ignitable concentrations of
flammable gases, vapours, liquids or
dusts can exist all of the time or some
of the time and for a long time under
normal operating conditions .required
equipment (annexe II):category
1=
ia only !
intrinsic Safety
ZONE 1G/21D
ATEX probability = high
• Where ignitable concentrations of
flammable gases, vapours,liquids or
dusts can exist some of the time
under normal operating conditions.
• required equipment (annexe II):category 2 =
is or e .
d or
ZONE 2G/22D
ATEX probability = low
• Where ignitable concentrations of
flammable gases, vapours,liquids
or dusts are not likely to exist
under normal operating conditions.
• Required equipment (annexe II):category 3
(minimum protection).
DANGEROUS ZONE:example
• In accordance with the new regulation
Required equipment
ATEX:
CATEGORY 3
CATEGORY 2
CATEGORY 1
GAS and
EQUIPMENT
CLASSIFICATION.
Explosion groups
• The IEC and the CENELEC
(European committee of electrotechnics
normalisation) drew up a classification of
flammable products :
• the I group only concerns the
equipment used in firedamp
mines.
• the IIB group includes all gases
except hydrogen, acetylene,
carbon sulphide and nitrile of ethyl
• the IIC group includes all gases
and vapours.
classification of temperatures
• "T" followed by the number from 1 to 6
indicates the class of the maximum temperature
of surface that can bear the device, taking care of
a security’s coefficient : no surface of the
equipment in contact with the gas/air mixture
should exceed the MIT of the gas.
• Each flammable gas or vapour has a specific
Minimum Ignition Temperature.
•
This classification is relevant to a given surrounding temperature ( 40°C).
classification of temperatures
• Example:T6
Gas
detectorcasing
Internal
fault
85°C
max
Gas detected
ignition T° >85°C
Temperature classes
• To ease the selection of equipment, six different temperature
classes have been created for both gases and equipment.
IEC
CENELEC
T6
T5
T4
T3
T2
T1
(group II)
maximum
temperature
85°C 100°C 135°C 200°C 300°C 450°C
of surface
Function of Explosion protection
• The function of the discipline of explosion
prevention is to ensure that there is a negligible
probability that a means of ignition and any
significant quantity of a potentially explosive
atmosphere can occur in the same location at the
same time.
• Both mechanical and electrical methods are
imployed and have been classified under 4
categories:
4 categories of protection
• Non-incendive equipment (comparable with n)
(EN 50021)
• Flameproof or explosion-proof
equipment (d) (EN 50018)
• intrinsic safety equipment (i)
(EN 50020)
• increased safety equipment(e)(EN 50019)
NON-INCENDIVE EQUIPMENT(n)
• Transmitters « n » for zones 2 (gas) and
22(dusts)
• In accordance with regulation CEI 79-15:
minimum protection (non sparking)
• Typical applications:instrumentation,
control gear, electronic systems,
measurement and control.
FLAMEPROOF or explosion-proof
equipment (d)= containment
• Elements sensitive to produce a spark are
enclosed in casings resistant to an eventual
internal explosion.
• This material is in accordance with the
decree of March 28, 1960:
FLAME PROOF TRANSMITTERS:
spécifications of installation
• Transmitters « d » for zones 1 and 2(gas)
and 21 and 22(dusts)
• The cable will be mecanicly protected
• Transmitter ’s body will be connected to the
earth
• If connections are in classified area:made in
certified housing.
• Typical applications:
switch gear, motors , pumps.
equipment with intrinsic
safety (i) = prevention of ignition
• Intrinsic safety aims to limit the level of
energy release under any circumstances
( MIE !…..)
• It is the ideal equipment in particular in
an atmosphere with hydrogen.
INTRINSIC SAFETY:
FAULT CONDITIONS
• The definition of intrinsic safety includes reference to fault
conditions.
• Two categories of intrinsically safe instrument are defined
in the standards: ia and ib.
• The categories differ in two principal respects:
- the number of specified faults which the instrument
can sustain without producing a risk of ignition
- the values of the safety factor which are applied to
the ignition data used in the design...
INTRINSIC SAFETY:
FAULT CONDITIONS
• Category « ia »:
• instruments of this category must be incapable of
causing ignition in normal operation or with a
single fault or with any two independent faults. A
safety factor of 1.5 must be applied to relevant
ignition data for normal operation or for a single
fault and a safety factor of 1.0 for the two-fault
condition.
• Intrinsic safety category « ia » is the only method
of protection approved for zone « 0 or 20 ».
INTRINSIC SAFETY:
FAULT CONDITIONS
• Category « ib »:
• instruments of this category must be incapable of
causing ignition in normal operation or with a
single fault . A safety factor of 1.5 must be applied
to relevant ignition data for normal operation or
for a single fault.
• Intrinsic safety category « ib » is generally
approved for zone « 1 or 21 ».
Intrinsic safety TRANSMITTERS:
spécifications of installation
• Transmitters for zones 0,1 or 2(gas) and 20,21
and 22(dusts)
• Obligatory powered by an intrinsic
source:28V/300 ohms
• If connections are in classified area:made in
certified housing.
• Typical applications: instrumentation, control
gear, electronic systems, measurement and
control.
Intrinsically-Safe Systems
I.S. systems require installation of power-limiting barriers
Barriers will ground the system power supply
Ground-fault monitoring systems are incompatible
equipment with increased safety (e)
• Transmitters « e » for zones 1 and 2(gas) and 21 and
22(dusts).
• This material is realised so that the occurrence of
accidental sparks is highly improbable
• protection by insulator : the whole equipment is dipped
in a resin or a liquid to be totally separated from the
ambient atmosphere.
equipment with increased safety
spécifications of installation
• Transmitters « e » for zones 1 and 2(gas)
and 21 and 22(dusts)
• The cable will be mechanically protected
• Transmitter ’s body will be connected to the
earth
• If connections are in classified area:made in
certified housing.
• Typical applications: motors, light fittings.
Types of protection:
symbols and zones
OG/20D
Type of PROTECTION
Non-incendive
immersion
in oil
internal under pressure
pulverulent
filling
flameproof
casing
increased safety
intrinsic safety
1G/21D
n
2G/22D
X
O
p
q
d
e
i(a/b) X(« a » only)
X
X
X
X
X
X
Index of protection
• Dust, water and impacts damage the
equipment. Device’s casing is protected
against this outside parameters.
• So the IEC defined a list of different
degrees of protection and a numeration
Degree of protection
IP xxx = degree of protection of casings of electric equipment
against solids
0
no special protection
0
1
larger than 50 mm.
1
2
larger than 12 mm.
2
3
larger than 2,5 mm.
4
larger than 1 mm.
against liquids
mechanical protection
No special protection
0
Protected against
No special protection
1
Impact energy = 0,225 joule
dripping water falling
vertically any angle up to 15°
2
Impact energy= 0,375 joule
3
rain falling vertically any
angle up to 60°
3
Impact energy= 0,5 joule
4
splashing water, splashed
from any direction
5
Impact energy= 2 joules
7
Impact energy = 6 joules
9
Impact energy = 20 joules
5
Dust protected
5
6
Dust- tight
6
7
8
dripping water
water projected from a nozzle against
the equipment from any direction
heavy seas or powerful
water jets
immersion under defined
conditions
submersion
POISONING RISKS
Where can we be poisoned ?
• In every place where these toxic products
are:used,manufactured,transformed,stored
• Examples of toxic gas emission sources :
• Combustion (CO/NO/NO2/SO2)
• Incineration (H2S/NH3/HCL)
• Fermentation (H2S/NH3)
Example:toxicity caused by carbon monoxide in terms of
time and concentration
c
% CO
in the air
a
r
b
o
n
m
o
n
o
x
i
d
e
0,16
0,14
0,12
0,10
c
o
n
c
e
n
t
In danger of death
0,08
Death
r
a
t
i
0,06
o
n
i
i
0,04
Headache and nausea
Perceptible effect
No perceptible effect
n
a
a
t
m
o
s
p
h
e
r
0,02
0,00
0
1
2
Time in a toxic atmosphere
3
4 hours
4 Hours
Concept of LIMIT VALUES
• The Short Term Exposure Limit: STEL
• Time-weighted average: TWA
•
Gases and vapours limit values are in volume (ppm:part per million).
STEL
• The STEL is admitted value for the medium
in time, concentrations in which a worker is
exposed for less than 15 minutes:
• SUBSTANCES with IMMEDIATE
EFFECTS.
TWA
• TWA : the average concentration of
contaminants over a specified time period
(8 hours : 5 days a week):
• SUBSTANCES with main CUMULATIVE
EFFECTS
Effects of carbon monoxide (CO)
exposure
ppm level*
35
200
400-600
1000-2000
2000 à 5000
effects
Max. permissible level
Slight headache
Headache, disconfort
Staggering,heart
palpitation
Unconsciousness, death
time
8 hours
3 hours
1-2 hour
1.5 hour
0.5 – 1 hour
*values are approximate
CO has an affinity for human blood hemoglobin that is over 200 times greater than oxygene.
Effects of chlorine (CL2)
exposure
ppm level*
1
3-6
15-30
40-60
60 à 1000
effects
Max. permissible level
Stinging eyes, nose,
throat
Serious irritation
Respiratory damage
Serious injury, death
*values are approximate
time
8 hours
minutes
minutes
30 minutes
minutes
Toxic risks :generations
Risks with OXYGEN
• The oxygen is essential for the life, it
represents 20.9 % of the air that we
breathe.
• A deficiency of oxygen is as important as an
oxygen enrichment, each variation will
provoke important effects on human beings.
Effects of OXYGEN :
• 20.9 % --> Normal concentration
• 19 % ---> tiredness and yawn...
• 14 % ---> pulse up, impaired co-ordination, perception and
judgement...
• 10 % ---> Nausea, mental failure, fainting,
unconsciousness, ashen face, blueness of lips, and vomiting
• 8 % ---> Coma in 40 seconds, convulsions, respiration ceases,
death
• 3-5 %
---> life expectancy; 3-5 minutes.
An oxygen deficiency in confined
spaces can be explained by:
• A defect of the ventilation system or a lack
of ventilation
• the presence of another gas in full quantity
(accidental leak)
• an oxygen consumption during a chemical
reaction such as combustion
• Inertion
Where are you exposed ?
Confined areas:
• Soldering and cleaning
of tanks
• Reparation of furnaces
• Visit of inert stocking
• Penetration in silos
• Premises without
ventilation
In the trenches, the low
places:
• in research of leak
• in visit of control
• in the cellars and the
sewers...
A moderate oxygen enrichment:
• can however cause accidents
• It provokes an EUPHORIA causing a
modification of the sense of DANGER and
vision !
Effects of OXYGEN:
• > 22 % ---> euphoria, modification of the sense of danger
and vision = maximum safe level (OSHA)
• < 22 % --> no respiratory troubles,
•
20.9 % --> oxygen content in “AIR”
PROCESSES using OXYGEN
• For its energetic properties:
• During combustion, to have more calories or a
higher T° than air…
• In oxypropane, oxyacetylene flames,examples of
applications:
- welding, soldering, surface hardening, forming
- stripping, flame spray coating
- oxycutting
- iron and steel industry, foundry, glassworks burners
- blast enrichment in blast furnaces.
PROCESSES using OXYGEN
• For its oxidant properties:
• in the iron and steel industry, it ’s used in pure
oxygen converters, to refine cast iron and steel
• in non-ferrous metal metallurgy, it ’s used to
refine copper, roast sulfurous ores, assay carbon
in metals
• in the paper industry, to bleach paper pulp.
Functional Safety and Safety
Integrity Level
SIL
EN 61511 - EN
61508 – EN 50402
The Standards
Application to Gas detectors
GAS ANALYSIS
METHODS
Catalytic sensor
The catalytic sensor, also referred to
as the catalytic bead sensor, is
commonly used to detect and
measure combustible gases from
0-100%LEL.
Example of Combustion
Methane (Natural Gas)
100 %
by volume
0%
AIR
9
Example of Combustion
Methane (Natural Gas)
100 %
by volume
0%
L
E
L
AIR
100 % LEL or 5% by volume
“Same Thing”
9
Example of Combustion
Methane (Natural Gas)
0%
L
E
L
100 %
by volume
15% by
volume
Explosive
Mixture
AIR
100 % LEL or 5% by volume
“Same Thing”
9
Example of Combustion
Methane (Natural Gas)
0%
L
E
L
100 %
by volume
15% by
volume
Explosive
Mixture
UEL
Too Rich to Burn
100 % LEL or 5% by volume
“Same Thing”
9
Common Combustibles
Methane
Hydrogen
Acetylene
Propane
Butane
Pentane
Hexane
LEL
5.0%
4.0%
2.5%
2.1%
1.9%
1.4%
1.2%
UEL
15% by volume
75%
100%
9.5%
8.5%
7.8%
7.5%
Catalytic sensors
• The sensor is composed by two
platinium spirals, both plated with
a ceramic coating (alumina)
• one of the pellistor is soaked with
a special palladium catalyst that
causes oxidation : detector
(sensing bead)
• while the other one is not treated
in order to forbid oxidation :
compensator ( reference element).
• Those two filaments and their
supports are fixed in a
« flameproof » body of cell.
Flame arrestor
Physical principle :
Wheaststone bridge
• The working principle of these
sensors is based on flammable gas
oxidation on the surface of a
catalytic element with electric
heating
• The current passes through the
spirals in order to reach 450°C
temperature that allows gas
oxidation
• when fuel gas has burned in the
detector, oxydation causes a
temperature increase only in the
treated pellistor and not in the nontreated one ( reference), causing
unbalance in the bridge circuit.
mV
15 % O2 mini
Catalytic sensors :
typical response curve
SUP
?
CATALYSE : note
• Flammable gas oxidation: must be used in
environments containing
a concentration of oxygene (O2) >
15% .
Catalytic Gas Sensors: ... Note !
• The sensor can be poisoned so that it cannot
respond to a flammable gas if exposed to
lead, silicone or certain other gases…
• The presence of inhibitors or poisons is the most
common cause of problems in gas detection
systems and, for this reason, it ’s necessary to pay
attention in order to avoid any contamination.
Catalytic Gas Sensor
Poisons/Inhibitors
• Inhibitors ( H2S, SO2, halogenated compounds) causes a
temporary sensitivity loss of the sensor
• Poisons affect catalytic sensor response & longevity and
cause a permanent reduction of the sensor sensitivity that may
be completely damaged
• Erosion, impervious covering, or plugging active sites
• Impact depends on poison type, level, time of exposure
• Known catalytic sensor poisons:
–
–
–
–
–
silicone oils, greases, resins (RTV adhesive)
halogens ( halon, chlorine, fluorine, bromine, freon)
phosphate esters
tetraethyl lead, trichlorobenzene
acid and pvc vapors, other corrosive materials …
ADVANTAGES
• The principle is simple, it uses a real
phenomenon
• valid for all flammable gases
• very short response time ( <15s.)
• very good repeatability
• very good reproducibility
• low cost
The thermal conductivity sensor
Has been used in instruments for
measuring gases above the %LEL
range and for leak detection.
Thermal conductivity
• Measuring the thermal conductivity of gases was
one of the earliest forms of gas detection and it ’s
suitable for % volume levels of certain binary
mixtures : two different gases, one of which can be
air .
• TC gas detectors operate by comparing the
thermal conductivity of the sample with that of a
reference gas ( usually air)
• this principle of detection , without chemical reaction, can
be used in an atmosphere with or without oxygene.
Thermal conductivity : principle
• The sensor consists of two elements, both
comprised of a wire coil.One element (detector) is
exposed to the atmosphere, whereas the other
element (reference) is sealed in a standard gas
atmosphere such as nitrogen or air.
Air + gas
Housing of cell
D
air
C
Thermal conductivity : principle
• The reference element compensates for
changes in temperature.
• The elements are heated to an operating t°
of approximately 250°C.
ADVANTAGES
•
•
•
•
•
•
High concentrations measurement ( 100% v/v)
With or without oxygene
possibility of detection: helium ….
No poisonning
long life time
resistant filaments.
Limitations of use
• This technique is only suitable for gases and
vapours whose thermal conductivity is significantly
different from air !
• Thermal conductivity sensors are used primarily in
portable gas leak detectors.
TCOD IR (CO2)
OLCT IR
INFRARED
ABSORPTION
INFRARED SENSORS :
• The non-dispersive
infrared sensor,
commonly referred as the
infrared sensor, is based
on the principle that gases
absorb light energy at a
specific wavelength,
typically in the infrared
range.
INFRARED SENSORS
• Gases that contain more than one type of atom absorb
infrared radiation.
• Gases such as carbon dioxide (CO2), carbon monoxide
(CO), methane (CH4) and sulphur dioxide (SO2) can
be detected by this means …
• But gases such as oxygen (O2), hydrogen (H2), helium
and chlorine (CL2) cannot.
INFRARED ABSORPTION
Emitter
Receiver
Optical
filter
IR Source
( filament / semi-conductor
or laser)
c
Measurement path lenght
•When flammable gas passes between the source and detector, the gas
absorbs infrared radiation and a lower intensity is registered at the detector
•The gas concentration is directly proportional to the amount of energy
absorbed and this absorption is illustrated by the BEER LAMBERT
formula.
ADVANTAGES in
summary
•
•
•
•
Instantaneous response time
no poisoning
no need of oxygen
no interferent gases
Limitations of use
• Exposure to high concentrations may saturate
the instrument for a finite time.
• Cannot detect monatomic or diatomic
homonuclear molecules : mercury, chlorine and
other halogens ...
C1100
Typical measurement tasks
• Combustion efficiency monitoring : CO/CO2
• Continous Emission monitoring systems:
CO/CO2/SO2 and helium
• Process control: CO/CO2 and total hydrocarbons
• Landfill gas monitoring : CH4/CO2
• Plant protection: CH4/C3H8/C4H10
• Distilleries and breweries : CO2
• Personnal protection : CO2
• Automotive emissions.
SEMI-CONDUCTORS
SEMI-CONDUCTORS
Collector
Platinum Coil
Heater
Heater Control
Schematic Diagram of a Bead-type Sensor
•This semi-conductor (SNO2 for example) is placed on the surface of a substratum
(tube or plate).
A
PRINCIPLE
• A filament is heated by an electric current,
• the substratum increases its temperature
until it reaches 300 to 500 °C.
• The sensitivity of SnO2 to different gases
varies with the temperature.
• This temperature will be chosen to work
with the maximum operation sensitivity.
PRINCIPLE
• Signal= induced variations of
electric conductivity,
by absorption of gas, on the
surface of a metallic oxide.
When gas enters the sensor it
reacts with the oxide coating
which causes a decrease in
resistance between the two
electrodes.
Detected GASES
• Toxic and flammable gases: VOC ,
hydrocarbures (toluène,xylène …), vapors of hydrocarbures
(essence,kérosène…), cétones (2-butanone…), esters, (acétate
of méthyle, éthyle éther…), alcools (méthanol…)
• FREONS.
• example : toxic products with low concentrations,
ADVANTAGES
•
•
•
•
High sensitivity
very good stability of the signal
long life time (~ 5 years)
low cost
• used to measure a wide range of gases and
vapours.
• Commonly used in low cost, hard-wired gas
detection systems
Limitations of use
• Wide range of sensitivity (interference) to different
gases
• after exposure to high gas concentrations the sensor
may need a recovery time of several hours and may
have irreversible changes to its zero gas reading and
sensitivity
• exposure to basic or acidic compounds, silicones,
organo-lead, sulphur compounds and halogenated
compounds may have a significant effect on sensitivity
• Oxygen concentration,humidity and temperature may
have a significant effect on sensitivity.
Electrochemical gas
sensors
with liquid or gelled
electrolyte.
Electrochemical gas sensors
• Are widely used for the gas detection of
toxic gases at the ppm level and for oxygen
in levels of % of volume .
Toxic electrochemical
sensors
In summary …
• This method is based on the
measurement of the current
established between a sensing
electrode and a counter electrode.
• A reference electrode is often used
to stabilize the measurement.
• Gases react electrochemically to
the sensing electrode: gases are
reduced or oxidised.
Oxygen electrochemical
sensors
• There are two fundamental
variations in fuel-cell oxygen
sensor designs:
• Partial atmospheric pressure, is that
fraction of the total atmospheric
pressure due to oxygen
• Capillary-pore, these sensors are
much less influenced by changes in
presure than partial pressure oxygen
sensor designs.
Mechanisms of oxygen sensor failure
• Oxygen sensors may be affected by prolonged
exposure to acid gases
• Such as carbon dioxide (CO2)
• Most oxygen sensors should not be used
continuously in atmospheres containing more
than 25%CO2
• Limitation of operations in extrem cold or
excessivily hot temperatures…
PID Sensor Technology
A PID sensor works differently
than other sensors and often used in
situations where high sensitivity
(sub-ppm levels) and limited
selectivity (broad-range coverage)
is desired.
Detector Operation
UV Lamp
A photoIonization device contains
of a vacuum lamp that emits UV
light at a specific energy .Some
common lamps avaible are 9.8eV,
10.6eV and 11.7eV.
Detector Operation
- The UV light is generated by the
excitation of the gas contained
- within the bulb (Krypton and
argon are two gases commonly
used).
UV Lamp
- The gas in the lamp is excited with an electrical
field or a radio frequency field.
PID Lamps
• The VX500 has a 10.6 eV lamp
• A 11.7 lamp has a short life and needs special
treatment as dehydrating and frequent
calibration, it is basically only suited for leak
detection
Gas Out
(No Longer Ionized)
e
-
e
-
e
e
-
-
e
e
Gas In through filter
e
-
UV Lamp
-
-
e
e
-
e
10.6 eV
-
e
-
- Electrode
e
+ Electrode
Gas Out
(No Longer Ionized)
-In addition to the lamp the PID contains two electrodes a positive and a
negative electrode :
- The negative electrode is often referred to as the collecting electrode
- The positive electrode is referred to as the biased electrode
- The distance between the two electrodes is 20 000 of an inch.
Gas Out
(No Longer Ionized)
e
-
e
-
e
e
-
-
e
e
Gas In through filter
e
-
UV Lamp
-
-
e
e
-
e
10.6 eV
-
e
-
- Electrode
e
+ Electrode
Gas Out
(No Longer Ionized)
- Gas molecules that pass through the light emitted from the ignited
lamp are ionized if their ionization potential is less than the ionization
potential of the lamp.
Detector Operation
UV Lamp
If the molecule’s ionization potential is above that of lamp,
then nothing happens !
Detector Operation
UV Lamp
If the molecule’s ionization potential is less that of lamp,
then the molecule is ionized !
Detector Operation
+
-
UV Lamp
When a molecule is ionized an electron is removed forming a
positively charged ion and an electron.
Principle of Operation
Gas Out
(No Longer Ionized)
e
-
e
-
e e-
e
e
Gas In through filter
e
-
UV Lamp
-
-
e
e
-
e
-
e
-
e
- Electrode
+ Electrode
Gas Out
(No Longer Ionized)
The charged particules then move to the oppositely charge electrode.
Detector Operation
+
negative
electrode
The positive and negative ions are collected on
Electrodes which produce a signal.
This signal is directly proportional to the amont
of ions present at the electrodes!
-
positive
electrode
current
PID Principle of Operation
Gas Out
(No Longer Ionized)
Display
Amplifier
e
-
100 PPM
e
-
e
e
-
-
e
Gas In
e
e
-
UV Lamp
-
-
e
e
-
e
-
e
-
- Electrode
e
+ Electrode
Gas Out
(No Longer Ionized)
The signal is then displayed in parts per million (ppm) on the
Instrument display.
PID Principle of Operation
Gas Out
(No Longer Ionized)
Display
Amplifier
e
-
100 PPM
e
-
e
e
-
-
e
Gas In
e
e
-
UV Lamp
-
-
e
e
-
e
-
e
-
- Electrode
e
+ Electrode
Gas Out
(No Longer Ionized)
As the ions leave the chamber they recombine with an electron and
The molecules exit in the same state as they entered !
PID sensors
PID Lamps
Lamp
Energy
Gas Fill
Window
Material
Window
Characteristics
Expected
Life
(hours of
operation)
10.6 eV
Krypton
Magnesium
Fluoride (MgF2)
Hydrophillic window
material, degraded
transmittance with
continued exposure to
moisture.
6,000 hours
typical
11.7 eV
Argon
Lithium
Fluoride (LiF)
Window material slightly
soluble in water, seriously
degraded in presence of UV
light
40 to 80
hours
typical
Maximum
150
What does a PID detect?
VOC’s or Volatile Organic Compounds
• Volatile: readily vaporizable at a relatively low
temperature.
• Organic: of, relating to, or containing carbon
compounds.
• Compound: something formed by a union of
elements. As a rule of thumb organic
solvents are VOC’s
Volatile Organic
Compounds
Alkanes
Alkenes
Aromatics
Alkynes
Terpenes
Examples
Butane
(a paraffin)
Examples
Ethylene
(Ethene)
Examples
Toluene
Examples
Acetylene
(welding gas)
(Ethyne)
Examples
1,8-Cineole
(Eucalyptus
Oil)
Reactivity
Slow
Reactivity
Fast
Reactivity
Medium
Reactivity
Slow
Reactivity
Fast
Sources
Liquid Fuel
Exhaust
Solvents
Natural Gas
LPG
Sources
Exhaust
Chemical
Feedstock
Sources
Liquid Fuel
Solvents
Sources
Exhaust
Biomass
burning
Sources
Natural
Vegetation
•
More accurate description: A VOC is Any hydrocarbon, except methane
and ethane, with a vapor pressure equal to or greater than 0.1 mm Hg
Sensitivity and
accuracy of a PID
• PID is capable of sub-ppm level detection of most
volatile organic compounds (VOCs) (typical
resolution 0.1 ppm)
• PID output per unit concentration ie. mV/ppm
• A PID has the sensitivity but not the accuracy
Ionization Potentials and the Lack of
PID Selectivity
Chemical Name IP (eV)
Benzene
Toluene
m-Xylene
Ethylbenzene
Ammonia
Methylene
Chloride
Carbon
monoxide
Oxygen
Water
9.25
8.82
8.56
8.77
10.20
Detected
with a 10.6
eV lamp
YES
YES
YES
YES
YES
Detected
with a 11.7
eV lamp
YES
YES
YES
YES
YES
11.32
NO
YES
14.01
NO
NO
12.08
12.60
NO
NO
NO
NO
Selectivity
• PID’s are not selective : Any molecule with an
IP less than the IP of the lamp will be ionized.
• There is a need to consider:
– What VOC’s am I likely to see?
– What of these has the lowest acceptable level?
– Calculate with response factors
Troublesome Conditions
• Presence of water vapor in sample stream
causes quenching of the detector signal
due to UV absorption and can short out the
two electrodes
• Oxygen and methane are also UV
absorbers. Significant changes in their
concentration can cause both gain and
background changes in the PID signal
Effect of Environmental
Conditions on PID Signal
• Variation in pressure and temperature will have an
effect on PID response. These effects will be
compensated by the instrument.
• For maximum accuracy: Calibrate
instrument in environmental conditions as
close to sample conditions as possible
PID Lamps Require
Periodic Cleaning
• Dust, dirt, or oil residue on lamp window will
degrade the the performance of the PID.
• The frequency of cleaning will depend on the
application. As a rule of thumb, under normal
conditions the lamp should be cleaned after every
40 hours of service.
Method of Cleaning
1 - Methanol
Solvent
cleaning with methanol, will save time and
may be sufficient depending on the type and amount
of residue on the lamp window. Use a q-tip and
gently clean lamp window.
Method of Cleaning
2 - Abrasive

Abrasive cleaning or Polishing “cuts” away a very
thin layer of lamp widow and will restore the lamp
window to like new condition.

After cleaning, the lamp requires a burn-in period until
the output of the lamp stabilizes.

- Lamp burn-in is 24 hrs.-
Photo, or the “light source”
• Handle the lamps by
grasping their bodies.
• Never touch the lens.
Touching the lens
transfers oil which will
decrease the UV
output.
Troublesome Compounds
• Compounds that have a tendency to
condense on the inner surfaces of the
detector can cause signal drift.
• Ethylene behaves erratically
• Ammonia causes severe degradation of
detector performance
PID Applications
•
•
•
•
•
•
•
Petrochemical
Oil and Gas
Hazmat
Aviation
Fire Departments
Environmental
Drug Enforcement
DETECTOR TUBES
•
•
•
•
•
Measure of low concentrations
specificity to one gas
easy to use.
Low cost
is often the complement of the standard gas
detection systems.
Dust particules detection :principles
• Backscattering:LASER light backscattered by
dust particules (analyser)
• POSTDIFFUSION: LASER light postdiffused by
dust particules (analyser)
• in-situ measurement of smoke opacity
• Particules detection by triboelectric effect (probes)
TECHNICAL
INFORMATIONS
Effects of Air Currents and
Barriers
Number of detectors :
• The number of detectors required for an
application depends on a number of factors:
• plant layout, air flow pattern, type of gas to
be monitored, degree of protection ...
ATMOSPHERIC
TESTING
Atmospheric Testing
Sample When / Where?
prior to entry
top, middle & bottom
continuously during entry**
prior to re-entry
Sample Why?
TOP
MIDDLE
BOTTOM
stratification / weights / mix
-There may be no hazardous atmosphere within the space
whenever any employee is inside the space
Atmospheric Testing
METHANE (lighter than air)
Atmospheric Testing
METHANE (lighter than air)
CARBON MONOXIDE (slightly lighter than air)
Atmospheric Testing
METHANE (lighter than air)
CARBON MONOXIDE (slightly lighter than air)
HYDROGEN SULFIDE (heavier than air)
Atmospheric Testing
-Before an employee enters the space, the internal
atmosphere shall be tested for the following
conditions in the order given:
Sample What?
What Levels?
– Oxygen content
19.5 - 23.5%
– Combustible Gases
10% LEL
– Toxic Gases
Depends on gas type
Testing recommendations for gas
monitoring instrumentation.
• How often gas monitoring instruments
be tested and calibrated ?
Testing recommendations for gas
monitoring instrumentation
• Gas monitoring instrumentation should be
treated like any other piece of lifesaving
equipment.
• It should be tested and calibrated on a regular
basis.
• The safest approach to testing gas monitors is to
function test or calibrate them prior to each
day’s use (mines …) !
When it comes to testing and calibrating
gas monitoring equipment ?
•
•
•
•
Things to consider :
Instrument use
Abuse experienced in the field
Gas exposures in fields ( high
levels )
• Operature use ( shocks or
abuse …)
Testing recommendations for gas
monitoring instrumentation
• A function test consists of
• A calibration has become a
exposing each sensor in the
very simple, sometimes
gas monitor to a known
automated, process
concentration of gas in excess • A calibration consists of
of the lowest alarm set-point.
exposing the instrument sensors
• The instrument should
to a known concentration of
respond to the gas
gas, making appropriate
concentration by going into
response adjustments to ensure
alarm
the instrument it reading
accurately :
• If the sensors do not respond
to the applied gas: the
• zero setting in a pure air and
instrument will be calibrated
span setting with a known
…
concentration of gas.
Testing recommendations for gas
monitoring instrumentation
• ISC/OLDHAM recommend a regular test
according to the using :before each day’s
usage sensitivity must be tested on a
known concentration of gas
• and a minimum interval of calibration every 6
months ( industries) or every year (domestic
market) .
Summary
u
Complete Systems
ISC/OLDHAM provides complete
Gas and flame detection, and dust
monitoring systems for your applications.
ISC/OLDHAM can supervise the installation
of these systems and execute it’s
Commissioning and start-up.