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GAS TANKERS
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Authorised Gas Tester Assessed Competency
Course
Presented by: Presented by:
John M Sonley, C.Sci C.Eng John M Sonley, C.Sci C.Eng MIGEM MEI C.Chem FRSCMIGEM MEI C.Chem FRSC
JMS Consultants
Gas Molecules
Properties of Flammable GasesFLAMMABILITY AND IGNITION
Most natural gases are mainly methane - typically 70 - 90 % methanemixed with smaller amounts of the other hydrocarbon gases
Flammability Limits for Methane in Air
Ignition of methane
Flammable mixtures of methane and air may ignite if heated above the auto-ignition temperature , which for methane is 538oC
- without any other source of ignition being presentFlammability of Other Gases – BS EN 61779-1:2000
GasEthane - Propane -Butane -Pentane -Hydrogen -
LEL2.5 %vol.1.7 %vol.1.4 %vol.1.4 %vol.4.0 %vol.
0% Methane( Clean Air)
LEL 4.4%
% Vol. Methane % LEL Methane
20% LEL = 0.88% methane in air
40% LEL = 1.76% methane in air
60% LEL = 2.64% methane in air
80% LEL = 3.52% methane in airUEL 17%
‘BUMP’ Test
2.2% methane in air
= 50% LEL
Response Factorsof the catalytic flammable gas sensor
to a range of hydrocarbons
Gas at its LEL Concentration Meter Reading % LEL
Methane 100
Ethane 68
Propane 55
n-Butane 59
n-Pentane 46
Hexane 37
Heptane/Octane 38
Nonane 31
Hydrogen 77
Condensate
or Solvent
Vapour Cloud
Containment of hazard
……. use sampling system
Containment of:
Gases/Vapour/Liquids
Other ‘heavy’ gases:
Carbon dioxide: R.D. = 1.5
Chlorine: R.D. = 2.5
Units of Measurement
Oxygen Flammables Toxics
10 10 10
10% vol 10% LEL 10 ppm 0.44% vol 0.001% vol
Clean Air
Oxygen = 20.95% vol.
Nitrogen = 78.08% vol.
Properties of Harmful and Toxic Gases
Large amounts of natural gas and some other gases displace oxygen from the air we breathe and cause suffocation
APPLICATION / WHERE FOUND
Pipeline and vessel purging operationsPipe freezing operations
Argon arc welding
GAS
Nitrogen
Argon
ASPHYXIANT GASES
Hazards of Oxygen
<19% Oxygen
‘Deficiency’
>23% Oxygen
‘Enrichment’
TOXIC GASES
GAS APPLICATION / WHERE FOUND
Condensate / heavyhydrocarbon vapours
Hydrogen sulphide
Carbon monoxide
Carbon dioxide
Sulphur dioxide
Chlorine
Benzene
Hydrazine
Crude oil residues, oil condensate residuesand sludges
Found widely in reservoir gas, crude oil vapours and around stagnant water
Associated with combustion processes
Fire extinguishers and deluge systems
Sulphur recovery plants, flare stacks - formed when hydrogen sulphide is burned
Water purification processes
Occasionally found in crude oil gases and vapours
Used in water treatment processes
Small amounts of some gases and vapours found in the oil industry poison our bodies- we measure these amounts in parts per million - ‘ppm’.
For example : 10 ppm = 0.001% 1000 ppm = 0.1 %
Properties of Harmful and Toxic Gases
For Toxic Gases …
C.O.S.H.H.
Control of Substances Hazardous to Health
Workplace Exposure Limit
WEL
8 HOUR LIMIT 15 MINUTE LIMIT
WELs published in HSE document EH40 each year
Control of Substances Hazardous to Health - COSHHWorkplace Exposure Limits for Toxic Gases
WEL
8 Hour Limit 15 Minute Limit
GAS
Carbon Monoxide
30 ppm 200 ppm
Hydrogen Sulphide 5 ppm 10 ppm
Light PetroleumHydrocarbons( Condensates )
500 ppm -
Medium PetroleumHydrocarbons( Sludges)
250 ppm -
Carbon Dioxide 5000 ppm 15000 ppm
Sulphur Dioxide No longer listed No longer listed
Chlorine 0.5 ppm 1 ppm
Benzene 3 ppm(1 ppm – 2003)
-
Hydrazine 0.02 ppm 0.1 ppm
A number of the listed Workplace Exposure Limits are cancer causing, carcinogenic. As a principle, no exposure to concentrations greater than the WEL shall be permitted, and for carcinogenic compounds, ‘adequate control’ should be to as low a level ‘as is reasonably practicable’.
Properties of Hydrogen Sulphide
Occurrence Important Properties•In crude oil - ‘Sour’ crude•In reservoir gas - ‘Sour’ gas•In stagnant water
•Deadly toxic gas at high concentration•Colourless•Smells of bad eggs at low concentrations•Paralyses sense of smell at high concentrations
Effects and Symptoms of H2S
0.15ppm -------------------------------- threshold of smell - 0.000015% vol.5ppm ----------------------------------------------------- 8 hour exposure limit10ppm ------------------------------------------------- 15 minute exposure limit20ppm ---------------------------- strong ‘bad - eggs’ smell, eye irritation100ppm ------------------------------- loss of sense of smell in 3 - 15 mins200 - 300ppm ----------------------- instantaneous loss of sense of smell (that’s why you cannot use your nose as a gas detector!)700ppm or more -------breathing ceases, loss of consciousness and death unless resuscitated
REMEMBER! - 700ppm sounds a lot but its only 0.07% vol.- -there may well be 99.93% vol. fresh air as the remainder
H2S
Physical Properties of Hydrogen Sulphide
1.Flammable gas:
LEL = 4.0% vol = 40,000 ppm!
2. Reacts with iron in an inert atmosphere to create : ‘Pyrophoric Iron Sulphide’ – a fire and explosion hazard
3. Heavier than air:
Relative Density = 1.2
(Air = 1)
Industries with SRB Problems
Hazards of Carbon Monoxide - CO
1. Produced by incomplete combustion
2. Haemoglobin in your blood prefers CO to oxygen – 240 times more! – to form carboxyhaemoglobin
3. Colourless gas of neutral density
4. Odourless gas – it doesn’t smell of exhaust fumes!
5. Flammable gas: LEL = 10.9% vol. in air
6. Smokers have 5 – 10% of their blood permanently complexed as carboxyhaemoglobin
The Catalytic Gas SensorUSED IN PORTABLE AND FIXED GAS DETECTORS
TO DETECT AND M EASUREMETHANE AND OTHER FLAMMABLE GASES IN AIR
The Catalytic Gas Sensor ( Pellistor )
Pellistors measure combustible gases in air at concentrations of upto the Lower Explosive Limit. Flammable gases are burnt on the hot
catalytic detector element and the resulting heat generated increasesthe electrical resistance of the platinum wire coil. This change inresistance produces a reading on the detector which is directly
proportional to the amount of flammable gas present in the atmosphere
Limitations
( Hot work and Confined Space Entry Gas Testing )
Catalyst Platinum wirecoil
1. Cannot be used in inert gas atmospheres - needs 13% or more oxygen
2. Fails - to - danger due to Poisons and Blockage of the flame arrestor Silicones Water De-greasing fluids Drilling fluids Hot lube oils / WD40 Oil 3. Methane calibrated detectors seriously under-read concentrations of the higher hydrocarbon gases and vapours e.g. condensate vapours
4. Older gas detectors can give ambiguous readings at high gas concentrations
Approx. 1 mm
550 oCApprox.
‘Old Wives’ Tale’ – Don’t ‘bump’ test the detector too often because it ‘wears out the sensor’ – untrue!
Catalytic Sensors - EEV ‘pellistors’
A Simple ‘Explosimeter’
Gas Detectors don’t like water !
Corrosion of catalytic sensors
Catalytic fixed gas sensor
The Thermal Conductivity Gas SensorUSED IN PORTABLE GAS DETECTORS TO MEASURE THE PERCENTAGE
OF FLAMMABLE GAS MIXED WITH AN INERT GAS ( NITROGEN )DURING PURGING OPERATIONS
Principle
Thermal conductivity sensors detect a gas by comparing the heat conducting propertiesThermal conductivity sensors detect a gas by comparing the heat conducting propertiesof that gas with the heat conducting properties of a ‘ reference’ gas, usually nitrogenof that gas with the heat conducting properties of a ‘ reference’ gas, usually nitrogen
Typical Application - entry into a vessel that contains natural gas
Natural gas
Purge vessel
with nitrogen
Less than7.5% methane
Purge vessel
with air
20.9%oxygen
(a) Permit to Enter issued
(b) Entry made, work completed
20.9%Oxygen
Purge vessel
with nitrogen
Less than 7.7% oxygen
Purge vessel
with natural gas
Natural gas
Step 1. Step 2. Step 3.
Step 4. Step 5. Step 6.
Limitations
1. For accurate results, the detector should be calibrated with the natural gas in use and be zeroed with the inert (purge ) gas being used
2. The detector is sensitive to changes in sample gas flow rate
IMPORTANT
Thermal conductivity based gas detectors are calibrated ‘ % volume ’and CANNOT be used for Hot Work and Confined Space Entry gastesting - - - - - - they are too insensitive!
in nitrogen
Principle of Infra-red Gas Detection
Sieger ‘Searchpoint’infra-red ‘point’ detector
Sieger ‘Searchline’Open-path detector for
flammable gases
PLMS ‘Line of Sight’Infra-red Flammable Gas
Detector
Installation ofOpen Path Gas
Detection
Draeger Multiwarn II- infra-red flammable
gas sensor
The Infra Red Gas SensorUSED IN PORTABLE AND FIXED GAS DETECTORS
TO DETECT AND MEASURE HYDROCARBONS AND CARBON DIOXIDE
Broad-band infra red light from a source shines through a window and into a cuvette where ambient air to be monitored diffuses or is pumped. The light is reflected from a concave mirror, back through the window on to an angled beam splitter. Part of the light penetrates the beam splitter, passes through a narrow band interference filter tuned to the absorbtion wavelength of the gas being detected and through to a detector. The remainder of the light is reflected by the beam splitter through an equivalent filter tuned to a reference wavelength, slightly away from the absorbtion wavelength of the gas being detected and through to a reference detector.
Features• Can be configured to be Fail – Safe, unlike catalytic detectors• Electrically more stable than catalytic detectors• Will operate in inert gases as well as air – purging applications• Cannot be used to detect hydrogen• Interference filters available for several gases• Widely used in ‘Line of Sight’ or ‘Open Path’ gas detectors
Electrochemical Oxygen Sensor
Oxygen electrochemical cells are self powered, metal - air ‘batteries’ consisting of an anode, a gel electrolyte and an air permeable cathode. Oxygen within the air supply diffuses in a controlled manner through a barrier, chemically reacts within and the reaction produces a voltage proportional to the concentration of oxygen
Limitations• Selective, but some ‘cross-sensitivity’ •Gas ‘batteries’ which become ‘flat’ and need replacement •Temperatures greater than 400C cause rapid failure and leakage •Toxic gas sensors - radio transmitter interference •Oxygen sensors - air pressure pulses cause false alarms
USED IN PORTABLE AND FIXED GAS DETECTORSTO DETECT AND MEASURE : % OXYGEN
AND TOXIC GAS CONCENTRATIONS (ppm) OF:
HYDROGEN SULPHIDE CARBON MONOXIDE SULPHUR DIOXIDE CHLORINE
+ + + + + + + + + + + + +CATHODE + + + + + + + + + + + + +
ELECTROLYTE
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ANODE _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
LoadResistorV
4e-
Capillary DiffusionBarrier
Air
The Electrochemical Cell
H2S Paper Tape Detector
Chemical Stain Tubes
Chemical Stain Tubes
Draeger Chemical Detector TubesUSED MAINLY WHEN TESTING CONFINED SPACES AND VESSELS
FOR TOXIC AMOUNTS OF THE FOLLOWING SUBSTANCES :(a) Vapours from crude oil residues, condensates and sludges
(b) Residual motor spirit vapours(c) Paint and other solvent fumes and vapours
(d) Carbon monoxide and carbon dioxide(e) Hydrogen sulphide and mercaptans
(f) Benzene and other aromatics
Limitations1. Tubes are gas specific, choose the correct tube2. Tubes are concentration specific, choose the correct tube3. Instructions enclosed with each box of tubes4. Tubes can only be used once5. Accuracy only plus/minus 25%, don’t use for oxygen tests6. Single test only - not a continuous monitor7. Shelf life - typically 2 years, see ‘use-by’ date on box
Kitagawa Chemical Stain Tube
Gastec Chemical Stain Tube
Calibration Gases
2.2% methane in air – stable gas mixture
Low ppm mixtures of many reactive toxic gases in air – NOT stable
Examples: H2S, SOx, NOx Cl2 – these gas mixtures require special cylinders,
regulators and tubing
Explosive Atmospheres Certification
1. The EEx label is on the outside of the certified product – some detectors need leather cases
2. Don’t infringe the EEx certification by a ‘cowboy’ repair
GAS DETECTOR APPLICATIONSAND
TYPES OF PORTABLE GAS DETECTOR
1. HOT WORK Gas Testing
Gas Test
Flammable GasDetector Principle Unit of
MeasurementMethane
( and otherflammable gases)
Catalyticor
Infra-red – NOT for Hydrogen!
% LEL
2. CONFINED SPACE OR VESSEL ENTRY Gas Testing
Gas Test Detector Principle Unit of Measurement
Oxygen
Methane or other
flammable gases
Toxic gases or vapours
Electrochemical cell
Catalytic orInfra-red
Electrochemical cellChemical stain tube
% O2
% LEL(flammable gas in air)
ppm toxic gas
3. INERT GAS PURGE Gas Testing
Oxygen
Methane
Electrochemical cell
Thermal conductivity % volume methane in inert gas
% O2
Pre-issue Testing of DetectorMechanical Checks
BEFORE USING A PORTABLE ELECTRICAL GAS DETECTOR 1. Check body of detector for physical damage and loose components
2. Check gas inlet port for water / mud / oil contamination
3. Check that flammable atmospheres safety label (BASEEFA or others) is attached and recalibn. date is OK
4. Connect and check aspirator bulb and tubing for wear, blockage, leakage or water / oil contamination
5. Check any user accessible filters for dust, oil or water contamination BEFORE USING A DRAEGER CHEMICAL TUBE DETECTOR
1. Check that bellows spring(s) is not broken or dislodged by squeezing bellows and observing
2. Check that you have chosen the correct tube for your application - - correct gas, correct concentration and correct ‘use-by’ date
3. Check that the bellows do not leak by inserting unbroken tube into the aspirator ( with or without the extension tube )compress bellows and confirm that bellows do not expand.
- - - and if your detector fails any one of these checks return it to the issuing authority
Pre-issue Testing of DetectorElectrical / Gas Checks
BEFORE USING A PORTABLE ELECTRICAL GAS DETECTOR1. Switch on and study any diagnostic messages
2. Check condition of battery using built - in indicator
3. Check functioning of audible and visual alarms during warm - up (when applicable) - cancel alarms
4. Note reading in clean air to be 20.9 or 21% for oxygen and ‘zero’ for flammable and all toxic gases
5. If detector has an electrical aspirating pump, seal gas inlet with finger and check the operation of the flow failure indicator or note change in sound of the pump
6. Check correct functioning of a catalytic sensor by aspirating a standard test gas through the detector and noting : (a) activation of both audible and visual alarms at the set alarm level, usually 20% LEL (b) meter indicates plus/minus 5% LEL of the correct reading : with 2.2% methane in air reading is 45 - 55% LEL with 1.1% methane in air reading is 15 - 25% LEL7. Check correct functioning of a thermal conductivity sensor by aspirating a 50% by volume methane in nitrogen gas mixture through the detector and noting a reading of 50 plus / minus 2 % vol. - - - and return faulty detectors to the issuing authority
VA 16
TRUSTTRUST THE INDICATION THE INDICATION ON YOUR GAS DETECTOR - -ON YOUR GAS DETECTOR - -
- - - PERHAPS THAT LITTLE - - - PERHAPS THAT LITTLE READINGREADING IS NOTIS NOT
‘‘JUST ZERO DRIFT’JUST ZERO DRIFT’ ! !
TRUSTTRUST THE INDICATION THE INDICATION ON YOUR GAS DETECTOR - -ON YOUR GAS DETECTOR - -
- - - PERHAPS THAT LITTLE - - - PERHAPS THAT LITTLE READINGREADING IS NOTIS NOT
‘‘JUST ZERO DRIFT’JUST ZERO DRIFT’ ! !
- - and finally- - and finally ! !- - and finally- - and finally ! !
Why are you carrying out a gas test?
Answer : To certify that the atmosphere is safe to work in
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