Tanker familiarisation gas detecting instruments 2006

Tanker Familiarization Gas detecting Instruments Gas Indicators    Measurement of Hydrocarbon concentration Catalytic Filament Combustible Gas indicator (CFCG) is used to measure HC gas in air at concentration below LFL ( % LEL) Non catalytic heated filament gas indicator and Refractive Index meter – used for measuring HC gas in excess of LEL or oxygen deficient (inerted) atmosphere Catalytic Filament Combustible Gas indicator (CFCG)    Sensing element is catalytic metal filament heated by an electric current When a mixture of HC in air is drawn over the filament, gas oxidises on the hot filament and makes it hotter This increases the resistance and change of resistance provides a measure of concentration of HC gas in the mixture Catalytic Filament Combustible Gas indicator (CFCG)     Works on Wheatstone bridge with sensor element forming one arm of the bridge Deflection is shown on a scale calibrated to read – 100 % LEL Some instrument may have extended range of 1-10% LEL Follow Manufacturer’s detailed instructions for the correct use and calibration Catalytic Filament Combustible Gas indicator (CFCG)     If concentration of HC is more than about twice the LEL, there is insufficient oxygen in the mixture to burn the HC gas completely Due to above, needle will point to max reading on the scale and falls back to reading near zero Continuous observation of the needle is necessary to avoid overlooking this kind of response For the same reason, this instrument does not give reliable reading in inerted or oxygen deficient atmosphere Catalytic Filament Combustible Gas indicator (CFCG) - Checks         Check instrument prior use in fresh air Also check after renewing filament Calibrate as per manufacturer instructions Check sensing lines for any leakages Leak test by pinching aspirator if fitted Only instruments fitted with flashback arrestors in inlet and oulet of detector filament should be used No filters other than cotton filters to be used in inlet side Water trap or water absorbent material may be used if gas is very wet Non Catalytic heated filament Gas Indicator    Sensing element is non catalytic hot filament Composition of surrounding gas determines the rate of loss of heat from the filament, and hence its temperature and resistance Sensor filament forms one arm of Wheatstone bridge Non Catalytic heated filament Gas Indicator     Reading is shown as % HC Not affected by gas concentration in excess of working range Follow Manufacture’s detailed instructions Note reading under no flow condition and atmospheric pressure Non Catalytic heated filament Gas Indicator Checks  Calibrate regularly prior use and follow manufacturer’s instructions  Reading taken with gas at high pressure will vary significantly from actual reading Refractive Index Meter      Optical device depending upon the refractive index of the gas sample and air A beam of light is divided into two Recombined at eye piece Recombined beam exhibit an interference pattern which appears to the observer as a number of dark lines to the observer One light path is through chamber filled with air and other filled with sample gas Toxic gases     Chemical indicator tubes Sealed gas tube filled with proprietary filling designed to react with specific gas and to give visible indication of concentration of that gas Bellow type fixed volume displacement hand pump Color change along the length is the measure of the toxic gas Toxic gases    Must follow manufacturer’s detailed instruction for correct use Pumps and tubes of same manufacture to be used Cross sensitivity of one gas with other – to consult the manufacturer for detailed guidance Measurement of Oxygen   Required to know the atmosphere in an enclosed space or cargo tanks, whether inerted or gas free Most common type of oxygen analyzer in use are: Paramagnetic sensor Electrolytic sensor Selective chemical absorption iquids Paramagnetic sensors  Substances having a magnetic susceptibility greater than are paramagnetic They are drawn into a magnetic field.( Substances having a negative magnetic susceptibility are diamagnetic They are repelled out of a magnetic field ) Magnetic susceptibility is the degree of magnetization of a material in response to a magnetic field Paramagnetic sensors    Oxygen is strongly paramagnetic whereas most other gases are not Sample cell has a light weight body suspended in a magnetic field When sample gas is drawn through the cell, the suspended body experiences a torque proportional to the magnetic susceptibility of the gas Paramagnetic sensors    An equal and opposing torque is produced by an electric current passing through the coil wound round the suspended body Equalizing current is a measure of magnetic force and is measure of magnetic susceptibility of the sample ie related to its oxygen content Before use, calibrate with nitrogen or co2 to purge the sample cell for zero check and with air for 21% oxygen for span Paramagnetic sensors      Analyzer readings are directly proportional to the pressure in measuring cell Calibrated for specific atmospheric pressure Reading errors can be significant for pressure variation Can avoid by reducing the sample air pressure to atmospheric pressure Filter should be clean and dry for smooth gas flow Electrolytic sensors    Oxygen content measured by output of an electrolytic cell Oxygen diffuses through a membrane into the cell causing current to flow between two special electrodes separated by a liquid or gel electrolyte Current flow is related to the oxygen concentration in the sample Electrolytic sensors    Analyzer readings are directly proportional to the pressure in the measuring cell but only small errors are caused by normal variations in atmospheric pressure Certain gases may affect the sensor and give rise to false readings SO2 and oxides of nitrogen if concentration more than 0.25% by volume Electrolytic sensors   Mercaptans and H2S can poison the sensor if levels are greater than 1% by volume Poisoning does not occur immediately but over a period of time For such cases, reference to be made to manufacturer’s instruction Chemical absorption liquids    A known volume of sample gas is brought in contact with a liquid which absorbs oxygen, causing a volume change in liquid The relationship between these volumes is a measure of oxygen content in the sample Not recommended for checking oxygen level in ullage space due to high concentration of hydrocarbon gases Personal oxygen monitors     Capable of continuously measuring the oxygen content of the atmosphere Employ an electrolytic sensor Automatically provide an audible and visual alarm when atmosphere becomes deficient in oxygen so as to give the wearer adequate warning of unsafe conditions These should be tested at regular interval Precaution for use of gas measuring instruments    Due to their vital importance, need to maintain and test carefully as per Manufacturer’s instructions Each time an instrument is to be used, a check is made of batteries, zero setting and calibration Monitor the function and response closely to get accurate reading Precaution for use of gas measuring instruments     Material and condition of sample lines affect the accuracy of gas measurement Use only recommended hose types Must be resistant to water Cracked or blocked tubes, contaminated tubes with cargo residues – all affect readings, so need to regularly inspect, check and replace sampling tubes if necessary Precaution for use of gas measuring instruments    ‘Dead spots’ in any enclosed space opr cargo tanks – where ventilation or purging is less than the average in the bulk of the tank eg bottom of tank, corners as per structural members etc Sample lines to be long enough to reach these spots Proper use of filters in the sampling line for filtering moisture – follow makers advise closely as per supplied instruction booklet ... Toxic gases     Chemical indicator tubes Sealed gas tube filled with proprietary filling designed to react with specific gas and to give visible indication of concentration of that gas Bellow... heated filament gas indicator and Refractive Index meter – used for measuring HC gas in excess of LEL or oxygen deficient (inerted) atmosphere Catalytic Filament Combustible Gas indicator (CFCG)... absorbent material may be used if gas is very wet Non Catalytic heated filament Gas Indicator    Sensing element is non catalytic hot filament Composition of surrounding gas determines the rate of