GAS /OXYGEN DETECTORS

GAS /OXYGEN DETECTORS
1. INSTRUCTION
Before the personnel enter into the enclosed spaces where is possibility of anoxia, the
gas or oxygen content inside must be checked by the specific gas or oxygen contents
meter to confirm safety.
These instruction manuals define the correct maintenance and handling of the gas or
oxygen content meter.
It is recommended to refer to Chapter 11 and 18 in ISGOTT (RD-0711) which has been
supplied to Oil tankers and Gas carriers under our management.
2. BASIC KNOWLEDGE OF GAS /OXYGEN DETECTORS
2-1 LEL(LFL) and UEL(UFL) %
A mixture of Hydrocarbon and air cannot be ignited and burn unless its
composition lies within the range of gas in air concentrations known as the
“Flammable range”.
The lower limit of this range, known as the lower flammable limit (LFL) or lower
explosive limit (LEL), is that hydrocarbon concentration below which there is
insufficient hydrocarbon gas to support and propagate combustion.
The upper limit of the range, known as the upper flammable limit (UFL) or upper
explosive limit (UEL), is that hydrocarbon concentration above which there is
insufficient air to support and propagate combustion.
2-2 Flammable limit of VOL %
Very roughly the gas mixture from crude oils, motor or aviation gasolines
and natural gasoline type products can be represented respectively by the pure
hydrocarbon gases propane, butane and pentane. The Table gives the
flammable limits for these three gases.
Gases
Flammable limits Vol. % hydrocarbon in air
Flammable limit
Upper (UFL) Vol. %
Lower (LFL) Vol. %
Propane
9.5
2.2
Butane
8.5
1.9
Pentane
7.8
1.5
This information is very relevant to the ease with which vapours disperse to a
non-flammable concentration in the atmosphere.
In practice the lower and upper flammable limits of oil cargoes carried in tankers
can, for general purpose, be taken as 1 % and 10% by volume respectively.
2-3 Measurement of Hydrocarbon Gas in Air / in Inert gas or Nitrogen
The Catalytic Filament Combustible Gas (CFCG) Indicator is used for
measuring hydrocarbon gas in air at concentrations below the lower flammable
limit (LFL). The scale is graduated in % LFL.
A CFCG (接触燃焼式可燃性ガス) indicator must not be used for measuring
hydrocarbon gas in inert atmospheres.
Two types of instrument are available commercially for measuring hydrocarbon
gas concentrations in excess of the LFL or oxygen deficient (inerted)
atmospheres - the Non-Catalytic Heated Filament Gas Indicator and the
Refractive Index Meter -.
The sale is graduated in % volume hydrocarbon gas.
3. ATMOSPHERE CHECKING PRIOR TO ENTRY INTO ENCLOSED SPACES
3-1 Enclosed Spaces
An enclosed space is one with restricted access that is not subject to
continuous ventilation and in which the atmosphere may be hazardous due to
the presence of hydrocarbon gas, toxic gas, inert gas or oxygen deficiency.
This definition includes;
cargo tanks, ballast tanks,
fuel tanks, water tanks, lubricating oil tanks,
slop and waste oil tanks, sewage tanks,
cargo hold,
cofferdams, duct keel, chain locker, void spaces and trunk,
pipelines or fittings connected to any of these.
3-2 Accident
Many of the fatalities in enclosed spaces have resulted from entering the space
without proper supervision or adherence to the procedures. In almost every
case the fatality would have been avoided if the simple manner had been
followed.
The rapid rescue of personnel who have collapsed in an enclosed space
presents particular risk. It is a human reaction to go to the aid of a colleague in
difficulties, but far too many additional and unnecessary deaths have occurred
from impulsive and ill-prepared rescue attempts.
3-3 Condition for Entry
Before initial entry for inspection, cold work and hot work into any enclosed
space which is not in daily use, the atmosphere should be checked with an
oxygen analyzer to check that the normal oxygen level in air of 21 % by volume,
and with gas detector to check that not more than 1 % LFL of hydrocarbon gas
and no presence of any toxic gases.
4. HANDLING OF OXYGEN CONTENT METER AND GAS DETECTOR
4-1 Preparations
The oxygen content meter and gas detector should be handled according to the
instruction book of the meter , and make preparations as follows.
1) To fit necessary assemblies
2) To check power of batteries
3) To set range and mode switch
4) To adjust Zero (0) point
5) To check indicating in normal ie; 21 % for oxygen in air
4-2 Cautions
If extension tube is used for observation in the tanks, the duration (or times of
hand pumping) of which the sample gas or air is taken in the instrument, shall
be confirmed. Its duration must be indicated at the tube.
Observer should pay attention for the level of liquid in the tanks to avoid sucking
of such liquid into the instrument.
Care should be taken to obtain a representative cross-section of the
compartment by checking at several depths and through as many deck
openings as practicable. When checking are being carried out from deck level,
ventilation should be stopped and minimum period of about 10 minutes should
be allowed to elapse before reading are taken.
5. MAINTENANCE OF GAS DETECTOR AND OXYGEN CONTENTS METER
5-1 Calibration
1) Gas Detector
The response of Catalytic filament combustible gas indicator (CFCG) should
be checked with a mixture of hydrocarbon gas in air such as 50 % LFL(LEL)
test gas.
The response of Non-Catalytic heated filament gas indicator should be
checked with a mixture of hydrocarbon gas in air such as 5 to 10 % Vol. test
gas.
Some gas detector is compatible with above systems. In this case both
types of test gas should be used for, which testing method has to be referred
with instruction book of the maker.
2) Oxygen Content Meter
It is essential that each time an instrument is to be used, a check is made
of batteries (if fitted) and zero setting.
Calibration is simple on all type of oxygen content meter, using atmospheric air
as standard. Zero (0) calibration can be checked with pure nitrogen.
5-2 Renewal of Sensor
The sensor must be replaced in accordance with the maker’s instruction,
usually valid of the sensor is limited within 2-5 years depend on the each type of
the instrument.
5-3 Record of Calibration and Renewal of Sensor
Calibration and replacement of the sensor should be recorded using by
RCD-3-9.
5-4 List of Instruction for GAS DETECTOR and OXYGEN CONTENT METER
Maker & Type
Detected Gases
Range of Observation
RIKEN OX-1
Oxygen
0-25 %
Oxygen
0-25 %
RIKEN GX-111
Oxygen
0-25 %
Portable sample-
Combustible Gas
0-100 % LEL In Air
Carbon Oxide
0-100 ppm
Portable Oxygen
Analyzer
RIKEN OX-62B
Portable Oxygen
monitor
drawing detector
/Alarm for detector
of combustibles,
carbon oxide,
hydrogen sulfide
and oxygen
deficiency, battery
powered
(CO)
Hydrogen Sulfide
0-29 ppm
(H2S)
30-125 ppm
RIKEN GP-226
Combustible Gas
Hand held
0-100 % LEL In Air
0-20 % LEL In Air
portable gas
0-1.8 % / 0-0.36 Vol % LPG In Air
detector
/Alarm
TOKA GC-707
Combustible Gas
0-100 % Vol In Air/Inert gas
Combustible Gas
0-100 % Vol In Air/Inert gas
Portable gas
concentration
meter
RIKEN R1415(HC)
0-100 % LEL In Air/Inert gas
Portable HC gas
detector
KITAGAWA AP-1
Precision gas
detector
Appropreate

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