3.14.1 Certain fixed and portable instruments
are required for the safe and effective operation of an inert gas
system. It is desirable that all instruments should be graduated to
a consistent system of units.
3.14.2 Clear instructions should be provided for
operating, calibrating and testing all instruments and alarms. Suitable
calibration facilities should be provided.
3.14.3 All instrumentation and alarm equipment
required in compliance with regulation 62 should
be suitably designed to withstand supply voltage variation, ambient
temperature changes, vibration, humidity, shock, impact and corrosion
normally encountered on board ships.
3.14.4 The arrangement of scrubber instrumentation
and alarm should be as follows:
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.1 The water flow to the scrubber should be monitored
either by a flow meter or by pressure gauges. An alarm should be initiated
when the water flow drops below the designed flow requirements by
a predetermined amount and the inert gas blowers should be stopped
automatically in the event of a further reduction in the flow. The
precise setting of the alarm and shutdown limits should be related
to individual scrubber designs and materials.
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.2 The water level in the scrubber shall be monitored
by a high water level alarm (see regulation
62.19.1.2). This alarm should be given when predetermined limits
are reached and the scrubber pump shut down when the level rises above
set limits. These limits should be set having regard to the scrubber
design and flooding of the scrubber inlet piping from the boiler uptakes.
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.3 The inert gas temperature at the discharge
side of the gas blowers shall be monitored. An alarm should be given
when the temperature reaches 65°C and automatic shutdown of the
inert gas blowers should be arranged if the temperature reaches 75°C.
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.4 If a precooler is necessary at the scrubber
inlet to protect coating materials in the scrubber, the arrangements
for giving an alarm in .3 above should apply to the outlet temperature
from the precooler.
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.5 To monitor the scrubber efficiency, it is recommended
that the cooler water inlet and outlet temperatures, and the scrubber
differential pressures are indicated.
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.6 All sensing probes, floats and sensors required
to be in contact with the water and gas in the scrubber should be
made from materials resistant to acidic attack.
3.14.5 For the deck water seal, an alarm should
be given when the water level falls by a pre-determined amount but
before the seal is rendered ineffective. For certain types of deck
water seals, such as the dry type, the water level alarm may require
to be suppressed when inert gas is being supplied to the inert gas
distribution system.
3.14.6 The pressure of the inert gas in the inert
gas main shall be monitored (see regulation
62.19.1.9). An alarm should be given when the pressure reaches
the set limit. The set limit should be set having regard to the design
of cargo tanks, mechanical nonreturn valve and deck water seal.
3.14.7 The arrangement for oxygen analyser, recorder
and indicating equipment should be as follows:
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.1 The sampling point for the oxygen analyser
and recorder unit should be located at a position in the pipework
after the blowers and before the gas pressure regulating valve specified
in regulation 62. At the chosen position
turbulent flow conditions should prevail at all outputs of the blowers.
The sample point should be easily accessible and be provided with
suitable air or steam cleaning connections.
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.2 The sampling probe should incorporate a dust
filter in accordance with the instrument manufacturer's advice. The
probe and filter should be capable of being withdrawn and cleaned
or renewed as necessary.
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.3 The sensing pipe from the sampling probe to
the oxygen analyser should be so arranged that any condensation in
the sensing pipeline does not prevent the gas sample reaching the
oxygen analyser. Joints in the pipeline should be kept to a minimum
to prevent the ingress of air.
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.4 Any coolers required in the sensing pipes should
be installed at the coldest point in the system; alternatively, in
certain instances it may be prudent to heat the sensing pipes to prevent
condensation.
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.5 The position of the analyser should be so chosen
that it is protected from heat and adverse ambient conditions, but
it should be placed as close as practicable to the sampling point
to reduce the time between the extraction of a sample and its analysis
to a minimum.
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.6 The recording unit and repeater indication
required by regulation 62.16 should
not be located in positions subject to heat and undue vibration.
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.7 The resistance of the connecting cables between
the analyser and the recorder should be in accordance with the instrument
manufacturer's instruction.
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.8 The oxygen analyser should have an accuracy
of ± 1% of the full-scale deflection of the indicator.
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.9 Dependent on the principle of measurement,
fixed zero and/or span calibration arrangements should be provided
in the vicinity of the oxygen analyser fitted with suitable connections
for portable analysers.
3.14.8 A sampling point should be provided between
the automatic gas pressure regulating valve and the deck water seal
for use with portable instruments.
3.14.9 The inert gas pressure sensor and recorder
should obtain the signal from a point in the inert gas main between
the deck isolating/nonreturn valve and the cargo tanks (see regulation 62.16.1).
3.14.10 When the pressure in the inert gas main
forward of the nonreturn devices falls below 50 mm water gauge means
shall be provided to give an audible alarm or to shut down the main
cargo pumps automatically (see regulation
62.19.8).
3.14.11 The alarms required by regulation 62.19.1.7 should be given on
the navigating bridge and in the machinery space.
3.14.12 In accordance with regulation 62.17 portable instruments shall be provided for
measuring oxygen and flammable vapour concentration. With regard to
the hydrocarbon vapour meter, it should be borne in mind that meters
working on the catalytic filament principle are unsuitable for measuring
hydrocarbon concentration in oxygen-deficient atmospheres. Furthermore,
meters using this principle cannot measure concentrations of hydrocarbon
vapours above the lower flammable limit. It is, therefore, advisable
to use meters using a principle which is not affected by oxygen deficiency
and which are capable of measuring hydrocarbon concentration in and
above the flammable range. For measuring below the lower flammable
limit, provided sufficient oxygen is present, the catalytic filament
meter is suitable.
3.14.13 All metal parts of portable instruments
and sampling tubes requiring to be introduced into tanks should be
securely earthed to the ship structure while the instruments and sampling
tubes are being used. These portable instruments should be of an intrinsically
safe type.
3.14.14 Sufficient tubing etc. should be provided
to enable fully representative sampling of a cargo tank atmosphere
to be obtained.
3.14.15 Suitable openings should be provided in
cargo tanks to enable fully representative samples to be taken from
each tank. Where tanks are subdivided by complete or partial wash
bulkheads, additional openings should be provided for each such subdivision.