Page 57 - International safety guide for oil tankers and terminals

HAZARDS OF PETROLEUM

Such pressurisation may occur when testing for gas in the following
conditions:
---ใช้เพื่อการศึกษาเท่านั้น---
Inert gas under high pressure or at high velocity, such as from a purge
pipe or high velocity vent.
atmosphere after it has been in an air conditioned environment. Timeย์นาวี
Hydrocarbon gas mixtures at high velocity in vapour lines or from a

high velocity vent.
The above is also relevant when using multi-gas instruments. For
example, when an infra-red sensor is being utilised for taking a % Vol gas
reading, any pellistor sensor in the instrument may suffer damage if the
inlet gas stream into the instrument is at a pressure or has a high velocity.

งานห้องสมุด ศูนย์ฝกพาณิช
Condensation
The performance of pellistors may be temporarily affected by
condensation. This can occur when the instrument is taken into a humid

should be allowed for instruments to acclimatise to the operating
temperature before they are used.

Combustible Mists
Pellistor instruments will not indicate the presence of combustible mists
(such as lubricating oils) or dusts.

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2.4.3.3 Instrument Calibration and Check Procedures
The instrument is set up in the factory to be calibrated using a specific
hydrocarbon gas/air mixture. The hydrocarbon gas that should be used for
calibration and testing should be indicated on a label fixed to the
instrument.

Guidance on calibration and on operational testing and inspection of gas
measuring instruments is given in Sections 8.2.6 and 8.2.7 respectively.

2.4.3.4 Precision of Measurement

The response of the instrument depends upon the composition of the
hydrocarbon gas being tested and, in practice, this composition is not
known. By using propane or butane as the calibration gas for an
instrument being used on tankers carrying stabilised crude oil or
petroleum products, the readings provided may be slightly in error by
giving a slightly high reading. This ensures that any reading indicated will
be “on the safe side”. (See also Section 8.2.6.)

Factors that can affect the measurements are large changes in ambient
temperature and excessive pressure of the tank atmosphere being tested,
leading to high flow rates which in turn affect the pellistor temperature.

The use of dilution tubes, which enable catalytic filament indicators to
measure concentrations in over rich hydrocarbon gas/air mixtures, is not
recommended.

2.4.3.5 Operational Features

Older instruments are fitted with flashback arresters in the inlet and outlet
of the detector filament chamber. The arresters are essential to prevent
the possibility of flame propagation from the combustible chamber and a
check should always be made to ensure that they are in place and fitted

© ICS/OCIMF/IAPH 2006 23