Page 63 - International safety guide for oil tankers and terminals

HAZARDS OF PETROLEUM

Sensing devices distributed throughout the spaces to be monitored. Signals
are taken sequentially from each sensor by a central control.
---ใช้เพื่อการศึกษาเท่านั้น---
A gas measurement system installed in the central control room. Samples
of the atmospheres to be checked are drawn sequentially, usually by
vacuum pump, through sample lines to the central gas measurement
carried out using equipment that has been calibrated and tested and that has์นาวี
system. It is important to ensure that there is no leakage of air into the
system as that would dilute the samples and cause misleading readings.
Infra-red sensors located in the space being monitored with the electronics

necessary for processing the signals located in a safe location, usually the
central control room.
งานห้องสมุด ศูนย์ฝกพาณิชย
Fixed gas detection units are usually fitted as a means of detecting leakage
and not for gas testing prior to entry. Gas testing for entry should only be

appropriate indicator scales. Some fixed gas detection units do meet these
criteria. (See Section 10.10.2.)

2.4.9 Measurement of Oxygen Concentrations
Portable oxygen analysers are normally used to determine whether the
atmosphere inside an enclosed space (cargo tank for example) may be
considered fully inerted or safe for entry. Fixed oxygen analysers are used for
monitoring the oxygen content of the boiler uptakes and the inert gas main.
ึ

The following are the most common types of oxygen analysers in use:
Paramagnetic sensors.
Electrochemical sensors.

All analysers, regardless of type, should be used strictly in accordance with the
manufacturer’s instructions. If so used, and subject to the limitations listed
below, the analysers may be regarded as reliable.

2.4.10 Use of Oxygen Analysers

2.4.10.1 Paramagnetic Sensors
Oxygen is strongly paramagnetic (i.e. it is attracted by the poles of a
magnet but does not retain any permanent magnetism) whereas most
other common gases are not. This property means that oxygen content
can be measured in a wide variety of gas mixtures.

One commonly used oxygen analyser of the paramagnetic type has a
sample cell in which a lightweight body is 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. An electric current passing through a coil wound around the
suspended body produces an equal and opposing torque. The equalising
current is a measure of the magnetic force and is thus a measure of the
magnetic susceptibility of the sample, i.e. related to its oxygen content.

Before use, the analyser should be tested with air for a reference point of
21% oxygen and with nitrogen or carbon dioxide for a 0% oxygen
reference point.

© ICS/OCIMF/IAPH 2006 29