Page 55 - International safety guide for oil tankers and terminals

HAZARDS OF PETROLEUM

instruments using infra-red sensors that can perform the same function as
the Tankscope.
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2.4.3 Flammable Gas Monitors (Explosimeters)
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Modern flammable gas monitors (Explosimeters) have a poison resistant
flammable pellistor as the sensing element. Pellistors rely on the presence of
oxygen (minimum 11% by volume) to operate efficiently and for this reason
flammable gas monitors should not be used for measuring hydrocarbon gas in
inert atmospheres.

2.4.3.1 Operating Principle

A simplified diagram of the electrical circuit incorporating a pellistor in a
Wheatstone Bridge is shown in Figure 2.1.

Unlike early Explosimeters, the pellistor unit balances the voltage and
zeros the display automatically when the instrument is switched on in
fresh air. In general, it takes about 30 seconds for the pellistor to reach its
operating temperature. However, the operator should always refer to the
manufacturer’s instructions for the start up procedure.

A gas sample may be taken in several ways:
ึ
Diffusion.
Hose and aspirator bulb (one squeeze equates to about 1 metre of
hose length).
Motorised pump (either internal or external).

Flammable vapours are drawn through a sintered filter (flashback
arrestor) into the pellistor combustion chamber. Within the chamber are
two elements, the Detector and the Compensator. This pair of elements is
heated to between 400 and 600ºC.

When no gas is present, the resistances of the two elements are
balanced and the bridge will produce a stable baseline signal. When
combustible gases are present, they will catalytically oxidise on the
detector element causing its temperature to rise. This oxidation can only
take place if there is sufficient oxygen present. The difference in
temperature compared to the compensator element is shown as % LFL.

The reading is taken when the display is stable. Modern units will indicate
on the display when the gas sample has exceeded the LFL.

Care should be taken to ensure that liquid is not drawn into the
instrument. The use of an in-line water trap and a float probe fitted to the
end of the aspirator hose should prevent this occurrence. Most
manufacturers offer these items as accessories.

Only cotton filters should be used to remove solid particles or
liquid from the gas sample when hydrocarbons are being
measured. Water traps may be used to protect the instrument
where the sampled gas may be very wet. Guidelines on the use of
filters and traps will be found in the operating manual for the
instrument. (See also Section 2.4.13.3)

© ICS/OCIMF/IAPH 2006 21