Figure 9.3(b) Combustible gas indicator — calibration



reading will be misleading and that the gas being sampled is above the lower flammable limit.

Some instruments may have sensor filaments whose catalytic action may be spoilt by the presence of other gases such as halogenated hydrocarbons (halon) sometimes used for fire extinguishing. Whenever opportunity arises, instruments should be checked against each other and any doubt resolved by a calibration kit. It should be noted that the batteries fitted within such instruments should only be changed in gas-safe areas.

Non-catalytic heated filament gas indicators

Since the action of the catalytic gas indicator depends upon combustion with air, it cannot be used for inerted atmospheres because of oxygen deficiency. Instruments suitable for such use, while operating on a similar Wheatstone Bridge principle, contain a filament sensitive to variations in heat conductivity of the sample which varies with its hydrocarbon content. Such meters usually register over the range 0 to 25 per cent hydrocarbon vapour by volume and are useful for monitoring inerting operations.

Multipoint flammable gas monitors

The catalytic and heated filament flammable gas indicators are widely used as port­able, hand-aspirated instruments. They are intrinsically safe. Their main purpose is for testing cargo tanks, void spaces and other enclosed spaces and this is most often carried out during gas freeing operations and before entry by personnel.

The catalytic instrument is also used in multi-point form for continuous monitoring of air-filled or air-ventilated spaces such as compressor rooms, motor rooms, machinery spaces and cargo holds. In multi-point form, the indicator is installed on ships' bridges or in cargo control rooms. These instruments draw samples sequentially from points in the various spaces monitored. The indications may be automatically recorded and individual alarms are provided when a low percentage of the Lower Flammable Limit is detected.


Figure 9.4 Infra-red gas analyser

Where void spaces are inerted continuously with nitrogen, the catalytic type will not function and aninfra-red analyser is often provided as the central multi-point instrument. Figure 9.4 illustrates the principle of a typical infra-red analyser. This instrument employs the property of hydrocarbon gas to absorb infra-red radiation. Two similar nickel/chrome emitters within the instrument beam provide infra-red radiation to two separate channels, one through the sample cell and one through a reference cell free of hydrocarbon. The two channels are alternately blocked by a semi-circular beam chopper driven by an electric motor. The transmitted radiation from both channels passes to a detector cell in which the gas is heated by the received radiation. The resultant rise in pressure is detected by the sensitive membrane of a condenser microphone. As a result of the chopping of the two beams and the absorptive effect of any hydrocarbon in the sample cell, the output of the microphone is an alternating current signal, directly related to the hydrocarbon content of the sample. This signal is amplified and recorded and, when gas is detected, actuates the alarm for the point being sampled.

9.7.4 Toxicity detectors

Toxic gas detectors usually operate on the principle of absorption of the toxic gas in a chemical tube which results in a colour change. A common type of toxic gas detector is illustrated in Figure 9.5. Immediately prior to use, the ends are broken from a sealed glass tube. This is inserted into the bellows unit and a sample aspirated through it. The reaction between the gas being sampled and the chemical contained in the tube causes a colour change. Usually, readings are taken from the length of the colour stain against an indicator scale marked on the tube. These are most often


Дата добавления: 2018-02-28; просмотров: 460; Мы поможем в написании вашей работы!

Поделиться с друзьями:






Мы поможем в написании ваших работ!