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PART XII
SOME SAFETY CONSIDERATIONS WITH INERT GAS SYSTEMS

Backflow of Cargo Gases

68. (1) To prevent the return of cargo gases or cargo from the tanks to the machinery spaces and boiler uptake, it is essential that an effective barrier always be present between these two areas; in addition to a non-return valve, a water seal and vent valve should be fitted on the deck main; it is of prime importance that these devices be properly maintained and correctly operated at all times.

(2) An additional water seal is sometimes fitted at the bottom of the scrubber (see also subsection 21(6)).

Health Hazards

Oxygen Deficiency

69. (1) Exposure to an atmosphere with a low concentration of oxygen does not necessarily produce any recognizable symptom before unconsciousness occurs; the onset of brain damage and death can follow within a few minutes; if oxygen deficiency is not sufficient to cause unconsciousness, the mind nevertheless is liable to become apathetic and complacent; even if the victim notices these symptoms and attempts to escape, the physical exertion will aggravate the weakness of both mind and body; it is therefore necessary to ventilate thoroughly to ensure that no pockets of oxygen-deficient atmosphere remain; a steady reading of 21 per cent oxygen is required for a worker to enter.

Toxicity of Hydrocarbon Vapours

(2) Inert gas does not affect the toxicity of hydrocarbon gases and the problem of toxicity is no different from that on ships without an inert gas system; because of possible gas pockets, regeneration, etc. gas-freeing must continue until the entire compartment shows a zero reading, with a reliable combustible gas indicator or equivalent, or a 1 per cent reading of the lower flammable limit, should the instrument have a sensitivity scale on which a zero reading is not obtainable.

Toxicity of Flue Gas

(3) The presence of toxic gases such as sulphur dioxide, carbon monoxide and oxides of nitrogen can be ascertained only by measurement; however, provided that the hydrocarbon gas content of an inerted tank exceeds 2 per cent by volume before gas-freeing is started, the dilution of the toxic components of flue gas during the subsequent gas freeing can be correlated with the readings of an approved combustible gas indicator or equivalent; if, by ventilating the compartment, a reading of 1 per cent of the lower flammable limit or less is obtained in conjunction with an oxygen reading of 21 per cent by volume, the toxic trace gases will be diluted to concentrations at which it will be safe to enter; alternatively, and irrespective of initial hydrocarbon gas content, ventilation should continue until a steady oxygen reading of 21 per cent by volume is obtained.

Tank Pressure

70. When an inerted cargo tank is maintained at a positive pressure, personnel should be advised of the practical hazards; such pressure must be adequately reduced before any tank-lids, ullage plugs or tank washing openings are opened.

Electrostatic Hazards

71. (1) Small particulate matter carried in flue gas can be electrostatically charged; the level of charge is usually small, but levels have been observed well above those encountered with water mists formed during tank washing.

(2) Because cargo tanks are normally in an inerted condition, the possibility of electrostatic ignition has to be considered only if the oxygen content of the tank atmosphere rises as a result of an ingress of air, or if it is necessary to inert a tank that already has a flammable atmosphere (see section 30).

Repair of Inert Gas Plant

72. (1) Because inert gas is asphyxiating, great care must be exercised when work on the plant is undertaken; although the worker may be in the open air, inert gas leaking from the plant could render him unconscious very quickly; before any equipment is opened, therefore, the inert gas plant should be completely gas freed.

(2) If any unit (e.g. the inert gas scrubber) is to be examined internally, the standard recommendations for entering enclosed spaces must be followed; blind flanges should be fitted where applicable, or the plant completely isolated.

Hazards from pyrophoric iron sulphide

73. (1) Bearing in mind the reduction of oxygen in ullage spaces compounded by the operation of inert gas systems, research has led to the conclusion that there is a significant risk of pyrophoric deposits forming in inerted tankers carrying sour crude oil; furthermore, that pyrophoric deposits can form with crude oils having a low hydrogen sulphide content and that no minimum safe level of hydrogen sulphide content can be identified; and, finally, that pyrophors which have formed during a loaded passage can persist during the subsequent ballast voyage.

(2) Thus, while various factors (such as lack of sufficiently thick deposits of iron oxide) may inhibit pyrophor formation and while the correct operation of the inert gas plant will prevent the possibility of ignition, the degree of risk is judged to be sufficiently high to require the precautions in subsection 54(2) in case of inert gas system failure.

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