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Figure 2.18 Flammable vapour zones — a liquefied gas spill




The region (B) immediately adjacent to the spill area (A) is non-flammable because it is over-rich. It contains too low a percentage of oxygen to be flammable. Region (D) is also non-flammable because it is too lean; containing too little vapour to be flammable. The flammable zone lies between these two regions as indicated by (C).

2.21 SUPPRESSION OF FLAMMABILITY BY INERT GAS

Whereas increasing the oxygen concentration in a flammable mixture causes a broad­ening of the flammable range and a lowering of the energy necessary for ignition, decreasing the oxygen causes the flammable range to be narrowed and the minimum ignition energy to be increased. If the oxygen availability is reduced to a sufficient extent, the mixture will become non-flammable no matter what the combustible vapour content may be. Figure 2.19 illustrates this concept for a typical hydrocarbon gas mixtures with air and nitrogen. The mixtures are represented on the horizontal axis by the percentage oxygen content in the total mixture. The diagram provides much useful information. The narrowing of the flammable range as the oxygen is reduced


Figure 2.19 Flammable limits of gas mixtures in air and nitrogen


can be seen from the shape of the area labelled flammable. It is also clear that an oxygen content of less than that at the left hand extremity of the flammable envelope renders the mixture non-flammable. This value, for most hydrocarbon vapours, is around 10 to 12 per cent by volume. However, on a gas carrier for an atmosphere to be adequately non-flammable, less than 5 per cent (sometimes 2 per cent) by volume oxygen is needed. This allows for a degree of poor mixing and pockets of gas remaining in some areas of the tank.

The diagram is also useful in illustrating proper inerting and gas-freeing procedures. For example, assume that a tank atmosphere is determined to be at point A. If the tank is then gas-freed directly with air, the composition of the tank atmosphere will move along the line AB to the fully gas-free condition at point B. In so doing, the atmosphere passes through the flammable envelope. This can be avoided by first inerting the tank along, say, the line AC to a point below the critical dilution line. Aerating to point B may then be undertaken without the tank atmosphere passing through the flammable envelope. This result can only be safely achieved if regular measurements are taken, using properly calibrated instruments to evaluate the atmosphere throughout the tank at the various stages. In this process, it is important to use reasonable margins of safety since the shape of the flammable envelope is ill-defined for mixtures and any non-homogeneity of the tank atmosphere must be allowed for. Also, the varying range of flammable limits for the different gases must be considered (see Table 2.8). The flammable envelope data, as given in Reference 2.1, can also be helpful on a grade by grade basis.

2.22 SOURCES OF IGNITION

The principal method of protection against fire and explosion on gas carriers and on jetties is achieved through design and operational procedures. These should be planned to control atmospheres and to avoid spills or leakages. However, added protection is essential and can be provided by means of controlling sources of ignition. Sources of ignition can also result from human error. Some of the principal sources of ignition are outlined below.


Smoking

Illicit smoking can be a source of ignition in hazardous areas; therefore smoking must always be restricted to approved locations. These regulations must be enforced during cargo-handling operations, particularly when visitors are present who may not appreciate the nature of the cargo being handled. Smoking regulations should also ban the carriage of matches and lighters within hazardous areas.

Hot Work and Cold Work

Hot and cold work should only be permitted under conditions of strict control. This can best be achieved by the use of appropriate work permits. Atmospheres in areas which could become hazardous should be continuously monitored during hot and cold work operations. This should preferably be carried out with instruments which are capable of alarming automatically on the detection of flammable vapour.

Safety Tools

The use of safety tools designed for spark-free use in hazardous areas can create a false sense of security. Made from soft copper alloys, these tools are often referred to as non-sparking but it should be appreciated that fragments of steel and grit can easily become imbedded in the heads of these tools. The use of such tools is, therefore, not recommended.



Static Electricity

As is the case with many other hydrocarbon liquids, a static electrical charge can be built up within a liquefied gas as it is being pumped. It has been found that the charge will increase as pumping velocity rises. This phenomenon occurs due to charge-separation between layers within the fluid. The charge is then retained for some time within the liquid mass by its non-conducting property. The danger of such charges is that they can attain sufficient potential to create incendive sparks and, particularly in cargo tanks, electrical arcing is possible. It is, therefore, vital that the handling of gas cargoes only takes place in spaces having atmospheres outside the flammable range. On gas carriers, such atmospheres are always maintained in the over-rich condition.

Problems with static electricity can also arise within vapour flows but only when the gas is contaminated with debris, dust particles or when a condensed mist is present. In such cases it is the debris (or the mist which forms as it exits to atmosphere) which attains a static charge. Vapours which can attain a static charge in this way include carbon dioxide (as a fire extinguishing agent) and steam.

Liquid hydrocarbons which are most prone to static build-up are called static accumulators. For a physical description of this phenomenon see Reference 2.4.


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