The Various hazards associated with carriage of chemicals.
Hazards associated with carriage of chemicals:
Toxic Vapour given off by a flammable liquid will burn when ignited provided it is mixed with certain proportions of air, or more accurately with the oxygen in air. But if there is too little or too much vapour compared to the air, so that the vapour-and-air mixture is either too lean or too rich, it will not burn.
The limiting proportions, expressed as a percentage by volume of flammable vapour in air, are known as the lower flammable limit (LFL) and the upper flammable limit (UFL), and the zone, in between is the flammable range.
- In addition, a flammable liquid must itself be at or above a temperature high enough for it to give off sufficient vapour for ignition to occur. This temperature is known as the flash point. Some cargoes evolve flammable vapour at ambient temperatures, others only at higher temperatures or when heated.
- Safe handling procedures depend upon the flammability characteristics of each product. Non-combustible cargoes are those which do not evolve flammable vapours.
- Volatile and Non Volatile Cargoes.
- If a cargo is being handled at a temperature within 10C of its flashpoint, it should be considered volatile.
- Therefore a cargo with a flashpoint of 80C should be considered volatile if handled at a temperature of 70C or above.
➢ HEALTH HAZARDS:
- Toxic means the same as poisonous. Toxicity is the ability of a substance, when inhaled, ingested, or absorbed by the skin, to cause damage to living tissue, impairment of the central nervous system, severe illness or, in extreme cases, death. The amounts of exposure required to produce these results vary widely with the nature of the substance and the duration of exposure to it.
- Suffocation: suffocation is unconsciousness caused by lack of oxygen, Any vapour may cause suffocation, whether toxic or not, simply by excluding oxygen in air. Danger areas include cargo tanks, void spaces and cargo pumprooms. But the atmosphere of a compartment may also be oxygen deficient through natural causes, such as decomposition or putrefaction of organic cargo.
- Anaesthesia: Certain vapours cause loss of consciousness due to their effect on the nervous system. In addition, anaesthetic vapours may or may not be toxic.
- Additional health hazards: Additional health hazards may be presented by non-cargo materials used on board during cargo handling. One hazard is that of frostbite from liquid nitrogen stored on board for use as atmosphere control in cargo tanks. Full advice on dealing with frostbite is contained in the MFAG .
Another hazard is that of burns from accidental contact with equipment used while handling heated cargoes.
Self Reaction:The most common form of self-reaction is polymerisation. Polymerisation generally results in the conversion of gases or liquids into viscous liquids or solids. It may be a slow, natural process which only degrades the product without posing any safety hazards to the ship or the crew.
Such a reaction is called a run-off polymerisation that poses a serious danger to both the ship and its personnel. Products that are susceptible to polymerisation are normally transported with added inhibitors to prevent the onset of the reaction.
An inhibited cargo certificate should be provided to the ship before a cargo is carried. The action to be taken in case of a polymerization situation occurring while the cargo is on board should be covered by the ship’s emergency contingency plan.
Reaction with water: Certain cargoes react with water in a way that could pose a danger to both the ship and its personnel. Toxic gases may be evolved. The most noticeable examples are the isocyanates; such cargoesare carried under dry and inert condition. Other cargoes react with water in a slow way that poses no safety hazard, but the reaction may produce small amounts of chemicals that can damage equipment or tank materials, or can cause oxygen depletion.
Reaction with air: Certain chemical cargoes, mostly ethers, may react with oxygen in air or in the chemical to form unstable oxygen compounds (peroxides) which, if allowed to build up, could cause an explosion. Such cargoes can be either inhibited by an anti-oxidant or carried under inert conditions.
Reaction with other cargoes: Some cargoes react dangerously with one another. Such cargoes should be stowed away from each other (not in adjacent tanks) and prevented from mixing by using separate loading, discharging and venting systems. When planning the cargo stowage, the master must use a recognised compatibility guide to ensure that cargoes stowed adjacent to each other are compatible.
Reaction with other materials: The materials used in construction of the cargo systems must be compatible with the cargo to be carried, and care must be taken to ensure that no incompatible materials are used or introduced during maintenance (e.g. by the material used for replacing gaskets).
Some materials may trigger a self-reaction within the product. In other cases, reaction with certain alloys will be non-hazardous to ship or crew, but can impair the commercial quality of the cargo or render it unusable.
Acids, anhydrides and alkalis are among the most commonly carried corrosive substances. They can rapidly destroy human tissue and cause irreparable damage. They can also corrode normal ship construction materials, and create a safety hazard for a ship. Acids in particular react with most metals, evolving hydrogen gas which is highly flammable.
The IMO Codes address this, and care should be taken to ensure that unsuitable materials are not included in the cargo system. Personnel likely to be exposed to these products should wear suitable personal protective equipment.
Most animal and vegetable oils undergo decomposition over time, a natural process known as putrefaction (going off), that generates obnoxious and toxic vapours and depletes the oxygen in the tank.
Tanks that have contained such products must be carefully ventilated and the atmosphere tested prior to tank entry.
It must not be assumed that all vapours produced by cargoes liable to putrefaction will in fact be due to putrefaction; some may not be obvious, either through smell or appearance of the cargo. Carbon monoxide (CO), for instance, is colourless and odourless and can be produced when a vegetable or animal oil is overheated.