HAZARDS AND PROPERTIES OF CHEMICALS

This gives an introduction to the range of hazards normally associated with the properties of chemicals that are carried as cargoes, and the precautions necessary to minimise or avoid those hazards.

INTRODUCTION

Chemical tankers are required to transport a wide range of different cargoes, and many tankers are designed to carry a large number of segregated products simultaneously. The operation of chemical tankers differs from any other bulk liquid transportation operations, in that on a single voyage a large number of cargoes with different properties, characteristics and inherent hazards may be carried. Moreover, in port several products may be handled simultaneously at one berth, typically including different operations such as discharge and loading as well as tank cleaning. Even the less sophisticated chemical tankers are more complex to operate than oil tankers. Transportation of bulk chemicals by sea not only requires specialist ships and equipment, but also specialist crew training, both theoretical and practical, in order for those involved to understand the characteristics of the various chemicals and be aware of the potential hazards involved in handling them. A particularly important aspect of this requirement is the provision of a data sheet, or cargo information form, giving details specific to a substance, to be held on board whenever that substance is carried by the ship.

The following notes are only intended to indicate the most common characteristics and hazards of chemicals transported by sea in bulk. More detailed information on the main hazards and properties will be found in relevant chapters and appendices, where guidance is also given on the precautions to be taken.

FLAMMABILITY

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 (see Definitions for further details). Combustion of a vapour-and-air mixture results in a very considerable expansion of gases which, if constricted in an enclosed space, can raise pressure rapidly to the point of explosive rupture.

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.

As mentioned, the fire risk presented by a flammable cargo depends upon the oxygen content of the atmosphere above it. By filling the ullage space in a cargo tank with an inert gas such as nitrogen or the output of an oil fired inert gas generator/ the oxygen content can be reduced to a level at which the atmosphere will no longer support combustion of flammable vapour. This is known as inerting a tank. But it is important to remember that an inerted atmosphere may become flammable again if air is admitted, for instance during routine measuring or on venting the mixture to atmosphere or during gas freeing with air.

An inert atmosphere must not be considered as being without hazard, however, as without enough oxygen it will not support life either. Any person entering a tank which has been inerted must always follow strict procedures for entry into enclosed spaces.

HEALTH HAZARDS

1 Toxicity

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. Acute poisoning occurs when a large dose is received by exposure to high concentrations of a short duration, i.e. a single brief exposure. Chronic poisoning occurs through exposure to low concentrations over a long period of time, i.e. repeated or prolonged exposures. Toxicity is objectively evaluated on the basis of test dosages under controlled conditions, and expressed as threshold limit values (TLVs).

Prevention of exposure is achieved through a combination of cargo containment, which prevents toxic fumes or liquid from contaminating the workplace.

2 Asphyxia

Asphyxia is unconsciousness caused by lack of oxygen, and means suffocation. Any vapour may cause asphyxiation, whether toxic or not, simply by excluding oxygen in air. Danger areas include cargo tanks, void spaces and cargo pump rooms. But the atmosphere of a compartment may also be oxygen-deficient through natural causes, such as decomposition or putrefaction of organic cargo , or rusting of steel in void spaces such as cofferdams, forepeak and after peak tanks.

3 Anesthesia

Certain vapours cause loss of consciousness due to their effect on the nervous system. In addition, anaesthetic vapours may or may not be toxic.

4 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.

4 REACTIVITY

A chemical may react in a number of ways; with itself, with water, with air, with other chemicals or with other materials.

1 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, or it may be a rapid, exothermic reaction evolving large amounts of heat and gases. Heat produced by the process can accelerate it. 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 polymerisation situation occurring while the cargo is on board should be covered by the ship's emergency contingency plan.

2 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 cargoes are 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.

3 Reaction with air

Certain chemical cargoes, mostly ethers and aldehydes, 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.

4 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.

5 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.

5 CORROSIVENESS

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.

6 PUTREFACTION

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.

7 PHYSICAL PROPERTIES

1 Specific gravity

Cargo tanks on a chemical tanker are normally designed to carry cargoes of a higher specific gravity than an oil tanker. Sometimes the design strength even differs between tanks on the same ship.

The information regarding tank strength may be found on the classification society's certification of the ship, and the master must be familiar with any restrictions that may be imposed on loading heavy cargoes. Especially important is the risk of slack loading a tank because this can lead to sloshing forces that may cause damage to the tank structure or its equipment. Likewise, the tank's design capacity must be strictly observed: exceeding it is dangerous. Note that the cargo's specific gravity and its vapour pressure must be considered together.

2 Vapour pressure and boiling point

At any given temperature every liquid exerts a pressure called the vapour pressure. The liquid will boil when its vapour pressure equals the external atmospheric pressure. In a closed cargo tank a liquid will boil when the vapour pressure is equal to the external vapour pressure plus the pressure setting of the pressure/vacuum (P/V) valve. The tanks and vent systems are designed to withstand this pressure, plus the hydrostatic pressure of the cargo. Cargoes that exceed the normal atmospheric pressure at

37.8°C (100°F) should not be loaded into a tank that is not specially designed for that duty. Where a P/V valve set point can be varied, the correct setting should be confirmed. Vent line systems must be checked for correct operation at regular intervals, as structural damage can easily result from malfunction or blockage due to freezing of cargo vapour, polymer build-up, atmospheric dust or icing in adverse weather conditions. Flame screens are also susceptible to blockage, which can cause similar problems.

The higher the vapour pressure the more vapours will be released, a fact that may require use of personal protective equipment.

3 Freezing point

Most liquids have a defined freezing or solidification point, sometimes described as the melting point. Some products, such as lubricating oil additives, vegetable and animal oils, polyols etc. do not have a defined point, but a freezing or melting range. For such cargoes, viscosity is used as a measurement of the product's liquidity or handling characteristics, and the term pour point is used instead. Cargoes with a freezing point higher than the ambient temperature of the ship's trading area will need to be heated in order to remain liquid.

The structure and equipment of a ship normally impose a limitation on the carriage of heated cargoes.

Exceeding this limitation could damage the tank coating or its structure. Excessive heat will also create thermal stresses, and the risk of cracking will increase. (Note that moderate heat increases steel strength; it is expansion forces that are the immediate limiting factor.) Caution should be exercised when carrying high heat products because cargo in non-insulated pipes and vents may freeze and clog the systems. Heating arrangements must be operated in accordance with design safety precautions; for example, pressures inside heating coils in tanks must be kept higher than the cargo pressure, and any interceptor tanks between heating return lines and the engine room must be checked regularly to detect any contamination. For certain cargoes, heating coils must be blanked off in accordance with IBC Code requirements.Uninsulated cargo pipes used for high heat products pose a further safety hazard, as they may cause severe burns if touched.

4 Cubic expansion

Liquids will expand as temperature rises, or contract when temperature falls. Sufficient space must be allowed in the tank to accommodate any cubic expansion expected during the voyage. A useful formula is:

Filling ratio (% full) = 100 (1 - RT) - S

where R = coefficient of expansion per °C (from cargo data sheet)

T = expected maximum temperature rise in °C (during voyage)

S = safety margin, usually 2% of tank capacity.

Vent line systems must be checked at regular intervals. Their design capacity is based on vapour flow only; structural damage may result if vent systems become full of cargo liquid due to thermal expansion.

5 Vapour density

Vapour density is expressed relative to the density of air, as heavier or lighter. Most chemical cargo vapours are heavier than air. Caution must therefore be exercised during cargo operations, as vapour concentrations are likely to occur at deck level or in lower parts of cargo pumprooms.

6 Solubility

Solubility is expressed in different ways: either as a simple yes or no, as slight, or as a percentage, but always in relation to water. Solubility is temperature dependant. A cargo with low solubility will form a layer above or below a water layer depending on its specific gravity. Most non-soluble chemicals are lighter than water and will float on top but some others, such as chlorinated solvents, are heavier and will sink to the bottom. Chemicals that are heavier than water can cause a safety risk in pumprooms when the overlying water is disturbed, and in drip trays. Even in cargo tanks they may be trapped under water in pump wells, and pose a danger even after the tank atmosphere is tested and found safe for entry.

7 Electrostatic charging

Certain cargoes are known as static accumulators, and become electrostatically charged when handled. They can accumulate enough charge to release a spark that could ignite a flammable tank atmosphere. 

8 Viscosity

The viscosity of a cargo determines how easy it is to pump, and the amount of residue that will be left after unloading. Viscosity is related to temperature and, in general, a substance will become less viscous at higher temperatures, but note that certain cargoes (such as luboil additives) show increased viscosity when heated.IMO standards define high and low viscosity substances, and require cargo tanks that have contained substances with a high viscosity to be pre-washed and the washings discharged to shore reception facilities.