FLAMMABILITY

When petroleum is ignited, it is the gas that is progressively given off by the liquid which burns as a visible flame. The quantity of gas available to be given off by a petroleum liquid depends on its volatility which is frequently expressed for purposes of comparison in terms of Reid Vapour Pressure. A more informative measure of volatility is the True Vapour Pressure but unfortunately this is not easily measured. It is referred to in this guide only in connection with venting problems associated with very volatile cargoes, such as some crude oils and natural gasolines. Petroleum gases can be ignited and will burn only when mixed with air in certain proportions. If there is too little or too much petroleum gas, the mixture cannot burn. The limiting proportions, expressed as a percentage by volume of petroleum gas in air, are known as the Lower and Upper Flammable Limits. These limits vary according to the different possible components of petroleum gases. For gas mixtures from petroleum liquids likely to be encountered in normal tanker trades, the overall range is from a minimum Lower Flammable Limit of about 1% gas by volume in air to a maximum Upper Flammable Limit of about 10% gas by volume in air. As a petroleum liquid is heated, the concentration of gas in air above it increases. The temperature of the liquid at which this concentration reaches the Lower Flammable Limit is known as the Flashpoint.

FLAMMABILITY CLASSIFICATION

There are many classification systems for defining the flammability characteristics of petroleum liquids, most of which are based on Flashpoint and Reid Vapour Pressure data. For the purpose of this guide, which deals primarily with the particular conditions associated with handling petroleum cargoes in tankers and terminals, the division of such liquids into the two broad categories of non-volatile and volatile, as defined below, is in general sufficient to ensure that proper precautions can be specified.

• Non-Volatile: Flashpoint of 60ºC or above, as determined by the closed cup method of testing.
• Volatile: Flashpoint below 60ºC, as determined by the closed cup method of testing.

If there is any doubt as to the characteristics of a cargo, or if a non-volatile cargo is being handled at a temperature above its Flashpoint minus 10ºC, it should be treated as volatile petroleum. Owing to their particular characteristics, residual fuel oils should always be treated as volatile.

GAS DENSITY

The gases from most petroleum liquids are heavier than air and inert gas, thus the possibility of layering of gases is a very important consideration in cargo handling operations. The density of the undiluted gas from a high vapour pressure distillate, such as motor gasoline, is likely to be about twice that of air and about 1.5 times that from a typical crude oil. These density differences diminish as the gases are diluted with air. Flammable mixtures usually contain at least 90% by volume of air and consequently have densities almost indistinguishable from that of air. More detailed information on the density of petroleum gases is given in Section 2.3.

TOXICITY

Toxicity is the degree to which a substance or mixture of substances can harm humans or animals. Toxic substances can affect humans in four main ways: by being swallowed (ingestion); through skin contact; through the lungs (inhalation) and through the eyes. Toxic substances can have local effects, such as skin or eye irritation, but can also affect other, more distant, parts of the body (systemic effects). The purpose of this Section is to describe the adverse effects associated with toxic substances to which personnel engaged in tanker operations are most likely to be exposed; to indicate the concentrations at which those adverse effects are expected to occur in humans through a single or repeated exposure; and to describe procedures for reducing the risks of such exposure. Although not strictly a matter of toxicity, the effects of oxygen deficiency are also described.

LIQUID PETROLEUM

Ingestion

Petroleum has low oral toxicity, but when swallowed it causes acute discomfort and nausea. There is then a possibility that liquid petroleum may be drawn into the lungs during vomiting and this can have serious consequences, especially with higher volatility products, such as gasolines and kerosenes.

Skin Contact

Many petroleum products, especially the more volatile ones, cause skin irritation and remove essential oils from the skin, leading to dermatitis. They are also irritating to the eyes. Certain heavier oils can cause serious skin disorders on repeated and prolonged contact. Direct contact with petroleum should always be avoided by wearing the appropriate protective equipment, especially impermeable gloves and goggles.

PETROLEUM GASES

Comparatively small quantities of petroleum gas, when inhaled, can cause symptoms of diminished responsibility and dizziness similar to drunkenness, with headache and irritation of the eyes. The inhalation of a sufficient quantity can be fatal. These symptoms can occur at concentrations well below the Lower Flammable Limit. However, petroleum gases vary in their physiological effects and human tolerance to these effects also varies widely. It should not be assumed that because conditions can be tolerated the gas concentration is within safe limits. The smell of petroleum gas mixtures is very variable and in some cases the gases may dull the sense of smell. The impairment of smell is especially likely, and particularly serious, if the mixture contains hydrogen sulphide. The absence of smell should never be taken to indicate the absence of gas.

Exposure Limits

The toxic hazards to which personnel are exposed in tanker operations arise almost entirely from exposure to gases of various kinds. A number of indicators are used to measure the concentrations of toxic vapours and many substances have been assigned Threshold Limit Values (TLVs), sometimes referred to as Permissible Exposure Limits (PELs). The term Permissible Exposure Limithas been discontinued in this publication, as operational procedures should be aimed at reducing personnel’s exposure to a minimum and not to a permissible level. Exposure limits may be set by international organisations, national administrations or by local regulatory standards. Any limits established by regulation should not be exceeded. Industry bodies and oil companies often refer to the American Conference of Governmental Industrial Hygienists (ACGIH) which has established guidelines on limits that should protect personnel against harmful vapours in the working environment. The values quoted are expressed as Threshold Limit Values (TLVs) in parts per million (ppm) by volume of gas in air. In spite of the fact that serious health affects are not believed likely as a result of exposure to TLV concentrations, the values are only guidelines. Best practice is to maintain concentrations of all atmospheric contaminants as low as is reasonably practicable. In the following text the term TLV-TWA (Time Weighted Average) is used. Because they are averages, TWAs assume short-term excursions above the TLV-TWA that are not sufficiently high to cause injury to health and that are compensated by equivalent excursions below the TLV-TWA during the conventional 8-hour working day.

Effects

The main effect of petroleum gas on personnel is to produce narcosis. The symptoms include headache and eye irritation, with diminished responsibility and dizziness similar to drunkenness. At high concentrations, these lead to paralysis, insensibility and death. The toxicity of petroleum gases can vary widely depending on the major hydrocarbon constituents of the gases. Toxicity can be greatly influenced by the presence of some minor components such as aromatic hydrocarbons (e.g. benzene) and hydrogen sulphide. A TLV-TWA of 300 ppm, corresponding to about 2% LFL, is established for gasoline vapours. Such a figure may be used as a general guide for petroleum gases but must not be taken as applicable to gas mixtures containing benzene or hydrogen sulphide. The human body can tolerate concentrations somewhat greater than the TLV-TWA for short periods.