Table 9.1 (a) Health data — cargo inhibitors
| Cargo Inhibitors
| Toxic effects
| ||||
| Substance | Flammable | Toxic | Typical TLV-TWA (ppm) | Corrosive/ Irritant | Effects on Nervous System |
| Hydroquinone | Limited | Yes | 1 | Very | Yes |
| Tertiary butyl catechol | Limited | Yes | — | Very | — |
Table 9.1 (a) provides similar information to that shown in Table 9.1 but covers the potential hazards of cargo inhibitors. Information on the type of inhibitor used in particular cargoes is given in 2.6.
Table 9.2 Additional health data — cargo liquid (effects on the human body)
| Substance | Frostbite | Chemical bum |
| Methane | Yes | — |
| Ethane | Yes | — |
| Propane | Yes | — |
| Butane | Yes | — |
| Ethylene | Yes | — |
| Propylene | Yes | — |
| Butylene | Yes | — |
| Isoprene | Yes | — |
| Butadiene | Yes | — |
| Ammonia | Yes | Yes |
| Vinyl chloride | Yes | — |
| Ethylene oxide | Yes | Yes |
| Propylene oxide | No | Yes |
| Chlorine | Yes | Yes |
This information is discussed further in 9.4 and 9.5.
9.2 FLAMMABILITY
9.2.1 Operational aspects
The single most hazardous aspect of liquefied gases is the flammable nature of their vapours. Much effort is put into ship design to ensure effective cargo containment so as to limit vapours escaping to atmosphere. In addition, ships and terminals have design specifications for electrical equipment so as to ensure that, within well-defined operating zones, such sources of ignition are eliminated. Furthermore, in the ship and terminal working environments, operational procedures should apply that limit other possible sources of ignition, such as those described in 2.22, to areas outside established safe distances (see also 9.2.2).
All liquefied gases transported in bulk by sea, with the exception of chlorine, are flammable. The vapours of other liquefied gases are easily ignited. The exception to this is ammonia which requires much higher ignition energy than the other flammable vapours. Accordingly, fires following ammonia leakage are less likely than with the other cargoes. However, in practice, it is usual to consider the possibility of ammonia ignition and to act accordingly.
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9.2.2 Emergency aspects
Because of the very rapid vaporisation of spilled liquefied gases, the spread of flammable vapour will be far more extensive than in the case of a similar spillage of oil. The chances of ignition following a spill of liquefied gas is, therefore, much greater. For this reason, many terminals establish ignition-free zones round jetties. The extent of these zones is based on a hazard analysis, taking into account local conditions and involving the dimensions of the gas cloud which could be so formed. To establish the size of such a cloud, it is necessary first to estimate the size of the maximum credible spillage. Such an estimation may be carried out in various ways and numerous methods are available. One simplified method is published in Guidelines for Hazard Analysis (see Reference 2.18). Results of such estimations at jetties, often show the need for safety distances in the order of several hundred metres.
The hazards to personnel in fighting oil cargo fires are well known and apply generally to liquefied gas fires. There are, however, some points of difference to note (see 2.20, 2.21 and 2.22). Radiation from liquefied gas fires, because of the rapidity of vapour production, can be intense and fire-fighting should only be attempted when personnel are wearing protective clothing suited for purpose.
9.3 AIR DEFICIENCY
9.3.1 Toxicity
General
Toxicity is the ability of a substance to cause damage to living tissue, including impairment of the nervous system. Illness or, in extreme cases, death may occur when a dangerous gas or liquid is breathed, taken orally or absorbed through the skin. (In general, the terms 'toxic' and 'poisonous' can be considered synonymous.)
Many substances can act as poisons and a person can be exposed to their effects by various routes. As a result, toxicology has branched into several specialised areas, one of which is industrial toxicology. In this area, the effects of chemicals in the air or on the body are evaluated.
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Toxic substances are often ranked according to a system of toxicity ratings. One such scale is shown below:
Unknown, for products with insufficient toxicity data available;
No toxicity, for products causing no harm (under conditions of normal use) or for those that produce toxic effects only because of overwhelming dosages;
Slight toxicity, for products producing only slight effects on the skin or mucous membranes or other body organs;
Moderate toxicity, for products producing moderate effects on the skin or mucous membranes or other body organs from either acute or chronic exposure;
and,
Severe toxicity, for products that threaten life or cause permanent physical impairment or disfigurement from either acute or chronic exposure.
In summary, toxic substances may result in one or more of the following effects:
1 Permanent damage to the body: With a few chemicals, such serious ill-effects may occur. Vinyl chloride is a known human carcinogen and butadiene is suspected of having similar effects.
2 Narcotics: A patient suffering from exposure to a narcotic product can be oblivious to the dangers around him. Narcosis results in ill-effects to the nervous system. The sensations are blunted, clumsy body movements are noticeable and distorted reasoning occurs. Prolonged exposure to a narcotic may result in loss of consciousness.
3 Corrosion/Irritation of the skin, lungs, throat and eyes.
Threshold Limit Values (TLV)
Research into toxicity considers such factors as:—
• The length of exposure
• Whether contact is by inhalation, ingestion or through the skin
• The stress of the person, and
• The toxicity of the product
As a guide to permissible vapour concentrations in air, such as might occur in terminal operation, various government authorities publish systems of Threshold Limit Values (TLVs). These systems cover many of the toxic substances handled by the gas industry. The TLVs, as published, are usually quoted in ppm (parts per million of vapour-in-air by volume) but may be quoted in mg/m3 (milligrams of substance per cubic metre of air).
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TLVs-TWA (see definitions below) for the main liquefied gases are given in Table 9.1. These are provided for purposes of illustration and help to identify the relative toxicity of vapours. However, it must be appreciated that the application of a specific TLV to the workplace is a specialist matter. It is not just the safe level which must be known;
it is also the resultant effect on the body which must be understood.
The most widely quoted TLV system is that of the American Conference of Governmental Industrial Hygienists (ACGIH). TLV systems promulgated by advisory bodies in other countries are generally similar in structure. The TLVs in most systems are republished annually and updated in light of new knowledge. The latest revision of these values should be made known to operating personnel by their management.
The ACGIH system contains the following three categories of TLVs which describe the concentration in air to which it is believed personnel may be exposed, under certain specific circumstances, without adverse effects:
(1) TLV-TWA. This is known as the Time Weighted Average. It is the concentration of vapour-in-air which may be experienced for an eight-hour day or 40-hour week throughout a person's working life. It is the most commonly quoted TLV. It shows the smallest concentration (in comparison to (2) and (3) below) and is the value reproduced in Table 9.1.
(2) TLV-STEL. This is known as the Short Term Exposure Limit. It is the maximum concentration of vapour-in-air allowable for a period of up to 15 minutes provided there are no more than four exposures per day and at least one hour between each. It is always greater than (1) above but is not given for all vapours.
(3) TLV-C. This is what is known as the Ceiling concentration of the vapour-in-air which should never be exceeded. Only those substances which are predominantly fast-acting are given a TLV-C. Of the main liquefied gases only the more toxic products, such as ammonia and chlorine, have been ascribed such a figure.
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The IGC Code (Chapter 19) gives a list of the more hazardous products. This is indicated for some cargoes where a toxic alarm (as well as a flammable alarm) is required to be fitted on ships (see also Appendix 2).
As explained earlier in this section, TLVs should not be regarded as absolute dividing lines between safe and hazardous conditions. It is always good operating practice to keep all vapour concentrations to an absolute minimum so limiting personal exposure.
9.3.2 Asphyxia (suffocation)
For survival, the human body requires air having a normal content of about 21 per cent oxygen. However, a gas-free atmosphere with somewhat less oxygen can support life for a period without ill-effects being noticed. The susceptibility of persons to reduced oxygen levels vary but at levels below about 19 per cent, impaired mobility and mental confusion rapidly occur. This mental confusion is particularly dangerous as the victim may be unable to appreciate his predicament. Accordingly, self-assisted escape from a hazardous location may be impossible. At levels below 16 per cent, unconsciousness takes place rapidly and, if the victim is not removed quickly, permanent brain damage and death will result.
In general, such a problem is limited to enclosed spaces. Oxygen deficiency in an enclosed space can occur with any of the following conditions:—
• When large quantities ofcargo vapour are present
• When large quantities ofinert gas or nitrogen are present, and
• Whererusting of internal tank surfaces has taken place
For the above reasons, it is essential to prohibit entry to any space until an oxygen content of 21 per cent is established. This can be assured by using an oxygen analyser (see 9.7.2) and sampling the atmosphere from a number of points. These should be at different levels and widely dispersed within the space. As appropriate for the space being entered, tests for hydrocarbon gas and carbon monoxide may also be required (see 9.8).
With regard to Table 9.1, it will be seen from the footnote that some gases are known as asphyxiant gases. This is because they have limited toxic side effects but can be dangerous if present in sufficient quantities so as to exclude oxygen. Accordingly, a
casualty having been exposed to these products is likely to be suffering from suffocation. Immediate action is necessary in such cases as outlined in 9.3.3.
If tank entry is absolutely necessary and the above gas-free condition cannot be ensured, personnel entering the space must be protected by breathing apparatus and should follow the advice given in the Maritime Safety Card (see Figure 9.6).
9.3.3 Medical treatment
The symptoms and medical treatment for casualties of asphyxia or from the effects of toxic materials are summarised in this section.
Medical treatment for exposure to gas first involves the removal of the casualty to a safe area. Where necessary it may also involve artificial respiration, external cardiac massage and the administration of oxygen. Professional medical treatment should always be sought in cases where casualties have been overcome by gas.
Further advice on these issues is available from the data sheets in Reference 2.1 and in the Medical First Aid Guide (MFAG) published by IMO (see Reference 1.7). The latter publication has a number of Chemical Tables associated with it. These tables categorise the main liquefied gases into groups as shown in Table 9.3.
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