THINK DETAILS FOR EXPLOSIVITY

                       Is there a threat to users of the sea?

As a general rule, access to the casualty area should be limited and activities in the area restricted.  Operating diesel engines are known to have been ignition sources of several explosive/flammable gas clouds and strict control on diesel driven craft should be maintained.  Restrictions should be lifted when the area is declared safe.  The size of the risk area can be estimated by using dispersion models or calculations expressing the explosivity of the substance in TNT equivalents (see MIDSIS-TROCS database) and/or through monitoring of the air quality.  There are certain limitations when using either dispersion models or calculations since certain peculiar behaviour patterns may arise due to specific atmospheric conditions or terrain roughness which may produce results different to the expected output of a model or from calculations.  Thus, results have to be treated with caution.

In the absence of using models or calculations, the following table provides crude estimates on the extent of the explosive risk in the downwind direction for gases (vapour pressure >100 kPa) which are commonly transported.  In the case where the amount released falls between the quantities (in tonnes) shown in the table, a decision would need to be taken on which quantity best represents the quantity spilled.

Table:  Estimated extent of the explosive risk in the downwind direction for different quantities of gases spilled

The spread of an evaporated vapour from a liquid spill (fast evaporator) can be estimated by multiplying the values in the table by the ratio of VP/100 where VP is the liquid’s vapour pressure in kPa at ambient temperature.

e.g. for a 10 tonne benzene spill at 15-20^oC ambient temperature (VP of benzene = 8 kPa at 15^oC).

Downwind explosive risk = 8/100 x 1000 = 80 m.

If it is decided to monitor, then an appropriate gas detection system for explosive gases would be used.  Some marine pollution control vessels are equipped with sampling gas detection systems with data logging which perform the task of providing information as to whether and how severely the air surrounding the ship or the filtered air inside has been contaminated with explosive air-gas mixtures and whether this presents a hazard to the personnel on board or to the vessel itself, making it necessary to abort the operation.

For outdoor use, on board a damaged ship, a portable explosimeter should be used.  It is important to check in the manual of the explosimeter if it can be used for the gas or vapour present in the air.  It also needs regular calibration and this should be done according to the manual instructions.  Certain factors may give rise to erratic readings:

-             lower readings than actual due to the low heat of combustion of the gas or vapour, e.g. carbon disulphide;

-             a decrease in the values of the readings during measurements due to polymer formation of the chemicals which accumulates on the sensor (polymerising chemicals such as styrene, acrylonitrile).  This problem can be anticipated for certain liquid chemicals since these are carried with inhibitor additions;

-             invalid readings due to a concentration of oxygen of < 19.5%;

-             total failure of the explosimeter due to corrosion or loss of catalytic property of the sensor caused by the chemical, e.g. halogenated hydrocarbons, hydrogen sulphide.

When monitoring an explosive gas cloud release, the edge of the cloud should be approached from different directions up to a distance at which the explosimeter reads 10% of the LEL and/or initiates the pre-set alarm mechanism.  The risk area begins where the explosimeter reads as a minimum 10% of the LEL.  The extent of the explosive cloud depends on the substance, temperature, wind velocity and local wind eddies, so that the position of the cloud may change.

There is no widely accepted method for the selection of a safe distance for response personnel in the risk zone of an explosive atmosphere.  Consulting experts in the field of explosives would be the most reasonable option.  If the explosive substance has ignited and is burning, hazardous smoke is formed.  In fires involving substances containing only carbon, hydrogen and oxygen, a minimum distance of 100m from all visible parts of the smoke should be used as a guide.  Fires involving substances containing chlorine or other halogens or nitrogen, a minimum distance of 500m from all visible parts of the smoke should be used as a guide (see Diagram below).

 Diagram:  Risk zone around fires where the minimum distance (d) is 100m for smoke originating from fires involving substances containing C, H and O where the minimum (d) is 500m for smoke originating from fires involving substances containing F, Cl, Br, N