THINK DETAILS FOR AIR TOXICITY

      .             Is time available to respond?

            One method of response is to ventilate and/or to knock down the vapour or gas cloud using a water spray or fog.  The air entrained in the water spray dilutes the cloud, lowers the gas concentration and reduces the extent of the hazardous zone.  Water spray techniques are suitable for:

-          assisting the diversion of toxic gas clouds from areas occupied by people;

-          providing heat to a cold gas cloud (e.g. LNG) to enhance buoyant dispersion.

-          absorbing certain gases (e.g. ammonia) into solution in the water;

-          diluting a continuous gas leak to below its Lower Flammable Limit (LFL);

-          protecting ignition sources by controlling the direction of flow of the flammable gas clouds;

-          diverting flammable clouds from areas, such as confined spaces where an explosion may occur on ignition.

Diagram:  Knockdown

of a soluble gas

Certain factors should be taken into account when applying a water spray as a response method:

-           caution should be adopted in using water sprays for diluting large flammable gas releases or instantaneous releases which require a corresponding scale and arrangement of water sprays.  The spray may not dilute to below the LFL but only increase the volume of the pre-mixed cloud;

-           the performance of water sprays in diluting and controlling the direction of gas clouds reduces with increasing wind velocity;

-           water sprays are not suitable to act as an impenetrable barrier; they only dilute gas clouds passing through them by mixing with air.  They will also not contain a high velocity jet leak, if placed too close to the source of the leak;

-           water spray may sometimes not prevent ignition since the turbulence and mixing caused by the water spray may increase the flame speed on ignition, however the water spray will assist the protection of people, structures and equipment from radiation heat damage should ignition occur.

Another response method is to make a gas cloud visible.  This method, although not applicable to all gas releases, was used following the decision to intentionally destroy sunken chlorine cylinders.  The rising chlorine gas was seeded with ammonia gas released upwind to make the chlorine gas cloud visible for a longer period of time than if the gas was released as pure chlorine gas.  When a gas cloud is made visible, its exact position, dimensions and dispersion pattern can be seen.

Diagram:  Overview of how the chlorine cylinders were destroyed and the cloud made visible in the

Sinbad incident, 1979 (refer to TREE 4 Think Detail - Apply appropriate methods)