THINK DETAILS FOR AIR TOXICITY

Gases

Examples:     Ammonia, vinyl chloride, chlorine, methane, propane, butane, LPG

General actions should be taken according to general first response found in  INFORMATION COMPONENT . In order to perform these and other actions it is necessary to have knowledge about how various gases are stored and how they behave, as well as how they escape from the container and how they spread out over large areas.

Water sprays may be used on some occasions to:
 

 

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Wash down (knock down) water soluble gas clouds
(cf.  Method C1)
 

 

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Reduce the risk of fire and explosion in flammable clouds of gases, by cooling down hot surfaces or putting out sparks and suppressing flame formation
 

 

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Stop, steer or disperse gas clouds which are sparingly soluble
or insoluble in water
 

At discharges of liquefied ammonia into water, part of the ammonia boils away very rapidly and moves in the wind direction as a dense, hazardous, white cloud. Approximately 60 % dissolves in the water when released above the surface and more than 90 % when released below. The dissolved ammonia forms alkaline ammonium hydroxide solution which is dangerous for the environment. Neutralizing agents should therefore be utilised to reduce environmental damage from ammonia discharges in confined, vulnerable water areas with low water turnover .
 
Dissolved ammonia forms a chemical equilibrium with water where most of the ammonia exists as ammonium ions NH4+  and hydroxide ions OH- (Figure 5 - 1). 

Figure 5 - 1   Dissolved ammonia in water


However, a minor fraction of  ammonia is un-ionized (neutral) NH3  and it is mainly this part that is responsible for the toxic effects on the aquatic organisms.

The fraction of un-ionized NH3  increases with higher water temperature and higher pH, from e.g. 0.2 % at 10oC and pH 7.0 to 15% at 25oC and pH 8.5. Some fish die after short exposure, at the latter temperature and pH, in a solution of 1g ammonia in 1 tonne of water.

 

Combating water soluble gas clouds
(Method C1)

 

Method

Water spray (“water mist”)

 

Application

Water soluble gases like ammonia and sulphur dioxide (Group GD)
.

 
Description

Small confined gas clouds of water soluble gases can be ”knocked down” (washed down), at low wind speed, by fine, scattered water sprays (water mist) cf. Figure 5 - 2

Figure 5 - 2   Small water-soluble gas clouds can be ”knocked” or washed down by water spray
 

Limitations

Applicable to small/limited gas clouds only.
Difficult to manoeuvre response vessels in narrow waterways.


 

Combating non-water-soluble gas clouds
( Method C1)

Method

Water spray (“water mist”)

Application

Non-water-soluble gases like methane, propane, butane, butadiene, ethylene, propylene, LPG mixtures, vinyl chloride (Group G).
 

Description

 

Small confined gas clouds of non-water-soluble gases can be stopped, steered, pushed and dispersed at low wind speed, by fine, scattered water sprays (water mist).

The risk of fire and explosion can be reduced by cooling down hot surfaces or putting out sparks and suppressing flame formation.
 

Limitations

Applicable to small/limited gas clouds only.
Difficult to manoeuvre response vessels in narrow waterways.


 

Fighting gas clouds by recondensing into the liquid state
( Method C1)
 

Method
Recondensing leaking gases by covering the container with a flat tarpaulin or collecting the jet stream with a tarpaulin made as a funnel or a cone.
 

Application
Mainly ammonia, sulphur dioxide and chlorine.
 

Description   (Picture source: Swedish Rescue Services Agency)
Releases of ammonia, sulphur dioxide and chlorine form aerosol clouds which are extremely dangerous for both health and environment. The method of recondensing is based on the fact that the aerosol droplets in the clouds, under certain conditions, can be merged into the liquid state.

Released gases from tank containers and similar reservoirs can be recondensed by covering with a tarpaulin according to Figure 5 - 3.

A release giving rise to a jet stream within easy reach, can be recondensed by a funnel or a cone (cf. Figure 5 - 4) with appropriate anchoring close to the point of release.

The jet stream is lead into the cone after which the recondensed liquid is collected in a pool (Figure 5 - 4, Figure 5 - 5 and Figure 5 - 6).
 
Clickable pictures


Figure 5 - 3
 Recondensing by means
of a tarpaulin


Figure 5 - 4
 A cone for recondensing
a gas release
 

 Clickable pictures


Figure 5 - 5
 The cone is anchored at the point of release
 


Figure 5 - 6
 The recondensed liquid is collected in a pool
 
Limitations
 Applicable to small/limited gas releases only.


 

Supervising a momentary release
of a hazardous gas cloud
( Method M1)
 

Method
Supervising a released gas cloud by ships, helicopters and aircraft and seeding the cloud by ammonia to make it better visible.
 

Application
Large gas clouds at a safe distance from populated areas.
 

Description
Figure 5 - 7 shows an example of using explosives after an accident off the coast of the Netherlands. (refer to think detail "Is time available to respond" and (Ref. 44).

Sunken steel cylinders containing chlorine were broken by explosives and the gas escaped through the water column into the air.

The rising chlorine gas cloud was strictly supervised by fully protected personnel. An exclusion zone was established  (Figure 5 - 8) and patrolled by ships, helicopters and aircraft.

Ammonia gas was released upwind as an indicator gas that seeded the chlorine gas cloud and made it clearly visible as a white smoke over a large area.

 
Figure 5 - 7


Figure 5 - 8
 

Limitations
 Very careful planning is necessary with safe distance to populated areas and strict warnings to seafarers and population.