Page 35 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

close to the release. As the temperature falls, the rate of release will reduce and in the case of some flammable
vapors, ice will form at the point of release.
2 Liquefied pressurized releases from pipelines may take considerable time to depressurize following an
emergency shutdown. When a vapor cloud is formed, particularly where the release is indoors, one option
would be to prevent ventilation. This is not generally recommended as it will increase the concentration
of flammable vapors and therefore increase likelihood of ignition. However, if the flammable vapors pose
additional health effects such as being toxic (such as ammonia and hydrogen sulphade), ventilation may
reduce the flammable hazard only to produce a much wider toxic hazard. In such instances, factors such as
the total quantities of vapors, location of the incident, wind speed and direction will all contribute to determine
the tactic that will take precedence.

Fig 23

Flammable vapours will only ignite when they encounter a source of ignition at concentrations within their
flammable range. The precise range is specific to each substance; consequently, flammable vapours with a wide
flammable range are more likely to create large ignitable vapour clouds than those with a narrow flammable
range. At operational incidents, a vapour cloud at concentrations above its upper explosive limit (UEL) should
still be regarded as a dangerous environment. A key control measure for flammable vapours is to reduce the
concentration in air below the lower explosive limit (LEL) and prevent ignition. Vapours in an uncontrolled state
will naturally spread and in doing so, dilute.

Ventilation: Good ventilation can assist in dispersing flammable vapours to minimize the size of any ignitable
plumes. This approach will need to be weighed against the generation of a larger plume and the potential to find
ignition sources. This depends on the LEL of the vapour.
Water sprays: Adding water in the form of fine spray or mist will create convection currents that will assist
in dispersing flammable vapours. Water mist will also act as a good absorber of heat if ignition occurs. Most
hydrocarbon fuel vapours have little or no solubility in water. Where vapours are water soluble, such as ammonia
and hydrogen sulphide, water sprays may be used to dissolve the vapour cloud out of the air.

Weather: Strong winds can disperse flammable vapours and gases, rain can dissolve soluble gases (atmospheric
scrubbing) and promote the mixing/dilution of any vapour plume.
Inerting gas: By replacing the air with a gas that does not support combustion, the risk of fire can be greatly
reduced or eliminated. This will not only reduce the concentration of flammable vapours but also reduce the
concentration of oxygen present. Portable monitoring equipment known as LEL meters or combustible gas
detectors can be used to detect the presence of flammable vapours.
Once a flammable vapour cloud has formed, the options available to reduce this hazard are limited, until vapours
have dissipated to a safe concentration.

Chemical protective clothing (CPC) is generally not suitable where heat, fire or flammable risks are present.
Personal protective equipment (PPE) for emergency teams may need to be sufficient to protect wearers in the
event of ignition of a vapour cloud that leads to a flash fire or uncontrolled vapour cloud explosion.
Occupational Hazards & Dangerous Chemicals
1 Classification of Air borne Contaminants
When chemicals are disseminated in air and contaminate it, they are called air-borne contaminants. They are
classified according to their physical state as under:

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CITS : Automotive - Mechanic Diesel - Lesson 01 - 04