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Monday
Nov092009

Vapor Density and Specific Gravity

Vapor Density and Specific Gravity are integral concepts that can tell you where your hazardous spill or release will go. If you understand how these relate to your product you can accurately predict and anticipate needed changes to your scene before citizens or responders become exposed. This of course, minimizes injury and contamination. Both vapor density and specific gravity are based on an arbitrary scale surrounding both sides of the numeral 1. Vapor Density measures the weight of a gas or vapor in air, using 1 as the weight of normal outside air. Specific gravity measures the weight of a liquid or solid in water, using 1 as the weight of normal water.

Vapor Density using this scale gives us immediate information allowing us to formulate our future plume dispersion plans. If the vapor density is determined from chemical information resources to be less than 1 a number of constants can be calculated. For most vapors lighter than air, the tendency upon release, {In addition to rising in the sky} is to dissipate and become weaker very fast. This is beneficial for a number of reasons.

  1. Evacuation is non-existent or confined to the immediate area.
  2. Exposure to un-protected persons is usually minor.
  3. Day-to-Day operations are minimally disrupted.
  4. Mitigation equipment of the released vapor is natural dispersion.

All four of these response issues decrease the drain on your man-power and equipment. Generally the contamination area is small and can be dealt with by few responders. Since confinement is impossible, and containment depends on the vessel releasing the vapor plume, tactics should involve the use of an air monitoring device. Once the plume direction has been evaluated it is critical to determine the location of the PEL {Permissible Exposure Limit} on the downwind side affecting civilians. Beyond this position evacuation is not necessary and harm to the public will be limited to an unpleasant irritating odor. By placing one firefighter at this point to prevent civilians from entering any area above the PEL, you have isolated the area of contamination from the public. This is one reason why chemical facilities are usually found with large area sites. If a plume dispersion scenario occurs, their fenced in grounds may be large enough for this to occur. In this case, their fence eliminates the need to post a firefighter or employee at the PEL limit downwind.

If the Vapor Density of the release is greater than 1, other issues are presented. Entrapment in confined areas by vapors, asphyxiation, and unstable air currents, are just some of the concerns. As you can see by the drawings, heavier than air vapors will fall near the ground right where we humans breathe. If they fall into a sub-level area we may be asphyxiated. Generally, these heavier than air vapors with a vapor density greater than 1 tend to stay concentrated for a longer time. As a result, they are more hazardous remaining above their PEL, and from the physics of being denser than air. Since there are uneven air currents near the ground surface, these toxic vapors can travel quite far and still remain above the PEL. If they are flammable, they can re-surface inside a building through basement drains, possibly finding an ignition source, such as the water heater or furnace creating explosion possibilities. Because of these properties, a response to a scene involving a vapor release greater than 1 will require more man-power and equipment. If in an industrial area, streets will need to be roped off. If in a residential area, door-to-door warnings may have to be implemented. These actions will all require a larger commitment from your department.

Specific Gravity involves the same type of arbitrary scale of 1, except now we are evaluating the weight of a material when mixed in any form of water. Sewer drains, lakes, oceans, etc, if the information resources data reveal a number less than 1, the material will float on top of water, much like gasoline in a puddle does. If the data reveals a value above 1, it will sink as in the example of Carbon Disulfide. Since most contaminants spilled have a specific gravity less than 1, you will find much re-mediation equipment and data for these types of spills. Booms, Vactors, Absorbent Pads are among some. The general format for these spills is the area is larger but the contaminant is easier to pick-up and retain. It does however require vast equipment and man-power for open water, less than 1 specific gravity, spilled contaminant. Defining “easier” becomes the issue. For spills with a specific gravity above 1, which sink to the bottom of a waterway you will not find much remediation data or equipment. These types of spills are less frequent and very expensive to mitigate the bottom of a waterway. The three most common treatments for this type of spill are:

  1. Natural environment absorption, which sometimes works and sometimes ruins a waterway.
  2. Specialized dive teams with imbibed absorption capabilities.
  3. Overall reduction of these chemicals uses, by switching to a non-hazardous alternative.

This concept is quickly becoming the industry standard for many hazardous chemicals. Ultimately, while this is best for people and the environment it may also eliminate the need for us Haz-Mat guys.

Haz Mat Mike

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