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Sunday
Mar072010

Vapor Pressure

     Having the vapor pressure of a contaminant at a Hazardous Materials response can do many things for your organization. The numeric value can tell you whether the product is a gas at atmospheric pressure, a mobile liquid such as an oil or fuel, or even if it is a volatile substance. The vapor pressure of a chemical is the “rate” at which off-gas vapors will evolve in the atmosphere. The higher the pressure {or numeric value} the faster these vapors will generate. Many solids have vapor pressures that allow vaporization without passing through a liquid state. This is known as the process of “sublimation”. Here, unsuspecting vapors can be emitted without the responder being forewarned by the usual liquid pool generating vapors from its surface. Many common volatile liquids that emit hazardous vapors exhibit this visual tendency. Seeing as how this is common, responders sometimes become complacent when no pooling of liquid is observed. 
     Vapor pressure is temperature dependent. Generally, as the temperature increases, the volume of the vapor pressure of the material increases. This exposes the responder to a higher volume of contaminant, and continues until one of three things occurs. Ignition, asphyxiation, or falling temperature is common. Either the responder is asphyxiated, or the vapors can ignite, and finally the least likely option, the temperature falls.
     Normal atmospheric pressure is 1 {one} atmosphere = 14.7 psi {pounds per square inch} = 760 mmHg {millimeters of Mercury}. By comparing this data to your chemicals value, you can determine its “state of matter”. If your value is above these limits, your material is a gas in normal air and either a liquifacted, pressurized, or both inside its vessel. If the value is below this number, the material is either liquid or a solid, contingent with other characteristics. This one variable can inform you on the likely characteristics, proper container, handling techniques, and shipping requirements.
     If you have the benefit of obtaining the products vapor pressure, you can determine the concentration inside a container without any detection devices. Visualize a container filled appropriately with the needed expansion area, inside the container known as “headspace”. Under normal, safe conditions, the product will exert pressure on the sides of the container in its liquid or gaseous state. If the product is “liquifacted”, the surface of the product will vaporize filling the headspace with vapors, exerting vapor pressure on the inside container walls above the liquid level. When this headspace becomes saturated with vapors above the condensation point for that liquid, they will condense back to liquid, falling into the solution. This principle is known as “Dynamic Equilibrium” and will continue until either the temperature rises or falls, the product is used, or becomes lost spillage from its original container. This physical rule dictates the law of vapor pressure.
     If the vapor pressure is known, you can calculate VP inside the headspace of any container. The formula is VP X 1300 = Headspace concentration in ppm. This is called the 1300 rule. Its accuracy is within + or - 1-2%. By having the contamination in ppm {parts per million} you can now look up the toxicity in your information resources to determine PPE, respiratory protection, and potential relief valve discharge. Your headspace VP will always be greater inside the vessel and lesser when released into the atmosphere. Remember, solids operate under these same rules of physics.
     A solid may have a VP of 10. If so, when you open that can, you will be exposed to 13,000 ppm. This may be above the level of exposure without proper respiratory protection. Consider Vapor Pressure {especially solids} before exposing your team to unknown hazards and always don the appropriate respiratory protection.
                                                                                      Haz Mat Mike


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