248 U
Unified Command. The process of determining overall incident strategies and tactical objectives by having all agencies, organizations, or individ- uals who have jurisdictional responsibility, and in some cases those who have functional respon- sibility at the incident, participate in the deci- sion-making process.
Unified Commanders (UC). Command level representatives from each of the primary responding agencies who present their agency’s interests as a member of a unified command team. Depending upon the scenario and incident timeline, they may be the “lead” Incident Commander or play a supporting role within the command function. The unified commanders manage their own agency’s actions and make sure all efforts are coordinated through the uni- fied command process.
Underflow Dam. Spill control tactic used to trap floating lighter-than-water materials behind the dam (specific gravity <1). Using PVC piping or hard sleeves, the dam is constructed in a manner that allows uncontaminated water to flow unob- structed under the dam while keeping the con- taminant behind the dam. Operationally, this is most effective on slow moving and relatively narrow waterways.
V
Vacuuming. A physical method of confinement by which a hazardous material is placed in a chemically-compatible container by simply vac- uuming it up. The method of vacuuming will depend upon the hazmats involved. Vacuuming is also a physical method of decontamination.
Vapor. An air dispersion of molecules in a sub- stance that is normally a liquid or solid at stan- dard temperature and pressure.
Vapor Combustion Unit. Is an air pollution con- trol device that destroys the contaminant by oxi- dation and emits carbon dioxide and water. It is usually an intermediate piece of equipment located between an enclosed ground flare and an incinerator for disposing of liquid and gaseous waste.
Vapor Recovery Unit. An air pollution device that recovers the contaminant and returns it to its original and reusable state.
GLOSSARY
Vapor Density. The weight of a pure vapor or gas compared with the weight of an equal vol- ume of dry air at the same temperature and pres- sure. The molecular weight of air is 29. If the vapor density of a gas is less than one, the mate- rial is lighter than air and may rise. If the vapor density is greater than one, the material is heav- ier than air and will collect in low or enclosed areas. Significant property for evaluating expo- sures and where vapors and gases will travel.
Vapor Dispersion. A physical method of con- finement by which water spray or fans is used to disperse or move vapors away from certain areas or materials. It is particularly effective on water- soluble materials (e.g., anhydrous ammonia), although the subsequent runoff may involve environmental trade-offs.
Vapor Pressure. The pressure exerted by the vapor within the container against the sides of a container. This pressure is temperature depend- ent; as the temperature increases, so does the vapor pressure. Consider the following three points:
1) The vapor pressure of a substance at 100°F is always higher than the vapor pressure at 68°F.
2) Vapor pressures reported in millimeters of mercury (mm Hg) are usually very low pres- sures. 760 mm Hg is equivalent to 14.7 psi. or one atmosphere. Materials with vapor pres- sures greater than 760 mm Hg are usually found as gases.
3) The lower the boiling point of a liquid, the greater vapor pressure at a given temperature.
Vapor Suppression. A physical method of con- finement to reduce or eliminate the vapors ema- nating from a spilled or released material. Operationally, it is an offensive technique used to mitigate the evolution of flammable, corrosive, or toxic vapors and reduce the surface area exposed to the atmosphere. Common examples include the use of firefighting foams and chemi- cal vapor suppressants.
Venting. The controlled release of a liquid or compressed gas to reduce the pressure and diminish the probability of an explosion. The method of venting will depend upon the nature of the hazmat.
Viscosity. Measurement of the thickness of a liq- uid and its ability to flow. High viscosity liquids, such as heavy fuels oils (i.e., #6 fuel oil), must
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