and it’s influence on the



Vapor density is a term that arises (or should arise) in every HAZWOPER course.  Vapor density is usually introduced along with a variety of other terms that help us to understand the way a chemical can be expected to act in the environment.  While vapor pressure and boiling point are useful in determining how quickly a chemical will produce a vapor or whether the chemical is in its gaseous state when released from containment, vapor density tells us what will happen to that gas or vapor upon release or when entering a container or room where the chemical has been stored.


Chemical vapors and gases can be divided into two groups based upon their vapor density:  chemical vapors and gases which are heavier than air, and chemical vapors and gases which are as light or lighter than air.  Using air as a reference point and assigning it a value of 1.0 vapor density, we can use a chemical's vapor density to judge the expected action of the chemical vapors or gases when we encounter them.  Chemicals that have a vapor density greater than one will be found in the bottom of storage containers and will tend to migrate downhill and accumulate in low lying areas.  Chemicals that have a vapor density which is the same or less than the vapor density of air will disperse readily into the surrounding environment.  Additionally, chemicals that have the same vapor density as air (1.0) tend to disperse uniformly into the surrounding air when contained and, when released into the open air, chemicals that are lighter than air will travel up and away from the ground.


One common chemical type that is encountered by both response personnel and service company personnel is petroleum fuel.  Response personnel tend to be better trained to recognize, evaluate and control chemicals based on their specific chemical properties.  Petroleum service workers are much more likely to lack a specific understanding of the influence of chemical properties on daily activities at a site and tend to rely heavily on experience and task specific training to guide site decisions.  With well more than a million underground storage tanks containing motor vehicle fuels and a huge number of above ground tanks storing petroleum products, this article will concentrate on the decisions affected by vapor density while making these storage vessels safe for hot work and entry.  Certainly, the following discussions apply to a host of chemicals sharing similar specific gravity, toxicity and/or flammability properties with petroleum products.


Liquid petroleum fuels include gasoline, diesel fuels, kerosene, jet fuels, aviation gasoline (avgas) and like products.   These products can be found at your corner gas station, airports, hospitals, power generation facilities, fuel distributors, and refineries and are stored and transported in tanks and pipelines.  Some chemical properties common to liquid petroleum fuels are flammability or combustibility (fire and explosion hazards), specific gravity less than 1.0 (float on water), presence of toxic components and vapor density greater than 1.0 (heavier than air).  The tanks and piping where these chemicals are stored may leak, require cleaning or require other maintenance activities to be performed on them.  Therefore, in order to make these tanks safe for removal, entry or performing hot work, it is important to understand how to reliably remove petroleum vapors and ensure that the atmosphere both inside the vessel and in the work area outside the vessel is safe.  Unfortunately, the answer to proper venting and testing of vapors is not well understood.   Before we discuss the proper ventilation and testing techniques, lets look at two examples that have been encountered by T.R. Consulting in the field.




The Illinois State Fire Marshal’s office regulates underground storage tanks containing petroleum fuels in Illinois.  When using the eductor method for evacuation of vapors from the tank, the state inspectors require the eductor to be turned off to take readings of the oxygen content and the percent of the lower explosive limit of the vapors within the tank utilizing an opening in the tube immediately beneath the eductor (venturi). 


For those not familiar with the eductor method of ventilation, the following explanation will be helpful.  An eductor is a venturi that is provided with a source of compressed air.  The shape of the venturi causes a vacuum to form in the tube extending up from the tank to the venturi when air from the compressor is forced through the venturi. Air enters the tank from another opening in the tank displacing vapors that are drawn through the tube extending into the tank to the venturi.  At the venturi, the vapors mix with the compressed air and the venturi discharges the vapor and air mixture into the air.  In the tube immediately beneath the venturi, there is a hole for insertion of monitoring instrument probes (prior to the point where the vapors mix with the compressed air).  When using the eductor method, four important rules must be followed.  #1 since petroleum vapors are significantly heavier than air, the tube beneath the venturi must extend near the bottom of the tank.  #2 because vapors and particulates that pass through the eductor system can generate static, the eductor system must be bonded to a grounded system.  #3 again, because petroleum vapors are heavier than air, the vapors must be discharged at some distance (regulations require discharge a minimum of 12 feet above grade and above any adjacent structure) above grade to prevent reaccumulation of vapors outside the storage vessel after discharge into the air.  #4 the readings taken with monitoring equipment must be taken with the eductor running.


If you are still having a hard time picturing this ventilation method, try thinking about taking a drink through a straw.  When you (the venturi) draw on the straw, the straw needs to extend to the bottom of the glass to extract all of the liquid from the glass.  The liquid is many times heavier than air and, therefore, accumulates at the bottom of the glass (illustrating point #1 above).  When you stop drawing and remove your mouth from the straw, the liquid drops out of the straw, again because the liquid is heavier than the air (illustrating point #4 above).  Petroleum vapors, though not as heavy as the liquid in your glass, are significantly heavier than air and act similarly to the liquid in your glass.  Thus, when the Illinois inspector requires the eductor to be turned off, the petroleum vapors settle out of the eductor tube and back into the tank.  The measured sample is in no way representative of the atmosphere within the tank.  One time, we tested the atmosphere at the monitoring access hole with the eductor off after only a brief period of tank ventilation and measured the LEL (lower explosive limit) at less than five percent (.05).  When the eductor was turned back on, another reading was taken and the actual percentage of the LEL registered seventy-five percent (.75).


What do we tell our clients who are affected by this problem?  Well, first we point out that arguing with a regulator is somewhat akin to wrestling with a pig in the mud.  Eventually, you’ll arrive at the conclusion that he likes it!  Arguing with a regulator is seldom productive.  We tell our clients to be sure that they get the proper readings with the eductor turned on before turning it off to please the inspector.  Then we tell them to check it again after turning on the eductor again. 




People who have graduated from institutions of higher learning (colleges and universities) are much more likely to be able to apply generic 40 hour HAZWOPER training to their particular situations than are field workers with only a high school diploma or less.  Many field workers fall into the latter category.  Unfortunately, the vast majority of 40 Hour HAZWOPER classes are generic and that leaves many without a true understanding of the hazards that confront them in the field.  Local community colleges and traveling seminars provide an open enrollment 40 Hour HAZWOPER program.  In a class of 20 students, few will have the same job functions, potential exposures, level of personal protective clothing or work within the same industry.  Thus, most of these programs are best suited to remediation or factory work. 


One new-hire working for a petroleum service company was on a site.  He had recently completed a 40 Hour HAZWOPER program offered by a company that sends instructors to various locations throughout the United States.  Upon completing the course, the first sites he had been sent to work at were in Illinois.  There, the inspectors from the Illinois State Fire Marshal’s Office had instructed him to turn the eductor off prior to taking readings with his combination combustible gas indicator and oxygen meter.  When asked whether he was familiar with the term “vapor density”, he replied, “They talked about it once near the beginning.  It’s something to do with how heavy the chemical is.”  When asked if the class had covered ventilation methods, he answered, “They said that you had to ventilate a confined space when someone was in it.”  When asked if the course had covered tanks, he replied, “Tanks were on the list of places that are confined spaces.  Other than that, no.”  When asked what he remembered about the course, he said, “They read us some regulations and talked about some chemicals that I had never heard of and couldn’t pronounce and then we were divided into groups and got dressed in moon suits.”  While the above are not exact quotes, they are as close as recollection will allow to the answers provided by this young man (who had not completed high school) when asked the above questions.  He further stated that the crew chief on the site had taught him everything he knew about tank work.  Unfortunately, with the help of the tank inspector in Illinois, some of that information was wrong.




Three methods of ventilation are commonly employed to purge tanks of vapors.  The information provided below is not intended as an instructional guide, but merely as a brief summary of the basic functions of each method.




The eductor method of ventilating a tank involves placing a venturi attached to a tube that extends into one opening of the tank.  Compressed air is supplied by a compressor to the venturi and the venturi creates a vacuum in the tube extending into the tank.  This creates a negative pressure within the tank that causes air to be drawn into the tank through another opening.  The fresh air displaces the vapors within the tank and the vapors are drawn into the venturi and discharged outside of the tank.  As with all ventilation methods, bonding of the ventilation device to a grounded system is a required safety measure.




The diffused air blower method introduces compressed air into the tank and forces vapors out another opening in the tank.  In this method of ventilation, a compressed air source is attached to a tube that extends into the tank.  The tube has holes along its length to ensure that air is jetted into the tank at different levels and angles in order to stir the vapors into the air so that they discharge through the other provided opening.  It is important to note that any contaminants discharged by the compressor, such as carbon monoxide, will be introduced to the tank during ventilation by the diffused air blower method.  Therefore, when using a compressor that is oil-driven (runs on diesel fuel or gasoline) to supply air to the diffuser, an oil and water separation filter must be used and a carbon monoxide monitor must sample the air being supplied to the diffuser as well.  Because this method introduces air to the atmosphere within the tank, it is necessary to turn off the diffuser and take monitoring instrument readings through an opening into the tank.  As with all ventilation methods, bonding of the ventilation device to a grounded system is a required safety measure.




Explosion proof or air-driven fans are commonly used to ventilate tanks (as well as sewers, manholes, etc.).  The fans can be used to blow air into the tank (sometimes through a canvas chute), to draw air out of the tank, or to both blow air into and draw air from the tank simultaneously.


Explosion proof fans are designed and tested (by a testing organization such as UL) to ensure that the electrical current that is used to energize the fan is incapable of causing ignition of any flammable vapors that might pass through the fan.  The air-driven fans use a source of compressed air to turn the fan blades.  As with all ventilation methods, bonding of the ventilation device to a grounded system is a required safety measure.




Above ground tanks which have stored chemicals with vapor density greater than 1.0, such as petroleum storage tanks, will often have access manways near grade level.  The vapors within the tank may accumulate to levels that exceed the height of the manway.  Removal of the manway access opening cover can cause these heavier than air vapors to escape through the opening.  This can result in many undesirable conditions including:  immediate and severe exposure of personnel to toxicity hazards, displacement of available oxygen (oxygen deficiency), and concentrations of flammable vapors around personnel and equipment.




This article is intended only as a discussion of the proper use of vapor density information.  It is not intended to supply all the information necessary to properly ventilate a space containing chemical vapors or gases nor as guidance regarding specific vapor density applications for any specific chemical except as used by a properly trained and experienced individual in conjunction with specific chemical data and within appropriate procedural guidelines.




Feel free to distribute this article to anyone who may be interested in the provided information or in services provided by the author, T.R. Consulting, Inc.


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