BOYLES LAW: When the temperature is kept constant, the volume of a gas is inversely proportional to the pressure upon it.
CHARLES LAW: When the external pressure is kept constant, the volume of a gas is directly proportional to its absolute temperature.
BASIC AND COMBINED GAS LAWS
Knowledge of the basic concepts of gaseous behavior is vital to understanding what action should be taken during an emergency involving a confined gas and why.
For our purposes, only those laws concerning the reactivity of gases to stimuli such as heat and volume are important. These laws are basically the result of experiments done separately by two scientists, Robert Boyle and Jacques Charles.
Boyle was concerned primarily with the relationship between pressure and volume in a gas. His experimentation proved that the pressure of a contained gas is directly proportional to the volume of space in the container. Therefore, by filling a one-cubic-foot container with gas at atmospheric pressure and then halving the volume to 1/2 cubic foot, we have also doubled the pressure. This principle is used when air is compressed to fill an SCBA bottle.
On the other hand, Jacques Charles experimented with heat’s effect on the pressure of a confined gas. His work proved that, put into the simplest of terms, as the heat of a confined gas rises, so will the pressure. Therefore, by applying sufficient heat to an adequately designed sealed container of gas, we will eventually raise the pressure enough to cause the relief valve to open and thus relieve the pressure on the container structure.
Combining both principles gives us a basis for understanding when dealing with a confined gas. We know a straightforward relationship exists between pressure, temperature, and volume when working with a gas. By confining a gas in a given volume and applying heat, for instance, either the pressure must increase or the volume must expand. When we discuss increasing heat of a gas confined in a container with elasticity, such as a balloon, we can safely discuss increasing volume to a point. When we’re talking about a container such as a storage container or cylinder, an increase in heat would cause an increase in pressure.
Understanding this principle should provide some insight into the hazards involved when the conditions of gas confinement are not normal or as designed.
COMMON PROPERTIES BASIC TO ALL GASES (The Science of Gases)
Since the beginning of man’s existence, he has had to learn to live and work with the matter that nature has placed on this earth. As time passed, he learned to alter this matter so that it was in a form that better suited his needs and improved his standard of living. Even though this ability to alter matter appears to be a human accomplishment and, in a sense, it was, man still had to adhere to the basic laws of nature.
As we progress through this course, you will better understand many of the theories, concepts, and principles we will discuss if you have some basic understanding of these laws.
We begin with a look at the two (2) basic properties of concern:
- the physical property and
- the chemical property of matter
The physical property is the size, shape, and form of a substance.
The chemical property is the ability a substance must react with other materials.
Since all matter does exhibit physical and chemical properties, and since fire is exhibiting both properties out of control, let’s discuss them one at a time.
The physical property of matter determines that matter can exist in three (3) states:
- solid
- liquid
- gaseous or vapor
Two (2) factors govern which state any given substance will be in. One is temperature, and the other is pressure.
Again, we see two (2) factors in Boyle’s and Charles’ experiments. The means of altering our physical state through temperature change is frequently experienced in our daily lives. Man has learned to use this process for his convenience. We lower the temperature of water below 32°F (0°C) to freeze or make the water into a solid state, which we commonly call ice. We use this, for example, to cool our drinks or as a playing field for sporting events. When this condition occurs naturally in our environment, it even hinders us in our job of fighting fires.
Water in liquid form exists at temperatures between 32°F (0°C) and 212°F (100°C). This state is necessary to sustain life, and we also use this liquid to extinguish fires.
When we raise the water temperature above 212°F (100°C) at atmospheric pressure conditions, it turns to the gaseous state, or steam as it is called. Man has also learned to trap this water vapor in a container, and raising its temperature creates a pressure higher than atmospheric pressure. This proved a significant discovery as he learned to power machinery to make his life easier.
It is not always to our advantage to alter the physical state by changing the temperature. In this case, we have an alternative means: a pressure change. The process of changing physical state by altering pressure centers around two (2) areas:
- altering the melting point or
- the boiling point of a substance
The melting point has little significance in our study of gases in this course; however, the boiling point does, so we will center our discussion only around the boiling point.
The normal boiling point of a liquid is the temperature of the liquid at which its vapor pressure equals the atmospheric pressure. We can raise the temperature at which a liquid boil by confining the vapor in a container and allowing the pressure to increase, or by increasing the pressure with a mechanical compressor. One point to remember.
AT ATMOSPHERIC PRESSURE (14.7 PSI) NO SUBSTANCE CAN EXIST AS A LIQUID IF THE TEMPERATURE OF THE SUBSTANCE IS ABOVE ITS BOILING POINT
Now that we have explored the physical states and conditions of those states, we can define a gas. By definition, a gas is one (1) of a substance’s physical states.
There is a precise and scientific definition of a gas. For our purposes and general uses, we can use this simplified definition:
A gas is a substance that exists only in the vapor or gaseous state at normal atmospheric pressure conditions approximately 50° to 70° F (10° to 21°C)
Remember, this is overly simplified, and there may be some exceptions; however, nearly all gases fall in this category, and as you can see, their boiling points would be lower than 50° to 70°F. This condition can be referred to as N.T.P. or Normal Temperature and Pressure conditions.
The next point we must make clear is that when we condense any substance from its gaseous state to its liquid state, its volume of space is reduced significantly. At atmospheric pressure, condensing steam to a liquid reduces its volume by approximately 1/1700th of its original volume.
If we consider the fact that when we store and transport any substance, the goal or objective is to get the most amount of material in the smallest container possible. Therefore, storing and transporting gases in the liquid rather than the gaseous state is much more convenient and economical.
Here, we can establish some ground rules. We must do one of two things to store and transport a gas as a liquid. We must either
- lower the temperature of the liquid to or below its boiling point at atmospheric pressure or
- raise the pressure to equal its respective vapor pressure at the liquid’s ambient temperature.
When we store and transport gases as a liquid, they fall into one of three (3) categories:
- Pressurized liquefied gases (liquid at ambient temperature and moderate to high pressure),
- Refrigerated liquefied gases (gases that need only be refrigerated to a moderate degree and with pressure from atmospheric to a few pounds),
- Cryogenic liquefied gases (gases that need only be refrigerated to a very low temperature).
Naturally, the last two would have to be stored and· transported in insulated containers to prevent the liquid from warming to ambient temperature. In contrast, the first one can be transported in uninsulated containers, allowing it to remain at ambient temperature. There are essential points to remember, as there will be two distinctly different methods of handling an incident based on those points.
Some examples of Pressurized Liquefied gases are LP gases, Chlorine, Ammonia, Vinyl Chloride, and others. Depending on the gas, the pressure will vary from about 3 or 4 PSI to about 120 PSI at 70°F.
An example of a Refrigerated Liquefied gas is Refrigerated Propane where the temperature would be -44°F at atmospheric pressure (14.7 PSI).
Some examples of Cryogenic Liquefied gases are Liquefied Natural gas (methane), Liquid Oxygen, Liquefied Argon, and Liquefied Hydrogen. The temperature of these gases will vary from about -450°F to -150°F at atmospheric pressure (14.7 PSI).
Not all gases are stored and transported in the liquid state. Many cannot be liquefied without being reduced in temperature, so they are sometimes bottled at normal temperatures and are only in the gaseous state. When we store them in this manner, we call them compressed gases. Some examples are Compressed air, Oxygen, Argon, CO2, etc.
When a substance reacts with itself to either decompose or polymerize, or when it reacts with another substance to satisfy its need to link up with something or to form a new substance, one or both of two things occur in relationship to gases.
One, large quantities of heat are produced, called combustion or fire, or two, there will be a physiological effect on living things (toxic effect or poisoning).
The heat effects of the Chemical reaction of propane with oxygen (fire) is one example, and the physiological effects of carbon monoxide gas are all too familiar to the Firefighters.
To recap the major points of this presentation :
1. Matter has two basic properties of concern to the Firefighter :
a. Physical
b. Chemical
2. The physical property has three states:
a. Solid
b. Liquid
c. Gaseous
3. The two factors that govern physical states are:
a. Temperature
b. Pressure
4. A gas is a substance whose physical state is the gaseous state at atmospheric pressure (14.7 PSI) and normal temperature 50 to 70°F. (This is called normal temperature and pressure N.T.P.)
5. Gases may be stored in their containers as a:
a. Pressurized liquid
b. Refrigerated liquid
c. Cryogenic liquid
d. Compressed gas
6. The chemical property of matter may produce:
a. Large quantities of heat
b. Toxic effect on living things
The better your understanding of these concepts, the more likely your actions will not result in disaster.
DEFINITIONS
A.P.I.: American Petroleum Institute
AS.M.E.: American Society of Mechanical Engineers
BLEVE: Boiling Liquid Expanding Vapor Explosion—a major container failure into two or more pieces at a moment in time when the contained liquid is at a temperature well above its boiling point at normal atmospheric pressure. A portion of the liquid is vaporized (often 1/3 – 1/2), and this large liquid-to-vapor expansion provides tremendous energy.
NOTE: This may occur even though the relief valve is operating!
BOILING POINT: The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure – the temperature at which a liquid becomes a vapor or a gas.
BULK PLANT: A property where flammable liquids or gases are delivered by tanker ship.
CRITICAL PRESSURE: The pressure required to liquefy a gas at its critical temperature.
CRITICAL TEMPERATURE: The temperature above which a gas cannot be liquefied by pressure alone – The temperature above_ which the material can exist only in a gaseous state.
D.O.T.: Department of Transportation
FIRE POINT: The temperature at which a liquid gives off enough vapor to continue to bum when ignited—usually a few degrees higher than the flash point.
FLASH POINT: The minimum temperature of a liquid at which it gives off vapor in sufficient quantity to form an ignitable mixture with air near the surface of the liquid or within the vessel used. The mixture will not support continuous combustion until or unless the fire point is reached.
FLAMMABLE OR EXPLOSIVE LIMITS: The lower limit (LEL) is the minimum concentration of gas or vapor in the air below which a substance does not burn when exposed to an ignition source. The upper limit (UEL) is the maximum concentration of the substance in the air above which ignition does not occur. The lower and upper limits are usually expressed in percent by volume of vapor in air.
IGNITION TEMPERATURE (Auto Ignition Temperature): The minimum temperature required to initiate or cause self-sustained combustion independently of the heating or heated element. The temperature at which the substance will ignite without any additional ignition source.
LIQUIFICATION : A process whereby a gas becomes a liquid when compressed OR both compressed and cooled.
LNG: Liquefied Natural Gas: A mixture of materials all composed of carbon and hydrogen. The principal component is methane (83%-99%) with lesser amounts of propane, ethane, and butane. LNG is nontoxic, but is an asphyxiant.
Approximate Properties:
Normal boiling point (NBP) ………………………….-260°F.
Density of liquid at NBP ………………………………3.5 lbs. per gallon
Density of vapor at NBP .( compared w/air at 70 °F) 1.47
Liquid to vapor expansion ……………………………..600 to 1
Flammable range …………………………….. ………5-15%
LNG is shipped as a cryogenic gas in insulated cargo trucks or marine vessels. It is stored in insulated ASME Code or API tanks;
LPG: Liquefied Petroleum Gas is a mixture of materials all comprised of carbon and hydrogen. It is applied to the segment of the gas family that is a vapor at atmospheric pressure and normal temperature but can be changed to a liquid under conditions of moderate pressure. Propane, Isobutane, normal butane, or mixtures of these three are referred to as Liquefied Petroleum Gases (also referred to as LPG or LP Gas).
Approximate Properties
Commercial Butane Commercial Propane
Vapor Pressure in PSIG at 70°F ………… 120 psi 17 psi
Vapor Pressure in PSIG at 100°F ……….. 205 psi 37 psi
Vapor Pressure in PSIG at 130°F ……….. 300 psi 69 psi
Specific Gravity of Liq. at 60°F……….. 509 psi 582 psi
Weight per Gallon liquid at 60°F ………. 4.24 lbs 4.81 lbs
Specific GravityofVaporat 60°F ………. 1.52 2.01
Boiling Point@ 14.7 psi -44°F 32°F
Flammable Limits
Lower 2.15% 1.55%
Upper 9.60% 8.60%
Liquefied Petroleum Gas (LPG), in domestic and recreational applications, is sometimes known as “bottled gas”, and is shipped as a liquefied gas in uninsulated DOT and CTC cylinders and ASME tanks and in DOT specification cargo trucks, railroad tank cars and marine vessels .
NTP: Normal temperature and pressure: 55-70°F at 14.7 PSI.
SPECIFIC GRAVITY: The ratio of the weight of a solid or liquid substance to the weight of an equal volume of water. The specific gravity of water is considered as one (1). A solid or liquid with a specific gravity less than one (1) will float on water. If more than one (1), it will sink.
Specific Gravity = Weight of substance
Weight of equal volume of water.
TRIPLE POINT: Point at which a substance can exist as a solid, gas, or liquid by manipulating temperature & pressure.
VAPOR DENSITY: Density of a gas or vapor compared to an equal volume of dry air. Air is rated as one (1). A figure greater than one (1) indicates the gas or vapor is heavier than air, a figure less than one indicates it is lighter than air (Vapor density figures do not always indicate behavior of vapor!)
Vapor Density = Molecular weight of a gas
29 (MW air)
VAPOR PRESSURE: In a closed container the motion of the molecules leaving the surface of the liquid is confined to the vapor space-above the surface of the liquid. As an increasing number re-enter the liquid, a point of equilibrium is eventually reached when the rate of escape of molecules from the liquid equals the rate of return to the liquid. The pressure exerted by the escaping vapor at the point of equilibrium is called vapor pressure .
VAPOR SPACE: That space above the liquid level in an enclosed tank. Whenever a tank is filled, a certain percentage of the capacity of the tank must be left unfilled to allow for expansion of the liquid contents. The percentage of capacity a tank may be filled is governed by size, location, and type of tank, as well as the type of product. The tank should not be allowed to become “liquid full” as only a slight temperature rise may cause the relief valve to open and discharge liquid! Liquid propane expands at the rate of 1.6% for each 10° F rise in temperature.
FLAMMABLE OR EXPLOSIVE RANGE: The numerical difference between the upper and lower flammable (or explosive) limits. Example: Acetylene has a LEL of 2.5%, Acetylene has a UEL of 81 %, and Acetylene has a range of 78.5%.
NOTE: When the mixture temperature is increased, the range widens.
GAS: Substance which exists in the gaseous state at so-called “NTP” (approximately 70° F and 14.7 PSI). A gas would be considered a substance or mixture of substances which, when in .the liquid- state, would exert a vapor pressure of 40 PSI or greater at 100°F as compared to a flammable liquid having a vapor pressure not exceeding 40 PSI at 100°F.
“Gas”: Gaseous material existing above the Critical Temperature
“Vapor”: Gaseous material existing below the Critical Temperature.
CLASSED BY CHEMICAL PROPERTIES:
- Flammable Gases: Any gas that will burn in the concentration of oxygen in the air.
- Nonflammable Gases: The many gases that will not burn in any concentration of air or oxygen – a number of these gases will support combustion (“oxidizers” such as oxygen), while some, such as nitrogen, argon, helium, etc., will not support combustion (“inert” gases).
- Reactive Gases: Gases which will either react with other materials or within themselves by a reaction other than burning and under reasonably anticipated initiating conditions of heat, shock, etc. Examples: Chlorine, fluorine, acetylene, vinyl chloride, etc.
- Toxic Gases: Gases that are poisonous or irritating when inhaled or contacted . Ammonia, carbon monoxide, hydrogen sulfide, etc. are examples.
CLASSED BY PHYSICAL PROPERTIES:
- Compressed Gases: Those which at normal atmospheric temperatures inside their containers, exist solely in the gaseous state under pressure. The pressure is basically dependent upon the pressure to which the container was originally charged and upon how much gas remains.
- Cryogenic Gases: Liquefied gases which exist in their containers at temperatures far below normal atmospheric temperatures, usually slightly above their boiling point at normal pressure, and correspondingly low to moderate pressures.
- Liquefied Gases: Those which at normal ·atmospheric temperature inside their container, exist partly in the liquid state and partly in the gaseous state, and under pressure as long as any liquid remains in the container.
CLASSED BY USAGE:
- Fuel Gases
- Industrial Gases
- Medical Gases
Source: The National Fire Academy (Document seem to be out of publication)