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Glossary of Petroleum Industry Common Terms & Symbols


Abel tester: A closed-cup flash tester for kerosene and other oils

Absolute pressure: Total pressure equal to gauge pressure plus 14.7 lbs./sq. in at sea level

Acidity: The presence of acid-type constituents whose concentration is usually defined in terms of neutralization number. The constituents vary in nature and may or may not markedly influence the behavior of the oil. (see neutralization number)

Air-Fuel Ratio: The ratio of air weight to fuel weight consumed in an internal combustion engine or furnace.

Aliphatic: A class of saturated or unsaturated carbon compounds, in which the carbon atoms are joined in open chains.

Aniline Point: The aniline point of a petroleum product is the minimum equilibrium solution temperature with an equal volume of freshly distilled aniline.

API: American Petroleum Institute.

API Gravity: Gravity (weight per unit volume) of oils as measured by the API scale. This standard was adopted by the API 5/4/22 as the standard for the American petroleum industries

Aromatics: Group of hydrocarbons of which benzene is the parent. They are called "aromatics" because many of their derivatives have sweet or aromatic odors.

Ash: Inorganic residue remaining after ignition of combustible substances determined by definite prescribed methods.

Asphaltenes: Insoluble, semi-solid, or solid particles which are combustible and are highly aromatic. Asphaltenes contain a high carbon to hydrogen ratio and entrap water, fuel ashes and other impurities.

ASTM: American Society for Testing Materials. Grade and quality specifications for petroleum products are determined by ASTM test methods.

Atomization characteristics: The ability of an oil to be broken up into a fine spray by some mechanical means.


Barrel: A unit of volume measurement used for petroleum and its products. 1 barrel = 42 U.S. gallons or 35 British gallons

Bbl: Abbreviation for barrel.

Benzene: An aromatic hydrocarbon which is a colorless, volatile, flammable liquid. Benzene is obtained chiefly from coal tar and is used as a solvent for resins and fats in dye manufacture.

BHP: Brake horsepower

Blender: A device for mixing two fuel oils to achieve a less viscous and more uniform fuel.

Blending: Mixing of two compatible fuels having different properties in order to produce an intermediate fuel

BS & W: Bottom sediment and water.

BS & W Monitor: An instrument which detects entrained water content in petroleum products wherein the water changes the capacitive reactance as a function of the dielectric constant.

BTU: British Thermal Unit. The amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit.

Bunker Fuel Oil: Heavy, residual fuel oil used in ships..


Calorie: The amount of heat required to raise the temperature of 1 gram of water by 1 degree centigrade, at or near maximum density.

Calorific Value: Amount of heat produced by the complete combustion of a unit weight of fuel. Usually expressed in calories per gram or BTU's per pound, the latter being numerically 1.8 times the former.

Catalyst: A substance which promotes a chemical reaction, but does not itself enter into the reaction.

Catalytic Fines Hard, abrasive crystalline particles of alumina, silica, and/or alumina silica that can be carried over from the fluidic catalytic cracking process of residual fuel stocks. Particle size can range from sub-micron to greater than sixty (60) microns in size. These particles become more common in the higher viscosity marine bunker fuels.

Cat Cracker: A large refinery vessel for processing reduced crudes or other feed-stocks in the presence of a catalyst, as opposed to the older method of thermal cracking, which employs heat and pressure only. Catalytic cracking is generally preferred since it produces less gas and other highly volatile byproducts. It produces a motor fuel of higher octane than the thermal process.

Centigrade: Temperature based on 0 for the temperature at which water freezes and 100 for the temperature at which water boils.

Centipoise: 0.01 poise or centistokes times specific gravity at the test temperature.

Centistoke: 0.01 stoke (see stoke)

Centrifuge: A machine using centrifugal force produced by high-speed rotation for separating materials of different densities. Applied to Diesel engine fuels and lubricating oils to remove moisture and other extraneous materials.

Cetane Index: An empirical measure of ignition quality. Defined as the percentage by volume of cetane in a mixture of cetane and methyl naphthalene which has the same ignition quality when used in an engine as a fuel under test.

CCR: Conradson carbon residue

CFR Diesel fuel testing unit: A standard engine employed in making cetane number tests of Diesel engine fuels.C/H Ratio: Carbon/Hydrogen ratio

Clarifier: A machine used for a liquid-sludge separation in which the particles with a higher specific gravity are separated form the lower specific gravity of the liquid. A clarifier bowl has one outlet for the light phase oil; the heavier phase particles are retained on the bowl wall.

Cloud Point: Temperature at which wax begins to crystallize from a distillate fuel.

Corrosion: Detrimental change in the size or characteristics of material under conditions of exposure or use. It usually results from chemical action either regularly and slowly, as in rusting (oxidation), or rapidly, as in metal pickling.

Cracked: Refers to a petroleum product produced by a secondary refining process such as thermal cracking or vis-breaking processes which yield very low quality residue.

cSt: Centistokes @ 50 Centigrade

Cutter stock: Flux Stock. A petroleum stock which is used to reduce the viscosity of a heavier residual stock by dilution.


Demulsibility: The resistance of an oil to emulsification, or the ability of an oil to separate from any water with which it is mixed. The better the demulsibility rating, the more quickly the oil separates from water

Density: Density is the term meaning the mass of a unit of volume. Its numerical expression varies with the units selected.

Desalter: The desalter mixes the hydrocarbon stream with a small amount of fresh water (e.g. 10% by volume) forming a water-in-oil emulsion. The resulting emulsion is subjected to an electric field wherein the water is coalesced as an under flow from the upper flow of a relatively water-free, continuous hydrocarbon phase. The desalted hydrocarbon stream is produced at relatively low cost and has a very small residual salt content. The performance of this unit can be improved with a demulsifier, such as 

Detonation: A violent explosion involving high-velocity pressure waves; in a gasoline engine, the spontaneous combustion of part of the compresses charge after spark occurs. Detonation usually produces a characteristic metallic sound, or knock.

Diesel index: Product of the API gravity and the aniline point (in degrees Fahrenheit) of a Diesel fuel, divided by 100; an indication of the ignition quality of the fuel.

Distillation:The process of heating a liquid to its boiling point and condensing and collecting the vapors

Doctor test: A qualitative method of detecting undesirable sulfur compounds in petroleum distillates, that is, of determining whether oil is "sour" or "sweet".


Electrolytic process: A process that causes the decomposition of a chemical compound by the use of electricity.

Emulsion: A liquid mixture of two or more liquid substances not normally dissolved in one another, one liquid held in suspension in the other. Water-in-oil emulsions have water as the internal phase and oil as the external, while oil-in-water have oil as the internal phase and water as the external.

Engler viscosity: A viscosity obtained by dividing the out-flow time in seconds for 200 ml. of the material being tested, by the time in seconds for 200 ml. of water at 68F (20C) to flow out of an Engler viscosimeter.


Fahrenheit: Temperature scale based on 32F for the temperature at which water freezes and 212F for the temperature at which water boils (180 difference). Conversion to Farhenheit from Celsius (centigrade) temperature scale is by the following formula: F = 9/5C + 32, where C is the temperature in Celsius degrees.

Final Boiling Point FBP: The highest temperature indicated on the thermometer inserted in the flask during a standard laboratory distillation. This is generally the temperature at which no more vapor can be driven over into the condensing apparatus.

Fire Point: The lowest temperature at which an oil vaporizes rapidly enough to burn for at least 5 seconds after ignition, under standard conditions.

Flash point: The lowest temperature at which a liquid will generate sufficient vapor to flash (ignite) when exposed to a source of ignition.

Force Majeure: A standard clause which indemnifies either or both parties to a transaction whenever events reasonably beyond the control of either or both parties occur to prevent fulfillment of the terms of the contract.

Fraction: A separate identifiable part of crude oil; the product of a refining or

Fuel oil: The heavy distillates from the oil refining process; used as fuel for power stations, marine boilers.

Fungible: Interchangeable. Products which can be commingled for purposes of pipeline shipment.


Gasoil: Designation for No.2 heating oils and diesel fuels. A clean distillate fuel oil.


Heat of Combustion Gross: Total heat evolved during complete combustion of unit weight of a substance, usually expresses in BTU per pound.

Heat of Combustion Net: Gross heat of combustion minus the latent heat of condensation of any water produced.

Heavy crude: Crude oil with a high specific gravity and a low API gravity due to the presence of a high proportion of heavy hydrocarbon fractions and metallic content.

Homogenizer: A mechanical device which is used to create a stable, uniform dispersion of an insoluble phase (asphaltenes) within a liquid phase (fuel oil).

HP: Horsepower

HHV: Higher heating value

Hydrometer: An instrument for determining the gravity of a liquid.


IBP: Initial Boiling Point. In a standard laboratory distillation, the temperature on the distillation thermometer at the moment the first drop of distillate falls from the condenser.

Innage: Space occupied in a product container.

IP: British Institute of Petroleum.

Ionization: The process of adding electrons to, or removing electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, and nuclear radiation can cause ionization.


Kinematic Viscosity: The ratio of the absolute viscosity of a liquid to its specific gravity at the temperature at which the viscosity is measured. Expressed in Stokes or Centistokes. Example: Viscosity, kinematic, cS @ 100F.....5.2


Latent heat: Heat required to change the state of a unit weight of a substance from solid to liquid or from liquid to vapor without change of temperature.

Layering: This occurs in tanks when a high density fuel is mixed with a low density fuel.

LHV: Lower Heating Value

Lifting: Refers to tankers and barges taking on cargoes of oil or refined product at the terminal or transshipment point.

Light Crude: Crude oil with a low specific gravity and high API gravity due to the presence of a high proportion of light hydrocarbon fractions and low metallic compound.

Light Ends: The more volatile products of petroleum refining; eg. butane, propane, gasoline.

Liter: A measure of capacity in the metric system equal to 61,022 cubic inches, 0.908 US quarts dry and 1.0567 US quarts wet.

Long Ton: An avoirdupois weight measure equalling 2,240 pounds.


Marine Diesel Oil (MDO): Marine Diesel oil is a middle distillate fuel oil which can contain traces often percent (10%) or more residual fuel oil from transportation contamination and/or heavy fuel oil blending. The MDO does not require heated storage.

MCR: Maximum continuous rating

MDO: Marine Diesel Oil

Metric Ton: A weight measure equal to 1,000 kilograms, 2,204.62 pounds, and 0.9842 long tons.

Mg/L: Milligrams per liter = ppm (parts per million) - expresses a measure of the concentration by weight of a substance per unit volume.

Middle Distillate: Term applied to hydrocarbons in the so-called "middle range" of refinery distillation. Examples: heating oil, diesel fuels, and kerosene.

Micron: A unit of length. One millionth of a meter or one thousandth of a millimeter. One micron equals 0.00004 of an inch.

Molecule: The smallest division of a compound that still retains or exhibits all the properties of the substance.

Motor Gasoline: A complex mixture of relatively volatile hydrocarbons with or without small quantities of additives, that have been blended to form a fuel suitable for use in spark-ignition engines.

mm: Millimeter

MSDS: Material safety data sheet - a document that provides pertinent information and a profile of a particular hazardous substance or mixture. An MSDS is normally developed by the manufacturer or formulator of the hazardous substance or mixture. The MSDS is required to be made available to employees and operators whenever there is the likelihood of the hazardous substance or mixture being introduced into the workplace. Some manufacturers prepare MSDS for products that are NOT considered to be hazardous to show that the product or substance is NOT hazardous.


Naphtha: A volatile, colorless product of petroleum distillation. Used primarily as paint solvent, cleaning fluid, and blendstock in gasoline production, to produce motor gasoline by blending with straight-run gasoline.

Naphthenes: One of three basic hydrocarbon classifications found naturally in crude oil. Naphthenes are widely used as petrochemical feedstock. Examples are: cyclopentane; methyl-,ethyl, and propylcyclopentane.

Neutralization number: The number that expresses the weight in milligrams of an alkali needed to neutralize the acidic material in one gram of oil. The neutralization number of an oil is an indication of its acidity.

NH3N: Ammonia nitrogen.

NPDES permit: National Pollutant Discharge Elimination System permit is the regulatory agency document issued by either a federal or state agency which is designated to control all discharges of pollutants from point sources into U.S. waterways. NPDES permits regulate discharges into navigable waters from all point sources of pollution, including industries, municipal wastewater treatment plants, sanitary landfills, large agricultural feed lots and return irrigation flows.


Oil: Crude petroleum and other hydrocarbons produced at the wellhead in liquid form

Olefins: Class of unsaturated paraffin hydrocarbons recovered from petroleum. Typical examples include: butane, ethylene and propylene.

OSHA: The Williams-Steiger Occupational Safety and Health Act of 1970 (OSHA) is a law designed to protect the health and safety of industrial workers and treatment plant operators. It regulates the design, construction, operation and maintenance of industrial plants and wastewater treatment plants. The Act does not apply directly to municipalities, EXCEPT in those states that have approved plans and have asserted jurisdiction under Section 18 of the OSHA Act. Wastewater treatment plants have come under stricter regulation in all phases of activity as a result of OSHA standards. OSHA also refers to the federal and state agencies which administer the OSHA regulations.

Oxidation: Combining elemental compounds with oxygen to form a new compound. A part of the metabolic reaction.

Oxidizing agent: Any substance such as oxygen and chlorine , that can accept electrons. When oxygen or chlorine is added to wastewater, organic substances are oxidized. These oxidized organic substances are more stable and less likely to give off odors or to contain disease bacteria.

Outage: Space left in a product container to allow for expansion during the temperature changes it may undergo during shipment and application. Measurement of space that is NOT occupied in a drum.

Ozonation: The application of ozone to water, wastewater, or air, generally for the purposes of disinfection or odor control.


Particulate: Free suspended solids.

PAH: Polycyclic aromatic hydrocarbons. (occasionally polyaluminum hydroxide)

PCB: Polychlorinated biphenyls; polychloro-biphenyls. Difficult to remediate chemical used in old-style transformers. Concentrated PCBs used to be referred to as "1268".

Pensky-Martens: A closed-cup test for flash points of oil.

Peristaltic pump: A type of positive displacement pump.

Petrochemical: An intermediate chemical derived from petroleum, hydrocarbon liquids or natural gas, such as: ethylene, propylene, benzene, toluene and xylene.

Petroleum: A generic name for hydrocarbons, including crude oil, natural gas liquids, natural gas and their products.

pH: pH is an expression of the intensity of the basic or acidic condition of a liquid. Mathematically, pH is the logarithm (base 10) of the reciprocal of the hydrogen ion concentration. The pH may range from 0 to 14, where 0 is most acidic, 14 most basic, and 7 is neutral. Natural waters usually have a pH between 6.5 and 8.5.

Phenol: An organic compound that is an alcohol derivative of benzene.

PIB: Product Information Bulletin. General information on a product.

Pollution: The impairment (reduction) of water quality by agriculture, domestic or industrial wastes (including thermal and radioactive wastes) to such a degree as to hinder any beneficial use of the water or render it offensive to the senses of sight, taste, or smell or when sufficient amounts of waste creates or poses a potential threat to human health or the environment.

Polymer: A chemical formed by the union of many monomers (a molecule of low molecular weight). Polymers are used with other chemical coagulants to aid in binding small suspended particles to form larger chemical flocs for easier removal from water. All polyelectrolytes are polymers, but not all polymers are polyelectrolytes.

Polymerization: Process of combining two or more simple molecules of the same type, called monomers, to form a single molecule having the same elements in the same proportion as in the original molecules, but having increased molecular weight. The product of the combination is a polymer.

Pour Point: Lowest temperature at which an oil will pour or flow under certain prescribed conditions.

ppm: Parts per million - the unit commonly used to designate the concentration of a substance in a wastewater in terms of weight ie. one pound per million pounds, etc. PPM is synonymous with the more commonly used term mg/L (milligrams per liter).

Purifier: A machine used for a liquid-liquid separation in which the two intermixed liquids which are insoluble in each other have different specific gravities. Solids with specific gravities higher than those of the liquids can be separated off at the same time. A purifier bowl has two outlets; one for the light phase liquid and one for the heavy phase liquid.


Ramsbottom coke: A carbon residue test originated by Dr. J.R. Ramsbottom in England.

Reagent: A pure chemical substance that is used to make new products or is used in chemical tests to measure, detect, or examine other substances.

Recycle: The use of water or wastewater within (internally) a facility before it is discharged to a treatment system.

Reduced Crude Oil: Crude oil that has undergone at least one distillation process to separate some of the lighter hydrocarbons. Reducing crude lowers its API gravity, but increases the handling safety by raising the flash point.

Reducing agent: Any substance, such as the base metal (iron) or the sulfide ion that will readily donate (give up) electrons. The opposite of an oxidizing agent.

Redwood viscosity: The number of seconds required for 50 ml. of an oil to flow out of a standard Redwood viscosimeter at a definite temperature; British viscosity standard.

Refinery: A plan used to separate the various components present in crude oil and convert them into usable products or feedstock for other processes.

Residual Fuel Oil: Heavy fuel oils produced from the non-volatile residue from the fractional distillation process. Heavy oils that are "leftovers" from various refining processes. Heavy black oils used in marine boilers and in heating plants.


Saybolt Furol viscosity: A viscosity test similar in nature to the Saybolt Universal viscosity test but one more appropriate for testing high=viscosity oils. Certain transmission and gear oils, and heavy fuel oils are rated by this method. The results obtained are approximately 1/10th the viscosity which would be shown by the Saybolt Universal method.

SSF: Seconds Saybolt Furol

SSU: Seconds Saybolt Universal

Short ton: An avoirdupois measure of weight equal to 2,000 lbs.

Slagging: Formation of hard deposits on boiler tubes and/or piston crowns, usually due to the presence of sodium, vanadium and sulfur. 

Sludge: Deposits in fuel tanks and caused by the presence of wax, sand, scale, asphaltenes, tars, water, etc. The "sludge" formed in a #6 fuel oil storage tank is mostly composed of heavy hydrocarbons. This type of sludge by breaking the sludge into small particles and re-suspending them in the fuel for more efficient combustion. The "sludge" formed in diesel storage tanks is a combination of water with fungus and bacteria, which grow on the unevenly mixed water/fuel interface. Adding Treatment to stored fuel promotes a clean separation of water and fuel, reducing the substrate upon which bacteria and fungus can grow. Since the bacteria and fungus bind to the separated water, they can be removed by draining the water from the storage tank. If draining the storage tank is impossible,  will emulsify the water into tiny droplets and break the sludge into such small particles that they will no longer clog filters and will efficiently burn.

Soluble: Matter or compounds capable of dissolving into a solution.

Solvent: A substance, normally a liquid, which is capable of absorbing another liquid, gas, or solid to form a homogeneous mixture.

Specifications: Term referring to the properties of a given crude oil or petroleum product, which are "specified" since they often vary widely even within the same grade of product. In the normal process of negotiation, seller will guarantee buyer that product or crude to be sold will meet certain specified limits, and will agree to have such limits certified in writing.   Generally the major qualities of oil for which a buyer would demand a guarantee are: API gravity (or specific gravity, in some cases), sulfur percentage measured by weight, pour point measured by degrees C maximum, viscosity min./max., BS&W percentage by weight, etc.

Specific gravity: Weight of a particle, substance or chemical solution in relation to an equal volume of water at 15C. Abbreviated as Sp.Gr.

Specific heat: The quantity of heat required to raise the temperature of a unit weight of a substance by 1 degree; usually expresses as calories/gram/C or BTU/lb./F.

Spec. Sheet: Specification Sheet. Detailed information of a product including, tests, color, odor, specific gravity, bacterial strains, other major ingredients, etc.

SIT: Spontaneous Ignition Temperature. The temperature at which an oil ignites of its own accord in the presence of air or oxygen under standard conditions.

SR1:Seconds Redwood # 1 @ 100 F

Stabilize: To convert to a form that resists change. Organic material is stabilized by bacteria which convert the material to gases and other relatively inert substances. Stabilized organic material generally will not give off obnoxious odors.

Static mixer: A motionless mixer which has a series of fixed, geometric elements enclosed within a tubular housing. The internal elements impart flow division and radial mixing to the media flowing through the housing to produce a uniform dilution of the production.

Stoke: The unit of kinematic viscosity

Straight-Run: Refers to a petroleum product produced by the primary distillation of crude oil, free of cracked components.

Stratification: Occurs in blended fuels that have a compatibility problem. It is usually experienced when paraffinic based oils are mixed with asphaltic based oils, causing asphaltenes to precipitate and settle to the bottom of the tank.

STP: Standard Temperature (25C) and Pressure (300 mm Mercury).

Sulfur: An element that is present in crude oil and natural gas as an impurity in the form of its various compounds.

Surfactant: Surface-active agent. The active agent in detergents that possesses a high cleaning ability. Used in a spray solution to improve its sticking and wetting properties when applied to plants, algae, or petroleum.


Tag-Robinson Colorimeter: An instrument used to determine the color of oils. Also a scale of color values.

TBN: Total Base Number

Thermal Value: Calories per gram of BTU per pound produced by burning fuels.

Topped Crude Oil: Oil from which the light ends have been removed by a simple refining process. Also referred to as "reduced crude oil".

Total Existent Sediment: Combination of inorganic and hydrocarbon sediments existing in a fuel as delivered.

Toxic: A substance which is poisonous to a living organism.

Toxicity: The relative degree of being poisonous or toxic. A condition which may exist in wastes and will inhibit or destroy the growth or function of certain organisms.


Ubbehohde viscosimeter: A suspended level apparatus for accurately determining the viscosity of a liquid.

Ullage: The amount which a tank or vessel lacks of being full.


Vanadium Inhibitor: An organic and/or inorganic metal bearing chemical intended to chemically and/or physically combine with the compounds formed during combustion of heavy fuel oil to improve the surface properties of the treated ash compounds.

Viscosimeter: A device for determining the viscosity of oil. There are several methods or devices in general use. Basically, a fixed quantity of oil is allowed to pass through a fixed orifice at a specified temperature over a measured time span and then compared to a standard liquid such as a calibration oil or water.

Viscosity: Measure of the internal friction or resistance of an oil to flow. As the temperature of an oil is increased, its viscosity decreases and it is therefore able to flow more readily. Viscosity is measured on several different scales, including Redwood No. 1 at 100F, Engler Degrees, Saybolt Seconds, etc. The most common method for designation of viscosity is kinematic viscosity, measured in centistokes, cst @ 50Centigrade. (See Saybolt Furol, Saybolt Universal, Engler, Redwood, Kinematic)

Vis-Breaking: A light thermal cracking process carried out on a fuel oil during the refining process to reduce product viscosity without blending.

Volatile: A volatile substance is one that is capable of being evaporated or changed to a vapor at a relatively low temperature. Volatile substances also can be partially removed by air stripping.

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