Characteristics and Hazards of Crude Oil


Petroleum, or crude oil, is a naturally occurring oily, bituminous liquid composed of various organic chemicals. 

It is found in large quantities below the surface of the earth and is used as a fuel and as a raw material in the chemical industry.


Petroleum is formed under the earth's surface by the decomposition of organic material.

Organic material arising out of:

  • The remains of tiny organisms that lived in the sea
  • To a lesser extent land organisms that were carried down to the sea by rivers
  • Plants that grow on the ocean bottoms


These combined with the fine sands and silts in calm sea basins went to form the vast reservoirs of crude oil. This process began many millions of years ago with the development of abundant life, and it continues to this day. The sediments grow thicker and sink into the sea floor under its own weight. 


As additional deposits pile up, the pressure on the ones below increases several thousand times, and the temperature rises by several hundred degrees.   The mud and sand harden into shale and sandstone. Carbonate precipitates and skeletal shells harden into limestone. 

The remains of the dead organisms are then transformed into crude oil and natural gas.

The range and complexity of naturally occurring petroleum is extremely large and the variation in composition from one reservoir to another shows quite a range.


Crude Oil


Northeastern Utah field (USA)


Upper Nile, Sudan (Africa)


Panhandle region, Texas (USA)



North Sea, UK


Southwestern Illinois, (USA)



Crude oil is graded by a specific viscosity range indicated as degrees API (American Petroleum Institute) gravity. 

The higher the API gravity, the lighter (low specific gravity) would be the crude.

For example, light crude oils have high API gravity's and low specific gravity.


All crude oils differ in the fractions of the various hydrocarbons they contain.


The specific molecules vary in shape and size from (Carbon) C1 to C80 or more. 


At the simplest, the one hydrocarbon compound has four hydrogen atoms bonded to the carbon atom to produce a compound CH4, or methane gas.


Crude oils with low carbon, high hydrogen and high API gravity are usually rich in paraffin's and tend to yield greater proportions of gasoline and light petroleum products; whereas those with high carbon, low hydrogen, and low API gravities are usually rich in aromatics.


Crude oils that contain appreciable quantities of hydrogen sulfide or other reactive sulfur compounds are called "sour." Those with less sulfur are called "sweet."


Some exceptions to this rule are West Texas crudes, which are always considered "sour" regardless of their H2S content, and Arabian high-sulfur crudes, which are not considered "sour" because their sulfur compounds are not highly reactive


Liquid hydrocarbons from natural wells may have nitrogen, oxygen, and sulfur in quantities from trace amounts to significant, as well as traces of metals.


Three broad classes of crude petroleum exist:

  • Paraffin types,

The paraffin types are composed of molecules in which the number of hydrogen atoms is always two more than twice the number of carbon atoms. (2n+2)

  • Asphaltic types,

The characteristic molecules in the asphaltic types are naphthenes, composed of twice as many hydrogen atoms as carbon atoms.

  • Mixed base types.

In the mixed-base group are both paraffin hydrocarbons and naphthenes.



The saturated open-chain hydrocarbons form a homologous series called the paraffin series or the alkane series. 

The composition of each of the members of the series corresponds to the formula CnH2n + 2, where n is the number of carbon atoms in the molecule. 

All the members of the series are un-reactive.  They do not react readily at ordinary temperatures with such reagents as acids, alkalis, or oxidizers.


The first four carbon molecules, C1 to C4, with the addition of hydrogen, form hydrocarbon gases:

  • Methane,
  • ethane,
  • propane,
  • Butane. 


Larger molecules C5 to C7 cover the range of light gasoline liquids;

  • C8 to C11 are napthas;
  • C12 to C19, kerosene and gas Oil;
  • C20 to C27 lubricating oils; and
  • Above C28 heavy fuels, waxes, asphalt’s, bitumen’s and materials as hard as stone at normal temperatures. 

Accompanying the gas compounds may be various amounts of nitrogen, carbon dioxide, hydrogen sulfide, and occasionally helium.


Alkene Series

The unsaturated open-chain hydrocarbons include the alkene or olefin series, the diene series, and the alkyne series. 

The alkene series is made up of chain hydrocarbons in which a double bond exists between two carbon atoms. 

The general formula for the series is CnH2n, where n is the number of carbon atoms. 

Similar to the paraffin series, the lower members are gases, intermediate compounds are liquids, and the higher members are solids. 

The alkene series compounds are more active chemically than the saturated compounds.  They react easily with substances such as halogens by adding atoms at the double bonds.

They are not found to any extent in natural products, but are produced in the destructive distillation of complex natural substances, such as coal, and are formed in large amounts in petroleum refining, particularly in the cracking process. 

The first member of the series is ethylene, C2H4.



Contain two double bonds between pairs of carbon atoms in the molecule.  They are related to the complex hydrocarbons in natural rubber and are important in the manufacture of synthetic rubber and plastics.  The most important members of this series are butadiene, C4H6 and isoprene, C5H8.


Alkyne Series

The members of the alkyne series contain a triple bond between two carbon atoms in the molecule.  They are very active chemically and are not found free in nature.

The first and most important member of the series is acetylene, C2H2


Cyclic Hydrocarbons

The simplest of the saturated cyclic hydrocarbons, or cycloalkanes, is cyclopropane, C3H6, the molecules of which are made up of three carbon atoms to each of which two hydrogen atoms are attached.  Cyclopropane is somewhat more reactive than the corresponding open-chain alkane, propane, C3H8.  

Other cycloalkanes make up a part of ordinary gasoline. The most important group of unsaturated cyclic hydrocarbons is the aromatics, which occur in coal tar.  The aromatics sometimes exhibit unsaturation, that is, the addition of other substances, their principal reactions bring about the replacement of hydrogen atoms by other kinds of atoms or groups of atoms.


The aromatic hydrocarbons include benzene, toluene, anthracene, and naphthalene.


Natural Gas occurs in mixtures of hydrocarbon gases and vapors, the more important of which are:

  • Methane,
  • ethane,
  • propane,
  • butane,
  • pentane and
  • hexane. 


Natural Gas is lighter than air, non-toxic and contains no poisonous ingredients. 


Breathing natural gas is harmful when there is not an adequate supply of oxygen in the atmosphere.



Methane (C1)

Methane, also referred to as marsh gas, is a gas composed of carbon and hydrogen with a chemical formula of CH4. It is lighter than air, colorless, odorless, tasteless and is flammable.  It occurs in natural gas and as a by-product of petroleum refining.  


Propane (C3)

Propane is a colorless, odorless gas of the alkane series of hydrocarbons, of formula C3H8.

It occurs in crude oil, natural gas, and as a by-product of refinery cracking gas during petroleum refining.  Propane does not react strongly at room temperature.  It does react, however, with chlorine at room temperature if the mixture is exposed to light. 


Butane (C4)

Butane, is the either of two saturated hydrocarbons, or alkanes, with the chemical formula of C4H10 of the paraffin series.  In both compounds the carbon atoms are joined in an open chain.  In n-butane (normal), the chain is continuous and un-branched whereas in i-butane (iso) one of the carbon atoms forms a side branch. 


LPG, Liquefied Petroleum Gas (C3 & C4)

Liquefied Petroleum Gas (LPG) is a mixture of the liquefied gases of propane (C3) and butane (C4).  

It is obtained from natural gas or petroleum.

It has a flammability range of 1.8% to 10% and the vapor has a density of 1.5 to 2.0 that of air. 

One volume of LPG liquid may form 2,300 to 13,500 time the volume of gas in air. 

LPG vapor is an anesthetic and asphyxiant in high concentrations. 


Gasoline (C5 to C11)

Commercial gasolines are a mixture of straight -run, cracked, reformed, and natural gasolines.

Gasoline is a mixture of the lighter liquid hydrocarbons that distills within the range of 38 to 204 ºC (100 to 400 ºF). 

The yield of gasoline from this process varies from, about 1 percent to about 50 percent, depending on the petroleum. 


Condensate (C4, C5, C6 & C - higher)

Condensate is normally considered the entrapped liquids in process or production gas streams due to temperature or pressure, in the typically in the range of C3, C4, C5 or heavier hydrocarbon liquids.  



Kerosene or sometimes referred to as Fuel Oil # 1- is a refined petroleum distillate. 

Kerosene’s usually have flash points within the range of 37.8 °C to 54.4 °C (100 °F to 130 °F). 

Therefore unless heated, kerosene will usually not produce ignitable mixtures over its surface.



Diesel or sometimes referred to Fuel Oil #2 is the fraction of petroleum that distills after kerosene; 

This is considered to be in the family of gas oils.


Lubricating Oils and Greases (C20 to C27)

Vacuum distillates or residual fraction of vacuum distillates are the main source of lubricating oils from the petroleum industry. 



Grease is a thick, oily, lubricating material that typically has a smooth, spongy or buttery feel. 

Lubricating greases are made by thickening lubricating oils with soaps, clays, silica gel or other thickening agents.  Greases range from soft semi-fluids to hard solids, the hardness increasing as the content of the thickening agent increases. 


Asphalt and Waxes (C28 & C- higher)

Asphalt is a bituminous substance that is found in natural deposits or as the residual of in petroleum or coal tar refining processes.  

It has a black or brownish-black color and pitchy luster. 

It is cement-like in nature varying in consistency at room temperature from solid to semisolid depending on the amount of light hydrocarbon fractions that have been removed. 



Wax is a soft impressionable semi-solid material having a dull luster and a somewhat soapy or greasy texture.  It softens gradually upon heating, going through a soft, malleable state before ultimately forming a liquid.  Paraffin wax is a mixture of saturated hydrocarbons of higher molecular mass, produced during the refining of petroleum. 


Gas and Fuel Oils (CI2 to C19)

Gas oil or fuels oil is a generic term applied to petroleum distillates boiling between kerosene and lubricating oils.  The name gas oil was originally derived from its initial use for making illuminating gas, but is now used as a burner fuel, diesel engine fuel, and catalytic cracker charge stock.  

Gas oils contain fuel oils such as kerosene, diesel fuels, gas turbine fuels, etc.


Non hydrocarbons that are found in crude oil:

  • Sulfur Compounds
  • Oxygen Compounds
  • Nitrogen Compounds
  • Trace Metals
  • Salts


Products obtained from the refining process:

  • Gasoline (MS)   -           Kerosene
  • Distillate Fuels   -           Diesel fuels
  • Residual Fuels   -           Coke and Asphalt
  • Solvents            -           Petrochemicals and
  • Lubricants


Hazard- Health

Hydrocarbon materials have several different characteristics that can be used to define their level of hazard.  Since no one feature can adequately define the level of risk for a particular substance they should be evaluated as a synergism.  It should also be realized that these characteristics have been tested under strict laboratory conditions and procedures that may alter when applied to industrial environments. 


The toxic hazards to which ships personnel are exposed during operations and carriage arise almost entirely from exposure to gases of various kinds.

Generally nearly all substances have been assigned:

  • Permissible Exposure Limits (PEL) and /or
  • Threshold Limit Values (TLVs).


The term Threshold Limit Value (TLV) is often expressed as a time weighted Average (TWA).


The use of the term Permissible Exposure Limit refers to the maximum exposure to a toxic substance that is allowed by an appropriate regulatory body.

The PEL is usually expressed as a Time Weighted Average, normally averaged over an eight-hour period


Short Term Exposure Limit (STEL), is normally expressed as a maximum airborne concentration averaged over a 15-minute period.


The values are expressed as parts per million (PPM) by volume of gas in air.


Toxicity can be greatly influenced by the presence of some minor components such as aromatic hydrocarbons (e.g. benzene) and hydrogen sulphide.

A TLV of 300PPM, corresponding to about 2%LEL, is established for gasoline vapours.


Concentration                         %LEL                         Effects

0.1% vol. (1,000 PPM)              10%                 Irritation of the eyes within one hour.

0.2% vol. (2,000 PPM)              20%                 Irritation of the eyes, nose and throat, dizziness and unsteadiness within half an hour.

0.7% vol. (7,000 PPM)              70%                 Symptoms as of drunkenness within 15 minutes.

1.0% vol. (10,000 PPM)            100%                Rapid onset of ‘drunkenness’, which may lead to unconsciousness and death if exposure continues.

2.0% vol. (20,000 PPM)            200%                Paralysis and death occur very rapidly.


Typical effects of exposure to petroleum gases

The smell of petroleum gas mixtures is very variable, and in some cases the gases may dull the sense of smell.

The impairment of smell is especially serious if the mixture contains hydrogen sulphide.

The absence of smell should therefore never be taken to indicate the absence of gas.


The TLV concentration is considerably below the lower flammable limit and combustible gas indicators cannot be expected to measure concentrations of this order accurately


The aromatic hydrocarbons include benzene, toluene and xylene. These substances are components in varying amounts, in many typical cargoes. The health hazard of aromatic hydrocarbons is not fully established but it is recommended that personnel engaged in cargo operations involving products containing them follow the precautions and procedures.


The Threshold Limit Value (TLV) or Permissible Exposure Limit (PEL), of an aromatic hydrocarbon vapour is generally less than that of other hydrocarbons


Repeated over exposure to high levels of Benzene vapour may have chronic effects, which can lead to disorders of the blood and bone marrow


Benzene primarily presents an inhalation hazard.

It has poor warning qualities, as its odour threshold is well above the Permissible Exposure Limit.

Exposure to concentrations in excess of 1,000 PPM can lead to unconsciousness and even death.


Benzene can also be absorbed through the skin and is toxic if ingested



The Permissible Exposure Limit (PEL) of hydrogen sulphide expressed as a Time Weighted

Average (TWA) is 10 PPM.


Concentration                                                 Effects

50-100 PPM                 Eye and respiratory tract irritation after exposure of one hour.

200-300 PPM                Marked eye and respiratory tract irritation after exposure of one hour.

500-700 PPM                Dizziness, headache, nausea etc. within 15 minutes, loss of consciousness and possible death after 30-60 minutes exposure.

700-900 PPM                Rapid unconsciousness, death occurring a few minutes later.

1,000-2,000 PPM           Instantaneous collapse and cessation of breathing.          


Note: Persons over exposed to H2S vapour should be removed to clean air as soon as possible. The adverse effects of H2S can be reversed and the probability of saving the person’s life improved if prompt action is taken.


For example a crude oil containing 70 PPM (by weight) hydrogen sulphide has been shown to produce a concentration of 7,000 PPM (by volume) in the gas stream leaving an ullage port above the cargo tank.

Thus, it is not possible to predict the likely vapour concentration from known liquid concentrations.


Prior to entry into a tank which has previously carried petroleum products containing hydrogen sulphide, the tank should initially be ventilated to a reading of less than 1% LFL on a combustible gas indicator and then checked using the appropriate instruments to ensure that there are no detectable traces of hydrogen sulphide.


Inert gas


Inert gas composition

Component                        IG from main boiler flue gas


Nitrogen (N2)                    83%

Carbon dioxide (CO2)        13%

Carbon monoxide (CO)      Present

Oxygen (O2)                     4%

Sulphur dioxide (SO2)        50 PPM

Oxides of Nitrogen (NOx) Present

Water Vapour (H2O)         Present

Ash and Soot (C)               Present

Dew point                         High if not dried

Density                             1.044


The main hazard associated with inert gas is its low oxygen content; it also contains trace amounts of various toxic gases which may increase the hazard to personnel exposed to it.  

Nitrogen dioxide is even more toxic with a TLV of 3 PPM.

Sulphur dioxide produces irritation of the eyes, nose and throat and may also cause breathing difficulties in sensitive people  

Carbon monoxide is an odourless gas with a TLV of 50 PPM.

It is insidious in its attack, which is to restrict oxygen uptake by the blood, causing a chemically induced form of asphyxiation.




As the amount of available oxygen decreases below the normal 21% by volume breathing tends to become faster and deeper. Symptoms indicating that an atmosphere is deficient in oxygen may give inadequate notice of danger.  Most persons would fail to recognise the danger until they were too weak to be able to escape without help.  

This is especially so when escape involves the exertion of climbing.

While individuals vary in susceptibility, all will impaired if the oxygen level falls to 16% by volume. Exposure to an atmosphere containing less than 10% oxygen content by volume inevitably causes unconsciousness. An atmosphere containing less than 5% oxygen by volume causes immediate unconsciousness with no warning other that a gasp for air.  

If resuscitation is delayed for more that a few minutes (about 4 minutes), irreversible damage is done to the brain, even if life is subsequently restored.


Hazards - Flammability


Burning/Igniting/Explosion - all of these are related to the property of a Hydrocarbon (Hc) gas to react with the oxygen in the air to produce carbon dioxide and water. This reaction between the two gives off enough heat to form a flame which travels through the above mixture.

When the gas above the liquid Hydrocarbon (Hc) ignites and burns, sufficient heat is generated to vaporize more liquid and the fire is thus fueled, thus it is actually the gas which burns and the effect is seemingly the liquid which is on fire.


This is one reason that, a flash back may occur if dousing a fire with water; water is a heat remover as such if sufficient quantities are not used, and the liquid Hydrocarbon (Hc) floating on the surface of the water may again get ignited, due to residual heat.



Open cup flashpoint.

A sample of the liquid is gradually heated in a special pot and a small flame is repeatedly and momentarily applied to the surface of the liquid.

The flashpoint is the lowest liquid temperature at which the small flame initiates a flash of flame across the surface of the liquid, thereby indicating the presence of a flammable gas/air mixture above the liquid.

For all oils, except some residual fuel oils, this gas/air mixture corresponds closely to the lower flammable limit mixture.


Closed cup flashpoint

The space above the liquid is kept closed except for brief moments when the initiating flame is introduced through a small port.


Because of the greater loss of gas to atmosphere in the open cup test the open cup flashpoint of a petroleum liquid is always a little higher (by about 6ºC) than its closed cup flashpoint.



Flashpoint of 60ºC or above as determined by the closed cup method of testing.

These liquids produce, when at any normal ambient temperature, equilibrium gas concentrations below the lower flammable limit.

They include distillate fuel oils, heavy gas oils and diesel oils.

Their (Reid Vapour Pressure) RVPs are below 0.007 bar and are not usually measured.



Flashpoint below 60ºC as determined by the closed cup method of testing

Some petroleum liquids in this category are capable of producing an equilibrium gas/air mixture within the flammable range when in some part of the normal ambient temperature range, while most of the rest give equilibrium gas/air mixtures above the upper flammable limit at all normal ambient temperatures.  


Examples of the former are jet fuels and kerosenes and of the latter gasolines and most crude oils. In practice, gasolines and crude oils are frequently handled before equilibrium conditions have been attained and gas/air mixtures in the flammable range may then be present.




If there is any doubt as to the characteristics of a cargo, or if a non-volatile cargo is being handled at a temperature above its flashpoint minus 10ºC, it should be treated as volatile petroleum.

(Example – If loading a grade of oil (flash point of 65ºC) then it will be treated as Volatile cargo – if the loading temperature is above 55ºC).


Owing to their particular characteristics, residual fuel oils should always be treated as volatile


Density of Hydrocarbon Gases:

Undiluted hydrocarbon gas is always heavier than air; it thus has a property to be dense in layers, with those in contact with the oil being denser than those at the boundary with air.

At the Lower Flammable limit, the gas has a density that is indistinguishable from air, that is, it is the nearly the same as air density.

A hydrocarbon and air mixture cannot burn unless the composition lies within the flammable range. This range is defined as that where the %volume of the gas in air is just sufficient to begin combustion to a concentration where the % volume exceeds a predetermined value where the mixture is incapable of burning.



A substance typically formed inside tanks by the corrosive interaction of sulfur compounds in the hydrocarbons and the iron and steel in the hull and structural. On exposure to air (oxygen) it ignites spontaneously, and in air/hydrocarbon mixture can cause an explosion. 


Some common petroleum materials and their flammable limits under normal conditions are listed below beginning with the widest ranges:

            Material            % Range          Difference

            Hydrogen          4.0 to 75.6         71.6

            Ethane              3.0 to 15.5         12.5

            Methane           5.0 to 15.0         10.0

            Propane            2.0 to 9.5          7.5

            Butane              1.5 to 8.5          7.0

            Pentane            1.4 to 8.0          6.6

            Hexane             1.7 to 7.4          5.7





Flammable limits, Propane, Butane, Pentane



Vapour Pressure

All crude oils and the usual petroleum products are essentially mixtures of a wide range of hydrocarbon compounds (i.e. chemical compounds of hydrogen and carbon).

The (Boiling point) BP of these compounds range from -162ºC (methane) to well in excess of +400ºC, and the volatility of any particular mixture of compounds depends primarily on the quantities of the more volatile constituents (i.e. those with a lower boiling point).


The volatility is characterised by the vapour pressure.

When a petroleum mixture is transferred to a gas free tank or container it commences to vaporise.

There is also a tendency for this gas to re-dissolve in the liquid, and equilibrium is ultimately reached with a certain amount of gas evenly distributed throughout the space.

The pressure exerted by this gas is called the equilibrium vapour pressure of the liquid, usually referred to simply as the vapour pressure.


The vapour pressure of a pure compound depends only upon its temperature.


The vapour pressure of a mixture depends on its temperature, constituents and the volume of the gas space in which vaporisation occurs.


The True Vapour Pressure (TVP) or bubble point vapour pressure is the equilibrium vapour pressure of a mixture when the gas/liquid ratio is effectively zero.

It is the highest vapour pressure which is possible at any specified temperature.


As the temperature of a petroleum mixture increases its TVP also increases.


If the TVP exceeds atmospheric pressure the liquid commences to boil.


The TVP of a petroleum mixture provides a good indication of its ability to give rise to gas.


Reid Vapour Pressure

A sample of the liquid is introduced into the test container at atmospheric pressure so that the volume of the liquid is one fifth of the total internal volume of the container.

The container is sealed and immersed in a water bath where it is heated to 37.8ºC.

After the container has been shaken to bring about equilibrium conditions rapidly, the rise in pressure due to vaporisation is read on an attached pressure gauge.

This pressure gauge reading gives a close approximation, in bars, to the vapour pressure of the liquid at 37.8ºC.

RVP is useful for comparing the volatilities of a wide range of petroleum liquids in a general way.  


It is, however, of little value in itself as a means of estimating the likely gas evolution in specific situations, mainly because the measurement is made at the standard temperature of 37.8ºC and at a fixed gas/liquid ratio. For this purpose TVP is much more useful; as already mentioned, in some cases correlations exist between TVP, RVP and temperature.