1. INTRODUCTION :
During the past decades, Crude oil (Petroleum) has been an essential factor for prosperity in human life. Since it is in short supply and expensive, the same is very much in the lime-light all throughout the world.
2. HISTORY :
Petroleum was formed million years age from organic matter of marine deposits. This organic matter was being continuously subjected to conditions of extreme pressure and temperatures. This caused its gradual decomposition into petroleum. The resulting petroleum or crude oil or rock oil accumulated in places favoured by geological conditions and was pressurised through ages. Now and then, however, petroleum and the related product – natural gas managed to penerate to the earth’s surface where oil seepages, tar pits and natural gas ‘wells’ led to their discovery.
Due to the complicated nature of processes leading to its formation, petroleum consists of multitude of substances, exclusively of carbon and hydrogen popularly known as ‘Hydrocarbons’.
3. COMPOSITION OF PETROLEUM :
Petroleum usually consists of mixture of thousands of different hydro-carbons with a widely varied number of carbons atoms. The mixture of hydro-carbons can be classified as Paraffins (i.e. straight chain compounds like Methane, Propane, Butane, etc.), Napthenes, (i.e. cyclic compounds like cyclohexane, cyclopentane, etc.) and aromatics (i.e. ring compounds like benzene, toluene, Xylene, etc).
4. PROCESSING OF CRUDE OIL (OR PETROLEUM OR ROCK OIL) :
As stated above, the various components of petroleum possess varying numbers of carbon atoms and hence different boiling points. Crude oil is processed in the Refineries by distillation, which separates the different hydrocarbons by a method based on their boiling points.
By variation of the distillation pressures, the boiling point of the products can be altered. Usually, several distillation columns are arranged in series, operating at different pressures. In these columns, several million tones of Crude oil are separated into ‘fractions’ or ‘cuts’ (i.e. products having similar boiling points).
The principal products obtained by such distillation (conventionally called as ‘refining’) are as under:
Refinery Gas – Gas dissolved in Crude Oil.
Gas (L.P.G.) – Mainly propane and butane.
Naphtha – Mixture of hydrocarbon liquids.
Gasoline – Known as ‘Petrol’ used in automobile industry.
Jet fuels – For lamps, stoves and jet planes.
Diesel oil – For diesel engines in heavy vehicles.
Furnace oil – For use as industrial fuel in boilers, etc.
Bitumen – Solid residue also known as asphalt/tar, used in road
construction and water proofing.
5. PETRO-CHAMICALS :
These are basic chemical intermediates manufactured by using refinery gas natural gas or hydrocarbon fractions derived from petroleum as their primary raw material.
The examples of petro-chemicals can be best cited from the chart on the adjacent/page, which also explains the basic refinery operation.
6. CHARACTERISTICS OF HYDRO-CARBONS LEADING TO FIRE & EXPLOSION :
In order to understand the hazards involved in hydrocarbon processing industry, first the terminology used to clarify the characteristics of hydrocarbons leading to fire or exposion must be examined. These then can be applied to typical hydrocarbon products processed, handled typical hydrocarbon products processed, handled or stored in the hydrocarbon processing industry or stored in the hydrocarbon processing industry.
6.1 FLASH POINT :
It is the lowest temperature at which a flammable liquid would give off enough vapours to form a flammable mixture with air when an ignition source is brought near the liquid surface.
Obviously, lower the flash point, higher is the hazard possessed by a particular liquid. Many hydrocarbon liquids such as crude oil, naphtha, etc. have flash points at or below the ordinary room temperatures and normally are covered by a layer of flammable vapours that will ignite immediately if a source of ignition is brought nearby.
It must be noted here that flammable liquid does not burn on its own, but the vapours from the liquid burn. Since, vapourisation increases with rise in temperature, a flammable liquid at elevated temperature is more hazardous than the same liquid at normal temperatures.
6.2. FLAMMABLE LIQUID
It is any liquid with a flash point upto 93*c. The Indian Petroleum Rules (as per Gazette
of India, July 26, 1976) divide flammable liquids into three classes :
Class A : Having flash point below or upto 23*C.
Class B : Having flash point above 23*C and upto 65*C.
Class C : Having flash point above 65*C and upto 93*C.
6.3. FLAMMABLE VAPOURS :
There are hydrocarbons that have come from the surface of liquid because of their low flash point or that have boiled off when liquid temperature rises above its boiling point. Vapours from flammable liquid are heavier than air expressed in terms of vapour Density.
6.4. VAPOUR DENSITY :
It is a measure of relative density or weight (compared with air expressed as unity) of a vapour or gas with no air present. Obviously, gas or vapour with a density less than one is lighter than air and will rise when released.
Ex : Ammonia (0.59), Hydrogen (0.069), Methane (0.6).
Vapours or gasses with density greater than one, will go down when released.
Ex : Propane (1.56) , Butane (2.04) Benzene (2.8)
6.5 AUTO-IGNITION TEMPERATURE :
It is the lowest temperature required to initiate or cause self-ignition or self-sustained combustion in the absence of a spark or a flame. Obviously, substances having higher auto-ignition temperature will be comparatively less hazardous.
6.6 SPECIFIC GRAVITY :
It is the ratio of the weight of a given volume of liquid to the weight of an equal volume of water.
Most flammable liquids will float on water where they can burn freely and flow over a large area. Others are heavier than water and water will extinguish the fire.
6.7 MATERIAL FACTOR :
Material Factor of a substance is a measure of its “Energy Potential”. The National Fire Protection Association (N.F.P.A.) follows a numerical system for identification of fire hazards of various materials based on their material factor.
This “Material Factor” is a function of “flammability” and “reactivity” of a given substance.
The “Flammability” depends upon the flash point or heat of combustion as its degree of measure, while the “Reactivity” interprets the instability of water reactivity of a particular substance.
The abbreviation definition of the various degree of hazards (“o” to “4” in each category) as per NFPA 704 M-1969 are as under :-
6.7.1 FLAMMABILITY : (Nf – MEASURE OF FIRE) :
4 – Very inflammable gases or liquids.
3 – Those which can be ignited at normal temperatures.
2 – Ignite, if moderately heated.
1 – Ignite, after considerable preheating.
0 – Those which will not burn.
6.7.2 REACTIVITY (Nr – MEASURE OF STABILITY) :
4 – Readily detonate or explode.
3 – Can detonate or explode, but require strong initiating force or confinement.
2 – Normally unstable, but will not detonate.
1 – Normally stable, but unstable at high temperature and pressure and reacts with
0 – Normally stable, but will not react with water.
6.7.3 By knowing the “flammability” and “Reactivity” of a particular material, its “Material
Factor” can be arrived at by using the following chart :
N R 0 1 2 3 4
0 14 24 29 40
4 14 24 29 40
10 14 24 29 40
16 16 24 29 40
21 21 24 29 40
For example, Ethylene has flammability factor 4 and Reactivity factor 2. Hence, its material Factor as per chart above will be 24.
Obviously, higher the Material Factor, higher is the fire and explosion hazard potential of a particular substance.
Few Material Factors of typical products in hydrocarbon processing industry are as under :
Ammonia – 4 (Nf-1, NR-0)
Benzene – 16 (Nf-3, NR-0)
Butane – 21 (Nf-4, NR-0)
Butadiene – 24 (Nf-4, NR-2)
Acetylene – 29 (Nf-4, NR-3)
Nitro-toluene – 40 (Nf-1, NR-4)
6.8. FLAMMABILITY LIMIT (EXPLOSIVE RANGE) :
The explosive range includes all concentrations of a mixture of flammable vapour gas in air (usually expressed in percentage by volume) in which a flash will occur or a flame will travel if the mixture is ignited. The lowest percentage at which it occurs is the lower explosive limit and the higher percentage is the upper explosive limit.
If such a mixture is confined and ignited, an explosion results. Many common flammable liquids and gases have very wide explosive ranges. That of ethylene oxide is 3% to 100% and that of hydrogen is 4% to 74% – which shows that only very lean or usually strong mixtures of these materials in air are free from an explosion hazard.