INRODUCTION :
The hydrocarbon processing industry is generally of gigantic size as compared to the normal chemical industry. The industry is susceptible to the risk of catastrophic financial losses because of :
(a) Its enormous concentration of capital investment;
(b) The magnitude of its earnings;
(c) The inherent fire and explosion hazards of hydrocarbon processes and products.
Today’s trend towards “jumboizing” the plant and storage capacities of hydrocarbon processing industry has further enhanced the risk potential of the industry. This puts an added responsibility on the shoulders of the Underwriter.
The hydrocarbon processing industry has laid its roots in India in the last two decades. Recent successes in oil and natural gas explorations have further accelerated the growth of this industry. This has made the Underwriter’s job a complex one.
For sound underwriting practice, the fire insurer must :
(a) Keep abreast of fast changing technical developments.
(b) Have highly trained and specialised Engineers for risk assessment, loss prevention and risk identification.
(c) Have thorough knowledge of risk management techniques.
From the fire and explosion hazard point of view, hydrocarbon processing complex can be divided broadly into five identifiable areas as under :
(a) Process Plants/Production Units.
(b) Bulk Tankages/Tank Farms.
(c) Utilities/Auxillaries, Miscellaneous Buildings.
(d) Material Handling facilities like loading/unloading areas and Warehouses.
(e) Flares and Waste disposals.
Before considering the hazards associated with the above identifiable areas, due consideration must also be given to the layout and site preparations for a hydrocarbon processing complex.
2. SITE PREPARATION AND LAYOUT :
The most important single requirement in choosing the site is that it should be large enough to accommodate the proposed developments and to avoid congestion.
In considering the possibilities of fire spread on the site, allowances should be made for the velocity and direction of the prevailing wind.
Despite attention being continuously paid to the elimination of fire hazard, a fire may spread from the adjoining or nearby premises. The risk can be minimised by providing a clear space between the risk and the neighbouring premises.
Even where the adjoining area is a waste land, hazards are not absent since an accidental fire can set fire to the dried vegetation. To minimise this hazard, the growth of vegetation. To minimise this hazard, the growth of vegetation in open space may be controlled and a boundary wall be constructed so that an effective fire break can be maintained.
Since the majority of hydrocarbons are heavier than air (vapour density more than 1), they tend to flow like liquids along the earth’s surface and if ignited, can flash back considerable distance to the source of leak which is usually the Plant or Strorage Tank.
They can also be accumulated in hollows, gullies, trenches, etc. upto hazardous concentration. In order to avoid such accumulation, it is advisable to choose a site with a levelled ground.
The process plants pose the highest hazard potential. In view of this, adequate separating distances should be maintained between “Process Plants” and Tank farms/bulk tankages, liquefied pressurised hydrocarbon storage areas and Utilities, auxillaries and miscellaneous buildings. Also, adequate separating distances should of differential hazards.
The processing units can be grouped together according to flammable material handled i.e. as per their degree of hazard. Ex: High pressure gas and liquified petroleum gas processing units should be grouped together. Similarly, ordinary flammable liquid processing units should be grouped together.
Bulk storage tankages/tank farms contain very huge quantities of hydrocarbon fluids. The intensity of fire and explosion in the Tank farm is, therefore, very severe. In view of this, adequately safe distances have to be maintained between tank farms and processing units, tank farms and liquidified petroleum storages, tank farms and utilities and between tanks of differential hazard potentials. Tanks possessing similar hazards can be grouped together bearing in mind that tanks are dyked individually.
The elevation of bulk storage tankages is equally important. Usually, they should be at a lower elevation as compared to the processing units in order to avoid flow of huge quantities of hydrocarbon fluids towards the processing units.
The Liquified Petroleum Gas (L.P.G.) storage areas should be given a special consideration as the L.P.G. is nothing but vapours compressed into liquid. When leaked/released, liquified petroleum gas is converted into vapours which are known to travel as far as 1500 metres. In view of this, L.P.G. storage areas must be as away as possible from the remaining complex. Loading and Filling stations of such gases like propane, butane and others should be far away from furnaces and other ignition sources.
All other comparatively non-hazardous occupancies such as Utilities, Stores and miscellaneous buildings should be grouped together within a certain area away from the processing plants, tanks and L.P.G. storage areas, with suitable spacing between the individual occupancies.
An important factor in planning the layout of the plant is to understand the local laws pertaining to location of hazardous installation. Hence, the local and also the Insurance authorities must be consulted while planning the layout of the complex.
3. HAZARDS IN PROCESS UNITS/PRODUCTION PLANTS :
The Process Unit, as the name indicates, processes various combinations of hydrocarbons. The inputs are also called as raw materials or feed stocks (ex. Crude oil in a Refinery). These feed stocks are subjected to either a chemical reaction or are forced to undergo physical processes, the changes involved thereby convert the raw materials into finished products (like Aviation turbine fuel in a Refinery) and side or by-products (like Furnace oil or fuel residue) formed during the changes simultaneously.
The three salient features of a Process unit are:
(a) High quantities of hydrocarbons
(b) Very critical conditions of temperature and pressures.
(c) The rapid movement of these hydrocarbons in various phases like gases, liquids and solids.
The production activities in hydrocarbon processing industries can be classified into two broad categories :
(A) UNIT OERATTIONS :
In these, the hydrocarbons undergo a sequence of physical separations based on their physical properties, Ex: Distillation (based on difference in Boiling Point), Absorption (based on physical affinities of the substances), Extraction (based on difference in solubilities), Crystallisation (based on difference in freezings point), etc.
(B) UNIT PROCESSING :
In these, the hydrocarbons undergo chemical change under specified conditions of temperature, pressure, in the presence of catalysts, etc. Ex: Cracking i.e. breaking of larger molecules into smaller molecules under very high temperatures, Alkylation i.e. process by which an alkyl group (i.e. Methyl, Ethyl or Propyl, etc.) is introduced into a hydrocarbon, Hydrogenation i.e. process by which a hydrogen molecule reacts with hydrocarbon; Polymerisation i.e. chemical reactions which produce large molecules by a process of repetitive addition.
It is the characteristic feature of hydrocarbon processing industry to have fully integrated single train units where both unit operations and unit processes are being carried out using the same feed stocks.
4. PROCESSING OF TYPICAL HYDROCARBONS MANUFACTURING :
In order to appreciate the hazards posed by a hydrocarbon process unit or production plant, production processes of a few typical hydrocarbons is worth studying.
(a) Naphtha Cracking
(b) Manufacturing of Polythylene.
(c) Ammonia Synthesis.
(d) Manufacture of Ethylene oxide by oxidation of ethylene.
(a) NAPHTHA CRACKING :
The liquid Naphtha (which is a complex mixture of hydrocarbons) is cracked in a Naphtha Cracker which is nothing but a fired heater at a temperature of about 700*C to 900*C. This leads to formation of lower molecular weight hydrocarbons such as Methane, hydrogen, carbons monoxide, acetylene, ethane, ethylene, propane, propylene, butane, butadiene and aromatics like benzene, toluene, xylene, etc.
These cracked gases are compressed to a pressure of 35 atmospheres. The compressed gases are cooled to a low temperature by propane/ethylene refrigeration and then fed to a series of distillation columns operating at different sets of conditions of pressures (0 atmosphere to 35 atmosphere) and temperatures from 150*C to 160*C) in order to separate the individual components.
The Methane, carbon monoxide and hydrogen can be further processed as a group to obtain Synthesis Gas which can be used for various synthesis products like ammonia or can be used as a fuel. The acetylene which is generally formed in lower quantity may be converted into ethylene by hydrogenation.
The production capacities of Naphtha crackers range from 125 tons per day to 600 tons per day of ethylene, and other co-products such as butane, propylene, butadiene and aromatics like benzene, toluene and xylene.
From the above description, it can be seen that
(i) The two major unit processes involved are cracking (Temperature upto 900*C) and hydrogenation.
(ii) Series of distillation columns operating between the pressure range of 35 atmosphere to vacuum and between the temperature range of -200*C to -160*C.
(iii) Huge quantities of liquid and gaseous hydrocarbons are held up in the equipment.
(b) MANUFACTURE OF POLYETHYLENE :
Ethylene, a valuable product, obtained from “Naphtha Cracking” is compressed to a pressure of 2,500 atmospheres in a series of huge compressors. The compressed ethylene is polymerized in a reactor over a catalyst to form polyethylene. The semi-solid mass of the polymer is then cooled, dried and given a particular size and shape to be used in plastic industry.
The very high pressures pose a tremendous explosion hazard while the ethylene which is very reactive and flammable (Material Factor – 24) poses a great fire hazard.
(c) AMMONIA SYNTHESIS :
The synthesis gas containing hydrogen and carbon monoxide may be obtained from Naphtha Reforming (as described in (a) above,) or furnace oil or natural gas reforming. The carbon monoxide is converted into carbon dioxide by using oxygen from steam or air. This carbon dioxide is absorbed in Mono ethyl amine solution and separated. This is sent to Ammonia-Carbon dioxide synthesis to form Urea.
The nitrogen (obtained by liquefying and distillating air) and hydrogen (obtained from synthesis gas/steam) are fed in 1 : 3 proportion in a Ammonia Reactor at a pressure of 100 – 1000 atmospheres and temperature between 500*C to 600*C over iron oxide catalyst. The ammonia thus obtained is then cooled, compressed, liquified, dried and stored at 80*F, with a pressure of 175 p.s.i. in Horton spheres.
The average plant capacities of Ammonia Plants are 100 to 1500 tonnes per day. The principal hazards posed are high temperatures in steam reforming, low temperatures in air liquefaction, presence of hydrogen and moderately high temperatures and pressures in synthesis reactor.
(d) MANUFACTURE OF ETHYLENE OXIDE :
Ethylene oxide is manufactured by direct oxidation of ethylene. The temperature in the reactor is 250*C and pressure of 4 to 5 atmospheres. The reaction is highly exothermic and takes place in a very short residence time of one second. The outcoming gases from the reactor are water washed under pressure. The absorbed ethylene oxide is removed by distillation operation. The average plant capacities are 30 to 100 tons per day.
The measure of hazards posed are – a very wide flammability range (3 to 100) of ethylene oxide, oxidation reaction and high reactivity of ethylene oxide (Material Factor 29)
The above cases give a very brief and broad idea of the complexity and severity of the process/production operations in a hydrocarbon processing industry.
Further, specific study may give a clear idea of the hazards involved in the plant operations.
5. HAZARDS IN UNIT PROCESSES :
Since, the unit processes essentially involve chemical changes, the nature of their reactivities vary considerably. This is quantified by what is known as “Reactivity Factor”. The Reactivity Factor amongst the various unit processes will be the lowest for the endothermic (heat absorbing) reactions such as cracking, reforming, etc.
However, a unit operation carried out under severe conditions of temperatures and pressures, handling of materials with higher material factors may pose a higher hazard as compared to the unit process.
Also, some peculiar unit operations like distillation, absorption and extraction which may operate under mild temperature-pressure conditions may perhaps pose a greater hazard due to the huge quantities of flammable materials (hold-ups) involved in the operations.
7. OTHER GENERAL AREAS & HAZARDS RELATED TO PROCESS UNITS/PRODUCTION PLANTS :
7.1 FIRED HEATERS
These are widely and most commonly used heat sources in hydrocarbon processing industry especially for endothermic reactions. The fired heater is an equipment/where the liquid to be heated is passed in tubes and heat is supplied by burning fuel outside on the shell side.
Due to the presence of live and naked flame obtained by controlled combustion of a variety of fuels, fired heaters are the most hazardous pieces of equipment in the Plant. The hazard is further aggravated due to their essential presence in the heart of the Plant, integrated with the train of process vessels. Therefore, a sincere attempt must be made to locate them upwind in one corner of the unit where exposure is reduced to the minimum possible level.
The operational hazards of fired heaters can be listed as under :
(a) Explosions due to simple faults like leakages in fuel flow controlling valves. If these are not leak proof, fuels can continue to pass and get ignited in the furnace chamber by the hot radiant walls.
(b) Similarly, if the fuel flow controlling valve is located far away from the fired equipment, fuel in the line between the furnace and the control valve may continue to pass into the fire box when burners may be already extinguished. This fuel may form vapour cloud or explosive mixture with air in the absence of combustion.
(c) Mechanical rupture of tubes carrying the material to be heated.
(d) Improper temperature controls may lead to cooling of hydrocarbons leading to poor heat transfer and forming an explosion hazard.
7.2 COMPRESSORS :
Compressors are used to compress huge quantities of hydrocarbon gases. For example, in Ammonia Plant a separate compressor house renders services to compress synthesis gas, nitrogen, oxygen, and ammonia. Similarly, in Naphtha cracking unit, all the gaseous products are compressed and then separated by distillation.
The compressors should be located down-wind and sufficiently far away from heaters. The operational hazards of compressors can be listed as under :-
(a) An internal explosion is possible if air is drawn into the system through leakages in packing glands, fittings, etc.
(b) Gas may also escape because of human failure or equipment rupture by over pressure.
(c) Hazards posed by huge quantities of lubricating oils in the system emerged out due to gigantic compressor capacities.
(d) Certain hydrocarbons reduce the viscosity of lubricating oils when they come into direct contact with lube oils. This may lead to inadequate lubrication leading to additional operational failure hazards.
7.3 PUMPS :
Hydrocarbon processing industry is a continuous process industry. In view of this, pumps are required to render service of transporting hydrocarbons round the clock and years together. This necessitates preventive measures and adequate maintenance of pumps which otherwise pose the following hazards :
(a) Leakage of flammable hydrocarbons through glands, packings etc.
(b) Inadequate disposal of leaking or by-passed hydrocarbons which may accumulate warranting to safety.
(c) Improper location of pump compounds like location in hazardous area or at a place having inadequate ventilation.
(d) The high pressure charge pumps must be isolated from major process equipments or other pumps.
7.4 PIPINGS :
Piping for a hydrocarbon processing industry is what blood vessels are for a human body. All the equipments are interconnected by piping in an integrated process unit. The utilities and storage tank farms render their services to the production plant by piping. The length of piping in a typical hydrocarbon processing industry can run into several kilometers.
Pipe racks in individual units normally run centrally within that unit, thereby splitting the unit two or more areas of equipment. Critical piping circuits such as transfer lines from fired heaters, condensing vapour lines, etc. need to be identified and have adequate space provided.
The hold-ups of hydrocarbons in piping therefore cannot be neglected and nor can thee hazards associated with pipings be ignored. To list them out,
(a) Improper piping designs neglecting the thermal and contraction allowances may lead to mechanical damages in piping systems.
(b) Choking up of pipelines leading to heat and material build-up in a particular section of process unit which in turn will lead to fire and explosion.
(c) Leakage in piping system at hazardous locations, untraceable leakages, leakages at inaccessible locations in the complex piping network may lead to vapour cloud formation and accumulation of flammable liquids/vapours in the form of pockets at particular locations in the Plant.
(d) Mechanical damage to the piping system can lead to shut-down of the Plant. Delayed shut-downs may again lead to leakages of hydrocarbons and accumulation of flammable vapours as well as vapour cloud formation.
(e) Majority of the hydrocarbon fluids when flowing through pipes can build up large static electricity potentials. If not adequately earthed, the accumulations of static electricity charges round the clock in the piping systems may lead to disastrous fires and explosions.
(f) Inadequately or improperly insulated pipes may create local hot or cold spots and the internally flowing hydrocarbons if leaked near such spots may hold a threat of fire and explosion.
(g) The major pipe racks should remain as unexposed to process equipment as possible.
8. CRYOGENIC PROCESSES :
These are essentially low temperature processes commonly used in the hydrocarbon processing industry for separation of liquified petroleum gases and air liquefaction units, etc.
The temperatures achieved in such processes by using refrigeration systems through auto-refrigerants like ethylene and propane are of the order upto – 160*C. The related hazards are:
(a) The refrigerants usually used like ethylene, propylene, propane, etc. have high “material factors”. These are required to be circulated to various sections of the entire processing unit and hence are also required in large quantities.
(b) At low temperatures, the selection of material of construction for equipment involving cryogenic operations should be very selective. Certain materials like carbon steel become very brittle at very low temperatures. These materials are then susceptible to developed leakages.
9. HAZARDS OF FLARE & FLARE STACKS :
The vapours of flammable liquids are generally heavier than air. Hence, if waste and surplus hydrocarbon gases released by safety valves and pressure relief valves of various equipments cannot be released to the atmosphere as they will settle down and may strike any hot surface in the plant leading to fire and explosion.
These gases are, therefore, collected by a network of pipelines and joined to a header which leads to a knock-out drum for separating any liquid from flare gas. A water-sealed vessel is provided to prevent flame from backing up from flare stack where the gas is safety disposed off by burning.
In Fertilizer industry, gases like hydrogen and methane which are lighter than air are only released. These can be safely vented off without burning.
To avoid the possibility of air diffusing down the flare stack during low flaring rates, a deliberate bleed of fuel gas should be given to the base of the flare stack. Steam can also be introduced at the base of flare for improved combustion.
Because a flare system provides such a convenient way to dispose of unwanted materials, frequently there is a tendency to regard it as a ‘general purpose waste gas siever’ than as an ‘emergency disposal system’. As a result, materials are sometimes routed through a flare which was not intended to accommodate them. This way either cause immediate difficulty or seriously impair reliability of flare facilities in subsequent emergency.
The hazards involved in flare and flare stacks are as following:-
(a) Hazards of internal explosions inside flare system due to ignition of combustible mixture.
(b) Liquid carryover from flare stacks.
(c) System obstructions i.e. the vapours or the liquid streams from relief valves of process equipment cannot be disposed off through a closed valve, should an obstruction to flow be present when over-pressure occurs, failure of vessels or other facilities is a distinct possibility.
(d) Low temperature failures – the entry of low boiling hydrocarbons may lead to a complete failure of flare line as a result of sudden drop in temperatures; coincident with stress risers and/or sudden impact.
(e) Hazards associated with the maintenance of the system.
(f) Inadequate/unsafe distance from the processing Unit.
10. BLOW DOWN AND EFFLUENT DISPOSAL:
Normally, hydrocarbon processing plants contain huge quantities of flammable fluids. In case there is a fire or a plant emergency in a particular section of the plant, the flammable liquid and vapour phase contents from all the integrated process equipment must be safely removed to blow down drums, pits or sumps and or flare or any other safe location.
Usually in case of emergencies, all the integrated process equipments are depressurized so that the majority portion of the liquid contents get vapourised and released by pressure relief /safety valves to flare where the gas is safely disposed off by burning.
For liquid based contents, the quantity of hold-up at any one time between two block valves must be ascertained and provisions made for dropping it down to blow down drums, pits, sumps, etc., the capacity of which must be at least 10% higher than the capacity of the largest hold-up between any two block valves.
Dropping of liquid based contents from the process vessels directly to the main Tank Farm should not be accepted as an adequate means of blow down.
Thus adequate blow down facility is an essential feature of a hydrocarbon processing industry. The absence of blow down facilities is a cordial invitation for catastrophic losses.
WASTE EFFLUENT DISPOSAL system is provided for collection of waste from spills, washings, equipment catch basins, traps, funnels, etc. This collected waste material is then taken to a remote and safe location and the flammable and hazardous materials are separated from water, etc.
The resultant mass is then processed to make it harmless and then it is disposed off. Generally, the main underground headers with manholes are located on either side of central pipe rack to enable the collection of waste. If required, separated headers for wastes of different types such as oily, caustic, acidic, etc. may be provided and the equipment should be grouped accordingly.
For liquid spills beneath heaters, special considerations must be given. If possible, the ground beneath the heater must be sloped, so that no oily spills will collect there or flow towards other heaters or equipment.
The underground effluent system must have water interceptors with water seal. In fact, the inlet of the prior header/channel opens at the bottom of the water seal of interceptors and the overflow from this interceptor (generally lighter hydrocarbons) leads to subsequent header/channel. This avoids the propogation of fire.
11. CONTROL ROOM:
The hydrocarbon processing industry as explained earlier is a fully integrated unit where hundreds of unit operations and unit processes are carried out simultaneously. All these processes are controlled by means of sophisticated instruments. All these instruments are housed in a single location so as to facilitate better control on the operation of the plant, i.e. in a Control Room.
The safety of the Control Room is absolutely a must in order to fight any emergencies in a running Plant. In view of this, and also due to numerous fires and explosions actually occuring in the Control Rooms, the basic concepts of design and location of the Control Room have considerably changed.
Control Rooms of new plants are of bunker type. Alternatively, they are of sturdy constructions with explosion proof classes in the window frames.
The Control Rooms, in practice nowadays, are located sufficiently away from equipments handling hazardous materials, but not so far away that the operative personnel feel reluctant to go to Plant for frequent plant checks.
It is very essential that if the Control Rooms are located near the Plants, then at least the plinth level of the Control Room should be adequately higher than the plant floor level. This is to avoid the entry of heavier hydrocarbons into the control room in the event of accidental leakages.
It is a safe practice to keep the Control Room under positive air pressure to avoid the entry of heavier hydrocarbons into the Control Room.
An increasing dependence on computers and remote controls which on the one hand may be considered to have increased operational safety, but on the other hand, reduces the manpower on site available for emergency of fire fighting purpose. Whilst automatic shutdowns are a good thing, the fires insurance underwriters is inclined to take a cynical point of view of these automatic devices as they do not always operate.
12. MAINTENANCE AND PLANT MODIFICATIONS:
Maintenance in the hydrocarbon processing industry is of vital importance. When we realise that present on-stream periods may be long as three years. This further emphasizes the importance of preventive maintenance.
The long on-stream periods reduce no doubt the hazards of Plant shut downs and plant start-ups. On the other hand, fire insurance underwriters are exposed to failures through fatigue effects in a measure never experienced before.
Due to modern sophisticated nature of hydrocarbon processing industry aided by instrumentation and computerisation, it is probably true to say that maintenance and supervisory staff is today more important than operational staff.
Psychologically, one feels that plant under maintenance shutdown is very much safe as compared to a running plant but, this is not true. The maintenance hazards listed below can explain how much precaution is required to be taken during maintenance period.
(a) Failure to replace an inspection cover can lead to an explosion, the next time it is switched on in the presence of a flammable vapour.
(b) Ignition sources introduced by repair and maintenance workers such as sparks, flames from cutting and welding equipment and heat from soldering and riveting operations and fractional sparks from various tools.
(c) Absence of work permit system. The procedure by which all the essential safety measures-precautions those are required to be taken before starting up of any maintenance job in the premises is known as “Work Permit” system.
(d) Not clearing the plant areas from the point of view of hazardous materials.
(e) Inadequate isolation of areas in which maintenance work is to be undertaken.
(f) Improper cleaning of equipment handling flammable fluids.
(g) Inadequate purging of equipment by inert medium.
(h) Bad housekeeping and improper waste disposal.
PLANT MODIFICATIONS OR MATERIAL ALTERATIONS:
The well known catastrophic loss of Flixborough was because of negligence exhibited during the period of plant modifications.
During the original design of plants, a lot of effort is put in to see that the design meets all the technical requirements. Similarly, effort is put in at the construction stage also.
In a running plant, the need often arises to carry out a minor modification which, by intent, would either increase safety or profitability of a company. It is important that during the execution of plant modifications, the same care is required to be taken as during the original design and construction.
The head of the operations department, safety superintendent, Project Manager and Engineering Inspection Department must equally be involved in carrying out any plant modifications.
13. ELECTRICAL EQUIPMENT:
There are three main approaches to the problem of electrical equipment in flammable atmosphere viz. to use apparatus which is either
(a) Flameproof
(b) Pressurised, or
(c) Intrinsically safe
(a) FLAMEPROOF EQUIPMENT:
The principle of flameproof enclosure relies on the fact that flame will not propagate through a gap which is smaller than a certain critical size. Each vapour and gas has its own characteristic critical gap size. If the apparatus is so constitened that no gaps exceed this critical size and explosion originating within an enclosure will not be propagated to the outside atmosphere. The enclosure is required to be mechanically made strong enough to take care of internal explosion.
(b) PRESSURISED EQUIPMENT:
The equipment is pressurised by injecting pure air drawn from a safe area under positive pressure into the enclosure. The air leaks out through small gaps in the apparatus casting and so prevents the ingress of the vapours and gases.
(c) INTRINSICALLY SAFE CIRCUIT:
Intrinsically safe circuit or part of a circuit is said to be so when any spark or thermal effect produced normally (i.e. by breaking or closing the circuit) or accidentally (for example, by short circuit or earth faulty) is incapable of causing ignition of prescribed gas or a vapour.
It must be noted that the above is not an equipment, but design feature of the system.
It is very necessary to classify areas according to the degree of probability of the presence of dangerous atmospheres in order to determine the class of electrical installation appropriate to the particular risks.
Division ‘O’ – Areas where dangerous atmospheres will be continuously present.
Division ‘1’ – Areas where dangerous atmospheres is likely to occur anytime.
Division ‘2’ – Areas where flammable atmospheres is unlikely to be present.
The hazards associated with electrical equipment are as under:
(a) Wrong assessment of an area according to the degree of probability of the presence of a dangerous atmospheres.
(b) Improper selection of the equipment suitable for a given division of area.
(c) Use of portable electrical equipment in hazardous areas.
(d) Use of electric heaters in hazardous areas.
(e) Use of non-flameproof lighting fittings in flameproof areas.
(f) Dangerous faults in electrical equipments due to corrosive atmosphere prevailing in industry.
(g) Inadequate maintenance of electrical equipment.
(h) Use of temporary installations.
(i) Use of unprotected cables and wire in flammable atmosphere.
14. STATIC ELECTRICITY:
Students must be aware about the basic causes, nature and hazards related to static electricity as discussed under the study courses of “General Fire Hazards” and “Chemistry & Electricity”.
Special reference is made here in connection with behaviour of static electricity with hydrocarbons. Static Electricity can be generated when flammable liquids are being moved while in contact with other materials. The generation of static electricity charges can occur when the liquid is discharged from a pipe or hose, transferred from one container to another, mixed, pumped or agitated.
Unless suitable static corrective measures are taken, static charges if accumulated during these operations may ignite flammable vapour air mixtures when a spark occurs in their presence. Flammable liquids having low flash point are considered to prevent static electricity hazard. Unheated flammable liquids with flash point of 93*C or less and other flammable liquids heated to within 10*C of their flash point require evaluation of static electricity. No static control evaluation of static electricity. No static control measures are generally required for unheated flammable liquids having flash point above 93* C
Pure gases generally do not generate static electricity while flowing. Hazards posed by static electricity in the hydrocarbon processing industry can be briefly summarised as under :-
(a) Inadequate electricity bonding and not grounding the electrical equipment handling or storing flammable liquids.
(b) Flammable liquids having high electrical resistance and low flash point require careful treatment to avoid static electricity build-up.
(c) Not avoiding splashing while filling the flammable liquids.
(d) Not providing electrical bonding for pipelines.
(e) Not allowing the flammable liquid to flow at a lower velocity in pipes.
15. BULK TANKAGES AND TANK FARMS:
There has been a trend towards installing larger capacity hydrocarbon storage tanks. Storage of 50,000 Cu. Mts. Capacity and above are not only used for crude oil storage, but also for intermediate and finished products. such large storage poses high risk of a exposure and warrants careful planning.
Most hydrocarbon vapours are heavier than air so that the location of the tank farms should receive consideration of the prevailing wind in order to minimize the possibility that released vapours and gases might drift through the plant.
Crude oil received from a pipeline or tanker is stored in tanks generally not less than three in number. Capacity of the tanks should be based on method and frequency of crude receipt. Intermediate storage is provided for start up operations and for units running on block operation.
Each tank farm should be provided with not more than two rows of tanks so that each tank is approachable from the road side for fire fighting. However, tanks of large capacity should be in only one row.
Generous spacing between individual tanks, between the tank farm and processing area, is a must. Radiant heat has damaged adjacent exposed tanks over one diameter away even when there has been no ignition of the second tank. A Common dyke has been the greatest single factor in determining the size of the risk. It has proved almost impossible to prevent the spread of fire to other tanks when they are not separated by individual dykes.
Tankfarm enclosure capacity should be such that it should be able to contain the complete contents of the largest tank in the tank farm in case of an emergency. Height of the enclosure (dyke) is generally limited to 2.0 metres since higher dykes hinder access during emergency and restrict pit ventilation. Additionally, fire walls of about 600 mm. height should be provided within the dyked area to isolate the small spillages from individual tanks.
The area within the tankfarm shall have an adequate slope not only for draining the rain water, but also for directing spills within the tank farm. The storm water drain from tank farme shall be connected to an oily water system through a valve so that the first rain effluent can be diverted to the waste water treatment plant. Such valves should be located outside the dyke.
Offside pumpstations are kept outside the tank farms by roadsides, not only because pump drivers are a source of ignition, but also because they are vital for transferring products to safe tanks in an emergency. Extra electrical points are provided on the roadside where mobile electrical pumps are planned to be used for pumping out heels of tanks and other emergencies. Mobile diesel driven pumps can also be used.
Following are the types of tanks which are most commonly used for the storage of hydrocarbon liquids.
(a) Fixed Conical Roof tanks
(b) Floating roof tanks.
The floating roof tanks as the name indicates have roofs which float on the surface of liquid stored. The floating roof tanks therefore eliminate the empty space above low flashing liquid surfaces thus avoiding formation of explosive mixtures in the vapour space.
All liquids having flash points less than the ambient temperatures must be provided with floating roofs. The use of inert gas in place of air in the vapour space of standard cone roof tanks in combination with a water spray or insulation would provide a means to minimise the effects of atmospheric pressure variations and maintain a more constant temperature in the vapour space, reducing thereby the fire hazard.
It should be borne in mind that none of the tanks storing class A and B products are of rivetted construction. Tanks having conical roof and storing class A and B products should not be fitted with mechanical stirrers or artificial heaters.
The heating of Class C liquids may be permitted provided the temperature does not exceed beyond its flash point.
The safety devices such as vents (to allow escape of vapours in case if excess vapour pressure is built up), breathers, flame arrestors, earthing and electrical bounding are also of immense importance for all types of storage tanks.
16. L.P.G. STORAGE
This storage is considered reliable when constructed to standards and codes and properly operated and maintained. There is remote risk of catastrophic failure. Apart from the pressure relieving system and other safeguards against overfilling, layout plays a very important part. The storage should be located downwind of process units and other important buildings. Also, it should be placed away from the process units, but well within the boundary wall.
The base under L P G storage should be impervious and sloped to the maximum possible gradient so that any spillage occurs, the spilled L P G gets collected in a sumprather than accumulating under the vessel and within grass growth. The spillage collection sump should at such a distance from the vessels that the flames from a fire in the sump will not impinge on a tank or vessel. The flames may develop at an angle of 45* to the vertical and of twice the length of the pool diameter. Collection sump capacity should be equivalent to about 5 to 10% of the volume of each vessel located at one enclosure. Enclosure height should not be more than 300 mm so that proper ventilation is provided.
The layout of horizontal storage tanks (bullets) should be such that the longitudinal axis does not point toward vital process areas or important, high-value structures. Experience has shown that tanks can rupture under fire conditions and so propelled considerable distance along their longitudinal axis due to rocket effect.
Bullets should be kept in one row only. If multiple rows are required, the vessels should be arranged to minimize the chances of any one vessel being propelled into another. Spheres are considered safer than bullets and can be located in one or two rows in a tank farm.
Roads should be constructed on all sides of the tank farm to provide direct approach to each tank from the roadside. Water draw-off (drain) from L P G tanks should be extended so that these terminate outside the shadow of the vessels in the collection sump and double valves should be provided on the drain.
Normally, it is recommended practice to connect the pressure relief system of the LPG spheres/bullets to the flare. The relief valve on the inter-connecting pipeline must however be kept in openlocked position.
As the heat intensity of L.P.G. fires is tremendous, the load bearing structural steel work of spheres and bullets must be protected by at least 50 mm. thick concrete carried upto the highest of the points where the load of the storage equipment is transferred to the supporting framework.
Also, the LPG storage vessels must be protected by fixed water spray systems.
AN LPG BOTTLING STATION should be located at a safe location with minimum possible ingress of truck traffic. A bottling station should be downwind of the plant. Adequate ventilation provision is necessary to avoid concentration of LPG gas on floors.
The station can be planned for stacking filled and empty cylinders in separate areas and the LPG filling testing facilities should be in a separate area on the downward side of the station. There should not be any depressions, etc. in the adjoining areas. The areas on all sides of the station should be hard paved to check growth of grass to an extent of 15 metres. Trucks handling LPG bullets must have flame arrestors fitted to their exhaust.
17. LOADING AND UNLOADING AREAS :
The precious raw materials, intermediates and finished products in a hydrocarbon processing plant are to be handled with precision and utmost care.
This is done generally in a specified location known as “Bulk Loading/unloading area”.
The hydrocarbon quantities to be handled in this area are in huge quantities. Every material requires special methods and precautions depending upon its phase (whether solid, liquid, gas or a presurrised liquid), properties like flash point, flammability range, explosive limits, etc. and mode of transportation.
The chief methods of transportations are super road tankers trans-continental pipelines, rail cars and the like.
Loading racks for dispatch of products should be located in separate blocks for isolation in the event of a fire away from tanks and process units.
The gantries of rail loading/unloading racks require careful planning with respect to rake composition, type of rail cars to be loaded, frequency of rake replacement, etc.
For control of fire emergency and protection against exposure fires, adequate spacing between two loading racks must be provided.
Truck loading racks must be as far as possible away from storage tanks and important structures. The trucks must not pass through any other important area between the main road and the rack.
The loading rack should be limited in size so as to accommodate not more than a given number of loading and unloading vehicles so that hazard concentration : is controlled in a limited manner.