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Safety and Good Practices - Shipboard Oil Storage Tanks

written by: sriram balu • edited by: Lamar Stonecypher • updated: 11/3/2011

Safety around oil tanks aboard ships is an ever-ongoing concern for ships engineers and the owning companies, but incipient risks can be hard to recognize. Do you know the symptoms of exposure to hydrocarbon gases?

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    Points to Remember for Fire Prevention aboard Ships

    1. Flame or smoldering sources:

    Many shipboard fires continue to be caused by the careless disposal of lighted matches and cigarettes, even though prevention is easily made by the provision of ashtrays of an approved design and the proper attitude among the seaman. Ashtrays should be provided and used only in authorized areas. Smokers should ensure that matches are extinguished and that cigarettes ends are properly stubbed out. Warning notices should be displayed where smoking is forbidden and the same should obeyed.

    2. Hot surfaces:

    While it may be termed “black heat,” it also is not generally realized that the heat from an ordinary electric light bulb can soon put the temperature above the safe ignition limit of some materials (if that heat is not allowed to dissipate by normal convention currents). A fire may be prevented by considering where the oil would spray from any burst pipe lines, especially high pressure pipes and fitting deflector plates over steam pipe lines, flue gas exhaust pipe lines, and in boiler casings. If it is not desirable to use a deflector plate, every pipeline carrying hot fluid must be lagged with some insulating material. General care must be taken in the maintenance of machinery such as pumps-glands, etc., which could overheat if faulty.

    3. Creation Of Spark:

    These include funnel sparks, friction, and mechanical sparks from grinding, cutting, chipping or welding, electrical sparks from switches and faulty brush gear, or short-circuiting. Particular care should be taken in gaseous conditions where even an electric torch may be unsafe.

    Gas-tight switches should have good gaskets, and flame arrestors should be kept in good condition. In the gas storage area we should use intrinsically safe equipment. This intrinsically safe equipment can be defined as an electrical circuit in which a spark or thermal effect (under normal operation or specified faults condition) is incapable of causing the ignition of a given explosive mixture.

    4. Electrical Protection:

    Any wiring/electrical equipment which is overloaded will become overheated, so correct protective devices are important and wiring in places that have difficult access must be inspected (and always be on the lookout for unauthorized wiring).

    5. Spontaneous combustion

    Apart from the risk in the gaseous cargo, rope-store, oilskin, dirty linen, or life-jackets lockers are likely risk areas. Keep these well ventilated to prevent heat build-up. Contaminations with natural oil or rotting due to dampness are the main culprits. So good housekeeping is essential in stores.

    6. Static electricity

    Static electricity is the electrical charge produced on dissimilar materials caused by relative motion between each when it contact. In gaseous condition, the importance of static electricity should be noted for tanks and piping, and there should be a continuity wire which is properly grounded.

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    Risk of Shipboard Explosion

    Flammability Range:

    A mixture of hydrocarbon gas and air cannot be ignited and burn unless its composition lies within a range of gas in air concentration known as the “flammable range.” The lower limit of this range, known as the lower flammable limit (LFL) is the hydrocarbon concentration below which there is insufficient hydrocarbon gas to support combustion. The upper limit of the range, known as the upper flammable limit (UFL) is the hydrocarbon concentration above which there is insufficient air to support combustion.

    Any Gas Concentration below LFL may be called as "Too Lean" to burn and any gas concentration above the UFL may be called as "Too Rich" to burn.

    Flammability Range 

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    Oil Storage Tanks:

    1. No signs of oil leak should be visible near man hole doors.

    2. The tank outlet valve must have a remote closing or quick closing valve.

    3. No hot work must be done near these tanks.

    4. All tank bulkheads/boundaries must be well marked with " no hot work" symbols.

    5. The tank vents must be placed in a safe place where the vapours do not cause any fire hazard.

    6. Vents must have a flame screen such that any fire spark entering the tank will have its energy drained.

    7. All Filling and transfer lines, valves must be clearly tagged and numbered to avoid any confusion.

    8. All tank vents must have save all trays to accomodate some quantity of oil when the tank overflows.

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    Precautions Carried out to Avoid Electrical Fires.

    1. Authorised persons only should interfere with electrical fittings. Personal electrical appliances usage should be avoided.
    2. Faulty fittings and wiring should be reported immediately to the head of the department.
    3. All electrical fittings should be firmly secured.
    4. Flexible leads should be secured properly to avoid being chafed or cut.
    5. Makeshift plugs, sockets, and fuses should not be used.
    6. Circuits should not be overloaded since these causes overheating failure of insulation, which resulting a short circuit current, which could start a fire.
    7. All portable electrical appliances, lighting, etc. should be isolated from the mains after use.
    8. Any Earth Faults must be attended immediately and rectified.
    9. All Ex (Intrinsically safe) items must be inspected regularly.
    10. All electric motors must be cleaned on their external surface, enabling the cooling fins to be clean.
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    Precautions against Personal Hazards

    Toxic gas

    Normally these hydrocarbon gases are toxic in nature; most of the liquefied petroleum gases are odorless. Propane and butane are “stenched” with chemicals like “Ethyl mercaptan” to aid in case of leaks. However sometimes the cargo is not stenched, depending on the receiver’s requirements. Most hydrocarbon gases dull the sense of smell after some period. The absence of smell, therefore, should never be taken to indicate the absence of gas. Dangerous concentration levels could be some times considerably below the lower flammable limits and the combustible gas indicators cannot be expected to measure concentration of this order accurately (so we use the dragger tubes which give the accurate measurement of the gas in terms of PPm).

    The following are typical effects at higher concentration:

    0.1 % - Irritation of eyes with one hour.

    0.2 % - Irritation of eyes, nose and throat, dizziness and unsteadiness with half an hour.

    0.7 % - Symptoms as of drunkenness within 15 minutes.

    1.0 % - Rapid onset of drunkenness which may lead to unconsciousness and death in about 5 to 10 minutes if exposure continues.

    2.0% - paralysis and death occur very rapidly.

    So care should be taken to note the concentration of hydrocarbon and make sure that it is within the safe working limits in the gas storage stations.

    Related Reading at Bright Hub

    Gas Detection Meters for Ships