วันจันทร์ที่ 22 กุมภาพันธ์ พ.ศ. 2553

อันตรายจากก๊าซ แอลพีจี / HAZARDS of LPG



HAZARDS of LPG
  • LPG is usually stored and transported as a liquid. Leakage of liquid results in the formation of large volumes of vapour as 1 volume of liquid produces approximately 250 volumes of vapour at atmospheric pressure.
  • Even small quantities of LPG vapour in air may form a flammable mixture. The limits of flammability of LPG are approximately 2% to 10% volume of gas in air and this results in LPG leakage forming large volumes of flammable gas—air mixtures. volume of vapour can form 10 to 50 volumes of flammable gas—air mixture. 1 volume of liquid can form 2,500 to 12,500 volumes of flammable gas air mixture.
  • LPG at ambient temperature and normal atmospheric pressures is normally a gas which is heavier than air. Commercial butane vapour and commercial propane vapour are approximately 2.0 and 1.5 times as heavy as air, respectively. LPG vapours will therefore sink to the lowest levels of the surroundings and flow along the ground or through drains or such-like passages. Under still air conditions the natural dissipation of accumulated vapour may be slow.
  • A small release of LPG into the atmosphere will result in a flammable mixture which may be ignited some distance from the point of leakage.
  • LPG has a low viscosity; hence it has little or no lubricating properties and it is more likely to find a leakage path than water or most other petroleum products. These facts are significant with respect to the design of plant and have particular relevance to the glands and seals of pumps, compressors and valves.
    Whilst LPG is colourless both in the liquid and vapour phases the cooling which takes place when the liquid evaporates results, in cases of leakage, in water condensation and even freezing of water vapour contained in the air; this appears as a white mist or cloud. The cooling which occurs on evaporation of liquid LPG causes frost burns to the skin anddamage to eyes if protection, e.g. gloves and goggles, is not used when contact with LPG is a possibility. Refrigerated LPG causes severe frost burns and eye damage even if little or no evaporation occurs.
  • LPG whether in liquid or vapour form is only slightly toxic. The vapour is an asphyxiant in high concen trations and is anaesthetic. Thus precautions must be taken in entering confined spaces such as tanks because of these properties of the LPG (and also because of its flammability and the risk of reduced oxygen levels).
  • Liquid LPG has a high coefficient of expansion. Adequate ullage must therefore always be provided in storage vessels (Static and transport) to prevent them becoming hydraulically full as a result of temperature rise. Pipelines and equipment which are normally liquid full must be protected, by operating procedures and/or safety equipment, against the high pressure which would result from the liquid's expansion with temperature rise.
  • A container which has held LPG and is nominally empty is still potentially dangerous. In this state the internal pressure is approximately atmospheric and should the valve be open or leaking a flammable mixture may result due to air diffusing into the container when emperatures fall or vapour issuing from the container when temperatures rise.
  • The density of liquid LPG is approximately half that of water, and when leakage occurs in low ambient temperatures the product could be carried away by water.
  • If water is used for hydraulic testing of storage etc. then its weight and not that of the LPG contents is relevant for foundation design.
  • The presence of water in LPG systems presents a hazard from icing under certain conditions (see 1.5).

    ODORIZATION
    Odorization is not required if harmful in the use or further processing of the LPG or if it will serve no useful purpose as a warning agent in such use. In other circumstances the odour should be distinctive, unpleasant and non-persistent and should indicate the presence of gas in the air at a level in excess of 20% of the lower limit of flammability. The odorization requirement can be met by the addition of controlled quantities of suitable odorizing agents, e.g. ethyl mercaptan, dimethyl sulphide. Care is necessary in handling those odorants which are toxic in concentrated form.

    PREVENTION OF WATER DEPOSITION
    Methanol or propylene glycol may be added to LPG to prevent water deposition and hydrate formation. For refrigerated product water and other trace components must be removed prior to refrigeration.

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