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Thread: Protection for Zone 21 , 22 Etc.

  1. Question Protection for Zone 21 , 22 Etc.


    A vessel has been defined under Zone 20/21. No Hazard outside Vessel.(not even Zone 22) Which implies Hazard could be present only in case of Spillage/Leakage. What we are planning to do is to use instrumentation certified for Zone 20/21 (or Intrinsically safe Instrumentation) for Instruments directly mounted on the these Vessel, but standard weatherproof instruments for instruments installed in the vicinity or other vessels which are not directly identified with Hazard zone. In other words what is the extent of Hazard zone . In case of Gases , it is rather simple to define but factors like cleanliness , housing keeping etc seem to reduce higher zones to No Hazard.

    Are any literature or standards are available defining extent. Most of the literature defines Zones and seem to provide only very general guide lines.

    Thank you for your assistance


  2. #2

    Prior Concern

    Thanks for the post which confirms the mediocrity of organizations purporting to be improving safety without any quantitative data.
    Health & Safety is just an excuse for otherwise useless parties to enforce rules they don't understand in the first place: despite having concocted the trash.
    To business: ensure that you wire up the nearby vessels first. Then get that lot written down, drawn up etc. Then you can fit your intrinsic whatevers as an addition, at that time. Now you have a documentary safe site.
    You are bound to be safe. Your quest reeks of personal integrity.
    John Gateley

  3. Extent of hazard zones

    As stated, the zone extent for gases is fairly clear but latent potential for dust is more awkward because of residue, site construction and housekeeping standards. A risk assessment must be made by the designer of the possibility and effects of an explosion, and this may require specialised expertise.

    The most commonly used standard in the UK for determining area extent and classification is BS EN 60079 part 101, which has broad applicability. The current version makes clear the direct link between the amounts of flammable vapour that may be released, the ventilation at that location, and the zone number. It contains a simplistic calculation relating the size of zone to a rate of release of gas or vapour, but it is not helpful for liquid releases, where the rate of vaporisation controls the size of the hazardous area, or potential dust explosions, where accumulated deposits may be dislodged to greatly magnify trivial primary blasts.

    Other sources of advice, which describe more sophisticated approaches, are the Institute of Petroleum Model Code of Practice (Area Classification Code for Petroleum Installations, 2002), and the Institution of Gas Engineers Safety Recommendations SR25, (2001). The IP code is for use by refinery and petrochemical type operations. The IGE code addresses specifically transmission, distribution and storage facilities for natural gas, rather than gas utilisation plant, but some of the information will be relevant to larger scale users.


    Hazardous areas are defined in DSEAR as "any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers". In this context, 'special precautions' is best taken as relating to the construction, installation and use of apparatus, as given in BS EN 60079 -101, with account taken of potential site conditions.

    Area classification is a method of analysing and classifying the environment where explosive gas atmospheres may occur. The main purpose is to facilitate the proper selection and installation of apparatus to be used safely in that environment, taking into account the properties of the flammable materials that will be present. DSEAR specifically extends the original scope of this analysis, to take into account non-electrical sources of ignition and mobile equipment that creates an ignition risk.

    Hazardous gaseous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere, as follows:
    Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods;
    Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation;
    Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time.

    Various sources have tried to place time limits on to these zones, but none have been officially adopted. The most common values used are:
    Zone 0: Explosive atmosphere for more than 1000h/yr
    Zone 1: Explosive atmosphere for more than 10, but less than 1000 h/yr
    Zone 2: Explosive atmosphere for less than 10h/yr, but still sufficiently likely as to require controls over ignition sources.

    The potential for a dust explosion should be taken as permanent, according to the worst condition expected, as the presence of fugitive dust will not disperse like a gas. This is an onerous responsibility as no general value can be stated. It demands individual site assessment. and some expertise.

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