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نحوه ی چیدمان ارت پیت ها در زمین پالایشگاه.

  • This topic has 1 reply, 2 voices, and was last updated 4 months, 2 weeks ago by محمد سعید جعفرپور.
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  • #8334
    Pouya
    Participant

      فرض کنیم یک پالایشگاه با چندین واحد پروسسی و یوتیلیتی طبق Specification های پروژه به ارت حفاظتی (PE) دو اهم نیاز دارد و طبق محاسبات (با توجه به مقاومت خاک منطقه و مشخصات و محیط زمین) به تعداد 30 Earth Pit نیاز داشته باشیم. نحوه ی چیدمان این ارت پیت ها چگونه باشد؟ آیا لازم است که در دورتا دور محیط پالایشگاه اجرا شوند؟ یا می توان آنها را با حفظ فاصله ی مناسب در یک زمین خالی و در یک سمت پالایشگاه اجرا کرد و تنها رینگ ارت را در Plant پخش نمود؟
      همانطور که اطلاع دارید در صورت وجود تانک ها و برج های روشنایی یا فرایندی مرتفع لازم است که در پای هر کدام حداقل یک ارت پیت اجرا شود.(جهت حفاظت در برابر صاعقه) همچنین در دورتا دور پست های برق موجود در پالایشگاه نیز ارت پیت هایی اختصاصی برای ترانسفورماتورها اجرا می شود. آیا بایستی این تعداد(N) را از تعداد محاسبه شده ی فوق کم کرد؟ یعنی آیا به منظور تامین 2 اهم مورد نظر میتوان از 30 عدد ارت راد مورد نظر N عدد کمتر را در دور پالایشگاه اجرا کرد؟

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    • #8335
      محمد سعید جعفرپور
      Keymaster

        بطور کلی از نظر صحت محاسبات منجر به 2 اهم ذکر شده، یک حداقل فاصله بین دو الکترود مطرح است که از طول میله الکترود نباید کمتر باشد. اما آنچه حداکثر فاصله الکترودها را محدود می کند گرادیان افزایش پتانسیل  ناشی از تخلیه الکتریکی در زمین است که به پارامترهای متعددی از قبیل مقاومت خاک، پراکندگی الکترودها و میزان جریان اتصال کوتاه بستگی دارد و بهترین راه اطلاع از آن انجام محاسبات کامپیوتری با نرم افزارهای اختصاصی است. در این ارتباط می توان به BS 7430 و IEEE 80 مراجعه نمود.

        در محاسبه مقاومت زمین کلیه الکترودهای مدفون به هر منظوری در محاسبات موثر هستند.

         

        9.6 Potential gradient around earth electrodes
        9.6.1 General
        Under fault conditions an earth electrode is raised to a potential with respect to
        the general mass of Earth that may be calculated from the prospective fault
        current and the earth resistance of the electrode; this results in the existence of
        potential differences in the ground around the electrode that might be
        damaging to telephone and pilot cables, whose cores are substantially at earth
        potential. Such a risk should be considered mainly in connection with large
        electrode systems, as at power stations and substations. The potential gradient
        over the surface of the ground is should also be considered because personnel
        or livestock can be in contact with two points sufficiently far apart that the
        potential difference constitutes a danger to life; cattle are most at risk.
        Figure 16 may be used to predict the ground surface potential gradient around
        an isolated 3 m rod electrode. The high potential gradient within the first few
        centimetres of the rod is clearly seen and, depending on the electrode voltage,
        a barrier to prevent contact by personnel or livestock may be advisable out to a
        radius of 1 m or 2 m.
        NOTE Further recommendations on reducing this gradient by placing the top of the
        electrode below ground level are made later in this Clause.The potential gradient on a perpendicular to a line of three electrodes may be
        taken from Figure 16; this shows that, for a given electrode voltage, although
        the initial gradient within about 2 m of the rods is slightly reduced, the general
        effect at greater distances is to increase both the ground potential and the
        gradient. However, for a given fault current the multiple electrode arrangement
        may be used for a lower resistance compared to a single electrode and results in
        considerably smaller ground potentials and gradients than those of a single
        electrode.
        The distribution of ground surface potential in the vicinity of a horizontal
        electrode (wire or strip) may be assumed to have gradients much smaller than
        those of a vertical electrode, the highest gradient occurring at a distance from
        the electrode equal to its depth. Likewise, the potential gradients between
        horizontal electrodes may be assumed to be smaller. Depth of laying, within the
        range normally used, may be assumed to have little effect on ground surface
        potential.
        It should be borne in mind that the ground potential contours associated with
        installations of more than one electrode consist of isolated areas around each
        electrode where the potential and its gradient are high and possibly dangerous;
        between these areas there are troughs of minimum potential which can
        nevertheless have a value which presents a danger to a person or animal
        touching conducting items in contact with either the electrode or the mass of
        earth at a more remote location, e.g. a metallic fence.
        Such variations in ground potential within and close to an installation may be
        considered unacceptable and measures to reduce them might be necessary.
        These measures may be more conveniently implemented by the use of
        additional electrodes in the form of buried horizontal strip or wire,
        possibly 0.25 m to 0.6 m below the surface (see Clause 5 for further comment on
        the laying of such electrodes).
        The large fraction of the electrode potential developed over the few
        centimetres of soil next to the electrode indicates how important it is to
        consider the resistivity of the material in this region. In high resistivity soils, for
        cases where the use of additional rods is not practicable and it is desired to
        reduce the electrode resistance by soil treatment or by replacement with
        concrete, only the soil in this region should be considered. Since soil conductivity
        is affected by soil density (except for very wet soils), good compaction, or lack of
        disturbance, of soil in contact with the electrode is important.
        Earth electrodes should not be installed close to a metal fence, unless they are
        used for earthing that fence; this is to avoid the possibility of the fence
        becoming live and thus dangerous at points remote from the substation, or
        alternatively giving rise to danger within the resistance area of the electrode by
        introducing a good connection with the general mass of the earth.
        In rural areas, danger to animals in the vicinity of earthed poles may be
        minimized by the use of insulated earthing connections and by having no earth
        electrode less than 0.6 m below the surface of the ground.

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