|EQUIPMENT FOR MINING, OIL AND CHEMICAL, INDUSTRIES
Methods of Safe Communication in Gas or Vapour Hazards
In coal mines, where the natural gas methane (fire damp) may occur, and in oil refineries, chemical plants, gas works, hospitals or any other place where a flammable gas or vapour may be present, it is a statutory requirement that safeguards be applied to electrical equipment to prevent ignition of the gas or vapour and consequent explosion or fire.
Two systems of protection against such dangers have been evolved: Flameproof (FLP) and Intrinsically Safe. A brief explanation of the difference between these two systems may be helpful; they are accepted as normal practice in coal mining, but the need for them in industry is strictly localised and those responsible for the application of safeguards have less in the way of accepted procedure to guide them. Neither system provides absolute safeguards: each demands correct installation, followed throughout the working life of the devices by strict observance of procedures ensuring safety.
It is necessary to realise at the outset that the two systems are quite separate and distinct: they must not be confused, nor may they be mixed. Any one installation must be either FLP throughout or intrinsically safe throughout. Occasionally a particular instrument or piece of apparatus may be made suitable for both systems in order to reduce cost of production. Such cases are rare and it must be appreciated that these exceptional dual-purpose instruments should be considered only in relation to the particular system in which they are being used.
The Flameproof Method
British Standard 229 gives this definition: A flameproof enclosure for electrical apparatus is one that will withstand, without injury, any explosion of the prescribed flammable gas that may occur within it under practical conditions of operation within the rating of the apparatus (and recognised overloads, if any, associated therewith) and will prevent the transmission of flame such as will ignite the prescribed flammable gas which may be present in the surrounding atmosphere." The first requirement is met by the provision of a substantial case, the second by ensuring that joints and other openings (such as bearings) provide a sufficiently long and restricted flame path to prevent external ignition.
In practice, a minimum length of path is specified, and the maximum safe gap between the faces of joints depends upon the category of gas for which the apparatus is approved. Gases and vapours have been classified, in order of their risk, into four groups, for the first three of which a maximum gap is specified (B.S. 229, Table 1). Group I includes only methane (fire damp) and is applicable particularly to coal mining. Group II covers many of the vapours of the oil and paint industries; Group Ill includes such hazards as municipal gas, and Group IV contains those gases for which type approval is not possible, the maximum permissible gap being too small to be practicable for manufacture. Where the use of a particular item is not restricted to the coal industry it is usual for the manufacturer to seek approval for its use in situations involving all three groups of gases; the maximum permissible joint-gap will therefore be that prescribed for Group Ill.
The provision, by the manufacturer, of apparatus certified as safe, is only the first step in the ensurance of safety. Such apparatus must be installed carefully to preserve those features on which safety depends the wiring has to be in seamless conduit, armoured or other approved cable, properly attached to the terminal chamber and sealed either by filling with compound or by a cable sealing gland, whichever is appropriate. The conduit or cable run needs to be carefully planned to avoid mechanical damage, which might break or cut a conductor and thus give rise to a risk of ignition. Throughout its working life a system must be so inspected and maintained that lines and apparatus remain fully safe. It is essential, when examining apparatus, to ensure that any enclosure is 'dead' before it is opened; similarly lines must be either isolated in sections or totally disconnected from all other circuits before any test is made. When any case or chamber is re-closed, care must be taken to ensure that the flame path gap or flange surfaces are scrupulously clean, that the cover bears smoothly and evenly on the body and the screws are tightened evenly so that there is no distortion of the cover. With gaps of the order of 0.005 in. quite a small foreign body or uneven tightening can introduce an element of risk, and it is a wise precaution always to test with a feeler gauge. For industries other than coal mining, British Standard Code of Practice CP.1003 is a guide to good practice.
Though B.S. 229 is not intended to apply in detail to telecommunication apparatus, the latter follows similar principles of design, which restrict the scope for reducing size and weight.
The factors involved in making a circuit intrinsically safe can be complex or fairly simple, according to the type of circuit and its components. Broadly speaking, safety is achieved partly by limitation of current and voltage, partly by diverting or absorbing some of the energy released on the collapse of magnetic field when an inductive circuit is broken. Inherent or added resistance is used to restrict the output of the source of energy, inductive windings may have ohmic or non-linear resistors, or rectifiers in shunt, and other direct current devices may be fitted with a copper sleeve or equivalent device. Except in a few special cases, such as the magneto calling generator and some testing instruments, the permissible voltage rarely exceeds 25 volts, the supply used for most purposes being equivalent to the old standard 17-cell 3-pint Leclanche battery. In actual practice, normal installation methods are usually followed: in many coal mines armoured cable is chosen because of its greater robustness, but design is as for bare line wires which may be broken or short circuited at any point.
Though the low energy requirements obviously prevent the use of this method for the protection of most power and lighting circuits, it is eminently suitable for telephone and bell systems. It avoids many of the serious difficulties of installation and maintenance associated with flameproof equipment in awkward situations or where constant extension of a line is needed.
Protective measures necessarily entail a slight loss in operating efficiency, but this is usually so small as to be imperceptible by the standards of performance of similar standard apparatus; speech is not affected and only on heavily loaded circuits is any difference in ringing observed; in any case, modem practice is to eliminate circuits employing large numbers of instruments in parallel.
In British coal mines, telecommunication and signalling circuits must be intrinsically safe; apparatus designed for such circuits cannot be certified as FLP in Group I.
No precautions beyond those required by normal good practice are needed in installation; maintenance also follows standard practice. Repairs involving any electrical part of the equipment, such as coils, resistors, generators, must be effected either by returning the whole instrument to the maker, or by replacing the faulty component by an identical part obtained from the manufacturer for that purpose.
It is well to realise that when testing either protective system on site, either intrinsically safe testing instruments must be used or the area must be proved gas or vapour free before testing.
Choice of System
In industrial applications, especially where working conditions are good, supervision informed and responsible, and inspection and maintenance meticulous, FLP will permit freer use of automatic and c.b. telephone systems. Public or main exchange lines may, in FLP devices, be taken direct into an industrial danger zone instead of through an intrinsically safe coupling device. The need for careful supervision and maintenance of FLP equipment must be stressed, for any slight defect of lines, of couplings to terminal chambers or of the flameproof gap at once introduces a serious hazard. FLP equipment can only be certified for gases in Groups I, II and Ill, and its use is not permitted with gases of Group IV. Intrinsically safe equipment can however be certified for use in certain gases in Group IV.
In the intrinsically safe system, which is based on the characteristics of the electric circuit elements, a fault does not necessarily make the circuit potentially dangerous: line breaks and short circuits are harmless; so also is any series disconnection external to the instrument, and safety shunts fitted internally are usually so arranged that their failure leaves the circuit safe. Safety factors are not usually confined to one element, but are spread through a circuit, so that failure in one should not make the circuit actively dangerous. Further, an electrical failure affects operation and so is often self indicating. These considerations do not minimise the need for a high standard of maintenance, but they are factors which weigh heavily when apparatus is installed in conditions of reduced accessibility or poor lighting.
Contents of a Typical Certificate
Certificates of Intrinsic Safety for telephone and signalling apparatus in firedamp (methane) are issued by the British Ministry of Power. Office desk telephones and similar items of comparatively light construction, which are unsuitable for use below ground, are certified "for surface use under cover".
Certificates of Intrinsic Safety for apparatus in gases or vapours other than methane are issued by the Factory Department of the British Ministry of Labour.
Certificates of Flameproofness in any gases or vapours are issued by the Ministry of Power.
Classification of Gases and Vapours
Groups II and Ill are being continually extended as knowledge of ignition by flame is amplified by research.
B.S. 1259 relating to Intrinsically Safe apparatus and circuits classifies industrial gases and vapours in four groups as shown on page 8. For mines applications, the apparatus is tested in a mixture containing approximately 8.3 per cent (by volume) of methane in air. For applications in the petroleum industry, the testing mixture is 3.9 per cent of pentane in air. Ethylene and. hydrogen are used as representative testing media for gases or vapours more combustible than pentane.
GROUPING OF GASES AND VAPOURS FOR FLAMEPROOF CERTIFICATION
Note: The grouping of AMMONIA is under consideration. In the meantime, for situations where ammonia may be present, enclosures constructed in accordance with the requirements for Group I are deemed to be appropriate.
CLASSIFICATION OF GASES AND VAPOURS
Last revised: December 18, 2010