Area Classification

Area Classification and Selection of Electrical Equipments By R.K.Sinha, DGM (Electrical),ONGC Ltd.,Mumbai Mumbai High Asset INTRODUCTION In most of the industrial plants such as Chemical plants, Refineries, Crude Oil & Gas processing plants, hazardous areas exist due to the presence of flammable gases . While designing such plants the first step is to finalize the layout of facilities and equipments. Area Classification drawings are then developed based on the layouts. The purpose of the area classification drawings is to clearly identify the hazardous and safe areas in the plant . For safe operations all electrical equipments must be selected keeping in view their area of application in the plant. It goes without saying that electrical equipments designed for hazardous areas are much costlier than the ordinary ones and hence the plant facility layout has a considerable bearing on the capital expenditure. SOME DEFINITIONS We know that for a fire to take place three things are essential : 1. Presence of fuel such as flammable gas 2. Presence of oxygen 3. Presence of heat ie. sufficient ignition energy to ignite the flammable mixture While discussing hazardous areas ,it is useful to have the following relevant definitions in mind : • Ignition Temperature : The lowest temperature at which ignition occurs in a mixture of explosive gas and air. • Flash Point : The temperature at which the liquid gives so much vapour,that this vapour, when mixed with air,forms an ignitable mixture. • Explosive Limits : The extreme values for the concentration of a flammable gas or air under atmospheric conditions, which can be ignited by an electric arc or spark. STANDARDS Most countries have developed their own standards and codes for Area Classification but internationally two main standards are being followed.These are : • The North American standards produced by the API and NFPA. • The International Electrotechnical Commission (IEC) standards. • API RP 500 is used in the United States whereas IEC 79 is popular in Europe. Area classification basically covers two aspects : 1. The probability of flammable atmosphere in the area 2. The type of gases / chemical vapours involved The North American API RP 500 defines the above two aspects as Divisions and Gas Groups whereas in IEC 79 they are termed as Zones and Gas Groups.

NORTH AMERICAN METHOD CLASSES The type of flammable material is classified as follows : Class 1 :Flammable gases or vapors are present in the air in quantities sufficient to produce explosive or ignitable mixtures. Class 2 : Combustible or conductive dusts are present. Class 3 : Ignitable fibres or flyings are present, but not likely to be in suspension in sufficient quantities to produce ignitable mixtures. (Group classifications are not applied to this class.) In the petroleum industry we are concerned with Class 1 mainly. DIVISIONS Class 1, Division 1 Location : A location in which ignitable concentrations of flammable gases or vapors are expected to exist under normal operating conditions or in which faulty operation of equipment or processes might simultaneously release flammable gases or vapors and also cause failure of electrical equipment. Class 1, Division 2 Location : A location in which flammable gases or vapors may be present, but normally are confined within closed systems; are prevented from accumulating by adequate ventilation; or the location is adjacent to a Division 1 location from which ignitable concentrations might occasionally be communicated. GAS GROUPS GROUP A GROUP B GROUP C GROUP D GROUP E GROUP F GROUP G

Atmospheres containing acetylene. Atmospheres containing hydrogen Atmospheres containing ethyl ether vapours, ethylene or equivalent gases Atmospheres containing gasoline, naptha, propane, acetone, natural gas or equivalent Metal Dust Coal Dust Grain Dust

EUROPEAN METHOD ZONES ZONE 0

ZONE 1

ZONE 2

NON-HAZARDOUS AREA

An area in which hazardous atmosphere is continuously present. In oil industry such a condition exists in confined spaces, such as the vapour space of closed process vessels, storage tanks etc. An area where explosive gas and air mixture is continuously present for a long period or is likely to occur in normal operation. An area in which explosive gas and air mixture is likely to occur only under abnormal operating conditions. For example : Gas Turbine enclosures. Areas not falling under Zone 0,1 or 2 are

considered as safe areas. Oil and gas pipelines laid in the open outside hazardous areas and that do not have any flange joints,which cannot become loose, are considered safe area. GAS GROUPS GROUP 1 GROUP 2A

GROUP 2B GROUP 2C

Covers gases produced in coal mines (mainly fire damp methane). Atmospheres containing acetone, ethane, hexane, ethyl acetate, ammonia, benzene, butane, diesel, propane etc. Atmospheres containing ethylene, town gas, ethyl ether etc. Atmospheres containing hydrogen, acetylene, ethyl nitrate, carbon disulphide.

In addition to the zones ( defining probability of occurrence of flammable mixture) and Gas Groups ( defining type of flammable gas) , the European Standard also has a Temperature Classification . • The external surfaces of explosion proof equipment must not exceed the temperature whereby they may be liable to become source of ignition for the surrounding atmosphere. • According to ignition temperature gases and vapours are divided into six temperature classes as follows : T1 450 deg C T2 300 deg C T3 200 deg C T4 135 deg C T5 100 deg C T6 85 deg C The ignition temperature of natural gas is approximately 480 deg Celsius. However, it goes down considerably if H2S is present. TYPES OF PROTECTION FOR ELECTRICAL EQUIPMENTS Electrical equipments are designed and manufactured with the following types of protections : Type ‘d’ (Flameproof) The enclosure will withstand an internal explosion of the flammable gas, which may enter it, without suffering damage and without communicating the internal flame to the external flammable atmosphere through any joints or structural opening in the enclosure. Type ‘e’ (Increased safety) A method of protection by which additional measures are applied to provide increased safety against the possibility of excessive temperatures and

Type ‘i’ (Intrinsically safe)

Type ‘p’ (Pressurized)

Type ‘n’ (Non-sparking)

Type ‘o’ (Oil Immersed)

Type ‘q’ (Sand filled)

Type ‘s’ (Special)

of the occurrence of arcs or sparks during the service life of the apparatus. A protection technique based upon the restriction of electrical energy within apparatus and the interconnecting wiring,exposed to a potentially explosive atmosphere, to a level below that which can cause ignition by either sparking or heating effects.Devices whose electrical parameters do not exceed any of the values 1.2 V,100mA,20 J or 25 mW. A method of protection using the pressure of a protective gas to prevent the ingress of an external flammable atmosphere to a space which may contain a source of ignition. A type of protection applied to an electrical apparatus such that,in normal operation,it is not capable of igniting a surrounding explosive atmosphere , and a fault capable of causing ignition is not likely to occur. A method of protection where the enclosure is made safe by oil-immersion in the sense that flammable gases or vapours above the oil or outside the enclosure will not be ignited. A method of protection where the enclosure of electrical apparatus is filled with a powdery material such that, if an arc occurs, it will not be able to ignite the external flammable atmosphere. Special methods of protection which may be a combination of above methods

SELECTION OF ELECTRICAL EQUIPMENTS Zone 0 Only Type ‘ia’ (Intrinsically safe) and some special type ‘s’ allowed. Zone 1 Apparatus suitable for Zone 0 and Intrinsically Safe ‘ib’,Flameproof (‘d’),Oil Immersed (‘o’),Pressurised (‘p’), Sand Filled (‘q’). Zone 2 Apparatus for Zone 0 & 1 plus Increased Safety (‘e’),Non-sparking (‘n’) allowed. INGRESS PROTECTION To complete the subject it is also worthwhile to touch upon the topic of Ingress Protection for enclosures of electrical equipments and switchgear. Ingress protection defines the level of protection of the enclosure against the ingress of solids and liquids. It is denoted as IP followed by two digits eg. IP 55. Here the first digit

specifies protection against ingress of solids whereas the second digit specifies protection against ingress of liquids. The following tables provides the details : SOLIDS 0 1 2 3 4 5 6 LIQUIDS 0 1 2 3 4 5 6 7 8

No protection Protected against solid objects upto 50 mm (eg. Hands) Protected against solid objects upto 12.5 mm (eg. Fingers) Protected against solid objects upto 2.5 mm (eg. Tools) Protected against solid objects over 1 mm (eg. Wires) Protected against dust (No harmful deposits) Totally protected against dust.

No protection Protected against vertically falling drops of water Protected against water spray upto 15 deg from the vertical Protected against water spray upto 60 deg from the vertical Protected against water spray from all directions Protected against water jets from all directions Protected against strong water jets from all directions Protected against immersion upto 1 Mtr depth. Protected against lengthy immersion under pressure.

SOME IMPORTANT AGENCIES API AMERICAN PETROLEUM INSTITUTE NFPA NATIONAL FIRE PROTECTION ASSOCIATION IEC INTERNATIONAL ELECTROTECHNICAL COMMISSION UL UNDERWRITERS LABORATORY CENELEC EUROPEAN COMMITTEE FOR ELECTROTECHNICAL STANDARDIZATION BASEEFA BRITISH ASSOCIATION OF SAFETY ON ELECTRICAL AND ELECTRONICS FLAMEPROOF APPARATUS DGMS DIRECTORATE GENERAL OF MINES SAFETY CMRI CENTRAL MINING RESEARCH INSTITUTE

SOME IMPORTANT STANDARDS API RP Recommended practice for Classification of Locations for Electrical 500 Installations at Petroleum facilities Classified as Class 1,Division 1 and Division 2. API RP Recommended practice for Classification of Locations for Electrical 505 Installations at Petroleum facilities Classified as Class 1,Zone 0,Zone 1 and Zone 2. API RP Recommended Practice for Design and Installation of Electrical Systems 14 F for Offshore Petroleum Facilities. IEC 79- Classification of Hazardous Areas 10 IS 5572 Indian Standard for Classification of Hazardous Areas

CONCLUSION It is imperative that all of us engaged in the pursuit of oil and gas clearly understand the disastrous implications of an electrical equipment wrongly selected or improperly maintained. Even if we select world class equipment/apparatus , it will not reduce the risks if not well maintained. So the onus is on the electrical maintenance engineers ( a thankless and much hassled community) to beware of each missing bolt from a flameproof equipment or a worn gasket in a field junction box. And my dear brethren (read electrical maintenance engineers) do we not know that 90% of the fires in the industry are thrust in our laps as caused by electrical short-circuits. After all, any fire burns out cables and wires also ,doesn’t it ? Very difficult to establish the cause and the effect later. So why give the world a chance ? Let us resolve to mitigate the risks with our knowledge, skills and commitment.