hazardous areas

Course 9050 - October 1996 Principles of Instrumentation and Control

Hazardous Areas 15 - 1

HAZARDOUS AREAS

Without danger we cannot getbeyond danger.

- Proverb

Principles of Instrumentation and Control Course 9050 - October 1996

15 - 2 Hazardous Areas

SynopsisThe definition of a hazardous area, conditions for an explosion and a classification ofcombustible materials precede a more detailed discussion of the properties of flammablegases, vapours, mists and dusts.

Ignition sources are touched upon and techniques for explosion-protection surveyed.The SAA classification of Hazardous Areas is overviewed in tabular form and again inmore detail with reference to international standards and terminology.We also look at the role of the SAA and the legitimacy of overseas standards.A full treatment of electronic barrier selection and usage, together with examples ofequipment markings, conclude the section.

v v v v v v vv v v v v v vv v v v v v vv v v v v v vv v v v v v v



Introduction

Hazardous Area

An area in which explosive gas/air mixtures are, or may be expected to be, present inquantities such as to require special precautions for the construction and use of electricalapparatus.

Hazardous Location (Classified)That portion of a plant where flammable or combustible liquids, vapors, gases or dustsmay be present in the air in quantities sufficient to produce explosive or ignitable mixtures.

There is a danger of an explosion or fire occurring wherever combustible materials are handled.This is graphically illustrated by the dreadful toll taken by coal mining explosions in the past . Forexample in England in the last century hundreds of lives were lost every year. In 1866, in a seriesof explosions in the Oaks Colliery, 361 people were killed in one accident alone.

The hazard today exists not only in the coal mining industry but also in many other industries.Major industries include the petrochemical, chemical, sewerage treatment, and grain handlingindustries; while smaller industries involve such areas as paint shops and dry-cleaning premises. Inmany cases the hazards occur in areas frequented by the public, for example petrol service stations.

In all these situations, electricity is used.

To prevent any of the electrical equipment becoming a source of ignition for an explosion, specialprecautions have to be taken in the design, construction, and installation of such equipment.

Hazardous Areas

The above are examples of what can be termed hazardous areas.

A hazardous area is defined as an area in which an explosive atmosphere is present, or may beexpected to be present, in quantities such as to require special precautions for the construction,installation, and use of potential ignition sources.

The explosive atmosphere may be caused by the presence of a flammable liquid or vapour or by thepresence of combustion dust in suspension or in layers.

In the design of industrial plants, every effort is usually made to minimise the extent of hazardousareas but it may be difficult to ensure that an explosive atmospherewill never occur.

Conditions For An Explosion

Three basic conditions must be present for a fire or explosion tooccur:

(a) A combustible material must be present in sufficientquantities.This can be a liquid, vapour, mist, gas, dust, fibresor flyings.

(b) The combustible material must be mixed with air or oxygenin proportions needed to produce an explosive mixture.

Fig. 15.1



(c) An ignition source of sufficient energy to ignite the explosive mixture must be present.The above may be defined by The Infernal Triangle.

Combustible Materials

Combustible materials which may lead to an explosive atmosphere comprise the following:

(a) Flammable liquids having a flashpoint of not more than 61C. Examples are petrol, kerosine,acetone, ethyl alcohol and paint thinner.

(b) Flammable vapours. The vapours from a flammable liquid constitute a flammable vapour.(c) Flammable gases. Examples are hydrogen, methane, liquid petroleum gas and natural gas.(d) Flammable mists. Droplets of flammable liquid may be dispersed in air so as to form an

explosive atmosphere

(e) Combustible dusts. Examples are the dusts from grain, sugar, wood, starch, coal, aluminium,and polypropylene.

(f) Fibres. Fibres are characterised by flexibility, fineness and high ratio of length to thickness.(g) Flyings. Flyings are waste fibres which fly out into the atmosphere during carding, drawing,

spinning, and other similar processes.

Properties of Combustible Materials

General

There are a number of properties of combustible materials which must be considered when thedegree of risk associated with a particular installation is being assessed.

The following describes briefly the relevant properties of combustible materials and how theserelate to the type and extent of a hazardous area. The properties discussed here are based on thebehavior of the most common materials and do not take into account unusual materials.

Gases, Vapours and Mists

Vapour pressure. Molecules in a liquid are in continual motion. Near the surface of the liquid theymay have enough energy to escape to form a vapour. This is called evaporation. For any quantityof liquid evaporating in an enclosed space some molecules will condense into the liquid. The rateof evaporation depends on the temperature, while the rate of condensation depends on the numberof molecules present per unit volume in the space immediately above the liquid.

A condition of equilibrium will be reached when the number of molecules returning to the liquidequals the number leaving it. The space is then said to be saturated, and the pressure exerted by thevapour is the (saturation) vapour pressure. At any given temperature the vapour pressure will varyfrom liquid to liquid, thus determining their volatility. For instance, at typical room temperatures,motor spirit has a high vapour pressure, and evaporates rapidly. This is in contrast with keroseneand diesel oil (distillate) which have respectively lower vapour pressures and lower evaporationrates (i.e. are less volatile).



For each liquid, the actual vapour pressure depends only on temperature; typically increasing by afactor in excess of 1.5 for each 10 degrees Celsius rise. Therefore, quite a small temperaturechange can cause a dramatic difference to the quantity of vapour that can be present in the air. Inturn, this can have a significant effect on the practical hazard of a material.

Where the vapour condenses as clouds in the air it is known as a mist. A mist will generally reverteither to a vapour or to a liquid. Therefore mists are often not considered as a separate entity whenassessing hazardous areas.

Boiling point. A liquid will boil when its vapour pressure equals the external pressure. The boilingpoint of a liquid is the temperature at which this occurs at standard atmospheric pressure - 101.3kPa.

A compound will normally exist as a gas if its boiling point is below normal ambient temperature.

Flashpoint. This is the lowest temperature at which, under certain standardised conditions, amaterial gives off sufficient vapour to form an explosive gas/air mixture in the air immediatelyabove the surface.

Flashpoint data are normally associated with liquids, although there are certain solids which giveoff sufficient vapour to form explosive mixtures with air. For these materials, and those whichsublime, i.e. pass from solid to vapour without normal intermediate liquid phase, flashpoint dataare associated with the materials in the solid form.

Ignition temperature. The ignition temperature of a solid, liquid or gas is the minimum temperatureat which the compound will ignite and sustain combustion when mixed with air, without initiationof ignition by spark or flame. The ignition is due to chemical reactions initiated by the temperatureof the local environment, and may therefore in practice be a result of the temperature of hot surfacesadjacent to the combustible material.A direct result of established ignition temperatures is the limitation of surface temperatures ofequipment in hazardous areas.

Explosive limits. Before an explosion can occur there must be a mixture of the flammable gas orvapour with air. Such a mixture iscapable of exploding only when itsconcentration lies within certainlimits.

These limits are known as the lowerexplosive limit (LEL) and the upperexplosive limit (UEL) and areexpressed as percentages of thematerial mixed with air by volume.

The range of mixtures between theLEL and the UEL is the explosiverange. Gas mixtures outside this rangeare non-explosive or non-flammableunder normal atmospheric conditions.

Relative vapour density. The relativevapour density of a gas is the mass ofgiven volume of pure vapour of gas Fig. 15.2



compared with the mass of the same volume of dry air, at the same temperature and pressure.

A vapour density less than 1, i.e. lighter than air, indicates that the gas or vapour will rise in acomparatively still atmosphere. A vapour density greater than one, i.e. heavier than air, indicatesthat the gas or vapour will tend to sink and may travel at low levels for a considerable distance.

Care should be taken in the application of vapour densities where they are in the range of about0.75 to 1.25. Gases or vapours in this range, particularly if released slowly, may be rapidly dilutedto a low concentration and their movement will be similar to that or the air in which they areeffectively suspended.

In practice, there are relatively few flammable gases and vapours with densities below 1.25 andthese are shown in the table. Consequently, the vast majority of flammable substances are in factheavier than air.

Minimum ignition energy. A certain minimum energy, which differs from one gas to another, isrequired for an explosion to occur. If a source of ignition, such as a spark, has an energy below thisit cannot cause an explosion.

The minimum ignition energy of a gas is the minimum energy required to ignite the most easilyignitable mixture of that gas. The minimum ignition energies of gas are typically in the range of0.019 mJ (for hydrogen) to 0.29 mJ (for methane).

Fig. 15.3



Table 15.1Summary

The distinction between gases, vapours and mists can generally be drawn as follows:

(a) A gas will not occur in liquid form at normal temperature and pressure as it is above itsboiling point.

(b) A vapour may be in contact with its liquid phases at normal temperature and pressure.(c) A mist is a cloud of condensed vapour. Generally it will revert either to a vapour, e.g. when

it touches a warm surface, or to a liquid. Thus it is not often considered as a separate entitywhen assessing hazardous areas.

Dusts

General. Combustible dusts are those dusts which are combustible or ignitable in mixtures withair. Inherently explosive dusts (such as gunpowder, propellant powder and lead styphnate) whichrequire only a specific level of energy for ignition, are not taken into consideration here. Suchdusts are hazardous whether airborne or not.

NOTE: Combustible dusts include dusts, fibres and flyings.



Dust explosions may be initiated by ignition of either a layer of dust or a cloud of dust. It is notunusual for an explosion which starts from the ignition of a dust layer to cause the dislodging ofdust accumulated on various surfaces, which then leads to a dust cloud explosion.

Layer ignition temperature. The layer ignition temperature, previously called the glowtemperature, is the lowest temperature at which a heated surface can ignite a layer of dust.

Cloud ignition temperature. This is the lowest temperature at which a dust cloud ignites. Formost, but not all dusts, this temperature is lower than the layer ignition temperature.

Minimum ignition energy. This is the minimum energy required to ignite a dust cloud. The lowestpublished ignition energy for combustible dusts is in the order of 5 mJ, although it may be possibleto achieve lower values under certain experimental conditions. In comparison, hydrogen has aminimum ignition energy of 0.019 mJ.

Other factors, such as particle size, moisture, inerts and resistivity may also need to be consideredin determining the degree of hazard involved. For instance, irregularly shaped particles producedby milling have a high area-to volume ratio. This means that they are more easily ignited andrepresent a more severe explosion hazard than spherical particles, such as those produced by spraydrying.

The presence of inert dusts reduces the rate of pressure rise and increases the minimum dustconcentration. The use of limestone dust in coal mines is an example of the practical application ofinerts.

Table 15.2



Ignition Sources.

An ignition source is a source of energy sufficient to ignite an explosive atmosphere. Commonsources of ignition are flames, welding and cutting operations, electrical and mechanical sparking,hot surfaces, glowing and smouldering combustion and spontaneous heating.

Sources of ignition caused by electrical means may be divided into two types as follows:

(a) An energy source such as electrical sparks and arcs.(b) A hot surface, e.g. the surface of an electric motor, solenoid or light fitting.These types of ignition sources are directly related to the minimum ignition energy and the ignitiontemperature respectively of the material concerned.

The first method involves the exclusion of the hazardous material, be it gas or dust, from theequipment so that a spark or hot surface inside the equipment cannot cause ignition. This is achievedbe sealing the equipment enclosure, by enclosed devices, or by filling the equipment with somesubstance which may be solid liquid or inert gas.

The second method aims to contain an explosion, if it does occur, in the equipment enclosure. Aflameproof enclosure is probably the best known and most widely used of all techniques, but isonly appropriate for gas hazards.

The third method uses energy limitation. Flammable gases and combustible dusts have minimumignition energies, below which it is not possible for a spark or arc to cause an explosion. If theenergy in an electrical circuit can be maintained below these levels, it cannot cause an explosion,Intrinsic safety is the most common technique used to achieve this.

The fourth method involves dilution of a hazardous gas atmosphere below LEL by ventilation. Itis not appropriate for combustible dust areas.

The last method aims to prevent an ignition source from occurring. The most common techniqueis increased safety. This is used for equipment, or parts of equipment, such as terminal boxes, thatdo not arc or spark in normal service.



Table 15.3



Table 15.4



International Standards

There are two basic groupings of standards: North American and elsewhere (IEC and ISO).Nomenclature differs between the two groups. It is valuable that people be completely familiarwith these differences and with terminology generally.

Australian standards use the terminology and definitions used by IEC (International ElectrotechnicalCommission).There has been a progressive move towards Internationalisation of electrical safety standards.Harmonisation is the term used. This move has been comparatively easy for new techniques suchas Intrinsic Safety, but is very difficult for techniques which have been in use for many years, suchas flameproofing.

CENELEC takes harmonisation one step further. Members of CENELEC not only use harmonisedstandards, but also accept other members test stations results. Hence an instrument tested say inItaly and certified as intrinsically safe will be certified without further testing in the U.K.

Australia is not a member of CENELEC.

Other important standards that are often seen in the Australian context is the U.S. NEMA standard.

Types of Protection

-More Detail

To avoid sources of ignition from electrical apparatus in hazardous areas due to sparks, arcs or hotsurfaces, the apparatus is constructed in the suitable type of protection.The largest area of applicationis for apparatus of protection type flameproof encapsulation. This apparatus has to have thefollowing properties:

* All joints leading outside have to be flameproof, i.e. the joint gaps and widths must be belowcertain values.

* The enclosure has to withstand an internal explosion without any remaining deformation

* The surface temperature of an enclosure must not exceed the ignition temperature of theambient gas-air mixture.

Type of protection pressurised apparatus prevents the explosive atmosphere getting into touchwith the source of ignition. This is achieved by maintaining clean air or a non-flammable inert gas

Table 15.5 Overview of Enclosure Standards Organisations



inside an enclosure at an over-pressure either with or without continuous flow of protective gas. Ithas to be ensured by means of monitoring and control devices that the electrical apparatus is turnedoff immediately in case of a pressure drop.

Type of protection intrinsic safety is only applicable where low energy is required. Thus themain application is in the measurement and control sector. According to European Standard andIEC intrinsically safe circuits are divided into categories ia and ib. Apparatus of category ib mustnot cause an ignition in case one failure occurs. For apparatus of category ia the same applies forany combination of two failures.

Oil immersion and powder filling are hardly used today.

Powder filling is however gaining more and more importance for the protection of electronicassemblies. Type of protection increased safety was developed in Germany and has woninternational recognition by being included in the European Standards as well as the IECRecommendations. Apparatus, where sparks or arcs or high temperatures can occur during normaloperation, cannot be this type of protection. So Increased safety is mainly applied for connectionand distribution technology. By means of constructional measures increased safety is achievedagainst the occurrence of failures, causing sparks or too high temperatures.

Terminals and electrical connections for instance are carried out in such a way that self-looseningand thus resulting sparks are not possible. Increased requirements also apply in regard to resistanceto tracking of the insulating materials as well as the necessary air and creepage distances. Furtherrequirements refer to the mechanical resistance and the dust and water protection. Often increasedsafety is combined with other types of protection. A special economical aspect is that the individuallyencapsulated switching elements can be built into and wired in an enclosure of increased safety.

A further type of protection moulding is to become part of the European standard.

Flameproof Enclosure

The parts, which can ignite and explosive atmosphere are placed in an enclosure, which can withstandthe pressure developed during an internal explosion of anexplosive mixture. This prevents the transmission of theexplosion to the explosive atmospheres surrounding theenclosure.

Applications: Switchgear, control and indicating equipment,control boards, motors, transformers, light fittings and otherspark-producing parts.Pressurised Apparatus

Entry of a surrounding atmosphere into the enclosure of the electrical apparatus is prevented bymaintaining inside the enclosure a protective gas (air,inert or other suitable gas) at a higher pressure than thatof the surrounding atmosphere. The overpressure ismaintained either with or without a continuous flow ofthe protective as.

Applications: As the above but especially for largeequipment and complete rooms.

Fig. 15.4

Fig. 15.5



Intrinsic Safety

The electrical apparatus contains intrinsically safe circuits, whichare incapable of causing an explosion in the surroundingatmosphere. A circuit or part of a circuit is intrinsically safe,when no spark or any thermal effect in this circuit, produced inthe test conditions prescribed in the standard (which includenormal operation and specified fault conditions) is capable ofcausing ignition.

Applications: Measurement and control equipment.Oil Immersion

The electrical apparatus or parts of the electrical apparatus areimmersed in oil in such a way that an explosive atmosphere,which may be above the oil or outside the enclosure cannot beignited.

Applications: Transformers (only used rarely now).Increased Safety

A type of protection in which measures are applied so as to prevent,with a higher degree of security, the possibility of excessivetemperatures and of the occurrence of arcs or sparks in the interiorand on the external parts of electrical apparatus, which does notproduce them in normal service.

Applications: Terminal and connection boxes, control boxeshousing Ex-modules (of a different type of protection) squirrelcage motors, light fittings.

Powder Filling

The enclosure of electrical apparatus is filled with a material in a finely granulated state so that, inthe intended conditions of service, any arc occurring within the enclosure of an electrical apparatuswill not ignite the surrounding atmosphere. No ignition shall be caused either by flame or byexcessive temperature of the surfaces of the enclosure.

Applications: Transformers, capacitors, heater strip connection boxes electronic assemblies.Moulding

A type of protection in which the parts which can ignite an explosiveatmosphere are enclosed in a resin sufficiently resistant toenvironmental influences in such a way that this explosiveatmosphere cannot be ignited by either sparking or heating, whichmay occur within the encapsulation.

Applications: Only small capacity switchgear, control gear,indicating equipment, sensors.

Fig. 15.6

Fig. 15.7

Fig. 15.8

Fig. 15.9



Local Authorities

When electrical equipment is to be used in a hazardous area, precautions which have to be takenagainst explosion are prescribed by the appropriate Australian State Authority which has responsibilityfor safety in the state. All these state authorities require that apparatus for use in hazardous areasmust be approved and/or certified. installation of the apparatus must comply with the mining orwiring regulations of the state in which the installation is located.

The Department of Mines in each state is responsible for regulations relating to electrical equipmentused in mines. Elsewhere, the Electricity Supply Authority has the equivalent responsibility.

In addition, the states Department of Labour and Industry has responsibility for regulations relatingless specifically to control of ignition sources in areas containing combustibles.

The electrical requirements for installations in mines are similar to those above ground, but belowground, there are additional requirements for the materials used in the manufacture of housing etc.Aluminium housings are not allowed below ground for instance.

The regulations of different states are very similar and are based on national (SAA) standards andcodes of practice where these exist. Australian standards cover explosion-protected electricalequipment for use in atmosphere containing gases, vapours and mists (Class 1) and dust excludingignition proof (Class 2).The S.A.A.

The Associations Committee on Electrical Equipment in Hazardous Locations (Committee EL/14) prepares standards, the committee is made up of representatives from industrial users, governmentauthorities, and manufacturing organisations.

Another committee which is similarly composed, Committee P/3, is responsible for certification ofelectrical equipment for hazardous locations. Committee P/3 will issue a certificate if the membersare convinced that a piece of apparatus conforms to the requirements of the relevant Australianstandard. The committee does NOT carry out any physical testing of equipment. Testing has to becompleted (by a separate test lab) before the committee considers certification.Testing

Committee P/3 recognises four test laboratories which can carry out testing to confirm that apparatuscomplies with SAA Standard.

- Department of Industrial Relations Londonderry Industrial Safety Centre N.S.W.

- British Standards Institution Technical Help for Exporters Service U.K.

- Factory Mutual Research (FM) Approvals Division Massachusetts U.S.A.- Underwriters Laboratories U.S.A.

It is important to recognise the distinction between accepting the results of an overseas test laboratoryfor conformance to Australian standards, and acceptance of overseas standards.

FM certified to US standards is valueless as far as Committee P/3 is concerned.

Committee P/3 meets every other month or so, and deals with certification expeditiously andefficiently.



SAA Standards

CLASSIFICATION OF HAZARDOUS AREAS

A FULLER DESCRIPTION

A hazardous area is one where an explosive atmosphere is, or may be expected to be, presentcontinuously, intermittently or due to an abnormal or transient condition.

Australian Standard AS2430 Parts 1 and 2 specifies 2 main classes of hazardous areas:

(a) Class 1 - Explosive Gas atmospheres(b) Class 2 - Areas which are hazardous because of the presence of combustible dust, fibres or

flyings.

(a) Class 1 - These areas are further divided into 3 zones as follows:(1) Zone 0 - Areas in which an explosive gas/air mixtureis continuously presentor present for long periods.

(2) Zone 1 - Areas in which an explosive gas/air mixture exists intermittently orperiodically under normal operating conditions and areas in which an explosivegas/air mixture may exist frequently because of leakage.

(3) Zone 2 - Areas in which an explosive gas/air mixture is not likely to occur andif it occurs it will exist only for a short time.

FURTHER SUBDIVISION INTO GROUPS OCCURS

Group I - Coal Mining

Group II - Other Industries

In these groups, limitations upon the surface temperature of the enclosure are imposed.

Group I 150C if coal dust can form a layer 450C if this risk avoided

Group II A range of temperature classes T1 - T6

(b) Class 2 - Hazardous dust flyings and fibresDivision 1

(1) Combustible dusts, fibres or flyings of an electrically conductive nature are present,regardless of particle size, or

(11) Electrically nonconductive combustible dusts, fibres or flyings of such fineness as to becapable of producing explosive mixtures when suspended in air, are present not insuspension but lying as settled dust, or which may be in suspension either continuously,intermittently or periodically under normal operating conditions in quantities sufficientto produce an explosive concentration, or where mechanical failure or abnormal operation



of plant may cause accumulations of such substances to be thrown into suspension inair in quantities sufficient to produce and explosive concentration.

Division 2

(1) Electrically nonconductive combustible dusts, fibres or flyings of such coarseness as tobe incapable of remaining in suspension in air in quantities sufficient to produce anexplosive concentration are present but where accumulation of such substances may besufficient to interfere with the safe dissipation of heat from electrical equipments, or

(11) Deposits of such dusts, fibres or flyings as may become susceptible to spontaneouscombustion or easy ignition due to carbonisation or excessive dryness resulting fromexposure to heat dissipated from electrical equipment.

Marking of Equipment

Correct marking on explosion-protected equipment is very important as it is the means of identifyingequipment and defining the hazardous areas in which the equipment may be safely used.

Information to be marked:

(a) The name of the manufacturer or his registered trade mark. This identifies the source of theequipment.

(b) Manufacturers type identification. This is normally a model number for the equipment andshould unambiguously identify the equipment.

(c) The symbol Ex followed by the letter (in lower case) which indicates the particular type ofexplosion-protection, which, for published Australian Standards, is as follows:

d - Flameproof enclosure

e - Increased safety

ia - Intrinsic safety, Category ia - higher risk zones, zone 0

ib - Intrinsic safety, Category ib - other zones

m - Encapsulation

n - Non sparking

p - Pressurised enclosure

pl - Purging

s - Special protection

v - Ventilation

EXPLOSION PROOF is an American equivalent to Flameproof, it is not a term that isrecognised by SAA. Do not use the term Explosion Proof.



Table 15.6

EXAMPLES OF MARKING

The following are examples of marking that would be acceptable:

Ex d IEx d IIB T5Ex d IIB T4 (TAMB = 50C)Ex d I/IIC T3Ex d e IIB T2Zone 1 Ex s IIB T5Ex ia IIC T6Ex (ib) IIBEx d (ib) IIC T3Ex d e (ia) IIB T4Ex m IIC T5 IP65

Note: E prefix (eg E ExdI) denotes CENELEC Approval (not used in Australia).Certification Exemptions (Simple Devices)Devices which never exceed 1.2V, 0.1A. 20uJ or 25mW need not be certified nor marked.

This category includes such things as RTDs thermocouples, pH electrodes, etc.

However if they are connected to other devices then the system as a whole has to comply withnormal standards.

Entity concept vs Integrated Systems

Entity concept equipment. The associated electrical equipment is connected to the intrinsically safeelectrical equipment in the hazardous area by cables. Each item of equipment is certified separatelyand the equipment to which each one may be connected may vary. Hence, not only the cableparameters but also the total parameters of the circuit to be connected must be defined for each itemof equipment.



Integrated systems. The associated electrical equipment is connected to the intrinsically safe electricalequipment in the hazardous area by cables. As these cables have both capacitance and inductance,they can store energy and hence their relevant parameters must be defined.

What barriers Do I Use

There is an increasing number of process plants handling potentially explosive materials such asmethane gas and petroleum.

This, together with stringent laws enforcing safe practice in factories , such as the Health andSafety Act means that safety techniques have to be applied where electrical equipment is introducedinto the process area.

In the field of low power electrical equipment (about 0.5W), for example process controlinstrumentation, the accepted technique is intrinsic safety (IS).This is an electrical circuit design technique, where under normal and fault conditions the circuit is

Fig. 15.10

Fig. 15.11



incapable of producing incendive sparks. Unlike other safety techniques, however, this conceptrelies on the design of the circuits in the safe as well as the hazardous area.

Ignition Curves

The basic design technique is to establish the gas in which the equipment is intended to be used,(for example hydrogen), and using a set of ignition curves, establish the safe currents and voltagesfor the circuit.

Where reactive components are involved it is also necessary to find, using other curves, the safecurrents and voltages for inductors and capacitors respectively. This is necessary because they canstore energy over a long period and release an incendive level in a short time.

Fig. 15.12

Fig. 15.13



Shunt Diode Safety Barriers

As stated above, it is necessary to consider the equipment in the safe area as well as in the hazardousarea.

Before the advent of barrier devices it was necessary to assess, and certify if necessary, all equipmenton the secondary of the mains transformers connected to the hazardous area apparatus. With theintroduction of computers, this was becoming an impossible task.

A device was therefore designed in the early 1960s that would limit the current and voltage to safelevels and, at the same time maintain these levels even when 250V a.c. was applied to its safe areaconnections.

The practical realisation of this circuit is shown in Fig. 15.13. The circuit is simple to understand.The voltage to the hazardous area is limited by D2 and the current limited by R2 in conjunction withD2.

The fuse/diode thermal characteristics are matched so that even when the application of 250V toterminals 1 and 2 while the fuse is blowing, the zener diodes limit the voltage to the hazardous area.

Two diodes are included in the circuit for redundancy purposes, and R1 limits the transient currentinto D2 while the fuse is blowing.Barrier Characteristics

Safety barriers are described by the maximum output voltage (D2) and the minimum output resistorvalue (R2).It is important to realise, when looking at the functional design of a system incorporating barrierdevices, that the maximum working voltage will normally be less than that of D2.

Choosing the most suitable barrier for a particular application involves a number of considerations,e.g. working voltage (the maximum voltage which can be applied between terminal 1 and earth for

Fig. 15.14



a specified leakage current), end-to-end resistance, power transfer, effect of leakage through thezener diodes, effect of d.c. or a.c. interference and earth faults, and safety of the system.

A frequent question is, what polarity barrier is required? The general rule is: if the negative supplyis earthed, which is the most common practice, a positive polarised barrier is required - if thepositive line is earthed a negative polarised barrier is required.

Ohms law is one of the most useful tools in establishing whether a system will operate satisfactorily.

The following examples show how safety barriers can be used to protect various pieces of equipmentin hazardous areas.

The figure above (Fig. 15.15) shows a two-channel, non-polarised barrier and floating instrumentinput circuit connected to a thermocouple. This combination rejects common-mode a.c. interference,d.c. leakage to the thermocouple and earth faults and is suitable for receiving instruments with aninput impedance exceeding 100 kilohm, i.e. the great majority.Certification for the use of the thermocouple with the barrier will be covered by the barrier systemcertificate.

This figure (Fig. 15.16) shows a scheme using another two-channel barrier, but this time being usedwith a 4-20mA signal, two wire process transmitter such as a flow transmitter. This barrier isemployed where a number of transmitters are to be powered from a common power supply.

Fig. 15.15

Fig. 15.16



Fig. 15.17

This diagram (Fig. 15.17) shows what is perhaps one of the most common functions beingencountered - the transfer of a switch status from a hazardous area to a safe area.

If the power supply is earthed, it shows the preferred arrangement. It is fail-safe in that an earthon either switch line de-energises the relay (and does not blow the barrier fuse).A key feature of the barrier shown is that the return channel contains series diodes instead ofresistors and therefore does not contribute to the fault energy of the combination.

Maximum power transfer is achieved by choosing a relay with a coil resistance equal to that of thebarrier channels, nominally 300 ohm.

This ensures that about half the supply voltage will be developed across the relay coil. With a 24Vsupply, a normally 12V relay will be required.

New Developments

The usual safety barrier contains only those components shown above, but recently developeddevices, for specific applications in process control can contain either a barrier and an additionalelectronic component or an IS reed relay.

One of the problems with these designs has been the possibility of a temperature rise inside thebarrier unit being caused by a fault in the electronic circuit, and the resultant application of a highvoltage. This, however, has been overcome by the use of thermal trips in the unit, analogous to thedesign of an IS mains transformer.

Recent developments also include an alternative mounting scheme for barrier devices.

The usual mounting arrangement for barriers is to mount them, using the two earth studs, on a highconductivity busbar. This is usually quite acceptable and causes no problems.



However, there are instances where the space available for a large number of barriers is restricted,for example on oil platforms. The electrical circuits are identical to conventional barriers but arebuilt into plastic mouldings and mounted onto a printed-circuit board by solder pins.

Fig. 15.18



Fig. 15.19

Case Study



Fig. 15.20



Fig. 15.21



Fig. 15.22



Case StudyBrace Yourself Against a Dust Explosion

Throughout the chemical process industries (CPI), substances such as plastics, fertilisers, fuels andpharmaceuticals, are routinely dried to powder or dust. When suspended in air or other oxidants,these dusts are capable of producing a dangerous and costly explosion.

While it is difficult to precisely define the explosion risk created during the handling of potentiallyexplosive materials, the risk is ever present. Explosions should be anticipated, and preventivemeasures must be taken in the design and selection of bulk-solids-handling equipment, such as dustcollectors, conveyors, bins, grain elevators, and size-reduction equipment, such as grinders, crushersand pulverisers.

Selection of the correct explosion protection cannot be left to chance. The risks include the possibleloss of life, damage to capital equipment, loss of production and increased insurance premiums.Many of the hazards can be effectively avoided by thoroughly testing the dusts to ascertain theirexplosion potential, and taking the appropriate protective measures.

The potential explosion hazard involved in dust handling has prompted testing, federal regulations,and insurance considerations. The sheer power of dust explosions was dramatically demonstratedby the destruction of several large grain silos in the U.S. in 1977. Although much has been done tominimise risk, such incidents are reminders of the devastation and loss of life that any explosioncan inflict inside an industrial plant.

To regulate dust handling in the CPI, the U.S. Occupational Safety and Health Administration(OSHA) requires the presence of protective systems, including hooding, ducting and dust-collectiondevices. Most major dust-collection vendors provide such systems.There are three pieces to explosion puzzle: a combustible dust, air or another oxidant, and a sourceof ignition. During operation, a cloud of finely divided particles is held in suspension in the vesselof a dust collector - making the perfect explosion environment, if other critical components (suchas volatile gases) were to be introduced.If a source of ignition initiates the combustion of a dust cloud, the gases in the cloud will rapidlyexpand, due to heat developed during combustion. If a dust-collector vessel constricts this gasexpansion, a rapid pressure build-up inside the collector casing will cause a violent explosion.

When the flame speed in an explosion is less than the speed of sound at the appropriate pressure,the explosion is called a deflagration. When the flame speed is greater than the speed of sound, theresult is a detonation. The majority of industrial explosions are deflagrations.Some Explode Some Dont

Many commonly handled dusts have been tested to determine the degree of explosion hazard thateach poses, and the results are published by the National Fire Protection Assn. (NFPA 68, Ventingof Deflagrations, 1988 Ed.) The explosion potential of a dust confined within a vessel can becharacterised by maximum explosion pressure (P

max) and the maximum rate of pressure rise (dp/

dtmax

).If the explosion potential of particular dust is unknown, a sample may be submitted to a commercialtesting laboratory for characterisation. To classify a dust in terms of risk it poses, the substance is



subjected to a source of ignition and described as a dust that either does or does not propagateflame.

If the dust does propagate flame, further tests are conducted to determine the following parameters:the minimum ignition temperature, the minimum concentration of dust (by weight) required for anexplosion, and the minimum ignition energy in joules.Also measured are the maximum permissible oxygen concentration (oxygen deprivation will inhibitignition of the cloud), the maximum explosion pressure and the rate of pressure rise. Pressures ashigh as 1,035 kN/m2 (or 150 lb/in2) and rates of pressure rise as high as 140,000 kN/m2/s (or 20,000lb/in2/s) have been measured in dust-explosion tests.In addition to the physical characteristics obtained from testing the dust itself, a number of otherfactors affect the explosion potential of a dust cloud:

Source of ignition - As intuition would suggest, a large ignition source causes more-rapid combustionthan a smaller one. A typical small source may be a spark or contact with a hot surface. A typicallarge ignition source may be a flaming dust cloud passing through the ducting.

Particle size - While the maximum pressure of a dust cloud is relatively unaffected by the size ofthe individual particles, the rate of pressure rise increases significantly as particle size decreases.Additionally, as particle size is decreased, the amount of exposed surface area increases which aidscombustion.

Fig. 15.23



Turbulence - Even under normal operating conditions, turbulence is created by air flow in a dustcollector during routine operation and cleaning processes. Although the maximum pressure insidethe vessel is only slightly affected by turbulence, the rate of pressure rise increases significantly asturbulence increases.

Moisture - The presence of moisture in the dust particles raises the minimum temperature requiredto ignite the dust cloud. The presence of moisture in the gas surrounding the dust, though, has littleeffect; that entrained moisture is quickly atomised after an explosion begins.

Hybrid mixtures - These are mixtures of flammable gases with dusts. Some dusts are not flammablein air, but become flammable with the addition of even a small amount of flammable gas. The gasincreases the risk of explosion, and considerably reduces the minimum ignition energy required toinitiate an explosion.

A Match to a Tinderbox

Sources of possible ignition, present in every process plant, must be identified and controlled tominimize the risk of a dust-cloud explosion. Flames and smoldering particles can arise fromdryers, grinders, furnaces, kilns, ovens and mechanical handling equipment, such as conveyors, orsimply from bad housekeeping.

Friction on equipment, such as bulk chemical grinders, creates hot surfaces, which can ignite a dustcloud. Spontaneous combustion is also a risk when certain materials are contained in bulk.

Finally, the movement of air in dust-collector ducts create static electricity. The static dischargefrom dusts can reach an energy level up to 50 millijoules (mJ). If the minimum ignition energyrequired to ignite a particular dust cloud is below that level, such static electricity may be all that isneeded to ignite the cloud. Some dusts can be ignited by an energy source of only a few millijoules.Defuse an Explosive Situation

Awareness of the risks involved in working with dusts and powders in a process environment is ofparamount importance. A prevention program aims to minimise each of the risks. In addition topreventive measures, a number of steps can be taken to reduce the destructive effects of an explosion:

Limit static buildup - To eliminate one possible ignition source, specific dust collector componentsshould be grounded, to continuously discharge static electricity as it builds up.

The development of epitropic filter fabric is a recent innovation that helps to disperse the build-up of static charges in a dust collector.

Epitropic filters are made from conductive filters that have been impregnated with carbon. Thefilters must be connected to an effective grounding point on the outside of the dust collector, Thefabrics resistance is less than 1 x 108 ohms. According to the NFPA 77, (Static Electricity, 1988Ed.), a resistance of 1 x 1010 ohms or less will provide an adequate leakage path to bleed away staticbuild-up in most applications.

Inerting - The addition of an inert gas to replace oxygen in a dust collector can prevent an explosion,ensuring that the minimum oxygen content required for ignition is never reached. In open-circuitdust-collection systems (those that bring in and exhaust air), however, inerting is often not economical,as the open system means a constant loss of expensive inert gases.

Explosion suppression - This protective method requires the early detection of an explosion, usuallywithin the first 10 milliseconds. Once ignition is detected, an explosion-suppression device injects



a pressurised chemical suppressant into the collector vessel, to displace the oxygen and impedecombustion.

Such suppression systems can be operated in conjunction with rapid-acting isolating valves that areoften used in both the inlet and the outlet headers of a dust collector. Such isolation valves are oftenused when toxic dusts are being handled. The valves prevent an explosion from sending toxic dustsinto the ducting leading into and out of the collector.

Explosion containment - There are specialised dust collectors on the market that have been designedto withstand the maximum pressure generated during and explosion. Commercially available dustcollectors, however, are often designed to withstand only 7-14 kN/m2 (1-2 psi); this is not sufficientto contain an explosion in progress.

Explosion relief - This method of alleviating rapidly building pressures is commonly used with dustcollectors. As pressure increases quickly leading up to an explosion, a relief vent opens to allow therapidly expanding gases to escape, effectively reducing the pressure build-up. Relief vents shouldbe located on the dirty (unfiltered) side of any filter, as the filter media itself acts as a barrier tothe expanding gases.

The necessary area for such a relief vent is a function of the vessel volume, vessel strength, and themaximum pressure that the vent closure can withstand, as well as the rate of pressure rise characteristicof the dust in question.

Pressure-Venting Options

Two main types of explosion vents are available: explosion-relief doors and bursting-panel reliefvents, Explosion-relief doors should be lightweight and retained by springs, by magnets, or bygravity alone. Flexible pop out panels also may be included in this category.

In most cases, the restraining pressure of the door is listed as the stated relief pressure. This aloneis not always a true representation of relief pressure.

In a recent series of tests carried out under actual explosion conditions, the inertia of the door wasfound to be an additional factor. Thus, the force needed to blow out these doors is often greaterthan first assumed. The total weight of the door assembly, including any insulation and permanentlymounted hardware should be as low as is practical, and in no case should it exceed 17 kg/m2 (NFPA68, Venting of Deflagrations, 1988 Ed.).One disadvantage of using an explosion-relief door is the considerable increase in force that maybe added by corrosion or freezing between the door, the casing and the restraint interfaces. The useof hinges as a means of restraint should be avoided, as the high negative pressures following anexplosion could result in the door swinging closed and causing an implosion of the filter casing.

Often held in place by a non-corrosive magnetic strip, the door is maintained in a vertical plane toeliminate water traps that could develop when used outside. Wire ropes are often used to preventthe door from flying off during an explosion.

In addition to explosion-relief doors, bursting-panel relief vents are also used to alleviate pressurebuildup inside a vessel, to prevent an explosion. In this method, a membrane of known burstingpressure is used instead of a door over the vent opening.

There are many membrane materials available. Comprehensive testing is required to select thematerials with the optimum bursting characteristics.



For dust collectors, the membrane must be: weatherproof; sufficiently resilient to withstand thenormal working pressures experienced within the collector; capable of withstanding abrasion andchemical attack from the dust being handled; and conductive to prevent the accumulation of a staticcharge.

ActivitiesDecipher: (i) Class I Zone I Ex ib IIC T4

(ii) Class II Division I D.I.P.Using the sample SAA Certification Sheets provided, determine the level of protection certified forthe Rosemount 1151 Pressure Transmitter. What barrier information is provided in the documents?

DiscussionUsing the article entitled Brace Yourself Against a Dust Explosion:

(i) Explain the dangers inherent in dust-collectors.(ii) What is the difference between detonation and deflagration?(iii) What physical characteristics of the dust collection system contribute to the likelihood

of explosion?

(iv) What are typical ignition sources.(v) Explain epitropic, inerting.(vi) What pressure venting options are available?



SummaryA classifiable hazardous location may be exposed to flammable/combustible liquids, vapours, gasesor dusts. Conditions for an explosion are summarised by Infernal Triangle. The relevant propertiesof combustible materials are:

(i) Vapour pressure - molecules on the surface of a liquid continually escape to form avapour and recondense. An equilibrium state results which is very temperaturedependent.

(ii) Boiling point - transition from liquid to gas phase. It is ambient pressure dependent.(iii) Flashpoint - lowest temperature at which an explosive quantity of vapour forms.(iv) Ignition temperature - lowest temperature, without spark or flame, at which a solid

initiates combustion.

(v) Explosive limits (LEL, UEL) - a region in which a potentially explosive mixture canignite.

(vi) Relative vapour density -mass of pure gas vapour

_______________________

mass of dry air

at same volumes, temperatures and pressures.

Is a measure of whether it tends to rise, fall or be suspended.

(vii) Minimum ignition energy - minimum energy quantity necessary to ignite a mixture ofgas or dust.

(viii) Layer ignition temperature - temperature at which a heated surface ignites a dust layer.(ix) Cloud ignition temperature - lowest temperature of which dust cloud ignites.

Ignition sources are of two types - electrical sparks/arcs

- hot surface.

Explosion protection techniques can be broadly divided:

(i) Exclusion - explosive mixture kept outside a vessel containing ignition sources.(ii) Containment - explosion dissipates within the enclosure.(iii) Energy limitation - involves intrinsic safety.(iv) Dilution - lowers mixture below LEL by ventilation.(v) Avoidance - e.g. inherently non-sparking.

The Standards Association of Australia uses IEC terminology and definitions but will not acceptCertifications of Safety Standards from other countries. The SAA area classification and markingsmay be summarised.

Class 1, 2 Zones (0), 1, 2Liquid/Vapours Period of Hazardor Dusts/Fibres



Appropriate protection markings for each area are typically:

Ex (lower case alphabet) (Roman Numerals)Explosion Protective Groups dependentProtection Type upon Industry

Type

T1-6 IP XXTemperature Enclosure InformationClass

To limit the current flow to a hazardous area, an electrical barrier can be employed. It may becomposed of zeners, fuses or relays and obviates the need for certification of feeder devices to thearea.



Test1. What are the three components of the Infernal Triangle?

a. __________________________________

b. __________________________________

c. __________________________________

2. Define or explain:

(i) Flyings: _________________________________________________________________________________________________________________

____________________________________________________________

(ii) Vapour Pressure: __________________________________________________________________________________________________________

____________________________________________________________

(iii) Flashpoint: _______________________________________________________________________________________________________________

____________________________________________________________

(iv) Explosion Containment: ____________________________________________________________________________________________________

____________________________________________________________

(v) Intrinsic Safety: ___________________________________________________________________________________________________________

____________________________________________________________

(vi) D.I.P.: ___________________________________________________________________________________________________________________

____________________________________________________________

(vii) Safety Barrier: ____________________________________________________________________________________________________________

____________________________________________________________



3. A particular gas (X) has relative vapour density 2.25 and LEL/UEL of 3.1/15.5 respectively.Its flashpoint is 3C and it has an ignition temperature of 305C. Minimum ignition energy is.17 mJ.

(ii) Is the gas lighter than air? (Yes/No)Ans: ________

(iii) X is mixed with air in the volume ratio 1:6. Will it explode if sufficient energy issupplied? (Yes/No)

Ans: ________

Third Printing: October 1996Second Printing: December 1993

First Printed: October 1991

SynopsisIntroductionHazardous AreasCombustible MaterialsIgnition Sources.International StandardsTypes of ProtectionLocal AuthoritiesThe S.A.A.TestingSAA StandardsWhat barriers Do I UseNew DevelopmentsCase Study - CertificationCase StudyBrace Yourself Against a Dust Explosion

ActivitiesDiscussionSummaryTest

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