unit 1 - general principles

Hazardous Area Electrical Training

Hazardous Area Explosion Protected Apparatus Installation, Maintenance & Inspection Training

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Unit 1

General Principles

Unit 01 - General Principles


Basic Requirements for Combustion to Occur:

• FUEL• OXYGEN

• SOURCE OF IGNITION


• FUEL • GAS - VAPOUR - MIST - DUST

• OXYGEN• 21% IN AIR BY VOLUME

• SOURCE OF IGNITION• ARC - SPARK - FLAME - HEAT


Basic Requirements for Combustion to Occur:GAS or VAPOUR..

OXYGEN(21% In Air).

SOURCE OfIGNITION.

FIRE FIRE TRIANGLE.TRIANGLE.


To prevent combustion occurring we can:

• Remove any one of the three combustion elements.

• Isolate or Separate the Source of Ignition from the gas/air mixture.


• Allow the explosion to take place & contain it within a robust enclosure.

• Allow the three elements to co-exist & ensure that the source of ignition is kept below specific

values.


Flammable (Explosive) Limits:

• Combustion will only occur if the Flammable mixture is between certain limits

• i.e. between the LEL and the UEL


The Lower Explosive Limit - LEL

When the percentage of gas by volume is below this limit,the mixture is too weak to burn.

i.e.

• insufficient fuel

and/or

• too much air

Unit 01 - General PrinciplesThe Upper Explosive Limit - UEL

When the percentage of gas by volume is above this limit, the mixture is too rich to burn.

i.e.

• insufficient air

and/or

• too much fuel.


Upper Explosive Limit

(UEL)

Too Rich

Insufficient Oxygen

Lower Explosive Limit

(LEL)

Too Weak

Insufficient Fuel


Flammable (Explosive) Limits:

• Different gasses and vapours will have different flammable limits.

• The greater the difference between the LEL and the UEL.(flammable range).

The more dangerous the material.


• We can operate safely with flammable mixtures above the LEL

• It is more practical to operate below the LEL.

Unit 01 - General PrinciplesFlammable (Explosive) Limits:

The Flammable Limits of Some Materials are given in the table

Material LEL % by Volume UEL % by VolumePropane. 2 9.5Ethylene. 2.7 34Hydrogen. 4 75.6Acetylene. 1.5 100Paraffin. 0.7 5Carbon-Disulphide. 1 60Methane 5 15

Unit 01 - General PrinciplesSources of Ignition:

• The possible sources of ignition are many and varied.• They include the following:

Electrical arcs/sparks Frictional sparks Welding activities Static discharges

Hot surfaces Cigarettes Batteries


Sources of ignition

• Engine exhausts

• Chemical reaction

• Thermite action

• Pyrophoric reaction

• Lightning strikes


Flashpoint:

Definition:

The lowest temperature at which sufficient vapour is given off a liquid, to form a flammable mixture with air that can be ignited by an arc, spark or naked flame.


Flashpoint:Some typical values are given in the table below;

Material Flashpoint oCPropane. <-20.Ethylene. <-20.Hydrogen. <-20.Acetylene. <-20.Diethyl Ether. <-20.Paraffin. +38.Carbon Disulphide. <-20.


The flashpoint of a material gives an indication of how readily that material will ignite in normal ambient temperatures.

Reference to tables taken from:

• The UK Code of Practice - BS 5345: Pt (1)• The IEC Code of Practice - IEC 60079-20


• Materials with Materials with highhigh flashpoints should not be flashpoints should not be overlooked as a potential hazard.overlooked as a potential hazard.

• If a flammable material is discharged If a flammable material is discharged under under pressurepressure from a jet its flashpoint may be from a jet its flashpoint may be reduced.reduced.


Ignition temperature:

Definition:The minimum temperature at which a flammable material will spontaneously ignite without a spark or flame.


Ignition temperature :

The surface temperature of equipment, indicated by its ‘T’ rating will not exceed the Ignition Temperature of the flammable atmosphere, which may be present around the equipment.

Unit 01 - General PrinciplesIgnition temperature:Some typical values for Ignition Temperature are given in the table below:Ref: BS 5345 Pt (1) and IEC 60079-20 codes of practice.

Material Ignition Temperature oCPropane. 470.Ethylene. 425.Hydrogen. 560.Acetylene. 305.Diethyl Ether. 185.Paraffin. 210.Carbon Disulphide. 102.Methane 595


Oxygen enrichment: • The normal oxygen content in air is around 20.95%.• Any value which exceeds this is deemed to be

oxygen enriched.

Typical examples of where Oxygen Enrichment may occur are :

• Gas Manufacturing Plants• Hospitals

• Oxy-Acetylene Equipment


Oxygen enrichment has Oxygen enrichment has threethree disadvantages: disadvantages:

(1) (1) It can lower the Ignition Temperature of flammable It can lower the Ignition Temperature of flammable materials as shown in the examples below: materials as shown in the examples below:

AirAir Increased OxygenIncreased OxygenMaterial;Material; Ignition Temp. Ignition Temp. 00C; C; Ignition Temp. Ignition Temp. 00C;C;

Hydrogen Sulphide.Hydrogen Sulphide. 470.470. 220.220.Acetylene.Acetylene. 425.425. 296.296.Ethane.Ethane. 560.560. 506.506.


(2) It can significantly raise the Upper Flammable Limit of the majority of gasses and vapours, thereby widening their Flammable Limits .

This is illustrated in the examples below:

Air Increased OxygenMaterial; LEL %; UEL %; LEL %; UEL %;Methane. 5. 15. 5.2 79Propane. 2.2 9.5 2.3 55Hydrogen. 4. 75 4.7 94


(3) Oxygen enrichment of a flammable atmosphere can allow it to be ignited with much lower values

of electrical energy.

Note:• Explosion protected equipment is tested in normal

atmospheric conditions.• Certification for equipment in an oxygen enriched

environment would be compromised.


DensityIf a flammable material is released, it is important to know whether the material will rise or fall in the atmosphere.

Since air is the standard referenceIts relative density is 1.


If a material is twice as heavy as air its relative density will be 2.

Therefore:

Materials with a relative density less than 1 will rise in the atmosphere.

Materials with a relative density greater than 1 will fall in the atmosphere.

Unit 01 - General PrinciplesDensity:

• Materials which rise in the atmosphere can collect in roof spaces.

• Those which fall can drift along at ground level and possibly into a non hazardous location.

• May collect in locations without ever dispersing.

• Detectors must be installed at correct level and location

Unit 01 - General PrinciplesDensity:Some typical values are given in the table below:

Material Relative Vapour DensityAir 1.Propane. 1.56.Ethylene. 0.97.Hydrogen. 0.07.Acetylene. 0.9.Diethyl Ether. 2.55.Paraffin. 4.5.Carbon Disulphide. 2.64.Hydrogen Sulphide. 1.19


Area Classification:

A hazardous area is defined as:

“An area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of apparatus.”


A non-hazardous area is defined as:

“An area in which an explosive gas atmosphere is not expected to be present, in quantities such as to require special precautions for the construction, installation and use of apparatus”.


Area classification-zones:

zoning is a means of representing the following:

• the frequency of the occurrence• the duration of an explosive gas atmosphere

It assists in proper selection and installation of apparatus


Evaluation of the area within each zone is based on• The grade of release• Presence of artificial ventilation• Extent and volume of release

Hazardous areas are, therefore, divided into three zones.They represent the risk in terms of:The probability, frequency and duration of a release.


Ref BSEN 60079-10

Classification of hazardous areas, are as follows:

Zone 0:“In this Zone, an explosive gas atmosphere is continuously present, or present for long periods”.

Unit 01 - General PrinciplesZone 1:“In this zone, an explosive gas atmosphere is likely to occur in normal operation”.

Zone 2:“In this zone, an explosive gas atmosphere is not likely to occur in normal operation and, if it does occur, it is likely to do so infrequently and will exist for a short period only”.

Unit 01 - General PrinciplesAlthough not specified in the standards, the following figures are generally accepted in industry.The duration of a gas release, on a normal annual basis, for the different Zones is as follows:

• Zone 0 = over 1000 hours / year

• Zone 1 = 10 to 1000 hours / year

• Zone 2 = <10 hours / year

Unit 01 - General Principles Zone Diagrams.

Zone ‘0’ Old Zone ‘0’Non-Hazardous

Zone ‘1’ Zone ‘2’


a = 3m from vent openings.

b = 3m above the roof.

c = 3m horizontally from tank.

Unit 01 - General Principles Sources of Release

Zone ‘0’

SPACE ABOVE LIQUIDIN A CLOSED TANK

- ZONE ‘0’PUMP GLAND

- ZONE ‘1’

FLANGED JOINT- ZONE ‘2’

WELDED JOINT- NON HAZARDOUS

Unit 01 - General PrinciplesTypical Platform Hazardous Areas

HAZARDOUS ZONE 1

HAZARDOUS ZONE 2

NON HAZARDOUS AREA


Gas/Apparatus grouping

In the IEC system, the group allocation for surface and underground (mining) industries are separate.

• Group I is reserved for the Mining industry

• Group II is for Surface industries

• This course deals with group II


Group II is further subdivided into the following:

• Group IIA i.e. Propane

• Group IIB i.e. Ethylene

• Group IIC i.e. Hydrogen

Unit 01 - General Principles• Two methods have been used to ‘group’ these

flammable materials• According to the degree of risk they represent

when ignited• The first involved determining the MIE for each

gas• This proved that Hydrogen and Acetylene were

the most easily ignited• Propane was the least easily ignited

Unit 01 - General PrinciplesThe second, involved determination of the MESG.

• Using a flameproof bronze sphere• The 25mm flame-path gaps could be varied• A gas/air mixture was introduced inside and outside• Under test, the gas inside is ignited• The MESG is the maximum gap which prevents

ignition of the outside gas• The more dangerous the gas the smaller the gap

Unit 01 - General PrinciplesApparatus for determination of M.E.S.G.

FLAMMABLEFLAMMABLEMIXTUREMIXTURE

FLANGEFLANGE1”(25mm)1”(25mm)

M.E.S.G.M.E.S.G.


Gas Group Representative Gas MESG MWG MIE(mm) (mm) (J)

I Methane (Firedamp) 1.17 0.5 280IIA Propane 0.97 0.4 260IIB Ethylene 0.71 0.2 95IIC Hydrogen/Acetylene 0.5 0.1 20

Unit 01 - General PrinciplesNote 1:Apparatus marked II, i.e. with no sub-division letter A, B, or C means that this can be used in all hazards

Note 2:Apparatus marked II XXX.XXX represents the chemical formulae or name of a material and should only be used in this hazard

Unit 01 - General PrinciplesNote 3:

• Apparatus marked IIA can only be used in IIA hazards

• Apparatus marked IIB can be used in IIA and IIB hazards

• Apparatus marked IIC can be used in IIA, IIB and IIC hazards.

Unit 01 - General PrinciplesTemperature classification:Is based on the maximum temperature which any relevant part of the apparatus, which may be in contact with an explosive gas will reach when operated within its normal design rating and ambient temperature.

Apparatus is classified into one of six temperature ranges See following slide


Temperature Codes are given in the table below:

Temperature Code Maximum Surface Temperature oC

T1.T1. 450.450.T2.T2. 300.300.T3.T3. 200.200.T4.T4. 135.135.T5.T5. 100.100.T6.T6. 85.85.


Equipment T-Rating must be below the ignition temperature of any gas / vapour surrounding it.

The T- Rating temperatures are based on a maximum Ambient temperature rating of 40 0C, for use in the UK.

NoteHigher Ambient Temperature Ratings are available


T-Rating and Ignition Temperature Comparison of Some Materials are given in the table below:

Material Ignition Temperature T - Rating

Methane.Methane. 595 595 00CC T 1 (450 T 1 (450 00C)C)Ethylene. Ethylene. 425 425 00CC T 2 (300 T 2 (300 00C)C)Cyclohexane. Cyclohexane. 259 259 00CC T 3 (200 T 3 (200 00C)C)Diethyl - Ether.Diethyl - Ether. 170 170 00CC T 4 (135 T 4 (135 00C)C)Carbon - Disulphide. Carbon - Disulphide. 102 102 00CC T 5 (100 T 5 (100 00C)C)

T 6 (85 T 6 (85 00C)C)

Unit 01 - General PrinciplesTemperature classification example:If apparatus is tested in a room with an air temperature of 20°C and the surface temperature reached 100°C (T5), this would be an 80°C temperature rise.However if the test had been conducted at 40°C this would then be calculated as 100°C + 20°C = 120°C (T4).The correct method would be to start at 40°C ambient with a maximum 60°C temperature rise to stay within the 100°C or T5 rating.

Unit 01 - General PrinciplesIn order to avoid infringement of the apparatus certification:

• The ambient temperature rating must be compatible with the environmental ambient temperatures

• The temperature rise must not be exceeded.

This is demonstrated in the following example:

Unit 01 - General PrinciplesTEMPERATURE CLASSIFICATION:

Ambient Air Temperature Less than Apparatus Ambient Rating;;

(T 5)(T 5)

10010000CC

40400 0 CC20200 0 CC

000 0 CC

606000CCTemp. Rise.Temp. Rise.

Air Air TemperatureTemperature

Ambient RatingAmbient Ratingof Apparatusof Apparatus

(T 5)(T 5)

10010000CC

40400 0 CC20200 0 CC

000 0 CC

808000CCTemp. Rise.Temp. Rise.

Correct Incorrect


Ingress Protection:Enclosures of electrical equipment are classified according to their ability to resist:

• The ingress of solid objects• The ingress of water

By means of a system of numbers known as the:• Ingress of Protection (IP CODE)• Ref BS EN 60 529.

Unit 01 - General PrinciplesThe Code consists of the letters IP followed by two numbers

• For example - IP54• The first digit is for protection from solid objects• The second digit is for protection from water

Therefore for IP 54 5 = Dust protected 4 = Splashed water from any direction

Unit 01 - General PrinciplesINGRESS PROTECTION: Abridged version of full table shown below;

First Numeral

Level of Protection

Second Numeral

Level of Protection

0 No Protection 0 No Protection

1 Protection against objects greater than 50 mm

1 Protection against drops of water falling vertically


2 Protection against drops of water when tilted up to 15º

3 Protection against objects greater than 2.5 mm

3 Protection against sprayed water up to 60º


4 Protection against splashed water from any direction

SOLID OBJECTSSOLID OBJECTS WATERWATER

Unit 01 - General PrinciplesINGRESS PROTECTION: Abridged version of full table shown below;

First Numeral

Level of Protection

Second Numeral

Level of Protection

5 Dust Protected 5 Protection against jets of water from any direction

6 Dust Tight 6 Protection against heavy seas – deck watertight

7 Protection against immersion in water 1m in depth and for a specific time

8 Protection against indefinite immersion in water at a specified depth

SOLID OBJECTSSOLID OBJECTS WATERWATER

unit 1 - general principles

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