Introduction

A1

A technical seminar

on

Intrinsic Safety

Introduction

A2Section A

The Flammable Facts

Gas, Temperature and Area Classification

Introduction

A3

Spark

Explosion Triangle

Source ofIgnition

Hazardouslocation

AirGas

SparkHeat

Introduction

A4Requirement for plant

classification

Plant and Installations are classified according to:-

The nature of the Hazardous Atmosphere

The probability that the Hazardous Atmosphere will be present

- Gas Classification- Ignition Temperature

- Area Classification

Introduction

A5Requirement for

equipment

classification

- Apparatus Group

- Temperature Classification

Equipment is classified according to:-

The maximum spark energy it can produce

Its maximum surface temperature

Introduction

A6Spark Ignition Characteristic

Lower Explosive Limit (LEL) (UEL) Upper Explosive Limit

Minimum Ignition Energy (MIE)

Ignition Energy(milli Joules)

Volume concentration (%)

1.0

0.1

0.01 0 10 20 30 40 50 60 70 80 90 100

Hydrogen-air(1 atmos.)

Flammable Range

Propane-air (1 atmos.)Ethylene-air(1 atmos.)

Introduction

A7Explosive limits

for material

s

*

Flammable Range Gas/air mixtures are ignitable over a range of concentrations called the

Flammable Range Material Lower explosive limit Upper explosive limit Minimum

LEL UEL ignition energy

propane 2 % 9.5 % 180µJ ethylene 2.7 % 34 %

60µJ hydrogen 4 % 76 % 20µJ * of most easily ignitable mixture

Introduction

A8Flammable Range

Methane

Propane

Ethylene

Hydrogen

GAS 0 10 20 30 40 50 60 70 80 90 100

EXPLOSIVE LIMITS (% gas in air by volume)

Introduction

A9Gas classification: the two main systems

Representative(test) gas

Gas classificationIgnitability

IEC countries(including Europe) USA & Canada

AcetyleneHydrogenEthylenePropane

Methane

Metal dustCarbon dustFlour, starch,grain

Fibres & flyings

Group IICGroup IICGroup IIBGroup IIA

Group I (mining)

(Dusts notclassified)Discussions anddraft documentsin circulation

Class I, Group AClass I, Group BClass I, Group CClass I, Group D

(No classification)

Class II, Group EClass II, Group FClass II, Group G

Class III

Moreeasilyignited

Introduction

A10Temperature

classification

Flammable materials have a temperature at which ignition will take place, even in the absence of an external source

This temperature is called the

Ignition Temperature of that material

Apparatus must be selected such that it will not expose the gas-air mixture to a temperature exceeding the ignition

temperature

Introduction

A11Temperature

classification

(IEC/European)

T1T2T3T4T5T6

450 C 300 C200 C135 C100 C 85 C

BetterApparatus

Temperatureclassification

Maximumsurface

temperature

o

o

o

o

o

o

Tamb = 40oCunless otherwise

stated

Operation abovestated Tamb will

require re-assessment

Introduction

A12Temperature

classification in North

America

T1T2T2AT2BT2CT2DT3T3AT3BT3CT4T4AT5T6

450300280260230215200180165160135120100 85

T class

Apparatus is marked with either T rating or maximum surface temperature

User has responsibility to ensure that the T-rating is below the Spontaneous

Ignition Temperature ( SIT )

° C

Tamb = 40oC

Introduction

A13Gas/equipment compatibility

Ammonia 630 Hydrogen 560 Methane 537

Propane 470 Ethylene 425 Butane 365

Gas Ignition Temperature

700

600

500

400

300

200

100

T1

T2

T3

T6 T4

T5

ApparatusTemperatureClassification

Cyclohexane 259

Diethyl Ether 170

Carbon Disulphide 95

Introduction

A14Gas Ignition Temperature

T4 for apparatus is usually OK Exception:

Carbon Disulphide,CS2 requiring T6

Don't confuse T Class with maximum operatingtemperature!

Introduction

A15Flashpoint Definition

The flashpoint of a flammable liquid is defined as the temperature at which the free

surface of the liquid emits sufficient vapour to be ignited by a small flame

Flashpoint

Note: Don’t confuse Flashpoint with Ignition Temperature(eg for kerosene, Flashpoint = 38°C; Ignition Temp. = 210°C)

Introduction

A16Properties of Gases

Extract from BS.5345 Part 1

Compound Vapour LFL UFL Ignition T Apparatus Density Temp C Class Group

Taken from IEC 79-10:1995

AcetoneAmmoniaButaneCarbon DisulphideCyclohexaneDiethyl EtherEthyleneHydrogenKeroseneMethanePropane

2.000.592.052.642.902.550.970.07

0.551.56

2.0151.51.01.21.72.74.00.75.02.0

13288.5607.8363475.65159.5

535630372 95259160425560210537470

T1T1T2T6T3T4T2T1T3T1T1

IIAIIAIIAIICIIAIIBIIBIICIIAI / IIAIIA

Introduction

A17Why Area Classify?

Probability of

Gas/Air Mixture

being present

Probability of

Source of

IgnitionX =

Acceptable

Risk

Reasoning : The higher the probability of the gas/air mixture being present demands a lower probability of having a source of ignition and vice versa. Hence in Zone 0 we use IS; in Zone 2 we perhaps use Type N or Exq,Exo,Exm, etc

Introduction

A18Area classifica

tion in IEC/Euro

pean countries

Areas are classified with regard to the probability of a potentiallyexplosive atmosphere being present and the length of time

for which it is likely to exist

Zone 0:

Zone 1:

Zone 2:

in which an explosive gas-air mixture is continuously present or present for long periods

in which an explosive gas-air mixture is likely to occur in normal operation

in which an explosive gas-air mixture is not likely occur in normal operation, and if it occurs it will exist only for a short time

Introduction

A19Area classification

in North America

Division 1Hazardous atmosphere is likely to be present in normal operation

Division 2Hazardous atmosphere is unlikely to be present in normal operation

Introduction

A20

Why area Classify ?• Why go to all the trouble with areas & equipment?• If we know the probability of hazardous area & probability

of source of ignition , we know whether we are taking an acceptable risk.

• Eg. Probability of flammable atmosphere = high• If apparatus insures never sparks = acceptable risk

Introduction

A21Personnel Hazards

For Hydrogen Sulphide, H2S:a very common danger when dealing with oil products

Property VolumeConcentration

Smell detection level 0.1 ppmSafe working level 10Serious health danger level 50Lower Flammable Limit 40,000 (4%)Upper Flammable Limit 460,000 (46%)

Note! Figures for safe working levels vary, the above are from a Middle East location.

Introduction

A22Area Classification Example

Bund wall

Tank vents

Liquid surface

Area Classificationfor a fixed-roof tankwith heavier than air

vapourZone 1

Zone 0

Zone 2

Key

Introduction

A23Thermocouple in a hazardous area

Inside of pipe orprocess vessel

Zone 0Zone 1Outside of pipe

Zone 2

Division 2

Division 1

Introduction

A24Which documents

Within UK, BS EN60079-10 : 1996 IEC79 - 10 : 1995

Book; -

Classification of hazardous locationsby A.W. Cox, F.P. Lees, and M.L. Ang

Published by IChemE ISBN 0 85295 2589

Introduction

A25Explosion protection for dusts

About 70% of dusts occuring in industry are flammable.

Dusts and powders typically require mJ of spark energy for ignition,so a more likely mechanism is hot surface ignition.

Whereas the majority of flammablegases have ignition temperatures above 350 C, some dusts ignite at150 - 200 C.

Ingress protection alone for equipment is not enough: must protectagainst ignition by raised temperatures of enclosures.

Is the dust flammable?

What is its ignitiontemperature?

Dusts need milliJoulesof spark energy to ignite

Ingress protection aloneis not enough

oo

Introduction

A26Ignition Properties of Dusts

Type of Dust Ignition Temperatureof dust cloud (oC)

Minimum Spark Energyrequired for ignition

of cloud ( milli-Joules )

Minimum Explosiveconcentration

( gms / cu.metre )

Aluminium, milledZincPolystyreneUrea resinCocoaCoffeeCotton seedGrain dustSugarCoalCorkSulphurWood flour

550

600490450420410470430350610470190430

35

3851255357045453045303035

15

650158010016080303060451520

Introduction

A27

Following the introduction of new European Directives a group of newStandards are being prepared.

Two standards have been published, they are;

EN 50281-1-1 : 1998 Electrical apparatus protected by enclosures - Construction and testing

EN 50281-2-1 : 1998 Test methods for determining the minimum ignition temperatures of dust

In draft stage are;

prEN 50281-1-3 Classification of areas where combustible dust are or may be present

IEC are working alongside the European Standards committee

IEC/European Dust Standards

Introduction

A28Comparison of dust classification systems

Grade of release BS6467 IEC 1241 DIN VDE 1988 ATEX 137 0165: 1991

Continuous 20

Primary Z 21 10

Secondary Y 22 11

Area classification of dusts and powders must be treated differentlyfrom gases and vapours:

Dusts don’t disperse with timeVentilation can convert layers into clouds

Introduction

A29Area Classification - Dusts

Zone Z An area in which combustible dust is, or may be, present as a cloud handling or cleaningoperations in sufficient quantity to be capable of producing an explosive concentration of combustible or ignitable dustin a mixture with air.

Zone Y An area not classified as Zone Z, in which accumulationsor layers of combustible or ignitable dust may be presentunder abnormal conditions and give rise to ignitable mixtures of dust and air.

Definitions taken fromBritish Standard 6467: Part 2: 1988

during normal processing, handling or cleaning

under abnormal conditions

Introduction

A30Dust Hazards

Relevant Standards for Dusts

BS 6467 "Electrical Apparatus with protection by enclosure for use in the presence of combustible dusts" Part 1 (1985) "Specification for apparatus" Part 2 " Guide to selection, installation and maintenance"

ISA-S12.10 Area classification in hazardous dust locations

BS 7535 : 1992 Guide to the use of electrical apparatus complying with BS 5501 or BS 6941 in the presence of combustible dusts

Introduction

A31Intrinsic safety and dust hazards

Hazardous Area Suitable apparatus

Zone Y

Zone Z

Intrinsically safe withdegree of protectionat least IP5X *

Intrinsically safe withdegree of protectionat least IP6X *

Source: BS 7535 : 1992; Clause 2.7

* Note: do not confuse with NEMA categories

Introduction

A32

Spark

Explosion Triangle

Source ofIgnition

Hazardouslocation

AirGas

SparkHeat