hazardous area instrumentation

7/30/2019 Hazardous Area Instrumentation

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Haz

ardous AreaInstrumentation

FIRE

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TABLE OF CONTENTS

1WHY CLASSIFY HAZARDOUS AREA?.............................................................................................3

2AREA CLASSIFICATION STANDARDS ...........................................................................................4

3EUROPEAN STANDARD OF AREA CLASSIFICATION...................................................................5

4AMERICAN STANDARD OF AREA CLASSIFICATION....................................................................6

5WHAT HAPPENS IF YOU IGNORE AREA CLASSIFICATION ?......................................................8

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1 WHY CLASSIFY HAZARDOUS AREA?

The most destructive element in nature is Fire.

The primary cause of fire is Inflammable materials. Inflammable or Flammable materials,

as is commonly called, create a Fire Hazard only if (based on common Fire triangle

knowledge) there is also a simultaneous presence of (initial or continuous) Source of

ignition and (undiminished) Combustible Air/ gas. The Flammable material can be solid

(including dust), liquid or gas or a combination of them in a homogeneous or heterogeneous

or in any morphed form.

(Note: There is a difference between flammable and combustible liquids based on flash

point but this article treats both as same for the purpose of simplicity)

But avoidance of fire by avoiding Flammable material is not a relevant discussion since

these Flammable materials are also sources of energy & product-of-use manufactured in

the industrial process. The hazard caused by Flammable materials is a function of number

of factors/ properties such as flashpoint, flammability Index, auto ignition temperature,

combustion energy released etc. Therefore some possibilities exist in reducing the risk of

fire by getting rid of unwanted materials to reduce the higher limits of Fire Hazard based on

the properties cited above.

The presence of air cannot be avoided, as earths atmosphere is an infinite reservoir of air

and its 29% component of Oxygen is the most combustible part of it integrally present,

apart from our obvious human need of it to survive. But action to reduce the presence of

other unwanted vapors and gases that heighten the fire hazard (e.g. Oxygen rich

atmosphere) may be taken.

Manual source of Ignition in the presence of flammable materials such as matches may becontrolled by its absence enforced by Industrial Security in the area of flammable materials.

But the most common source of Ignition is Electrical energy in the form of arc or spark or

even heat plasma in the industrial plant. The major potential Electrical ignition source is

sparking in electrical equipments such as Motors. The other potential source of ignition of

flammable mixtures that concerns us as Instrument Engineers is that from Electrical /

Electronic (process control) Instrumentation, controller, communication, computer systems

and instrument auxiliaries. These are required to be controlled in order to avoid or

insure/ensure against a Fire Hazard.

A hazardous area is a space in which a flammable atmosphere may be expected to be

present at such frequencies as to require special precautions for the control of potential

ignition sources including fixed electrical equipment. By classifying the properties ofFlammable materials with combustion air and correlating with their associated ignition

(electrical) energy, a method of scientifically classifying (Fire) Hazardous area can be

evolved along with their hazard radii of influence & spread or dispersion. Then mechanisms

& devices can be developed to control/ prevent fire hazard and its spreading or dispersion

from the hazard radii. This is the objective behind the (Electrically Fire ) Hazardous Area

Classification.

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2 AREA CLASSIFICATION STANDARDS

The Hazardous Area Classification, now simply called as Area Classification, is

standardized so that any industry can relate to it and classify their Fire hazard. It must be

remembered at all times that the Area classification is a risk based approach and is not a

certainty, since many permutations & combinations of properties (e.g. climate changes,

ventilation etc.) can alter the degree of fire hazard. Manufacturers provide standardized

solutions and devices to control or totally mitigate the Fire hazard based on such

classification.

Since the first energy industry with major fire hazard material (coal & methane gas) was the

underground coal mining, many of the Codes & standards of Area Classification and

associated Protection systems, evolved from coal mining industry. In India it is CMRS

(Central Mining Research Institute), Dhanbad that was responsible for evolving standards

(that are now incorporated as part of Indian Standard) and act as certifying authorities onmeasures to meet the classified Hazardous Area.

The British were the first to classify Flammable materials but the Germans were the first to

extensively relate products to meet Hazardous area. And the Americans were the first to

broaden both and occupy the market with products to meet the classification. Until recently,

the European code & practice was generally considered the most comprehensive. Now a

consensus to unify and universally harmonize the requirements of classification has

practically brought parity in classification between the major players essentially the

European and American Code of practices & Standards, with some local idiosyncrasies as

per past practice and to protect local product manufacture.

Any one standard may be used for Classification but the rule requires that areas classified

as per various standards cannot overlap but only abut. Generally the norm is not to use

more than one standard for a given industrial plant location, to avoid code-rules-mismatchproblems.

Electrical Engineers with active participation of Chemical Engineers are responsible for

classifying the Hazardous area. The classification is marked on a geographically surveyed

plot plan of the Industrial plant provided by a Civil engineer marked with firm details of

locations of equipments & machinery along with all auxiliaries & accessories including

buildings. Instrument Engineers need to pick up the Hazardous area correctly from this

marked classification drawing and specify in the data sheet &/or procure products to meet

the requirement.

It is important to remember that the Area classification is for a three dimensional space

and the classification is a division of a facility into Hazardous and Non-Hazardous areas.

The explosive nature of Flammable atmosphere is a special case as per some variation incodes, since there could be explosion (pressure wave) without fire but many use the word

explosion protection to qualify that equipment provide Explosive-Flammable-Fire

protection.

The code for Area classification is different from the code/ Standard for protection devices.

This part will be covered under a separate article. In Instrument data sheets either the area

classification only or the Area Classification and the Protection Code required are specified.

All devices for Hazardous area such as flameproof protection or explosion protection

devices are always to be procured labeled as suitable for area specified & with code

designations only. Labeling code is also laid out in standards. Protecting a plant with

hazardous area classification with coded labeled devices is the first requirement to be

satisfied for Plant (life) Insurance. More on this will be covered in a companion article.

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As the Indian standard is based on European codes (mainly British) and as the general

preference nowadays is to specify as per European Codes and as American classification is

moving towards harmony with European codes, the European code is discussed first.

3 EUROPEAN STANDARD OF AREA CLASSIFICATION

3.1 The Institute of Petroleum , London has issued an AREA CLASSIFICATION CODE FOR

INSTALLATIONS HANDLING FLAMMABLE FLUIDS Part 15 of the Institute of Petroleum

Model code of Safe Practice in the Petroleum Industry in its 2nd edition dated August 2002

and generally available as a reference in most engineering libraries.

It is also the European Classification code under harmonized European Union or EU Codes

(although language rendered may be different in different countries).

3.2 In this classification, Three important attributes are taken into account for classification and a

relationship established

1) Zone classification based on Likelihood of Occurrence and Duration of a

flammable atmosphere.

2) Grade of Release based on release source frequency and duration in

unrestricted open air.

3) Fluid Category

Note :

Table 1 Table 2 Table 3

Zone Zone Explanation Grade ofRelease

Gradeexplanation

FluidCatego

ries

Fluid categoryDescription

Zone 0 Flammable

atmosphere

continuously present

or present for long

periods

Continuous Release that is

continuous or

nearly so or

occurring even

frequently but for

short periods

A Any flammable

liquid that on

release would

vaporize rapidly &

substantially

Zone 1 Flammable

atmosphere likely to

occur in normal

operation

Primary Release that is

Periodically or

Occasionally

anticipated to

occur in normal

operation

B Any flammable

liquid not in Cat. A

but at a

temperature

sufficient for boiling

to occur on release

Zone 2 Flammable

atmosphere not

likely to occur in

normal operation but

even if it occurs, will

exist only for a short

period

Secondary Release that is

unlikely to occur

or anticipated to

occur under

normal operation

C Any flammable

liquid not in Cat. A

or B but on release

be at a temperature

above flash point or

form a flammable

mist or spray

Non-

Hazardous

Areas that is not any

of the above

G (i) Methane rich

natural gas

G (ii) Refinery Hydrogen

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1. Flammable liquids are further classified as Class 1- Petroleum, Class 2- Petroleum,

Class 3 Petroleum etc. based on their flash point as stated earlier and correlated to Fluid

category above but not shown in the above table for clarity.

2. The IEC method of grouping gases as Group 1, Group IIA, IIB and IIC etc. is not included

in Part 15 (rev. 2002) but included in API RP 505 (1997) as the link to European standards.

3.3 Flammable dust atmospheres that are classified as Zone 20, 21 and 22 are not included in above

table and are outside the scope of this article.

3.4 The practical means to interpret the code definitions stated above has led to some guide line

basis for quick checks :

Zone 0 : > 1000 hours per annum (10%)

Zone 1 : > Between 10 and 1000 hours per annum(0.1 to 10%)

Zone 2 : < 10 hours per annum(0.01 to 0.1%)

Unclassified : Less than 1 hour per annum(0.01%)(Percentages are based on year normalized to 10,000 hours)

3.5 Zone and Fluid category are only specified. Although Grade of release is a property attribute, it

is embedded in zone Classification.

* Continuous grade, normally but not necessarily, leads to Zone 0

Primary Grade, normally but not necessarily, leads to zone 1

Secondary grade, normally but not necessarily, leads to zone 2

3.6 Fluid category is very difficult to interpret from the above code for non-chemical engineers.

So, we interpret it from a product classification code called Apparatus & Enclosure sub-

grouping to be covered as a separate topic.

3.7 A means to relate the classification to safety standards to be followed is also now available

through another guideline called ATEX 100a and will be covered in other topics.

3.1 Area Classification Drawings will show Area Classification symbols as given below. The

area bounded by a cloud denotes extent and area boundary.

3.2 Zone 0 Zone 1 Zone 2 Non-hazardous

3.3 So Zone and Gas group must be specified in data sheet as a minimum as per this standard.

4 AMERICAN STANDARD OF AREA CLASSIFICATION

4.1 The American Petroleum Institute API has two standards - API RP 500 and RP 505. Theseoriginate from or traceable to other previous U.S. national standards such as National

Electric Code or NEC, National Fire Protection association or NFPA etc.

API RP 500: Recommended Practice for Classification of Locations for Electrical

Installations at Petroleum facilities classified as Class 1, Division 1 and Division 2 (Nov.

1997)

API RP 505: Recommended practice for classification of locations for Electrical installations

at Petroleum facilities classified as Class1, Zone 0, Zone 1and Zone 2.(Nov. 1997)

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API RP 505 is normalized to IEC and thus the link between two broad standards of Area

Classification is established.

4.2 API RP 500

4.2.1 Classification tables

Table 1 Table 2 Table 3

Class Class

Description

Division Division

Description

Gas (as per

NEC) & Fluid

Group

Representativ

e Gas Group

Atmospheres

Description

Class 1 Flammable

gases or

vapors are ormay be

present to

form

explosive or

ignitable

mixtures

Division 1 Flammable

gas or vapor

is expectedto exist under

normal

operating

conditions or

under faulty

operation or

faulty

electrical

equipment

Group A Acetylene

Division 2 Flammable

gas or vapor

may bepresent but is

contained

within closed

system or not

allowed to

accumulate

by proper

ventilation

Group B Hydrogen

series

Group C Di-ethyl etherGroup D Gasoline

Unclassified or

Non-

Hazardous

Areas not

classified as

Division 1 or

Division 2

Liquid- Class 1

Petroleum

Class for liquids

is based on

flash pointLiquid- Class 2

Petroleum

Liquid- Class 3

petroleum

Crude oil-

Mostly

planned as

Class 1

4.2.2 Class 2 Combustible Dust atmospheres and Class 3- Combustible Fibers and Flyings are

not in above table for clarity and is outside the scope of this article.

4.2.3 Area Classification Drawings will show Area Classification symbols as given below. The


Division 1 Division 2 Special Division 1

So Class, Division and Gas group must be specified in data sheet as a minimum, as per

this standard.

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4.3 API RP 505

4.3.1 Classif ication Table

Class detail is as per API RP 500. Zone detail is essentially as per IP 15. Division detail is

not required as it is covered by Zone detail. Fluid categorization i.e. Gas grouping is as per

IEC 70-10 (but Flammable Liquid Categorization is same as in IP 15 or API RP 500).

Gas

Grouping

Class

Description Remarks

I Not used but denotes

underground gas such as fire

damp (Methane)

II A Atmospheres containingacetone, ammonia, ethyl

alcohol, gasoline, methane,

propane etc.

MSEG- Max. ExperimentalSafe gap and MIC Max.

igniting current ratio values

decide gas grouping

II B Atmospheres containing

acetaldehyde, ethylene etc.

II C Atmospheres acetylene,

Hydrogen etc.

4.3.2 Area Classification Drawings will show Area Classification symbols as given below The


Zone 0 Zone 1 Zone 2 Unclassified

So Class, Zone and Gas group must be specified in data sheet as a minimum as per this

standard. Class is implied as Class 1 if not specified explicitly.

5 WHAT HAPPENS IF YOU IGNORE AREA CLASSIFICATION ?

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Courtesy: MIT Web site

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