Classification of and Protection for Hazardous Locations

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Classification Classification Of And Protection For Hazardous Locations

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : El Electrical File Reference: EEX20801

For additional information on this subject, contact W .A. Roussel on 874-1320

Engineering Encyclopedia

Electrical Classification Classification of and Protection for Hazardous Locations

CONTENTS

P AG E S

PRINCIPLES OF COMBUSTION .....................................................................1 Fire Triangle ............................................................................................1 Flammable and Combustible Liquids......................................................2 Ignition Temperature ...............................................................................3 Explosive Range......................................................................................6 POTENTIAL IGNITION SOURCES IN A HAZARDOUS LOCATION ........................................................................................................9 Arcs and Sparks.....................................................................................10 Hot Surfaces ..........................................................................................11 HAZARDOUS LOCATION LOCATION CLASSIFICATIO CLASSIFICATION N SYSTEM ............. ................... ...........12 .....12 Factors Used To Determine The Classification Of A Location.................................................................................................12 Specific Location Classifications ..........................................................13 Class I Hazardous Locations......................................................13 Groups ...................................................................................................13 Divisions................................................................................................15 Class I, Division 1......................................................................16 Class I, Division 2......................................................................17 Differences Between Class I, Class II, And Class III Hazardous Locations ............................................................................17 PROTECTION PROTECTION METHODS FOR ELECTRICAL EQUIPMENT ............. ................... .......20 .20 Fundamental Protection Philosophy......................................................20 Confining the Explosion........................................................................20 Isolation.................................................................................................21 Energy Limitation..................................................................................22 Miscellaneous........................................................................................22 Summary of Protection Techniques and Their Application ............................................................................................23 GLOSSARY .....................................................................................................25

Saudi Aramco DeskTop Standards


Electrical Classification of and Protection for Hazardous Locations

PRINCIPLES OF COMBUSTION This section provides information on the following topics that pertain to the principles of combustion: Fire Triangle • Flammable and Combustible Liquids • Ignition Temperature • Explosive Range • Fire Triangle Fires and explosions result from the chemical reaction among the three elements shown in Figure 1: Fuel • Oxygen • Heat (Ignition Source) •

Fuel

Heat (Ignition Source)

Oxygen Figure 1. Fire Triangle


1



Fires and explosions can occur only in locations in which all three elements of the fire triangle are present. In locations that Saudi Aramco classifies as hazardous, the fuel source is normally a flammable gas, a vapor from a flammable or combustible liquid, a combustible dust, or an easily ignitable fiber. One of these fuels must then be mixed in air or oxygen in a proportion that will produce a flammable or explosive mixture. The source of oxygen is normally the atmosphere surrounding the hazardous location. Finally, an ignition source of sufficient energy must be present to ignite the mixture. Ignition sources are discussed in more detail later in this module. Flammable and Combustible Liquids Liquids are classified as flammable or combustible on the basis of the ease with which they release vapors. The ease with which a liquid releases vapor is referred to as volatility: the more volatile a liquid is, the mo re easily it releases vapor; the less volatile a liquid is, the less easily it releases vapor. The volatility of a liquid can be defined in terms of flash point: technically, the temperature at which a liquid has become hot enough to release sufficient vapor to cause ignition when a flame is applied. Liquids can also be classified as flammable or combustible in relation to flash point: •

The flash point of a flammable liquid is below 100°F (37.8°C).

•

The flash point of a combustible liquid is at or above 100°F (37.8°C).

Figure 2 lists the flash points of the most common flammable and combustible liquids that occur in Saudi Aramco plants and facilities.


2



Flash Point o

Material

F

o

C

Acetone

-4

-20

Benzene

12

-11

Di-isobutylene

23

-5

Fuel Oils

100 to 270

38 to 132

Gasoline

-45

-43

Heptane

25

-4

Jet Fuels

--

--

A and A-1

110 to 150

43 to 66

B and JP4

-10 to 30

-23 to -1

JP5

95 to 145

35 to 63

JP6

100

38

Methanol

52

11

Toluene

40

4

Xylenes

81

27

Figure 2. Flash Points of Common Flammable and Combustible Liquids Ignition Temperature The ignition temperature of a solid, a liquid, or a gaseous substance is the minimum temperature that is required to initiate self-sustained combustion. Ignition temperature is more correctly referred to as auto-ignition temperature (AIT) or spontaneous ignition temperature (SIT) because a substance will, at its AIT or SIT, “automatically” or “spontaneously” ignite due to the heat energy that is present. The published ignition temperatures for materials are extremely conservative because they have been determined under ideal conditions. The ignition temperature of a substance can change substantially due to the following variables: •

The percentage of the vapor or gas in the air.

•

The shape and the size of the space where the ignition occurs.

•

The type and the temperature of the ignition source.

•

The catalytic or other effects of the materials that may be present.


3



•

The oxygen concentration.


4



Figure 3 shows the ignition temperatures for common vapors and gases found in Saudi Aramco plants and facilities. Ignition Temperature Material

o

F

o

C

Acetone

869

465

Acetylene

581

305

Benzene

928

498

Butane

550

288

Carbon Monoxide

1128

609

Di-isobutylene

736

391

Ethane

882

472

-

-

No. 1

410

210

No. 2

494

257

No. 3

505

236

Gasoline

536 to 880

280 to 471

Heptane

399

204

Hexane

437

225

Hydrogen

968

520

Hydrogen Sulfide

500

260

-

-

JP4

464

240

JP5

475

246

JP6

446

230

Methane

999

630

Methanol

725

385

Naphtha

550

288

Propane

842

450

Toluene

896

480

Xylenes

986

530

Fuel Oils

Jet Fuels

Figure 3. Ignition Temperature of Common Vapors and Gases


5



The AITs (SITs) of agricultural and ch emical dusts are similar to the AITs (SITs) of flammable vapors and gases. Figure 4 shows the ignition temperatures for common chemical dusts.

Ignition Temperature Chemical Dust Material

Acetoacetanilide

o

F

o

C

824

440

1022

550

Benzoic Acid

824

440

Dicyclopentadiene dioxide

784

420

Hydroxyethyl cellulose

770

410

Nitrosoamine

518

270

Sorbic acid

860

460

Stearic acid, aluminum salt

572

300

Sulfur

428

220

Adipic Acid

Figure 4. Ignition Temperatures of Common Chemical Dusts Explosive Range All flammable gases, and all flammable and combustible liquids that are heated above their flash point are potentially ignitable. However, ignition will only occur when the fuel-air concentration (vapor/air mixture) is in the explosive range. The explosive range for a flammable gas encompasses all fuel-air concentrations between the lower flammable or explosive limit (LFL or LEL) and the upper flammable or explosive limit (UFL or UEL). For fuel-air concentrations below the LFL (LEL) and above the UFL (UEL), no sustained ignition will occur when the fuel-air mixture is exposed to an ignition source. The LFL (LEL) and the UFL (UEL) for a flammable gas are usually expressed in terms of percentage by volume of gas or vapor in air under normal ambient conditions. The explosive ranges of materials are highly variable as demonstrated in Figure 5. Acetylene, an example of a material with a broad explosive range, has an LFL (LEL) of 2.5% by volume in air and an UFL (UEL) of 100%. This explosive range shows that no air or oxygen is needed for acetylene to ignite explosively since a volume consisting of 100% acetylene will ignite. Gasoline, an example of a material with a narrow explosive range, has a LFL (LEL) of 1.4% by volume in air and an UFL (UEL) of 1.6%. This explosive range shows that a relatively small amount of gasoline vapors in air is required to form an explosive mixture.


6



Flammable Limits % by Volume Material

Lower Limit

Upper Limit

Acetone

2.5

13

Acetylene

2.5

100

Benzene

1.3

7.9

Butane

1.6

8.4

Carbon Monoxide

12.5

74

Di-isobutylene

0.8

4.8

3

12.5

Fuel Oil No. 1

0.7

5

Gasoline

1.4

7.6

Heptane

1.05

6.7

Hexane

1.1

7.5

Hydrogen

4

75

Hydrogen Sulfide

4

44

Jet Fuel (JP4)

1.3

8

Jet Fuel (JP6)

0.6

3.7

Methane

5

15

Methanol

6

36

Naphtha

1.1

5.9

Propane

2.1

9.5

Toluene

1.2

7.1

Xylenes

1.1

7

Ethane

Figure 5. Upper and Lower Flammable (Explosive) Limits for Common Vapors and Gases


7



Figure 6 shows curves for two flammable gases, propane and hydrogen, with ignition energy in joules plotted against volume concentration in air. Ignition energy is the energy that is needed to ignite flammable and combustible materials. It is expressed as a finite quantity that depends on: •

Material.

•

Concentration.

•

Time period over which the energy release occurs.

The parabolic shape is characteristic of all gases and vapors. This figure is useful in demonstrating some of the terminology used in the text as well as reminding everyone that the mere presence of a flammable material does not make an area hazardous from an explosive perspective.

Propane-air, 1 ATM

1.0

rgy (mj) 0.1

Hydrogen-air, 1 ATM

0.01

0

10

20

30

40

50

60

70

80

90

100

Volume Concentration (%)

Figure 6. Ignition Energy vs. Volume Concentration for Propane and Hydrogen


8



POTENTIAL IGNITION SOURCES IN A HAZARDOUS LOCATION A finite quantity of energy is needed to ignite flammable and combustible materials. The energy that is necessary depends upon the particular material involved, the concentration of this material in the air or with an oxidizing agent, and the time period over which the energy release occurs. Much laboratory experimentation has been conducted to determine the minimum ignition energy required to ignite many materials. Figure 7 shows the results of some of this work for a few common materials. It should be noted that these energy levels were recorded under the most ideal conditions. For every day conditions found in the refinery environment, the energy levels required for ignition of any of these materials is considerably higher - in excess of 1 watt. Considerably more energy is required to cause ignition of finely divided dusts, and even more energy is needed for solid materials. This need for greater energy is due to the fact that the same homogeneous mixture which occurs with gases and vapors does not occur with dusts and solids, making the ignition process much less efficient.

Flammable Material

Minimum Ignition Energy, Millijoules (Milliwatt-Seconds)

Acetylene

0.017

Hydrogen

0.017

Ethylene

0.08

Methane

0.3

Figure 7. Minimum Energy Needed to Ignite Common Flammable Gases This section provides information on the following potential ignition sources that are associated with electrical equipment: •

Arcs and Sparks

•

Hot Surfaces

Additional information on these subjects is found on pages 43 through 48 of the supplemental textbook, "Electrical Installations in Hazardous Locations," by Peter J. Schram and Mark W. Earley.


9



Arcs and Sparks Electrical arcs and sparks are potential ignition sources because they produce heat. These sources can be described as follows: •

An electrical arc is a sustained luminous discharge of electricity across a gap in a circuit or between electrodes.

•

An electrical spark is a luminous electrical discharge of very short duration between two conductors that are separated by a gas, such as air.

The duration of the luminous discharge of electricity is the only major difference between an electrical arc and an electrical spark. Electrical arcs and sparks can be generated from a number of sources including the following: •

Arcs due to intermittent contact in electrical equipment.

•

Static electricity (except lightning).

•

Lightning.

The latter two are caused by natural phenomena and are not included in this discussion. Electrical equipment such as motor controllers, circuit breakers, switches, and relays produce electrical arcs as part of their normal operation. These electrical arcs occur each time a motor controller, circuit breaker, switch, or relay performs a switching function. Except for certain signaling, communications, and data processing circuits where the energy released at the contacts is maintained below the energy required to cause ignition, the arc at the contacts of the switching equipment is capable of causing ignition. The contacts of such equipment must be protected to prevent the propagation of explosions to the surrounding flammable atmosphere. In addition to the anticipated arcs at the contacts of switching equipment, arcs can occur as a result of insulation breakdown between live parts at different polarities, or between live parts and grounded metal. Arcs also can occur when wires break or fuses open. For example, an electrical conductor can burn open when subjected to a high level of current, such as a ground fault or short circuit. If the protective system does not isolate power, and if the voltage is high enough, an arc can occur across the open space.


10



Hot Surfaces A hot surface refers to any surface at an elevated temperature in electrical equipment that is exposed to the surrounding atmosphere. A hot surface is another potential source of ignition. Certain types of electrical equipment can produce high surface temperatures during operation. If the external surface temperature of an explosionproof enclosure is above the ignition temperature, the external surface itself can act as an ignition source such that the protective enclosure is inconsequential. High surface temperatures can be created by the normal operation of electrical equipment such as a resistance heater or an incandescent lamp in a light fixture. High surface temperatures can also be created by the abnormal operation of electrical equipment such as when a rotor stalls or when an arcing fault occurs inside a motor enclosure. Electrical equipment that is certified for use in explosive atmospheres has been examined and tested to assure that the potential for ignition due to high surface temperature has been prevented or minimized.


11



HAZARDOUS LOCATION CLASSIFICATION SYSTEM Factors Used To Determine The Classification Of A Location For a description of non-hazardous and hazardous locations read the section titled "What Hazardous Locations Are Not" on page 11 and “What Hazardous Locations Are” on page 15 of the supplemental textbook "Electrical Installations in Hazardous Locations" by Peter J. Schram and Mark W. Earley. A location needs to be classified relative to the presence of flammable gases, vapors and the like, only if electrical equipment is to be installed in that location. The classification of a location will serve as the basis for the minimum requirements that the electrical equipment must meet to be installed in the location. The factors to be considered in determining whether or not a location is classified as hazardous are as follows: •

The likelihood that flammable gases will be present; the quantity of flammable gases expected; the rate at which flammable gases and vapors could be released.

•

The adequacy of the ventilation in the location under consideration.

•

The total volume of the location under consideration.

These three factors are interdependent and must be considered as a group when determining if a location need be classified as hazardous. Certain flammable material containment systems are considered not subject to failure; therefore, the surrounding location is not classified as hazardous. Such systems include: •

All-welded closed piping systems without valves, flanges, or similar devices.

•

Closed piping systems constructed with continuous metallic tubing without valves, fittings, flanges, or similar devices.


12



Specific Location Classifications One of the most common hazardous location classification systems used, and the one used at Saudi Aramco facilities, identifies the type of material present, the specific category of material present, and the probability of its presence. This is stated using the class, group, and division designations, respectively. Class I Hazardous Locations Class I hazardous locations are defined as locations in which flammable gases or vapors are or may be present in quantities sufficient to produce explosive or ignitable mixtures. This section further describes the system used to define Class I hazardous locations into groups and divisions and the importance of group and division designations to equipment protection techniques. Groups Class I hazardous locations are further defined into groups according to the type of flammable gas or vapor present in an atmosphere. The groups were originally established based on tests to determine gap width and lengths required to prevent transmission of the explosion to the surrounding atmosphere. These tests led to the determination of explosion pressure and pressure rise times which affected the flame path lengths and widths required to prevent explosion transmission. The wide range of data also led to the concept of having different groupings of gases based on their explosion characteristics. This test work relates directly to the explosionproof housing protection technique which relies on enclosures designed to be strong enough to withstand internal explosion pressures and with flame path lengths and gap widths conservative relative to the results obtained in the abovedescribed tests. The groupings allowed different design criteria based on the same test results and the grouping of gases as described below. Class I hazardous locations are divided into four groups, with each group containing materials of similar explosion characteristics. The characteristic gas for each group was chosen based on the most commonly available commercial materials at that time: •

Group A hazardous locations have atmospheres containing acetylene.

•

Group B hazardous locations have atmospheres containing hydrogen and other gases and vapors with equivalent characteristics.

•

Group C hazardous locations have atmospheres containing diethyl ether and other gases and vapors with equivalent characteristics.

•

Group D hazardous locations have atmospheres containing gasoline and other gases and vapors with equivalent characteristics.


13



Many flammable gases and vapors were placed in one of the four chosen groups. A study of the chemical characteristics (such as the structure, weight, and other factors) of these flammable gases and vapors showed that the gases and vapors in different groups were distinctly different from each other, but that flammable gases and vapors within a particular group were similar to each other. This finding led to the current practice of grouping unknown gases and vapors by their chemical characteristics rather than by a formal test program which was found to be unnecessary for many materials. The original work was based on establishing requirements for flame paths in explosionproof enclosures by determining maximum experimental safe gaps (MESG) for many materials. Later work in determining minimum ignition energy (MIE) of materials from electrical arcing and sparking showed a close correlation to the MESG data. This correlation allowed researchers and practitioners to conclude that the gas groupings were valid whether explosion protection of electrical parts was by means of containment (explosionproof enclosures) or by limiting available electrical energy in the circuits (intrinsic safety) located in the classified location. Figure 8 lists the group classifications of the common flammable gases and vapors found in Saudi Aramco plants and facilities. Additional information on groups can be found on pages 33 through 37 of the supplemental textbook, "Electrical Installation in Hazardous Locations", by Peter J. Schram and Mark W. Earley.


14



Material

Group Designation

Acetone

D

Acetylene

A

Benzene

D

Butane

D

Carbon Monoxide

C

Ethane

D

Ether

C

Fuel Oils

D

Gasoline

D

Heptane

D

Hexane

D

Hydrogen

B

Hydrogen Sulfide

C

Methane

D

Methanol

D

Naphtha

D

Propane

D

Toluene

D

Figure 8. Group Classifications of Common Flammable Gases and Vapors Divisions The criterion for designating a location as a Class I hazardous location is the probable presence of flammable gases or vapors in a location where electrical equipment is intended to be installed. Once the decision is made to classify a location, the location is assigned a group designation according to the specific gas or vapor that is present in the location. The final description of a Class I hazardous location is to designate the location Division 1 or Division 2. This final classification must consider the likelihood or probability of the presence of a combustible gas or vapor during normal operations.


15



Class I, Division 1 As defined in API Recommended Practice 500 (RP 500), Class I, Division 1 locations are locations that meet one or more of the following criteria: •

Ignitable concentrations of flammable gases or vapors exist under normal operating conditions.

•

Ignitable concentrations of flammable gases or vapors may exist frequently because of repair or maintenance operations or because of leakage.

•

Breakdown, faulty processes, or faulty operation of equipment might lead to the release of ignitable concentrations of flammable gases or vapors and might also be the cause of simultaneous failure of electrical equipment that could become a source of ignition.

Additionally, API Recommended Practice 500 (RP 500) provides the following examples of locations that would be classified as Class I, Division 1: •

Locations where volatile flammable liquids or liquefied flammable gases are transferred from one container to another.

•

Interior locations where volatile flammable solvents are used.

•

Locations containing open tanks of volatile flammable liquids.

•

Inadequately ventilated locations where flammable gases accumulate or volatile flammable liquids stand or collect.

•

Interiors of refrigerators and freezers in which volatile flammable m aterials are stored in open, lightly stoppered, or easily ruptured containers.

•

All other locations where ignitable concentrations of flammable gases or vapors are likely to occur during normal operation.

For additional Class I, Division 1 considerations, read the section of API Recommended Practice 500 (RP 500) titled "Classification Criteria" (part 4.3).


16



Class I, Division 2 As defined in API Recommended Practice 500 (RP 500), Class I, Division 2 locations are locations that meet one or more of the following criteria: •

Locations where volatile flammable liquids, vapors or gases are handled, processed or used, but are normally confined in closed containers or closed systems from which they can only escape in cases of accidental rupture or breakdown of such containers or in cases of abnormal equipment operation.

•

Locations where ignitable concentrations of flammable gases or vapors are normally prevented by positive ventilation but might become ignitable through failure or abnormal operation of the ventilating equipment.

•

Locations adjacent to Class I, Division 1 locations (and not separated by a vaportight barrier) and to which ignitable concentrations of gases or vapors might occasionally be transferred unless such transfers are prevented by adequate positive pressure ventilation from a source of clean air, and effective safeguards are provided to prevent ventilation failures.

Class I, Division 2 locations usually include locations where volatile flammable liquids or flammable gases or vapors are used in containment and would not pose a risk of explosion unless there was a malfunction of both the containment of the flammable materials and any suitable (for Division 2 locations) electrical equipment. For additional Class I, Division 2 considerations, read the section of API Recommended Practice 500 (RP 500) that is titled "Classification Criteria" (part 4.4). Additional information on divisions can be found on pages 31 and 32 of the supplemental textbook, "Electrical Installation in Hazardous Locations", by Peter J. Schram and Mark W. Earley. Differences Between Class I, Class II, And Class III Hazardous Locations The differences between Class I, Class II, and Class III hazardous locations are best described by their associated definitions. The definition for a Class I hazardous location is given here for comparison with the de finitions for Class II and Class III locations as follows: •

•

Class I locations are those locations in which flammable gases or vapors are or may be present in quantities sufficient to produce explosive or ignitable mixtures. Class II locations are those locations that are hazardous because of the presence of combustible dusts.


17



•

Class III locations are those locations that are hazardous because of the presence of easily ignitable fibers or flyings that are not likely to be in suspension in the air in sufficient quantities to produce ignitable mixtures.

These definitions point out that the only major difference between the different classes of hazardous locations is the type of fuel that is present. For further information on the differences between Class I, Class II, and Class III hazardous locations, read the following sections of the supplemental textbook, "Electrical Installations in Hazardous Locations", by Peter Schram and Mark W. Earley: •

Section 2-4, Class I Locations pages 16-32

•

Section 2-5, Class II Locations pages 32-40

•

Section 2-6, Class III Locations pages 40-41

The table in Figure 9 summarizes the characteristics of Class I, Class II and Class II hazardous locations.


18



Divisions Classes

I - Gases, Vapors, and Liquids

Groups

A: Acetylene B: Hydrogen, etc. C: Ether, etc.

1

Explosive concentrations may be normally present and are hazardous.

D: Hydrocarbons, fuels, solvents, etc. II - Dusts

E: Metal dusts (conductive* and explosive)

Ignitable quantities of dust normally is or may be in suspension, or F: Carbon dusts (some are conductive dust may conductive, * and all are be present. explosive)

2

Explosive concentrations are not normally present (but may accidentally exist).

Dust is not normally suspended in an ignitable concentration (but may accidentally exist). Dust layers are present.

G: Flour, starch, grain, combustible plastic or chemical dusts (explosive) III - Fibers and Flyings

Textiles, woodworking, etc. (easily ignitable, but not likely to be explosive)

Ignitable concentrations may be present as a result of fibrous material being handled or used in manufacturing.

A location where flyings or fibers are stored or handled in storage (exclusive of manufacturing).

* Note: Electrically conductive dusts are dusts with a resistivity less than 105 ohm-centimeter. Figure 9. Summary of Class I, II, III Hazardous Locations


19



PROTECTION METHODS FOR ELECTRICAL EQUIPMENT This section provides information on the various methods of protection for electrical equipment for use in hazardous locations as follows: •

Fundamental protection philosophy

•

Confining the explosion

•

Isolation

•

Energy limitation

•

Miscellaneous

•

Summary of protection techniques and there application

•

Comparison of the three major techniques

Fundamental Protection Philosophy The fundamental protection philosophy has two parts: the first concerned with location and the second with precautions. When planning any facility, every effort should be made to locate electrical equipment outside of the hazardous location. This approach has several benefits, with the primary one being a reduction of the risk of explosion to nearly zero regardless of the circumstances. There are, however, many occasions when protection by location is not possible. When this condition exists, then extraordinary precautions are required to reduce the risk of explosion that may be caused by electrical equipment to an acceptable level. The acceptable level or acceptable risk is addressed by the second part of the fundamental philosophy: that is, safety experts worldwide agree that any protection technique applied to electrical equipment should require two independent events, each of low probability, between safe operation and a potential explosion. All protection techniques in use today meet this fundamental criterion as will be demonstrated in the following discussion. Confining the Explosion This method has one technique - the explosionproof housing. In this case, it is fully expected that ignitions or explosions will occur within the enclosure because no attempt is made to prevent entrance of flammable materials. However, the enclosure is designed to withstand the explosion pressure from any internal explosions for the highest gas rating of the enclosure (Groups A, B, C, or D), and any openings in the enclosure are tight enough and long enough to sufficiently cool any products of combustion to a temperature that will not ignite the external atmosphere. Such enclosures are gas group dependent, with the design requirements for Group D materials being the least restrictive.


20



The “two independent event” criterion applied to this technique is subtle, but effective as the following indicates: 1.

The first level of protection obviously consists of the design requirements to be applied to the enclosure.

2.

The second level of protection is more subtle and relates to the tightness of the enclosure and the nature of the electrical parts within. Most enclosure designs are relatively tight, designed for outdoor use to prevent entrance of the elements. Such enclosures have at least restricted breathing and would require presence of flammable materials for long periods of time to enter the enclosure. Assuming that flammable materials do gain entrance, either of two conditions would exist: (1) if there are ignition-capable arcs and sparks occurring in normal service, any entering flammables will simply burn off before they reach an explosive concentration; or (2), if there are no normally arcing or sparking parts, the mixture will sit harmlessly unless there is a concurrent equipment failure resulting in an ignition-capable energy release. If the latter should occur, then the enclosure would be called upon to perform its intended function - to confine the explosion. In either event, the requisite two fault criterion is satisfied. Isolation

There are several equipment design techniques that may be used to satisfy the objective of the isolation method: to isolate the ignition source from the flammable mixture. Techniques include oil immersion, sealing and restricted breathing, potting, and encapsulation. The most common technique is pressurization, also known as purging. This technique is based on maintaining a pressure within the enclosure using a protective gas, normally air, at a level higher than the external pressure. Any flow, therefore, is from within to the outside atmosphere such that no externally present flammables can enter the enclosure. The three levels of protection are as follows: 1.

The application of the pressurizing gas to maintain the internal pressure.

2.

Automatic shutdown at loss of pressure, activated by a pressure sensitive cut-off switch that is applied when there are normally ignition-capable arcing parts or hot surfaces in the enclosure.

3.

A visible or audible alarm at loss of pressure that is activated by a pressure sensing device when the internal parts are non-sparking and that is suitable for use in a Division 2 hazardous location.

The faults as described above make such equipment suitable for use in a Class I, Division 1 hazardous location without regard to the gas group present.


21



Energy Limitation The energy limitation method has two techniques that may be applied: intrinsic safety and non-incendivity. The former is intended for use in Division 1 hazardous locations while the latter is intended for use in Division 2 hazardous locations. For intrinsic safety, two levels of protection are provided if we assume that two faults must occur within the circuitry to cause an increase in the energy levels present. But even after two faults, or one fault with the application a factor of safety of 1.5 to voltage, current, or a combination of both as appropriate, no ignition of the test gas is allowed. The intrinsic safety technique is sensitive to group classifications of hazardous locations since it is based on ignition energy, which increases from Group A to Group D. Miscellaneous Several miscellaneous protection techniques are used which do not fit into any of the above categories. These techniques include: •

Sand filling (a type of isolation)

•

Increased safety

•

Dust-ignitionproof

•

Continuous dilution

Each of these protection techniques is used in specific locations. For example, increased safety is a popular European technique used for non-sparking motors and lighting fixtures. Such designs are suitable for international Zone 1, as shown in Figure 10, which includes the international classifications of hazardous locations.


22



Summary of Protection Techniques and Their Application Figure 10 provides a summary of the various protection techniques and locations where they may, and may not be used. External Location Classification Type Of Protection

Non-Haz

Zone 2

Zone 1

Zone 0

Special Protection Ex s

X

X

X

Some

Intrinsic Safety Ex ia

X

X

X

X

Intrinsic Safety Ex ib

X

X

X

Flameproof Ex d

X

X

X

Pressurized Ex p

X

X

X

Increased Safety Ex e

X

X

X

Encapsulation Ex m

X

X

X

Hermetic Seal Ex h

X

X

X

Type N, Non-incendive Ex n

X

X

Restricted Breathing Ex n

X

X

Unprotected Electrical Equipment

X

Some

Figure 10. Summary of Protection Techniques


23



The table in Figure 10 lists the international designations for the types of protection and the hazardous locations. An “X” in a box means that pertinent type of protection is generally permitted in the X'ed zone classification. The Special Protection, Ex s (special constructions), category can include double protection techniques, such as an explosionproof enclosure that is also pressurized. This use of double protection techniques would be acceptable for use in a Zone 0 location.


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GLOSSARY AIT

Auto- (Apparent) ignition temperature. The ignition temperature of a substance required to initiate or cause self-sustained combustion.

combustible liquid

A liquid with a flash point at or above 100°F (37.8°C); a liquid that releases ignitable concentrations of vapor less easily than a flammable liquid. (See flammable liquid.)

explosionproof

Pertaining to electrical equipment that is designed so that an explosion of flammable gas within the enclosure of the equipment will not ignite inflammable gas outside the enclosure.

flammable liquid

A liquid with a flash point below 100°F (37.8°C); a liquid that releases ignitable concentrations of vapor more easily than a combustible liquid. (See combustible liquid.)

flying

An airborne combustible material such as jute, hemp, rayon, and similar materials.

grade level

A datum or reference level such as ground level.

hermetically sealed

An enclosure sealed through use of a fusion joint rather than through use of a gasket joint. Sealed by welding, silver soldering, brazing, or similar techniques).

The minimum concentration of a material in air that will allow lower explosive propagation of a flame when the gas/air mixture is in contact with an limit (LEL) or ignition source. lower flammability limit (LFL) MESG

Maximum experimental safe gap. The maximum gap between mating surfaces that just prevents explosion propagation determined by explosion test experimentation.

MIE

Minimum ignition energy. The least amount of energy required to ignite a gas-air mixture at its most easily ignited con centration.

propagation

Spreading out and affecting a greater area.

pyrophorics

Materials that ignite spontaneously in co ntact with air.

SIT

Same as AIT above.


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The maximum concentration of a material in air that will allow upper explosive propagation of a flame when the gas/air mixture is in contact with an limit (UEL) or ignition source. upper flammability limit (UFL) vapor pressure

The pressure measured in psia (kilopascals) that is exerted by a volatile liquid.

vapors

Common term for use in referring to the gas emitted from the surface of a flammable or combustible liquid.

volatile

Descriptive of a liquid that releases vap or with relative ease.


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ADDENDUM A TABLE OF CONTENTS

API Recommended Practice 500 (RP 500) Section 4 - Classification Criteria API Recommended Practice 500 (RP 500) Section 5 - Extent of a Classified Location


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Classification of and Protection for Hazardous Locations

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