HAZARDOUS AREAS

PROTECTION TECHNIQUES

Neil Dennis

Properties of Materials

Flash point & boiling point

Vapour/gas density

Vapour pressure Flammable range

Ignition temperature

Molecular size

Ignition energy +

560

360

310

210

0

100

200

300

400

500

600

Auto Ignition Temperature (Hot Surface)

Deg

rees C

elc

ius

H2 LPG Petrol Kero (Jet)

0

0.5

1

1.5

2

2.5

Ignition Energy mJ

H2 LPG Petrol Kero (Jet)

Less than 2

0.019

Hazardous Area Classification

Electrical Equipment Design

Electrical Equipment – Gas/vapours

Temperature Class

Related to gas/vapour ignition temperature (inversely)

- T1 = 450C

- T2 = 300C

- T3 = 200C

- T4 = 135C

- T5 = 100C

- T6 = 85C

Equipment Group

Factor related to molecular size

Gas/vapour categories:

IIA, IIB and IIC

Sets characteristics for:

- Ability of gas/vapour to pass through small gaps

- Sensitivity to ignition by sparks

Examples: Methane and H2S

Methane H2S

Flash Point: gas gas

Gas density to air: 0.55 1.19

Flammable range: 4.4% – 17% 4% - 44.5%

Ignition Temperature: 600oC 260oC

Temperature class: T1 T3

Safe Flame Gap (MESG) : 1.14mm 0.63mm

Gas Group: IIA IIB

Electrical Ignition

Friction

Chemical reactions

Hot surfaces

Arcs and sparks

Static electricity

Lasers

Used or Created by electrical

equipment

See AS/NZS 60079-14 for requirements

Electrical Equipment - Protection Principles

• Containment (of hot particles)

• Exclusion (of gas/vapour from electrical components)

• Energy limitation (below that required to cause ‘big’ sparks)

• Ignition avoidance (increasing safety factors to avoid conditions that

cause heat or sparks)

• Dilution (capture vapour/gas and remove it)

Flameproof Equipment ‘Ex d’

Oldest technique, Robust equipment but needs to be handled with care.

Flameproof does not mean weatherproof.

Vapour can enter equipment, but if ignited, flames (hot gases) do not leave the enclosure.

Sensitive to ‘gaps’ and surfaces providing controlled release.

Flameproof Equipment - Principles

Gas is cooled below ignition temperature of outside gas

Surfaces cool hot gas on exit Vapour enters

enclosure as it heats up & cools (transpiration)

Electrical equipment may ignite vapour

Explosion forces on lid

Spark

Ex d - When and Where – gas/vapour only

For

Anything that sparks e.g.

relays, contactors – only technique (But see also Ex de)

Unrestricted use for motors (VVVF)

Anything not otherwise possible.

Robust

Against

Use in corrosive environments

Service difficulties

Cost

All bolts in place.

Check cables correctly secured (fully engaged with correct glands)

Ex d - example

Ex d Faults – Surface Finishes

Tolerance to failure for a 25mm flamepath may be as little as 0.01mm (0.4 thousands of an inch)

Calibration label indicates service

technician does not understand flameproof

issues.

Environmental Performance

Equipment should be installed and maintained for environmental performance. (To maintain explosion protection rating)

Increased Safety ‘Ex e’

• ‘Modern’ technique

• Increasing protection factors over conventional industrial equipment for; • Overheating

• Sparks (clearances and creepage distances etc.)

• Not applicable for equipment that creates sparks (e.g. switches)

• Subject to specific wiring rules to maintain safety issues e.g. circuit protection characteristics, derating of cables, termination details.

Ex e Equipment - gas/vapour only

Ex e equipment allows for the use of alternative materials and construction which may be suited to some environments.

OH6/9

Mixed Techniques Ex de

Combinations of explosion protection in one device

Allows for special constructions

Exe Connections

Miniature Exd Switch

OH6/20

Ex e enclosure flexibility

Ex e faults

Ex e - When and Where – gas/vapour only

For

Junction boxes, Lights, Motors

Contactors, circuit breakers etc as Exde)

Style and materials flexibility.

Cost

Against

VVVF and SS rules for motors

Careful selection requirements.

Installation sensitivities

Some limitations

Intrinsic Safety – ‘Ex i’ – Gas/vapour/dust

Low energy technique (too small to create ignition capable sparks).

Three levels of security.

- Exia - Suitable for zone 0/20 (inside tanks)

- Exib - Suitable for zone 1/21

- Exic - Suitable for zone 2/22 only

Secure under recognised fault conditions (Ex ia – 2 faults, Ex ib – 1 fault, Ex ic – no fault tolerance)

Allows freedom in some types of equipment used

Penalty is complex wiring rules and design issues.

Intrinsically Safe Circuit

P=VI E= ½ CV2 E= ½ LI2

All power and energy needs to be accounted for (including wires and electronics).

Calculations need to be recorded

Only certified Intrinsically Safe electronics can be used in I.S. circuits. However switches and LEDs are ‘simple devices’ that do not need certification.

Wire Capacitance

Switch Wire

Inductance I.S. Barrier

Device Inductance & capacitance

‘Zener Barriers’ require special earthing

provisions

‘Transformer Barriers

Safety segregation distances between wiring as part of ‘fault security

I.S. Barriers

Intrinsically Safe ‘Barriers’ allow safe use of equipment in a hazardous area but must be installed outside a hazardous area.

Ex i Faults

Zener Barrier earthing not up to standard

I.S. wiring not segregated from

other wiring.

Labelling and wire numbering required

Ex i - Faults

Ex i - When and Where – gas/vapour/dust

For

Low power devices only (few watts), instruments, LEDs, simple switches and sensors

Zone 0

Style, function and materials flexibility.

Cost

Against

Power limitations

Complex selection and design requirements.

Installation sensitivities

Non Sparking ‘Ex n’ – gas/vapour only

Only allowed for zone 2

Uses increased security, but is close to conventional industrial standards

Can use a number of ‘sub-styles’ for protection

Not preferred for many sites

Ex n - When and Where – gas/vapour only

For

Junction boxes, Lights, Motors, electronics etc.

Style and materials flexibility.

Cost

Against

Zone 2 only

VVVF and SS rules for motors

Careful selection requirements.

Installation sensitivities

Some limitations

Others

Ex td – dust protected; dusts only,

Ex m – encapsulation;

Gas/vapour/dust,

small items only

Ex p – pressurisation;

Gas/vapour/dust

for anything really special or otherwise can’t be done.

Questions or Questionable?