Hazard location equipment for non-engineers

7/30/2019 Hazard location equipment for non-engineers

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Hazardous Locations


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Introduction

Hazardous locations are those where there is ahazardous environment. The Codes forhazardou environments are specifically

concerned with the possibility of a fire orexplosion due to forseeable or avoidable humanerrors like:

Improper installation, selection, and design

Lack of proper maintenance Improper use

Carelessness or oversight


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Hazards

Fire is rapid oxidation/combustion which results inproducing heat, smoke, and light

An explosion is a violent and sudden expansion ofgases produced by rapid burning; it produces a very

strong force when shut in a small space and generallyhas a loud, sharp noise and may have a supersonicshock wave

The fundamental nature of electricity is to create aspark or generate energy which provides ignition in theright mix of fuel and air which leads to a fire orexplosion Both sparks AND heat have to be considered in electrical

equipment design


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Category System

Class I: Gases and flammable liquids such as gasoline

Class II: Dusts and particles such as coal, flour, grain, and paint pigments

Class III: Fibers such as carpet fiber

Within these categories, the type of material also defines severalsubclasses depending on material properties

Division 1: A situation where a hazardous atmosphere is present eithercontinuously or for long periods of time under normal operation, or inwhich they exist frequently because of repair and maintenance or leakage

Division 2: A situation where a hazardous atmosphere is not normallypresent except in the case of an accidental rupture or breakdown ofequipment, or there is an adjacent Division 1 area that can potentially spill

over. Unclassified: Not normally containing a hazardous atmosphere under

normal or abnormal conditions. Example: Piping without valves, checks,meters, or similar devices would be unclassified. Pneumatic coal linesdesigned and maintained according to NFPA 85 is unclassified

See NFPA 30A, 36, 51, 52, 54, 55, 58, 61, 85, 88A, 99, 407, 409, 495, 496,

654, 655 for specific classification guidance. See NFPA 30, 497, and 499 forgeneral guidance. All can be viewed on NFPA web site (www.nfpa.org)
http://www.nfpa.org/http://www.nfpa.org/http://www.nfpa.org/


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European Classifications

European system uses zones instead of

class/division

Does not have a direct equivalent to division 2

NEC allows either system (choose one)

European equipment ratings

Rated by the terms ATEX or Ex

European Manufacturers can self-certify without

any third party verification (CE)

Not acceptable to MSHA or OSHA


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Solutions

There are MANY approaches, and each has particularadvantages and disadvantages

As a PHA team, it is better to ensure that electricalequipment meets hazardous location requirements

and avoid a specific implementation Some methods: dust tight, dust-ignition proof,

nonincendive, explosion proof, hermetically sealed, oilimmersion, purged, pressurized, intrinsically safe,quartz/sand filled, encapsulated, ventilated

Each method has developed to serve specificequipment and atmospheres. Each has advantagesdepending on the classification and the process


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Method Overview

The following notes outline details on the major designmethods for performing safe electrical installations inhazardous locations as background material

Some (but not all) equipment will be specifically certified

for use with a particular method by a certifying agency (FMGlobal, TUV)

Some equipment is automatically approved Example: TEFC (Aurora standard) motors are approved for use in

Class II, Division 2 areas

Some methods have greater flexibility or higher powerlimitations at the expense of cost and weight. Others (oilfilled) are equipment specific.

Againa PHA team will want to ensure that the hazardouslocations are documented and that the electrical team is

aware of them, not specify solutions


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Coal Mine History

The use of electricity in the explosive environment wasinitially done in the mining industry where theefficiency and power of electricity was quicklyrecognized

The early approach was to burn any fire damp(methane) gases to reduce the risk of an explosion.Fire men were young miners who were covered withwet sacking and go ahead of the others with long wicksand hold them up in the highest parts of the mine to

burn off methane. Methane flames are also a differentcolor which was one of the first combustible gassensors

The method was effective but considered primitive

Flare towers are still in common use in refineries today


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Alternative Coal Solution

The first improved technique wasventing (mine ventilation) which isstill a major concern forunderground mines today

The second was the Davy safetylamp. This lamp had a wire meshthat cooled the hot gases whichprevented the methane around a

lantern from igniting 1800s, first example of explosion

proof method


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Pressuring/Purging Methods

Most flexibleany equipment can be used in any sizeenclosure

Can be high cost to operate and maintain Often less expensive due to flexibility and enclosure size

Methods are to pressurize an enclosure to preventhazardous atmospheres from entering, or to purgewith enough fresh air to achieve the same effect

Usually requires some sort of monitoring and control

to shut down if the purge/pressurizing system fails Must consider maintenance access especially with

pressurized systems, and repressurizing/repurging

Can require process calculations to verify effectiveness


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Explosion Proof Method

Dr. Ing Carl Beylingdeveloped flame proof in1908.

An explosion proofenclosure is designed towithstand the pressure ofan internal explosion

Also called dust ignitionproof in dusty areas

Very effective butenclosures are cast, heavy,expensive, and requireexpensive wiring withspecial seals


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Flameproof Gap

Misconception: Not gas tight

Allows gas/dust to exist inside equipment

Hot gas jets through the opening which causes it to cool

Openings are designed so that an escaping flame will becooled to the point where it will not ignite any gas or dustoutside the enclosure


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Nonincendive Method

Every make/break component (relay, switch,etc.) must: Be nonarcing

Use an electrical circuit that limits the power sothat the arc cannot cause anything to ignite

Be sealed so that a spark cannot cause ignition The only remaining concern is heat, which is

addressed by enclosure design

Enclosures must be weather proof and impact-resistant

Wiring must also be rated for the hazardouslocation

Note: Many intrinsically safe ventilation fansand other parts are actually mislabeled and arenonincendive


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Intrinsically Safe Method

Dates back to Senghenydd Coal Mine Disaster onOctober 13, 1913. Just prior to the explosion,older wet cell batteries were upgraded to drycell batteries (note: no MOC!)

It turns out that the higher current output of thedry cells created a spark large enough to ignitemethane in a coal mine. Older wet cells wereincapable of causing ignition.

Intrinsically safe was born! To date, there are no recorded cases of IS

equipment causing a fire or explosion


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IS Design

Barriers Barriers electrically limit both current and voltage to within

the safe limits required by the intrinsically safe design

Barriers must be located within the non-hazardous area

Generally this is less than 1.3 Watts (low power)

Some portable devices on the market have a specialbattery (back to the old wet cell concept) which isdesigned to allow the IS power supply inside thehazardous area

Some PLC hardware has barriers integrated into it as aconvenience. However manufacturers (Honeywell, AB)charge far more for convenience and it is usually lessexpensive to use a separate barrier


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IS Devices

Field devices must be designed to not accumulate energy to a levelwhere a spark or heat could form

Devices must be either rated as IS or else be a simple apparatus: All standard wiring (no special wiring required for IS)

Passive components: switches, junction boxes, resistors, and some

simple semi-conductor devices Sources of generated energy that do not exceed 1.5 volts, 100 mA, and

35 mW (thermocouples and photocells)

LVDT type position sensors

Magnetic pickup coils if the inductance does not exceed the limit on ISequipment

Vibration sensors were previously excluded but are now allowed

Most 120 VAC equipment cannot be approved but most 24 VDCinstrumentation and controls are approved


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IS Wiring

IS wirng is simply standard wiring methods

No special wiring is required


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Hazardous Location ComparisonMethod Power Wiring Best Use Cost

Intrinsically

safe

Very limited Standard LED pilot lights, push

buttons, most field

instruments, radios

Low

Nonincendive Unlimited Special Medium size motors,

lights, power relays

Medium

Explosion

proof/dust-

ignition proof

Limited Special Junction boxes, smaller

motors

Medium

to high

Dust tight Unlimited Tray/Conduit Standard process

equipment

Low

Sealed/oil

filled/sand

filled

Limited Special Transformers, some

motors, some medium

voltage equipment

Low

Purged/Pressur

ized

Unlimited Unliimted Entire MCCs, control

rooms, custom equipment

Low to

high

Documents

Hazard location equipment for non-engineers