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NEBOSH International

Technical Certificate in Oil and Gas

Operational Safety

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• Failure Modes

• Other Type of Failures

• Safety Critical Equipment Controls

• Safe Storage of Hydrocarbons

• Fire Hazards, Risks and Controls

• Furnace and Boiler Operations

Element 3Hydrocarbon Process Safety 2

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Failure Modes

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Failure ModesCreep

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Failure Modes

Or, more generally

Measured in Units of Pa

Dimensionless quantity

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Stress-Strain Curves for Tensile Loading of a wire

Strain

Stressa (elastic limit)

aa/b

c d

c (ultimate tensile strength)

dd (breaking point)

b (Yield point)

Stiff brittle material

Ductile material

Elastic material0

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In the ELASTIC region, Hooke’s law:

Failure Modes

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Stress can arise from, for example:

• Periodic fluctuations in operating pressure• Temperature cycling• Vibration• Water hammer• Periodic fluctuations of external loads

Leading to various types of failure.

Failure Modes

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Needs: • a susceptible material• a corrosive environment (specific to the material)• enough tensile stress to induce the condition

Failure Modes

Stress Corrosion Cracking

Material Cracks on simultaneous exposure to stress and:

Aluminium alloys chloride

Mild steel nitrates

Copper and its alloys ammonia

Also - Corrosion Fatigue (from cyclic stresses)

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Thermal Shock

• Rapid and extreme temperature changes.

• Thermal differences – uneven expansion.

• Stress generated overcomes material strength –cracking and failure

• e.g. failure of weld

Failure Modes

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Brittle Fracture

• Very sudden – no warning.

• Due to structure of the material or timescale of loading –material does not slip

• Cracks quickly spread through the material (may be audible)

Failure Modes

Some factors that promote brittle fracture:

• Low temperature• Impact or “snatch” loading• Residual tensile stresses• Inherent material brittleness

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• No signs of deformation

• Fracture surface:

― ‘bright’― sometimes with ‘chevrons’ ― sometimes with lines and ridges

Failure Modes

Brittle Fracture - Characteristics

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“Safe Operating Envelope”

the limits/boundaries of what is considered safe operation –specified at design stage

Failure Modes

Knowledge of Failure Modes is used to establish safe operating envelope in Initial Design, Process and Safe Operation:

- Maximum safe design loads for vessels, pipework, etc.- Calculate required material thickness- Material selection- Component shape (stress concentration)- Etc.

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Failure of Annular Rim of storage tank

• Corrosion from within

― Due to sea water content (leads to pitting)― Due to high sulphur content (bacterial corrosion)

• Tank settlement into/onto a foundation

― joints and protective finishes affected by the movement (corrosion).

Failure Modes

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Weld Failures – the need for regular inspection and NDT

Other Types of Failures

Visual Inspections

― Naked eye

― A magnifying glass

― A microscope

• Need good light source

• Protective coatings and finishes removed

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Other Non-Destructive Testing

Dye Penetrant

• Uses a three-part spray-can system to clean the area and highlight defects.

• Works on many non-porous materials but only detects surface flaws.

• Often used before and with other methods.


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Magnetic Particle

• Magnetises the component

• Applies magnetic particles or ink

• Defects show as magnetic field is distorted

• Defect tends to cause a concentration of the magnetic field which attracts more particles than surrounding materials


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Eddy Current

• Uses principle of electromagnetic induction.

• When high frequency AC current passed through conductor (e.g. copper coil), fluctuating magnetic field develops.

• If brought close to another electrical conductor, induces current in it.

• Defects, e.g. cracks cause variation in eddy current.

• Having second coil enables detection of changes in induced eddies (used to determine depth of crack, etc.)

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Ultrasonic


Uses generator transmitting pulses of high-frequency sound (ultrasound).

Transmitted in a probe head – in contact with material surface (some contact liquid used).

Detects sound reflected from within the material - output displayed on oscilloscope.

With calibration – can indicate location and depth of defect within the material (not just surface), so access to only one side of material needed.

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Radiography (X-ray or Gamma)


Rays transmitted through material onto strip of photographic film.

Image produced on film (radiograph) – indicates locations of defects as intensity differences:

• Locates internal defects.

• Provides permanent record.

• Expensive, radiation hazard, requires expertise.

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Pressure Testing

• Finished pressure system subjected to pressure test (typically 1.5 x normal working pressure)

• Liquid used rather than gas

• Must be cleaned afterwards


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Use of Strain Gauge (electrical sensitivity)

• A ‘strain gauge’ is attached to the item and its electrical sensitivity is measured.


• Can be left in place to monitor – recording changing stress levels

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Thermal Imaging Camera

• Small variations in heat can be shown on a colour screen

• This can detect the existence of faults in an object or part


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We’ll cover:

• Emergency shut-down (ESD) equipment and systems

• Safety integrity levels (SIL) for instrumentation

• Procedures for by-passing ESD

• Blow-down facilities

• Closed and open drain headers, sewers, interceptors.

Safety Critical Equipment Controls

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The Fire & Gas systems:

• Can detect hazardous events (flame, smoke etc.)

• Set off alarms to alert control personnel

• Set off the ESDs to minimise consequences

• They operate through a number of fire and gas detectors


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Emergency Shut-Down Equipment and Systems

• Monitor and detect faults in process and service systems

• When detected, will shut-down to prevent escalation of hazardous event

• Will protect people and property on the installation from damage


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ESD equipment and systems

• Should be independent from normal production controls

• Control valves should be independent within ESD systems – not used for dual-control or shut-down

• Shut-down and blowdown valves should fail to safety

• Pipework isolator valves should fail closed


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• Blowdown valves should fail open if power supply or control signal is lost

• Safety case will need to justify where fail-safe is not integral to ESD system

• Where by-pass is provided around shutdown valves (for maintenance) they should be locked closed and handwheels removed

• Hydraulic return line valves should be locked open


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Safety Integrity Levels (SIL) for Instrumentation


Increasing probability of failure to perform (its safety functions) on demand (PFD)SIL4 SIL1SIL3 SIL2

SIL is an index of tolerability of failure to perform

SIL needed depends on estimated risk reduction needed for acceptability/tolerability

SIL4 has highest integrity (highest probability that will perform when needed, e.g. where major accident potential)

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Procedures for Bypassing ESD

• Operate under permit conditions, authorised by competent person with justification

• Adequate risk assessment needed


• Minimise bypass time

• Continued application to be monitored and controlled

• Critical controls needed at shift hand-over

• Bypasses to be tested or correct functioning; Full testing after reversal

• Bypasses to be entered in bypass log

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Blowdown Facilities and Flare Types

Blowdown

• removal of liquid from process vessels and equipment (through flares or to tanks) to reduce likelihood of fires or explosions


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• Liquid blowdown should not go to flares designed for gases (flare flame-out; wide discharge spread)

• Route liquid blowdown to facilities to handle large quantities of liquids e.g. storage tank


Beware - gases from liquids can be released (pressure may rupture tank)

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Flaring


Can act as a safety device that will protect vessels and pipework from overpressure.

Many different types – fixed, portable, self-supporting, some just for gas, others liquid and gas, etc.

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Steam-assisted flares (common in refineries)

• Single burner tips

• Elevated above ground to burn vented gas in a diffusion flame

• Steam injected into combustion zone - promotes turbulence for good mixing- introduces air into the flame


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Air-assisted flares

• Use forced air (from fan) for combustion and mixing

• Give a relatively smoke-free flame

• The burner has many small gas orifices in a spider-shaped pattern inside the top of a steel cylinder

• Fan speed can vary to alter the amount of combustion air


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Non-assisted flares

• Simple flare tip (no steam or air mixing)

• Have limited gas streams with a low heat content

• Have a low ratio of hydrogen/carbon that will burn well without producing lots of smoke

• They manage with less air to give complete combustion and have lower combustion temperatures


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Pressure-assisted flares

• Uses vent steam pressure to assist with mixing the combustible fuels at the burner tip

• With enough vent steam pressure they can be used on flare tips that would have used steam or air to give a smokeless discharge

• They have a number of burner heads that operate depending on the amount of gas discharged

• Normally have burner at ground level – so need safe location


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Enclosed ground flares

• Burner heads enclosed in internally insulated shell

• Helps cut down smoke, noise, luminosity, heat radiation and protect from the wind

• Adequate mixing is achieved by a high nozzle pressure-drop so air or steam not needed

• Flare tip height must be adequate to create enough draught to give enough air for smokeless combustion and to disperse thermal plume


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Flare monitoring

• To ensure integrity of the emission and the flame

• Monitoring equipment, e.g. thermocouple sensors, UV flame sensors, remote flame sensors and flue analysers

• Placed in the flame for continuous monitoring


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• Clean flame, where possibly only gas is being burned.


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• Dirty flare

• Steam is often injected into the flame at the tip of the stack to reduce the black smoke (but makes them noisier)


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Drains

• Open drains – for non/low-hazardous (e.g. rain water)

• Closed drains – hazardous, e.g. Offshore, drains


• Closed drains should not be interconnected with any open drain

• Sampling and monitoring needed

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Sewers

• Collects sewage (and organic food waste from galleys), directed through a treatment plant

• Often involves maceration and chlorination of the waste

• Treated sewage mixed with sea water and untreated ‘domestic’ water and discharged through a sewage caisson


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Interceptors (oil/water separators)

• Used to collect and separate oil from contaminated water (e.g. rainwater from hazardous areas)

• Have a series of settling bays Water flows through Oil stays on the top and accumulates Oil sucked out and disposed of

• ‘Cleaned’ water must meet legal limits before discharged to sea/rivers (monitoring)

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Overfilling

Failure of operator to monitor filling (when filling manually)

Failure of pumping system to shut off

Failure/absence of sensors and alarms

Blockage or lack of adequate tank venting or relief systems

e.g. Buncefield fire and explosion

Safe Storage of HydrocarbonsHazards and Risks

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Effects of pressure and vacuum

Over-pressurisation can cause stress on joints and seals in sealed tanks

Floating roof tanks can have roofs lifted or torn

Vacuum can cause implosion of vessel

Safe Storage of Hydrocarbons

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Failure of Tank Shells

Explosion (ignition of flammable contents)

Wind loading and earthquakes

Corrosion (annular rim etc)

Poor construction (materials, welding) and installation

Operational errors (over-pressurisation or vacuum when filling or emptying)

Deformation of the structure can cause failure

Settlement can affect foundations and tank bases


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External Floating Roof Tank

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Internal Floating Roof Tank

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Fixed Roof storage Tank

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Bunding of Storage Tanks- some considerations:

110% of the capacity of the largest tank in the bund


Impervious to liquid being stored

Drainage (e.g. rain water, spills) via locked valve and interceptor

Maintained (vegetation, etc.)

Electrical equipment (pumps, etc.) explosion protected

Crash barriers or bollards (collision protection)

Wall height to take account of ventilation, access, etc.© RRC Training

Tank filling:

‘Top’ filling – splashing, aeration, electrostatic charge

‘Bottom filling’ – pressure may cause tank failure

Overfilling!

Escape, fire, explosion, environmental damage (Buncefield!)

Use level sensors/alarms, shut-down, bunds, spill kits, etc.


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Fire-resistant

Walls up to 15mm thick.

Pressure relief (on top)


Pressurised/Refrigerated Vessels for LPG/LNG

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Loss of Containment and Consequences

Jet (spray) fires

Fuels - Gas, 2-phase, flashing liquids and pure liquids

Characteristics depend on fuel, release rate, etc.

Water content may render fire more unstable

Directed onto structures can cause, e.g. vessel failure

Confined vs. Unconfined


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Pool Fires

Outdoor fires will be well ventilated (combustion controlled by the fuel)

Enclosed fires may become under-ventilated (combustion controlled by the ventilation)

‘static’ or ‘running’ fires


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1. Vaporisation of HC (e.g. LNG vessel leak)

2. Concentration build up (above LEL)

3. Ignition (source with > MIE)

4. Explosion - over-pressure, blast wave, thermal radiation, fire, debris as airborne missiles


Hydrocarbon Vapour Clouds – generation and potential effects

Explosion types: Detonation vs. Deflagration

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Unconfined Vapour Cloud Explosions (UVCE)

Large quantity of flammable gas/vapour released into atmosphere

Cloud ignited before it can be dispersed below LEL

Explosion (usually a deflagration) - Shock waves and thermal radiation, damage, e.g. Flixborough


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Confined Vapour Cloud Explosions (CVCE)

Vapour cloud contained (e.g. vessel or building)

Ignition

Explosion pressure wave may rupture vessel/building walls

Requires only small quantity of vapour

Considerable damage – but usually localised


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Boiling Liquid Expanding Vapour Explosion (BLEVE)

Typical sequence of events:

Fire heats vessel containing flammable liquid (e.g. LPG)

Internal pressure rises – Relief valve operates

Vapour escape reduces vessel liquid level

Fire rapidly heats vapours above the liquid surface

Vessel wall above liquid level weakens and fails < 20 mins –sudden uncontrolled vapour release

Vapour cloud explodes – thermal radiation, blast wave, flying debris


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Pipeline Monitoring


Supervisory Control And Data Acquisition (SCADA) systems– industrial computer systems – that monitor and control (in this case) oil and gas transportation in pipelines.

Detection systems can detect change in flow at leak or tapping point (theft/damage)

Simplest pipeline inspection method – ‘walk the line’

CCTV

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Decommissioning of Plant (an Overview)

Decontamination – using water/air, steam, detergents etc

Dismantling

Disposal (if no longer needed) – including of contaminants

Site clearance/remediation (e.g. contaminated land) and verification


Factors to consider in decommissioning plan: Health and safety, Environmental impact, Technical feasibility, Cost effectiveness

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Decommissioning Old Oil and Gas Wells

Obtain all relevant site information (for possible re-use of the installation)

Effect on marine environment?

Costs? (plugging and abandoning)

Select optimal disposal method and disposal contractors


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Decommissioning of Topside Production Equipment

Removal of deck support structures, drilling decks and plant

Processing and transportation of oil and gas pipelines

Services, welfare and accommodation facilities

Re-use should be a first choice rather than disposal


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Removal and Disposal of Deck and sea-bed support (Jacket) Structures

Cost of removal vs. leave where it is (may present shipping hazard, may contaminate fish stocks etc)

Deck packages can often be removed in modules

Require use of lifting vessels to load onto transporters

Possible use of explosives on pilings and legs (but consider effect on marine life)

Jacket now supports marine eco-system (‘artificial reef’), so some can be left (sea-bed removal may be difficult)


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Pipeline & Power Cable Decommissioning

There are environmental as well as technical issues

Best method depends on location of pipeline, depth buried and/or depth of water

Consider other nearby pipelines, sea-bed structures and the marine environment

Consider removal of onshore-offshore power cables


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SIMOPs can occur due to:

Contractor/Maintenance activities -same location/time

Emergencies (fire/explosion)

Platform and vessel operations

Weather or environmental impacts


Management of Simultaneous Operations (SIMOPS)

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Stakeholder meeting - draw up plan of operation

Appoint overall responsible person (e.g. OIM )

Assign other specific responsibilities; how liaison is achieved; duration for each operation


Managing SIMOPS

Risk assess the project

Each party assembles own work file (covering work in their area)

Project Review meeting

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Each work file will include, for example:

Method statement Drawings/schematics (if applicable) Asset lists for the work The constraints identified for each activity An organisation chart identifying key personnel Main hazards and control measures Communications MoC procedures (for any deviations from plan) Emergency response Etc.

Safe Storage of HydrocarbonsManaging SIMOPS cont’d

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The Project review meeting:

Hazard identification and risk assessment (HIRA)

Consider any clashes of activity

Determine hierarchy of controls

Determine roles and responsibilities for all in the operations; lines of reporting and control


Managing SIMOPS cont’d

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Next steps:

Create Interface documents

Conduct pre-operations briefing

Daily meetings during the work

Operated under single permit-to-work system

Close out process (including review of ‘lessons learned’)

Safe Storage of HydrocarbonsManaging SIMOPS cont’d

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Consider:

Lightning

The fire triangle

Static electricity

Identification of ignition sources

Hazardous area classification ‘zones’

Electrical equipment and tools for use in hazardous areas

Fire Hazards, Risks and Controls

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Lightning

Major static electrical discharge

Superheats surrounding air –bright flash

Audible shock wave (thunder)

Protection – grounded lightning rods


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The Fire Triangle

Potential Consequences: explosion, thermal radiation, shock wave (as discussed earlier- CVCEs, etc.), fires

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Classification of FiresFire Hazards, Risks and Controls

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Stages of Fire (Combustion)


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Electrostatic charges

Static accumulates, e.g. from fuel flowing inside a transfer pipe

Static discharges

Ignition of fuel/air mixture in vicinity (if discharge energy is high enough)

Precautions, typically: good earthing/bonding, use of conductive materials for pipes/vessels; additives in fuels


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Identifying Ignition Sources, e.g.

Open flames

Sparks (electrical switches, grinding tools, internal combustion engines)

Static

Friction

Etc.


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Control and Mitigation of Vapour Phase Explosions:

Building design (Structural protection for personnel; blast panels)

Plant and process design (keep conc. below LEL; eliminate ignition sources; blast resistant equipment; explosion relief/venting devices; spillage containment)

Isolation, Inerting and suppression

Segregation of flammables (storage) and minimise inventory

Procedures (mop up spills)

Monitoring (to detect vapour concentrations in flamm range)


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Zoning and Hazard Area Classification

Zone 0 – a place in which an explosive atmosphere consisting of a mixture of air with dangerous substances in the form of gas, vapour, mist is present continuously, or for long periods of time, or frequently (Zone 20 for dust)


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For DUSTS, equivalent zones are 20, 21 and 22, respectively

Zone Description: a place in which an explosive atmosphere consisting of a mixture of air with dangerous substances in the form of gas, vapour, mist is

0 present continuously, or for long periods of time, or frequently

1 likely to occur in normal operation occasionally 2 not likely to occur in normal operation but, if it does

occur, will persist for short period only

Zoning and Hazard Area Classification for gas, vapour, mist

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Selection of equipment to be used in the hazardous area:

Zone 0 or zone 20 – category 1 equipment

Zone 1 or zone 21 – category 1 or 2 equipment

Zone 2 or zone 22 – category 1, 2 or 3 equipment


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Intrinsically Safe Equipment (Type ‘i’)

Energy level insufficient to produce incendiary spark.

Two categories:‘ia’ - allows for two simultaneous faults (more stringent)‘ib’ - allows for only one fault

Only ‘ia’ equipment can be used (exceptionally) in Zone 0 if sparking contacts are not part of the equipment.

Examples: instrumentation and low energy equipment.


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Flameproof Equipment (Type ‘d’)

Totally enclosed

Casing can withstand internal explosions without igniting surrounding flammable atmosphere.

Suitable for Zones 1 & 2(Not Zone 0)

Heavy and expensive; requires regular maintenance.

Examples: motors, lighting, switchgear and portable handlamps.


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Furnace and Boiler Operations

Boilers and furnaces are widely used to generate and distribute steam and hot water

There are hazards and risks associated with operating boilers and furnaces, including those arising from the loss of pilot supply, over-firing and flame impingement

Further problems arise from over pressurisation of the fire-box, low tube flow and the control of the tube-metal temperature (TMT)


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Fire-tube boiler© RRC Training


Water-tube boiler

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Hazards and Risks of Boiler Operations

Loss of pilot gas supply –Building up an explosive atmosphere. Flame detectors are used to ‘watch’ the pilot flame.

Low tube flow –Low flow (hot water or heated air) causes temperature & pressure rise - potential explosions


Control of tube-metal temperature (TMT) –Otherwise excessive stresses placed on boiler tubes Need to manage water level in the boiler. Low water level can lead to explosions

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Boiler explosions:BLEVE – very high steam pressures

Fire box explosions:Occur after flame out when firebox is hot. Damages internal boiler tubes - structural failure, steam leakage.


Flame impingement:Heating flame directly touches boiler surfaces (heating coils, pipework)Causes erosion, corrosion, cracking, failure. Prevention: proper adjustment of flame

iog1 element 3

Documents

rrc trainingfailure

various types of failure

susceptible material

timescale of loading

material strength cracking

safe operation

tensile loading

movement corrosion