DESIGNS TO DESIGNS TO PREVENT FIRE & PREVENT FIRE &

EXPLOSIONEXPLOSION

LECTURE 11LECTURE 11

Eliminate Ignition SourcesEliminate Ignition Sources

Typical ControlTypical Control Spacing and LayoutSpacing and Layout Spacing and LayoutSpacing and Layout Work ProceduresWork Procedures Work ProceduresWork Procedures Sewer Design, Diking, Weed Sewer Design, Diking, Weed

Control, HousekeepingControl, Housekeeping

ProceduresProcedures

Fire or FlamesFire or Flames Furnaces and BoilersFurnaces and Boilers FlaresFlares WeldingWelding Sparks from ToolsSparks from Tools Spread from Other Areas Spread from Other Areas

Matches and LightersMatches and Lighters

Eliminate Ignition SourcesEliminate Ignition Sources

Hot SurfacesHot Surfaces Hot Pipes and EquipmentHot Pipes and Equipment Automotive EquipmentAutomotive Equipment

Typical ControlTypical Control Area ClassificationArea Classification Grounding, Inerting, Grounding, Inerting,

RelaxationRelaxation Geometry, SnuffingGeometry, Snuffing ProceduresProcedures

ElectricalElectrical Sparks from SwitchesSparks from Switches Static Sparks Static Sparks

……………………………………………… LightningLightning Handheld Electrical EquipmentHandheld Electrical Equipment

Typical ControlTypical Control SpacingSpacing ProceduresProcedures

What else can be done?What else can be done?

InertingInerting Controlling static electricityControlling static electricity Explosion-proof equipment & instrumentsExplosion-proof equipment & instruments VentilationVentilation Sprinkler systemsSprinkler systems

InertingInerting

Process of adding Process of adding inert gasinert gas to combustible mixture to to combustible mixture to reduce reduce concentration of oxygenconcentration of oxygen below limiting oxygen concentration below limiting oxygen concentration (LOC)(LOC)

Inert gas- Inert gas- nitrogennitrogen, , carbon dioxidecarbon dioxide, , steamsteam(sometimes)(sometimes) Inerting begins with Inerting begins with initial purge of vessel with inert gasinitial purge of vessel with inert gas to to

bring oxygen concentration down to safe concentrationsbring oxygen concentration down to safe concentrations Commonly used control point=4% below LOC~6% oxygen if Commonly used control point=4% below LOC~6% oxygen if

LOC is10%LOC is10%

Methods of inertingMethods of inerting

Vacuum purgingVacuum purging Pressure purgingPressure purging Combined pressure-vacuum purgingCombined pressure-vacuum purging Vacuum & pressure purging with impure nitrogenVacuum & pressure purging with impure nitrogen Sweep-through purgingSweep-through purging Siphon purgingSiphon purging

Vacuum purgingVacuum purging

Not used for large storage vessels because they are not Not used for large storage vessels because they are not designed for vacuumsdesigned for vacuums

Reactor~designed for full vacuum(-760 mm Hg gauge OR 0.0 Reactor~designed for full vacuum(-760 mm Hg gauge OR 0.0 mm Hg absolute)mm Hg absolute)

Steps in vacuum purging:Steps in vacuum purging: Drawing vacuum until desired vacuum is reachedDrawing vacuum until desired vacuum is reached Relieving vacuum with inert gas~NRelieving vacuum with inert gas~N22 or CO or CO22

Repeat steps 1 & 2 above until desired oxidant Repeat steps 1 & 2 above until desired oxidant concentration is reachedconcentration is reached

Concentration after Concentration after jj purge cycles, vacuum and relief is given purge cycles, vacuum and relief is given by:by:

yy00=initial oxidant concentration=initial oxidant concentration yyjj=final target oxidant concentration=final target oxidant concentration PPHH=initial pressure=initial pressure PPLL=vacuum pressure=vacuum pressure nnHH=number of moles at P=number of moles at PHH

nnLL=number of moles at P=number of moles at PLL

j j

L Lj

H H

n Py y y

n P

Total moles of inert gas added for each cycle is constant. For j Total moles of inert gas added for each cycle is constant. For j cycles, the total inert gas is given by:cycles, the total inert gas is given by:

2N H L

g

Vn j P P

R T

Example 7.1Example 7.1

Use a vacuum purging technique to reduce the oxygen Use a vacuum purging technique to reduce the oxygen concentration withing a 1000-gal vessel to 1 ppm. Determine concentration withing a 1000-gal vessel to 1 ppm. Determine the number of purges required and total nitrogen used. The the number of purges required and total nitrogen used. The temperature is 75 degrees F, and the vessel is originally temperature is 75 degrees F, and the vessel is originally charged with air under ambient conditions. A vacuum pump is charged with air under ambient conditions. A vacuum pump is used that reaches 20 mm Hg absolute, and the vacuum is used that reaches 20 mm Hg absolute, and the vacuum is subsequently relieved with pure nitrogen until the pressure subsequently relieved with pure nitrogen until the pressure returns to 1 atm absolutereturns to 1 atm absolute

Pressure purgingPressure purging

Vessels can be pressure-purged by adding Vessels can be pressure-purged by adding inert gas under inert gas under pressurepressure

After the added gas is diffused throughout the vessel, it is After the added gas is diffused throughout the vessel, it is vented to the atmosphere~usually down to vented to the atmosphere~usually down to atmospheric atmospheric pressurepressure

Vessel is Vessel is initially at Pinitially at PLL and is and is pressurized using a source of pressurized using a source of pure nitrogen at Ppure nitrogen at PHH

nnLL=total moles at atmospheric pressure (low pressure)=total moles at atmospheric pressure (low pressure) nnHH=total moles under pressure (high pressure)=total moles under pressure (high pressure) Initial concentration of oxidant (yInitial concentration of oxidant (yoo) is computed after the ) is computed after the

vessel is pressurized (1st pressurized state)vessel is pressurized (1st pressurized state)

j j

L Lj

H H

n Py y y

n P

2N H L

g

Vn j P P

R T

Example 7.2Example 7.2

Use a pressure purging technique to reduce the oxygen Use a pressure purging technique to reduce the oxygen concentration in the same vessel discussed in Example 7.1. concentration in the same vessel discussed in Example 7.1. Determine the number of purges required to reduce the oxygen Determine the number of purges required to reduce the oxygen concentration to 1 ppm using pure nitrogen at a pressure of 80 concentration to 1 ppm using pure nitrogen at a pressure of 80 psig and at a temperature of 75 degrees F. Also, determine the psig and at a temperature of 75 degrees F. Also, determine the total nitrogen requiredtotal nitrogen required

Combined pressure purgingCombined pressure purging

Purging cycles for a pressure-first purge (Fig 7.3)Purging cycles for a pressure-first purge (Fig 7.3) Purging cycles for evacuate-first purge (Fig 7.4)Purging cycles for evacuate-first purge (Fig 7.4)

j j

L Lj

H H

n Py y y

n P

Vacuum and pressure purging with Vacuum and pressure purging with impure nitrogenimpure nitrogen

Previous equation only applies for pure nitrogenPrevious equation only applies for pure nitrogen Nitrogen 98%+ rangeNitrogen 98%+ range Remaining impurities=oxygenRemaining impurities=oxygen

1 1

L L

j j oxyH H

P Py y y

P P

Advantages & disadvantagesAdvantages & disadvantages

Pressure purging is faster because pressure differentials are Pressure purging is faster because pressure differentials are greater. However uses more gas than vacuum purginggreater. However uses more gas than vacuum purging

Vacuum purging uses less inert gas because oxygen Vacuum purging uses less inert gas because oxygen concentration is reduced primarily by vacuumconcentration is reduced primarily by vacuum

Combined pressure-vacuum purging~less nitrogen is used Combined pressure-vacuum purging~less nitrogen is used compared to pressure purging, especially if the initial cycle is compared to pressure purging, especially if the initial cycle is a vacuum cyclea vacuum cycle

Sweep through purgingSweep through purging Adds purge gas into a vessel at one opening and withdraws the Adds purge gas into a vessel at one opening and withdraws the

mixed gas from the vessel to the atmosphere from another mixed gas from the vessel to the atmosphere from another openingopening

Commonly used when vessel not rated for pressure or vacuumCommonly used when vessel not rated for pressure or vacuum Purge gas is added and withdrawn at atmospheric pressurePurge gas is added and withdrawn at atmospheric pressure

V=vessel volumeV=vessel volume CC00=inlet oxidant concentration=inlet oxidant concentration QQvv=volumetric flow rate=volumetric flow rate t=timet=time Reduce oxidant concentration from CReduce oxidant concentration from C11 to C to C22

1 0

2 0

ln

v

C CQ t V

C C

Example 7.3Example 7.3

A storage vessel contains 100% air by volume and must be A storage vessel contains 100% air by volume and must be inerted with nitrogen until the oxygen concentration is below inerted with nitrogen until the oxygen concentration is below 1.25% by volume. The vessel volume is 1000ft3. how much 1.25% by volume. The vessel volume is 1000ft3. how much nitrogen must be added:nitrogen must be added:

assuming nitrogen contains 0.01% oxygenassuming nitrogen contains 0.01% oxygen If it is pure nitrogenIf it is pure nitrogen

Siphon purgingSiphon purging

Sweep-through process requires large quantities of Sweep-through process requires large quantities of nitrogen~expensivenitrogen~expensive

Siphon purging is used to minimize this type of purging Siphon purging is used to minimize this type of purging expenseexpense

Starts by filling vessel with liquid-water or any liquid Starts by filling vessel with liquid-water or any liquid compatible with productcompatible with product

Purge gas is added to the vapor space of the vessel as the Purge gas is added to the vapor space of the vessel as the liquid is drained from vesselliquid is drained from vessel

Static ElectricityStatic Electricity Sparks resulting from Sparks resulting from static charge buildupstatic charge buildup (involving at (involving at

least one poor conductor) and least one poor conductor) and sudden dischargesudden discharge Household Example: Household Example: walking across a rugwalking across a rug and and grabbing grabbing

a door knoba door knob Industrial Example: Industrial Example: Pumping nonconductive liquid Pumping nonconductive liquid

through a pipethrough a pipe then subsequent then subsequent grounding of the grounding of the containercontainer

Dangerous energy near flammable vaporsDangerous energy near flammable vapors 0.1 mJ0.1 mJ

Static buildup by walking across carpetStatic buildup by walking across carpet 20 mJ20 mJ

Double-Layer ChargingDouble-Layer Charging Streaming CurrentStreaming Current

The flow of electricity produced by transferring The flow of electricity produced by transferring electrons from one surface to another by a flowing electrons from one surface to another by a flowing fluid or solidfluid or solid

The larger the pipe / the faster the flow, the larger the The larger the pipe / the faster the flow, the larger the currentcurrent

Relaxation TimeRelaxation Time The time for a charge to dissipate by leakageThe time for a charge to dissipate by leakage The lower the conductivity / the higher the dielectric The lower the conductivity / the higher the dielectric

constant, the longer the timeconstant, the longer the time

ControllingControllingStatic ElectricityStatic Electricity

Reduce rate of charge generationReduce rate of charge generation Reduce flow ratesReduce flow rates

Increase the rate of charge relaxationIncrease the rate of charge relaxation Relaxation tanks after filters, enlarged section of pipe Relaxation tanks after filters, enlarged section of pipe

before entering tanksbefore entering tanks

Use bonding and grounding to prevent dischargeUse bonding and grounding to prevent discharge

ControllingControllingStatic ElectricityStatic Electricity

GROUNDING

BONDING

Explosion Proof EquipmentExplosion Proof Equipment

All electrical devices are inherent ignition sourcesAll electrical devices are inherent ignition sources

If flammable materials might be present at times in an If flammable materials might be present at times in an area, it is designated XP (Explosion Proof Required)area, it is designated XP (Explosion Proof Required)

Explosion-proof housing (or intrinsically-safe Explosion-proof housing (or intrinsically-safe equipment) is requiredequipment) is required

Area ClassificationArea Classification National National

Electrical Electrical Code (NEC) Code (NEC) defines area defines area classifications classifications as a function as a function of the nature of the nature and degree of and degree of process process hazards hazards presentpresent

Class IClass I Flammable gases/vapors presentFlammable gases/vapors present

Class IIClass II Combustible dusts presentCombustible dusts present

Class IIIClass III Combustible dusts present but not Combustible dusts present but not likely in suspensionlikely in suspension

Group AGroup A AcetyleneAcetylene

Group BGroup B Hydrogen, ethyleneHydrogen, ethylene

Group CGroup C CO, H2SCO, H2S

Group DGroup D Butane, ethaneButane, ethane

Division 1Division 1 Flammable concentrations normally Flammable concentrations normally presentpresent

Division 2Division 2 Flammable materials are normally in Flammable materials are normally in closed systemsclosed systems

VENTILATIONVENTILATION

Open-Air PlantsOpen-Air Plants Average wind velocities are often high enough to safely Average wind velocities are often high enough to safely

dilute volatile chemical leaksdilute volatile chemical leaks

Plants Inside BuildingsPlants Inside Buildings Local ventilationLocal ventilation

Purge boxesPurge boxes ‘‘Elephant trunks’Elephant trunks’

Dilution ventilation (Dilution ventilation (1 ft1 ft33/min/ft/min/ft22 of floor area) of floor area) When many small points of possible leaks existWhen many small points of possible leaks exist

Sprinkler system typesSprinkler system types Antifreeze sprinkler systemAntifreeze sprinkler system

A wet pipe system that contains an antifreeze solution and A wet pipe system that contains an antifreeze solution and that is connected to water supplythat is connected to water supply

Deluge sprinkler systemDeluge sprinkler system Open sprinklers and an empty line that is connected to Open sprinklers and an empty line that is connected to

water supply line through a valve that is opened upon water supply line through a valve that is opened upon detection of heat or flammable materialdetection of heat or flammable material

Dry pipe sprinkler systemDry pipe sprinkler system A system filled with nitrogen or air under pressure. When A system filled with nitrogen or air under pressure. When

the sprinkler is opened by heat, the system is depressurized, the sprinkler is opened by heat, the system is depressurized, allowing water to flow into the system and out the open allowing water to flow into the system and out the open sprinklersprinkler

Wet pipe sprinkler systemWet pipe sprinkler system A system containing water that discharges through the A system containing water that discharges through the

opened sprinklers via heatopened sprinklers via heat

SummarySummary

Though they can often be reduced in Though they can often be reduced in magnitude or even sometimes designed out, magnitude or even sometimes designed out, many of the hazards that can lead to many of the hazards that can lead to fires/explosions are unavoidablefires/explosions are unavoidable

Eliminating Eliminating at leastat least one side of the Fire one side of the Fire Triangle represents the best chance for Triangle represents the best chance for avoiding fires and explosionsavoiding fires and explosions