CONCEPTS TO CONCEPTS TO PREVENT FIRES & PREVENT FIRES &

EXPLOSIONEXPLOSION

CHAPTER 7CHAPTER 7

Objective Objective

ExamineExamine the designs to prevent fires and the designs to prevent fires and explosion and explosion and compare compare the advantages the advantages and disadvantages of each designs.and disadvantages of each designs.

The specific topics include:The specific topics include:

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 combustible mixture to to reduce concentration of oxygenreduce concentration of oxygen below limiting below limiting oxygen concentration (LOC)oxygen concentration (LOC)

Inert gas- Inert gas- nitrogennitrogen, , carbon dioxidecarbon dioxide, , steamsteam(sometimes)(sometimes)

Inerting begins with Inerting begins with initial purge of vessel with initial purge of vessel with inert gasinert gas to bring oxygen concentration down to to bring oxygen concentration down to safe concentrationssafe concentrations

Commonly used control point=4% below LOCCommonly used control point=4% below LOC Example: 6% oxygen if LOC is10%Example: 6% oxygen if 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 Vacuum & pressure purging with impure

nitrogennitrogen 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 vacuums and usually can withstand a pressure of designed for vacuums and usually can withstand a pressure of only a few inches of wateronly a few inches of water

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

Vacuum purging is a common procedure for reactorsVacuum purging is a common procedure for reactors 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 CO2 2 to atmospheric to atmospheric

pressurepressure 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 Total moles of inert gas added for each cycle is constantconstant. For j . 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 within a 1000-gal vessel to 1 ppm. concentration within a 1000-gal vessel to 1 ppm. Determine the number of purges required and total Determine the number of purges required and total nitrogen used. The temperature is 75 degrees F, and the nitrogen used. The temperature is 75 degrees F, and the vessel is originally charged with air under ambient vessel is originally charged with air under ambient conditions. A vacuum pump is used that reaches 20 mm Hg conditions. A vacuum pump is used that reaches 20 mm Hg absolute, and the vacuum is subsequently relieved with absolute, and the vacuum is subsequently relieved with pure nitrogen until the pressure returns to 1 atm absolutepure nitrogen until the pressure returns to 1 atm absolute

Pressure purgingPressure purging

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

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

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

One practical advantage of pressure purging vs One practical advantage of pressure purging vs vacuum purging is the potential for vacuum purging is the potential for cycle time cycle time reductionsreductions

The pressurization is The pressurization is much more rapidmuch more rapid compared compared to the relatively slow process of developing a to the relatively slow process of developing a vacuumvacuum

Also, the capacity of vacuum systems decreases Also, the capacity of vacuum systems decreases significantly as the absolute vacuum is decreased.significantly as the absolute vacuum is decreased.

Pressure purging, however, Pressure purging, however, uses more inert gas.uses more inert gas. Therefore, the best purging process is selected Therefore, the best purging process is selected

based on cost and performancebased on cost and performance

Example 7.2Example 7.2

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

Combined pressure-vacum Combined pressure-vacum purgingpurging

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

1 1

L L

j j oxyH H

P Py y y

P P

Advantages & disadvantagesAdvantages & disadvantages

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

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

Combined pressure-vacuum purging~less Combined pressure-vacuum purging~less nitrogen is used compared to pressure purging, nitrogen is used compared to pressure purging, especially if the initial cycle is a vacuum cycleespecially if the initial cycle is a 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 Sweep-through process requires large quantities of nitrogen~expensiveof nitrogen~expensive

Siphon purging is used to minimize this type of Siphon purging is used to minimize this type of purging expensepurging expense

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

Purge gas is added to the vapor space of the vessel Purge gas is added to the vapor space of the vessel as the liquid is drained from vesselas the liquid 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 electrons The flow of electricity produced by transferring electrons from one surface to another by a flowing fluid or solidfrom one surface to another by a flowing fluid or solid

When a liquid or solid flows through a pipe (metal or glass), When a liquid or solid flows through a pipe (metal or glass), an electrostatic charge develops on the streaming materialan electrostatic charge develops on the streaming material

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 The voltage difference between two conductive materials is The voltage difference between two conductive materials is

reduced to zero by bonding the two materials, that is by reduced to zero by bonding the two materials, that is by bonding one end of a conducting wire to one of the materials bonding one end of a conducting wire to one of the materials and bonding the other end to the second materialand bonding the other end to the second material

Bonding and grounding reduces the voltage of an entire Bonding and grounding reduces the voltage of an entire system to ground level or zero voltagesystem to ground level or zero voltage

This also eliminates the charge buildup between various parts This also eliminates the charge buildup between various parts of a system, eliminating the potential for static sparksof a system, eliminating the potential for static sparks

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