Inerted Vessels: Preventing Inerted Vessels: Preventing Hazards Caused by Gas BuoyancyHazards Caused by Gas Buoyancy

2008 Mary Kay O2008 Mary Kay O’’Connor Process Safety Center Connor Process Safety Center International Symposium International Symposium College Station, TexasCollege Station, TexasTuesday, October 28, 2008Tuesday, October 28, 2008

presented bypresented by

Russell A. Ogle, Ph.D., P.E., CSPRussell A. Ogle, Ph.D., P.E., CSP

rogle@exponent.comrogle@exponent.com(630) 743(630) 743--77047704

IntroductionIntroduction

••

““InertingInerting””

refers to the removal or refers to the removal or prevention of combustible mixtures in prevention of combustible mixtures in vesselsvessels

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Perfect mixing assumptionPerfect mixing assumption

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Buoyancy effects can impede perfect Buoyancy effects can impede perfect mixingmixing

Hot Work Safety: Hot Work Safety: HazardsHazards

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Fires and explosionsFires and explosions

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AsphyxiationAsphyxiation

Hot Work Safety: Hot Work Safety: Standards and GuidelinesStandards and Guidelines

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NFPA 51B: NFPA 51B: Fire Prevention during Welding, Cutting Fire Prevention during Welding, Cutting and Other Hot Workand Other Hot Work

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NFPA 69: NFPA 69: Explosion Prevention SystemsExplosion Prevention Systems

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AWS F4: AWS F4: Safe Practices for Welding and Cutting of Safe Practices for Welding and Cutting of Containers and PipingContainers and Piping

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API 2015: API 2015: Safe Entry and Cleaning of Petroleum Safe Entry and Cleaning of Petroleum Storage TanksStorage Tanks

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NFPA 327: NFPA 327: Procedures for Cleaning or Safeguarding Procedures for Cleaning or Safeguarding Small Tanks without EntrySmall Tanks without Entry

Layers of ProtectionLayers of ProtectionSystem Safety System Safety HierarchyHierarchy

Hazard ControlHazard Control ExamplesExamples

Eliminate hazardEliminate hazard Fill vessel with water or Fill vessel with water or sandsand

Passive safeguardsPassive safeguards Blind flangesBlind flanges

Active safeguardsActive safeguards Inert gasInert gas, vessel , vessel cleaningcleaning

WarningsWarnings Atmospheric monitoringAtmospheric monitoring

Procedural safeguardsProcedural safeguards Procedures, PPE, Procedures, PPE, training, supervisiontraining, supervision

Specify the Inerting Design Objective

Select the Inerting Medium and Method

Determine the Minimum Quantity of Inert Gas

Promote Good Mixing During Inerting Procedure

Verify with Atmospheric Monitoring

Design Strategy for Design Strategy for Inerting Process VesselsInerting Process Vessels

Purging and Inerting of Purging and Inerting of Process VesselsProcess Vessels

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PurgingPurging––

Number of volume exchangesNumber of volume exchanges

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BlanketingBlanketing––

MakeMake--up flow rateup flow rate

••

Control volume analysisControl volume analysis––

Perfect mixing assumptionPerfect mixing assumption

Departures from Perfect Departures from Perfect Mixing Caused by BuoyancyMixing Caused by Buoyancy

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Unexpected intrusion of air into inerted Unexpected intrusion of air into inerted vesselvessel

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Internal obstruction creates stagnant Internal obstruction creates stagnant pocket of flammable gaspocket of flammable gas

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Density stratification during inert Density stratification during inert blanketingblanketing

Size and Shape Influences Size and Shape Influences Mixing PatternsMixing Patterns

Stagnant zones

Stack effect

Densimetric Froude NumberDensimetric Froude Number

gDVFr

V, V, characteristic velocitycharacteristic velocityρρ'='=[[ρρ

aa --ρρ

00 ]/ ]/ ρρ

0 0 ,, modified densitymodified density

ρρ

aa ,, ambient gas densityambient gas densityρρ

0 0 ,, inert gas densityinert gas density

g, g, gravitational accelerationgravitational accelerationD,D, characteristic dimensioncharacteristic dimension

Geometric FactorsGeometric Factors

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OpeningsOpenings––

Promote outward flow of light gasPromote outward flow of light gas

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Aspect RatioAspect Ratio––

Large height/diameter promote flow of light Large height/diameter promote flow of light gasesgases

••

Flow obstructions Flow obstructions ––

Can create stagnant zonesCan create stagnant zones

Imperfect Mixing in a Large Imperfect Mixing in a Large VesselVessel

Stagnant zone

Inert gas

Case Study #1Case Study #1

Unexpected intrusion of air into a vessel with Unexpected intrusion of air into a vessel with an inert atmosphere blanketan inert atmosphere blanket

Nitrogen blanket temporarily Nitrogen blanket temporarily stoppedstopped

Contractors to remove residual Contractors to remove residual polypropylene with vacuum hosepolypropylene with vacuum hose

Manway open for several hoursManway open for several hours

Air displaced NAir displaced N

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Flammable mixture formed with Flammable mixture formed with residual flammable gasresidual flammable gas

Ignition source: smoking materialsIgnition source: smoking materials

Flash fire causing one fatalityFlash fire causing one fatality

Prevent Leakage into Prevent Leakage into or out of Vesselor out of Vessel

Maintain continuous Maintain continuous nitrogen flownitrogen flow

Nitrogen flow Fr > 1 to Nitrogen flow Fr > 1 to avoid air intrusionavoid air intrusion

Beware asphyxiation Beware asphyxiation hazardhazard

Case Study #2Case Study #2

Internal obstruction creates a stagnant Internal obstruction creates a stagnant pocket of flammable gaspocket of flammable gas

Reactor vessel recently contained Reactor vessel recently contained ethylene oxideethylene oxide

Contractors to remove paint from Contractors to remove paint from manway with grindermanway with grinder

Tank blanketed with NTank blanketed with N

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CGI measurements taken before CGI measurements taken before beginning hot workbeginning hot work

Abandoned pipe connected to the Abandoned pipe connected to the bottom was not identified bottom was not identified

Case Study #2 (cont.)Case Study #2 (cont.)

Internal obstruction creates a stagnant Internal obstruction creates a stagnant pocket of flammable gaspocket of flammable gas

CGI sample tube did not reach tank CGI sample tube did not reach tank bottombottom

EEthylene oxide thylene oxide ““hidhid””

in the blinded in the blinded pipe section during the nitrogen pipe section during the nitrogen purgepurge

Ethylene oxide (denser than Ethylene oxide (denser than NN

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), ), migrated out of the pipe via natural migrated out of the pipe via natural convection and diffusion into the convection and diffusion into the lower section of the vessellower section of the vessel

Anticipate Stagnant ZonesAnticipate Stagnant Zones

Look for dead endsLook for dead ends

Blinded connectionsBlinded connections

BafflesBaffles

Distribution platesDistribution plates

Aggressively sample Aggressively sample the vessel atmospherethe vessel atmosphere

Bottom or floorBottom or floor

Behind obstructionsBehind obstructions

Case Study #3Case Study #3

Density stratification during blanketing: Density stratification during blanketing: lighter nitrogen blanket on top of a heavier lighter nitrogen blanket on top of a heavier fuelfuel--air mixture air mixture

Reactor was first rinsed with water, Reactor was first rinsed with water, then with xylenethen with xylene

Vessel blanketed with nitrogenVessel blanketed with nitrogen

Work required an oxyacetylene Work required an oxyacetylene cutting torchcutting torch

Not all of the penetrations into the Not all of the penetrations into the pipe had been blinded offpipe had been blinded off

Some organic solid (crust) was Some organic solid (crust) was observed on the vessel walls and on observed on the vessel walls and on the internal steam coilsthe internal steam coils

Boiler(inactive)

Valve

PolymerizationReactor

Hot Work

Case Study #3 (cont.)Case Study #3 (cont.)

Boiler was started to supply steam Boiler was started to supply steam to another processto another process

Leaky valve allowed steam into Leaky valve allowed steam into vessel steam coilsvessel steam coils

Hot coilsHot coils

volatilized solvent trapped volatilized solvent trapped in the solid crustin the solid crust

Solvent vapor ignited from the Solvent vapor ignited from the welding torch resulting in a flash fire welding torch resulting in a flash fire and explosionand explosion

The nitrogen blanket did not sweep The nitrogen blanket did not sweep air out of the vessel, it only provided air out of the vessel, it only provided a gaseous barrier on top a gaseous barrier on top

Boiler

Valve

PolymerizationReactor

Hot work

Leak

Xylene Vapor

Avoid Density StratificationAvoid Density Stratification

Use gas buoyancy to Use gas buoyancy to promotepromote

good mixinggood mixing

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If inert gas is If inert gas is less denseless dense

than vessel atmosphere: than vessel atmosphere: ––

Inject inert gas from the bottomInject inert gas from the bottom

––

Extract it from the topExtract it from the top

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If inert gas is If inert gas is more densemore dense

than vessel atmosphere:than vessel atmosphere:––

Inject inert gas from the topInject inert gas from the top

––

Extract it from the bottomExtract it from the bottom

Specify the Inerting Design Objective

Select the Inerting Medium and Method

Determine the Minimum Quantity of Inert Gas

Promote Good Mixing During Inerting Procedure

Verify with Atmospheric Monitoring

Design Strategy for Inerting Design Strategy for Inerting Process VesselsProcess Vessels

Consider Buoyancy EffectsConsider Buoyancy Effects

••

Determine the relative gas densitiesDetermine the relative gas densities––

Molecular weightsMolecular weights

––

TemperatureTemperature

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Rank the gases from heaviest to lightestRank the gases from heaviest to lightest

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How can buoyancy interfere with good How can buoyancy interfere with good mixing?mixing?

Lessons LearnedLessons Learned

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Prevent leakage into or out of vesselPrevent leakage into or out of vessel

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Anticipate stagnant zonesAnticipate stagnant zones

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Avoid density stratificationAvoid density stratification

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Promote good mixingPromote good mixing

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Perform aggressive air monitoringPerform aggressive air monitoring