Inerted Vessels: Preventing Inerted Vessels: Preventing Hazards Caused by Gas Buoyancy Hazards Caused by Gas Buoyancy 2008 Mary Kay O 2008 Mary Kay O ’ ’ Connor Process Safety Center Connor Process Safety Center International Symposium International Symposium College Station, Texas College Station, Texas Tuesday, October 28, 2008 Tuesday, October 28, 2008 presented by presented by Russell A. Ogle, Ph.D., P.E., CSP Russell A. Ogle, Ph.D., P.E., CSP [email protected][email protected](630) 743 (630) 743 - - 7704 7704
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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
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
••
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
••
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