flixborough slides

7/21/2019 Flixborough Slides

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The Flixborough, UK,

Cyclohexane Disaster, 1 June 74

A Vapor Cloud Explosion (VCE) Case

Study

Presented to ES-317y at UWO in 1999.

Dick Hawrelak


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Flammability Hazards

Bursting shock wave - TNT equiv.

Fireball - thermal hazards

BLEVE - flying fragments

Flash Fire - thermal hazards

Vapor cloud explosions - TNT equiv.


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Two Classes Of Hydrocarbon

Vapor Cloud Explosions, VCEs

Detonation Class VCE - High level

explosion. Deflagration Class VCE - Low Level

explosion.


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Detonation Class VCE

Flammable vapor is ignited in a congestedplant area with vertical confinement.

High flame acceleration leads to detonation.

The sharp impulse force can be equivalentto a TNT explosion.

Damage will be radial from the explosion

epicenter.

Overpressure range, PSO, 10 to 5 psig.


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Deflagration Class VCE

Flammable vapor is ignited in uncongestedarea with open space.

Low flame speed leads to deflagration.

Impulse force longer duration and not as

damaging as a detonation.

Damage may be directional from the

explosion epicenter.

Overpressure range, PSO, 5 to 0.5 psig.


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Flixborough Reactors


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Flixborough Flowsheet


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The Incident

A 20 inch diameter temporary by-pass pipeJack-knifed and failed under thermal

expansion stress.

40 of 120 tonnes of cyclohexane escapedinto the congested reactor support structure.

Within two minutes, the vapor cloud ignited

and a Detonation Class VCE took place (35tons TNT equiv).


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Thermal Expansion Jack-Knife

l


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VCE Results

Flammable Hazard V1.2

0.10

1.00

10.00

100 1,000 10,000

Distance From Vessel, Feet

P

S

O

i

n

p

s

i

g

Clancey

Gugan

Flix Pts.

Edge Of Cloud


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Flammable Hazard V.12

Type of Damage Overpressure Radial

PSO, psig Dist., Ft.

Flixborough Type Damage 10.01 376

Distillation Tower Overturned 7.25 468

Piperack Bent - Piping Breaks 6.09 527

Limit Of Major Plant Eqt Damage 5.00 603

Steel Panel Building Demolished 3.48 772API Tank 50% Full Uplifts 3.00 854

Non Reinf Conc Blocks Shattered 2.51 965

Lower Limit Serious Struct Damage 2.20 1,053

Man Knocked Over - No Ear Damage 2.00 1,125

Corrugated Steel Panels Buckle 1.00 1,804

Glass Damage Shattered 0.50 2,892

Limit Minor Structural Damage 0.41 3,334

Limit of Glass Failure 0.15 6,721


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Basis: Know Your Insurer's Expectations, Fire Protection Design, Hydrocarbons Processing, August 1977,

p-103, by Robert W. Nelson. Updated from Lihou's Table 4 - 8 Sep 96

Overpressure, psig

Equipment 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10 12 14 16 18 20 22

Control Rm, Stl Rf a c d n

Control Rm, Conc Rf a e p d n o

Cooling Tower b f o

Tank: API Cone Roof d k u

Instrument Cubicle a l,m t

Furnace Heater g i t

Reactor: Chemical a i p t

Filter h f v t

Regenerator j i,p t

Tank: Float'g Roof k u d

Reactor: Cracking j i t


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The Consequences

28 plant people were killed.

53 people were wounded and required

medical treatment.

1,800 houses were damaged in the rural

area beyond the plant fence line.

Property damage was $425MM in US

funds.


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Events Leading To The Incident.

Two months before the incident, R-5 was

found to be leaking.

A 6 ft. long crack had developed.

A water hose stream was directed to the

crack to cool and quench the small

cyclohexane leak.


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Events Contd

The cooling water contained nitrates which

encourage stress corrosion of certain carbon

steels. Thus, by trying to relieve the situation, the

quenching was actually acting as a promoter

of corrosion. Ultimately, the reactor had to be removed

from service.


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Events Contd

There was no experienced works manager,WM, available on site at the time of the

removal of R-5.

The previous WM, a good maintenanceengineer with 25 yrs of experience, had quit

because an anticipated promotion was given

to an outside person.


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Events Contd

As there was no experienced mechanicalengineer on site, those remaining decided to

fast track or scratch pad a solution for

the intended by-pass. They sketched a full-scale by-pass line in

chalk on the maintenance floor.

No stress analyses calculations wereperformed on the by-pass connection.


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Events Contd

The by-pass line was quickly installed andthe plant put into start-up mode.

Shortly after start-up, the by-pass line failed

causing 40,000 lbs of cylcohexane to leakinto the confined spaces of the reactor

support structure.

Within two minutes, the vapor cloudexploded.


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Lessons Learned

The main root cause of this incident was theuse of cooling water with nitrates to quench

cyclohexane leaks on the reactors.

Another root cause was installing a by-passline, or any line for that mater, without

stress analysis. This is a recipe for disaster.

A third root cause was management mustrecognize when they are vulnerable to

critical manpower changes.


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Lessons learned Contd

More control is required to conduct good

engineering practices once the plant is up

and running. Poor location and poor construction of the

control room.

Plant was too congested at the design stage.

Must minimize hazardous inventories.


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Lessons Learned Contd

Process hazard review required at regular

intervals.

Plant must adhere to pressure vesselregulations.

Require emergency planning with the

community.


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Deterministic Pre-planning

Flammable Hazard V1.2 Defines vapour cloud characteristics

between UEL and LEL.

BLEVE shock wave, thermal andfragmentation analysis.

Flash Fire thermal analysis.

VCE analysis.

Space separation (ISBL, OSBL and green

belt).


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Possible Exam Questions

How does a Detonation Class VCE differ

from a Deflagration Class VCE?

Describe the characteristics of the two typeof explosions.

What were the three root causes of this

incident?

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