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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


Flammability Hazards • Bursting shock wave - TNT equiv. • Fireball - thermal hazards • BLEVE - flying fragments • Flash Fire - thermal hazards • Vapor cloud explosions - TNT equiv.


Two Classes Of Hydrocarbon Vapor Cloud Explosions, VCEs • Detonation Class VCE - High level

explosion. • Deflagration Class VCE - Low Level explosion.


Detonation Class VCE • Flammable vapor is ignited in a congested  plant area with vertical confinement. • High flame acceleration leads to detonation. • The sharp impulse force can be equivalent to a TNT explosion. • Damage will be radial from the explosion epicenter. • Overpressure range, PSO, 10 to 5 psig.


Deflagration Class VCE • Flammable vapor is ignited in uncongested area 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.


Flixborough Reactors


Flixborough Flowsheet


The Incident • A 20 inch diameter temporary by-pass pipe Jack-knifed and failed under thermal expansion stress. • 40 of 120 tonnes of cyclohexane escaped into the congested reactor support structure. • Within two minutes, theVCE vaportook cloud ignited and a Detonation Class place (35 tons TNT equiv).


Thermal Expansion Jack-Knife


VCE Results Flammable Hazard V1.2 10.00 Clancey Gugan P S O   i n 1.00   p s i g

Flix Pts. Edge Of Cloud

0.10 100

1,000 Distance From Vessel, Feet



Flammable Hazard V.12 Type of Damage

Overpressure PSO, psig 10.01 7.25 6.09

Radial Dist., Ft. 376 468 527

Limit Of Major Plant Eqt Damage Steel Panel Building Demolished API Tank 50% Full Uplifts Non Reinf Conc Blocks Shattered Lower Limit Serious Struct Damage

5.00 3.48 3.00 2.51 2.20

603 772 854 965 1,053

M arnruKgnaote cd keSdteOevl ePra-nN Co eo ls EBaurcD ka lemage Glass Damage Shattered Limit Minor Structural Damage Limit of Glass Failure

2 1..0 00 0 0.50 0.41 0.15

1 1,,1 82 05 4 2,892 3,334 6,721

Flixborough Type Damage Distillation Tower Overturned Piperack Bent - Piping Breaks


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, Overpressu re, 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


Control Rm, Stl Rf




Control Rm, Conc Rf




Cooling Tower


Tank: API Cone Roof Instrument Cubicle






n d






k a



Furnace Heater


Reactor: Chemical






t i






Tank: Float'g Roof


Reactor: Cracking


v i,p


t u


d i




Plot Plan Showing VCE Impact


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.


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.


Events Cont’d  • 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.


Events Cont’d  • There was no experienced works manager, WM, available on site at the time of the removal of R-5. • The previous WM, a good maintenance engineer with 25 yrs of experience, experie nce, had quit

 because an anticipated to an outside person. promotion was given


Events Cont’d  • As there was no experienced mechanical engineer 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 were  performed on the the by-pass connection.


Events Cont’d  • The by-pass line was quickly installed and the plant put into start-up mode. • Shortly after start-up, the by-pass line failed causing 40,000 lbs of cylcohexane to leak into the confined spaces of the reactor support structure. • Within two minutes, the vapor cloud exploded.


Lessons Learned • The main root cause of this incident was the use of cooling water with nitrates to quench cyclohexane leaks on the reactors. • Another root cause was installing a by-pass line, or any line for that mater, without stress analysis. This is a recipe for disaster. • A third root cause was management must recognize when they are vulnerable to critical manpower changes.


Lessons learned Cont’d  • More control is required to conduct good

engineering and running.practices once the plant is up • Poor location and poor construction of the

control room. • Plant was too congested at the design stage. • Must minimize hazardous inventories.


Lessons Learned Cont’d  • Process hazard review required at regular

intervals. • Plant must adhere to pressure vessel regulations. • Require emergency planning with the community.


Deterministic Pre-planning Flammable Hazard V1.2 • Defines vapour cloud characteristics

 between UEL and LEL. • BLEVE shock wave, thermal and fragmentation analysis. • Flash Fire thermal analysis. • VCE analysis. • Space separation (ISBL, OSBL and green



Possible Exam Questions • How does a Detonation Class VCE differ

from a Deflagration Class VCE? • Describe the characteristics of the two type of explosions. • What were the three root causes of this incident?

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