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
10,000
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
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.
between UEL and LEL. • BLEVE shock wave, thermal and fragmentation analysis. • Flash Fire thermal analysis. • VCE analysis. • Space separation (ISBL, OSBL and green
belt).
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?