Main Cause: Temporary bypass pipe between Reactor No. 4 and
Reactor No. 6
After the No.5 reactor was removed for repair, a
temporary bypass pipe was installed between the No.4 reactor
and the No.6 reactor. The bypass pipe required an "S" shape,
because the reactors were mounted on a sort of staircase.
Overview of the bypass between Reactor No. 4 and Reactor
Reactor train vessels R4 to R6 with the temporary bypass pipe
with the liquid levels as for full flow. (Source: Teng-yang et al.,
Details of the support for the cranked by for the cranked bypass
Forces on bypass pipe
Successive stages of a crack-initiated two-stage bellows and pipe
bridge failure.
Sights and Sounds in the Flixborough Bypass Pipe Bridge
Water Sprays
A 20 inch pipe and bellows had developed an inner crack from a
6 ft crack in the mild steel cladding which had been caused by
nitrate corrosion from a temporary spray of cooling water (to
which nitric acid had been added for pH control).
Zinc embrittlement of stainless steel
Many of the stainless steel pipes taken from the disaster site had
suffered cracking due to a process of embrittlement caused by
zinc. The zinc had come into contact with the steel whilst it was
under stress and elevated temperature. It is plain that a relatively
small but fierce fire can, if there is a source of zinc nearby cause
a sudden catastrophic failure.
Sources of zinc
Possible sources of zinc suggested were:
(a) galvanised stairways and walkways
(b) galvanised wire securing the lagging
(c) metal primer chromate paint.
How Zinc can possibly transfer to stainless steel
(a) An oxide film on the stainless steel resisted zinc penetration:
a crack or defect in the oxide film allowed zinc to penetrate.
(b) Oxidation of zinc droplets inhibited penetration.
(c) Applied stress was necessary for rapid zinc penetration.
(d) In most of the experiments liquid zinc was applied to a
stainless steel specimen, but zinc vapour was shown to be
capable of giving embrittlement.
(e) Zinc from a wire can cause embrittlement; the wire needs to
be close to the specimen but contact is not essential.
Contributory cause
 Management error
 Failure by the local management to understand the hazards
of the cyclohexane process.
 No mechanical engineer on-site. Changes to a design should
be overseen and authorized by properly qualified personnel
Failings in technical measures
Plant Modification / Change Procedures: HAZOP
Design Codes Pipework: use of flexible pipes
No pressure testing was carried out on the installed
pipework modification.
Plant Layout: positioning of occupied buildings
Plant was too congested at the design stage.
Poor location and poor construction of the control room.
Control Room Design: structural design to withstand major
hazards events
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