© Institution of Chemical Engineers0260-9576/08/$17.63 + 0.00

1 | Loss Prevention Bulletin 100 August 1991

Ethylene oxide plant explosion, 3 July 1987 BP Chemicals, Antwerp, BelgiumBE Mellin

Case study

Background

The ethylene oxide unit at Antwerp was commissioned in 1969. The process (see Figure 2.2) involved the oxidation of ethylene with oxygen over a silver catalyst in multi-tubular reactors at pressures between 15 to 20 bar. In addition to ethylene oxide, by-products such as carbon dioxide, water and minor quantities of aldehydes were produced by the reaction. Reaction gases were then scrubbed with recycled water to absorb ethylene oxide, whilst the unconverted feeds recycled to the reactors. A part of these unconverted feeds is first passed through a carbon dioxide removal system before recycling to the reactors. Ethylene oxide was recovered from the scrubber water in a stripping column by a combination of pressure reduction and live steam injection. The unrefined ethylene oxide was then compressed prior to the purification section where the remaining water and residual light end components, (ethylene, carbon dioxide, etc) were removed to produce low grade ethylene oxide. The low grade ethylene oxide product was further refined in the final purification column, which removed

Summary

At 1908 hours on 3 July 1987, the final purification distillation column of the ethylene oxide unit on the BP Chemicals factory at Antwerp, failed explosively. Fourteen people were taken to hospital, no-one suffered serious injuries and all were released following treatment. The explosion was accompanied by an enormous fireball, which initiated numerous secondary fires within the unit and on neighbouring units. The shock waves and missile damage resulting from the explosion also caused serious damage to the ethylene oxide unit (see Figure 2.1) and other neighbouring derivative units, whilst minor building damage was experienced to property in neighbouring factories and in the local village. The quick and effective response of the Site Emergency Response Team and the Local Authority Emergency Services minimised the resultant equipment and environmental damage by successfully containing and preventing any subsequent escalation of the incident. All fires were under control within 30 minutes of the explosion. Investigations into the cause of the incident were carried out by both BP Chemicals and the Belgium Regulatory Authorities.

the aldehydes to produce high grade ethylene oxide. The final purification column operated at 3.5 bar. In June prior to the incident, the ethylene oxide unit had undergone a planned three week shut down to allow a routine planned maintenance programme. The unit was re-commissioned on 24 June. All operating data indicated normal conditions up to the time the column suddenly exploded.

Cause of the incident

Due to the extensive damage caused to plant equipment and the reports that the unit had been operating normally up to the incident, the investigation in identifying the cause of the incident was a very lengthy and a difficult process since all possible event scenarios needed to be considered and evaluated. BP Chemicals obtained expert assistance in this task through consultants and industrial specialists. Following a very long and exhaustive investigation, it was concluded that the final purification column had failed on being subjected to a rapid overpressure caused by the decomposition of the ethylene oxide in the bottom section of the column. That is ignition of the ethylene oxide in the base section of the column led to a decomposition reaction resulting in a deflagration which rapidly over pressured the column causing its failure. No conclusive evidence was found to prove a unique source of ignition for the decomposition reaction. However, after careful investigation and taking into account the results of extensive analytical research and a comprehensive fault-tree evaluations, the investigation team by a process of elimination narrowed the potential sources of ignition to either:

• An ethylene oxide leak on one of the insulated large flanges on the base of the column which had been opened up during the shutdown period. The escaping ethylene oxide trapped in the ‘free space’ beneath the mineral wool insulation reacted with the ‘active material’ in the lagging. The resulting ‘adiabatic’ heating from the subsequent reaction ignited the escaping ethylene oxide. The subsequent heat from the under lagging fire raised the metal temperature to a temperature sufficiently high to decompose the ethylene oxide inside the tower.

or

• A hot spot was generated in accumulated reactive material located in an internal quiescent area at the column base. The hot spot was initiated by the exothermic reaction

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Loss Prevention Bulletin 100 August 1991 | 2

Figure 2.1: Damage to the ethylene oxide unit

Figure 2.2: Ethylene oxide unit block diagram

Ethylene

Oxygen

E.O.reactor

Scrubber E.O.stripper

CO2

removal

Compressor

E.O.purification

E.O. to low grade storage

E.O. to high grade storage

E.O.redistillation

Recycle

© Institution of Chemical Engineers0260-9576/08/$17.63 + 0.00

3 | Loss Prevention Bulletin 100 August 1991

polymerisation of ethylene oxide catalysed by rust present in the section of the column.

Conclusions

The investigation team reported that there was no conclusive evidence to completely exclude either of the previous scenarios, but were satisfied that circumstantial evidence pointed to ignition having most likely resulted from heat generated externally by an undetected leak of ethylene oxide from a large flange at the base of the tower. The flange most likely to have started this sequence of events was the 610 mm manhole (see Figure 2.3) cover as this has been opened up during the shutdown period and had been insulated prior to start up. It was reported that positioning of the gaskets had indeed posed some problems. Some of the main lessons learned from this incident are as follows:• Mineral wool, which is commonly used as fire insulating material, can be a potential source of hazard where it is used on insulating equipment on ethylene oxide because it has a high surface area and can absorb a lot of moisture. Minor leaks of ethylene oxide, if left undetected, may well react with water contained in open cell structure type insulation such as mineral wool. If near adiabatic conditions exist then a hot spot can build up in the lagging that may well be sufficient to ignite the escaping ethylene oxide.• The accumulation of rust in quiescent areas of the column, e.g. nozzles, manways or other dead zones etc, particularly those located in unwetted areas must be avoided at all costs.

• Materials of construction of equipment and associated pipework must be such as to minimise the generation of rust, or be kept substantially rust free.

• The practice of insulating large flanges on ethylene oxide duty can increase the overall fire and explosion hazard on account of their leak potential and the possibility that small leaks may remain undetected.

• High leak potential areas, i.e. flanges, nozzles and small diameter piping on ethylene oxide installation, which contain gaseous ethylene oxide and where the internal flow is insufficient to provide adequate cooling are particularly susceptible to a potential explosion hazard in the event of fire.

Editorial comment: This was identified as the cause of the incident described in the previous article.

• Those parts of equipment which are identified as having high leak potential, i.e. flanges, should be subjected to a comprehensive leak checking programme commensurate with the potential hazards.

• The response of the site and the Antwerp Emergency Services were excellent and were successful in effecting rapid containment of the incident and minimising consequential damage.

Recommendations

As a result of the conclusions the following recommendations were made, namely that:• All ethylene oxide purification columns and their associated pipework on ethylene oxide duty should be designed in such

a way as to minimise the number of nozzles and connections. Should there be a requirement to insulate small flanges then these must be equipped with leak indicators or detectors. In addition, the integrity of the electrical bonding across such connections must be maintained.

• The ethylene oxide purification columns, should be designed to minimise column inventory. The trays to be of stainless steel construction to prevent excessive rust promotion.

• Large flanges on ethylene oxide duty in high fire risk areas should not be insulated on account of their high leak potential and the likelihood that escaping material may become trapped in the insulation and remain undetected.Such large flanges should instead be protected with leak and fire detection systems supplemented by “heat activated” sprinkler systems.

• All equipment and pipework on ethylene oxide duty should be designed and installed to minimise the presence of “stagnant” zones of ethylene oxide, to avoid areas where rust and polymer can accumulate.

• The design criteria on which the insulation standards for equipment and pipework on ethylene oxide duty are based must be clearly defined and specified and fully understood. The criteria must take into consideration:

- The fire resistant properties of the materials to be used

- Materials must not be capable of initiating a hot spot reaction or should not substantially lower autodecomposition temperatures of ethylene oxide

- Materials must possess low moisture absorbing and retention properties

- Materials should not contain or generate chloride

It is therefore recommended that insulation used on ethylene oxide duty should be of a two-layer design,

Aluminium shielding

Manhole cover

Insulation rockwool 160 (thickness 120mm with chicken wire backing)

Aluminium pinsto hold insulation

Flat face to prevent ingress water

The manhole insulation is formed by two half cylinders that are preformed and covered with a layer of Rockwool mat (thickness: 120mm)

Silicones to seal

Free space

Clips to close the two halves

Free space

Column wall

Figure 2.3: Insulation of the manhole

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Loss Prevention Bulletin 100 August 1991 | 4

Figure 3.2: Aerial view of the site showing the extent of the damage

an inner layer of material of a close cell structure (e.g. cellular glass), and an appropriate outer layer of fire resistance material sufficient to provide the necessary fire protection. The inner and outer layers to be separated by a vapour barrier to avoid ingress of water and chlorides.

• Leaks of ethylene oxide from equipment and pipework must be prevented at all cost, therefore a “comprehensive” combustible gas detection system coupled with a detailed leak testing inspection programme using portable gas leak detectors should be in place to ensure the mechanical integrity of such installations and to facilitate the rapid detection of any leakage.

• Operating conditions relating to acceptable standards of the internal cleanliness of ethylene oxide equipment, prior to being put into service to be clearly defined and consistent with the control of hazards.

• Maintenance requirements especially in relation to the prevention of leaks from flanges (i.e. types of gaskets, installation standards) must be clearly defined and consistent with the control of the hazards. To this effect it is recommended that spiral wound teflon or graf oil gaskets with stainless steel inner and outer guide rings be used for all flanges up to 900 mm diameter size. For larger size flanges, envelope type gaskets with a teflon or graf oil filler should be considered. Spiral wound gaskets with an inner guide will provide additional protection against unwinding of the spirals.

Torque wrenches should be used to bolt up large flanges to pre-determined stress level. Figure 3.1: Aftermath of the explosion