Explosion in a polyethylene manufacturing plant

In a polyethylene manufacturing plant, the explosion of a flammable cloud (isobutene, ethylene, hexane and hydrogen) escaped from a reactor during maintenance caused 23 fatalities and 314 injured workers. The explosion is equivalent to that of 2.4 t TNT and comparable to an earthquake of 3.5 on the Richter scale.

The main explosion was followed by multiple other explosions (including those of 2 isobutene tanks) and fires which will only be under control after 10 hrs of emergency response. This long time was due to water supply difficulties, from not availability of power to too low pressure in the line.
The accident occurred while a maintenance operation, carried out by a specialized subcontracting company, had started since the previous day: the aim was to clean up settling branches connected to the manufacturing reactor during operation. These branches allow the polymer deposits to be recovered so that they do not clog the reactor. A specific procedure has been defined for this operation, specifying in particular that the branches must be isolated by ball valves (actuated by compressed air) placed between them and the reactor.

The investigation carried have shown that the accident resulted from the rejection of 40 tonnes of process gas to the atmosphere (99% of the reactor content) via an isolation valve of a settling branch that remained open, and which ignited within 90 to 120 sec on contact with one of the multiple sources of ignition nearby. The opening of the valve is due to the reversal of the connections of the compressed air pipes actuating it (error prior to the operation in progress): the valve was open while the actuator in the control room indicated that it was closed. However, safety standards require that in the event of an intervention on an operating reactor, the isolation device consists of a double valve or a closed flange, which was not provided for in the intervention procedure. The workshop was devoid of gas detectors.

INITIATING CAUSE was the release of process gas during maintenance on a blocked reaction loop line.
The instrument line to a shut valve was being connected when the valve opened and the massive release occurred. The valve actuating hoses were found connected the wrong way round.
ROOT CAUSES were failure to perform a proper risk assessment and to use it in an effective safe design. Management failed as well, in reviewing safety procedures and setting up safeguards to mitigate mistakes in maintenance. Contributing CAUSE to the escalation was the malfunctioning of the fire-extinction water system.

Reaction vessel, line and valve

The accident resulted from a release of process gas during maintenance on a blocked reaction loop line. The instrument line to a shut valve was being connected when the valve opened and the massive release occurred.

The explosion was equivalent to that of 2.4 t TNT and comparable to an earthquake of 3.5 on the Richter scale.
The explosion kills 23 people, within a radius of 75 m, and leaves 314 injured including 185 employees and 129 subcontractors.

The material damage is considerable, the 2 polyethylene units covering an area of 4,000 m² are destroyed, debris is found up to 10 km from the place of the explosion, the windows of the constructions located within a radius of 2.5 km are broken, houses and buildings are damaged within a perimeter of 6 to 7 km.

The total cost of damage is estimated at $ 750 million, that of operating losses at $ 700 million.

From OHSA investigation :
After an explosion and fire on October 23, 1989, OSHA initiated an investigation to determine the cause of the accident.

At the conclusion of the investigation (April 19, 1990), OSHA issued 566 wilful and 9 serious violations with a combined total proposed penalty of $5,666,200 to the chemical company and 181 wilful and 12 serious violations with a combined total proposed penalty of $729,600 to the maintenance contractor on the site. Both employers filed a timely "Notice of Contest".

1. The company had made no use of hazard analysis or an equivalent method to identify and assess the hazards of the installation.

2. Separation distances between process equipment plant did not accord with accepted engineering practice and did not allow time for personnel to leave the plant safely during the initial vapour release and that the separation distance between the control room and the reactors was insufficient to allow emergency shut down procedures to be carried out.

3. The ventilation intakes of buildings close to or downwind of the hydrocarbon processing plants were not arranged so as to prevent intake of gas in the event of a release.

4. There was a failure to minimize the exposure of personnel.

5. The plant had no fixed flammable gas detection system despite the fact that the plant had a large inventory of flammable materials held at high pressure and temperature.

6. An effective permit system was not enforced for the control of the maintenance activities.

7. The sole isolation was a ball valve which was meant to be closed but was in fact open. There was no double block system or blind flange. The practice of not providing positive isolation was a local one and violated corporate procedures.

8. The practice of relying for fire on the process water system and the failure to provide a dedicated fire water system meant that the fire water system was vulnerable to an explosion.