Damaged compressor : dual-stage compressor compressing ethylene from 3 to 33 bar. It operates alternately with the main compressor. On the day of the accident, it had been in operation for 43 days since its last maintenance five months prior. It was located next to two other compressors (Nos. 2 and 3). The latter two had been shut down and automatically isolated upon activation of the gas detectors, which was not the case for the damaged compressor No. 1, which had to be isolated manually from the control room.
In a petrochemical complex, an ethylene (a highly flammable gas) compressor was in stable operation when a sudden drop in pressure occurred at 5:33 p.m., accompanied by a loud noise. The gas detectors in the zone became saturated, and a 2nd level alarm was triggered in the unit’s control room, as well as in the control room controlling the nearby compressor and at the safety station. The compressor began to vibrate. Its motor stopped but not its ethylene supply.
Confusion over which compressor was leaking
Not knowing which compressor was causing the leak, the shift crew of compressor No. 1 contacted the crew in charge of compressor No. 2. The latter crew persuaded them it was compressor No. 2 that was to blame. Crew No. 1 left their control room to help them. The alarms and parameters indicating the malfunction of compressor No. 1 (pressure drop) were not taken into account since the control room had been deserted.
The two shift crews, joined by the internal fire brigades, approached the area of the accident but were unable to enter due to the deafening noise. They encountered a flammable cloud of ethylene measuring 4 m high x 100 m, with visible droplets. The firefighters protected the nearby units with water curtains. Around 5:45 p.m., the shift leader 1 consulted with the shift leader in charge of the neighbouring unit. A leak on the ethylene supply network was suspected. Two operators, equipped with hearing protection, moved through the cloud, protected by a water curtain, to reach the network’s manual shut-off valves. They were able to close the valves manually, ending the leak. The cloud rapidly dispersed. The operators returned to the control room and closed the local supply valve of compressor 1. 8 t of ethylene (354 kg of flammable mass) was released in 21 minutes.
Valve ejection due to incorrect tightening and a non-compliant seal
The hatch and the valve porthole of the 2nd stage of compressor 1 were found 6 m away. One stud from the hatch was severed (sudden brittle-type rupture), while the other five studs remained in place, three of which were missing their nuts and exhibited torn threads. These studs are compliant but had never been replaced since the compressor was commissioned 16 years ago. An expert assessment showed that the valve’s copper seal was not annealed at the time of its installation, contrary to procedure and the other seals on the equipment. It was therefore not as flexible and less able to absorb stresses. This defect, combined with a bolt tightening error on the verge of plastic deformation, led to fluttering in the stack and its rupture.
The operator is inspecting the other compressors, overhauling and replacing the compressor studs for which the technology has changed (rolled threads instead of cut threads). It is updating the compressor maintenance plans with the subcontracting company. The studs are replaced every three maintenance operations. The damaged compressor was returned to service two months later.
Technical and organisational lessons learnt
The information allowing quick identification of the equipment responsible for the leak was not exploited as no one was present in the control room, and there was no reporting via the mobile phones of the shift operators in the field or via the complex’s other control rooms.
If the emergency stop had been activated, the leak could have been stopped more quickly as it shuts down both the motor and the ethylene supply. The operators believed that the motor shut-down because of vibration was sufficient. After the accident, the compressor was equipped with an emergency stop triggered by gas detection. However, the manual emergency stop remains in operation should this detection system fail.
Despite the risk of igniting the cloud (UVCE, Flash Fire), the internal emergency plan was not triggered, the personnel were not evacuated, and on-site traffic was not restricted. The public emergency services and the surrounding municipalities were not alerted. At the time, the safety supervisor on duty felt that the situation was under control owing to the weak wind and the absence of subcontractors on site on a Saturday (no work, little vehicular traffic). The administration was notified more than 24 hours after the event. The external alert procedures were reviewed.