At around 9:30 a.m., ammonia (NH3, toxic, corrosive) began leaking from a drain valve on a pipe used to supply ammonia to a Horton sphere at a chemical plant. The alert was given by an employee. A cloud of ammonia formed. Gas detectors registered a concentration above 100 ppm and triggered the alarm. An employee activated the emergency stop, cutting off the sphere’s supply and activating its sprinkler deluge system. At 9:34 a.m., the detectors near the leak registered that the 300 ppm threshold had been exceeded. The ammonia network’s isolation valves closed. The internal emergency plan was implemented at 9:35 a.m. Emergency services set up a cordon. The plant’s 250 workers and 312 people at two nearby facilities sheltered in place. The firefighters sprayed the area of the leak to knock down the fumes. The sprayed water was channelled to a bund. The supply line was drained and the discharge was stopped. At 12:17 p.m., an ammonia concentration of 20 ppm was measured. Both the internal emergency plan and the order to shelter in place were lifted.

Mildly sickened by the ammonia fumes, two contractors were taken to hospital. An estimated 300–500 kg of ammonia was released during the 8-minute leak. Production, which had been halted, resumed at around 9:00 p.m.

The leaky drain valve was located between two block valves on the sphere’s supply line. As part of scheduled checks on the sphere, it had been emptied and isolated from the ammonia supply. Both block valves had been locked in the closed position and the drain valve had been locked in the open position. However, the integrity of the first block valve was faulty. The leak occurred while ammonia was being transferred from the unloading station directly to the production unit.

The operator is looking into ways to improve communication in the event of a similar accident.