Explosion in an ammonia plant

The event took place in the on one out of the seven reactors for ammonia synthesis.
The day of the accident, preparation works for the decennial pneumatic test (previous pneumatic test was carried out in 1978) were started on that unit. The reactor was put out of service and a slow pressurisation with process gases (hydrogen and nitrogen) was carried out at ambient temperature. It was necessary to carry out many pneumatic cycles of pressurisation and depressurisation because of leaks from some flanges.
After any cycle, the bolts of the flanges were tightened and a purging with nitrogen at 8 bar was carried out.

Eventually, a slow pressurisation with process gases was in progress at ambient temperature when the explosion occurred, 20 minutes after the start, at an internal pressure of 250 bar.

The explosion caused one fatality and one injury, destroyed the upper part of the column; metallic parts of the reactor were thrown in a radius of many metres around it. It also caused the complete destruction of the building where the reactor was located.

The original investigation report is not available. According to the eMARS report (see reference), some experts suggested the possibility that accident occurred due to an internal explosion (deflagration) in the reactor.
The ROOT CAUSE could be tentatively attributed to an inadequate testing procedure (too long period between testing, use of flammable gas for testing, failure to correctly identify hazards and mitigate risks)

Ammonia synthesis reactor

The steel reactor had been manufactured in 1929 and since then It had been regularly tested.

DESCRIPTION OF THE REACTOR
It had a volumetric capacity of 8,000 litres, was 7.7 metres high and had a diameter of 850 mm, with a design pressure of 300 bar.

The material damages were estimated at approximately 20000 millions of lire (approximately 10 million of euros).
eMARS reports 1 fatality and 3 injuries, ARIA 1 resp. 1, Rigas&Amyotte reports 2 resp. 3, and 15,000 evacuated persons.

After the accident, on-site arrived the local fire brigade and the police. The ammonia production was interrupted. The ammonia production was interrupted and the whole factory was stopped by the authorities to establish the causes of the explosion.
On June 14 the production in the other plants not involved in the explosion started again.