Explosion in a chemical plant

An explosion occurred at a plant where p-nitrophenol (PNP) is reduced to p-phenetidine with hydrogen. A drum supplying PNP to a hydrogen reduction reactor burst into flames, and part of the insulation was damaged and fell off. The upper part of the PNP drum and piping connecting the drum to the reduction reactor were destroyed by the fire.
The explosion took place due to the convergence of several events. Workers did not firmly close a valve on discharge pipes of para-nitrophenetole (PNP) and forgot to close a valve fitted with an air-operated flow meter, so that hydrogen got into the PNP discharge pipes. With defects on the welded part of the pipes and corrosion by chlorine in PNP, the pipes got fractured to allow hydrogen to leak into the insulation cover of the PNP tank, where it accumulated to form explosive gas, which was ignited by electrostatic sparks generated when hydrogen was ejected.

INITIATING CAUSE was the failure to perform two manual operations required before starting the reaction, allowing hydrogen to flow into the supply line of the product.
CONTRIBUTING CAUSE was that this line was made of the wrong choice of the stainless steel material and affected by severe damage which caused its collapse. This damage was related to manufacturing defects and to the corrosion of the chemicals, which had already leaked in the past
ROOT CAUSE is related to design shortcoming, lack of effective operative procedures and failure to learn from previous events.

valves, pipe, reactor

The plant was approved as a manufacturer of dangerous materials on December 25th, 1990. The accident occurred a year and a half after the start of operation.

DESCRIPTION OF THE CHEMICALS

p-Phenetidine (4-ethoxyaniline) is a chemical compound with the molecular formula C8H11NO. It is one of the three isomers of phenetidine. It is used as an intermediate in the synthesis of pharmaceutical drugs, dyes, and the sweetener. It has high toxicity.
[https://en.wikipedia.org/wiki/P-Phenetidine#:~:text=p%2DPhenetidine%20(4%2Dethoxyaniline,Precautionary%20statements]

P-Nitrophenol (PNP) is a significant industrial chemical used in making dyes, pesticides, and pharmaceuticals, but it poses health risks (carcinogenic, mutagenic). It's recognized by its formula C₆H₅NO₃.
[https://en.wikipedia.org/wiki/4-Nitrophenol]

The PNP storage and piping system were partially destroyed by fire.

Time sequence:
• 09:30 on May 30th, 1990; Charging toluene and catalyst, and next PNP to the reduction reactor started.
• 14:00 Charging finished. Nitrogen purging of the reactor was performed.
• 15:20 Hydrogen displacement in the reactor started.
• 15:30 Whistling was heard from somewhere around the PNP drum. The cause of the whistling was checked but it was not clear. Steam was supplied to the jacket of the reduction reactor, and a temperature rise started.
• 15:40 Hydrogen displacement was completed.
• 15:48 Reduction reaction started.
• 16:05 Flames spouted from upper and lower parts of the PNP drum.
• 16:10 The fire was extinguished by employees.
• 16:15 A fire brigade confirmed the fire was extinguished.
As precautionary measure an emergency shutdown of the entire factory.

This is a typical case of several independent causes contributing to the incident.
(1) The most important lesson learnt is related to the need to learn from previous near misses and smaller incidents of the past. Part of the piping work had already failed due to corrosion in the past and had been replaced with a better materials. In absence of a Management of Change approach and of a thorough risk re-assessment, the rest of the pipes was left with the original bad materials.
(2) The possibility that employees can make errors when executing preparatory manual actions has to be taken into account in the overall risk assessment, including the consequence. A risk assessment should be able to identify the critical aspects requiring preventing measures.

1. All similar piping was replaced with that made of SUS316L, and welds were inspected with X-rays.
2. Valves with an air flow meter were installed to be monitored from a central control room.
3. Valves were replaced with those that are easily checked for an open/closed state.
4. Inspection windows were made on piping with insulation for daily checks.
5. The pressure and temperature of steam trace of a supply line of the PNP drum were lowered.
6. The standards of operation procedure and education for employees were prepared and carried out without exception.