Explosion in an ammonia production plant

The event occurred at the hydrogen production unit of the ammonia production plant. An air-hydrogen mixture formed in the building hosting a water pump which fed water to a vessel that contained hydrogen gas at 30 bars. The triggering event was the blowing out of a gasket in the water pump.
The pressure caused a back flow of water through the pump and out of the failed gasket. The hydrogen followed the same path, reaching the leakage point after approximately 3 minutes. The discharge of gas lasted 20 - 30 seconds before the explosion. The total discharge of hydrogen inside the building was about 10 - 20 kg.
Most likely a hot bearing ignited the gas cloud. The main explosion was very violent, and it is likely that the gas cloud detonated. 3.5 - 7 kg of hydrogen must have been burning during the explosion (no quantitative evidence that the cloud detonated, but from the damage observed, experience from other accidents and detonation experiments seemed most likely).

The roof of the building was lifted by an estimated 1.5 meters before resettling. The displacement of the roof caused a guillotine break of a 350 mm diameter pipe connected to the vessel that was the source of the original gas discharge. The gas composition in the vessel was 65 - 95 % hydrogen. This resulted in a large horizontal jet fire lasting about 30 seconds, outside the pumps building. Minor explosions occurred in the plant culvert system.

The INITIATING CAUSE was the loss of confinement of the water system, which allowed also the release of hydrogen, due to the failure of a gasket on the water pump.
A wrong procedure caused the overheating of the pump and the failure of a gasket. The accident occurred when operators were trying to switch from one water pump to another. They did not realise that the inlet valve on the low-pressure side of first pump was closed. This resulted in a series of failed attempt to bring the situation back to normal.
The root CAUSE was a combination of operational error, technical failures and weakness in the design. Moreover, the lack of diagnostics and automation did not helped personnel to understand the situation.

hydrogen vessel, water pump

DESCRIPTION OF THE HYDROGEN PRODUCTION UNIT
The hydrogen feedstock required to produce ammonia was produced in this plant by partial oxidation of a heavy hydrocarbon fuel. The main explosion took place in the purification section of the hydrogen plant, which consisted in a wash tower where CO2 was removed from hydrogen by absorption in water. The gas concentration at the gas feed inlet into the tower was 65% (vol), hydrogen plus mainly CO2 and CO. At the outlet, the hydrogen concentration was approximately 95%, with a rest 5% of CO. The wash tower and the hydrogen pipelines were located outdoors. The two water pumps required for the flow into the tower were inside a building (100 m long, 10 m wide, and 7 m high). The capacity of the pumps was 2600 m³/hr and water pipeline diameter was 600 mm. The incident started when personnel switched the operation from one pump to another.
[Bjerketvedt and Mjaavatten, A HYDROGEN-AIR EXPLOSION IN A PROCESS PLANT: A CASE HISTORY http://conference.ing.unipi.it/ichs2005/Papers/100096.pdf]

The accidental sequence resulted in the destruction of the building (100 m long, 10 m wide and 7 m high) and two fatalities. The explosion caused large number of fragments representing a severe hazard. Glass windows were broken up to 700 m from the centre of the explosion. Within a radius of 100 m all ordinary windows were broken.

20 kg of hydrogen was released, out of which 7 kg burnt in the initital detonation.

The local fire brigade extinguished the ensuing fire two hours later. No employees or emergency responders were injured. Air monitoring at the perimeter of the refinery did not detect elevated concentrations of any hazardous chemicals.

This gas explosion is one of the largest industrial hydrogen explosions reported till today. The main conclusion from the investigation can be summarized as follows: the accident occurred due to a combination of operational error, technical failures and weakness in the design.

No recommendation is available, but the investigators also issued advices for an optimal investigation process.
1. Documentation of the damage has to start immediately, and should be done by an explosion expert and a structural response expert(s).
2. Take many photographs, both of the area view and the specific damages. Use a professional photographer and make systematic records of locations and directions of all the photos taken.
3. Organise a fragment map, showing the original position of the fragments and where they landed. Fragments can be a good indicator of where the explosion occurred and of the magnitude of the explosion