Explosion on a water pump due to hydrogen backflow

The event occurred at a unit producing ammonia, in a plant manufacturing fertilisers. It started when a gasket in a water pump was blown out. The pump was feeding water to a washing tower for the purification of hydrogen. The tower was containing hydrogen gas at pressure of 30 bars. This pressure caused a back flow of water through the pump and out through the failed gasket. The hydrogen followed the water and reached the leakage point after about 3 minutes. The discharge of gas lasted some 20 to 30 seconds before ignition. An explosion followed the ignition, and it is likely that the gas cloud detonated. The pump was placed inside a building, which was destroyed by the explosion. Through a domino effect, the building was destroyed and its roof lifted. This broke the purified hydrogen line out of the washing tower, causing a jet flame.

DETAILED ACCIDENTAL SEQUENCES
The accident started when operators were trying to change over from pump B to pump A:
(1) The operators did not realised that the inlet valve on the low-pressure side of pump A was closed. Only the small bypass valve was open.
(2) When pump B was stopped, the water supply from pump A was too low to maintain the water level in the absorption tower, resulting in its automatic shut-down.
(3) A second attempt to start pump A resulted in abnormal vibrations of the pump.
(4) The operators finally discovered that the inlet valve was closed and tried to open it but failed because of the large pressure difference across it.
(5) Smoke was coming from the pump bearing, which had turned red-hot. At the same moment, water started to leak from a flange between the pump and the closed inlet valve.
(6) Pump A was stopped. The interlock system should then have closed the block valve, which however failed in 40% open position.
(7) The check valve did not close either, because the water back flow was relatively low.
(8) The water leak lasted for about 3 minutes before hydrogen gas from the tower reached the leaking gasket.
(9) Hydrogen was discharged into the building for 20 to 30 seconds before the explosion occurred.
(10) (Bjerketvedt at al., ICHS2005) estimated to total mass of the hydrogen discharged in a range from 10 to 20 kg hydrogen, and the quantity which participated to the explosion from 3.5 to 7 kg. The total mass of the hydrogen discharge was estimated at 10 to 20 kg hydrogen and 3.5 to 7 kg of hydrogen was estimated to have burned in the explosion, which was very probably a detonation. The pressures inside the building must have reached at least the order of 10 bars. 60 meters of the sidewalls were blown out and the roof was lifted more than one meter.
(11) The lifting of the roof tore off a 350 mm diameter pipe coming from the wash tower. This new release resulted in a jet fire with an initial flame length of 50 meters lasting for approximately 30 sec.

The INITIATING CAUSE was the overheating of a water pump and the failure of a gasket, with consequent release of water first, then gas reach in hydrogen.

The IGNITION SOURCE was almost certainly a hot bearing for the first combustion event. The ignition source of the second combustion, above the building roof, is not provided.

The ROOT CAUSE was a combination of operational error, technical failures and design weakness (an in-depth root cause analysis is provided by Bjerketvedt at al., ICHS2005).

water pump, hydrogen tank

DESCRIPTION OF THE FACILITY
In the ammonia plant hydrogen was produced by partial oxidation of a heavy hydrocarbon fuel. The main explosion took place in the purification section of the hydrogen plant. This section contained a wash tower where CO2 was being removed from the H2 by absorption in water.
The hydrogen concentration in the gas feed to the absorption tower was typically 65% (vol). The remainder of the feed was mainly CO2 and CO.
At the outlet of the tower, the hydrogen concentration was approximately 95% hydrogen and 5% CO.

The wash tower and the hydrogen pipelines were located outdoors. The two water pumps A and B were located inside a 100 m long, 10 m wide, and 7 m high building. The capacity of the pumps was 2600 m3/hr and the water pipeline diameter was 600 mm.

One worker died and two were severely burned.
Window glass was broken up to 700 m from the centre of the explosion.
Concrete blocks, originally part of the north wall of the building and weighing 1.2 metric tons were thrown up to 16 meters.
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.

An employee noticed a visual alarm few seconds before the explosion, but could not communicate it.

3.5 to 7 kg of hydrogen estimated to have burned violently in the explosion.
No quantitative evidence that the cloud detonated, but from the damage observed, experience from other accidents and experiments detonation seems most likely.
The explosion was followed by a large horizontal jet fire lasting about 30 seconds.

This gas explosion was one of the largest industrial hydrogen explosions reported till the date. The accident occurred due to a combination of operational error, technical failures and weakness in the design.
1. The explosion caused large number of fragments representing a severe hazard
2. 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.
3. The window fragments represent a severe hazard to humans
4. The explosion was followed by jet fire. Domino events such as fires are common after gas explosion.

The investigators draw these conclusion regarding the analytical methodology:
a. Documentation of the damage has to start immediately,
b. Explosion expert and a structural response experts must be involved.
c. Take many photographs, both of the area view and the specific damages.
d. Use a professional photographer and make systematic records of locations and directions of all the photos taken.
e. Organise a 10 fragment map, showing the original position of the fragments and where they landed.
f. Fragments can be a good indicator of where the explosion occurred and of the magnitude of the explosion
g. Damage indicators can be of great help in investigating accidental explosion.

The equipment damage was assessed at 9.2 millions of US$ and the damage due to production losses at 7.3 millions.