Explosion on a water pump due to hydrogen backflow

The event started when a gasket in a water pump was blown out.
The water pump was situated inside a 100 m long, 10 m wide, and 7 m high building. The pump was feeding water to a vessel containing hydrogen gas at pressure of 30 bars.
The accident occurred when operators tried to change over from pump B to pump A. They did not realise that the inlet valve on the low-pressure side of pump A was closed. Following a series of attempt to solve the situation, a valve remained partially open and the gasket failed.
This pressure caused a back flow of water flow through the pump and out through the failed gasket. The hydrogen reached the leakage point after about 3 minutes.
The discharge of gas lasted some 20 to 30 seconds before the explosion occurred. The total mass of the hydrogen discharge was estimated at 10 to 20 kg hydrogen.
The ignition source was almost certainly a hot bearing. 3.5 to 7 kg of hydrogen are estimated to have burned violently in the explosion. The explosion was very violent and it is likely that the gas cloud detonated. 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 up more than one meter.
As the roof lifted, it 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 accident occurred due to a combination of operational error, technical failures and weakness in the design:
Initiating operational event: operator's wrong procedure
Initiating technical event: overheating pump and failure of a gasket.

water pump, hydrogen tank

The accident occurred when operators tried to change over from pump B to pump A. They did not realise that the inlet valve on the low-pressure side of pump A was closed.

DESCRIPTION OF GAS CONTENTS
The gas feed to the absorption tower was typically 65% hydrogen, at the outlet 95%, the rest CO2 and CO.

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.

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 to production losses at 7.3 millions.