Explosion in a hydrogen production plant

The incident occurred at a hydrogen production plant, consisting of an electrolyser, a gasholder for the temporary storage of the hydrogen, compressors and gas dryers and final storage in four high pressure storage banks.
Explosions occurred at different locations inside the hydrogen pipes feeding the high-pressure storage, causing the failure of welds and the connections at the storage system. These multiple confinement losses released hydrogen in large quantities from two of the four storage banks. The escaping hydrogen auto-ignited, resulting in an explosion and fire.

The cause of the initial internal explosion was the formation and auto-ignition of an oxygen-hydrogen flammable mixture.
The damage to the whole high-pressure section of the installation was considerable. No injuries.

The cause of the initial explosions was due to the spontaneous ignition inside the high-pressure feed pipes of an explosive mix of hydrogen and oxygen. The oxygen presence in the system was coming from the hydrogen electrolyser. The prime cause of it was blockages in the water feed lines to some electrolytic cells, allowing high levels of oxygen to mix with the hydrogen as electrolyte levels fell. The low-purity hydrogen was not detected by the low-pressure purity analyser due to a failure of the isolation transformer.

The ROOT CAUSE can be attributed to an inadequate safety design of the installation and the lack of reliable online diagnostics which could have been able to provide the information on progressing deterioration of the hydrogen purity due to oxygen presence. Also the safe operation of the electrolyser lacked of proper performance detection.

pipe, fittings, jonts, storage tank

One hour before the explosion, an operator attended the hydrogen generation plant as part of a routine plant inspection. The plant was functioning correctly and no obvious problems were noted.

DESCRIPTION OF THE PLANT
The plant consisted in an electrolyser, a gasometer or gasholder for the temporary storage of the hydrogen, and a hydrogen conditioning system delivering the gas at high pressure.
(1) The plant was able to continuously deliver hydrogen at a purity of 99.75% and at a rate of 510 m3 per day.
(2) The electrolyser consisted in two banks of alkaline electrolytic cells.
(3) The gasholder was a low-pressure storage vessel with movable bell -ceiling and a capacity of 28 m3 of gas. The tank was filled with water functioning as a water seal. Hydrogen from the cells entered the gasholder above the water line, causing the bell to rise.
(4) The purity of the hydrogen was monitored between the cells and the gasholder. If purity was falling below 99.5%, the hydrogen was vented to the atmosphere.
(5) Two compressors were used to drawn hydrogen from the gasholder and to deliver it at 12.4 MPa, via high-pressure (HP) dryers, to the high-pressure storage system.
(6) Four storage banks were available for storage, each consisting of 16 cylinders with a capacity of 460 liters per bank. Each cylinder was fitted with burst discs as pressure relief measure.

The the high-pressure pipe work had ruptured or parted at the several locations: between two valves adjacent to a compressor, at the pressure regulating valve near the compressor room, above a high-pressure dryer, at the manifold inlet of one of the banks, at various bottle fittings in the same bank.

Pipe swelling was noted at a regulating valve (compressor side) and at the non-return valve of a dryer.
Damage was noted a compressor blow-down pneumatic valve and at a dryer blow-down pneumatic valve

Twelve of the sixteen cylinders in one storage bank and six cylinders on a second bank had been affected by the heat of the fire and many of the cylinder burst discs had ruptured.

The vacuum and purge pipe work around the two storage banks had been destroyed by the fire. The cladding at the eastern end of the building had been blown out above two storage banks. Debris from the site was distributed for 20 meters.

Recommendation regarding the electrolyser
Individual cell purity should be monitored regularly, as measure of cell performance deterioration. Specific contaminants (particularly magnetite) may settle at the bottom of the cells block the water makeup ports and the drain valves. This was found to be the case on 6 makeup ports and on most drain valves. The blocked drain valves made it very difficult to clean the cells and to unblock the makeup ports. The diaphragms on the cells showing low-purity results will have to be replaced.

Recommendations on the purity analysers
The analysers have a critical role in guaranteeing safety of the installation. They must be of a type which fails safe in case of loss of power. Moreover, a risk assessment is required, to determine the level of risk associated with their failure associate d to a loss of sample flow.

(1) Replacing the existing low-pressure line analyser with a fail-safe model ensures that the vent valve is opened on low purity signal.
(2) Fail-safe purity analysers must be installed also on the high-pressure section, either between the stages of the compressor or immediately after the compressor.

Recommendation on the gasholder - Replace the existing pot-type water seals with open trough water makeup. This would reduce the risk of water entering the gas lines. While the original design foresaw the inlet and outlet of the gas pipes at 50 mm above the water line, they were found only at 20 mm. A small variation in the water level could allow water into the lines.

Recommendation on the high-pressure storage – The storage system should be installed in an open area, and not semi-enclosed building as in the case of this plant.