Explosion at a pharmaceutical plant

The event occurred at the hydrogen storage facility of a pharmaceutical plant. It started with a leak, experienced as a loud whistling lasting 20 to 40 seconds.
The leaking hydrogen mixed with air forming a flammable cloud, which ignited producing a very bright, red-orange fireball measuring 15 to 20 m in diameter visible for 3 minutes. Witnesses in a break room, located 100 m away from the pit, felt the shock wave and the heat from the blast.
When the fire brigades arrived, 2/3 of the pit was engulphed in flames, with flames high 3 to 4 m.
All the hydrogen in the associated tanks burned completely.

Since the event early on a Saturday morning, when the plant was almost empty, no injury occurred, only property damage.

The INITIATING cause of the event was the sudden mechanical failure of the storage cylinder due to hydrogen embrittlement.

The tube steel had a high tensile strength of and had at least partially a martensitic microstructure, unsuitable for hydrogen applications. The surface of the cylinder's inner wall has pitting, and even corrosion craters. The surface roughness resulting from this condition increases the surface area of the steel in contact with H2 and facilitates the absorption of this gas.
The post-incident investigation concluded that the tube rupture due to static fatigue resulting from hydrogen embrittlement of the steel on the front support cradle.

The ROOT CAUSE was probably a combination of wrong design (the tube characteristics were not for operation in hydrogen environment) and in operation and organisation shortcomings, including a lack of a properly performed risk assessment. The company had recycled the tube without any documentation of its previous history. Its remaining operative lifetime had not been assessed and the possibility of embrittlement and the fatigue stress due to pressure cycling had not been considered.

gaseous hydrogen storage, steel cylinder

The affected tube was manufactured in 1939 (almost 50 years before). It had been used before as nitrogen storage and was recycled in this plant as hydrogen storage. The last hydraulic test of the cylinder was performed in 1985.

DESCRIPTION OF THE STORAGE UNIT
The storage unit was providing hydrogen to several production units.
The gas was produced in a nearby plant and passed first through a 70 m³ buffer tank, which supplied the storage facility via a pipe at 35 bar. The hydrogen was then compressed to 150 bar and stored in 8 tubes with a total capacity of 35 m³. At a nomina pressure of 15, this gives a maximal hydrogen capacity of approximately 380 kg. These tubes could be isolated from the rest of the supply system by a pneumatic closing automatically in the event of an emergency or upon an air supply failure.
The tubes were installed in a concrete pit dug into the side of an embankment and which is protected by a fibre cement roof.

The tube which failed was in the middle of the pit and above 2 other tubes, below another one. It had the following characteristics:
Outside diameter: 0.57 m
Wall thickness: 12.6 mm
Length: 15 m
Water Volume: 3 m³ (3000 litres)
Operating pressure: between 120 and 150 bar
Material: standard steel with density of 7,850 kg/m³

Ten or so cylinder fragments were found after the accident: 6 in the pit and 2 outside the plant (one piece of 183 kg at 22 m and another piece of 33 kg 150 m away). One of these fragments severed the compressed air line supplying the storage facility's isolation valve.
Propelled to the side opposite its rupture, the exploding cylinder collided the pit's east wall, and then flew back in the opposite direction.

The blast effect of the explosion resulted in property damage:
1) Broken windows and store fronts up to 515 m away;
2) Damage to building cladding, walls or ceilings up to 350 m away;
3) Displaced roofing tiles up to 280 m away.

Thermal effects of the fireball were negligible:
4) Burn marks on supports, cylinders and the metal uprights of the storage tank roof;
5) Temperature rise of the dry foam lining the wall of the nearest workshop

The fire brigade was called, and the unit was shut down via an “emergency stop” switch.
The pit was flooded with foam. and the hydrogen line supplying the storage facility isolated.

Several factors contributed to the hydrogen embrittlement of the steel in this accident:
A) The pressure-dependent hydrogen sorption activation on the surface of the steel.
B) The purity of the hydrogen gases. The gas used on this site had a very low oxygen content (0.5 ppm) which aggravated the hydrogen embrittlement phenomenon, because presence of oxygen inhibits it.
C) The additional stresses generated by the action of the tube casing against its support structure, during filling and emptying.
D) The mechanical performance characteristics of the steel used for the tube.
E) The previous operative history. the tubes were recycled, but there was no record of the age, cycles, pressure and stresses previous experienced by the tubes.

To be able to assess the suitability of old components for a new application, knowledge on their manufacturing and operative history is required, e.g. the steel composition and microstructure, the manufacturing details and previously experienced strain-stress records and environmental history. it would be impossible to assess.
The failed tube was used as a buffer and experienced in its new application a high number of pressure cycling. Evaluating if it was fit for use would was requiring a quantitative assessment of its remaining fatigue resistance and the expected degradation, impossible without knowing the previous history.
Failing to do so, it should have been replaced.

IMMEDIATE MEASURES TAKEN
The operation of the units connected to the storage facility was resumed by supplying hydrogen directly through an existing 35 bar pipe. Before commissioning, the piping underwent a leak test with helium followed by a hydraulic test coupled with a dye penetrant inspection to search for external cracks. Impact tests (resiliency) and crystallographic examinations in addition to these tests. All the examinations showed no traces of hydrogen embrittlement.

INSTECTION BY AUTHORITIES
The authority in charge of the inspection of this type of Classified Facilities identified similar storage facilities in the region. Special attention was given to older equipment and their history, as well as the characteristics and composition of the steel. Nearly 80 internal defects were detected on one of the site's hydrogenation units (made of Hastelloy alloy) where the explosion occurred.
The method used to inspect the hydrogenation units regarding hydrogen embrittlement risks:
(1) Hardness measurement of the inside wall.
(2) Internal and external metallographic replicas.
(3) 100% examination of welds using the eddy current method.
(4) Ultrasonic examination (mesh size: 50 mm x 50 mm) to detect internal defects.

CORRECTIVE MEASURES OF PLANT OPERATOR
The plant operator developed new operating procedures for its hydrogen piping, concerning the pressurisation phase and a safe emergency shutdown, by burning hydrogen to reduce piping pressure to 1 bar, followed by flushing with nitrogen. Two automatic shutoff valves controlled by independent pressure gauges were installed, protecting the supply line on a hydrogenation unit.