Hydrogen explosion at a refuelling station

This incident occurred at a hydrogen refuelling station with on-site hydrogen production based on reforming of city gas and natural gas. The part affected was a coil in the heat exchanger belonging to one of the two hydrogen production units. The unit was shut down and under preventive maintenance and was resuming operation.
The coil broke at a weld, releasing hydrogen, which flowed into the exhaust vent and ignited due to static electricity, causing an explosion at the exit of the vent pipe.
The investigation revealed that the coil broke at a weld located at a bending point, due to local concentration of stress and fatigue. These factors were mainly due to the geometry of the coil, a weld of difficult manufacturing, and the fact that during pressure transients, water condensation occurred, increasing the local stresses.

The INITIAL CAUSE was the failure of a subcomponent due to concentrated and repeated high stresses.
The ROOT CAUSE was shortcoming in design, which did not considered the effect of operation transients and the possibility of local condensation of water.
CONTRIBUTING FACTOR was the difficulty to execute the weld, which probably introduced some manufacturing defect.

coil, heat exchanger

The facility had been shut down for preventive maintenance and was resuming operations.

DESCRIPTION OF THE FACILITY
Production capacity: 91,986 m3/day
The operative condition were a normal pressure of approximately 82 MPa, and a process temperature around 300°C. External temperature in the range -40 to 50°C
The affected coil had an outer diameter 60.3mm, thickness 3.9mm, and was made of seamless nickel-chromium-iron alloy heat exchanger tubes (NCF800HTB)

No injury, no property damage. The exit cap of the vent pipe was deformed.

The detailed sequence of event:
At 15:15, the operator started up the hydrogen production equipment (both production Units).
In conjunction with the start-up of the hydrogen production equipment, the feed compressor was also started up by the automatic start-up program.
At 15:17, during the ignition preparation process to start the air blower, a popping sound was heard near the hydrogen production equipment (Unit B).
At 15:18, the operator confirmed that the hydrogen production equipment (Units A and B) was still operating. At about 15:20, the operator confirmed that the top of the exhaust gas chimney of the hydrogen production unit (Unit B) was deformed.
At about 15:42, the operator stopped the operation of the hydrogen production units (Units A and B).
At about 15:49, the operator reported the situation to the Head Office Hydrogen Station Technical Group.
At about 16:07, the operator performed visual inspections of the hydrogen production units (Units A and B) and gas leak detection.
At about 16:50, the Hydrogen Station Technical Group notified the government's high-pressure gas department.
At the time of the report, the hydrogen production unit (Unit A) had been given permission to operate after safety was confirmed.
At about 17:30, the operator purged the system of the hydrogen production unit (Unit B) with nitrogen.
At 17:39, the operator restarted the hydrogen production unit (Unit A) and started operation.

(A) The process of condensation/evaporation of water can cause repeated thermal stress, which can lead to fatigue cracks, especially at places where there is already a stress concentration, such as at the at the toes of welded joints.
(B) If the toes of the welding are difficult to manufacture, they may become the starting point of cracks, so it is necessary to consider introducing seamless pipes into the shaping of the pipes.
(C) Since hydrogen can easily ignite once leaked, monitoring system and mitigating measures must be in place also in place where normally hydrogen is not expected.
(D) All the hydrogen production units of similar design and operation at refuelling stations must be carefully checked and, if deemed required, measures must be adopted.

Measures to prevent cracks
(1) The manufacturer of the hydrogen production equipment was requested to improve the operation by temporarily increasing the reformer burner heat so that the temperature at the outlet of the raw material heater could never fall below 250°C.
(2) The heat exchanger coil was replaced by a seamless tube made of the same material.
Additional safety measures
(3) A flammable gas leak detector was installed in the exhaust gas line of the heat exchanger.
(4) A monitoring system was installed to monitor the evolution of the pressure in the system when switching from normal hydrogen production operation to the standby operation occurring outside business hours.
(5) A ground connection was added to the exhaust gas line of the heat exchanger.