Hydrogen accidental release into a compressor room

Hydrogen was accidentally released into the compressor room of a hydrogen refuelling station. The fixed gas detector in the compressor room issued a value of 48% vol. of he Low FLammability Level, causing all station to shut down. It did not come to ignition.

The incident comprised various phases:
(1) After completing the replacement of the automatic shut-off valve gland packing in the compressor unit, the compressor was tested for tightness. The procedure foresaw a manual intervention of the operator, to close the pressurisation line at the nominal pressure of the test. However, this action was performed too late, the compressor was over-pressurised and safety valve (pressure relief valve) at the compressor outlet activated. Because the compressor interlock system was switched off due to the tightness test, the activation of the PRD did not trigger the foreseen safety actions and the station operation could continue with business-as-usual.
(2) The activated safety valve was removed for overhaul and maintenance, and the opening of the pipe fitting on the discharge line was left open.
(3) The day after, since the compressor could not operate, it was decided to execute the vehicles refuelling by differential pressure charging, using the residual hydrogen pressure in the hydrogen storage.
(4) Before doing so, a dispenser pressure maintenance test was performed. After the test, the dispenser was depressurised by sending the hydrogen to the vent. However, the dispenser depressurisation line was still connected to the safety valve release line that had been removed the previous day. Therefore, the discharged hydrogen flew back and was released into the compressor room.

The INITIATING CAUSE of the accidental release of hydrogen was a combination of manual operations (the first and second day) and ad-hoc decision (second day).

The ROOT CAUSE could be identified by a lack of guidance and shortcoming in the safeguards which should have mitigate deviations from normal operations. Moreover, the decision to keep refuelling under abnormal circumstance appears having been affected by a lack of overview of the situation resulted from the mishap of the previous day.

safety valve, compressor

DESCRIPTION OF THE FACILITY AND THE AFFECTED COMPONENT
The production capacity of the refuelling station is unknown.
Its nominal delivery pressure was 35 MPa, in a temperature range between 5 C and 35 C.

The filling hose consisted of an innertube made of resin, a reinforcing layer made of high-strength fibre and outer cover (resin). The whole was then wrapped by a metallic spring. The purpose of the spring was to protect the filling hose's external surface, i. e. preventing mechanical damage, for example due contact with surrounding objects, to abrasions due to the hose being dragged and to deformation due to stepping on it. Moreover, the spring had also the function to prevent static electricity caused by the non-conductive layers.

No personal injury occurred. or property damage

This event highlights the challenges encountered by operative teams when facing new situations not described by the normal operative instructions and procedures.

An interlock is a mechanism that automatically intervene in case of unexpected events or improper operation, such as operational or procedural errors. Disabling an interlock at any moment of an operative phase introduces risks which have not been considered during the safety design phase. Therefore, disabling an interlock must require approval from a supervisor or site manager, in accordance with the construction plan and work procedure manual. The re-start of the operation must also be prevented before interlock is activated again.
All the actions occurring in the period in which the interlock is deactivated must be subjected to a properly designed temporary control system.

The KHK report notes as well that In Japan, the definition of a “high-pressure gas accident” is when a legally required safety device (e.g., a safety valve) is activated and high-pressure gas leaks from a high-pressure gas equipment. The activation of the safety valve in the first part of this incident was not reported. To ensure that everyone correctly understands the definition of a high-pressure gas accident, it must be specified in rules and manuals and thoroughly communicated.

1. The maintenance manual was updated to include information on releasing and restoring the compressor interlock, and on closing valves with plugs or other devices when they are removed.
2. The emergency procedures manual was updated to include information on notifying the prefectural governor without delay if a safety valve is activated. All relevant personnel were also informed and an explanation was given at a meeting.
3. While there was an "operation manual" for automatically starting the compressor (for steady-state operation), there was no manual for operating the compressor with a forced shutdown (for non-steady-state operation, mainly for differential pressure charging). Therefore, an "operation manual (for differential pressure charging)" was created.