leaks from a threaded joint

At a hydrogen production facility, hydrogen stored in a 40 MPa hydrogen storage unit leaked from the main body of a fitting of the storage pressure and flow control system. The leak was not detected by the fixed hydrogen safety detector but discovered by safety officer using a portable gas detector during a routine tightness test. By using soap water, the officer located the leak near the main cone-and-thread type fitting of the 40 MPa gas accumulator unit. The safety personnel stopped the use of the 40 MPa gas storage unit and depressurised the piping system.

The INITIATING CAUSE was a leak in a threaded joint of the hydrogen storage control system.

The facility had already experienced three leak events on the same component, which had been replaced with another of the same producer, model and batch.

The investigations following previous leaks had identified that an initial defect in the joint manufacturing process had progressed due to hydrogen embrittlement, leading to the leaks. However, despite corrective measures, additional leaks occurred on 11 May and 4 June 2022. Various hypotheses were formulated: hydrogen embrittlement, hydrogen erosion, pitting corrosion, fatigue cracking, stress corrosion cracking and non-metallic inclusions. Based on microstructural observations and of the operative conditions, the study concluded that the first 5 hypotheses were unlikely and determined that the leak was caused by non-metallic inclusions. It was assumed that many non-metallic inclusions remained in the base material during the fabrication, and the interfaces between the non-metallic inclusions and the base material represented crack onsets due to stress concentration, leading to the leak.

Based on these findings, it can be concluded that the ROOT CAUSE was lack of manufacturing quality and control.

threaded joint

In addition to the leak on June 4, 2022, hydrogen leaks occurred from fittings (cone-thread type) of the same manufacturer, model, and lot in similar locations in the 40MPa gas storage unit on
• October 21, 2021 (KHK event report 2021-603),
• November 4, 2021 (KHK event report 2021-644),
• and May 18, 2022 (KHK event report 2022-354).
The compressed hydrogen station began production in 2003, and no fittings had been replaced until the leaks on October 21 and November 4, 2021. As a preventative measure against the recurrence of the leaks on November 21 and November 4, the fittings were replaced with unused fittings (same manufacturer, model, and lot number). However, leaks occurred on May 18 and June 4, 2022.

DESCRIPTION OF THE FACILITY
The hydrogen refuelling station Production Capacity was 489,084 m3/day (Business Facility) and 232,741 m3/day (Facility)
Nominal Use Pressure 40 MPa
Nominal Use Temperature 35 C

This is almost a near miss.

Threaded joints and fittings are the most prevalent connection solutions adopted by hydrogen refuelling stations. There are few manufacturers of fittings for high-pressure hydrogen environments worldwide. If a fitting-related incident occurs at one hydrogen station, there is a high possibility that similar accidents will occur not only at the same station but also at other hydrogen stations. It is important for hydrogen station stakeholders to quickly share information about the accident.

Operators must increase attention to material quality control (including cleanliness) when selecting fittings. This is of particular importance due to the high diffusivity of hydrogen, which can leak through microstructural paths inaccessible to bigger gas molecules. The diffusion process is also time-dependent, so that the detection strategies have to take into account this.

The gas detector installed in the gas storage unit (400 ppm alarm, 1000 ppm automatic shutoff) did not respond to the four leaks. The reason for this was probably the very small leaking flow: 10 ppm. At 15 cm away from the suction port the petable detector was already not detecting hydrogen. The start-of-the-day inspections were a good example of a daily additional control aiming to early detection of deviations.

Following contacts with several fitting manufacturers regarding hydrogen leaks from fittings originating from non-metallic inclusions, the operator of the HRS replaced all fittings of the cone-thread type used in the 40MPa gas storage unit with those from a different manufacturer that had not experienced leaks originating from non-metallic inclusions.