A single-stage regulator "failed" while flowing hydrogen gas from a standard 200 cu.ft. gas bottle. The regulator had functioned properly prior to the event through several on-off cycles. During the event, a solenoid valve was opened to allow hydrogen to flow, when a rather loud noise was noted and gas began flowing out of the pressure relief valve on the side of the regulator. It was noted that the low-pressure gauge on the regulator was "pegged" at the high side (>200 psi). The valve on the bottle was shut off, and hydrogen flow was immediately stopped. Hydrogen flowing out of the relief valve did not ignite. With the bottle shut off, the regulator was removed and replaced with another regulator of the same type, and activities continued.
The failed regulator was later taken apart to try to determine the cause of failure. All the internal parts appeared to be intact with the exception of a small elastomeric ring that seals the internal "nozzle" to the "seat assembly." This elastomeric ring was deformed and had become lodged in the nozzle orifice, preventing the seat assembly from properly seating and allowing high pressure hydrogen to continuously flow into the low-pressure side of the regulator. The regulator has a pressure relief valve as protection and it operated properly, relieving the pressure in the system. Fortunately, nothing downstream of the regulator was damaged. It is not known what led to the failure of the elastomer ring.
The manufacturer was contacted, and the event/observations were described and discussed. Damage to the elastomeric ring on that valve stem, thus limiting its ability ro seal, is a rare event. The ring material was specifically selected for hydrogen/methane service and subjected to tens of thousands of open-close cycles. The manufacturer doesn't require a periodic maintenance program for these regulators but occasional replacement of the elastomer seal and o-ring on the outer part of the regulator body is recommended.
Because the bottle was located outside at the time of the event, and the hydrogen did not find a source of ignition while venting through the relief valve, nothing serious happened. The failed regulator was replaced and operations continued. However, if this had happened indoors or an otherwise enclosed space, the outcome could have been much worse.
The installed pressure relief valve and the small size of the orifice in the regulator (although allowing high-pressure gas to the low-pressure side of the regulator the mass flow rate is rather low) should be adequate protection of the rest of the system.
The key aspects of what can be learned from this near-miss can be emphasized as follows:
- A regulator is not a safety device. Without additional protection, downstream components can be exposed to pressures exceeding the set pressure up to the full bottle pressure. If items downstream of the regulator are not rated for full bottle pressure, it is recommended that protection be added to the system.
- Pressure relief device discharges need to be routed to a safe location. In the event of a pressure relieving event, it is important for the flow to be directed away from personnel, preferably such that the shut-off valve can be accessed safely.
- Adequate ventilation is an important consideration in the layout of a compressed gas system. Inert gases (as potential asphyxiants), toxic and flammable gases can pose a significant hazard if not properly ventilated.