Maintenance on the low-pressure venting system was not occurring at regular intervals. Ventilation integrity is now checked before starting an experiment.

Excessive venting of hydrogen from the tank due to lower facility consumption, in combination with extreme temperature conditions, placed thermal stress on the gland nut, causing a leak. The low consumption of hydrogen resulted from the shutdown of some production equipment and the delay of additional production equipment coming online. The tank size is too large for the facility's current hydrogen demand.

The hydrogen supplier will conduct annual training on handling all types of gases used by the facility and will include the local fire department in this training. The facility will continue daily rounds to look for visual evidence of leaks, and the hydrogen supplier will exchange the 9,000-gallon tank for a smaller 4,500-gallon tank to significantly reduce pressure build up from lower usage.

Disable the fueling process if the operator does not follow the proper sequence of steps in the fueling procedure. Improve the fueling procedure to make it inherently impossible for the sequence to be done improperly with the same result.

Several recommendations were outlined by the investigating committee to govern future operations of the hydrogen compressor in the synthetic liquid fuels laboratory:

1. Install a cutoff that will shut down the compressor when the suction pressure drops to a positive pressure of one to two inches of water.
2. Install an oxygen analyzer with an alarm as close to the compressor suction inlet as possible.
3. Attach all cylinders to the filling rack, discharge the contents to the atmosphere, and evacuate before filling.
4. Analyze as soon as possible the contents of at least one cylinder of each rack of cylinders charged.
5. Eliminate the recycle connection from the suction side of the compressor to the blowdown pot.
6. When compressing hydrogen, disconnect all interconnecting lines from the hydrogen suction line except the inert gas line.
7. Establish more definite procedures and responsibilities for maintenance repairs, construction, and operations.

1. Consider periodic testing of breakaway device.
2. Consider determining shear force limit of vehicle receptacle adapter fittings.

Mechanical pressure gauges tend to be imprecise if only used in a narrow portion of the full scale. Digital transducers, although slightly more expensive, offer much more precision. The event happened because the set pressure was only 10% of full scale, and the error of the mechanical gauge was over 5%.

While this is not the reason for the event described, control of the charging pressure is one of the most crucial parameters. Although the storage is at low pressure, the pressure increase upon temperature increase can be much steeper than the ideal gas law would predict, depending on the charging conditions.

The chosen trigger point of 35 bar for the pressure relief valve is very low. Based on the tank design, at least 60 bar would be acceptable.

Additional discussion about working with reactive metal-hydride materials in the laboratory can be found in the Lessons Learned Corner on this website and in the Hydrogen Safety Best Practices Manual.

Corrective actions included replacing the breakaway with a new one, which restored normal operation of the dispenser.

Verify and periodically inspect the pull/separation force adjustment if the breakaway is so equipped.

Additional information on equipment maintenance and inspection can be found in the Hydrogen Safety Best Practices Manual.

A gas detector was added in close proximity to the compressor shaft and a vibration switch is under consideration. Additional predictive measures are being considered to predict bearing failure. In addition, the manufacturer has been contacted and the bearing design is being analyzed to see if it can be improved.

Included inspection on monthly preventive maintenance plan and evaluated alternate materials for better cold-weather performance.

The fitting was an SAE straight thread and was likely loosened by torque applied to the fueling hose. After the incident, these fittings had additional means applied to restrict loosening, a cover installed to deflect any leakage, and means taken to restrict hose torque by using a different style nozzle. In addition, different fittings have now been deployed.

Metallurgical examination of the two failed disks by light optical microscopy (LOM), scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopic analysis (EDS) found them to be fabricated from pure nickel with evidence of extensive fracture. Each of the 24 tubes in the system is protected by a burst disk. Examination of another disk in the system that had not given way found that it, too, possessed surface fracture features, and they extended around the entire periphery of the rupture disk. Such defects are indicative of hydrogen embrittlement. An inspection of all vent circuits found that each of the 24 disks in service was made from nickel. Nickel is a material not recommended for hydrogen service in rupture disks.

Prior to the attempted use of the tube bank for hydrogen service, the vessel had been employed for helium service. The pressure vessel documentation accompanying the system indicated that the burst disks were made of stainless steel and rated to 10,000 psig. Careful physical inspection of system hardware is recommended on any system being adapted to hydrogen service. In this instance, inspection conducted prior to the transfer in service could have alerted operators to the need to install disks with the proper material, and therefore, have prevented the incident.

Relief of hydrogen gas should not lead to movement of the vent line sufficient to cause system damage. Corrective actions included increasing the line diameter and adding bracing between the lines and the system bulkhead to strengthen the components should other releases occur. The hardware that failed was of a commercial origin. Caution should be exercised to insure that all hardware is adequate for its designed purpose, even when procured from a commercial source.

More information on management of change can be found in the Lessons Learned Corner and also in the Hydrogen Safety Best Practices Manual. A web-based resource developed by Sandia National Laboratories to provide data on hydrogen embrittlement of various materials is available at Technical Reference for Hydrogen Compatibility of Materials.

Mounting hardware incorporated polymeric braces not suitable for long-term exposure to sunlight and temperature extremes. With time, the polymeric materials had disintegrated, allowing the mounting brackets to become loose. In addition, the mounting brackets were all oriented with a degree of freedom in the same direction such that drag forces from strong wind coming from just the right direction were able to dislodge the vent line and blow it down. Periodic inspections and maintenance operations failed to pick up the deteriorating hardware.

Hardware design must be adequate for weather conditions and materials selection must be compatible with temperature excursions and solar-UV exposure conditions. Operations must include periodic inspection of mounting hardware.

Emergency procedures must address conditions that include the presence of a hydrogen leak that may pose a hazard to personnel attempting repair operations. Procedures were developed by:

• Determining the approximate temperature and release rate of the hydrogen emanating from the damaged vent,
• Finding computed hydrogen dispersion information based on diffusion and wind (see combustible cloud length as a function of release rate in Edeskuty, Frederick J. and Walter F. Stewart, Safety in the Handling of Cryogenic Fluids, Plenum Press, New York, 1996), and
• Using the dispersion information to establish a safe working area for repair operations and an exclusion zone around the hydrogen release point.

The possible outcomes from new maintenance scenarios can be predicted by using an accurate simulation. The proposed filter change-out maintenance was studied to identify conditions to which the catalyst might be exposed and a mock-up of the filter, it's mounting/housing, and catalyst was assembled. Conditions selected to represent the worst case that could be encountered during a maintenance operation were reproduced. For the conditions of temperature and pressure, this included simulating the state of the catalyst. Oxygen was removed from the catalyst with a dry nitrogen purge, and followed by a graduated hydrogen purge from 0.5 to 10 % concentration. Hydrogenation was completed by then subjecting the catalyst to a purge of 100 % concentration of hydrogen. De-ionized water was vacuum-degassed for 10 minutes to remove oxygen, then saturated with hydrogen (by bubbling) at pressure. The simulation of pre-change-out filter conditions was completed by adding the hydrogen-saturated water to the hydrogenated catalyst within the mock-up system. The simulation of conditions introduced by the proposed maintenance was accomplished by draining the water and introducing a purge of oxygen-enriched air (ISS ambient conditions). The drained water and purged air were captured for laboratory analysis to check for thermal degradation and toxic byproducts of Teflon. Gas Chromatography with Mass Spectral detection (GC-MS) revealed no fluorinated species above the detection limits in the gas phase, and Ion Chromatography (IC) identified only small amounts of fluorinated compounds in the liquid phase (not indicative of a handling hazard). Post-test thermal gravimetric analysis (TGA) of samples indicated that insufficient heat was generated in the tests to thermally decompose the Teflonized catalyst. The conclusion is that when charged catalyst is handled wet, reactions with air are reduced to the point of permitting safe handling. A caution is noted. While deliberate simulation of conditions that would result in rapid exposure to dry charged catalyst was not performed, the hydrogenation step, if not initially done gradually, but with 100 % concentration hydrogen, will produce smoke and steam, suggesting any procedure that introduces rapid exposure to air could result in high temperatures and potentially hazardous by-products.

Maintenance on PEM technology systems that involves accumulated catalyst within filters may be safely performed by reducing the rate of exposure of catalyst materials to air or oxygen, always keeping a coating of water on catalyst surfaces and making sure discarded catalyst is properly disposed and not allowed to dry out, especially in the presence of volatile and flammable materials. Catalyst materials exposed for a time in a hydrogen environment will absorb substantial amounts of hydrogen, becoming "activated". Subsequent exposure to air or oxygen without mitigating steps can cause high temperatures to occur on catalyst surfaces, creating a potential fire hazard. Do not expose Teflon to high temperatures.

1. Hydrogen safety training should be provided to local emergency responders.
2. Liquid hydrogen installations should be inspected by facility personnel on a frequent basis, consistent with NFPA 55, to verify proper operation and inspect for physical damage or leaks. If there are problems, contact the servicing company immediately.
3. Industrial gas companies that design, install, and maintain liquid hydrogen installations should follow the guidelines set in the Compressed Gas Association (CGA) publications: G-5.4 - Standard for Hydrogen Piping Systems at Consumer Locations, G-5.5 - Hydrogen Vent Systems, H-3 - Cryogenic Hydrogen Storage, and H-5 - Installation Standards for Bulk Hydrogen Supply Systems.
4. Burst disks are highly sensitive to any form of back pressure.

The SS 24-inch pipe that failed was replaced with 1-1/4 Cr 1/2 Mo alloy pipe that is corrosion-resistant to SCC. A revised HTS bypass piping layout was installed to prevent the hazardous conditions that lead to the failure. A detailed hazard review and evaluation of all of the materials of construction in the hydrogen plant process gas system led to preventively changing several other pieces of piping and equipment items in the SMR process to 1-1/4 Cr 1/2 Mo.

1. Redundant safety systems prevented this event from becoming an incident. The 1%-hydrogen-concentration-level-triggered fan was backed up by a 2%-hydrogen-concentration alarm. The alarm is continuously monitored (24/7) by a remote Network Operations Center (NOC).
2. Since this event, a pressure switch has been added to alarm in case of a fan failure and is also continuously monitored by the remote NOC.
3. Future standards will require two ventilation fans, one running continuously and the other triggered to start when a 1% hydrogen concentration is reached.

Adequate ventilation of battery charging facilities is addressed in the Lessons Learned Corner on this website.

To prevent a similar flashback, the following measures have been taken:

1. The valve is opened slowly to avoid a major leading shock into the venting line.
2. A special nozzle (Laval or similar) is mounted directly after the opening valve. This prevents shock reflections from entering the premixed zone.

Many accidents reported from paper mills have much in common with this incident. Microorganisms in the process water with pulp produce hydrogen gas that mixes with air to form an explosive atmosphere. The ignition source is typically sparks produced by hot work, but ignition by electrostatic discharges in a cloud of mist in a storage tower has also been reported (see Explosion Caused by Microbial Hydrogen Formation).

Provided the hazard posed by bacterial hydrogen production has been recognized, there are several preventive and/or mitigating measures that can be considered for reducing the risk:

1. It is advantageous to avoid anaerobic and stagnant conditions in the solution by providing sufficient circulation and/or aeration (bubbling air through the solution), but this requires monitoring and could result in foaming or generation of biogas.
2. The formation of an explosive atmosphere may be avoided by providing sufficient natural or forced ventilation to keep the hydrogen concentration below the lower flammability limit (LFL), or by using an open-top floating roof tank to remove the space where an explosive mixture could form.
3. Forced ventilation can be used, but this solution requires monitoring, and care must be taken to avoid introducing a potential ignition source.
4. The free space above the liquid level can be blanketed with an inert gas to prevent the formation of an explosive mixture, but this is a relatively expensive solution that requires monitoring, and the inert atmosphere will promote anaerobic conditions that favor bacterial hydrogen production.
5. Strict enforcement of hot work procedures, including gas measurements and the use of non-sparking tools, may prevent certain ignition sources, and especially those that are directly associated with personnel risk.
6. Technical solutions such as grounding to prevent build-up of electrostatic charges and installation of flame arresters on all openings in the tank to prevent ignition by lightning, etc. can also reduce the risk, but the possibility of having an explosive atmosphere inside the tank should still be regarded as a severe hazard.
7. Risk management strategies should include a continuous focus on preventive maintenance, mandatory safety training for all workers, regular reviews of risk assessments, and learning from previous accidents in related industries.

A tool is provided for removing the cylinder cap that cannot contact the valve.

Consider design review of all adapter fittings.