This incident highlights the need to properly design safety interlocks. These safety interlocks need to be carefully incorporated into the initial building/plant designs and should consider all of the unexpected occurrences, such as the electrolysis cell bank losing power in this case, and the potential ramifications of such occurrences.

The investigative report noted that the explosion could have been prevented by (among other things) a continuous gas analyzer test of oxygen and hydrogen product purity. The continuous analyzer should be interlocked to shut the electrolyzer down when product purity falls below some nominally critical values. This incident, illustrates the need for more widespread use of hydrogen analyzers, and the inverse relationship between hydrogen accidents and regular maintenance.

The lessons of this event fall into five categories: (1) proper in-plant communications during events, (2) proper valve application for use with hydrogen, (3) excess flow check valve set point, (4) heating and ventilation and air conditioning (HVAC) maintenance and flow testing, and (5) hydrogen line routing. The operator is examining ways to improve communications in the plant during events and the training of personnel in reading portable instruments.

As another corrective measure, the operator is examining the use of other types of valves, such as valves with a diaphragm or bellows rather than conventional stem packing, in lines containing hydrogen. The operator is also examining the set point for the excess flow check valves on the hydrogen lines. These check valves are designed to limit the flow of hydrogen in the event of a large leak so that when combined with proper ventilation in rooms with hydrogen lines, hydrogen levels would remain within specified limits throughout the plant.

This plant had HVAC flow balancing problems during the preparation for plant startup. Generally HVAC flow balance is based on the heat loads and the resultant room temperatures under normal and accident conditions. However, this event demonstrates that hydrogen concentrations also may need to be considered to set a lower limit on the ventilation in rooms that contain hydrogen lines.

These events show the importance of preventing combustible gas mixtures from accumulating in piping. In both of the above described events, hydrogen and oxygen gases apparently accumulated to a combustible level which then catastrophically failed these piping systems. Proper venting or other considerations to prevent accumulation of combustible gases in piping high points might alleviate conditions leading to hydrogen combustion.

Work documentation (work orders and baseline drawings) should reflect the current system configuration.

Recommendations

1. Develop procedures for temporary change configuration control of high-pressure systems. The overall work process should be included in one work authorization document.
2. Re-emphasis to all personnel that current procedures be followed. If the work order is not written to reflect the current system configuration, stop work, revise work order, and have the work order properly reviewed prior to continuing work.
3. Write a procedure for proper clamp removal and installation and train technicians to the procedure.

Batteries stored on a charger can explode during use if overcharged.

Recommendations

1. Use automatic current limiting or timed circuit chargers when charging batteries.
2. Operators should be aware of safe practices and proper battery charging instructions.

Personnel should be properly supervised, and supervisors should be aware of the activities of their personnel. Personnel must be motivated to adhere to established policies and procedures. All personnel associated with potentially hazardous work should receive necessary safety training.

• Battery power supplies require adequate ventilation for all operations, or the battery box should be designed to:
  • Eliminate all sparking devices
  • Ventilate during charging operations
  • Provide inert gas purge and pressurization methods, and
  • Provide adequate pressure relief
• Give consideration to other power supply methods.
• Organizational operating procedures and staffing should as necessary ensure that all equipment designs and work procedures are reviewed by an independent engineer. When hydrogen equipment and procedures are modified, the changes should be reviewed.
• Safety manuals should be revised to incorporate information on storage battery handling and operations safety practices. Safety expertise should be made convenient. Provide adequate training.

Immediate Corrective Actions

1. Fuel cell test stand was shutdown and sent to manufacturer for investigation.
2. Carbon dioxide fire extinguisher installed in laboratory.
3. Formal process hazard review performed

Long-Term Corrective Actions

1. Replaced all flexible tubing in test stand with stainless steel (316).
2. Replaced water knockout device with latest manufacturer's design on both the H₂ and O₂ lines to reduce the risk of gas flame by leached catalyst.
3. Installed a “High-pressure trip – Carbon dioxide” fire suppression system in the lab.
4. Installed a redundant hydrogen detector inside the test stand.
5. Installed an external hydrogen delivery system with stainless steel feed lines into the lab.
6. Revised the Standard Operating Procedure for the fuel cell test stand.

In addition to resealing the glove box window, a positive pressure of argon gas was maintained inside the glove box while the course of action was planned. Subsequently, the glove box was cleaned up by specialized hazardous materials personnel using natural bristle brushes and plastic utensils. Also, Teflon-coated magnetic stirring bars were used to separate the milling balls from the powder while avoiding metal-to-metal contact.

While no direct evidence has been obtained, it is possible that a small leak in the antechamber seals or back diffusion from the vacuum pump occurred to expose the NaAlH₄ material to oxygen and/or water vapor. Similar sudden reactions within a glove box have been noted by other researchers working with NaAlH₄ where contamination by oxygen / water vapor was suspected. A possible material mechanism is detailed in “Ashby's warning” published in Chemical and Engineering News, V47 (1), 1969. In general, researchers working with NaAlH₄ or other reactive hydrogen storage materials should take extra precautions with regards to sealing and vacuum pump type/performance when holding such materials under vacuum for extended periods of time.

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.

• A SOP should be developed which prohibits drivers from backing into the fill position.
• As a precautionary measure to mitigate similar events in the future, piping barriers should be installed which protect critical H₂ piping components/systems from delivery truck collisions.

• When truck drivers are carrying new trailers, with new load distribution characteristics, they need to exercise caution, especially on hazardous roadways such as the one described in this occurrence. If the trailer stack would have been smashed and plugged or if the trailer shell would have been punctured and ignited, a much more severe accident would have occurred.
• H₂ truck drivers need to be educated on the explosive characteristics of H₂ they are carrying, so they will have a built in incentive to drive with caution in similar situations.
• Company policy should be clearly established to encourage safe driving practices under all conditions. Unsafe driving, in contrast, will not be tolerated.

• All tank trailers should have a safely accessible auxiliary shut off valve in case of spills.
• Emergency personnel need to have access to all of the appropriate protective clothing, including shoes. Liquid hydrogen is stored at 20.28 Kelvin or -423.166 °F, and is cold enough to freeze surrounding air at these temperatures.

Standard procedure must be followed in all cases. Assumptions are made at great risk. Risk also increases with complacency.

Standard procedures must be followed at all times. The importance of doing so should be frequently reinforced through safety communications to all staff.

The use of inerting gas or other means of separation should be employed when conducting mechanical work where hydrogen gas could be present. More importantly, per CGA S1.3, the vessel should be equipped with a dual relief system that can isolate one side from the other and allow a rupture disc to be changed without exposing the operator to hydrogen.

In the second incident, the cracking of the outer mild steel vacuum jacket was more than likely related to the coefficient of thermal expansion of steel, which defines how much the material will contract when its temperature is decreased. The temperature of cryogenic liquid nitrogen is at -195.8 °C (-320.44 °F), and the linear coefficient of thermal expansion of 1020 steel at room temperature is 12 x10^-6 1/0 °C. Thus, the significant contraction in the steel due to the instantaneous temperature reduction created localized stresses, which cracked under the vacuum pressure of the system. Some other method of controlling the fire should have been employed. In addition, the metal would have been made much more brittle due to the low temperature.

All relevant personnel should receive at least basic training on the proper selection of fire extinguishing techniques for the given scenarios they are likely to encounter.

Liquid nitrogen should not be used to put out a hydrogen fire. It is very difficult to put out a gaseous hydrogen fire, plus had the liquid nitrogen not cracked the nearby vessel's shell, it certainly could have cracked the original vessel. It could also have plugged up the stack by freezing at liquid hydrogen temperatures.

Extra caution should be taken working around elevated pressure or low-temperature fluids and storage. Values should be checked and then verified by a second party, if possible.

All valves and connectors should be clearly labeled to minimize chances for mis-connection. All technicians must be trained on proper procedures for both taking systems offline and bringing them back online.

Valves for compressed hydrogen gas service are discussed in the Hydrogen Safety Best Practices Manual.

First, it appears that the system may not have been vented properly. CGA G-5.5 should be used for determining safe locations based on the variables of the specific setup. Also, if the compressor was tied to a storage system, a backflow prevention device may have limited the amount of gas that was released. Finally, it appears that equipment was left in place from previous activities. Such equipment should be evaluated to make sure that it is appropriate and safe for use in new processes.

Because of the near invisibility of a hydrogen flame in daylight and hydrogen's extremely low ignition coefficient, if a known leak is present (e.g., an audible hissing), ignition should always be presumed. The primary cause of this incident derives from the technician improperly performing hot work in the vicinity of a charged flammable gas line. Given the location of the flammable gas line, an alternative to performing hot work or relocating the hot work should have been considered. If such work was necessary at this location, it should have been performed only after the gas supply was verified closed (along with a lock and tag). Also, if this latter option was chosen, then the system should be checked for leaks prior to turning the gas back on.