Hydrogen is unlike conventional fuels such as gasoline or propane, which are heavier than air and tend to accumulate at ground level. Hydrogen is lighter than air and will accumulate near the ceiling or roof area, or in pockets at these locations. When the buoyancy of hydrogen is not properly taken into account in the design of facilities, hydrogen leaks can become more dangerous than leaks or spills of conventional fuels. As a result, even slow releases of hydrogen in such facilities could lead to the formation of a flammable concentration at the ceiling.
Setting: Laboratory
Description: During a standard testing procedure, a 3,000 psig relief valve actuated at normal line pressure, releasing gaseous H2. The gaseous H2 combined with air, resulting in an explosion which damaged the test facility.
Lessons Learned: The site needs to implement a good Operational and Readiness Inspection procedure. System inspection deficiencies need to be identified, and (if possible) reviewed by a second party before future tests are conducted. This inspection could include the following:
A proper vent system design is as important as the relief valve itself. A vent system should be able to operate with no consequence at all. Proper facility design and venting to a safe location would have made this a near-miss instead of an incident.
Setting: Laboratory
Description: A H2 air explosion occurred near a H2 compressor, located outside. Gaseous H2 had been released from a vent stack when a relief valve was actuated. The source of ignition was not known, but considerable damage was inflicted onto the system by the ensuing fire and explosion. Following the explosion, the shut-off valves were closed and the system was vented.
Lessons Learned: 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.
Setting: Laboratory
Description: A hose clamp failed on a low-pressure vent line from a hydrogen reactor experiment and effluent was leaked into the laboratory. Unburnt hydrogen in the effluent stream triggered the low-level hydrogen alarm. The hose clamp was resecured and other hose clamps were checked for proper tightness.
Lessons Learned: Maintenance on the low-pressure venting system was not occurring at regular intervals. Ventilation integrity is now checked before starting an experiment.
Setting: Chemical Plant
Description: No description given.
Lessons Learned: It is important to understand the requirements and standards associated with safe equipment design (especially electrical equipment containing an internal ignition source with flammable gas) in potentially explosive atmosphere environments. Misinterpretation of requirements and standards can lead to serious consequences. If the application of a standard is not fully understood, it is advisable to contact the author of the standard to remove any misunderstanding and not try to interpret the rules.
Gas detection instrument location is critical to proper functioning. Light gases like hydrogen rise in air, and gas detection needs to be at the high point in the potential source area so that even small leaks can be detected. Various alarm/action thresholds below the lower flammability limit (LFL) of the flammable gas give additional warning of a possible problem in the event of a gas leak. The following is a summary of how equipment involved in this incident should have been installed.
The gas chromatograph should not be installed in a sealed cabinet, but should follow explosive atmosphere design standards to have forced ventilation with a minimum flow rate of 12 times the cabinet volume per hour and to exhaust outside the building. With this change, the analysis room can remain in its current configuration and it does not fall under the explosive atmosphere regulations.
The fixed gas detector must be installed in the cabinet sealed volume, and must comply with explosive atmosphere regulations. The gas detector must be connected to an interlock system and set with two threshold levels; the first at 25% of the LFL (which sends an alarm) and the second at 50% of the LFL (which closes the hydrogen isolation safety valve). For this gas detector, the 100% LFL threshold level is set at 4% hydrogen in air.
To minimize the consequences of a possible leak of hydrogen inside the cabinet, it is recommended that the hydrogen isolation safety valve be installed outside the analysis room.
The explosive atmosphere regulations also require the installation of a door switch that stops the supply of electricity and flammable gases whenever the door is opened (for example, when performing maintenance). This door switch limits the risk of creating an explosive atmosphere in the room that is not regulated under explosive atmosphere standards.
Setting: Commercial Facility
Description: A water treatment plant used an electrolytic process to generate sodium hypochlorite (NaOCl) from sodium chloride (NaCl). The strategy of using liquid sodium hypochlorite for disinfecting water instead of gaseous chlorine (CL2) is popular because the liquid is generally safer and falls under fewer OSHA and EPA standards. The further idea of generating the liquid sodium hypochlorite on an as-needed basis and in limited quantities also has certain obvious safety advantages.
One of the disadvantages of the electrolytic process is that hydrogen gas is also created as a byproduct. The hydrogen is supposed to be vented, by design, to the atmosphere before the liquid sodium hypochlorite passes into a holding tank.
For various reasons, in this instance it is believed that the hydrogen vent line was closed, thereby forcing the hydrogen gas into the liquid holding tank where it accumulated. In order to repair a leak in the tank, plant workers had drained the tank to within a few inches and then lowered an electric pump into the tank to remove the remaining liquid. When the switch was thrown to turn on the pump, the tank exploded. One worker was killed by the blast.
Lessons Learned: The mechanisms and rates by which hydrogen gas is generated and subsequently accumulated in the holding tank need to be fully understood by vendors and employees alike. Active venting, warning signs, and local alarms designed to activate when hydrogen ventilation lines are obstructed are essential.
Setting: Power Plant
Description: No description given.
Lessons Learned: The uncontrolled release of hydrogen occurred as a result of the rupture of the No. 6 hydrogen storage tube’s burst disc. This disc failed in response to being overloaded by mechanical stresses developed as water expanded and formed ice while in direct contact with the burst disc. It was the degraded condition of the vent cap (defective equipment) that enabled water to access the burst disc.
As a corrective action, eliminate burst discs from hydrogen storage assembly. Redesign venting system for the pressure relief valves to prevent or inhibit moisture build up and allow moisture drainage.
The investigation uncovered two instances where the supplier was in possession of information ("safety data") that, if successfully conveyed to plant management and subsequently acted upon, would have prevented or reduced the chance of occurrence of the subject incident. Specifically, the hydrogen supplier found ice in a vent pipe, and was aware that the vent caps were cracked (recall the cracks were painted). Had a requirement existed for this information to be communicated to the plant, then plant management would have had the opportunity to evaluate and potentially influence the supplier's maintenance and operations program.As a corrective action, contract documents for the hydrogen and nitrogen supplies will be modified to stipulate the following:
Suppliers of potentially hazardous equipment will provide plant management, for acceptance purposes, with written documentation describing the supplier’s preventive maintenance program.
The supplier shall provide the plant representative with a copy of a preventive maintenance report upon the completion of each PM check performed by the supplier. The supplier shall expeditiously rectify any identified deficiency.
Plant management will recommend to the Manager of Corporate Safety and Health that the above contract document modifications are implemented corporate wide.
Setting: Chemical Plant
Description: A rupture disc blew on a 20,000-gallon liquid hydrogen tank, causing the vent stack to exhaust cold gaseous hydrogen. Emergency responders were called to the scene. To stabilize the tank, the remaining hydrogen was removed from the tank except for a small volume in the heel of the tank that could not be removed manually. The tank vacuum was lost. Firemen sprayed the tank with water and directed a stream onto the fire exiting the vent stack. The water was channeled directly into the open vent stack, and the exiting residual hydrogen gas (between -423 F and -402 F) caused the water in the vent stack to freeze. The water freezing caused the vent stack to be sealed off, disabling the only exit for the cold hydrogen gas. After a time, the residual hydrogen gas in the tank warmed up, causing the tank to over-pressurize and rupture with an explosion known as a BLEVE (boiling liquid expanding vapor explosion).
Lessons Learned:
Some of the key lessons learned from these safety events are summarized below.
Proper ventilation can reduce the likelihood of a flammable mixture of hydrogen forming in an enclosure following a release or leak. At a minimum, ventilation rates should be sufficient to dilute a potential hydrogen leak to 25% of the lower flammability limit (LFL) for all operations and credible accident scenarios.
Passive ventilation features such as roof or eave vents can prevent the buildup of hydrogen in the event of a leak or discharge, but passive ventilation works best for outdoor installations. In designing passive ventilation, ceiling and roof configurations should be thoroughly evaluated to ensure that a hydrogen leak will be able to dissipate safely. Inlet openings should be located at floor level in exterior walls, and outlet openings should be located at the highest point of the room in exterior walls or the roof. If passive ventilation is insufficient, active (mechanical, forced) ventilation can be used to prevent the accumulation of flammable mixtures. However, no practical indoor ventilation features can quickly disperse hydrogen from a massive release by a pressurized vessel, pipe rupture, or blowdown.
Equipment used in active ventilation systems (e.g., fan motors, actuators for vents and valves) should have the applicable electrical classification and be approved for hydrogen use. If active ventilation systems are relied upon to mitigate gas accumulation hazards, procedures and operational practices should ensure that the system is operational at all times when hydrogen is present or could be accidentally released. Hydrogen equipment and systems should be shut down if there is an outage or loss of the ventilation system if LFL quantities of hydrogen could accumulate due to the loss of ventilation. If the hazard is substantial, an automatic shutdown feature may be appropriate.
More detailed information about ventilation system design is available on the Hydrogen Safety Best Practices web site
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