Hydrogen leak detection systems may be required by the Authority Having Jurisdiction (AHJ) or may be installed as a means for enhancing safety. Leak detection can be achieved by providing hydrogen (or flammable gas) detectors in a room or enclosure, or by monitoring the internal piping pressures and/or flow rates for changes that would suggest a leak is present in the system. Other methods include providing detectors in close proximity to the exterior piping or locating hydrogen piping within another pipe and monitoring the annulus for leaks.
Regardless of the method used, leak detection systems should, at a minimum, incorporate automatic shutoff of the hydrogen source when hydrogen is detected. For systems designed to monitor hydrogen concentrations in rooms or areas, the leak detection system should also warn personnel with visible and audible alarms when the environment is becoming unsafe.
The following safety events occurred in a variety of settings, but each of them illustrates some key lessons learned about the critical need for hydrogen detection equipment:
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: Commerical Facility, Hydrogen Storage/Use Facility
Description: A leaking liquid hydrogen cryogenic pump shaft during the process of filling a gaseous tube delivery trailer to 2400 psi at a liquid hydrogen transfilling location caused a series of explosions and a fire. After approximately 30 minutes of filling, the operator heard a single loud explosion and then saw flames and ripples from heat generation near the ground in the hydrogen fill area. The operator quickly actuated the emergency alarm system that shut down the cryogenic pump and closed the air-actuated valves on the cryogenic pump supply line...
Lessons Learned:
Setting: Laboratory
Description: Employees in the laboratory were conducting high-pressure, high-temperature experiments with animal and vegetable oils in a catalytic cracker under a gas blanket. They were using a liquefied petroleum gas burner to supply heat in the process.
Investigators believe that a large volume of hydrogen leaked into the room through a pump seal or a pipe union, spread throughout the laboratory, and ignited after coming into contact with the operating LPG burner some 10 to 15 feet away. The flash fire engulfed the people in the room...
Lessons Learned: This incident emphasizes the need for proper gas detection and ventilation systems, as well as fire suppression systems, in laboratories using and storing hydrogen. This is especially true when open flame burners are in close proximity. Experienced consultants/engineers should be involved in the design of gas detection and ventilation systems before hydrogen cylinders are employed in any laboratory. Laboratories also need to develop a Standard Operating Procedure, requiring periodic maintenance on hydrogen systems to check fittings, valves, and all critical components to ensure proper functionality at all times.
Setting: Battery Charging Facility
Description: The incident occurred in the forming room, where wet cell batteries were stored for charging on metal racks. The facility had a wet-pipe sprinkler system, but no automatic hydrogen detection equipment.
Lessons Learned: Although a functional fire protection system would have helped to extinguish the fire, a properly installed hydrogen detection system, coupled with a properly designed ventilation system, could have prevented the incident altogether.
Setting: Government Facility, Hydrogen Storage/Use Facility
Description: Only 25 minutes after the normal work shift ended, an explosion occurred at a hydrogen storage and use facility that had been in a non-operational mode for several months while undergoing modifications for future tests. No one was in the facility at the time of the explosion. The event was viewed about 30 seconds after the explosion by two engineers in a blockhouse 1000 feet away. Authorities were notified and calls were placed to other personnel needed to secure the area. About 8 minutes later, the engineers moved to a vantage point about 450 feet from the facility. There they viewed heat waves rising from a central location on the test pad, heard popping sounds similar to gaseous hydrogen (GH2) venting on a burn pond, and suspected that a hydrogen fire was in process. They returned to the blockhouse and then traveled via a safe route to where the hydrogen storage vessels were located. Two large manual supply valves were closed, isolating the GH2 source about 30 minutes after the explosion. The facility's other supply systems and utilities had been severed or ruptured. Shrapnel and debris were ejected up to 540 feet away. Teams of firefighters and emergency medical personnel were sent to the area to verify that no one was injured and to extinguish small residual fires.
Lessons Learned: Active GH2 sensors should be installed and continuously monitored in all enclosed buildings near GH2 sources. All buildings near areas where hydrogen is used should be designed to preclude GH2 entrapment (e.g., sloping roof with ventilation at the highest point).
Underground carbon steel lines beneath concrete pad areas should not be used for GH2 transmission. All GH2 lines are now stainless steel and above ground. - Any GH2 transmission lines buried underground should be proof-tested and leak-checked on a periodic basis.
Any below-grade piping installation should be in open trenches covered by grating...
Some of the key lessons learned from these safety events are summarized below.
A good practice is to set the detectors to alarm at 1 vol% hydrogen in air, which is 25% of the lower flammability limit (LFL). If automatic shutdown is incorporated into the system, manual reset should be required to restart the system.
Portable gas detectors are valuable for local leak detection. Portable detectors should be used for entry or re-entry into rooms in which an alarm has occurred to ensure that the hydrogen has dissipated.
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