Extensive industrial experience has shown that hydrogen degrades the mechanical properties of structural metals under certain conditions through a process called "hydrogen embrittlement", enabling the initiation and propagation of cracks. Degraded structural integrity of components, piping, and other equipment containing hydrogen is a critical safety issue, since such failures can lead to the unintended release of hydrogen gas.
The Lessons Learned database contains records of five incidents and near-misses where hydrogen embrittlement played a role. These safety events occurred in a variety of settings (two laboratories, a tube trailer, a power plant, and a government facility), illustrating that hydrogen embrittlement is a concern for people working in many different environments. Each safety event record is summarized below along with the lessons that were learned. Following the summaries, information is provided about a web-based resource that was developed by Sandia National Laboratories in 2005 to provide data on hydrogen embrittlement of various materials.
Setting: Laboratory
Description: A research laboratory experienced two similar air-actuated ball valve failures in a 6-month period while performing hydrogen release experiments. The hydrogen release system contained multiple air-actuated ball valves that were sequenced by a programmable logic controller (PLC) in order to obtain the desired release parameters. During an experimental release sequence, a PLC valve command failed to open the valve even though the PLC valve position confirm signal indicated that the valve had opened. On further investigation, the valve actuator and valve stem were found to be moving correctly, but the valve was not opening. The system was depressurized and purged with nitrogen, and the valve was removed for dismantling and inspection. In both incidents, a sheared valve stem was found. The valve stem failures occurred after 8-14 months of continuous hydrogen operation at pressures of 800 to 850 bar (11,603 to 12,328 psi). The two failed hydrogen valves were identical and were rated by the valve manufacturer for hydrogen service.
Valve stem material incompatibility with hydrogen was the suspected cause of this incident, although it could also have been a design flaw. Review of the valve manufacturer's drawings showed the valve stem material to be 17-4 precipitation-hardening stainless steel, which is reported in the literature as having a 90% reduction in fracture toughness in a hydrogen environment. No metallurgical analysis was done on the failed valve stems, so material failure was not confirmed. The valves were repaired with new 316 stainless steel valve stems, placed back in service, and operated satisfactorily for 18 months without failure.
Lessons Learned: 1) Ensure that equipment and materials exposed to hydrogen are compatible with a hydrogen environment (even hydrogen-service-rated equipment). 2) Equipment designs in a hydrogen environment that use the fracture toughness for 17-4 precipitation-hardening stainless steel may want to consider a 90% derating in published values.
Setting: Laboratory
Description: A bourdon tube ruptured in a pressure gage after 528 hours of operation in a liquid hydrogen system. The alarm sounded, the system was isolated and then vented.
Lessons Learned: The tube was 403 stainless steel, which is subject to hydrogen embrittlement. The laboratory decided that all gauges with bourdon tubes would be replaced with 303 stainless steel.
Setting: Hydrogen Tube Trailer
Description: An operation to increase the pressure within a hydrogen tube trailer to 6000 psig was in progress when a burst disk failed at approximately 5200 psig and hydrogen was released. A vent line attached to the burst disk was not sufficiently anchored and bent outward violently from the thrust of the release, causing considerable damage to the adjacent vent system components. Fortunately no one was in the proximity when the burst disk failed.
Following the incident, the damaged portion of the tube bank (consisting of 6 tubes) was isolated by valves on the system manifold. The operation was resumed with the unaffected portion of the tube bank (another 18 tubes) until a second burst disk failed.
Lessons Learned: Metallurgical examination of the two failed disks 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, which is not recommended for hydrogen service in rupture disks.
Prior to this incident, the tube trailer had been used 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. This incident could have been prevented if an inspection had been conducted prior to the transfer of the tube trailer to hydrogen service.
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. 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.
Setting: Power Plant
Description: Difficulties were experienced with two solenoid-operated globe valves in a charging system. When shut, the valves could not be reopened without securing all charging pumps. During a refueling outage, the two valves were disassembled and examined to determine the cause of the malfunction. It was found that disc guide assembly springs in both valves had undergone catastrophic failure. The springs, which initially had 25 coils, were found in sections of only 1-2 coils. Metallurgical analysis of the failed springs attributed the probable cause of failure to hydrogen embrittlement. The springs are made of 17-7 precipitation-hardening stainless steel. The valve manufacturer revealed that similar failures occurred on three previous occasions, and these spring failures were also attributed to hydrogen embrittlement.
Lessons Learned: These events indicate a potential licensee/vendor interface problem. Based on the information received, the vendor was not completely informed via the purchase specifications regarding the service condition to which the valve would be exposed. Further, all users of these valves were not notified of the initial problem through either oversight by the vendor or as a result of the valves being supplied through an intermediate source. To avoid similar incidents in the future, onsite personnel need to ensure that their vendors receive comprehensive specifications relating to the service conditions associated with all of the stainless steel valves implemented in applications susceptible to hydrogen embrittlement.
Setting: Government Facility
Description: Several workers sustained minor injuries and millions of dollars worth of equipment was damaged by an explosion after a check valve shaft blow-out. The valve failure rapidly released a large vapor cloud of hydrogen and hydrocarbon gases that subsequently ignited.
Certain types of check valves and butterfly valves can undergo shaft-disk separation and fail catastrophically or "blow-out," causing toxic and/or flammable gas releases, fires, and vapor cloud explosions. Such failures can occur even when the valves are operated within their design limits of pressure and temperature. Most modern valve designs incorporate features that reduce or eliminate the possibility of shaft blow-out. However, older-design check and butterfly valves, especially those with external appendages, may be subject to this hazard.
A number of design and operational factors may contribute to this hazard. The valve has a shaft or stem piece that penetrates the pressure boundary and ends inside the pressurized portion of the valve. This feature results in an unbalanced axial thrust on the shaft that tends to force it out of the valve if unconstrained. The dimensions and manufacturing tolerances of critical internal parts cause them to carry abnormally high loads.
These valves are subject to high cyclic loads and are susceptible to blow-out. In many incidents, the valve is repeatedly slammed shut with great force during compressor trips and shutdowns. Such repeated high stresses may causes propagation of intergranular cracks in critical internal components, such as dowel pins. The valve is subject to low or unsteady flow conditions that can result in increased wear of critical internal components.
Valves in high-pressure service lines may be more likely to undergo shaft blow-out. In this incident, system pressure at the failure point was approximately 300 psig. Valves used in hydrogen-rich or hydrogen sulfide-containing environments may be more susceptible to blow-out due to hydrogen embrittlement of critical internal components, particularly if these are made from hardened steel (as was the dowel pin in this incident).
Lessons Learned: Facilities should review their process systems to determine if they have valves installed that may be subject to this hazard. If so, they should conduct a detailed hazard analysis to determine the risk of valve failure. Detailed internal inspections may be necessary in order to identify high-risk valves. Facilities should consider replacing high-risk valves at the earliest opportunity with a blow-out-resistant design. If immediate valve replacement is impossible or impractical, facilities should consider immediately modifying the valves to prevent shaft blow-out. Valve manufacturers should be consulted in order to ensure that any planned modifications would be safe.
In 2005, Sandia National Laboratories (SNL) created the Technical Reference for Hydrogen Compatibility of Materials, a web-based resource that consists of material-specific chapters (individual PDF files) summarizing mechanical-property data from journal publications and technical reports. These data demonstrate the effects of hydrogen gas on engineering material properties such as strength, ductility, and fracture resistance. In each chapter, the data are presented as tables and plots, with text explaining the data and illuminating important trends.
The target audiences for this resource are component designers and standards development organizations. In addition to using the data, these audiences provide input to SNL on technologically relevant materials to include in the Technical Reference. These materials include carbon steels, low-alloy steels, stainless steels, and aluminum, as well as several specialty alloys. The Technical Reference currently has 22 chapters, and the content is updated and expanded as more data become available.
In the future, the Technical Reference will serve as the foundation for a new Hydrogen Materials Collaboration Database, which will be a mechanism for coordinating international research in hydrogen-compatible materials and components. This database will include discussion forums, archives of open-source content, reports from conferences or meetings, and new results on hydrogen compatibility testing that are pending review for incorporation into the Technical Reference. The objective of this Hydrogen Materials Collaboration Database is to enable global harmonization of test methods, facilitate research coordination, and accelerate deployment of hydrogen systems.
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