Evaluating the risk trade-offs of pressure relief devices in hydrogen systems

Abstract

Hydrogen is increasingly being used as an energy carrier because of its potential to reduce carbon emissions in transportation and heavy industry. Nevertheless, this transition necessitates establishing an infrastructure for storage, transporting, and distributing hydrogen. This infrastructure is typically equipped with pressure relief devices (PRD) to protect systems from uncontrolled pressure increases. Without PRDs, a substantial pressure increase has the potential to rupture the equipment and lead to a hydrogen release, which could lead to fires, explosions, and significant damage. However, recent incidents have shown that these PRDs can also be the root cause of leaks and releases. Therefore, there is a need to understand the conditions when PRDs increase the risk versus when these devices effectively mitigate the risk. This dissertation analyzes the risk profile of PRDs when installed on hydrogen systems. To do this, we establish the risk management needs by identifying pressure relief device incidents in hydrogen systems and standardizing the observed root causes. Then, we define a probabilistic failure model for PRD installed on hydrogen systems. With this model, we assess the risk provided by pressure relief devices, comparing the risk of different fueling station configurations and the risk mitigated by the device against the risks they provided. The evaluation of previous incidents indicated that the failure modes to focus from a risk perspective are failure to operate, spurious operations, and external leaks. From these failure modes, a failure mode and effect analysis (FMEA) and the probabilistic model developed indicate that spurious operations and external leaks are affected by hydrogen, increasing the failure rate over non-hydrogen uses of PRDs. The rate of failure to operate does not appear to be impacted. For the risk analysis, an important aspect to evaluate in this assessment is the difference between the American and European approaches toward PRDs, where European standards do not require the use of these devices, whereas the American counterpart does. This assessment illustrates that burst discs generate a higher local risk than pressure relief valves (PRV). This trend is also observed in the average individual risk, which results in a higher risk when using a burst disc than PRVs. Furthermore, the risks estimated when PRDs are not installed were lower than when these devices are installed, concluding that the European approach to PRDs is preferable from a risk perspective. This result is promoted by the fact that the demand frequency of the protective function of the PRDs (e.g., correct activation) in the tube trailer due to external fires (i.e., fires from leak of other devices impacting the tube trailer) was estimated as 2.88 x 10-5 events/yr (without considering PRD events), while the failure rate of PRD-induced hazard events (i.e., spurious operations and external leaks) was 7.37 x 10-3 failures/yr per device for PRVs and 5.38 x 10-2 failures/yr per device for burst discs. These failure rates resulted in a predicted 5.90 x 10-4 ignited events/yr per device for PRVs and 4.30 x 10-3 ignited events/yr (i.e., jet fire, flash fire, and VCE) for burst discs. With these values, the tube trailer would have from 183 to 920 times (for 9 to 45 tubes) more ignited events from PRD failures than demands when using PRVs and from 1,343 to 6,720 more ignited events when using burst discs. In this regard, this analysis recommends not using PRDs in hydrogen systems as they introduce more risk than their risk mitigation capacity. This research is the first of its kind in analyzing the risk profile of PRDs, and it has potential to impact hydrogen system designs as well as codes and standards. Additionally, the results can inform the risk management community about how to model the trade-offs when making decisions about using PRDs to control risks, to optimize asset integrity and maintenance strategies for systems where pressure relief devices and amendments required on hydrogen codes and standards concerning PRDs.