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Executive Summary

Compressed natural gas (CNG) has been safely used as a motor vehicle fuel for decades. As such, the technical requirements for CNG are well understood, whether for individual part specifications or throughout the broader automotive system.

Hydrogen is a relative newcomer to the motor vehicle fuel industry and is beginning to expand into the aviation field as well. Many of the technical requirements and equipment specifications for hydrogen are similar to those of CNG. However, the two fuels have different chemical properties, and hydrogen is stored and delivered at a higher pressure than CNG.

For an adequate comparison of technical requirements between hydrogen and CNG systems, a common set of terminology must be agreed upon before technical assessments can be made. The test regimens for components and systems, which are based on these commonly understood terms, can then be accurately evaluated against the same criteria.

The purpose of this report is to assess and address variations in pressure terminology for hydrogen and CNG codes and standards. An additional objective is to review pressure test levels for validation testing, based on hydrogen and CNG codes and standards, to identify any differences between regimens for the two fuels.

The existing pressure terminology and definitions from the relevant standards were recorded and assembled into a functionally distributed matrix presentation. Six equipment categories were examined:

1. Dispensing stations
2. Station components
3. Vehicle gaseous fueling interface
4. Vehicle systems
5. Gaseous fuel containers
6. On-board components

Overall, there was general agreement on the baseline pressure terms between hydrogen and CNG standards, with four exceptions:

• For baseline pressure, the ISO standards used hydrogen service level (HSL), while SAE, CSA and NFPA used a combination of nominal working pressure (NWP) and service pressure (SP). CSA/ANSI HGV 4.10:21 [1] used rated pressure (RP) as a baseline.
• The terms maximum operating pressure (MOP), also called maximum filling pressure (MFP), maximum allowable working pressure (MAWP) and maximum developed pressure (MDP) all had the same meaning throughout the documents. Again, CSA/ANSI HGV 4.10:21 only used rated pressure (RP) to define their devices.
• The term minimum component pressure rating, recommended by SAE J2579 [2] and ISO 19880-1:2020 [3], was only specified by name within the hydrogen vehicle interface documents SAE J2600 [4] and ISO 17268:2020 [5].
• The dispenser setpoint differs between hydrogen and CNG. The NFPA 2 [6] used 1.38 NWP for hydrogen, while NFPA 52 [7] used 1.25 SP for CNG. The rationale for the higher hydrogen value was to allow for a 10% tolerance over the MOP.

With the exception of the model codes NFPA 2 and NFPA 52, all of the standards had validation test requirements on par with the recommendations of the SAE J2579 and ISO 19880-1:2020 design standards. The model codes deferred to the product standards. These test requirements included the following general elements:

• Verification tests for baseline metrics
• Extreme and ambient temperature gas cycling test
• Leakage test/static pressure gas leak test
• Hydrostatic/pressure proof/ultimate strength/burst test

Both the hydrogen and CNG standards followed this process. While there were some differences in the testing pressures, they are insignificant enough to assume the two fuels follow the same criteria for validating their devices.

Analysis of the data from the relevant standards showed that the pressure test levels were essentially equivalent between hydrogen and CNG, with the exception of the dispenser set point.

All of the standards from the dispensing station and components, as well as the vehicle interface, referred to container pressure, calling it nominal working pressure, service pressure or hydrogen service level. All pressure testing was based off this number.

For on-board component standards, CSA/ANSI HGV 3.1:22 [8] separated the baseline pressures at the pressure reduction stage. Interestingly, CSA/ANSI HGV 4.10:21 [1] eliminated any difference between upstream and downstream by using rated pressure as a baseline for all pressure testing. All testing was based on the rated pressure of the device as designed.

Component and system-level validation test pressure requirements were the same for hydrogen and CNG. The component was to be exercised within its expected design envelope to the pressures anticipated by the system designer. Cycling was to be conducted at the intended usage extreme temperatures, and the pressures were to be cycled according to maximum anticipated design usage.

In summary, the pressure terminology and pressure test levels reviewed in this study represent foundational terminology for the well-established CNG industry. The development of hydrogen system validation test requirements heavily relies on the CNG knowledge base, and user confidence and historical performance provide a good starting point.