The Hydrogen Safety Panel is not aware of any detailed guidelines or requirements. There are general requirements for personal protective equipment (PPE) in relevant codes and standards, but these often defer to the manufacturer’s requirements or a material safety data sheet (MSDS) for specifics. Ultimately, PPE selection is a matter between the employer and the employee and may need to consider state and federal regulations, company policies and the employee’s work assignment. The information below is provided for consideration when working fuel cell systems equipped with hydrogen storage tanks and related hydrogen piping systems.
In general, the pressurized gaseous hydrogen system should be defueled and the high-voltage electrical system de-energized1 (in accordance with standard operating procedures) before any maintenance is performed. If this is done, then the PPE will likely need to be similar to repair garage activities for other (non-hydrogen) vehicles, and may include:
- Safety shoes
- Safety glasses
- Hand protection (to avoid cuts and abrasions)
- Long sleeve shirts and full length pants (Note: Personnel should avoid wearing clothing made of nylon or other synthetics, silk, or wool because these materials produce charges of static electricity that can ignite flammable gas mixtures.)
- Hard hats (when necessary)
- Hearing protection (when necessary)
If the portion of the hydrogen system to be repaired can be isolated (preferably by at least two means) and defueled prior to any maintenance activity, then it may be possible to avoid defueling the entire pressurized hydrogen system on the vehicle. In this case, additional PPE may be required depending on the proximity to other systems that are still pressurized:
- Flash fire resistive garments
- Safety glasses with side shields, goggles or face shields
It is important to consider that the above guidance will not cover every activity or circumstance. Specific activities should be evaluated by safety professionals to determine the necessary engineering controls, work practices and PPE.
Note 1: Since all FCVs have high voltage systems additional PPE may be needed if working on an energized high voltage electrical portion of the vehicle.
Safety codes globally have a requirement to provide a positive means to isolate energy sources and hazardous substances prior to performing maintenance. For gaseous hydrogen systems, methods such as a blind flange, a double block valve arrangement or a double block and bleed valve arrangement can provide that positive isolation.
Installing a blind flange requires breaking the supply line and inserting a solid insert that blocks the flow. The disadvantage of this approach is that it is more laborious than the other options and a method of isolation is needed to safely install the blind flange. The components involved are a lower cost than the other options but that cost is offset by the additional labor and system down-time required. For these reasons, they typically are only used for long term isolation.
A double valve arrangement is an effective approach that can be implemented quickly. A disadvantage of the double valve is that hydrogen may leak through the first valve and allow pressure to build between the valves without any indication. Aside from leading to a false sense of security, the pressure may also push its way through the second valve into the downstream plumbing and work area. While it may seem unlikely for two valves to leak, there sometimes is a common mode failure where both valves are damaged at the same time.
A double-block-and-bleed valve arrangement has a third valve to act as a means to vent, or "bleed" pressure between the two block valves. . In this configuration, leak through of the first valve cannot pressurize the second blocking valve, thereby eliminating the leak-through failure mode of the double block valve arrangement. For hydrogen systems, the outlet of the bleed valve should be routed to a safe venting location. A double block and bleed system can also be automated. In that situation the block valves are designed to fail closed and the bleed valve to fail open. Double block and bleed valves can also be used to safety depressurize and vent the downstream section prior to the isolation.
At least three of the ASME B31 piping codes are logical choices:
- ASME B31.1, Power Piping
- ASME B31.3, Process Piping
- ASME B31.12 Hydrogen Piping and Pipelines
Considerations for code selection include:
- Requirements imposed by the authority having jurisdiction, whether by direct reference or by reference from another applicable code or standard.
- Code(s) used for other piping systems at the site. The people who have to operate and maintain the piping will be better served with fewer piping codes. The piping codes are complex and have different requirements. The people who have to operate and maintain the piping will likely be more successful if they have to learn requirements from fewer codes.
In the absence of these factors, ASME B31.12 is probably the most logical choice.
All three codes are suitable for liquid and gaseous hydrogen at pressures 15,000 psi (100 MPa) and higher. For pressures higher than 15,000 psi (100 MPa), the designation of high pressure fluid service in accordance with Chapter IX of ASME B31.3 may be a more economical choice and should be considered.
The requirements of the code used for the original construction apply. The piping may meet the requirements of more than one code. In which case, the code used for changing the rating may be different than the original code of construction. In any case, the re-rated system should meet all of the requirements of the selected code. Note that if the original proof test of the system was not high enough meet the requirement for the new service, the piping will have to be tested at the higher pressure.
The code used for repair and alterations of an existing system depends on the code used for construction as well as on the requirements imposed by the jurisdiction. Note that getting a permit from the jurisdiction may be necessary for an extensive alteration.
|Code of Construction||Generally Accepted Code for Repair and Alterations|
|ASME B31.1||ASME B31.1, Nonmandatory Appendix V – Recommended Practice for Operation, Maintenance, and Modification of Power Piping Systems|
|ASME B31.3||API 570 – Piping Inspection Code: In-Service Inspection, Rating, Repair and Alteration of Piping Systems|
|ASME B31.12||ASME B31.12, Chapter GR-5 – Operation and Maintenance|
|Other Code||Unless otherwise specified, the same as requirements for new construction.|
Note that getting a permit from the jurisdiction may be necessary for an extensive alteration.