TIA 1783 points out a valid concern about how to address the electrical classification zone around a liquid hydrogen system. The existing requirements specify 3' around the outlet of the stack for Division 1 and 25' around the outlet of the stack for Division 2 area. These distances are historical and date back to the 1960's. They are a "one size fits all" simple approach that is easy to implement. Most existing systems use these distances for hazardous area plot plans and equipment selection. However, the distances are conservative for some systems, but also may not be sufficient for larger systems with higher flowrates.

These classified areas apply both to the system itself (for leaks) as well as the vent system outlets. This becomes more complicated since leaks are not expected, but may occur. In contrast, vent stacks are frequently used and hydrogen is expected to be vented since that is the stack’s function. Therefore, vent systems should be designed to properly vent the hydrogen to minimize impacts to personnel, facilities, and the environment.

The currently adopted editions of NFPA 2 are minimum requirements, but best practice for vent stacks would be to follow principles in NFPA 497 to ensure that the specific system and stack classified areas are developed with the actual parameters based on flow, direction, height, physical design, etc. The expected vent flows should be modelled and the classified areas developed accordingly, while using current NFPA requirements as a minimum for both the system and vents. NFPA 2 will eventually be updated with a new table in the future that is similar to, but not the same as, the proposal in TIA 1783.
 

BACKGROUND:
A Division 1 classified area expects hydrogen to be present for some portion of the time during normal operation. A Division 2 area only expects hydrogen to be present under upset conditions. Since systems are designed not to leak, and since leaks are not normal, the area around piping/tanks/etc. is normally considered Division 2. However, in certain areas such as fill connections and vent systems, hydrogen gas releases are expected. Hence, these areas are considered to be Division 1. The 3' extent of Division 1 area is somewhat arbitrary. A fill connection might have a small release of hydrogen during disconnection. Alternately, a vent stack could release much more substantial volume of hydrogen: a relatively small volume from normal operations to a very large release from a pressure relief device. TIA 1783 was correctly noting that since relief devices are of known sizes, then the classified areas should be based on the actual modelling of the relief rates and not just depend upon the traditional 3' or 25' distances.

For example, if it is known that a production system will vent a quantity that will result in a cloud that is 50' radius every time that it shuts down, then the classified area(s) should be much larger than the prescribed area. Similarly, if there is a very large relief device that isn't expected to operate, but might reasonably operate during the life of the system, then a similar analysis should be done.

TIA 1783 tried to express this in a new table but requires additional dialogue and analysis. Documents such as NFPA 497 (as referenced in the TIA), and API Standard 520 can be used as a best practice to develop the appropriate classified areas based on release rate models.

Classified areas are often shown in 2-D on drawings, but they are more accurately portrayed in 3-D (e.g. a "sphere"). In these situations, the height of a vent stack is a key dimension to ensure that the hydrogen cloud, radiation, and overpressure don't significantly create harm, especially considering hydrogen’s buoyancy. A properly designed vent stack should ensure that the momentum from the release further facilitates the upward direction from buoyancy, thereby reducing the extent of the classified area in the downward direction.

Systems should be sited in accordance with national and local standards such as NFPA 2, Hydrogen
Technologies Code. The nature of a trailer filling operation is not much different than a vehicle fueling
station, so the hazards are comparable and similar safeguards such as walls and sensors will apply. These
facilities might be larger and industrial standards and regulations will also apply based on quantity stored
at the location.

Each installation should be evaluated based on the results of a hazard analysis considering both of these
scenarios. Separation distances as listed in documents such as NFPA 2, Hydrogen Technologies Code, are
a minimum starting point but may need to be adjusted based on analysis. Recent work by NFPA 2 has
also included overpressure criteria, but the consequences can vary depending on system design and
surroundings.

Cylinders used within a laboratory can be used safely by meeting the requirements prescribed in NFPA 2,
Hydrogen Technologies Code, and NFPA 45, Standard on Fire Protection for Laboratories Using Chemicals.
Special consideration should be given to both safe handling and storage of cylinders. Regarding lecture
size cylinders, their small size can make them susceptible to damage and mishandling. A release from an
open valve or relief device can be hazardous, particularly if the potential hazard is underestimated.

Documents such as NFPA 2, Hydrogen Technologies Code, and the International Fire Code have quantity thresholds that differentiate requirements for the design of systems and enclosures. However, even the smaller quantities present a hazard under specific conditions, especially for systems that have the potential to release hydrogen into a confined or unvented space. Good engineering judgement must be used regardless of the size of system. Also, the quantity of hydrogen connected to the system from outdoor storage must be considered in the overall design.

Guidance for location of vent stacks is provided by NFPA 2, Hydrogen Technologies Code, which also references CGA G5.5, Hydrogen Vent Systems, for additional guidance. Minimum distances to vent stack outlets should be determined from dispersion and radiation analyses. The height of the vent stack and orientation of the release will affect the minimum separation distance.

Store flammable gas cylinders such as hydrogen, separated from oxidizing (e.g. oxygen), toxic, pyrophoric, corrosive, and reactive Class 2, 3, or 4 gases. Non-reactive gases, such as helium, may be co-located. See codes and standards such as NFPA 2 [7.2.1.1 Incompatible Materials] for further guidance.

The key concern with any hydrogen release is the risk of creating a flammable mixture. There should be no environmental issues if you properly vent hydrogen to a safe area where it is diluted in air below the flammability limit before contacting an ignition source. Very small quantities of hydrogen are frequently releasing into a fume hood. Releases have to be small enough so that the vent air is sufficient to dilute to below the lower flammability limit. The fume hood face velocity should be in excess of 100 ft/min (30 m/min). 

Larger quantities of hydrogen should be released through a properly designed and constructed hydrogen vent stack. Standards and codes such as CGA G 5.5 and NFPA 2 provide guidance for vent stack design and installation.

Outside storage is generally considered safer and is required for large amounts of gas. Stationary storage should be located outside at a safe distance from structures and ventilation intakes, and protected from vehicle impact. 

Hydrogen storage separation distance requirements are typically based on the quantity and pressure of the hydrogen or the piping diameter, depending on the type of storage. Consideration should be given to distances between multiple containers to prevent interaction during an unintended hydrogen release. More detailed guidance can be found in the applicable codes and standards such as NFPA 2, Hydrogen Technologies.

NFPA 2 provides Tables in Chapters 7 and 8 that specify the hazardous area classifications surrounding vent stack outlets. These are based on typical vent systems and flows, but are only applicable for smaller systems. The designer of a vent system should apply the principles of documents such as IEC 60079-10-1 (also required by NFPA 2) or NFPA 497 to evaluate larger vent releases where the hydrogen cloud will extend beyond the prescriptive distances.