Underground storage tanks can be either installed in a vault or directly buried. Both offer additional
protection from external impact and fire, but each has unique challenges. Vaults must be properly
ventilated and designed to not create an explosion or asphyxiation risk. Direct burial vessels should not
have any underground leak points and must be protected from corrosion. Both types of installation must
have hydrogen vents routed to a safe location above grade.

The Panel considers two approaches to be acceptable. 

1. Stainless steel (corrosion resistant) lines embedded in concrete. This keeps the piping out of direct contact with the earth and provides a degree of physical protection from activities such as digging. 
2. Lines enclosed within a PVC sleeve. The PVC sleeve would usually be directly buried but could also be embedded in concrete for additional protection.
   Cathodic protection is also an available and acceptable means of corrosion protection for underground lines, particularly for carbon steel. Each installation should be evaluated for local hazards and soil conditions to develop a thorough corrosion protection system. 
    

NFPA 2 has provisions for the use of LH2, and there are many existing fueling stations that store and use liquid hydrogen. One challenge for the use of LH2 in stations with small footprints is the code required separation distances from exposures. NPFA 2 has updated the separation distances with a risk informed approach in the 2023 edition.

Another possibility for public fuel stations would be to install the LH2 tank underground. Underground LH2 storage is permitted within NFPA 2 and requirements can be found in Section 8.3.2.3.1.7. It is certainly feasible to design and build a fuel station with an underground LH2 tank. 

It’s important to note that the NFPA definition of storage systems and resulting separation distance requirements are based on more than just the tank. The definition in NFPA 2 is an assembly of equipment that consists of, but is not limited to, storage containers, pressure regulators, pressure relief devices, vaporizers, liquid pumps, compressors, manifolds, and piping and that terminates at the source valve. The Hydrogen Tools portal (http://h2tools.org) has a variety of information, best practices, and lessons learned.