The conversion is based on the condition as determined from a variety of non-destructive techniques which are commonly used for pipeline mechanical integrity programs. Existing natural gas pipelines are frequently evaluated for conversion to hydrogen, hydrogen-natural gas blend, and other fluid services.
The conversion can be done safely if handled with the proper expertise and modifications.

Hydrogen has been transported safely through pipelines for over 50 years. There are dozens of pipeline networks in safe operation globally, with several individual networks that approach up to 1000 miles. 

Significant testing and some demonstration projects are underway to ensure safety. Some of the aspects under investigation include compatibility of the pipe and other materials, effect on odorants, and the need to modify existing residential and industrial equipment. Blending initially is at relatively low levels where its impact on the operations of the existing natural gas distribution system is minimized.

Because cast irons are relatively brittle materials, they should generally be avoided in industrial and
transmission pipeline applications. In low pressure applications like residential distribution piping
systems, the use of cast irons is probably acceptable.

Hydrogen affects the mechanical properties of most materials. For example, hydrogen reduces the
fracture toughness and increases the fatigue crack growth rate in steels. There is a significant amount of
research, analytical work, and codes and standards development being undertaken to improve our
understanding of how these materials can be utilized in pipelines. The results of the efforts will be
revealed as requirements in codes like ASME B31.12, Hydrogen Piping and Pipelines, and ASME B31.8,
Gas Transmission and Distribution Piping Systems, and then addressed by national regulations, such as
those provided by the US DOT Pipeline and Hazardous Material Administration (PHMSA).

Regarding the concept of introducing hydrogen gas into natural gas pipelines, this is indeed a hot topic and there are recent quantitative treatments of fatigue crack growth driven by pressure cycling and potentially accelerated by hydrogen.  Some analysis has shown that it can be acceptable to operate natural gas pipelines with a hydrogen blend.  However, this is highly dependent upon the pressure and wall stress.  Whereas low pressure distribution lines with low wall stress are more amenable, the higher pressures and wall stresses of major transportation pipelines may not be.  Deep cyclic stresses that might occur from using pipelines as storage  may also create additional issues since hydrogen can accelerate fatigue crack growth, especially for systems where both hydrogen and deep cycles were not anticipated in the original design.  Individual pipelines likely need to be evaluated based on their design and there is likely to be no single answer for this question of pipeline storage. 

    
For industrial propane tanks, the Panel needs more information about structural materials and their properties (including welds). All propane tanks are not built from the same materials or with the same construction techniques, so these tanks likely need to be evaluated on a case-by-case basis. Propane tanks are often built with techniques that leave potential internal features that are susceptible to the initiation of crack growth. In addition, and depending on the application, it’s likely that the cyclic pressure service will be significantly different for a tank in hydrogen service, particularly one that might be cycled deeply on a daily (or more frequent) basis. From the perspective of the low-pressure storage options, while it is tempting to repurpose old LPG vessels for hydrogen service, the Panel cautions against it due to the potential for hydrogen embrittlement in the steel/weld material. Also, a challenge with newer propane tank designs is that they are moving to thinner walls and higher strength steel, which is notably less resistant to embrittlement than older vessels, so counterintuitively, newer tanks might be less amenable to hydrogen than older tanks.   

Another concern for vessels in hydrogen service is the amount of non-circularity, or “peaking” of the longitudinal welds.  This is not as much of a concern for propane so is likely not to be part of a standard design specification. This manufacturing issue can have a severe effect on cyclic life and is well understood when designing and building high cyclic hydrogen vessels, but not propane. Besides the issue described above for the tank itself, there are similar potential embrittlement issues for PRVs, instrumentation, and other accessories on these tanks. Many incidents have been due to component failures in these accessories. 
In conclusion, propane vessels were not designed for H2 cyclic service. Almost by definition, the reason people want to use them is for storage. and that is likely to be over a wide pressure range. Fracture mechanics should be applied to all cyclic H2 service vessels, especially if they start from another service. Propane tank designs vary and unless a tank is specifically intended for H2, there is no guarantee of what might be used terms of materials and construction techniques.
 

It is possible to store large quantities of gaseous hydrogen above ground, but it will likely require a large footprint due to its relatively low density even at high pressure. Also, if the quantity equals or exceeds 10,000 lb., the facility will need to comply with OSHA 1910.119 process safety management requirements if located in the US. Similar regulations exist in Europe and Asia that increase the regulatory requirements as storage exceeds about 5 tons. Codes such as NFPA 2 aren’t intended to provide full guidance for large facilities and  systematic hazard and risk analysis should be completed to ensure safety. A large storage facility would have to be sited and permitted using methods and risk analysis as typically would be done for a large plant. The technology for gaseous storage is fairly well established and well proven from previous and smaller installations. The same types of tanks, from fully metallic to fully composite construction, would be selected and used based on the operating parameters and economics. Individual tanks may be larger, but large amounts of non-cavern storage are likely to consist of large arrays of multiple vessels due to manufacturing and transportation limitations. The key aspects for large storage systems are as follows: 

1. Materials of construction:  Material science for hydrogen pressure vessels is well established and would mirror smaller storage systems. 
2. Fatigue:  Gaseous storage systems are likely to cycle deeply to increase utilization of the high capital cost of vessels. Deep cycles will typically lead to relatively short intervals between inspections based on fracture mechanics. 
3. A Mechanical Integrity program:  Given the large amount of stored energy, a mechanical integrity program will be very important. The cost of inspections could be high based on quantity and size of vessels. 
4. Minimization of impingement: Piping should be designed such that leaks don’t impinge on a neighboring tank. Impingement fires offer high risk of failure. Fire barriers or intumescent paint could be used at the base for protection. 
5. Relief systems:  Design of relief systems from a fire could be challenging, as well as designing a safe vent system and stack for what are likely to be very large relief devices. 
6. Permitting:  Large quantities of hydrogen need to meet additional regulatory requirements and height of vessels can lead to additional review depending on location. 
7. Foundations:  These will be large and expensive due to weight, height, and seismic considerations. 
8. Pressure: Higher pressure means lower volume required, smaller and/or fewer vessels, and smaller plot space. Lower pressure requires less compression equipment and less compression energy requires less compression.
9. Installation: One overlooked aspect of storage is the cost to transport and then install the vessels including freight, concrete, rigging, and piping. These costs must be added to the capital cost of the vessels. Storage vessels are expensive to ship   due to size and weight.