# Inspection of Coated Hydrogen Transportation Pipelines
## 2 Background
### Brief Description of a Hydrogen Pipeline
#### 2.1.3 The Question of Blended Hydrogen Pipelines
Conversely, Hydrogen ageing was not a prime concern for PE (polyethylene) and PVC (polyvinyl chloride) pipelines, with most elastomeric materials used in the pipelines being suitable for use with hydrogen [18]. However, compared to methane (a natural gas), hydrogen mobility was higher in polymer materials [18]. Consequently, hydrogen escaping through the walls of plastic distribution pipes was responsible for major loss of gas from the system [18]. [PERSON] et al. [18] also discussed the consequences for end-use systems, safety, the durability of pipeline materials, the risk of leakage, and the downstream extraction of hydrogen from the blended pipeline--a key consideration for applications requiring pure hydrogen. While there have been discussions about potentially re-purposing natural gas pipelines for hydrogen transport, the economic and engineering feasibility of this has not been considered. Therefore, a summary of the challenges and opportunities of blended hydrogen pipelines is outlined in Table 3. \begin{table}
\begin{tabular}{c c} \hline \hline
**Hydrogen Level in Pipeline** & **Overall Risk Level** \\ \hline Less than 20\% & “Not significant” \\
20\% to 50\% & “Significant increase” in overall risk for service lines but the overall increase in risk in distribution mains is “moderate” \\ Greater than 50\% & Impermissible level of overall risk \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Hydrogen levels and their associated risks in blended pipelines [18]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Opportunities** & **Challenges** \\ \hline GHG (greenhouse gas) emissions are lowered if hydrogen is produced from sustainable energy sources [18] & Blending hydrogen into a methane pipeline could lead to a higher occurrence of “over-pressure, explosions, leakage and cracking [17]” \\ \hline Replacing conventional diesel fuel with hydrogen (in fuel cells) for transportation could reduce emissions of sulfur dioxides and nitrogen oxides and improve air quality [18] & The presence of hydrogen means that it can interact with pipeline steels and cause HE [17] \\ \hline The hydrogen-natural gas mixture, used as is for heat and electricity generation, is a cleaner fuel than pure natural gas [18] & Existing pipelines need to be modified by removing “undesirable parts,” replacing valve fittings and ideally coating the pipeline internally to allow hydrogen to be transported at high pressures [5] \\ \hline The cost of re-purposing NG lines for hydrogen transportation is significantly lower than building a new hydrogen line [5] & Operation of hydrogen lines requires thrice the compression power of NG lines and is more expensive [5] \\ \hline Obtaining relevant approvals and ensuring compliance with procedures would mean that a new hydrogen pipeline would take 5–7 years to complete from initial planning to commission [19] & Retrofitting “turbo-compressors” to handle gas with a volumetric hydrogen content of more than 40\% is not possible as yet [5] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Opportunities and Challenges of blended hydrogen pipelines.