D1C.2 Tightness of distribution pipes

As part of the national research programme HyDelta, an investigative study was carried out into tightness criteria for connecting pipelines if they are operated with hydrogen.

The study as described in this report is part of work package 1C “Pipes and Indoor Installations” and concerns research question 124. This is as follows :

• Do the leak tightness requirements for a natural gas network also apply to a network operated with hydrogen?
• If this is not the case, what should the criteria be?

The focus in this study is on service lines. The small leaks that need to be simulated (5 l/h on the basis of air) in main pipelines are so small (the route of the main pipeline is more extensive compared to a connecting pipe) that they will not lead to issues when hydrogen is used.  Such minor leaks will not enter buildings, but will evaporate into the open air.

The following activities were carried out with the aim of answering the above question:

• The leakage flow rates were measured at pressures of 30, 100 and 200 mbar with the media nitrogen, natural gas and hydrogen in four leaks that were created on purpose in connecting pipes with a nitrogen content of approximately 1 l/h. The ratios of the leakage flow rates were determined from these results.
• A theoretical consideration has been drawn up for assessing the risks of small hydrogen leaks using the maximum permissible leaks in NEN 7244-7 and the findings from the DNV GL report on the behaviour of hydrogen in leaks in the gas distribution network.
• The measurement results were reviewed in further detail and analysed, and subsequently combined with the theoretical risk assessment.

The conclusions from this study are as follows:

For new service lines, the same tightness requirements can be used for hydrogen as for natural gas. There is no reason to make the criteria more stringent.

For existing connecting pipes, the tightness requirements must be made more stringent. The maximum permissible leakage rate for existing hydrogen service pipelines is 74% of that for natural gas.  This has been determined on the basis of the following:

• The measured values give a factor of 1.83 higher leakage for hydrogen
• The assumption that the risks of ignition of a gas-air concentration < 8 vol% for hydrogen is lower than for natural gas with a concentration of 5.9 vol%

These tightness requirements can be incorporated by extending NEN 7244-7, table 4 for hydrogen, which would then read as follows

The leak tightness tests for a hydrogen network can be carried out by extending the following sentence (in italics) in VWI G-12:

An existing connecting pipe is considered leak tight if the test pressure drops ≤ 5 mbar during an overpressure measurement (at a constant temperature) (for hydrogen pipes: ≤ 4 mbar)