As part of the national research programme HyDelta, a study was carried out into the effects of air entering a hydrogen-filled distribution pipeline in the event of a pipe breakage. The research described in this report is part of the Work Package 1C "Pipes and indoor installations". As requested by the OGH2 (OnderzoeksGroep or Research Group H2), research question 187 “How can an existing gas distribution pipeline be safely decommissioned as a natural gas pipeline and (simultaneously) commissioned as a hydrogen pipeline during the conversion to a hydrogen network and what are the associated costs?” has been supplemented with an additional research question. During that research study, distribution pipes DN 100 and DN 200 were filled with hydrogen. The results in the context of research question 187 are described in a separate report. The research as described in this report is complementary to the research questions posed in the context of HyDelta WP1C. Aim and testing approach The aim of this research is to determine to what extent air enters a hydrogen distribution pipeline in the event of a pipe rupture. This was done by filling distribution pipes with a diameter of DN 100 and DN 200 with hydrogen. The concentrations (gas/air ratio) were measured in the pipes both with and without shielding of the entry opening. The pipes were filled with hydrogen and then opened on one end. Results The test programmes showed that after creating the leakage (opening a valve), hydrogen flows out of the pipe and air enters immediately. Furthermore, an explosive mixture was found almost immediately after the entry of air in both the DN 100 and the DN 200 pipe. This explosive mixture remained during the full measurement time of 90 minutes. The outflow of hydrogen and the entry of air is caused by the difference in density between hydrogen and air. The inflow of air into the DN 200 pipe was greater than the air inflow into the DN 100 pipe. Measuring at a distance of 25 metres from the opening: the explosive mixture formed more rapidly when testing the DN 200 pipe as compared to the smaller DN 100 pipe. The resistance of the hydrogen outflow and the resistance of the air inflow are smaller in a larger diameter compared to a smaller diameter. Like during the measurements without shielding there was hardly any wind and the effect of wind was therefore not measurable. The measurements show that an explosive mixture is formed almost immediately in an open depressurised pipe filled with 100% hydrogen. The release of hydrogen and the entry of air is caused by the difference in density between hydrogen and air. Comparing this to natural gas: natural gas is 9 times more dense than hydrogen. It is expected that, when using hydrogen, an explosive mixture will form in a depressurised pipe faster and over a greater length than when natural gas is in the pipe. Recommendations It is advisable to also investigate the effect of air entering a pipe when repairing a rupture in a natural gas-filled pipe in order to better understand the risks with hydrogen. In that additional study, it is also advisable to consider the influence of the opening’s size and the length between the opening and the inflatable stopper. Further research is needed to determine the consequences of an explosive hydrogen mixture igniting in a blocked section of the gas network. It can be determined whether an inflatable stopper offers sufficient stability to permanently block the gas flow in the event of an explosion. It is recommended to carry out these ignition tests with hydrogen and with natural gas.

Dit project is medegefinancierd door TKI Nieuw Gas | Topsector Energie uit de PPS-toeslag onder referentienummer TKI2020-HyDelta.