D1B.3A Ventilation in gas stations

As part of the national research program HyDelta, specific research has been performed to assess the role of ventilation in gas cabinets which are placed over a gas pressure reducing and metering station. This research as described in this report is part of the work package 1B “Gasstations” and involves research question 212/ D1B.3a which is described as; is the safety level for a gas cabinet (involving ventilation) the same for natural gas in comparison with hydrogen and to what extend are modifications required?

The goal of this research is to examine whether standardized gas cabinets overhauling gas pressure reducing metering stations and which are currently applied in the urban environment, are also suitable when these would be applied for hydrogen. This research question has been examined by measuring what gas concentration will exist when a predefined leakage is introduced in two different gas cabinets. In order to do so, two different gas cabinet configurations (1/2m³ and 4m³ gas cabinets) have been made available by Enexis (on behalf of the DSO’s). In both configurations, the gas  pressure reducing and metering station was positioned to mimic the situation in the field as realistically as possible. Via a separate inlet, the defined gas leak (of either natural gas or hydrogen) was introduced in the gas cabinet. During the tests, the dilution of the natural gas (or hydrogen) mixture and the transportation of gas was studied in relation to the ventilation. During all the tests, the gas concentrations have been measured inside and outside the gas cabinet.

Determining a representative gas leakage (for natural gas) has been a challenging exercise. A representative leakage is dependent on different factors including the configuration of the gasstation, the age of the installation and the maintenance scheme which it has had during its lifespan. By odorization, natural gas is smellable when the gas concentration is exceeding 10.000 ppm (1 vol%). Eventually, a leak opening of 1mm² was chosen (mentioned in an annex of the NEN 1059; 2019). Since a gasstation can operate at different pressures, five leak rates have been defined as reference points for this research bearing in mind to cover a clear, wide ranged comparison between the behavior of natural gas and hydrogen. For natural gas, the leak rates varied between 0,1 m³_n/h and 7,5 m³_n/h whereas for hydrogen, these were varied between 0,2 m³_n/h and 22,5 m³_n/h. It shall be noted that the volumetric late rate for hydrogen at a predefined leak opening is 1,2 to 3 times larger than natural gas, dependent on the pressure of the upstream system.

When the measurements for the gas cabinets are studied in more detail, the following conclusions can be drawn:

• When all the measurements for the 1/2m³ and the 4m³ gas cabinet are studies more closely, it can be concluded that hydrogen will more often lead to a flammable mixture at the ventilation openings in comparison with natural gas. This is more specifically applicable for the larger leak rates (≥ 6 m³_n/h). In itself, the logical explanation here is the combination of the larger volumetric leak rates as well as the wider flammability limits of hydrogen compared to natural gas. The question which arises from the above is also to what extend the leak opening and the selected leak rates are deemed realistic to represent a gas leakage. To facilitate this important dialog, this report will provide some recommendation about this matter later on.
• The influence of wind speed can be clearly seen by the bandwidth of the gas concentrations at the ventilation exits (ventilatieopeningen) of the gas cabinets.
• The behavior of a natural gas of hydrogen leakage is different for the two gas cabinets which were tested. It is remarkable to conclude that there is a different relation (linear versus logarithmic) when the gas concentration in the gas cabinets is compared to the gas concentration at the ventilation exits. The gas concentration in the gas cabinets is on average higher than the gas concentration at the ventilation exits for the 1/2m³ gas cabinet. For the 4m³ gas cabinet, the phenomena is exactly the other way around. The behavior involving ventilation is different when the two geometries are compared.
• One of the conclusions from this research program is that the selected leak opening will lead to quite significant volumetric leak rates. The 1 mm² considered in this, shall be indicated as an incident rather than a regular leakage/ malfunction. In close collaboration with the “stuurgroep”, an attempt has been undertaken to backtrack the originating source of the 1 mm² leak opening which has not resulted in identifying that specific source so far.
• In view of the ATEX analysis, the experimental results for the 1/2m³ and 4m³ gas cabinets have been reviewed with an expert from GasUnie. The advice on the approach/ outcome is to re-assess the volumetric leak rates as well as the leak opening according to the NEN-EN-60079-10-1 (Explosive atmospheres) where a leak opening (dependent on the boundary conditions) between 0,025 and 0,25 mm² shall be used (Table B.1 – “Suggested hole cross sections for secondary grade of releases”), which leads to substantially smaller volumetric leak rates at 8 bar pressure (according to calculations).

Recommendations:

• Additional research will be necessary to study the effects of smaller volumetric leak rates (more in line with practice). The results will allow the industry partners to distinguish an incident from a regular leakage/ malfunction and give a better view on the natural gas/ hydrogen concentrations that can be expected.
• Since the DSO’s apply a range of different gas cabinets in the urban environment, it would be beneficial to understand which specific gas cabinets are used most frequently before hydrogen is introduced in the gas infrastructure. During this step, all cabinets can be compared with a functional set of specifications (including the ventilation rate, the mechanical design, the expected gas leak rate) to allow a comparison with the gas cabinets tested in HyDelta.
• Based on the outcome of a research program in execution, focusing on (leakages) in 1000 gasstations on natural gas, it would recommendable to compare these results with this research “HyDelta Gasstations – Ventilation”. The outcome of this report will potentially need a re-assessment when it turns out that the selected volumetric leak rates in this research are substantially larger than a regular leakage in the field.