D6A.1 Outflow experiment results: concentration build-up at leakages between 50 - 1000 dm3/h

Hydrogen has different properties from natural gas. Its physical and chemical characteristics are known in detail, but there is still insufficient knowledge about the safety risk when applied in gas pipelines in the built environment.  However, this knowledge is needed to know whether different safety measures need to be taken than for natural gas. This required knowledge mainly relates to probabilities and consequences of the unwanted release of hydrogen. Mitigating measures are then aimed at reducing the probability of a hazardous situation arising and/or reducing its consequences.

In HyDelta 1.0 (the predecessor of this project), concentration measurements were therefore made to gain insight on the diffusion of hydrogen versus methane at small leak sizes of hydrogen and methane in the gas meter cabinet (up to 20 and up to 15 dm³/h, respectively). In the present study, which falls under HyDelta 2.0, the measurements were scaled up to higher gas outflows, namely 50, 100, 300 and 1000 dm³/h. The measurements were carried out in a container composed of a room (26 m³) and a hall (10 m³) containing a gas meter cabinet. The distribution of gases was measured with gas sensors attached according to a matrix with three height positions, three width positions and six length positions.

Experiments were conducted with the following variables:

The gas outlet was stopped as soon as one of the sensors registered more than 50% LEL. This safety measure was taken to prevent unwanted ignition of the gas. Extrapolation was used to estimate whether 100% would also be achieved if the gas outflow had not been stopped.

On this basis, the results of the experiments were classified according to final concentration into one of three categories: less than 50% LEL; between 50% and 100% LEL and greater than 100% LEL. As long as the concentration throughout the container is below 100% LEL, the situation is safe in any case because the gas-air mixture cannot ignite.

The measurement results for both hydrogen and natural gas lead to the following findings a. and b.:

a. Inside the gas meter cabinet

At a gas outlet in the gas meter cabinet of:

• 50 dm³/h or more, the concentration rises to more than 100% LEL if no ventilation grilles are fitted, but to less than 100% LEL if grilles are fitted in accordance with the standard.
• 100 dm³/h or more, the final concentration does exceed 100% LEL.

b. Outside the gas meter cabinet

At a gas outlet in the gas meter cabinet of:

• 50 or 100 dm³/h, the concentration outside the gas meter cabinet remains below 100% LEL;
• 300 dm³/h, the concentration rises above 100% LEL in part of the experiments (hydrogen: 3 out of 8; natural gas in 1 out of 8); and
• 1000 dm³/h, the final concentration exceeds 100% LEL in all cases.

The results lead to the following conclusions:

1.  The build-up of concentrations (hydrogen and natural gas) remains below 100% LEL in the gas meter cabinet and other rooms when the gas meter cabinet is fitted with the vents prescribed for natural gas, at a gas outlet of up to 50 dm³/h.

This means that a leak size in the meter cabinet of up to 50 dm³/h can be technically accepted. The ventilation grilles prescribed for natural gas are therefore more than sufficient for hydrogen.

It is recommended to install ventilation grilles in meter cabinets for hydrogen gas meter installations in accordance with the current standard NEN 2768 + A1 for natural gas meter cabinets.

2. Hydrogen has stronger stratification than natural gas.

Especially with low ventilation, this can lead to areas of high gas concentration ("dead spots") under the ceiling. Mechanical ventilation can reduce the likelihood of this. Both extraction and air supply can reduce the likelihood of such zones: 

• With extraction high in the room, the highest concentration is discharged.
• When air is blown in, the gas is more mixed with air so dead ends are avoided.

3. The usefulness of a gas stopper cannot be demonstrated with this study

Based on this study, no recommendation can be made on whether or not to apply a gas stopper, as the gas stopper only intervenes well above the gas outflow used in the project.

4. Mechanical ventilation is highly effective

The ventilation flow rates determined afterwards, calculated on the basis of a change in concentration, are often higher than the set ventilation flow rates. This means that mechanical ventilation often appears to be even more effective than expected on the basis of the set air supply or air exhaust. Some possible causes were found for this, however, an unambiguous explanation cannot be given based on the results.

The results lead to the following possibilities for follow-up research: 

• Experiments with different set-up locations for gas meters than examined here
• Experiments with small spaces other than a gas meter cabinet (e.g., a crawl space, kitchen sink or riser cupboard).

Experiments or CFD calculations to understand the causes of higher than expected ventilation (see above under 4).