D1A.2 Quantitative risk assessment, the effect of ventilation in small leaks and recommended risk-mitigating measures for hydrogen in the built environment in the Netherlands

To estimate the risks associated with the use of hydrogen in the distribution network and the built environment compared to the use of natural gas, it is important to know the differences in chances and consequence. The chance relates in particular to the probability of a dangerous situation occurring; the consequences can be expressed in damage caused by that hazardous situation (typically fire or explosion).  Mitigating measures are aimed at reducing the chance of a dangerous situation arising or lowering its consequences.

To this end, the Hydelta programme defines the 'Hydrogen and Safety' work package in which the main objective is formulated as:

Identifying the risks of hydrogen leaks in homes and in the distribution network and defining mitigating measures on the basis of the risks.

To answer this main question, this report relies heavily on previously published work from the United Kingdom (UK) where similar research has been conducted. In the first phase of the research, an analysis was made of the work from the UK and a translation was made to the Dutch situation. To give insight in the risks of hydrogen in the English distribution network, a quantitative risk analysis was made (QRA). Supporting experimental research was carried out to improve this QRA model. Based on the results of the model, recommendations have been made for mitigating measures for the UK, that ensure that the risk of the hydrogen infrastructure does not exceed the risks of the current natural gas infrastructure.

In the Hydelta work package, the same approach was followed to make an initial assessment of the risks in the Dutch situation. In the first deliverable of the work package, [1] the studies in the UK were analysed and differences with the Dutch situation were identified. Based on this analysis and existing questions about the safety of hydrogen in the built environment from the Dutch grid operators, a start was made in the second phase of the research in the work package with a QRA model for the Dutch (hydrogen) distribution network. In addition, an experimental programme has been set up to provide insight into the effect of ventilation on the accumulation of dangerous concentrations in the event of small leaks. It should be noted that this is an initial exploration: given the time available within the Hydelta programme, further research in next phases of Hydelta will be needed to further validate and fine-tune the models with additional research.

The report is divided into three coherent parts. Part I describes the setup of the QRA model. The effect of ventilation on small leaks is described in Part II, based on a set of experiments that have been carried out. Finally, Part III provides an overview of mitigating measures that can be considered in the pilot projects.

Part I 'Quantitative risk analysis'

The model that has been developed considers the gas distribution system for pressures up to and including 8 bar, as operated by the distribution grid operators. More specifically, in the model we focus on the underground mains in the distribution network and the service pipes between the mains and the meter setup in the houses. The model is based on the composition of the Dutch distribution network, with different pressure regimes, material types, diameters and lengths, as well as on the failure data of recent years for the corresponding natural gas network. This report describes the model. The assumptions used for the Dutch situation and the associated results of the quantitative risk analysis will be further described in the next phase of HyDelta.

Part II "Effect of mitigating measures on hydrogen accumulation"

It is known from the studies from the UK that the greatest risk for the use of hydrogen in the built environment is caused by an explosion due to accumulation of hydrogen at concentrations above 10 vol%. To prevent this build-up of the concentration, ventilation is an important parameter. The workshops held with the Dutch grid operators showed that the dispersion of hydrogen in the event of leaks inside homes and the associated influence of ventilation require additional recommendations for the implementation of pilots in the short term. To provide more insight into this issue, a test set-up has been built in a container that measures the effect of (low) ventilation rates on the accumulation in different rooms in the event of small leaks. In the experiments, both the outflow of hydrogen (up to 20 dm³/hour) and methane (up to 15 dm³/hour ) were investigated. The current limit for permitted leaks for natural gas is 5 dm³/hour. The results lead to the following conclusions:

1. Build-up of concentrations remains well below the LEL in the largest tested room (36m³).  The LEL for hydrogen is 4 vol% gas in air. In the experiments in the entire space of the container (36 m³), maximum concentrations of 6% LEL (= 0.24 vol% H2 in air) are measured. For both hydrogen and methane, the concentration at the top of the container is the highest. In none of the experiments a dangerous concentration was measured (near or above LEL).
2. Opening a door or ventilation opening is an efficient way to reduce concentration.  We tried to make the container as gas-tight as possible in order to simulate a very poorly ventilated room. When the ventilation rate is greater than 5 times per hour, it may be considered "Good" (NPR7910-1). In homes the ventilation rate often does not exceed 2 times per hour, where it should be considered moderate (NPR7910-1). In cases where the ventilation rate is less than 1 time per hour it is referred to as "no ventilation" (NPR7910-1). By measuring how fast the concentration decays after an outflow test, the ventilation rate of the container was determined. For hydrogen, it was 0.2 /hour. Opening a vent on the side of the container results in a ventilation rate of 1 /hour, while opening the door of the container leads to a ventilation rate of 15-20 / hour. Opening the door halves the concentration in about 1-2 minutes. Ventilation by opening a vent, it takes about half an hour, while with no ventilation the concentration halves in a few hours after stopping the supply.
3. Leakage in a meter cupboard also results in concentrations lower than the LEL. In the second phase of the experiments, the container was divided into 2 compartments (10 and 26 m³) with a meter cupboard with door with ventilation grills in the smallest compartment. The 10 m³ thus represents a typical hall. The outflow of the gas is always at the bottom of the meter cupboard. It is observed that the largest increase in concentration is in the upper part of the meter cupboard, but that it levels during the experiments. The concentration in the adjacent room then increases by the gas dispersing through the opening of the meter cupboard door to this room. The highest hydrogen concentration in the meter cupboard is achieved when both the ventilation grills in the meter cupboard door and the ventilation opening in the container are closed, with the concentration levelling off to a value of approximately 45 %LEL in the hydrogen tests. Opening the ventilation in the meter cupboard door leads to a significant decrease in the average hydrogen concentration in the meter cupboard (43% to 20% of the LEL). A decrease in the outflow rate from 20 dm³/hour to 15 dm³/hour also results in a decrease in the average hydrogen concentration in the meter cupboard. Only if the ventilation grills in the door of the meter cupboard are closed and the maximum gas supply is used, the safety value of 50% LEL set for the experiments is reached and the experiment is aborted. In the experiments, this only occurred with methane with an outflow of 15 dm³/hour. The maximum concentration at the top of the meter cupboard is generally lower for hydrogen compared to methane.
4. Closing the gas supply leads to a rapid decay, followed by normal ventilation. In both the experiments with hydrogen and methane, it is observed that as soon as the gas supply in the meter cupboard is closed, the concentration in the meter cupboard drops rapidly. The gas disperses quickly, even with closed ventilation openings in the door, to the adjacent room. This seems to happen faster with hydrogen than with methane. As soon as the concentrations in the meter cupboard and adjacent room are equal, the ventilation of the adjacent small space is leading in a further reduction of the concentration.
5. The measured concentrations could be detected by H₂ sensors or odorization.  The concentrations of hydrogen measured in the experiments are generally very low, far below the LEL. In the analysis it appears that with the small leaks up to 20 dm³/hour no dangerous situations arise. However, to be sure that such small leaks are noticed and do not extend over much longer periods of time than the hours used in these experiments, it is important to know whether detection mechanisms are effective. Based on previous research, an estimate has been made of the limits at which CO sensors, which can be used as hydrogen sensors, will alarm, as well as when the gas can be smelled. The figure below shows the average concentration in the meter cupboard (left) and in the larger room (right). It also shows the concentration bands in which the H₂ sensors are expected to react and the limits for smellability of the gas.

It is likely that gas air will already be detectable at concentrations below 20 %LEL. It could be recommended to place sensors at the top of the room, where the higher concentrations prevail. With this, a small leak will be noticed in time and the risk can be further reduced.

In case of leakage, ventilation by means of a grid or opening a door is an effective way to reduce concentration.

Part III "Safety recommendations for hydrogen pilot projects"

During the Hydelta programme, several pilot projects will be set up in the Netherlands to gain experience with hydrogen in the built environment. It is of great importance to execute these pilots and demonstration projects in a safe manner. Based on the results of this work and the knowledge already gained in various pilot projects, an overview has been made of the mitigating measures that can be considered for future pilots. We focus on measures for an infrastructure with a maximum working pressure of 8 bar in transition from natural gas to a hydrogen, for pilot projects and for permanent installations. Initially, the focus is on the pilot projects and these measures are conservatively inclined. Basis for this set of recommendations is that the majority of the methods that are used for the design/construction and operation of the natural gas infrastructures are also suitable for hydrogen. Extra attention is required to limiting "large leaks". The recommendations are grouped around the different phases of the project: preparation, design, implementation and operation. Both measures for the distribution network and 'behind the meter' in the homes are mentioned.  Based on the experiences from the pilots and future insights, we need to re-assess at a later stage to what extent these measures should also be applied in a future regular hydrogen distribution network. 

In this report dm³/hr refers to normal dm3/hr. In the experiments, a mass flow controller is used that has a slightly higher maximum flow rate for hydrogen than for methane.

LEL refers to the lower explosion limit of gas in air and can also be translated LFL (lower flammability limit). For hydrogen the LEL is 4vol%, for methane 5% vol. is used.