D2b.1 Purging hydrogen distribution pipelines with large diameters

As part of the national HyDelta research programme, a study was conducted into the purging of distribution pipelines with diameters larger than DN200.  

The research described in this report forms part of HyDelta 4.0, work package 2 “Optimal grid concept at 16 bar”, and concerns substudy WP2B: Purging of large diameters for industrial distribution gas grids. 

Purging activities are applied to prevent the formation of flammable gas mixtures in pipelines during the introduction of hydrogen. At present, such purging activities mainly take place within pilot projects in the built environment and typically involve pipelines with diameters up to DN200. 

When a pipeline is filled with air, it is common practice to first completely purge the pipeline with nitrogen in order to displace oxygen. Hydrogen is introduced only after sufficient inerting has been achieved. For this procedure, a recommended hydrogen purging speed of 1.0 m/s applies [1]. 
If a pipeline is already filled with natural gas, it can be purged directly with hydrogen, as this does not lead to the formation of flammable mixtures [2]. 

In the construction of industrial hydrogen distribution gas grids, pipeline diameters larger than DN200 may be applied. In such cases, the amount of nitrogen required for full inerting can become substantial. For this reason, this study investigated whether alternative purging techniques are available. In addition, field tests were carried out to determine whether purging with a nitrogen buffer (also referred to as pulse purging) can be used as an effective method to keep air and hydrogen separated during the purging process. 

Based on the conducted research, the following conclusions are drawn:  

Theoretical research 

Several techniques can be considered to make a distribution pipeline hydrogencarrying without the formation of flammable gas mixtures. From a practical perspective, the following techniques are considered the most applicable: 

Depending on the grid configuration, one of these techniques may be more suitable than the other. Full purging is a technique with which grid operators are familiar. 

Directly purging an air-filled pipe with hydrogen is a possibility that does not require nitrogen. The disadvantage of this is that flammable mixtures are formed. However, the question is to what extent it is realistic for these mixtures to ignite and what the consequences would be. It is advisable to conduct additional research into the possibilities of ignition and the effects of an unexpected ignition. Variables to be carried out in such an investigation include the hydrogen/air ratios in the gas, pipe length, diameter, and ignition sources. 

For a complete overview of the various techniques, including techniques considered less suitable, see Chapter 3 and Attachment I. 

Practical research 

A DN400 pipe filled with nitrogen can be commissioned to carry hydrogen without stratification occurring. This has been demonstrated at flow velocities of 0.5 m/s and above.  

Pulse purging proved to be an effective method for making a distribution grid hydrogen‑carrying. Both the length of the nitrogen buffer and the purging speed are determining factors in preventing the formation of hydrogen–air mixtures. The length of the pulse (buffer) must be at least 100 metres, while the hydrogen start‑up purging speed should be at least 1.0 m/s. At lower start‑up purging speeds and/or shorter buffer lengths, hydrogen–air mixtures may form. This is caused by the lower density of hydrogen compared to nitrogen, which results in hydrogen tending to flow over the nitrogen buffer. Interruptions during the preparation of the nitrogen buffer have a detrimental effect on its effectiveness. In such situations, a buffer length of 100 metres is insufficient, and flammable mixtures may occur. Examples of interruptions are applying the buffer in two parts or a period of stagnation of the nitrogen buffer. Such interruptions may occur under practical conditions. 

Branches connected to a main pipeline must be purged separately with nitrogen. When a nitrogen buffer is installed in the main pipeline, the air contained in a branch is not fully displaced. During subsequent hydrogen purging, this trapped air may enter the main pipeline, which can lead to the formation of flammable mixtures. 

The presence of a bridge or a pipe line below ground level within a buffer does not cause any adverse effects. Due to the very slight differences in density between nitrogen and air, no interaction occurs at the interface between nitrogen and air. At the transition from nitrogen to hydrogen, however, interaction between the gases does occur, just as it does during regular purging. Following an upward flow in the pipe, the length of the hydrogen/nitrogen mixture decreases. Following a downward flow in the pipe, the length of the mixture increases. This more or less cancels each other out.  

Recommendation for practical application 

Based on the conducted research, the following recommendations are made regarding commissioning hydrogen large-diameter pipes (up to DN400) by means of pulse purging.