f Hy4Heat Domestic Hydrogen Purge Procedures - Work Package 4

The aim of this project was to review the current purge standards for UK domestic installations, in particular IGEM/UP/1B, and carry out experiments to assess the validity of those standards for use in hydrogen in order to understand and recommend safe purge practices for hydrogen in a domestic environment.
This report provides the results and conclusions relating to the relative safety of purging domestic installations to hydrogen compared to Natural Gas and the implications of releasing any purged gas
into an enclosed volume representing a small room.
The two high-level findings from this work are:

• changeover to hydrogen will result in an increased risk of flammability inside the installation pipework
• changeover to hydrogen will result in a reduced risk of a build-up of flammable gas in any room where purging occurs.

This work recognises a number of important differences between hydrogen and methane. Hydrogen is more flammable than methane in air, particularly at high concentrations. Hydrogen can maintain a flame at 4%, but does not support general three dimensional deflagration until about 8.5%. Above about 18%, given increased obstruction within the combustion zone, or passage of the flame along a pipe the deflagration can transition to detonation. The flame speed suddenly increases to ~2000m/s. It is very important to avoid this situation although the inventory in a domestic pipe is low. This risk of detonation can occur at concentrations up to 59% and the risk of deflagration remains to 75 %.Methane behaves very differently to this with a range of deflagration of about 4.8 to 16% and a detonation range only marginally narrower. Initiating detonation is however much more difficult with methane.
The risks with hydrogen are associated with a wide range of flammability, with methane the risks are smaller and mainly in lower concentrations of gas in air. Because of this it is particularly important to ensure hydrogen pipes are appropriately purged.