f 301 HyPurge Safe Tooling, Final Report

This project has investigated equipment used to carry out purging of gas networks with a view to providing tooling for commissioning SGN’s H100 Fife project.
It has built on work, including the HyPurge NIA2_SGN0008, and Lot 1 of the Hydrogen Skills and Standards for Heat projects. The project has further advanced the body of purge theory founded and developed in those previous projects. The HyPurge project showed that direct purging was feasible with hydrogen; this project has investigated some of the hazards presented and recommended tooling to mitigate those hazards.
Flame arrestors are specified for certain network operations involving Natural Gas. It is recommended that the current procedures regarding flame arrestors are kept for hydrogen, and a range of flame arrestors suited to hydrogen use has been identified.
Purge tables specify minimum speeds for purging, related to pipe diameter. These minimum purge speeds are used to suppress the buoyancy driven effect of a less dense gas to preferentially flow over a denser gas. The lower buoyancy of hydrogen suggests an increase in purge speed of 1.7x those recommended for methane. This increase is not required in smaller diameters (100 mm and below), where it has been found that diffusion effects dominate purge performance, resulting in greater flexibility for purging. Therefore, purge tables have been produced giving recommended minimum purge speeds for methane and hydrogen according to the PE pipe diameters proposed in the H100 Fife project.
A purge stack with additional features to assist with hydrogen purging has been developed in this project. The features include a restriction at the end of the stack to mitigate burn-back in the event of a vent ignition. Specific restriction sizes are linked to the diameter of network pipes being purged and each individual restriction is tailored to achieve the correct purge speed for the given network pipe diameter. A pressure gauge on the stack indicates sufficient back pressure, showing the correct purge flow is being achieved. The stack also includes a hydrogen wHystle (developed by Steer independently) to provide feedback on purge progress in real time.
A review of non-sparking tool requirements has been carried out. Purge operations are such that it is unlikely that non-sparking tools will provide a significant reduction in hazard. The conclusions from this are that the current recommendations from SGN’s mainlay procedures on non-sparking tools and ignition prevention will be suitable for hydrogen use.
A preliminary investigation into the consequences of in-pipe ignitions has been carried out. The investigation has shown that the overpressures generated are affected by several different factors. The proportion of the pipe that contains the flammable mixture affects the ability of the system to absorb the overpressure through non-flammable gas buffer zones. Once detonable zones increase in size, then the absolute length of the detonable zone, in relation to pipe diameter, becomes a dominant factor. The most significant hazard to be prevented is an in-pipe detonation, therefore the volume of detonable mixture is an important factor that may result in a limit to the permitted length for direct purging in a given pipe diameter.
The hazards presented during purging have been investigated and three specific hazards have been studied. These are ignition of the vent, in-pipe ignition and burn back from a vent ignition into the pipe. Although none of these events are likely to occur, ignition of the vent is the most likely and the consequence of this is similar with hydrogen and methane. In-pipe ignition is the event with the greatest consequence and although very unlikely, this should be avoided.
Proposed further work includes: data mining from the body of purge studies to date, identification of the growth of flammable and detonable zones vs. purge length, a study into static electricity generation and consequence testing on ignitions in a variety of 90 mm and 125 mm PE pipes of different lengths.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating, which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz