This project is aimed at developing two novel methods for sealing SF6 leaks from equipment that are currently difficult to address with available solutions. The intention is that these solutions will be designed to be flexible in terms of the leaks they can stop or significantly reduce, they should be easy to apply and readily removable if necessary.
The two solutions are a low melting point metal alloy cast with a modular design of mould for small bore pipe work and a graphene impregnated elastomer which may be applied as a tape and in a spray to address flange leak from gas insulated busbars.
Benefits
The outputs of the project deliver benefit through reduced impact on global warming and as a result of reduced volumes of SF6 required for top ups and gas call outs for top-ups.
Learnings
Outcomes
Workstream 1
No dielectric change/gas composition change as a result of the leak repair.
The gas analysis results after the leak repair do not suggest that the detected levels of nitrogen (N2) were higher than the ones that were detected during the preliminary tests. Moreover, the outcomes of the high voltage performance testing do not indicate the presence of conductive particles within the test sample volume following the casting process. The results suggest that there were no conductive particles deposited on the surface of the electrodes used within the high voltage test vessel. The results also suggested that the insulating performance of the gaseous insulating medium was not affected following the casting process. The variation found in the breakdown (U50%) among the different test cases was less than 3 %, which can potentially be attributed to the statistical variations of the test results.
No significant gas ingress into the pipes as a result of the leak repair.
The gas analysis after the high voltage testing did not indicate any N2 contamination, above the expected levels in the interconnected volumes of the test rig and the stainless-steel vessel.
Conductivity of the joint improved after the repair.
The obtained resistivity measurements indicate that the resulting encasement provides a more effective equipotential bond across the joint which results in an improved conductivity following the repair.
Workstream 2
The originally planned outcome of targeting a kit which required a single site visit to survey and seal a leak became unrealistic within the early stage of this project as the complexity of the challenge became clear, and therefore this overall objective has not been met. However, the learning and development through this project has had the following benefits and outcomes for leak sealing of SF6 leaks from small bore pipes:
- Six SF6 leaks sealed as part of this project, without the need for an outage or de-gassing.
- Different mould designs used, demonstrating the versatility of the technology against SF6 leaks.
- Greater understanding of how access constraints influence the applicability of the technology.
- A range of tools supporting achieving both alloy containment and high mitigation which could be further developed.
- Test data giving confidence that SF6 leaks sealing can be achieved against a flowing leak using the technology on a wider range of scenarios, thus enabling more complex challenges to be tackled.
- Mould development journey leading to a range of options in the toolkit with significantly reduced bespoke development timescales for similar geometries and hence much improved time to deployment. This has been proven by being able to seal four leaks just two weeks after the site survey. It is estimated that development timescales for a geometry similar to those covered in this project has been reduced from months (prior to the project) to weeks. It is estimated that development timescales for small bore pipework geometries not covered by this project, but utilising the developments in this project will be reduced by up to 50 % and will be driven by part lead times and capacity rather than solely extended development time.
It was intended that the leak seal pilots would cover all three TO networks. In practice they were delivered at NGET and SSEN-Transmission substations. SPEN’s chosen location for repair was too complex, note, however that this leak has subsequently been sealed using a bespoke version of the low melting point alloy repair.
Workstream 3
Two series of ethylene propylene diene terpolymer (EPDM) compounding systems with the addition of graphene nanoplatelets were developed and processed successfully, to formulate an ideal candidate to be used as a sealing material for SF6 flanges. One batch contains carbon black contents of 150 phr (per hundred rubber), and the graphene nanoplatelets added to it. The other batch replaced the carbon black with the graphene. From the characterisation regarding their microstructure, mechanical properties and SF6 permeability, the best candidate selected was the EPDM filled with 20 wt% graphene nanoplatelets without carbon black. Its microstructure shows the nanoplatelets were dispersed homogeneously and in-plane oriented within the polymer matrix. The three most important mechanical properties which are excellent for sealing applications are: tear strength of 30 kN/m, a hardness of 72 Shore A and a compression set of 12% after 24 hours at 70 °C. The SF6 permeability tested is in the order of 10-18 mol/m.s.Pa which is one order of magnitude lower than reported data for low density polyethylene (LDPE), showing its high potential for the tape application. This sample was confirmed to have good processability by a specialist in rubber compounds (CLWYD Compounders).
Overall, the replacement of carbon black with graphene nanoplatelets can generally benefit rubber compounding systems for various applications, providing 2-3 times improvement due to its nanostructure and geometry. It is promising to implement such compounding systems for sealing applications such as mitigation of SF6 leaks too. Moreover, such systems can readily be applied into other rubber systems (such as nitrile rubber) for new installations filled with other gases, such as gas mixtures of O2/N2/CO2/C4F7N.
Recommendations for further work
The low melting point metal alloy leak sealing solution would benefit from further work to address complex geometries. This could include evaluation of new seal materials for a better mould or a development of a spray application where there is no space to fit a mould.
A digital twin for leak sealing scenarios could accelerate the time to provide a leak sealing solution for complex situations.
The developed technique may also have application for higher pressure pipes, such as those found in air systems or on liquid filled joints found on fluid filled cables.
Seal longevity has not yet been investigated.
The graphene impregnated rubber solution has demonstrated effectiveness as a solution for sealing flanges. Neither the spray nor paint applied solution is suitable for sealing bolts and further work is required to develop a complete solution for sealing a bolted flange on GIS.
The graphene rubber could be developed for very low permeability gaskets in new equipment or for replacement purposes in older equipment.
Lessons Learnt
Workstream 1
As part of WS1, a testing regime was established to evaluate the performance of metal alloy leak seals. The dielectric change testing was conducted again at the end of WS2 Stage 2.
Workstream 2
Data gathering as part of the project has highlighted a very high degree of variation in geometries and surrounding equipment for small-bore pipework installations. A single modular design for all eventualities would not be possible.
3D smartphone scanning is a useful addition to a survey toolkit. It allows issues with access restrictions to be assessed ahead of deployment.
Moulds made up of fewer parts can be smaller and work in more constrained environments. It is also better able to withstand pressure. That being said, it is critical that the air in the mould ahead of injection is able to vent during the process.
Rubber seals are better than 3D printed seals, they provide better sealing owing to their compressibility but extended exposure to the liquid alloy should be minimised to prevent permanent deformation.
Larger bore pipework (more than 1 inch) is more challenging for seals. This may be improved by use of a flux on the surface prior to injection. Careful removal of the mould plays a role in the effectiveness of the seal.
Workstream 3
In WS3, several lessons were identified:
- Mechanical properties of potential leak sealing materials should also be considered as well as the gas barrier performance.
- When modifying a test rig for evaluating leak sealing performance, the potential for the rig to leak (not just in the test part) should not be overlooked.
- Graphene impregnated elastomer is suitable for sealing SF6 leaks when used as a wrap.
- Spraying the elastomer is less effective as it can only be done with the uncured elastomer, and it lacks mechanical strength. Curing in situ after application is not a good solution.
- Liquid elastomer may be applied as a barrier for SF6 but has higher permeability than the tape. The time to apply and cure the painted rubber is likely too long to be practical in a final solution.
- The painted solution has a poor compression set; if it is subjected to compression, it does not return to its original shape, which can affect long term performance as a sealant.
Dissemination
A presentation on the metal alloy leak sealing technology was given to other utilities attending the Eurodoble conference in Rome in October 2022.
A similar presentation was given to transmission utilities at the ITOMS conference in Belgium in April 2023 for which NGET’s representative, Derrick Dunkley, was awarded Best Presentation.
On completion of WS1, a press release was made by National Grid to promote the work which had already started to be used within the business. https://www.nationalgrid.com/novel-anti-leak-tech-helps-keep-national-grid-substations-service-during-sf6-repairs. This was picked up and reported by, among others, Energy Live News, Utility Week, Industrial News, and Current News websites.
A test object used in WS1 was on view at the Energy Innovation Summit in Glasgow in September 2022 and a presentation on the work being undertaken on WS1 was shared at the Energy Innovation Summit in Liverpool in 2023. The technology was demonstrated at Innovation Zero at London Olympia in April/May 2024.
The partners involved in developing the metal alloy leak seal were recognised with a bronze award by the IET at the annual E&T awards in 2024.
A paper titled “High-efficiency enhancement of SF6 barrier of EPDM elastomers reinforced by graphene nanoplatelets” has been submitted to the American Chemical Society journal ACS Sustainable Chemistry & Engineering.
A presentation covering the graphene project was given as part of a National Grid sponsored event covering all of NGET’s SF6 and SF6 alternative related research at the University of Manchester which was attended by UK TOs and DNOs, and invited industry experts. Presentations were also given by GE, Hitachi, RTE, Siemens and Sintef in September 2024 at the Graphene Institute in Manchester.
Both workstreams will be presented by Gordon Wilson in brief at a keynote presentation at UHVNet 25 in Liverpool in June. Further academic publications are planned.
The project was highlighted as a case study in NGET’s annual innovation summary in 2023 (page 28) and 2024 (page 26).
