Corrosion causes material loss from steelwork on lattice OHL towers, which can be minimised through the application of an optimal painting regime. Approximately 1,200 towers are repainted every year on NGET’s network of OHLs. Current NGET policy dictates that high pressure water jetting may only be employed up to a height level with the bend line of a tower (i.e. a height beneath the lowest phase conductors). Above the bend line, preparation requires wire brushing and anti-fungal treatment (where required) due to concerns about the use of high pressure water jetting in proximity to live conductors. This project investigates the viability of adopting alternative approaches for surface preparation of steel lattice towers prior to painting, specifically: High pressure water jetting and Dry ice (CO2) cleaning.
Benefits
Financial:
Project business case is built based on the conservative costs provided by one of the suppliers on NGET’s tower painting framework. Baseline is considered based on the annual re-painting target of 1200 towers, out of which 90% of towers are cleaned using algae wash for the whole body. For 10% of annual volume, water jetting is being used below the bend line of the tower (that is, tower bases) and algae wash for above the bend line. If the alternate methods are proven successful, it is anticipated that water jetting for the whole tower body can be implemented on 63% of towers, while the remaining towers would have access constraints required to facilitate the alternate methods. It is assumed that cost of alternative approached will reduce to 75% of the initial value due to widespread use in BAU. Considering these, the innovation method has a benefit of approx. £254,000 for NGET, and in turn, to UK consumers.
Environment, Health & Safety:
Current practice involve anti-fungal treatments that are harmful through an irritant risk to working parties and wire brushing that has the potential to cause harm due to the creation of airborne fibres. It is anticipated that alternate methods provide better health and safety conditions to the workers and the environment through the use of more environment friendly materials, which are not quantified at this stage.
Learnings
Outcomes
Year 2021/22
Preparatory steps of XPS and SEM analyses were done on steelwork samples collected from the network to understand the nature and extent of surface cleaning that needs to be achieved by alternate methods. First outcomes of the project are expected in July and August 2022 in the form of risk assessments for alternate methods along with initial tests in HV lab conditions.
Year 2022/23
The outcomes for the project include data on the critical breakdown voltage using waterjet and CO2 cleaning equipment.
- The introduction of water into the gap improved the critical breakdown voltage.
- The introduction of dry ice into the gap indicated no change in critical breakdown distance.
- All measurement data gathered indicates that there will be no increased safety risk to personnel using wet spray / dry ice cleaning as long as standard safety clearances are maintained.
- The risk should be further reduced in practice as in the tests the spray was directly aimed at the HV electrode, whereas in reality the spray would be aimed at the tower.
Lessons Learnt
Year 2021/22
An interim learning from the work carried out so far is the increased understanding of the composition of the materials present on a typical tower steelwork surface (such as organics from algae deposit, zinc and ferric oxides etc.). The alternate methods being trialled in this project will need to be effective against all these materials to be able to clean the surface to necessary standards.
Year 2022/23
This project has provided additional information and data on the safety of using high pressure water spraying and CO2 cleaning for preparing towers which may affect future projects. The results indicate that wet spray improves the critical breakdown voltage and for dry ice there is no change in critical breakdown distance when the dry ice spray is on or off. This preliminary data indicates that there is no added risk in using dry ice in the air gap.
Based on the lessons learnt, the next set of testing work could also include the impact of water conductivity on the breakdown voltage, only applicable to water jet spray. There is also a need to perform live trials of the system, within a controlled tower environment, which will also aid in the changes required for policy and training, before considering incorporating into business as usual. There could also be huge benefits in being able to de-risk these novel preparation methods on cross-arms, and this would minimize the need for outages.
Dissemination
Year 2021/22
The delivery of Milestone 2 is currently in progress. Avenues for dissemination will be explored when first publishable outcomes are achieved (in Milestones 3 and onwards).
Year 2022/23
A paper on the test results has been written and a poster created for the ISH2023 conference which will be presented in August 2023.
