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
Interim report A, reviewed whether the use of water-jet or carbon dioxide cleaning equipment in the vicinity of live transmission voltage conductors will in anyway impact the risk to workers. The review is not intended to provide a definitive view on the likely change in risk but instead is used to inform the laboratory testing that would take place as part of the wider project. A subsequent report was then produced that summarises the findings and seeks to conclude whether there is any likely increase in risk to an overhead line worker.
Interim report B+D covers the high voltage testing of both the water and dry-ice spray systems. In accordance with the test program, the original proposal suggests the testing of the spray system for three different voltage levels applicable to NGET, namely 132kV, 275kV and 400kV. Testing within the labs was intended to answer the question of the use of the system and the inherent risk to the safety of the workers. The live line standard IEC 61472 provides a good example of how the risk of flashover across a gap can be estimated. It focuses on switching transients (given these are where the real risk usually lies) and uses a probabilistic approach to ensure the risk to a live line worker is no different to that of a worker in a standard substation. Taking this into account the testing done at Manchester involved understanding the risk of breakdown across a typical gap, when dry, and when using water jet and dry ice. Testing used positive switching impulse given this is the critical voltage that derives the size of a gap on an AC overhead line. It must be highlighted that the risk of flashover due to AC voltages is small, as a 0.75m gap is sufficient for AC case (343kV peak phase to earth voltage) based on Annex G of IEC 60071-2 – significant margin in respect of 3.1m safety clearance. Thus, within the tests, the main emphasis was on the switching transients.
The risk of breakdown is based on analysing if, for the same gap, there is a larger risk of breakdown when the spray is being used. If the laboratory tests show that the breakdown voltage of a gap is increased with water / CO2, the presumption will be that there is no increase in the risk to the worker (assuming they continue to respect safety clearances as managed by the site/NGET Senior Authorised Person).
Initial tests were performed on the jet spray. As part of the test the U50 breakdown voltage was calculated for a range of distances between a rod and plane gap (when dry and with water being sprayed between the gap). This U50 is defined at the critical breakdown voltage, which defined the 50% probability of breakdown across a certain gap. If the critical breakdown voltage decreases across a certain gap whilst using water spray (in comparison to when the gap is dry), it is deemed that there is an increased risk of flashover to the worker. However, tests indicated no increased change in the breakdown voltage, when dry and with the use of the water spray, highlighting no increased risk due to the use of such sprays within the lab experiments.
Interim report C - The first section of this report details the surface analysis work performed on several steel samples provided by National Grid. X-Ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) techniques were used to investigate the condition of the surface to provide background information prior to cleaning trials.
Alternative cleaning techniques which could potentially remove surface corrosion and debris better than the current water jet system were investigated. Dry ice cleaning was demonstrated by Cryogen in 2022 and then wet grit blasting, and further dry ice trials were performed in 2024 by ESL. Samples which had been cleaned with the various methods were painted with the primer used by National Grid and subjected to paint pull-off testing.
The report also details a site visit which took place in August 2022 in Lancashire to learn more about the current cleaning process and its limitations in terms of cleaning efficiency and safety. Information gathered on this visit was used by Manchester University to understand what they had to replicate in HV laboratory tests.
The final section of the report investigates the environmental effects of using jet wash cleaning and dry ice cleaning. Dry ice cleaning only uses carbon dioxide which is created as a byproduct of other industrial process, so no new carbon dioxide is released into the environment. However, data on the energy required to convert the carbon dioxide into solid dry ice pellets is not readily available and is likely to be energy intensive.
Recommendations for further work
Dry ice cleaning and wet-blast cleaning have been compared with waterjet cleaning in terms of the efficiency with which the surface is prepared for repainting and the consumption levels of cleaning media.
Wet grit blasting (Torbo system recommended by ESL) is the only cleaning method that could remove all layers of corrosion and paint quickly. Removing all corrosion and old paint gives the primer the best chance of adhering to the steel substrate to best protect the steel substrate from corrosion. However, the pull-off test data did not back these claims up and showed that the wet grit blasting samples performed no better than water jet cleaning. The initial theory is that the wet grit blasting samples have only one layer of primer on, whereas the other cleaning methods, such as dry ice, do not remove old layers of paint so this may affect the pull-off testing as there are more layers of paint to pull from the surface, causing a higher force to be recorded.
The cost of consumables for wet grit blasting is significantly less than dry ice cleaning and the equipment is currently being used in other countries for tower cleaning. The drawback with wet grit blasting is the blasting media which is left on site. Further investigations would be needed to look at the environmental effect of grit blasting.
Additional work should be performed on the following:
· Further adhesion trials comparing water, CO2 and wet blast cleaning to better assess the variable test results achieved.
· Assessment of the implications of implementing the use of high-pressure water jetting, CO2 cleaning (and wet blast cleaning) as operational practices
· An assessment of the likely carbon footprint of the above activities.
· Evaluation of the technical/commercial/environmental impact of the differing approaches
Lessons Learnt
The testing done at Manchester involving water jet and dry ice indicated no increased change in the breakdown voltage of the air gap. In addition, for dry ice cleaning to be successful, the cleaning of the steel would need to be significantly better to allow for cost savings in other areas (such as requiring towers to be painted less frequently or extending the lifespan of the steel).
Dissemination
A paper was presented at the ISH2023 conference in Glasgow.
Title: Impact of dry ice and water on safe electrical distances and impulse withstand voltage
Authors: Vidyadhar Peesapati, Ian Cotton, Chorphaka Plaengpraphan, Anusha Arva, Ed Brown, Peter Hansen, Shaun Costello, Gerry Boyce
Abstract: In order to minimise the time for tower surface preparation, and to improve health and safety conditions for the workers and the environment, National Grid Electricity Transmission have started an innovation project where high pressure water and dry ice jet sprays are being explored as potential alternatives to abrasive or chemical cleaning. The use of such technologies can also reduce the time required to re-paint individual towers. However, it needs to be assessed if the use of water and dry ice sprays could potentially increase the risk of flashovers. The current paper will look to determine the impact of using dry ice and water sprays for tower surface preparation ahead of re-painting activity. The paper will look to determine the safe electrical distances and voltages, based on laboratory tests of a typical rod-plane gap with the introduction of water and dry ice sprays between these gaps.
