Guest blog by Christopher Harvey Industrial Placement Student
I am a second year engineering student from the University of Sunderland and have a 12-month placement at National Grid, where I’m working on the T-pylon project. Part of my work developing the T-pylon concept has involved the visual amenity aspect of the structure, specifically surface finish options and coating systems.
Through research and detailed discussions with a variety of suppliers, I’ve investigated a number of surface finish options, taking the fabrication and construction requirements of the T-pylon into account. I received this industry learning in workshops with experienced landscape architects, consents officers, Bystrup (www.bystrup.dk) and key stakeholders, to help develop a distilled palette of finish options. This palette would thereby ensure the aesthetic appeal of the T-pylon in truly complementing the diverse natural landscapes of the UK.
With a minimum design life of 80 years, the T-pylon and therefore any finish option we choose needs to meet environmental and sustainable expectations of the 21st century. As a possible future addition to the UK landscape, we remain conscious about the T-pylon’s visual impact. At circa 35m in height, the T-pylon will arguably be viewed more often below the skyline, unlike its equivalent lattice pylon with heights of approximately 50m. It is therefore crucial to ensure the T-pylon blends in with the landscape.
The finish options we are considering include weathering steel (Cor-tenTM) as used in Antony Gormley’s “Angel of the North”, a wet painted solution, hot dip galvanising (HDG) and metalised alloy spray. Traditionally, the individual metal sections of lattice pylons have been galvanised and then coated with a vinyl green-grey paint.
Cor-tenTM steel is typically used for structures designed where low maintenance requirements are desirable e.g. motorway bridges. This steel alloy develops a natural orange-brown hue over time which will allow the pylon to blend in with autumnal and woodland landscapes. The use of Cor-tenTM steel with the T-pylon as a sustainable design option is enhanced thanks to low maintenance needs over the 80 year design life, leading to less regular site visits and no re-coating requirements, thus less traffic and local site disruption.
With strength approaching that of standard structural steel, the development of an orange-brown layer does not make the structure any weaker. The T-pylon will remain as strong as the day it was installed, able to withstand the design loads from high winds in excess of 80 miles per hour.
Unlike raw or carbon steel, after the formation of the protective corrosion coat (“patina”), the Cor-tenTM steel effectively stops the corrosion process from repeating because the chemical composition of the steel alloy prevents any strength loss. Hence, it may look weathered but the T-pylon will stay strong. It is possible to increase the thickness of the steel alloy if an extended design life is required or if the T-pylon is located in a potentially harsh environment. However, suppliers advise that Cor-tenTM¬ should not be used within areas of high salinity.
We are engaging with landscape architects, visual consultants and reviewing local and national planning guidelines to create a small palette of colours, in addition to Cor-TenTM, to support our overhead line routing consultations. This is in stark contrast to the current singular option of grey paint which has been a standard option over previous decades with lattice pylons, and will bring concurrent design considerations to overhead line routing. We are considering shades of light greys (e.g. RAL 7035) and greens that will allow the T-pylon to harmonise with a variety of potential settings.
Another departure from current practice will be the way we paint the T-pylon. For lattice pylons, teams of workers manually apply a very thick glutinous paint, with brushes, over the galvanised sections. Since we can’t climb the T-pylon, the factory will apply the initial coat of paint ensuring a high quality application and finish.
The robustness of the first coat on the thick sections of the T-pylon will mean that compared to the lattice, the T-pylon may not require re-painting for 30-40 years, against the current 15-20 years, with the lattice. We are examining different re-painting regimes, including the use of robots or working from a mobile elevated platform. We can remove any blemishes to the painted coat during the design life on site, as per standard practice.
With the complexities of maintenance in mind we are working with Bystrup, DS-SM and paint manufacturing companies (Hempel Denmark, Hempel UK and Spencer Coatings) to develop novel paint solutions that could theoretically last 80 years, minimising maintenance. Options include multi-layer epoxy paint with similar coloured layers so as the ultra violet light from the sun degrades the finish, no discernible difference will be seen. Alternately, we could apply a base coating of a metal alloy spray to ensure the toughness to abrasion and longevity of the paint finish.
Hot Dip Galvanising (HDG)
HDG is an established process used for lattice pylon members prior to painting because of the benefits of longevity and reduced maintenance requirements. However, there is one key problem with using this process on the T-pylon. HDG is limited by the size of the molten zinc baths into which the metal sections are submerged. This is because only relatively small sections of objects are galvanised at the moment, e.g. lampposts and crash barriers.
The UK Galvanising Association confirmed that the bottom sections of the T-pylon are too large (width, length and weight) to be galvanized in the UK, and would require shipping sections to Europe or even further afield. Only ~75% of the suspension pylon and ~50% of the tension pylon can be galvanised in the UK. If we went ahead with this solution, it would mean transporting a pylon to multiple locations to be coated, ahead of the entire pylon being painted, to create a uniform appearance. However, then we would only have to return to site to re-coat the larger lower sections (once they were deployed), which is a benefit over paint only finish.
Metal Alloy Spray
Metal alloy spray provides a very similar level of protection to HDG, but uses a a spray gun to apply the coating rather than submersing the sections in a molten zinc bath. Traditionally, we’ve carried out this process with either pure zinc or aluminium, although both of these materials have drawbacks as a protective coat.
We are investigating a new type of alloy that combines the best aspects of the zinc and aluminium finishes with none of the drawbacks. This finish may last 80 years and would have the advantages of HDG. Best, we wouldn’t need to transport the sections to multiple locations, depending on the manufacturers coating ability. We could resolve any blemishes to the metalised coat during the design life with zinc based paints as the metal alloy spray process cannot be carried out on site because of the strict application and environmental control requirements.
We’ve started the process to get some of the surface finish options into accelerated aging tests for reference on their performance as protective coatings. Main actions thus far are organising samples of the novel paint solutions for testing at EA Technologies accelerated aging test labs and perusing a sample of the metal alloy spray for testing.
By testing samples we can benchmark against our current experiences and have coating systems for comparison. As we have little experience with the likes of Cor-ten and metal spraying, it is important that we get an idea of how they perform before we build structures with them.