Lightning strikes on floating roof tanks are a common – and potentially devastating – hazard for petrochemical sites.
These storage tanks commonly hold large quantities of petroleum products, such as crude oil, and have an open-topped cylindrical steel shell with a roof that floats on the surface of the stored liquid. They are vulnerable to the direct and indirect effects of lightning. A nearby lightning strike, for example, can cause electrical currents to flow, or arc, across the tank shell and roof, igniting flammable vapours. The danger to lives is real, while the impact on a site through property losses and downtime can be disastrous.
Whereas old lightning-prevention approaches have a mixed record of mitigation, newer methods offer significantly greater protection.
The old …
In the past, to combat the risk of storage tank fires, oil and gas companies have typically used metal strips called ‘shunts’ that earth-out, or ground, the roof to the tank wall.
However, the bond from these shunts is not always sound and they can lack the desired contact between the roof and wall. They are also prone to wear and tear, as well as being sensitive to tank deformation. In addition, the floating roof can drift slightly off centre and disconnect some of the shunts from the shell.
The upshot is that, without a properly grounded roof, the risk of fire and or explosions in the event of lightning storms is high.
… and the new
Substantially cutting the risk of sustained lightning arcs requires a trusted and low-resistance connection between the tank shell and roof. The connection must also operate regardless of the tank shell’s condition.
Fortunately, newer technology called Retractable Grounding Assembly (RGA) has proven highly effective in preventing lightning strikes on storage tanks. It uses a sophisticated spring-loaded reel that is fixed to the tank wall, along with a retractable cable that is attached to the floating roof and which responds as the roof moves up and down.
The key benefits of the latest RGA models include:
- an ability to pre-tension them at the factory – so no onsite tensioning is required;
- corrosion resistance – the use of aluminium cable originally developed for marine applications increases corrosion resistance compared with bare and tinned copper cables and requires little upkeep compared with shunts; and
- ease of installation – it takes only about two hours to install the RGA system on new or existing tanks. The system is designed to retrofit easily on to any tank, even those in service, and is also easy to inspect.
RGA has earned the plaudits of the oil and gas industry on a number of fronts – it has helped improve safety, prevent production downtime and product losses, and curb the financial devastation associated with lightning strikes to floating roof tanks.
As with any system, proper installation of such systems is crucial. If it is done incorrectly, the benefits from the retractable conductors diminish. For that reason, it is advisable to do your homework and get the RGA system installed correctly through the engagement of a highly trained engineering team.
Armed with the latest technology, your industrial site can repel lightning – and protect the business’s people and profits.
Delta Fire Australasia specialises in the design, installation and servicing of fire-suppression systems on petrochemical and other industrial sites. Visit www.deltafire.com.au for more details.