Safety of stored crude oil focuses on fire prevention techniques
For fire prevention and safety, storing flammable liquids with high vapour pressure, particularly crude oil, poses difficult challenges. As crude oil storage facilities reach capacity, with more countries routinely storing oil both on land and anchored on ships off the coast, the safety of these storage facilities has become a priority.
Failed safety measures within a fuel storage facility can result in a large-scale fire, explosions and thick vapour clouds. These cause danger to people and infrastructure, with damage and repair costs potentially running to millions of dollars. In addition, environmental impact is a concern.
Focusing on fire prevention, rather than firefighting, can reduce costly evaporation losses and minimizes the need for flaring, reducing the likelihood of a large-scale fire and mitigating the effect on the environment.
If an oil storage tank has a full surface fire, the traditional response is to drain the contents as quickly as possible, while resources are mounted to provide adjacent tank cooling. However, draining can take many hours and mounting a manned response carries inherent risks, particularly near the tank or tanks.
Fire protection for flammable liquids usually involves water mixed with something similar to a soap concentrate (called Aqueous Film Forming Foam or AFFF) to create a thick foam blanket on the surface of the flammable liquid. However, a downside to AFFF is the breakdown of the foam blanket. Obtaining full surface coverage ahead of the foam breaking down can be difficult.
While application of AFFF can be effective in the best conditions, it can be less effective in extremely hot or cold environments where water is either scarce or freezing. This can also pose extraordinary problems when active, water-based fire protection systems, such as water deluge systems fail to perform. This problem is compounded in extremely cold climates where temperatures of -40º F are common.
Traditional internal floating roof systems are often used in oil storage tanks. A key drawback to this is the reduction in the useful height of the tank, sometimes by as much as nine feet, with the need for legs, pontoons or additional seals. There is also a 11.1 psi vapour pressure limit for traditional floating roofs, set by the US Environmental Protection Agency (EPA), and above this, additional control devices are required.
Large oil storage tanks are frequently equipped with rim seal fire protection but not full surface fire protection, usually because the amount of water required for full surface protection exceeds the available water supply. Another common issue is tank farms containing many tanks. These are not commonly equipped to handle multiple full surface tank fires as the water requirement becomes extraordinary and not feasible in many cases.
Another solution is oil tank spacing to reduce heat transfer to adjacent tanks in the event of a fire. This, however, results in a decrease of available space, limiting the overall storage capacity at locations where space is at a premium.
Very high vapour pressure products like Liquified Natural Gas (LNG) and Liquified Petroleum Gas (LPG) pose a different challenge. LNG and LPG storage can result in releases forming a vapour cloud, that if ignited can cause potential accidents to people and storage facilities nearby. Some applications require possible large releases to be directed to remote pits or sumps where the LNG can boil off safely. Remote pits in themselves have inherent risks due to placement and flashing, while they can freely flow a long distance due to variable wind speed and direction.
Flare stacks used for burning off or venting excess vapour from oil storage tanks reduces the pressure and prevents a buildup of highly volatile vapour. This has been recognized as a major environmental problem, generating significant amounts of greenhouse gases.
Composite spheres provide passive fire protection
Engineered composite spheres from Trelleborg called DryFoam are one of the latest developments in passive fire protection technology. A UL listed solution, demonstrating it has met the required safety standards from Underwriters Laboratories. DryFoam has been designed to effectively suppress vapors for a wide range of hydrocarbons, providing a short- and long-term solution for both polar and non-polar fuels.
While initially developed for extreme cold climates, the engineered spheres offer unique benefits that can be applied in virtually all climates and with different hydrocarbons, including LPG and LNG. It provides up to 98% vapor suppression and can mitigate a vapour cloud scenario using a relatively simple and passive method. The technology addresses the need for a non-water-based passive fire protection solution and the challenge of safe oil storage.
DryFoam spheres have multiple coatings making them capable of tolerating continuous, direct, long term exposure to hydrocarbons, including toluene, crude oils, diesel, ethanol, acetone, and gasoline. The outer coating consists of an oleo-phobic material that resists hydrocarbons. When spheres are exposed to high temperatures, such as flames from above, they will intumesce, creating a thermal barrier and protecting the liquid underneath. The spheres exposed to fire expand and interlock with adjacent spheres, creating a crust that forms a tighter vapour barrier.
The effectiveness of DryFoam in a passive fire protection application has been proven with multiple tests conducted in several labs and large-scale test sites. The tests demonstrated passive vapour suppression on both polar (ethanol) and non-polar fuels, in applications including storage tanks, sumps, horizontal tanks, and barges.
The DryFoam spheres offer a passive solution to a full surface fire scenario when applied either as a standalone solution on top of the flammable liquid, or on top of an existing internal floating roof providing a second layer of protection. When applied on top of an existing internal floating roof, spheres would remain in place. In a sunken floating roof scenario from a seismic event or weld failure, the spheres provide an effective vapour suppression barrier, replacing the role of the sunken roof. This is valuable in high risk situations or where tanks are near to high value assets and where maximum protection from a full surface fire is required, challenging conventional tank spacing rationale and providing the option of tighter tank spacing.
LNG & LPG
In addition to hydrocarbon storage, DryFoam is a proven solution for passive fire protection for both LPG and LNG, mitigating against a Vapuor Cloud Explosion (VCE). In these applications, the engineered spheres reduce the volatile vapor cloud resulting from a large spill by up to 90% using only passive means.
The material's vapour suppression and thermal insulating properties reduce the boil off rate when placed in a pit or sump in advance of any spill, suppressing LNG vapours by allowing the LNG to flow between the spheres to the bottom of the vessel. The spheres float on the surface of the LNG or LNP, providing effective passive vapour suppression from flashing, mitigating the risk of a vapour cloud scenario. Moreover, if the residual vapors were to ignite, the heat is also significantly reduced. Sumps pre-filled with this technology could be located closer to storage or transfer locations, with the confidence that the risk of a vapour cloud scenario has been mitigated.
Utilizing a combination of fire prevention solution provides the greatest protection and risk reducing strategy against the challenge of safely storing volatile hydrocarbons.
DryFoam engineered spheres can be utilized for their vapour suppression properties, providing an additional passive protection strategy to storage tanks where utilizing traditional methods prove difficult, for example, the challenge of irregular shaped obstructions within a tank, or storage facilities with spacing issues. They prove fail-over assurances in the event of a sunken roof scenario.
Choosing the best solution for the storage environment, will ensure tank integrity can be maintained for a longer period of time, supporting the drive for increased safety.
For more information about DryFoam, visit Trelleborg's Applied Technology division.
DryFoam sphere specifications
- Sphere diameter of 3mm provides optimal benefits
- Specific Gravity of ~0.15. The low specific gravity allows multiple layers creating a floating bed of spheres
- Intumescent coating triggered at ~350 ºC if exposed to fire
- Oil phobic repelling all common flammable liquid fuels
- Anti-static coating
- Non-toxic cured spheres can be crushed to ~1/30 volume for landfill disposal
- Uniform external force to crush, nominally 120 psi
- Point contact force to crush 8- 10 lbf