Tanks and drums containing hazardous and flammable liquids require secondary and tertiary protection to prevent a potential incident. That should be obvious, but, unfortunately, it all-too-often takes an incident to drive home the importance of keeping potentially deadly chemicals like fuels and lubricants safe and protected from threats of spillage or explosions.
As a common-sense safety measure, storage of hazardous and flammable liquids should be stored in areas where walls and floors, and penetration joints are leak-tight. Surfaces should be free from any cracks, discontinuities and joint failures that may allow relatively unhindered liquid trans-boundary migration.
As a priority, it’s highly recommended that existing storage facilities should be checked on a routine basis for any damage or disrepair, which may render the structure less than leak-tight.
In case after case, however, there are too many chemical related incidents and near misses. For example, in one case back in 1999, 16.561 tonnes of a 30 per cent solution of sodium cyanide was released at a site through a leak in a holding tank.
Of that total quantity, only 4.260 tonnes were recovered with the remaining material lost to the ground and water. The ultimate recommendation called for improvements to the secondary containment area.
The incident does not state what level of protection this site had, if any, but it can be assumed that since three quarters of the leaked material escaped, the protection was either not there, its chemical resistance was insufficient, or this storage facility developed cracks, allowing for the chemicals to seep out.
National Codes of Practice
Following dangerous occurrences varying in scale from minor near misses to those with catastrophic consequences, many countries have adopted codes of practice directed at installing and maintaining suitable secondary containment.
The U.S. Environmental Protection Agency (EPA), for instance, refers to stationary tank bunds in the Resource Conservation and Recovery Act (RCRA) Subpart J, Tank Systems (40 CFR 264. 193).
“Secondary containment systems must be: Designed, installed, and operated to prevent any migration of wastes or accumulated liquid out of the system to the soil, ground water, or surface water at any time during the use of the tank system; and be capable of detecting and collecting releases and accumulated liquids until the collected material is removed.”
To meet these requirements, the secondary containment area must be: “Constructed of, or lined with, materials that are compatible with the waste(s) to be placed in the tank system[…]” and, be “free of cracks or gaps.”
In the U.K., Control of Pollutions Regulations 2001 also states that “the container must be situated within a secondary containment system which satisfies the following requirements[and that its base and walls be impermeable.”
Secondary containment areas are typically constructed using concrete, because it is cost-effective and provides good structural strength. However, due to its porosity, concrete can be easily permeated and has poor chemical resistance, making it susceptible to deterioration through chemical attack.
In addition, concrete is highly prone to cracking due to substrate movement and freeze-thaw cycles.
Barrier Coatings for Secondary Containment Areas
As concrete does not address the requirement for chemical resistance, an additional barrier atop is needed to prevent potential spillages from permeating the secondary containment area. Over the years, a variety of solutions have been trialled, from bitumen- based paints to epoxy-resin based systems.
The right solution would depend on the type of media stored within the tank, size of the containment area, expected traffic and weather conditions, among others.
Where the highest chemical resistance is required in cases of extremely aggressive chemicals, such as concentrated mineral acids, alkali, amines and alcohols; solvent-free epoxy novolac resin based coatings are typically specified.
The drawback of these coatings, however, has long been associated with the very feature that made them chemically resistant – their rigidity. The chemical reaction between the base and solidifier creates an almost impenetrable “physical barrier.”
Subsequently, once hardened and cured, these epoxy systems become completely liquid-impermeable and will have excellent resistance to immersion and exposure to a wide range of oil and chemical spillages. Rigidity of these coatings, however, also makes them inflexible and not best suited for heavy trafficked areas, or in cases where the underlying concrete develops cracks.
Cracks in concrete
Concrete can develop cracks for many reasons, from excessive loading, to thermal expansion/contraction, or during freeze-thaw cycles which lead to the concrete’s movement and settlement. A rigid coating would crack with the concrete, thus terminating chemical protection in case of a spill. Recent advancements in polymer technology have resulted in the development of a hybrid epoxy coating, which combines high cross-linking with rubbery domains in the polymer chain, giving the coating a desired degree of flexibility.
New Material Development
One of the recently introduced coatings, called Belzona 4361, to incorporate these features comes from Belzona Canada of Richmond Hill, Ontario.
To determine the coating’s crack-bridging abilities, the product was first tested for elongation. When cured at 20°C, the coating’s residual elongation was recorded at 20 per cent, which would be sufficient to bridge a typical crack. To ensure the coating maintains its flexibility at low temperatures, a mandrel bend test was also performed, resulting in a pass at temperatures down to 0°C.
To further test the coating’s crack-bridging abilities, it was submitted for a long-term testing at the University of Stuttgart, Germany. The university carries out testing to award a German Federal Water Act (WHG) Approval which is part of a German water law for protecting surface water and groundwater.
Only chemical containment coatings with WHG Approval can be used in areas where strict regulations are in place, in order to protect ground water against chemical pollutants. The testing takes two years to complete, and consists of a combination of crack-bridging, chemical resistance and aging tests.
Crack-bridging tests are first performed by creating a crack within the concrete and ensuring the coating remains intact. This is followed by chemical resistance testing where the chemical is positioned onto the test coating so that the crack in the concrete is directly underneath.
Signs of chemical attack are visually observed, in particular to see if the chemical reagent attacks the test coating severely enough to penetrate through the crack due to the reduction in film thickness over the crack.
To replicate real-life exposure or aging, the coated test blocks are stored in damp sand and placed outdoors. After six months and two years respectively of aging exposure, crack-bridging and chemical resistance tests are repeated. Belzona 4361 passed the crack-bridging and chemical resistance tests after six months of aging exposure, which will be repeated again to complete the two year’s testing.
Chemical resistance was tested by coating rods and immersing them in specified chemicals for a period of up to 12 months. The coating is suitable to resist aggressive chemicals, as protection is only required to last until the leaked chemical can be recovered from the bund. Best practice reports in some countries do not specify a universal length of time the coating needs to resist the spilled chemical and some documents state 72 hours as an acceptable length of protection.
Chemical Protection in Action
Following its introduction in 2015, the coating was applied to protect the most critical assets; one being at a secondary containment area in a U.S. power plant. It was coated after the existing chemical protection weakened. The original coating was used to contain spillages from a 93 per cent sulphuric acid tank experiencing splashes, spills and poor clean-up procedures. In addition to movement, small gaps formed between the floor and bottom of the wall inside the containment area.
The power station in question had already been using a variety of materials to help solve a leak problem, but it recently opted to finally solve the problem with Belzona 4361 due to its chemical resistance, flexibility and good adhesion facilitating long-term sealing between the facility’s wall and floor.
One-hundred per cent solid epoxy materials adhere well not only to concrete, but can also be used to protect a metal substrate from the chemicals.
Added flexibility of the coating expands and contracts in sympathy with the underlying metal substrate.
As the industry keeps improving the safety of their operations, material manufacturers need to keep up and continue to innovate by utilising novel raw materials.
Of course, provision of an adequate secondary containment area is only one of the many improvements that can be done to manage hazards and minimise risks.
Some of the other areas to consider include system automation and software, with leak detection technologies and alarm sounding.
Such systems can dramatically help reduce human error.
Information for this Special Report provided by Marina Silva, Belzona Polymerics Ltd., U.K. Belzona also has a Canadian office in Richmond Hill, Ontario.