A new foam solution for rim seal fires
Published: 16 September, 2014
PAPER: Rim seal fire protection for floating roof tanks with new developments in fire fighting foams containing water-soluble high molecular weight polymers, by Armi Haran, President, Orchidee International.
An oil floating roof tanks vapours collect in the rim seal between the floating roof and the tank shell. In case of damage to the rim seal the vapours are enriched with oxygen from the air, resulting in a hazardous mixture that can be ignited by lighting or sparks produced by static electricity.
For such incidents – and to avoid a complete tank fire – it is important to have early detection as well as the most suitable extinguishing agent.
A few fire fighting agents that can be considered for this purpose are:
- Gas agent: in the past it was Halon, but following their phase-out there are a few Halon replacements that can be considered as an option.
- Foams: AFFF, AR/AFFF and fluorine-free foams.
- New fire fighting foams containing water-soluble high molecular weight polymers.
Foams – how they work
When foam is produced, the resulting bubbles increase the surface area of the gas-liquid. The surfactants contained in the foam formulation have a strong tendency to adsorb (ie accumulate on the surface/interface of a solid or liquid, forming a thin film), leading to a notable decrease in surface tension. The lower the surfactant’s tension, the easier it is to create this surface or interface area.
An increase in adsorption (which corresponds to the number of adsorbed molecules by unit of surface) is usually linked to two factors: the cohesion capability of the molecules and their hydrophobic properties (the driving force leading to adsorption). A greater compactness in the adsorption layers can be achieved by the addition of water-soluble natural or synthetic polymers.
The film that forms between the foam bubbles is subject to complex stretching stresses. As the film gets thinner, the speed of drainage typically decreases. As the speed of drainage is inversely proportional to the viscosity of liquid and film area, it is possible to slow down the drainage with thickening agents and by the use of large polyhydric bubbles.
There are also several forces at play that produce a repellent effect, preventing the bubbles from coming closer to each other, resulting in the drainage of the film. These forces are the result of the interaction of the double layers that are produced by the adsorption of surfactants on each of the two surfaces of film.
The main properties of fire fighting foams are stability, vapour suppression, and burn-back resistance.
AFFF formulations containing water-soluble fluorosurfactants and hydrocarbon surfactants are effective at extinguishing non-polar solvent fires. Since the surfactant-containing solution draining from the foam is not soluble in the non-polar solvent, thin films form on top of burning fuel such as Heptane, gasoline or Avgas, acting as a vapor suppressor, and preventing re-ignition of the fuel. However, AFFF formulations are totally ineffective on polar solvent fires such as Isopropanol or acetone, which are water-soluble solvents.
The addition of a polar-solvent insoluble foam stabiliser in AFFF is a good way to obtain alcohol-resistant (AR) AFFF formulations. These alcohol-resistant additives precipitate in contact with the polar solvent, giving rise to a protective membrane that prevents the foam from dissolving into the polar solvent. This membrane plays the role of a vapour barrier that extinguishes the polar fuel and prevents its reignition by keeping the foam hydrated. The foams stabilisers usually used in AR-AFFF concentrates are polysaccharides (or other high molecular weight synthetic polymers), which increase the viscosity of the foams. Another option for AR foams is to increase the content of the fluorosurfactants. Both these options are used in the production of foams worldwide, but are not the best solution.
Using option 1 – polysaccharides – increases the viscosity of the foam which in turn reduces its fluidity on the surface of the rim, so extinguishing time is longer than with a lower viscosity foam. In the second option – increasing the fluorosurfactants – the foam will have a low viscosity but it will contain too much fluorine, which persists in the environment. The lower the fluorosurfactant content the better the foam is for the environment.
A new development for foams containing water-soluble high molecular weight polymers
Following two years of research and development, Orchidee International's R&D team has succeeded in perfecting a foam stabiliser that provides excellent results when incorporated in low viscosity AR-AFFF foams and AFFF foams.
This stabiliser is a fluorine-containing polymeric complex (FPC) composed of water-soluble, high-molecular weight, grafted polymer, as well as a polymeric fluorosurfactant. The result is a foam that can fight hydrocarbon and polar solvent fires without the addition of any polysaccharides. The polymeric fluorosurfactant is based on a C6-carbon chain fluorotelomer product, preventing the toxicity, persistence and bioaccumulation footprint of PFOS- and PFOA-related contaminants.
PFC is based on an associative polymer (ie a macromolecule with attracted groups either attached to the ends or randomly distributed along the backbone) that has been hydrophobically modified and has surfactant-like qualities (HM-Polymer).
The addition of a polymeric fluorosurfactant to the HM-polymer results in hydrophobic properties. As the HM-polymer is sensitive to interactions with surfactant molecules, it forms a complex (FPC) with the polymeric fluorosurfactant molecules and micelles. The hydrophobic properties of the FPC containing the hydrophobic segments of HM-polymer in contact with the exposed hydrocarbon/fluorocarbon areas of micelles (playing the role of non-covalent interpolymeric cross-linkers), are shown in Figure 1 (below).
The co-adsorption of polymeric fluorosurfactant molecules and HM-polymer result in an appropriate stabiliser for fire fighting foams. Due to its highly hydrophobic character, FPC improves the extinction and burn-back on hydrocarbon fires. Moreover, thanks to its alcophobic character and therefore its insolubility in polar solvents such as Isopropanol or acetone, PFC forms a visible strong film at the surface of these polar solvents. The use of PFC to stabilise bubbles formation represents a new approach to reducing environmental impact (due to the reduced fluorine-content) while at the same time retaining performance.
The following tables I and II show some Class B fire test results effected with a low viscosity AR-AFFF /3x3 containing FPC based on C6 Fluoroalkyl chain, according to EN 1568-3 and EN 1568-4 Standards.
Orchidee has developed new low-viscosity foams that contain high molecular weight polymers with low fluorine content that we believe provide the best solution for high risks areas such as rim seals in floating roofs tanks, offering high resistance on hydrocarbons fuels and polar solvents. This type of foam is suitable for use with all types of rim seal protection systems as well as pressure vessels, CAFS (compressed air foam systems) and fire proportioners.