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Properties of high viscous foam agents and their influence on proportioning systems

From fluorinated to fluorine free foam agents

The success in firefighting depends on various factors. Most important is the right choice of foam agent. In the past AFFF and AFFF-AR foam agents were most widely used. They had in common, that they used fluorine as one of the base components, as it has very good extinguishing properties. They had the advantage, that a broad variety of different chemicals could be extinguished with one foam agent. 

Over the last years legislation has been passed in various countries, banning fluorine in foam agents. This means that companies must find fluorine free replacements. These are widely available, but presently have the disadvantage that they are not drop-in replacements for the fluorinated foams. In many cases these non-fluorinated foam agents (FFF) do not yet have approvals for a bandwidth of chemicals. In a lot of cases different non-fluorinated foam agents need to be used where formerly one fluorinated product did the job.

Viscosity

One physical property that a lot of the fluorine free foam agents have in common is an increased viscosity. High viscosity foam agents have been used in the firefighting industry for decades, such as in AFFF-AR, but with the broad transition to non-fluorinated foam agents the topic of viscosity must be considered more widely. The term viscosity refers to the resistance of a fluid to deformation. The higher the viscosity, the thicker, less flowable the fluid is. The lower the viscosity, the thinner, more flowable it is.

Pseudoplastic foam agents

Low viscosity foam agents, such as AFFF, Hi-Ex, Class A or multipurpose are characterized as Newtonian fluids. High viscosity foam agents, such as AFFF-AR and FFF are often pseudoplastic fluids. Pseudoplastic means that they change their viscosity with the speed and turbulence they are moved with. 

The viscosity of a pseudoplastic foam concentrate can be characterized as a function of dynamic viscosity and shear rate, whereby the shear rate is the rate at which a fluid is moved or “worked” during flow, i.e. due to the friction between the fluid and the wall of the pipe work or the turbulence induced by pipe angles or restrictions. As with individual viscosity values a dynamic viscosity / shear rate curve is always associated to a specific temperature. 

Image 1 shows the difference between a Newtonian and a pseudoplastic fluid. FM Approvals, as an example, has defined the shear rates of 5 1/s to 600 1/s as relevant for foam agents and proportioning systems. 

Image 1 Difference between a Newtonian and a pseudoplastic fluid 

In the following we will have a look at highly viscous foam agents in general and what to consider when deciding on a proportioning system.

Viscosity curve

The viscosity of foam agents has a major influence on the correctness of proportioning. Highly viscous foam agents have a significantly higher pressure-loss while flowing than low-viscous foam agents do. This rules many proportioning technologies out which are based upon pressure ratios. Proportioning technologies that are suitable for high-viscous foam agents are generally such that use displacement pumps. Besides foam agents, every proportioning system is characterized by a viscosity curve which describes the ability up to which viscosity fluids can be processed. The curve will depend on the technical characteristics of the individual proportioning technology and system design. 

Dimensioning of the suction line

As an example, the viscosity of a pseudoplastic foam agent will vary depending on the suction line. A small suction line diameter or a long suction line with multiple angles will generate a high shear rate, resulting in a big pressure drop. Hereby not only the diameter of the suction line itself must considered, but any restriction in the line. This for instance includes the outlet of the foam agent container. A 2” outlet of an IBC will be the limiting factor, even when using a 4” suction line.

According to our experience the permissible flow velocities for Newtonian foam concentrates in the suction line are in the range of 1,0 m/s to 1,2 m/s. For high viscosity pseudoplastic foam concentrates they are 0,6 m/s to 0,8 m/s. So, to achieve the same volume flows, larger pipe diameters must be used for high viscosity foam agents. 

In addition, consideration should be taken for the sizing of the suction line to accommodate the differences in pressure drop under the operating conditions of lowest and highest expected water flow as well as lowest and highest expected foam concentrate temperature in the storage container and suction line. 

Taking the viscosity curve into consideration is especially important when evaluating a proportioning system that shall function at varying flow rates. 

Image 2: Viscosity curve of a proportioning system and two differing foam agents 

In Image 2 the proportioning system is fully capable of proportioning the foam agent 1 but may have difficulties with foam agent 2. The curve of foam agent 2 exceeds the curve of the proportioning system above a shear rate of about 90 1/s. For verification whether the combination of the proportioning system and foam agent 2 can be used, physical tests are recommended. Always bear in mind, that you should use the viscosity curve of the lowest anticipated temperature that the system may be exposed to.

Air entrapment

One other big challenge in handling high viscosity foam agents is air entrapment. With high viscosity, pseudoplastic foam agents the air bubbles, that may be induced during transport or refilling, can remain in the liquid and thus have an influence on the proportioning rate. 10% air entrapment means a 10% reduced proportioning rate, so 3% effectively becomes 2.7%.

Image 3: Air trapped inside a high viscous foam agent

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Influence of high viscous foam agents on proportioning systems

Following, different types of proportioning systems will be compared for the effects when transitioning from a Newtonian to a high-viscous, pseudoplastic foam agent.

Venturi proportioner

When using self-inducting venturi proportioners the limiting factor for switching to a high viscous foam agent will be the limited suction capability of the venturi device. Suction lines will generally need a big diameter and be very short, if they will be functional at all, as the suction capability of the system is only generated by the water flow. In addition, it must be anticipated that the pressure drop in the extinguishing water line will be very high. 

Bladder tank with wide range proportioner 

If you want to continue using a bladder tank with a wide range proportioner, the pressure drop in the connection line between bladder tank and proportioner may be a limiting factor due to the friction loss. In any case the pipework of the foam concentrate line and the venturi orifices will need adaptation to the high viscosity of the foam agent. Very common is the use of wide range proportioners with a gear pump, where excess foam agent is returned to the foam agent tank. As with all systems, suction and foam agent injection lines must be increased in diameter to allow for a better flow of high viscous foam agents. Otherwise, the adjustments of the wide range proportioners must be adapted to the properties of the new foam agent, to assure correct proportioning rates at varying flows. Gear pumps generally work well with higher viscous fluids. An increase in the back pressure of an extinguishing system will though cause foam concentrate leakage at low flow rates.

Electronic proportioner

Electronic proportioners with a gear pump, where excess foam agent is returned to the foam agent tank, will need an adaptation of the suction and foam agent injection lines. Otherwise, the system will need to be recalibrated to the properties of the higher viscous foam agent.

Mechanical proportioner

Proportioning systems using a water motor with a gear pump will need new pipe work to accommodate the high viscous foam agent. In addition, the minimum and maximum water flows must be reviewed, as gear pumps have a varying amount of foam concentrate leakage dependent on the water flow and the back pressure in the extinguishing water system. High back pressures and low water flows may have an increased effect on the leakage, so the dimensions of the gear pump for the required water flows should be reviewed. 

When using a water motor driven proportioner with a piston pump, the suction line between the foam agent tank and the pump must be adapted. Otherwise, no changes need to be anticipated, as the piston pump does not have any leakage. Suppliers of water motor driven proportioners with a piston pump use two different pump types. One of them are regular industry pumps that are designed for pumping low viscosity Newtonian fluids like water. The other are pumps specifically designed for high viscosity pseudoplastic fluids like foam agents. The latter, specifically designed pumps, will generally have higher efficiency levels than the regular industry pumps, when using high viscosity foam agents.

The FireDos GEN III water motor driven proportioning systems exclusively use in house designed piston pumps that are engineered and tested for use with both low viscous Newtonian and high viscous pseudoplastic foam agents. In addition, all systems are equipped with foam agent return lines for eco-friendly testing of the of the proportioning rate without having to produce premix or foam.  

Image 4: Water motor driven proportioner with a piston pump and a foam return line for eco-friendly testing of the proportioning rate
1 = water motor 2 = proportioning pump 3 = coupling 4 = ball valve 'flushing' 5 = ball valve 'returning / proportioning' 

For all proportioning systems it should be observed that the above-mentioned viscosity curves (see image 2) for the proportioners and the foam agents are compatible. A changeover from a Newtonian to a high-viscosity foam concentrate is possible with the right planning for almost all proportioning technologies. The changeover must be carefully planned and implemented, and it is recommended to involve specialized companies for the changeover.

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