Firefighting foam: let’s get into the flow

Thick & Gloopy: An example of high viscosity foam concentrate typical of some foam concentrates.

Ensuring a successful transition to high viscosity fluorine-free foam concentrates with FM approved proportioners. Fire practitioners should carefully consider all the hardware variables that contribute to their effectiveness. Design standards from authorities such as the National Fire Protection Association (NFPA), Factory Mutual (FM) & the European EN standard correctly emphasise application rates & times, but some of the foam/foam hardware interface areas need equal scrutiny.

This article will discuss & highlight the way that some of the physical properties of more recently developed foam concentrates need to be considered, especially in retrofit situations.

Many in the fire protection industry have concluded that the long-running debate around fluorine-free foam, as opposed to fluorine-based foams, is resolved in favour of fluorine-free. The momentum appears unstoppable as the EU has clarified its restrictions in Directive 2017/1000, the state of California has outlawed them by 2022 & Norway is one of 5 countries who have restrictions in place to combat the threat posed by long term groundwater contamination. If we take Norway as an exemplar; they have substituted with fluorine-free alternatives in the offshore sector & the discharge of fluorine-based products has reduced by 98%^.1

As much as some may want to try to hold onto the status quo, it’s worth remembering how environmental factors also drove the demise of Halons as an ozone depleter & CO2 as a more immediate hazard to health. The environment usually trumps performance.

The re-formulation to fluorine-free has meant that several of the alternatives are now very viscous, especially the alcohol-resistant type, this has implications for existing foam proportioning systems & for designers of new installations. Nor are municipal & industrial firefighters immune as bulk storage & deployment via induction systems can also be undermined by extremely viscous products. In many ways, this has gone full circle in that the early alcohol-resistant foams were also very viscous. At this point, it may be helpful to offer a brief resume of the two main strands of concentrates that have been prevalent over the last 40-50 years:

The protein family: derived typically from animal hoof & horn to provide burn-back resistance requires aspiration & is not film-forming. Fluoroprotein developed to provide additional burn-back resistance. The pinnacle of the protein family has been film-forming fluoroprotein (FFFP) & film-forming fluoroprotein (alcohol-resistant) for use on polar solvents (FFFP-AR). Although film-forming it requires aspiration on the more demanding polar solvent fires.
The synthetic detergent family: derived from detergents with added surfactants & polymers to provide a free-flowing & film-forming foam that can be used unaspirated in many situations with polar solvents still requiring aspiration. The standard abbreviations include AFFF for aqueous film-forming foam &, like their cousins above, AFFF-AR for the alcohol-resistant variety. Like all good families, they have their advocates & detractors but the head of this particular family has now been undermined by the restriction on the use of fluorine-based polymers or surfactants with the introduction of the various country directives.
Let’s talk Foam solution: this is the mix of water to concentrate & is typically 3% concentrate to 97% water. 1% foam concentrates, by ratio, have been available for many years although they are associated with fluorine-based foams, 1% fluorine-free is probably best described as an emerging technology & one that has its sceptics when it comes to larger deep-seated fires.

Figure. 2. A typical pressure loss curve for FireDos foam proportioners, based on a minimum flow of 600-900 LPM (160-240 gpm) to a maximum of 8000 LPM (2113 gpm) at 10 bar (145 psi).

Foam proportioning

Bringing water and concentrate together is the next chapter in our story. It’s important to understand that increasing viscosity will potentially affect proportioning before this stage. Described as a foam solution, it is applied through your chosen discharge device. Selecting a foam proportioner that allows flexibility in pressure, flow, and viscosity is the preferred option. If the foam proportioner can itself be driven by water, all the better because this eliminates another potential point of failure. For this reason, balanced pressure proportioners with electric foam concentrate pumps are not ideal.

In physics & hydraulics, there is no such thing as free energy. However, we can help ourselves by analysing different water driven foam proportioner systems so that hydraulic efficiencies are maximised as much as possible. With many fire pump systems delivering 7-10 bar at source & fixed foam discharge devices requiring a minimum of 2.5 bar (plus pressure losses in long pipe runs & from height), it’s easy to see how further losses in proportioning the foam can add to additional cost in compensating for them. A loss of no more than 1.5 bar at this stage would be deemed highly efficient for a water-motor driven proportioner. It is important to carefully examine the efficiencies of water-motor driven foam proportioners as the type of piston, or gear pump can potentially adversely affect the outcome.

The importance of foam concentrate suction piping

With regard to suction, I think it’s important here to be sceptical when foam concentrate manufacturers talk of “drop-in” replacements for fluorine-based foam. With existing water-motor driven systems for foam proportioning, which use the energy of the incoming water supply to drive a rotor and is directly coupled to a piston pump, the suction line to the foam concentrate tank should have been determined based on the viscosity of the original fluorinated foam such as AR-AFFF or AFFF. However, this will not have anticipated the potentially higher viscosity of the replacement fluorine-free foam. This can dramatically affect & undermine correct proportioning, which is critical to the proper functioning of any foam-based fire protection system.

The crux of the challenge therefore is that the increased restriction inherent in the new foam concentrate may require a re-calculation of the suction line diameter from concentrate tank to proportioner. The objective is to maximise suction efficiency whilst still ensuring accurate foam proportioning. FM approved, “water-motor driven variable viscosity foam proportioners”, like those from FireDos, provide peace of mind through rigorous testing in conjunction with the new generation of foam concentrates.

FireDos Gen III water-motor driven foam proportioners are designed for both high & low viscosity foam concentrate. As a result, higher viscosity foam concentrates may not require any modification to the system. However, it is good engineering practice to check the new foam concentrate viscosity versus the old to determine if any suction piping modifications are required. Your local distributor should have the expertise to review the system in conjunction with the datasheets from your chosen foam concentrate supplier.

Should you find with older systems that suction pipe runs are not ideal, distributors’ engineers have the expertise to reverse engineer where appropriate. A successful review will give you the confidence that the system will work as intended. The emphasis should be on correctly dimensioning the suction line between the water-motor driven foam proportioner and concentrate storage tank to ensure correct proportioning.

If you have a bladder tank or balanced pressure proportioner system, it is likely that modifications would also need to be made with a change in viscosity. On a positive note, it also needs to be acknowledged that not all the new fluorine-free foams are highly viscous.

A cutaway of a FireDos water-motor driven foam proportioner highlighting the ability to recirculate foam concentrate without discharging for testing purposes.

Key points to consider on suction lines:

Foam agent suction line should be as short and straight as possible
Avoid ups and downs
Avoid blind lines
Construct the suction line as simple as possible
Avoid a joint suction line for multiple tanks
Avoid a joint suction line for multiple foam pumps
Calculate the dimensioning of the suction line
Connection to the foam tank must match the size of the suction line
All fittings to match suction line nominal bore.
The suction line must be vacuum-tight

Is foam discharge testing necessary?

Once the changes have been made you’ll want to function test the system, ideally without discharging foam which requires the foam solution to be held on site before instructing a certified waste disposal company to remove it for high-temperature incineration. This can be avoided by installing FM approved models of water-driven foam proportioner system which enable testing without discharge by means of FM approved flow meters & a foam return line to the concentrate tank. This will save you all the costs associated with disposal of test foam. Critically, FM are rigorous in testing a range of viscosities from low to high over the entire flow and pressure range & in accordance with the tolerances given in both NFPA 11 & EN13565-1. This highlights the benefit of a positive displacement plunger or piston pump, as an integral component of the FireDos Gen III, to handle both low & high viscosity foams.

Plunger or gear pumps: What is better?

Whilst gear pumps are classed as positive displacement pumps; they do not have the same inherent qualities to cope with low viscosity foam concentrate and changing pressure or flow rate conditions. This means gear pumps, in general, although highly capable as high viscosity pumps, cannot accurately proportion low viscosity fluids through a wide range of changing operating conditions. At differing flows and back pressures, or system-generated pressures, proportioning rates cannot be accurately determined. This is critical for foam firefighting when the foam proportioning ratio is the key to success in a firefighting event.

All the above applies equally to fire truck based proportioners for mobile deployment, for semi-fixed systems into pre-piped plant areas utilising trailers/skids or via monitors & hand lines. High viscosities can be especially problematic in central Europe where Winter temperatures are low & storage conditions may not allow the foam to be as free flowing at short notice.

Whilst foam fire protection systems can be said to be more evolution than revolution, it’s comforting to know that the more advanced water driven foam proportioners on the market today can be reset to accept further evolutions in foam technology, whether that be for concentrates of less than 1% or changes in flow characteristics. With many thousands of these systems installed worldwide, it must be time to ensure that you are ready to accept the new generation of fluorine-free foams.

For more information, go to www.firedos.com

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