Firefighting foams viscosity
What is foam concentrate viscosity?
First, the viscosity is the physical property that characterizes the flow resistance of fluids. The lower the viscosity is, the more fluid it is. Viscosity may impact transport in pipes and injection process.
There are various ways to measure foam viscosity. It depends on the type of foam concentrate. Viscosities of firefighting foam concentrates are separated into 2 categories:
- Newtonian firefighting foams:
Their movement are generally proportional to the force applied. A Newtonian fluid have constant viscosity, it is not affected by shear rate. Liquids, like gasoline or water, flow very readily.
Newtonian foams have kinematic viscosity, usually expressed as Centistokes, in cSt unit; or in mm²/s.
Generally, AFFF and class A foams are Newtonian (not viscous), such as BIOEX AFFF BIOFILM and class A foam BIO FOR.
- Non-Newtonian firefighting foams: There are commonly called pseudo-plastic foams or viscous foams.
Non-Newtonian foam concentrate viscosity is not constant and is related to the shear rate. It shows an instantaneous decrease in the apparent viscosity with an increase in the shear rate. Non-Newtonian firefighting foams will require a larger amount of energy to move, especially when priming the pump. It becomes fluid as the aspiration progresses.
A highly viscous fluid is very thick compared to low viscous fluid. Liquids such as shampoo are viscous and flows slowly without force.
Pseudoplastic foams have dynamic viscosity, usually expressed as millipascal seconds, in mPa.s unit; or in Centipoise, in cP unit. One cP is equal to one mPa.
I.e., some AR-AFFF and FFF (Fluorine-Free Foams) are pseudo-plastic foam concentrates.
Pseudoplastic foam viscosity may be measured using a rotary viscometer, cone and plate viscometer, or Rheometer. Viscosity measurement is done at a defined temperature.
To have an idea of viscosity in product of our everyday life, here is some examples:
|Fluids||Viscosity in cP|
|Water (at 20°C)||1|
|Gasoline (at 25°C)||0.6|
|Shampoo (at 20°C)||530|
|Glycerin (at 25°C)||934|
|Liquid honey (at 20°C)||10 000|
What is the difference between low viscosity and Newtonian firefighting foam?
A low viscosity foam concentrate is not a Newtonian foam concentrate. It means that the foam concentrate is still viscous but less than another one.
What effect does temperature have on non-Newtonian foam concentrate?
For non-Newtonian foams, the viscosity decreases as temperature increases. Thus, upon heating, non-Newtonian foam flow more easily.
On the contrary, when the temperature decreases, the foam concentrate is getting more viscous.
What type of firefighting foam are viscous?
The foam concentrate viscosity is depending on foam concentrate formulation.
Certain components (for example polymers) given the AR (Alcool-Resistance) property are responsible of an increase of foam concentrate viscosity. That’s why most of AR-AFFF (or also called AFFF ARC) and AR-FFF are viscous foam concentrates.
Nevertheless, it’s not a universal truth. For instance, BIOEX FILMOPOL 1N is an AR-AFFF foam concentrate, but due its formulation without gum, this product is Newtonian.
What are the relations between foam viscosity and foam equipment?
The success in firefighting depends, to a great extent, on combination of fire extinguishing system (mixing and discharge devices), fire foam concentrate, temperatures, water flow and pressure.
The foam concentrate is selected depending on the fire risk faced (class A combustible material, class B water-miscible fuel, class B water-immiscible fuel…). Foam physical properties have to be compliant with foam equipment settings. Indeed, foam viscosity may impact transport in pipelines and injection processes.
Foam concentrate viscosity is not handled in the same way on foam equipment. Some of them are calibrated to handle non-Newtonian foam concentrates (for example LEADER MIX AB foam proportioners), and others may require special settings.
EN 1568 standard tells that if a pseudoplastic foam with a viscosity above 120 mPa.s (at a lowest temperature of use and at 375 s-1) or a Newtonian foam above 200 mm2/sec (at a lowest temperature of use and at 375 s-1), it could need special adjustment on proportioning systems.
If foam discharge and mixing devices are not fitted to Non-Newtonian foam concentrate, they can have difficulties to provide the correct amount of foam agent and underdose it. It will result in trouble or longer fire extinguishment.
Regarding large fire installations, pressure and length of pipe are important factor for correct proportioning of Non-Newtonian foam.
Finally, when viscous foam agent is handled with non-adapted pump, it may cause air trapping into the concentrate. In this case, the volume of foam will increase and the foam concentrate will not mix properly, thus it will reduce the chances of successful fire extinguishment.
BIOEX offers both Newtonian and Non-Newtonian firefighting foams compatible with most of foam equipment available on the market.
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