Viscosity and Firefighting Foam By Joan Leedy, Technical Director, Dyne Fire Protection Labs
The following sections are from NFPA 25 The Standard for the Inspection, Testing, and Maintenance of Water-based Fire Protection Systems, 2023 edition:
11.2.13.1 Inspection of the foam concentrate shall verify the type and viscosity has not changed from the foam concentrate used to obtain system certification and is suitable for the current hazard (i.e., alcohol resistant).
11.3.1.1 Foam concentrate testing shall be conducted annually.
11.3.1.2 Samples of foam concentrates shall be sent to the manufacturer or an approved testing laboratory for quality condition testing.
11.3.2.1 Proportioner Tests. An operation test of the foam proportioner shall be conducted annually.
11.3.2.5.7 The foam concentrate induction rate, expressed as a percentage of the foam solution, shall be within minus 0 percent to plus 30 percent of the manufacturer’s listed concentration or plus 1 percentage point, whichever is less.
Note the first section referenced above, emphasizes the importance of measuring and monitoring the viscosity of the foam concentrate in a firefighting foam system. So why is this important?
In years past, proportioner tests were conducted by discharging the system; sampling the foam generated; and determining the concentration based on the refractive index. Today however, due to concerns with discharging large quantities of solution, it is the norm to conduct the annual proportioning test with a pseudo fluid or water. And since the actual foam is not being used, it becomes critical to monitor the viscosity of the stored foam.
The viscosity of the foam concentrate must not increase or decrease significantly or the foam will not proportion within the recommended specifications as shown below.
What Causes Changes in Viscosity?
- Dilution with water. Water can mix with foam concentrate because of malfunctioning equipment, such as a tear in a bladder tank. Significant dilution with water will decrease viscosity and render the foam ineffective. However, a minor dilution can actually cause an increase in viscosity if the polymer in the formulation is not fully hydrated.
- Temperature fluctuations. Foams are designed to be stored within specific temperature ranges. If the temperature is outside of these ranges or if there are significant temperature fluctuations, viscosity can increase.
- Mix two incompatible foams. When foam tanks need to be filled, the compatibility of the foams being mixed must be considered.
- Age or evaporation. – Foam will degrade as it ages and is also subject to evaporation over time. Many of the manufacturers of the legacy foam concentrates, claimed a shelf life of 20 years or more. With the new SFFF formulations, the shelf life is shorter.
- Incompatible materials of construction. – Corrosion or dissimilar metals can cause viscosity issues.
When purchasing foam, a certificate of analysis can be requested from the manufacturer. This will provide the baseline viscosity for foam. Subsequent testing can be compared to this baseline number as the foam ages. Note, there are different methods for measuring viscosity and the method should be considered when making these comparisons.
Make sure to test your foam stocks annually and make sure the viscosity is included in the testing!
Questions? Contact us today by calling (800) 632-2304 or emailing dyne.lab@nfpaglobal.com.