The National Fire Protection Association (NFPA) “Standard 11 Low- Medium- and High-Expansion Foam” recommends a foam system proportion at no less than the rated concentration and no more than 30 percent above the rated concentration. Note, this same criteria is also a requirement of Underwriters Laboratory (UL) specification 162 “Foam Equipment and Liquid Concentrates”. Table 1 provides a list of acceptable concentration ranges for typical foam concentrates meeting the NFPA and UL recommendations.

Table 1 – Acceptable Flow Concentration

Rated Concentration Acceptable Flow Concentration
1% 1.0% - 1.3%
2% 2.0% - 2.6%
2.75% 2.75% - 3.6%
3% 3.0% - 3.9%
6% 6.0% - 7.0%

Furthermore, NFPA recommends the concentration be verified when the system is installed and annually thereafter. But what do you do if your system is not proportioning at an acceptable concentration?

The following is a list of the more common reasons a system may not be proportioning correctly.

  • Water flow - When the sample was taken, the system may have not been flowing at its rated flow or for the time required for the system to stabilize. All proportioning systems, whether a bladder tank, pump system or educator, are designed to proportion foam correctly but only when operated at a specific water flow range. Many times inspectors run the system at a low flow for a short time in order to generate less foam solution - which is often costly to dispose of. If this lower flow is below the rated flow range or if the system has not operated for a long enough time to stabilize, a system will not proportion correctly. To determine the rated flow range, contact the manufacturer of the proportioning equipment or refer to the UL directory for the listed flow range.
  • Foam concentrate – To determine the concentration of the foam solution, NFPA recommends measuring its refractive index or conductivity and comparing the values to those of standard solutions made by mixing the foam concentrate and water at known concentrations. When using this method, the foam concentrate and water used to make the standard solutions MUST be representative of the foam concentrate and water proportioned by the system. A bladder tank proportions foam by expelling concentrate from the top of the tank. If the foam concentrate samples used to make the standard solutions are pulled from the bottom of the tank, the standard solutions might not be representative of system solution.
  • Equipment and foam compatibility – NFPA 11 recommends the foam concentrate used in the system be listed for use with the equipment. When a foam and equipment are listed, it ensures the user that the system was tested by an independent laboratory and the system proportioned the foam correctly. There may be cases where the foam has been changed to handle a new hazard. If the new foam is more or less viscous than the previous foam, the orifice may not be sized correctly. Again, check with the equipment manufacturer to make sure the equipment is listed with the foam concentrate.

There can be other reasons in addition to those listed here that may cause a system to proportion incorrectly. Feel free to call Dyne at (800) 632-2304 to discuss your particular system and issue – we will be happy to provide assistance.