Foam Proportioning Test Below Design Percentage
A flammable-liquid warehouse is protected by a foam-water deluge system. The design is 1,900 L/min water flow with 3% AFFF from a bladder tank and a ratio controller. The foam concentrate tank is 1,000 L and was last filled 14 years ago; the label says the concentrate has a 10-year shelf life. During the annual proportioning test, the refractometer reads 1.8% at the design flow. The bladder tank pressure gauge is normal, the proportioner orifice is clear, and the water flow meter confirms 1,900 L/min. The owner's maintenance technician proposes adding concentrate manually during a fire to 'make up the difference.' The system serves a rack storage area with Class II and Class III liquids. The insurance carrier has asked for a report.
1. Calculate the required foam concentrate rate at design flow and compare it with the test result.
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At 3% proportioning, the required concentrate rate is 0.03 × 1,900 L/min = 57 L/min. At the measured 1.8%, the system is delivering 0.018 × 1,900 = 34.2 L/min. The shortfall is 57 − 34.2 = 22.8 L/min, or about 40% below design. That is not a small calibration error. Low proportioning reduces foam expansion, drain time, and the ability to blanket the liquid surface, so the system may fail to control a fire in the rack storage arrangement. The test result should be reported as a failed proportioning test, not accepted because foam still appears at the discharge device. The refractometer reading also needs to be corrected for temperature and concentrate type per the manufacturer's instructions, but a drop from 3.0% to 1.8% is well outside normal test tolerance.
2. List probable causes of the low proportioning and the corrective actions.
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Check the bladder tank level, not just the pressure gauge; a tank can show normal pressure with an empty or low bladder. Verify that the correct 3% concentrate is in the tank and that it has not been diluted or contaminated. Although the orifice is clear, inspect the proportioner for wear, incorrect orifice size, or a stuck ratio controller. Confirm the water flow meter calibration and that the test was run at the design flow. The concentrate is 14 years old against a 10-year shelf life, so aging or viscosity change is a likely contributor; obtain a lab analysis or replace it. After correcting the cause, re-run the proportioning test at design flow and at a lower flow if the listing requires, and document the results. Do not adjust the proportioner by guesswork; use the manufacturer's calibration procedure. The insurance carrier should receive the failed test, the root cause, and the corrective action.
3. Evaluate the proposal to add concentrate manually during a fire. Why is it unacceptable?
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Manual addition during a fire is not a proportioning method. It cannot maintain a stable 3% ratio as water flow changes, it exposes personnel to fire and chemical hazards, and it depends on someone being present and able to reach the tank during an emergency. The system is required to be automatic and reliable, with listed proportioning equipment. Adding concentrate by hand also does not correct the underlying failure and may create an unsafe concentrate-rich or concentrate-lean stream. The correct response is to repair or replace the proportioner, restore the concentrate to serviceability, and re-test. If the system must remain out of service during repairs, establish a documented impairment with a fire watch and notify the AHJ and insurer. The owner should not rely on an improvised manual action to satisfy a design that is already below the listed proportioning rate.