Bladder-Tank Proportioning Failure and Fluorine-Free Foam Conversion
An aircraft maintenance hangar protects a 12,000 ft2 floor with a foam-water deluge system supplied by a bladder-tank proportioner and a 500 gal tank of 3% AFFF. The documented design basis is 1,400 gpm over the most remote four zones for 10 minutes. During the annual proportioning test, a refractometer sample taken at 600 gpm read 2.0%, while a sample at 1,400 gpm read 3.1%. The bladder was last refilled 11 years ago, and the tank tag shows no concentrate testing since installation. The owner's state has a PFAS ban taking effect in 18 months, so the owner wants to replace the AFFF with a listed fluorine-free (F3) concentrate and reuse the existing tank, proportioner, piping, and discharge nozzles. The installing contractor calls the swap a drop-in because both concentrates are nominally 3%. You are asked to review the test results and the conversion proposal before the owner commits.
1. The 600 gpm sample reads 2.0% against a 3% nominal concentrate. Is the system out of compliance, and what do you verify before concluding the proportioner is defective?
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A reading of 2.0% on a 3% concentrate is one-third below nominal, far outside any reasonable measurement tolerance, so as tested the system is not proportioning correctly. But do not condemn the proportioner on one sample. First validate the test: confirm the refractometer was calibrated against the curve for that specific concentrate and that sample temperature was corrected; confirm the sample came from the correct test valve after steady flow was established; confirm the 600 gpm flow was actually measured, not estimated from nozzle count. If the test method is sound, the likely cause is turndown. Most bladder-tank and venturi proportioners are listed over a flow range, and below the bottom of that range the pressure differential across the venturi is too small to induct the correct amount of concentrate, so the percentage falls off. The 3.1% at design flow fits that pattern: acceptable at rating, weak at low flow. The fix is not a bigger tank. It is a proportioner listed for the full range of flows the system can actually see, including a single-zone discharge, plus a concentrate that is within its tested shelf life.
2. The owner proposes reusing the tank, proportioner, piping, and nozzles with a listed 3% F3 concentrate. What must be verified, and what would make the proposal unacceptable?
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Treat the 'both are 3%' claim as marketing, not engineering. Verify, in this order: (1) The F3 concentrate is listed for foam-water deluge service and the manufacturer publishes compatibility with the specific proportioner, bladder material, and discharge nozzles. F3 foams differ in viscosity and expansion behavior from AFFF and are not drop-in for every venturi; the orifice or the whole proportioner may have to change. (2) The listed proportioning percentage and the design flow still fit the tank for the 10-minute duration; a higher percentage or a higher design flow can make the 500 gal tank short. (3) No mixing. Drain and flush the tank, bladder, and piping per the manufacturer; residual AFFF can affect foam quality, and the old concentrate is PFAS waste requiring regulated disposal. (4) Foam quality must be re-established: expansion ratio and 25% drainage time per the listing, and a concentration measurement method valid for F3. A refractometer calibrated for AFFF is not automatically valid for an F3 concentrate, so the manufacturer's specified method must be used. (5) The deluge valve, nozzles, and pipe are compatible. If the manufacturer will not list the existing proportioner with the new concentrate, the proposal is unacceptable.
3. What acceptance testing would you require after the conversion, and what documentation goes to the authority having jurisdiction?
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Require a full flow test at the design demand and at the minimum flow the system can experience, with concentration measured at both points by the F3-appropriate method. Measure expansion ratio and 25% drainage time on a discharged sample. Demonstrate the full 10-minute duration with the tank level recorded before and after, and confirm the proportioner's listed flow range brackets every measured flow. Functionally test each zone valve, the deluge valve, detection, and alarms. Record the concentrate batch, manufacture date, and listing in the as-built package, and submit the revised design, the manufacturer's compatibility letter, the disposal record for the AFFF, and the test results to the AHJ. Do not sign the acceptance until the low-flow concentration is inside tolerance, because that is the condition the original system failed.