Foam Eductor Back-Pressure / Hose-Lay Limit
In-line eductor limit: maximum back pressure = 0.65 x inlet.
Example
You enter
- Eductor inlet pressure (psi, ~200) 200
- Rated eductor flow (gpm, = nozzle flow) 95
- Downstream hose coefficient C (1.75 in ~ 15.5) 15.5
- Nozzle operating pressure (psi) 100
- Elevation to nozzle (ft, negative = downhill) 30
You get
- Maximum allowable back-pressure 130
- Downstream friction loss per 100 ft 14.0 psi per 100 ft
- Max length (ft) 121.4
Details, formula, and sources
Max hose length = (0.65 x inlet - nozzle - 0.434 x elevation) / (C x (Q/100)^2) x 100. If the back-pressure exceeds ~65% of inlet the eductor stops drawing foam concentrate entirely (not less, none) while water keeps flowing. A planning aid, not incident command.
max_back_pressure = 0.65 x inlet_pressure; FL_per_100 = hose_coefficient x (eductor_flow / 100)^2; max_length = (max_back_pressure - nozzle_pressure - 0.434 x elevation) / FL_per_100 x 100 (0 if the budget is non-positive).
IFSTA / eductor manufacturer data (TFT/Elkhart) in-line foam eductor back-pressure limit, by name.
The eductor back-pressure relations are public fire-service training / manufacturer material; the eductor manufacturer data governs the actual limit.
Estimate. AHJ and licensed professional govern.
Field names used by the API: inlet_pressure_psi, eductor_flow_gpm, hose_coefficient, nozzle_pressure_psi, elevation_ft, max_back_pressure_psi, fl_per_100_psi, max_length_ft
- Back-pressure ceiling downstream back-pressure must stay below about 65% of the inlet or proportioning stops entirelyIFSTA / eductor manufacturer data
- Flow match the eductor's rated flow must equal the nozzle's floweductor manufacturer data (TFT/Elkhart)