Firefighting Nozzle Reaction

PAUL GRIMWOOD

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By evaluating maximum flow capability for a hose-line that could be effectively directed and safely handled whilst advancing and working inside a fire-involved structure It was observed that there was a maximum nozzle reaction force that could be handled by one, two and three firefighters as follows - 

One firefighter - 266N (60 lbf) 

Two firefighters - 333N (75 lbf)

Three firefighters - 422N (95 lbf)

These were interesting findings and from these I was able to establish baseline flows for interior firefighting operations. To achieve this it became necessary to take acceptable pumping practice into consideration without contravening the limitations placed upon european pumps, hoses and equipment available at that time. This resulted in baseline flows of 277 lpm (73 gpm) on 45mm hose-lines with 12.5mm nozzles, 650 lpm (172 gpm) on 70mm hose-lines with 20mm nozzles and 750 lpm (200 gpm) on 70mm hose-lines with 25mm nozzles, as advanced by two-man crews.

However, these 'base-line' flows were rarely, if ever, achieved in practice as tradition had established a base-line pumping pressure of 3-4 bars (45-60 lbs psi) to which a small adjustment may have sometimes been made for frictional loss and pressure head. Actual firefighting flow-rates were in fact far lower than had been previously thought. - Ref: SRDB Codes of the period.

Interestingly, similar research has been carried out by other fire departments, notably San Francisco, Los Angeles and Chicago, who proposed that a safe and practical baseline flow for a workable firefighting hand-line would be around 550 lpm (150 gpm). More recently (1996), the City of St. Petersburg in Florida USA have established that, for their purposes, the ideal baseline flow is around 600 lpm (160 gpm) using a 7/8" (22mm) nozzle with a 50 lbs psi nozzle pressure on a 45mm (1 3/4") hose-line. This set-up will create an acceptable reaction force of 266N (60 lbf) and offers a hose-line that is easily advanced and maneuvered for interior position. 

However, the change to combination fog/straight-stream or automatic nozzles brings a demand for higher nozzle pressures to achieve similar flows and with that comes an increased reaction force. A baseline flow of 600 lpm (160 gpm) being discharged from a combination/automatic type nozzle operating at 7 bars (100 psi) NP will produce a reaction force of 356N (80 lbs lbf) which would cause a two-man team to struggle with any workable advance of the line. There are combination/automatic nozzles available that have been adjusted to provide rated flows at lower nozzle pressures but be sure to test these yourself as manufacturer's 'rated' flows are sometimes unachievable! Top US branded nozzles must meet the stringent demands of NFPA standards and Low-Pressure combination nozzles are able to achieve their rated flow-rates at factory-set nozzle pressures of just 5 bars. This would enable a flow of 600 lpm (160 gpm) to be achieved with a reaction force of just 303N (68 lbs lbf) which is more easily handled and advanced by a two-man team.

The firefighter is able to calculate the amount of nozzle reaction (NR) by resorting to various formulae -

NR (Newtons) = 1.57 x NP x d squared/10 (European Smooth-bore), or;

NR (Newtons) =  0.22563 x lpm x Sq.root of NP (European Combination fog/jet or automatic Nozzles)

These are metric formulae where P = Nozzle Pressure; d = Nozzle Diameter; lpm = Flow in Litres Per Minute and NR is in Newtons.

In the USA different formulae are used as follows -

NR (lbf) = 1.57 x d squared x NP (US Smooth-bore), or;

NR (lbf) = 0.0505 x gpm x square root of NP (US Combination fog/straight or automatic Nozzles)