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**Fire Ground Hydraulics**

Whether you are using selectable, single, or automatic nozzles, a basic understanding of fire ground hydraulics is imperative. Hydraulic calculations for all of these nozzle types is the same. This section contains formulas to determine different aspects of fire ground hydraulics. More information can be obtained from other sources; IFSTA Fire Streams Manual NFPA Fire Protection Handbook Manufacturer’s technical data

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**Fire Ground Hydraulics?**

175? Pump Pressure? 120? P.S.I.? Loss per 100 ft.? Hose Diameter? Elevation Loss? Device Loss? Fire Ground Hydraulics is one of the most misunderstood sciences in the fire service. However, to develop effective fire streams, it is necessary to understand a few basic concepts. This section presents many of these concepts. Other sources of information have helpful shortcuts in estimating many of the results presented in this section. GPM? 200? 150? Nozzle pressure?

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**Fire Ground Hydraulics**

Single, Selectable Gallonage, & Automatic Pump Discharge Pressure equals Nozzle Pressure + (Total Pressure Loss) PDP = NP+(FL+DL+EL) NP = Nozzle Pressure FL = Friction Loss DL = Device Loss EL = Elevation Loss Determining Pump Discharge Pressure is the same for all nozzle types; Single and selectable gallonage and automatic variable gallonage nozzles. To derive the pump discharge pressure for a particular layout, a few factors need to be known or measured. Nozzle Pressure. Friction Loss in the hoseline. Pressure losses in appliances or plumbing. Pressure loss (or gain) due to changes in elevation.

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**Friction Loss Hoseline Friction Loss FL = CQ2L**

FL = Hoseline friction loss in PSI C = Friction loss coefficient* Q = Flow rate in hundreds of GPM (GPM/100) L = Hose length in hundreds of feet (Feet/100) * Refer to NFPA Fire Protection Handbook Hoseline friction loss can be figured closely using information from the NFPA Fire Protection Handbook. The friction loss coefficient for various hose sizes and construction types are presented in tables in the handbook. In this formula, Q is the flow rate in hundreds of gallons per minute. (example: for 150 GPM flow, Q would equal 1.5) In this formula, L is the length of hose in hundreds of feet. (example: for 150 feet, L would equal 1.5) Refer to reference material for figuring multiple or complex hose lays.

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**Hose Friction Loss (per 100 feet of hose)**

Hose line friction loss is shown on this chart for typical hose sizes. If exact friction loss figures are required, tests can be performed on the actual hose being used, using calibrated gauges and flow measuring instruments.

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**Elevation Pressure Loss Device Loss**

EP = 0.5H 0.5 = A constant H = Height in feet Device Loss Appliance Friction loss in PSI Very simply, elevation pressure loss or gain is a function of gravity. A rule of thumb for elevation pressure loss is a 5 psi loss for every 10 feet of building height (10 feet is approximately one story). The inverse of elevation loss is elevation gain, due to the nozzle being below the level of the discharge outlet. This is a gain of 5 psi for every 10 feet the nozzle is below the discharge. Device loss is the actual friction loss in an appliance such as a gated wye, Siamese, reducer, etc. Preconnect piping can be a big determining factor when figuring pressure losses. Older apparatus and rear discharges may be plumbed with small diameter piping.

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**Calculating for Pump Discharge Pressure Selectable or Single Gallonage or Automatic**

125 gpm at 100 psi NP on 150 ft. of 1¾” hose PDP = NP+TPL TPL = FL = CQ2L TPL = (15.5)(1.25)2(1.5) TPL = 36 PDP = PDP = 136 In this example, we calculate the pump discharge pressure. The nozzle pressure is 100 psi. The flow rate is 125 GPM. The coefficient for 1¾” hose is 15.5 In this example, there is no elevation loss or appliance loss, therefore total pressure loss is simply the friction loss in the hoseline. As you can see, the friction loss is 36 psi. Add 36 to the nozzle pressure of 100 psi Pump Discharge Pressure is 136 psi.

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**Calculating for Pump Discharge Pressure Selectable (Change Flow Setting) or Automatic**

150 gpm at 100 psi NP on 150 ft. of 1¾” hose PDP = NP+TPL TPL = FL = CQ2L TPL = (15.5)(1.5)2(1.5) TPL = 52 PDP = PDP = 152 In this example, we calculate the pump discharge pressure for the same layout, only we change the flow to 150 gpm. The nozzle pressure still needs to be 100 psi. The flow rate is 150 GPM. The coefficient for 1¾” hose is 15.5 In this example, there is no elevation loss or appliance loss, therefore total pressure loss is simply the friction loss in the hoseline. As you can see, the friction loss is 52 psi. Add 52 to the nozzle pressure of 100 psi Pump Discharge Pressure is 152 psi. Note: If the pump discharge pressure is not changed for the selectable, the flow would no longer be 150 gpm. The automatic nozzle would maintain 100 psi, so this calculation becomes simple.

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**Calculating for Pump Discharge Pressure Master Stream 1½” Smooth Bore Tip**

psi NP on 300 ft. of 3” hose PDP = NP+TPL TPL = FL+DL TPL = CQ2L+Device Loss FL = (.677)(5.98)2(3) = 73 DL = 25 TPL = = 98 PDP = 80+98 PDP = 178 This example calculates Pump Discharge Pressure for a master stream, 1½” smooth bore tip. Flow is psi Nozzle Pressure. For this example, Total Pressure Loss will include the Friction Loss of the hoseline and Device Loss of the master stream device (monitor, deck gun, deluge gun, etc.) Coefficient of 3” hose, coupled 2½” is .677 Device Loss is 25 psi. This make the Total Pressure Loss at 98. Add 98 to the Nozzle Pressure, 80. The pump discharge pressure is 178 psi.

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**Hoseline Friction Loss**

Coefficients from NFPA Fire Protection Handbook Manufacture, construction, age & condition of hose are determining factors Actual coefficients of friction can be determined Test your hose for actual coefficients Refer to IFSTA Fire Streams Manual for procedures The coefficient can be found in the NFPA Fire Protection Handbook or the IFSTA Fire Streams Manual. Manufacture, construction, age, and condition of hose can effect actual friction loss. Actual friction loss can be determined by measuring the loss in hose line being used. The IFSTA Fire Streams Manual is a valuable source for more information.

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**Single & Selectable Nozzles**

Pump pressure charts can be made for any hose lay Hydraulic calculations can be pre-planned Communication is key to safe operations When operated at correct pressures, they will deliver the stated flow For multiple lines of different size, flow &/or length, pump to the maximum required pressure & gate the other discharge valves to correct pressure for each line Hydraulic calculations can be pre-determined for standard hose lays and nozzles used. Pump Pressure charts are a valuable tool and should be accessible to the pump operator. If operated according to hydraulics calculations, single and selectable nozzle will deliver stated flow. Complex multiple hoselays of different sizes, flows, lengths, etc, should be pumped to the highest needed Pump Discharge Pressure and other discharges gated to desired pressure.

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**Automatic Nozzles Hydraulic calculations are simpler**

Nozzle pressure remains constant (typically 100psi) If calculations are incorrect, automatic nozzles will deliver the available flow Maximum reach with available water As nozzles are throttled, pump operator should avoid “chasing” the pressure gauge The goal should be to maintain the minimum pressure necessary for the line When using automatic nozzles, hydraulic calculations are simpler because the nozzle pressure remains constant. If hydraulic calculations are not correct, automatic nozzles will deliver the available flow with effective stream reach of the flow available. The pump operator should avoid “chasing” the pump pressure, as nozzles are throttled. The pump operator’s goal should be to maintain the minimum pressure necessary for the line(s).

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