pfd430

One more addition to my earlier post. Technology has certainly changed since we were required to calculate friction loss as more and more engines are now being supplied with Flow Meters. These devices, when calibrated (annually) provide a pretty accurate indication of the flow through the plumbing and out the nozzle. Pumping to the desired GPM flow is easier, provided the nozzle man on the line gives the MPO some time to adjust the flow by opening up the nozzle fully PRIOR to entering the building. It only takes an extra 5 seconds after bleeding out the line, to set the engine pressure which corresponds to the appropriate desired flow. However you determine the Engine pressure, remember to set the governor or pressure relief valve if you are flowing water from more than one line from the engine.

The answer to your question is dependent upon 3 factors 1. Length of hose - which in the formula is L and usually expressed in 100' lengths - so in your example it is 2 2. The diameter of the hose and the associated coefficient 1-3/4" hose has a coefficient of 15.5 3. The GPM Flow desired - The more water, the higher the friction loss. For your question, let's assume 175 GPM - which is expressed in the formula as Q2 (squared) The formula for friction loss is FL = C x (Q)2 x L Friction loss for your question is expressed below: FL= 15.5 * 2* (1.75)2 FL=15.5 x 2 x 3.06 FL= 94.86 Therefore given that you are using a standard fog nozzle which requires 100 PSI at the tip (some work at 75) the Engine Pressure required to obtain the required 175 GPM Flow would be 195PSI. If you are operating from a hydrant this is not a problem. However if you are working from a tank or draft, remember that the efficiency of a centrifugal pump decreases as pressure goes up. Look at the test plate on your pup panel, at 200 PSI the pumps capacity is reduced 70% so if you have a 1500 GPM pump, at 200 PSI capacity is reduced to 1050 GPM. Similarly if you have a 1000 GPM rated pump, at 200 PSI the capacity is reduced to 700 GPM. Finally - good planning will help reduce the required pressure and increase the Engine Pressure required. Some ideas include: Changing to a Solid Bore Nozzle. These nozzles require only 50 PSI at the tip to deliver the required flow so swapping out nozzles would reduce the Engine pressure required to 150PSI increasing the capacity of the pump to 100%. Changing out the first two lengths of hose to 2-1/2" Diameter hose which has a lower coefficient (2) would also reduce the required engine pressure, and provide the interior companies with the comfort of the 1-3/4" hose and Fog Nozzle. FL = C x (Q)2 x L / 100 For 2-1/2" Hose the Friction Loss would be: 2 x 1.752x 1 or 6 PSI per 100 Feet As we determined above the Friction Loss for 1-3/4" hose is about 48 PSI so by changing out 100' of 1-3/4" for 2-1/2" at the truck would reduce the engine pressure required for the desired flow of 175 GPM to 100 PSI at the nozzle, + 54 PSI friction Loss which would result in an Engine pressure requirement of 154 PSI. Change to solid bore and you can bring it down to 100 PSI. Knowing these options is the difference between a knob turner and puller and a good MPO. I hope that answered your questions. I will come back to this forum if you have any more. One last thing. After 40 years as a volunteer, in a very busy department, I would never expect that for the 1 or 2% of the time knowing these formulas is required the MPO would go through these calculations. But they must be aware that if you needed to add an extra 100' of hose to your example, the engine pressure would be 300 and that becomes very dangerous as you are approaching hose test burst strength and exceeding it with the resulting water hammer resulting from opening and closing the nozzle any other way but VERY slowly which never happens on the fire ground.