Fire Engine Academy

Calculating Pump Discharge Pressure

Fire Engine Pump Discharge Pressure (PDP) is the pressure set at the pump to achieve the desired pressure at the nozzle furthest from the pump (while considering the change in elevation and friction loss). Having a good understanding of pump discharge pressure is essential to becoming an effective fire engine operator. Learning these principals will help you with much more than calculating the correct pressure at the nozzle. Understanding Pump Discharge Pressure will also help you gain a big picture understanding of fireline safety and hydrodynamics (fluids in motion).

In our example we will use a wildland firefighting example. All the calculations work the same for structure fires, vehicle fires and wildfires.


Definitions:

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Calculating Pump Discharge Pressure

Simple Pump Discharge Pressure Example:

Your crew has just deployed 3 Hose Packs on a small brush fire. Each hose pack has one 100 ft 1 ½” supply line, one 100 ft 1” attack line, one 1 ½” Gated Wye, one 1 ½” to 1" reducer, and one 10-20 GPM combination nozzle. Assume that all nozzles are flowing at 20 GPM and the fire is burning atop a small hill (40 feet above the fire engine).

Objective: Determine the proper Pump Discharge Pressure for nozzle with the highest friction loss.


Step 1 - Draw a picture of the hoselay

Step 1 Determining Pump Discharge Pressure
Step 2 - Add PDP formula to drawing

NP +/- H + FL + A = PDP Provide enough space on the right side of formula to enter numbers and perform calculations.

Step 2 determining Pump Discharge Pressure
Step 3 - Enter nozzle pressure & GPM used by each nozzle

Add 100 psi as NP in the Pump Discharge Pressure formula. Most combination nozzles are rated at 100 psi. Use 100 psi for this problem.

20 GPM for each open nozzle on the hose-lay

Step 3 in determingin Pump Discharge Pressure
Step 4 - Determine the GPM in each hose

Use the picture to determine the GPM passing through each section of hose, enter it beside each section of hose, and underline that number.

Step 4 in determining Pump Discharge Pressure


Step 5 - Calculate the friction loss for each hose

Use a Friction Loss Calculator to determine the Friction loss for each section of hose. Enter the friction loss calculation to the right each hose's GPM.

We have provided a small Friction Loss Cheat Sheet or compare friction loss calculators

Step 5 in determining Pump Discharge Pressure Friction loss cheat sheet
Step 6 - Calculate the friction loss total

Find the path from the fire engine to the nozzle with the highest friction loss. Circle each of the PSI friction loss values for every hose in that path. Add up those values (19.4 psi) and enter the sum as A in the Pump Discharge Pressure formula.

Step 6 in determining Pump Discharge Pressure Friction loss cheat sheet
Step 7 - Appliance Friction Loss.

Each organization has their own procedures for determining Friction Loss. I am not going to tell you what is right or wrong on this example. Ask your trainer or supervisor how they want you calculate hardware friction loss. Add each appliance's friciton loss in the path PDP you are calculating and enter appliance friciton loss as "A" in the PDP formula.

Keep in mind, hose-lay hardware usually accounts for a very small percentage of friction loss on a hose-lay and no two types of hardware have the same friction loss. Many organizations chose to leave hardware out of the Pump Discharge Pressure calculations (which is easiest). Others will sometimes add 5psi loss for each appliance (gated wye & reducer or inline t) the water passes through. You can learn more about Hardware Friction Loss.


Step 8 - Head Pressure & Calculate Total.

Determine the elevation gain or loss (in feet) for the last nozzle in the selected (circled) path If there is a gain in elevation, multiply by 0.5 and add the result as (+) H (20 psi) in the Pump Discharge Pressure formula If there is a drop in elevation, multiply by 0.5 and subtract the result as (-) H in the Pump Discharge Pressure formula

Add NP, H, FL, and A to get the Pump Discharge Pressure (139.4 psi). Enter 139.4 into your formula.

Step 7 in determining Pump Discharge Pressure

Advanced Topics & Additional Considerations

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Methods used to determining friction loss calculations:

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