Friction is the force resisting the relative motion of solid surfaces, fluid layers and material elements sliding against each other. Friction loss is the pressure loss due to the friction. Distance, diameter, and the GPM / volume, all affect friction loss.

In firefighting, friction between the water and the inside surface of the pump, connected appliances and fire hose all cause turbulence. This turbulence reduces the energy produced by the pump (PSI). The higher the GPM/LPM passing through a hose or pipe, the more turbulence and friction loss will result.

As a result of the friction loss, the PSI at the working end of the hose will be lower than at the pump – which thereby lowers the water output fighting the fire. The type of nozzle being used — smooth-bore, automatic nozzle or adjustable gallonage — will determine the working PSI required at the tip to hit the desired flow, anywhere from 50 - 200 PSI. Not accounting for friction loss will result in lower working flow and impact hose performance.

Let us direct you to a great write up by FireRescue 1 on friction loss:

"One of the pump operator’s primary goals is to provide the fire attack crew with the necessary water flow to get their job done. There are plenty of things that the pump operator must get done in the early stages of a fire, and none are more important than this...

Pump operators should commit these five friction loss principles to memory:

1. All fire hose has friction loss.
2. There is no set friction loss for any one size of hose.
3. Friction loss is determined by the GPM flowing, the size of the hose and the length of the hose. All three factors must be considered.
4. Most of the fog nozzles have flow rates ranging from 95 to 200 GPM. Don’t get stuck on one flow rate; the fire will dictate which flow rate to use.
5. It’s not PSI that puts the fire out; it’s providing the correct GPM that the incident commander needs."

We're here to make your job easier by calculating the friction loss on Mercedes Textiles hose here. 

We use an exclusive weave process for our Mertex® inner hose linings, developed exclusively for the manufacturing of our fire hose. We weave a molten lining material directly into the fabric jacket at the loom, which offers a smoother waterway — free from adhesives, glues or other bonding agents — that yields greater water flow at the discharge end. We also manufacture our own couplings, which allows us to engineer them specifically to minimize turbulence and friction loss.