How to calculate and overcome friction loss
There are two ways to calculate friction loss: the theoretical method or the fireground method — here's the fireground method
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.
One of the pump operator’s critical tasks is determining what pump discharge pressure to set for each supply or hand line. To determine this, the pump operator must first know the total gallons per minute flow, that is, the desired result on the working end of the hose.
The type of nozzle being used — smooth-bore, automatic nozzle or adjustable gallonage — will determine the gpm. Once the pump operator knows what the desired gpm is, then they must know what size hoses are being used, the lengths of the hoses and any appliances that are part of the hose layout, such as a gated wye.
Armed with that information, the pump operator can then calculate the friction loss, the remaining ingredient for getting the right mixture to their firefighting colleagues.
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.
In its fire service application, the friction is water sliding against the interior surfaces of the pump, any connected appliances — gated wyes, manifolds or a water thief — standpipes and fire hose.
Pump operators should commit these five friction loss principles to memory.
- All fire hose has friction loss.
- There is no set friction loss for any one size of hose.
- 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.
- 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.
- It’s not psi that puts the fire out; it’s providing the correct gpm that the incident commander needs.
Getting comfortable with friction loss
Truly understanding friction loss is critical for all pump operators, and it’s frequently presented to new pump operators as something akin to understanding quantum physics. In reality, understanding friction loss and its place in properly supplying hose lines and fire streams is not that daunting of a task.
The basic challenge for the pump operator is to develop the proper pump discharge pressure necessary to overcome the friction loss in a fireground set up to ensure that the firefighter on the nozzle will have the appropriate amount of water to suppress the fire.
There are two ways to calculate friction loss: the theoretical method or the fireground method. The former uses mathematical equations, while the latter uses rules of thumb that don’t require much in the way of mathematical calculations.
Theoretical calculations are generally best used for pre-fire planning, developing specifications for pumping apparatus and calculating problems ahead of time, such as creating pump charts. Theoretical calculations are typically not an efficient means of calculating friction loss on fire scenes.
Many times, instructors teaching pump operations bring out the theoretical method, along with its equations, at the beginning of the training process. It’s their mistaken belief that knowing how the watch is constructed is important for the watch owner to know before they learn how to tell time using the watch.
I was very fortunate at the beginning of my firefighting career to join a department where that was not the case.
My pump operator instructors focused early on developing my skills in using such tools as the hand method for calculating friction loss. They also emphasized memorizing the friction loss for the pre-connected hand lines and typical hose layouts that I would encounter as a pump operator.
A hose configuration that would be employed at an apartment or condominium complex, for example, was 200 feet of 3-inch supply hose, connected to a gated-wye, supplying two 13/4-inch fire attack lines of 200 feet each with a 100-psi adjustable fog nozzle on each line set for 150 gpm.
The hand method
This is an extremely valuable tool for both learning and teaching what the friction loss is for various sizes of hose and various gpm flows. Below is one example of a hand method for calculating friction loss in various sizes of hose.
Using the hand method, for each 100-foot length of 1¾-inch hose flowing 200 gpm, the friction loss is 48 psi: 2 x 4 x 6 = 48 psi. For a 3-inch supply line flowing 300 gpm, the friction loss per 100-foot section would be 9 psi: 3 squared equals 9 psi.
Memorizing this much simpler than it sounds. Just memorize the friction loss for the gallonage settings on the nozzles used in your department. Most 100-psi adjustable fog nozzles for use with 1¾-inch hose have four settings: 95 gpm, 125 gpm, 150 gpm and 200 gpm.
All you must remember is the friction loss for each, 14, 24, 35 and 62, respectively. Those are the amounts of friction loss per 100 feet of hose based on the gallonages above.
Thus, the friction loss per 100 feet of 1¾-inch hose when flowing 95 gpm is 14 psi; for a 200-foot line, your friction loss is 28. It works the same way for the other gallonage settings.
Change the flow setting on the nozzle to 150 gpm on the same 100-foot line and the friction loss is 35, if you increase the hose length to 200 feet, the friction loss becomes 62 psi.
Don’t forget, this is just the friction loss in the hose; it is not the pressure that you must generate from the pump at the engine. To figure the required discharge pressure, add up all friction loss — in the hose and any appliances — plus the required nozzle pressure.
Thus, for that apartment complex layout I mentioned earlier, the calculation would look like this:
Know your friction loss points
Do your homework about the different hose loads, appliances and nozzles used in your department and what each contributes to friction loss, that is, what are the friction loss points. A best practice employed by many skilled pump operators is creating a cheat sheet containing what those friction loss points look like.
Source: University of Alaska, Fairbanks. The Physics of Fireground Hydraulics. April 2011. [Available on-line]
My fire service colleague with the Henrico County (Va.) Division of Fire, Taylor Goodman, offers another perspective on friction loss. There is friction loss within the internal plumbing of the fire apparatus itself.
“Even pumpers ordered from the same manufacturer using the same spec can have differences in their internal plumbing,” said Goodman, “resulting in different required PDPs [pump discharge pressures] from one engine to another.”
Per Goodman, the only way to truly determine an accurate PDP for a specific pumper is to test each discharge with a flow meter. Time consuming, for sure, but more accurate.
Goodman also said that some of the nation’s leaders in fire streams and their applications, such as Dennis LeGear, have determined that there is no common national standard in determining the internal diameter between manufacturers for different diameter attack lines.
“This creates several issues when you teach a nationally accepted method for determining friction loss, then put a flow meter on it and find that you're actually 20 pounds off,” said Goodman.
According to Goodman, there's a grassroots effort to update the standards, as well as working with the manufacturers, to bring some consistency to those internal diameters and anticipated friction loss within the internal plumbing of fire apparatus.