Real World Hydraulics

By Kirk Allen

What is Real World? Some consider it as everything that happens outside the classroom. Others use it as an excuse to ignore the classroom. I’m sure you have heard this before: “Going to that class is a waste of time because what happens in the “real world” is not the same as what happens in the classroom.” Having been involved in both “real world” testing and classroom instruction, I am confident that the key to fixing many of our challenges in the fire service is to question what we were told and to demand proof that the information is based on fact and not just opinion.


Engine Company: You’re an Engineer; Now What?

Friction Loss Rules of Thumb

Every Pump Operator’s Basic Equation

The most basic hydraulic challenges in this country continue to be addressed the same way today as they were 50 to 100 years ago. A simple thumbs-up or thumbs-down gesture 99 percent of the time signals whether the flow is good, according to my surveys, even though we teach numerous mathematical formulas for accomplishing desired flows. Why do we use our thumb instead of applying the mathematical formulas when setting our desired flows?  Are the formulas that bad?  Will our thumb hold up in court? Is there a better way?

Desired Flow

This article defines “desired flow” and identifies some of the most common problems that can interfere with maintaining this flow. The goal on the “real world” fireground is to apply the desired flow to the fire. If we do that, we should win the battle quickly. However, often, we do not get the fire out, and we attribute it to a long list of reasons. But, what we are finding is that the main reason is that the “desired flow” may not have been the desired flow at all–it was never really established.  

We teach day in and day about friction loss for this or that length of hose, appliance, nozzle, and so on. Unfortunately, no one is asking the most important question that impacts every aspect of that teaching: “for what flow?” It’s all about application rate or gallons per minute (gpm). Knowing all the friction loss information in the world will do us no good if we don’t know the flow. As flow goes up, so does friction loss, yet the very equation we are being taught has no reference to gpm:

Desired Pressure = Friction Loss + Nozzle Pressure + (plus or minus for elevation + appliance). But, for what flow rate?

Key Questions

“Says who?  and “With what proof?” –two questions we have learned over time may well be the key to getting to the facts instead of settling for opinions. Just because a person is standing in the front of a room teaching you does not mean that person is always right. We must challenge the teaching. Let’s assume the instructor lists the following basic hydraulic conditions. How would the student verify that condition by obtaining proof?

• The discharge is open. Student: Is the discharge really open just because you pulled the handle out?

• The pressure is 100 pounds per square inch (psi). Student: Is the pressure gauge really 100 psi just because the needle is on 100?

• The friction loss (FL) is XX per 100 feet. Student: Is the FL really XX in that 100 feet of hose? How do you know?

• Each section of hose is 50 feet long. Student: Is the hose really 50 feet?

• The friction loss in that appliance is 10 psi. Student: Is the FL really 10 psi for that appliance?

Discussion: Did the fire kick your tail all night because it was big and hot, or did the fire get big and hot because you did not get on the fire the water flow you thought you were providing? How many after-incident critiques have addressed the simple question, “What were we flowing at that fire?” Determining that answer may well point to the real reason you spent all night at the fire.  We have allowed ourselves to live up to the name of “firefighter” instead of performing as a fire “suppressor.” It may sound corny, but think about what we are really trying to accomplish.

Fire suppression is our goal. This is accomplished through heat absorption as it relates to our flow rates, yet the vast majority of our teaching has been about pressure, friction loss, elevation, and the like, none of which absorb heat!  During measured flow testing, we must establish not only a desired flow rate but also identify problems in the system that make getting that flow rate difficult, if not impossible.

We pull a lever and see water come out the end of the nozzle, and we “assume” our discharge valve is open. Have you ever opened the valve and looked inside the waterway to see if it actually was open all the way? (Please don’t do that with the pump engaged.J)  Depending on the materials of the valve and the linkage, wear may be preventing the valve from opening all the way, thus restricting the flow.

We read a pressure gauge on the panel and assume it’s accurate. How often do you check those gauges? Simply capping the discharges a couple of times a year and opening all of them can tell you if you have a gauge problem. Bring the master to 150 psi and look at all the gauges to see if they, too, are reading the same pressure. We have found gauges that were as much as 30 psi off; yet, without proper testing, we had no idea they were off that much.

Fire academies across the country are teaching and drilling into students’ heads a test answer of friction loss per 100 feet of hose. Is anyone asking, “For What Flow?” or “For which model of hose?” Some of the “experts” hold that it’s only a ballpark figure that will get you close.  

Fifteen brands of hose necessitated 12 different pressures to get the same flow rate. Was it close?  Those pressures ranged from as low as 20 psi per 100 feet up to 64 psi for a flow rate of 180 gpm. So, the next time you’re told it gets you in the ballpark, make the person prove it with the equipment you use!   

We open a box of hose, pressure test it, and now even flow test it and then put it on the engine and call it good. We are excited that we finally spent some money and got new hose. You pay for it by the foot. When was the last time you pulled out a tape measure and checked to see what the real length was?  We have opened brand new boxes of hose that were to be 50 feet long; in reality, they were only 47 feet in length.  Amazingly, we have had chiefs share stories of hose lengths being as short as 38 feet and being charged for 50 feet. Measure your hose!

“Ten psi per appliance!” For what flow rate?  A standard 2½-inch to 1½-inch reducer can have as much as 40 psi of friction loss depending on the flow rate.  Measure it.


A Quantitative Approach to Selecting Nozzle Flow Rate and Stream, Part 1 | Part 2


Negligence is one of the greatest sources of civil litigation. Can you place your hand on the Bible before a judge and jury and tell them with any confidence what your flow rates were the night of the fire where lives were lost? All too often, we send our brothers and sisters into battle to slay the dragon without being able to state with confidence what gpm we were flowing, let alone flowing what the standards say we should be flowing.

Most know how many miles per gallon they get in their vehicle or understand the importance of balancing their checkbook, yet ask them what their flow rate was at the last fire they went to, and you get the look of a deer caught in the headlights.


Kirk Allen has served eight years as the Kansas (IL) Fire Protection District chief and is an active EMT. Having spent a career in the U.S. Air Force, he acquired a background in aircraft propulsion, is an FAA-licensed air frame and power plant technician, and gained extensive knowledge in aircraft crash rescue. He has served 23-plus years in and with volunteer and career fire departments in the areas of hazmat and fireground hydraulics. He has been a hydraulics instructor for the past 23 years.

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