Fireground hydraulics has been around all of my life(pushing 60). Just like CPR, it has changed through the years.  My question is , do you really use it.  I was an engineer for 28 years,I understood how the use the formula, and never did use it on a fire.  Reason being, in the heat of the battle and without a calculator it was impossible to use.  Besides that without the GPM part of the equation it can't be used .  I found that a pump chart designed from actual flow tests worked bet.  Why does it still exist? Teachers of this math even tell you that you will never use it.  Don't we have enough to learn in this job. What do you think?

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OK Now that I have clarified my view on fireground hydraulics which to me meant formulas only, I would like to post a friendly challenge. Using the fireground hydraulics formulas tell us how you would come up with the correct PDP for the following hose evolutions.

1. A 25 story building with a building pump flowing multiple handlines .
2. A fixed master stream(deck gun) with a 1.75 inch tipflowing 800 gpm.
3. A relay pump operation using 5" hose aproximatley 1200 feet long with an unknown flow requirement. The reason why the flow is unknown is because the relay just started and the flow requirements are growing with the fire demand.
wow, thats a lot of math. we just use psi scale and don't worry about the actual GPM flow. Our rigs are calibrated often so when we give the designated psi we assume we're gettin the proper flow.
50psi for 1 3/4" nozzles 20psi/ length friction loss and 5 psi/floor above the 1st. 6 lengths of 1 3/4" hose w/ solid bore nozzle we give 170psi
2 1/2" hose 40psi for nozzle 5psi/length for friction loss and 5psi/floor above the 1st.
i took the time to read everyones response and the dominant statements seems to be that delivery of water at the proper pressure seems to be the concern vs. knowing the science and math of it. i agree with those that say if you can put the pump in gear and get the water to where it needs to go at the proper pressure your're doing ok. but using my department as an example, becomming a "state certified engineer" and "qualifed pump opeator on all department appratus=ENGINEER" is a big deal for some reason. i have never understood why it was necesary for me to "test & certify" on every truck in the department or else i could not operate it. i mean, THERE IS NO DIFFERENCE BETWEEN THE PUMPS, THEY ARE ALL THE SAME! and we use them the same. so a driving test i could understand but proving i can operate a single stage on engine A the single stage on engine B and the same for engines C-D&E seemed like a waste of time espically when engines C&D were exactly the same. i guess the TO needed to justify his job.

learing the haydaulics has its place but those that have talked about cheet sheets, and SOP's have it right in my opinion, i do think that the subject of hydraulics has gone a little bit too far and had become too complicated and i do admit that in my carrear i used it very little except when gaugeing supply options. in those cases i used my expierence of knowing the nuances of the truck i was operating and what its minium/maxium capabilities were.

i have learned some tricks to maximize water supply (such as using all the ports from a hydrant to all connections on the intakes for example) {thats one 5"- five' pony and 2- 50' 3" to the intakes}. i was taught that by a guy that KNEW NOTHING of hydraulics but knew the pump. the first time i used that to supply a LDH (off a blue top hydrant) which was supplying a manifold which was 1000 ft away from the fire. the bossess were suprised that one 1500gpm pumper was able to do that. they looked at my connection and said that my hookup was not SOP i asked...why do you care? you got water, it was at the proper pressure with some to spare. i asked what i was suppying? they told me one engine using a deck gun with a 1'1/2 tip, and a tower ladder who had the stick working and 2 1-3/4 hand lines going. i was suprised too BUT i was not worried about the friction loss or the associated math, i dropped a 5' from the scene to the hydrant and didnt ask questions when they told me to charge it or raise or lower the pressure and that was that.

this was a great topic thank you for posting it Paul. i believe this will provoke more on this subject and i'm an "semi" old firegeezer too
OK, I see your point, Most of these would be solved with estimates of the formulas and rules in real life but I always try to know the rules before I make adjustments to them.

1. I think I read it to be I'm pumping into a FDC to supply a fixed fire pump? What kind of nozzles (NP = 50 or 100?) Standpipe FL, ROT = 25, ROT. FL due to El = 5 per floor, ROT = 50# Hose size and length? 100' of 2 1/2" @200 GPM FL = 8# (only figured once) How long is my lay into the FDC? two 100' lengths of 3" FL = 4# (only figured once) PDP = 137 psi? 140 for government work. Reality, pump to maintain 50 psi at the FDC.

2. Rule of Thumb (ROT) NP = 80 plus FL in plumbing (lots of variabliity there) 25 - 50# = PDP 105 - 130 psi. real world = Look at the flow meter

3. That's why we use 5" hose, to avoid formulas, start at 10 psi FL per 100 feet with at least 20# residual = PDP of 140# Don't exceed 150 psi. Engineers have to communicate to adjust as demand changes.

FYI, I'm a truckie at heart so I might have missed something. I didn't look up the formulas either.
Here is another viewpoint on 20psi residual used in any formula.

20 psi residual is the fire services general rule of thumb... textbooks haven't caught up to the technology used in today's apparatus though.

If you are using a newer Class 1 Electronically Controlled Pump Throttle and Pressure Relief system, you need to use (40) psi rule of thumb for residual, when you are operating in the PRESSURE MODE. In this mode, it is the only time you are afforded a pressure relief valve safety feature. The computer will calculate pressures and anticipate potential cavitation sometimes before 20psi and idle down the engine and pump even though you are no where near actual cavitation.

If you are using a Class 1 pump's computer in the RPM MODE, you are afforded no pressure relief protection, and you can pull the residual pressure down to (0) actual cavitation. I find in my travels many operators do not fully understand the features of the computer controlled system.
Maybe this is a good reason to understand a little bit about the calculations. In order to get to 40 psi residual in the last situation, if the demand is 1,000 gpm or more, you might have to be go over the 150 psi PDP rule for LDH. If you have manifolds in the equation, almost for sure. Do you push the limits or call for additional resources? Do you let the other engine know so they can change modes? (We almost always use Pressure mode and probably don't spend enough time explaining when to use RPM.) Just a thought.
Steve when I did our last LDH drill (we only have 4" supply line) I quickly showed that you can simulate a cavitation and pump shut down in that mode. If you are trying to supply 1000 gpm for flow we may need to lay dual 4" supply lines (depending on the distance) to not overwork the PDP. We took a pumper/tanker with 2400' of supply line and we were able to spilt load it (1200') the same way we did back in the dual 2.5" days.

Another consideration with the new computers is with a poor hydrant (low gpm and psi) as we can easily overun that residual and can quickly shutdown your attack lines when running in the wrong mode.

I have seen a pump run as set, idle down on it's own and the operator has no idea what is happening because they had residual... Now in the rpm mode, it is old school without a pressure relief (pump it until total cavitation)
We'll have to play with that next time we do pump training. I guess there is still a place for Ross Valves!
While the debate rages on, Mother Nature is heard screaming in the background.......I don't care what nozzle you use.....I don't care what size line you have....I don't care about the chrome on your rig.....just show me the gallons per second you are going to apply into the rockin' and rollin' fire space. It will be different for me, it will be different for you. Standpipe pressure? What's my engineering pumping up to me? Staffing? Tactics of Fire Stream Management? Hose sizes and lengths? Combination Nozzles or a piece of metal with a hole in it? 100 psi, 75 psi, or 55 psi? All these questions...I'm so confused! But at the end of the day...after we are all done arguing about these most tactical issues.....I still hear Mother Nature screaming....gimmie more.
You have no idea for SURE what you are flowing, regardless of what formulas you use, until you actually take the time, and evaluate your apparatus (different models, makes, plumbing configurations), your hose (mix-matched, or standard...it DOES make a difference)and your nozzles using your standard lay-outs and minimal/maximal distances using a calibrated digital flow meter.

Otherwise it is only mathematical speculation.

BUT if your not paying attention to FLOW (gpm)and going after the fire based on the desire to actually kill it, rather than standing face to face and engaging in hostile battle with some hose-line that is marginally effective, your wasting your time, and endangering your crews.

It's all so simple, yet seems to confuse so many. That 1.75" is NOT meant to be the only water spewing tool we have.

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