# Thread: Help with engine pressure

1. ## Help with engine pressure

Hi, I'm new to the forums here and I was wondering if anyone could help. I am a fire science major in college and I have a paper to fill out for engine pressure. I am usually all right with these equations, but this one just confuses me.

There are 2 engines. Engine #1 has a 300ft. 3" hose with a 1 3/8" solid stream tip nozzle. The nozzle is supposed to be used on a building's 5th floor.
Engine #2 is attached to the previous engine by a 600ft. 4" hose and also connected to a fire hydrant for water supply.

Can anyone help me with these? I am very overwhelmed by this equation.

2. Figure the engine pressure for the attack engine.

1) Find the gpm for the 1 3/8 nozzle at 50 psi, if it is a handline, 80 psi if it is a deluge gun.

2) Figure the friction loss and elevation loss for the 3 inch hose. remember not to count the first floor for the elevation loss.

3) Since you figured out the gpm's above you can determine the gpm's you need to flow in the 4 inch to the attack engine. Find the friction loss for that flow in the 4 inch and add your 20 psi residual to it. That is your supply engine's engine pressure.

I hope this helps. I prefer to give this kind of help than to give you the answer.

3. 1. I would say that a 1 3/8" smooth bore with 3" hose behind it must be considered a master stream....so 80 PSI at the tip.

2. You can use Freeman's Formula or the Rule of Eights to figure out the total GPM coming out of the fire attack line. You need the GPM to figure out friction loss, which for 3" hose is quanity squared ([GPM/100]squared) per hundred feet.

3. Engine Pressue = Nozzle Pressure (80) + Friction Loss + Elevation (we're taught 5 PSI per floor)

4. Supply Engine Pressure = Residual (we're taught 20 PSI for 4" and 50 PSI for 3" + Friction Loss for 4" ([GPM/100]squared divided by 5)

4. Firemedic515;1123907]

1. I would say that a 1 3/8" smooth bore with 3" hose behind it must be considered a master stream....so 80 PSI at the tip.

Not necessarily. Big Paulies has been advocating nozzles that big for large flows on handlines for years. The original poster needs to find out of this is a master stream or not and if it is he needs to add in friction loss for that deluge set to the formula.

2. You can use Freeman's Formula or the Rule of Eights to figure out the total GPM coming out of the fire attack line. You need the GPM to figure out friction loss, which for 3" hose is quanity squared ([GPM/100]squared) per hundred feet.

I would assume that he has formulas to use since he mentioned he is usually pretty good at the friction loss problems. I understood his question to mean more he had no idea how to set up the problem, not that he couldn't do the math.

3. Engine Pressue = Nozzle Pressure (80) + Friction Loss + Elevation (we're taught 5 PSI per floor)

You forgot the deluge set if you are assuming it is in fact a master stream.

4. Supply Engine Pressure = Residual (we're taught 20 PSI for 4" and 50 PSI for 3" + Friction Loss for 4" ([GPM/100]squared divided by 5)
Why is it necessary to have more residual for the 3 inch than the 4 inch? All the residual pressure is is a safety factor so that you don't collapse the supply line.[/QUOTE]

We mostly agree on how to accomplish this problem.

5. Just get CAFS...Problem Resolved...

6. Originally Posted by FireRescue61
Just get CAFS...Problem Resolved...
Um, no. If you seriously believe that you don't have to know anything about pumping just because you use CAFS all I can do is shake my head at that.

7. Originally Posted by FyredUp
Um, no. If you seriously believe that you don't have to know anything about pumping just because you use CAFS all I can do is shake my head at that.
That's why I put a wink at the top of the page! Just joking...

8. And as far as I understand there is is zero friction loss with a CAFS.

9. Thanks for the help guys, I appreciate it. At first I didn't know where to start with this particular setup, but now that it's broken down I think I can do it.

10. Originally Posted by FireRescue61
And as far as I understand there is is zero friction loss with a CAFS.
There is NEVER zero friction loss when something is moving through any kind of conduit, including air.

I missed the wink...sorry.

11. Originally Posted by Fire_Mike1991
Thanks for the help guys, I appreciate it. At first I didn't know where to start with this particular setup, but now that it's broken down I think I can do it.
Mike,

It is essentially 2 seperate problems. If you figure it that way it becomes much easier.

12. Originally Posted by Fire_Mike1991
Thanks for the help guys, I appreciate it. At first I didn't know where to start with this particular setup, but now that it's broken down I think I can do it.

for future reference, in any sort of relay or multi pumper hydraulic problem you need to solve; start with the Engine at the end of the line applying water to fire (or multiple engines if your instructor is cruel). Once you figure out the flow of the engine applying water to fire, you know what all then know what each engine in the supply chain is flowing so you can do the rest of the calculations.

13. ## Begin at the fire

Fire Mike: If you begin at the fire, it is generally the way to proceed. Nozzle flow will be specified if it is a fog or an automatic. You must calculate nozzle volume if a solid bore, and as mentioned above before calculating you must decide or know if it is a handline or a master stream. Your stated problem calls for a 1 3/8" Solid bore nozzle. Estimating reaction force for this SS nozzle = 1.5 d * d * Np. It will rapidly become apparent that this size nozzle will need at least 3 FF's and then it is marginal for controlling the line by hand. Its designation (hand line or Master stream) needs to be clarified by the instructor. For master streams, there is no reason for doing the calculation or even to have a flow chart. I use the following crutch for help. Most master streams start with a 1 1/4" tip and the #4 suggests to me 400 gpm. From there it is easy. Each 1/4 inch increase approximates an additional 200 gpm. 400, 600, 800, & 1,000 for a set of stacked tips of 1 1/4", 1 1/2", 1 3/4" & 2". Since your question dealt with 1 3/8" it is 500 gpm or half way between 1 1/4" and 1 1/2".

14. ## Collapsing a supply line

Fired up: Actually you can't collapse a supply that is being fed by a pressureized source. If you throttle up the attack engine until the pump begins to cavitate (near zero absolute in the impeller eye) the supply hose will constrict until there is 14.7 psi of friction loss due to turbulence and increased velocity in the narrowed portion (last foot) of the supply line connected to the attack engine. You can't suck this shut.!!! The problem becomes the high velocity trying to tear the lining out of the hose due to vibration and water velocity. If this occurs, you will violently slam the ripped out lining into the intake screen, and may even drive the screen right up into the eye of the impeller. Not a good result, especially if firefighters are doing an interior attack. Try it some time on a drill. If you have reached cavitation at draft, you can take a portable pump and run water into the intake opposite the suction sleeve and get additional water even though the engine is working far below atmospheric pressure. The line from the PP will not collapse.

15. Rule of thumb in my "world".Look at the guy on the end of the tip.Thumb up, more pressure.Thumb down, back it off a little.Arms crossed,shut it down.We had an engineer that knew "all that slide rule stuff" and confused us poor 'ol vollunteers.As for collapsing a pressurized supply line,you just gotta feel your hose.You know,it gets soft.

16. Originally Posted by fyrfytr240
Rule of thumb in my "world".Look at the guy on the end of the tip.Thumb up, more pressure.Thumb down, back it off a little.Arms crossed,shut it down.We had an engineer that knew "all that slide rule stuff" and confused us poor 'ol vollunteers.As for collapsing a pressurized supply line,you just gotta feel your hose.You know,it gets soft.
Pretty good system for yard breathers, kinda tough to sees the thumbs through smoke. Couple this logic with some 100 psi automatic fog nozzles and sooner or later you "poor 'ol vollunteers" will be standing around Jebahdiah's grave confused how he dun got dere!

17. Originally Posted by FireRescue61
Just get CAFS...Problem Resolved...
Let it come to the ground floor. No pesky calculations needed,hehe T.C.

18. Originally Posted by KuhShise
Fired up: Actually you can't collapse a supply that is being fed by a pressureized source. If you throttle up the attack engine until the pump begins to cavitate (near zero absolute in the impeller eye) the supply hose will constrict until there is 14.7 psi of friction loss due to turbulence and increased velocity in the narrowed portion (last foot) of the supply line connected to the attack engine. You can't suck this shut.!!! The problem becomes the high velocity trying to tear the lining out of the hose due to vibration and water velocity. If this occurs, you will violently slam the ripped out lining into the intake screen, and may even drive the screen right up into the eye of the impeller. Not a good result, especially if firefighters are doing an interior attack. Try it some time on a drill. If you have reached cavitation at draft, you can take a portable pump and run water into the intake opposite the suction sleeve and get additional water even though the engine is working far below atmospheric pressure. The line from the PP will not collapse.
Seriously, chill. Of course it won't go entirely flat but it will get mushy and flatten. You will get to a point where you run away from the water and the end result will be damage somewhere. The point is when you are supplying water to a pump under pressure, whether from a hydrant or through relay, you need to keep some residual pressure. 20 psi is the general rule of thumb around here although 10 is used in some fire departments.

19. Originally Posted by RFDACM02
Pretty good system for yard breathers, kinda tough to sees the thumbs through smoke. Couple this logic with some 100 psi automatic fog nozzles and sooner or later you "poor 'ol vollunteers" will be standing around Jebahdiah's grave confused how he dun got dere!
When 'ol volly Jeb spits out his chaw and calls to charge the line on my dept, pump man Jethro knows its a precon and revs the 'ol girl up to a predeeeterminateded psi. Good 'ol Jethro will holler into one of them new fangled radios if he needs more or less. Least that's how us bumpkin volly's here in VT does it...

20. Originally Posted by fyrfytr240
Rule of thumb in my "world".Look at the guy on the end of the tip.Thumb up, more pressure.Thumb down, back it off a little.Arms crossed,shut it down.We had an engineer that knew "all that slide rule stuff" and confused us poor 'ol vollunteers.As for collapsing a pressurized supply line,you just gotta feel your hose.You know,it gets soft.
Yeah that worked 40 years ago. Time to come into the 21st century and learn how to pump your trucks. A 2 hour rule of thumb class and some engine pressure sheets for your trucks and your equipment should have the guy running the truck in a good enough position to get you water.

21. Originally Posted by fyrfytr240
Rule of thumb in my "world".Look at the guy on the end of the tip.Thumb up, more pressure.Thumb down, back it off a little.Arms crossed,shut it down.We had an engineer that knew "all that slide rule stuff" and confused us poor 'ol vollunteers.As for collapsing a pressurized supply line,you just gotta feel your hose.You know,it gets soft.
Slide rule stuff? How about using flow meters to determine pressures? Of a pitot gauge and smoothbore nozzles to determine friction losses for your hoselines?

Seriously, your thought process has no business in the fire service. Why would you spend the money to have nozzles that flow a specific amount of water at a specific pressure and then do "thumbs up thumbs down' pump operations? The only reason I can come up with is just flat out laziness. If you spend a couple of hours one time you can get the data you need to do it right every time after that.

Oh, and thanks for setting back the attempt by most volunteers to be viewed as every bit as professional as their career Brothers.

22. I still use a formula I learned 40 yrs ago. With todays hose and nozzles it's overkill but it's easy to remember at Odark thirty and you'll always have plenty of water(and NP).30# per 100 small hose,15# per 100 big hose and 5# per 100 LDH. T.C.

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