Thread: Friction Loss Formula
01-13-2000, 11:18 PM #1DaronFirehouse.com Guest
Friction Loss Formula
I have lost my book on water streams from the academy and am trying to remember the formula that was in there on calculating friction loss in gpm's. I remember there were different coefficiences for different hoses. This formula figured the gpm loss per 100' of hose. I need the coefficienes for 1.5", 1.75" w/ 1.5 couplings, 2.5", and 3" w/ 2.5 couplings. We use cloth double jacket hose and maintenance free hose.
01-14-2000, 12:35 PM #2pompanofdFirehouse.com Guest
FORMULA IS C X Q2 X L
1 3/4" 15
2 1/2" 2
200' OF 1 3/4 HOSE WITH FOG NOZZLE?
C X Q2 X L
15 X 1 X 2
30 PSI LOSS
Q=100 PSI FOR FOG NOZZLE MIN.
NOZZLE PRESSURE 100 PSI + 30 PSI F/L
= ENGINE PUMP PRESSURE OF 130PSI......
I THINK 1 1/2 IS 22 OR 25 , WE DON'T USE IT SO I DIDN'T WRITE IT DOWN. HOPE THIS HELPED...
01-14-2000, 01:41 PM #3DaronFirehouse.com Guest
This formula is a little different than the one I was thinking of but it looks as if that will work too. The only problem is I am a little lost on the example.
200" of 1 3/4"
C x Q2 X L
15 X 1 X 2
The 15 makes sense for C it is the 1 for the Q that is throwing me off. I understand that the 2 if for 2 100' sections of hose but I don't see how you came up with the 1. I know we did this exact calculation a few years ago as we run 200' preconnects w/ 1 3/4" and we came up with 150 to 160 psi to get 100 psi NP on level ground. If you could explain a little more I would be thankful.
01-14-2000, 02:51 PM #4SteamerFirehouse.com Guest
Although I didn't post the formula, I think I can help you with the answer. There was a slight error in the original posting of the formula. The original formula showed that Q=quantity. It actually is equal to the quantity "in hundreds of gallons per minute."
The 2 means you square that number (Q).
Try this with the rest of the posted formula, and I think you'll have it.
Chillicothe (Ohio) Fire Department
[This message has been edited by Steamer (edited January 14, 2000).]
01-14-2000, 03:55 PM #5pikepoleFirehouse.com Guest
Yes, Q = quantity in GPM/100, so if your flowing 150 GPM then Q would equal 150/100 or 1.5. Then you must square the 1.5 to get 2.25. Another example would be a 250' 1.75 inch line flowing 150 GPM.
F.L.= C x Q2 x L
F.L.= 15 x (1.5)(1.5) x 250/100
F.L.= 15 x 2.25 x 2.5
F.L.= 85 (I rounded up)
Also the coefficient for 1.5" hose is 25. I hope this helps.
Keep it safe.
01-14-2000, 05:54 PM #6DaronFirehouse.com Guest
Yes, that makes sense. Is the answer I come up with after that formula friction loss in gpm's?
01-14-2000, 06:40 PM #7SteamerFirehouse.com Guest
The answer reflects the amount of pressure (psi) lost due to friction loss. That amount of pressure however, is directly affected by the amount (gpm) of water flowing through the hose. So, the higher the flow, the greater the friction loss. Try the formula with various volumes of water being flowed, and the principle should become evident.
Chillicothe (Ohio) Fire Department
01-14-2000, 08:59 PM #8DaronFirehouse.com Guest
Thanks for your help. I will play with them for a few days and see if I can't get back in the swing of it.
01-16-2000, 03:36 AM #9ChopsFirehouse.com Guest
It seems like you want to still know the GPM you have; easier said than done. Here are some basic rules:
Solid Stream Tips
GPM=29.83 x c x (diameter)^2 x sqrt.(pressure)
where 29.83 is a constant, c is a device loss coefficient (use .97), diameter is tip diameter in inches, and pressure is the pressure at the tip
Usually you would want 50 psi at the tip for a hand line and 80 psi if a master stream.
Automatic Nozzles are a different set of GPM calculations. The automatic nozzle was designed to allow it to flow a range of GPM, controlled by the pump operator. By raising the pressure at the tip, the GPM would increase significantly unlike the solid-stream conterparts. This idea has been expanded by one manufacturer to allow the pump operator to set a pressure and the nozzle operator to control the GPM buy fluxuating the bail.
So, GPM on automatic nozzles are what you want it to be; after looking at the other issues involved (Another story). Many manufacturers have slide cards to tell you what the GPM will be with different combos of pump pressure, hose length, and size. Some even have snazzy rules like "150' 1-3/4 hose, 1 GPM for every PSI at the pump". Sounds simple, not really.
01-17-2000, 12:02 AM #10DaronFirehouse.com Guest
Actually I did remember that formula but I was only aware of using the coefficents for calculating the gpm's from hydrants depending on the construction of the hydrant. Can you enlighten me on how this comes into play after the water leaves the hydrant or is this taken into account with the coefficents in the friction loss formula? Also, is the friction loss the same or close to the same for maintaince free hose as it is for cloth hose.
Most of the nozzles we use here are automatics except our deck guns which we usually have both available. We do have a wide variety of automatics though with a wide ranges of different gpm's on each.
Although most of my brothers would disagree, engineering and water supply tactics have always been one of my favorite parts of firefighting.
[This message has been edited by Daron (edited January 16, 2000).]
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