Need help with fire sprinkler calcs!

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jesseb143
Posts: 4
Joined: Sat Feb 19, 2011 9:31 am

Need help with fire sprinkler calcs!

Post by jesseb143 »

I know some of you will blast me, but this might be fun for others. I have to have fire sprinklers installed in a guest house I am building in the back yard. I have a 3/4 inch main coming from the city. I have about 75 PSI coming in. I have about 100 feet to get from the meter to the guest house. I can do it with only two 45 degree bends. The elevation rise to the guest house in the back yard is no more than 4 feet. Some installation companies are telling me I can get 35 GPM with 2 inch PVC coming off the 3/4 inch main. Others say I have to upgrade to a 1 inch main. This is a $7000.00 upgrade I would rather not have to do. Can anyone help me figure out the an idea of the GPM I would be able to get in the guest house.

Thanks for any help.

Jesse
admin
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Re: Need help with fire sprinkler calcs!

Post by admin »

Here is the report that I get from pressure drop calculator at:
http://www.pipeflowcalculations.com/pressuredrop/


Pipe pressure drop calculator CALCULATION REPORT
1. volumetric flow rate (q): q = 375.85056 gpm US
2. mass flow rate (w): w = 85364.88 kg/h
3. length(L): L = 100 ft
4. diameter (D): D = 2 in
5. pipe roughness (kr): kr = 0.01 mm
6. density (ρ): ρ = 1000 kg/m3
7. kinematic viscosity (ν): ν = 1.006 mm2/s
8. dynamic viscosity (μ): μ = 0.0010060001 Pas
9. local resistance coefficient (K ): K = 0.608
10. velocity (V): V = 11.699273 m/s
11. cross section area (A): A = 2026.8298 mm2
12. friction coefficient (f): f = 0.011412488
13. Reynolds number (Re): Re = 590778.3
14. boundary layer (δ): δ = 0.028387997 mm
15. pressure on the pipe start (p1): p1 = 90 psi
16. pressure on the pipe end (p2): p2 = 16 psi
17. pressure drop (p1-p2): p1-p2 = 74 psi


This calculation is based on you values which are:

1. pipe is 2" 100 ft long
2. with 2pcs 45deg bends K=2x0.3=0.608 (using K-factor calculator)
3. start pressure 75+15=90 psi - absolute pressure
4. pressure at the end 15 - for atmosphere pressure and plus 4 feet of WS = 1.7 psi

So at the end you will have about 370 GPM - which is I think ok.
Pipe flow calculations - since 2000
jesseb143
Posts: 4
Joined: Sat Feb 19, 2011 9:31 am

Re: Need help with fire sprinkler calcs!

Post by jesseb143 »

Is this possible? I thought I was looking for a number more like 35 GPM? 370GPM seems like a number that would never be questioned. My main is 3/4 inch, are you calculations taking this into account? I am not trying to argue, if you are right than I am going to save money for sure, but it just seems like so much water in a minute.
admin
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Re: Need help with fire sprinkler calcs!

Post by admin »

OK did you mean that you have 100 ft 2" line?

If so did you mean that you have 75 psi on the start oh 2" line?

If both are right than you have it.

What will happen upstream in 3/4" main line depends on the length of that pipe line and the pressure that you have on start of it. It might be that you will have high pressure drop in that 3/4" line and therefor you will not have 75 psi on the start of 2" pipe but it depends on the pump that supplies pipe on the start, if it can overcome the pressure drop in line after it.

Anyway for proper calculations you have to know for sure on some point of pipe line - available pressure at given flow rate - that is essential. Based on that value, you can calculate what will happen after that point and what pipe sizes you should use.
Pipe flow calculations - since 2000
jesseb143
Posts: 4
Joined: Sat Feb 19, 2011 9:31 am

Re: Need help with fire sprinkler calcs!

Post by jesseb143 »

So its all 3/4 inch right now. I checked the pressure at my first hose bib that comes up to my house from the main water line. At this hose bib it was 77psi. I am going to replace the 3/4 inch line from the main at the curb with a 2 inch PVC line. I will install a connector to attach to the my 3/4 inch main to expand it to 2 inch. So it will run 2 inch all the way from the main. I will Tee into it once to get my main house water and have two 45s to get it to the back yard. I am assuming the pressure from the main valve is close to this 77psi, since the hose bib is the closest place I can check pressure. I am probably missing something, please let me know if you have any recommendations on how to get accurate readings on my system.
admin
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Re: Need help with fire sprinkler calcs!

Post by admin »

Once you let more flow through main line 3/4", pressure will be lower than 77 psi that you have with zero flow. That is why you might need to change the main line.

What flow rate do you need to have?
Pipe flow calculations - since 2000
jesseb143
Posts: 4
Joined: Sat Feb 19, 2011 9:31 am

Re: Need help with fire sprinkler calcs!

Post by jesseb143 »

I need to have 35 GPM to pass the inspection. I have found some information of the internet that claims a 3/4 inch main can achieve 35 GPM, so if I can get that flow to my guest house 100 feet away, I will pass. I have had some installers tell me t hat the pressure will be the same anywhere in the system. So I guess it could work, but I would like to know a way to figure this out mathematically. Possibly I will just have to install it and fill a bucket up to find the answer. Thanks for the help, I appreciate your effort. Please feel free to give any advice or ask more questions if you have any.

Jesse
admin
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Posts: 375
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Re: Need help with fire sprinkler calcs!

Post by admin »

I think it will work also, but you can not have the same pressure in all system as you have pressure drop due to friction and minor losses.

Mathematics can be done but for the whole system as you pipe is just one part of piping network and if analised separately than you must know parameters in some point.
Pipe flow calculations - since 2000
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