Determining fire flow? 1

[degeng]

Just came across this forum, hope to be able to interact and contribute in the future. For now I have a basic question. I am working on a plan to extend 400 lf of 6" pvc line into a subdivision. The line will then tee with 2" lines perpendicular to the 6" going 400 lf in each direction, maybe 5 residential services each. There will be a fire hydrant placed off the 6" line near the 2" intersection. The 6" will tie into an existing 8" line via cut in tee.

Knowing the static pressure the water system maintains (60 psi), is it possible to calculate fire flow and pressure at the proposed hydrant and water services? Can this only be done with modeling software?

Thanks

 

[cvg]

you need several pieces of data including a) elevation of the ground at tie-in point b)elevation of ground at fire hydrant c)hydrant flow test or system curve at the tie-in point. Recommend that you extend 8" line to the new hydrant instead of 6" and you "might" be able to provide adequate fireflow.

 

[bimr]

As cvg posted, the minimum size water main is usually 8-Inch diameter.

 

[brewer101]

I have a similar problem. I need to install a hydrant on 850 LF of pipe at a new building. The 8" main I am connecting to is at 45 psi. I do not have flow information for this main. The hydrant is located at the same elevation as the main 850LF away. I used PSI to calculate head and it appears to me that we will not meet fire flow or residual pressure at the hydrant. Is this correct or am I off base?

 

[cvg]

you need a flow test, you can't calculate it just based on the pressure at the connection point.

 

[brewer101]

There is a hydrant about 20 feet from the tie in point, I assume this is an appropriate place to test since it is the only place to do so. It would seem somewhat odd to use this though since the hydrant has a 3' orifice and is on a 6' inch line not an 8" like the mainline.

 

[brewer101]

The Hydrant test records indicate an observed flow of 821 gpm and indicate the available flow at residual 20 psi would be 1570 gpm. How do I use this to calculate the flow and residual PSI at the proposed hydrant 800 feet away on the same line.

 

[CarlB]

Hopefully you also know the static pressure. With this you can model the supply system as a reservoir and a pipe with a friction factor, as would result in 20 psi at 1570 gpm. (reservoir elevation as would cause the static pressure). Now add 800 feet of pipe to this model, see what flow is available at 20 psi at that location. Include elevation differences in model.

 

[brewer101]

The static pressure on the 8" main is 40 psi, the elevation is flat from the end of the existing mainline to the connection to the proposed hydrant. So you are saying I convert the static psi to head and model it as a reservoir?

 

[CarlB]

I used a simple pressure pipe flow calculator.

P1=40 psi, P2 =20 psi, used C=140. Trial & error to get 1256 lf 8" pipe results in 1570 gpm.

Added 800 feet to pipe length, solved for discharge (P2=20 psi), Q=1203 GPM

 

[brewer101]

Thats really helpful in two ways, one it showed me methodology and I need to produce 1200 gpm @ 20 psi so I think we will need to do some more work to try and bring our factor of safety up.

 

[brewer101]

What calculator did you use to work on this?

 

[CarlB]

I used Haestad Methods "flowmaster"

But you can use an on-line calculator as well, just takes a little more trial and error to solve for pressure drop.

http://www.engineeringtoolbox.com/hazen-williams-water-d_797.html

 

[degeng]

Finally received the data from the fire flow.

static 81 psi
1" 80 psi 112 gpm
1.25" 79 psi 223 gpm
1.5" 77 psi 322 gpm
2" 69.5 psi 664 gpm
2.5" 57.5 psi 1115 gpm
4.5" 18 psi 1680 gpm

Fire hydrant will be on 6" pvc line at 300' from 8" tie in then branch 2" each way.

Need to estimate fire flow and customer flow. 8 customers each on 2" lines.