Determining Service Line Pressure From Hydrant Flow Test

[drewnje]

I just recently graduated and I am having to determine the water pressure of a 2" service line at the site of a new industrial facility. The facility will require at least 35 psi at the meter and is located 2,715 ft from the nearest fire hydrant. The site is 78ft higher in elevation than the hydrant as well. The only information I have is hydrant flow data. The hydrant (hydrant #1 on attachment) is tied into a 6" water main and just past the hydrant drops into the 2" service line and runs up the hill. So far all my attempts to use the hydrant flow data and find the end line pressure have resulted in huge head losses (over 7000ft using Hazen-Williams). Can a hydrant flow test be used to determine the pressure at the end of this line? Any help or advice would be greatly appreciated. From what I have calculated, this 2" service line running this far uphill is not possible; however, the line already exists and has been utilized in the very recent past by a facility across the street. Thanks.

 

[bimr]

The question is what is the water pressure at the desired water flow. Water mains should be sized for 3-5 ft/sec.

Note that the minimum operating pressure in the water system should be approximately 40 psi.

A 2-Inch water line will be able to provide pressure when flowing at 3-5 ft/sec. At 3 ft/sec (30 gal/min, you will have 81 ft of headloss. At 2 ft/sec (30 gal/min, you will have 43 ft of headloss.

The question is, what water flow is desired?

If you need a significant amount of water, it probably will not work.

 

[drewnje]

I need 18 gpm where the water line enters the building. Ok so I see where I was going wrong. I was looking at the hydrant flow test for a flow that would be flowing through the pipe. This is not accurate and the huge flow resulted in huge head loss. Thanks.

 

[cvg]

I would question the usefullness of a 1/2 mile long, 2 inch service line in this circumstance. You may want to upsize to a larger line. Assuming you are tieing into hydrant 1 or 2, your delivery pressure will barely make 35 psi. if you are connecting into hydrant 3 or 4 your pressure will be too low

 

[drewnje]

They are tying into hydrant 1, the line is actually already there and if they want to put a larger line in, the county is not going to cover the cost, but will require a 6" line the entire way. That is a cost they are trying to avoid.

 

[drewnje]

Is it possible to use the data I have to calculate the flow in the 2" line?

 

[CarlB]

Yes it's possible.
You need to model the existing system losses as well as losses in the new service. The hydrant test gave data to model the existing; 2 points on that curve are 75 psi at 0 gpm and 45 psi at 450 gpm. At the end of the 2" service you want to find what flow is available at 35 psi. Elevation loss of 78 feet is 34 psi, so at 0 flow the pressure is 41 psi at the "meter". There is 6 psi available (41-35), to be lost in existing system and new pipe. What pipe flow causes approx 5 psi pressure drop?

 

[drewnje]

But the 45 psi at 450 gpm is when a second hydrant is open as well, so this point wouldn't really be on the curve?

 

[CarlB]

Depends on proximity of the hydrants. Nearly all of the system loss should be in the pipes between reservoir/pumphouse to this location, so generally OK to flow one hydrant, read residual in an adjacent hydrant, account for elevation difference in readings.

 

[bimr]

The hydrant test is telling you that the distribution system is inadequate to supply the necessary flow for a fire situation. Hydrant No. 1 can only supply 450 gpm.

The hydrant test does not relate directly to service pressure. The hydrant test is a scenario where you would have one significant user of 450 gpm.

If you have the flow of 18 gpm as given, that would be 35 feet of headloss in the 2-Inch pipe. Adding in the elevation difference of 78 feet, which would give you a total of 49 psi pressure difference from hydrant No. 1. The resulting pressure is then 26 psi.

As cvg noted, this is probably unacceptable since the minimum service pressure should be approximately 40 psi.

 

[EngWade]

When doing this exercise, you have to make sure you carefully select an appropriate "C" value. Based on my Civil Engineering Reference Manual, the recommended "C" values used for a PVC pipe range from 120-150, with a suggested design value of 130. Applying the Hazen Williams equation with these varying values will result in significant differences. So, my advice is to dilligently cite all the assumptions and references you use in your calculations.