Fire flow test at single hydrant 1

[Rookie2]

Hi,

I am working on a residential subdivision where we are tying into an existing waterline that looks something like this-

exhyd1 exhyd2 exhyd3
0----------------0---------------0


Our site is adjacent to existing hydrant 2. In order to perform our fire flow calculations (1000gpm at 20psi required), the municipality provided us a flow test with the flow measured at hydrant 2, and pressure readings taken at hydrant 3. The existing waterline of course is part of a much larger system and we don't know how much of the flow at hydrant 2 is coming from either direction (hyd3 or hyd1).
We need to know this to accurately calculate the friction losses in order to calculate the available pressure at hyd2. This is a 6" line so friction losses are a major factor.

In order to eliminate the question of which direction the flow is coming, we are thinking of asking for the flow and pressure to be measured at hydrant 2. These are multiple outlet hydrants and the pressure would me measured at one outlet and flow at another. I've read about this testing method at

http://www.firehydrant.org,

and from what I've read I believe this method would be conservative, because there is additional headloss due to the fire hydrant assembly itself. Measuring flow at one hydrant and pressure at another eliminates this headloss.

I just wanted to get other opinions on this. Is this an acceptable way to do this? If not what is a better way to go about it?

I hope I've asked this question clearly, any input would be greatly appreciated.

Thanks

 

[RWF7437]

You can probably get another hydrant test closer to your point of connection but, with pipes only 6-inch diameter, you may have difficulty meeting your required 1000 gpm @ 20 psi. You might also consider modeling the system in the vicinity of your project using, say, EPANet, to see if your proposed system can deliver the required fire flow plus Maximum Day Demand.

good luck

 

[Rookie2]

Thanks for the response RWF7437,
I do plan on using EPANet, I've never used it before but I have used older versions of watercad. That leads me to another question-

I've been reading some of the older threads here, and many people recommend modeling the connection to the existing system as a storage tank. In the past I have always modeled the connection as a pump and reservoir. This is the method recommended in my version of Watercad's help file.

Could someone please point me to a resource that discusses modeling the connection as a tank, and the reasoning behind it? I have been searching the internet with no luck. I have questions about how to use the fire flow test results with this method.

Again, any help would be greatly appreciated.

 

[hydrae]

I would model the existing system as a reservoir, sitting at static head with a control valve that simulates the dynamic head and follows the delta Q = delta h^.54 relationship given by the fire flow test.
to be conservative add the length of pipe between hyd2 and hyd3
Hydrae

 

[bpattengale]

I've modeled the connection to the existing system as a resrvoir using WaterCAD. As hydrae said you set the reservoir at an elevation that will match the static head reading at the test location.

In your case you could model the system with a reservoir at hydrant 1 and another at hydrant 3. You could then adjust the model by changing the loses in the two pipes to match the flow data taken at hydrant 2. You could also include demands on the two pipes connected to hydrant 2 and adjust them based on the flow data. These would be your average day flows. This would be your base model to which you would add your connection for the subdivision.

You'd then make another scenario with the max day demands and add your subdivision with its max day demand and check fire flows.

This should provide you with a reasonable estimation of what the existing system can provide. Of course all of this assumes that whatever lines feed the 6" line have adequate capacity. Its a reasonable assumption short of modeling a large portion of the existing distribution system.

Modeling the adjacent system may or may not be worth your time, depending on the data available to you. If you don't have a lot of data on the adjacent system such as additional flow tests you're going to be making a lot of assumptions on the demands and what the system can actually provide. So in other words, if your assumptions are wrong or off a little the model is going to be junk.

 

[Rookie2]

Thank you all for the responses. This is very helpful.