USING A RESERVOIR TO MODEL THE CONNECTION TO AN EXISTING SYSTEM IN WATERCAD

Question

I'm using a Reservoir in WaterCad to model the connection point to an existing water main based on fire hydrant flow data. 
 This is a basic WaterCad question that has been discussed and it seems that everyone has a different opinion. I'm looking for insight on the use of a reservoir and fire flow test data to model the connection to an existing system using WaterCad. I'd like to confirm that this approach is valid along with the pros and cons of doing it this way. I do understand that you can use a pump curve to mimic the existing connection flow and pressure fluctuations. However; I'm only using a reservoir to model the connection. 
 The model was developed to evaluate fire flows required for a proposed multifamily development with a single looped 6" diameter water main. 
 Two hydrants were flow tested and the results are below: 
 FH-1 - Static PSI - 80, Residual PSI = 77, Pitot PSI = 62, Flow GPM = 3922 
 FH-2 - Static PSI - 80, Residual PSI = 72, Pitot PSI = 59, Flow GPM = 3826 
 Based on this data, I'm using 72 PSI and 3826 GPM to set the parameters for my reservoir. 
 Is this a valid approach? Pros and cons? Does the Pitot PSI need to be taken into account? 
 Any and all insight will be greatly appreciated!

Tom Walski · Answer

It's a matter of engineering judgment. How comfortable are you in your assumption that the hydraulic grade you measured will the value there during the design fire. 
 
My preference of course is to have a fully calibrated model of the system as the basis for design. 
 
One thing you have working in your favor is that you only got a 3 psi pressure drop at a flow of 3922 gpm. That means you must be sitting on a very large transmission main. Therefore, the pressure probably won't drop much during a large fire. If you were being fed by an old 6 inch main, the assumption of a fixed HGL would be much more unreasonable. 
 
In the end, you're the one whose seal will be on the plans. If you feel good about a fixed HGL, then use it.

Tom Walski · Answer

Pitot pressure is only used to calculate velocity head to convert to flow rate. 
 
4000 gpm from a single hydrant sounds very high. Were you using the 4.5 in. outlet and was the hydrant lateral very short?

Jesse Dringoli · Answer

Hello Rob, 
 I'm not sure if it was adjusted, you may need to check with the person or persons who did the hydrant test. If you know the discharge coefficient, you can use the equation that converts pitot pressure to flow to confirm if that was used. This is mentioned on page 185 (equation 5.1) of our book Advanced Water Distribution Modeling and Management . It's also referenced in this wiki article . 
 As far as the WaterCAD model, this would effect the inflow vs. pressure rating table that you establish with the "fake" pump and reservoir. As the downstream demand changes, the pump shifts on its curve to provide the corresponding pressure. If you're not seeing a significant pressure drop, then the exact flow amount from the recording may not have a large impact. You could also make a conservative assumption and use a reservoir at a fixed elevation.

Tom Walski · Answer

Jesse is right but the trick is figuring out where to set the reservoir HGL. Too low and you're wasting money being over-conservative; too high and you may not have enough pressure during an actual fire. 
 
I'm still trying to figure out what Rob means by "hydrant flow test results adjusted based on Pitot pressure". I've never heard of this or as lest not heard of it with those words. Exactly what are you adjusting and how are you adjusting it.

Tom Walski · Answer

Rob, 
 
You're not ignoring the Pitot pressure; you are using it to determine the hydrant discharge.