Reservoir Elevation

Hello Mark,

If you are using WaterCAD, the Reservoir (Inlet/Outlet) field is not used, so the value that you enter really will have no impact on the results. 

I would just enter the water surface elevation, in your case 650 feet. This will be the value that WaterCAD will use when determining the hydraulic grade. It is fine to use a reservoir at the source element if you can assume that the water elevation will not change over time. While you are currently running a steady state run, if you ran an extended period simulation and the water elevation would remain at 650 feet, then model this as a reservoir will be sufficient. 

Regards,
Scott

By only entering the surface elevation, it is then assumed that the intake for the pipe network is at the same elevation.

Regarding the pressurized flow in the system, this is very different from the actual case in which there is 50 ft of head between the piping intake and the surface elevation.

How can I model the scenario more accurately?

In this case the surface elevation of the tank (modeled as a reservoir) is at 650 ft, the pipe network intake is at 600 ft at the base of the tank.  In the model, the first link/pipe is between the reservoir and the first junction (FCV whish is also at 600 ft.)

Would it make sense to set the diameter of that first length of pipe to be equal to the tank diameter?  Would this more accurately consider the pressure at the FCV given the 50 ft of head above that junction where the actual pipe network begins?

Hello Mark,

I had responded to this in a private message. Thank you for the clarification on the setup, which was as follows:

"Top" @ 650'  and "Bottom @ 550'

Start --- Surface elevation in tank (modeled as a reservoir) at 650'  >>  out of bottom of tank at 600' into pipe network >>  passes through Gate Valve at 600' >> goes through series of pipe segments with bends ranging from 600' to 550' >>  passes through Butterfly Valve at 550' >> goes into river (modeled as a reservoir) at 550' --- End

The reservoir surface elevation is at 650' but the reservoir outlet (piping inlet) is at 600'.  Essentially my question is this:  When the reservoir is the source of the flow, how do I account for the 50 feet of head that is in the tank?  In other words, how do I model a tank with an infinite source?

Should I set my reservoir elevation to the actual surface elevation or should I set it to the elevation of the outlet (inlet into the pipes)?

A tank as an infinite source of flow should be modeled as a reservoir. The hydraulic grade at the reservoir will always be the value you enter for the elevation.

For the way to set up the reservoirs, the elevations of both reservoirs should be set at the water surface elevation. The water surface elevation is the hydraulic grade at the reservoir, even if the outlet pipe of the reservoir is 50 feet below this

Regards,

Scott

Just a few additional notes to add:

• Calculations are based on hydraulic grade and pipes are assumed to be flowing full. Pressure is determined based on the difference between the HGL and the physical elevation.
• If you want to model any significant resistance from the opening between the reservoir and the pipe, you can do that with a minor loss coefficient on the first pipe. Otherwise, the HGL at the first node would be equal to the reservoir elevation minus friction losses in that first pipe (something close to 650') and the pressure head at the first node would be something close to 50'.
• You will see additional headloss through the FCV if it is set to active and needs to throttle to reduce the flow to the desired setting. So, the HGL downstream of the FCV in that case would be something less than the 650' HGL from the reservoir water surface elevation.