View Full Version : Storage tank is demanding water

April 27, 2017, 08:56 PM
I have a network. The simulations are ok.
I want to ad a storage tank. But when I ad the tank, it acts as a big water demand.
Steady state:
When the tank is empty, it acts as a water demand. The simulation without the tank (left image) has pressure much higher than simulation with the tank (right)
Extended Period Simulation:
When I run EPS, the 1st time step has the big demand from the tank (to fill the tank), then simulations are ok.

Patrick Moore
April 28, 2017, 08:08 AM

Water will flow from high head (HGL) to low head (HGL), so if you add a tank and it has a large water flow into it, then generally this means your tank HGL is much lower than the system HGL without the tank and this is inducing a large water flow as water seeks to reach equilibrium as the tank water level is a known head water level that is controlling the head in the region.

Check the HGL's before and after and your tank values of elevation plus initial height are setting the HGL of the tank. Also check that against what is controlling the head in the first scenario and supplying water to the region and that should explain why the pressures are different.

It is possible you had a pumped only region before the tank and your pumps were pushed back on their curve due to lower demand which increased the HGL when operating that way. By adding the tank this would change where the pumps would operate on their curve and the discharge head would decrease and the flow would increase on the pumps due to the tank essentially now controlling the downstream HGL and allowing water not used in demand to flow into the tank.

Hope this helps you get a better sense of what is likely occurring based on the information provided.

Patrick Moore

May 4, 2017, 12:09 AM
Yes, the tank head is lower than the connection head, so it is ok to have water to the tank.
My doubts was about the high flow which also has high headloss. Even if the head difference is less than 1 m, I get a flow >23000 cmd and headloss 40 m/k-m. My doubts is why not to use a lower flow with lowers losses. All the pressure from the new connection gets dissipated in the first pipe, and the tank gets filled in the first time step.
I tried changing number of trials and accuracy.
I was thiking if there is maybe something to define in the tank. What is the meaning of Minimum Volume?

Patrick Moore
May 4, 2017, 08:44 AM
The minimum volume would be used to set the "zero percent full" level for the tank only if the minimum level is unknown.

From the help file:

A tank needs the following fields to be

Elevation -The
elevation (above sea level) of the bottom of the tank, ft (m).
Minimum Level
- The lowest allowable water level above the bottom of the tank (ex. 3 ft.), ft

Maximum Level
- The highest allowable water level
above the bottom of the tank (ex. 22 ft.), ft (m).

Initial Level -
The height of water above the bottom of the tank (ex. 12 ft.) when the
simulation is to begin, ft (m).Diameter - The
diameter of the tank, ft (m).
Minimum Volume
- Only required when minimum level is not known, ft3 (m3).

However, What is driving the flow is going to be two things 1) the head difference and/or 2) where your pumps fall on the pump curve when the tank is added.

Take a look at what is happening at your water supply points as the Tank is changing the operation. Also check the elevation of your tank as the initial level plus the elevation of the tank will set the tank HGL. Make sure it is as desired and it is not your elevation that is causing a large head difference.

Patrick Moore