Negative pressure, opposite flow of the water

Hello Edin,

A negative pressure means that the calculated hydraulic grade is below the physical elevation of the element. You should closely examine your node elevations and boundary conditions (reservoir/tank hydraulic grades) to make sure they are correct.

For example, your reservoir "R-1" has elevation of 410 m. This is the starting HGL of your source. From this source you have a pipe "C-1" which is 1411 m long, till junction "1" which has an elevation of 399.67. Now for a long distance like this, there is a headloss of 3847.89 m. For a huge headloss like this, the pressure at junction "1" would definitely be low (-375.88 bars).

Due to non-availability of sufficient head, you are getting negative pressures in the system.

Please refer this article to resolve this user notification;

Troubleshooting negative pressures at pumps, junctions, & other node elements

In your model I can see there is reverse flow in FCV-1. This is because you have set it to 10.89 L/s but the flow flowing to is -8.75 L/s.

I also see that you have set the "Engine Compatibility" to EPANET 2.00.12. Consider using the WaterGEMS 2.00.12 engine.

Here is a reference article on that;

Reverse flow in conduit where there should not be reverse flow

Hope this helps.

My node elevations are correct.

How can I reduce a headloss of 3847.89 m? I was using for friction method: Darcy-Weisbach, (e=0,002 mm), when I change to other friction method  Hazen-Williams (150 mm) I got lower headloss but pressure still in minues?

Any more tips?

Hello Edin,

The issue appears to be related to the diameter of the pipes. With the small diameters, large flows, and some of the long pipe lengths, this is leading to an excessive headloss. Assuming that the flows expected and the elevations are accurate, larger diameter pipes may be required here. 

As an example of what is happening. Pipe 1, which was referenced by Yashodhan above, as a diameter of 150 mm and a length of nearly 1500 meters. With the large flows, this means that the headlosses in the pipes are very large. Adjusting the roughness coefficient will have little impact here. A larger pipe would be needed to assure the headlosses are reasonable.

Regards,

Scott

Hello Edin,

The issue appears to be related to the diameter of the pipes. With the small diameters, large flows, and some of the long pipe lengths, this is leading to an excessive headloss. Assuming that the flows expected and the elevations are accurate, larger diameter pipes may be required here. 

As an example of what is happening. Pipe 1, which was referenced by Yashodhan above, as a diameter of 150 mm and a length of nearly 1500 meters. With the large flows, this means that the headlosses in the pipes are very large. Adjusting the roughness coefficient will have little impact here. A larger pipe would be needed to assure the headlosses are reasonable.

Regards,

Scott