# Thread: EPANET: Negative pressures in my model? (WARNING: Negative pressures at ??? hrs)

1. ## EPANET: Negative pressures in my model? (WARNING: Negative pressures at ??? hrs)

Dear All.

I am a Civil Engineering student, my professor asked me to run this hydraulic model by using EPANET.
It is a real time water distribution system, I've linked the junctions with pipes (manually) with Auto Length property.
There is about 300 junctions linked by pipes, all the data of (X,Y,Z) Junctions is given. So there is no ability to change (X,Y,Z).
But if necessary I can delete some junctions to let EPANET run the hydraulic model.

There is a problem when i run mine:
Negative pressures at ??? hrs.

Dear Engineers & Surveyors & anyone can help me.
Please have a look on my attached water system.

Regards.

2. Khalid,

If you get negative pressures it means the hydraulic grade line is lower than the elevations assigned. EPANET is used as the hydraulic engine of our InfoWater product which has a student license edition.

But this error is listed as it is often indicative of an error that should be checked in your model as in the real world demands could not be supplied if the actual pressure in the system was negative (i.e. as a vacuum). But to solve the hydraulic equations EPANET solves for the Head at each junction and calculates pressure as Pressure = (Head - elevation) * conversion factor depending on the units used like 1 psi = 2.31 ft

Since EPANET requires that all pipes flow full and that all demands are 100% satisfied negative pressures can occur, but given this is something that could be in error, it will issue a warning message if it occurs on demand junctions.

Look at what is controlling your hydraulic grade line if your elevation values are reasonable such as a tank head, or where a pump is operating. Something in your model is depressing the head below the elevation for one or more junctions and causing this error.

Patrick Moore

3. It looks like all your pipes have a diameter of 0.05 which is super small for your units of CMH and result in velocity and headloss values for pipes that are very very large.
(click if need larger image)
small pipe diameter.JPG

Check your units and pipe diameters as that is what is causing your negative pressures. You are losing all your available head right there.

BTW, When I import this EPANET file into InfoWater the diameters show up as 0.05 mm which is super small.

Look at this in the pipe report: headloss and velocities are huge. Your pipe diameters are not appropriate for the demands in the units used.
(click if need larger image)

Patrick

4. Thanks Patrick,
Yes the problem was with the diameter, which was very small.

5. I'd suggest 2 things:
Verify the units (0.05 mm is not real data)
A simple hydraulic grade verification. What is the head at your boundary condition? What is the elevation difference between boundary and node? That will give you an idea to check if your results are reasonable or not

6. Khalid,

Glad that was able to assist you

Patrick Moore