Attention please

All of my last explanations are genuine if you don't have any demand in your pipeline system.

If line is air tight(no air leakage), then important is the location of line ends, if ends, are placed at positive pressure, than sufficient flow would be possible even with negative pressure, in between line( as we empty the container, without tilting it, by plastic tube by sucking from, out side end, and by putting it lower, than suction side end)

Dear Mr. Pooyan

Your figure has shown that the elevation of start point is 1646 m and the end point of this pipeline has located in the elevation of 1607 m. Therfore you have a high hydraulic gradient. And this hydraulic gradient difference is suitable for a pipeline with length of 50 KM and in real project the pressure of flow in this kind of pipelines, can help to transition of flow but maybe you have a little discharge head-loss at the end point of your system and you can waiver from that....

Best regards

Attention please

All of my last explanations are genuine if you don't have any demand in your pipeline system.

Though viscosity of fluid also play important role, in getting the discharge needed, along with head differance.......

Dear Bhupendra Desai

As the figure is shown, this pipeline dose not have any pump or suction and transfer the water just with gravity. The up-side of pipeline is a reservoir with infinity capacity and the down-side is a tank with a FCV valve before the tank to control the discharge flow.

I thought, may be this phenomena behaviors as an air-pocket with not full-blocking (the fluid flows as a free-surface) but we don't have air leakage in the model.

Best regards

Dear Mr. Mohammadizadeh

As we know, when the EGL intersect the elevation line the fluid stop at the intersect point. In this figure we have a HGL line (HGL=EGL-(V2/2g), in generaly, the third part is very low and we can waiver it) which intersect the elevation, so we must have no fluid flow.

So, do you mean the pressure is satisfy for fluid, to pass the hump? But the pressure is negative and how could it done and if we think the pressure is zero (not negative) and the water flow just with it's velocity it means we have a "projectile motion" but it does not work.

Best regards

Dear Mr. Pooyan Nikeghbali

Maximum elevation between kilometer 10.00 and 15.00 is about 1633 m. And Hydraulic Gradient Level in start point of your pipeline system is 1646 m. It means this difference between these elevations (13 m) can  conquest on the negative pressure in mentioned distance. And the pipeline system can transmit the flow from start point to end point.

Regards

I am assuming you are modeling water flow. Such flows can be sustained at pressurized regime and negative pressures as long as the vapor pressure limit of water is not reached (roughly 2kPa abs.). If the pipeline is supported above ground, there may be air admission into the line because of negative pressures if you have a faulty pipe junction of physical damage to pipe walls. Otherwise, if the pipeline is buried and covered in the water table, contamination with external water may occur.

I assume that the pressure calculations are gauge pressures as this is typically the case.  Pressure going negative then just means below atmospheric pressure.  Therefore as Dr. Vasconcelos points out, flow is possible as calculated so long as the pressure remains above the vapor pressure of the fluid.  Subatmospheric pressure may have undesirable consequences, pipeline can admit air if not tight as pointed out also pipe walls could collapse inwards if wall thickness is not sufficient and the pipe is not perfectly circular.  Program is trying to alert you to some of these things which is not in its domain to assess.

ASSUMING A GRAVITY ENHANCED FLOW, JUST LIKE IN A TWO RESERVOIR PROBLEM, AND THAT HE HYDRAULIC GRADE LIME FROM WATER SOURCE TO THE DOWNSTREAM DISCHARGE POINT  DOES NOT FALL BELOW THE PEAK PIPELINE ELEVATION ALONG THE TRANSMISSION LINE BY ABOUT 4M ABSOLUTE WATER PRESSURE,  DISCHARGE  WILL ALWAYS TAKE PLACE WITHIN ACCEPTABLE AN ACCEPTABLE HEAD LOSS BETWEEN THE WATER SOURCE UPSTREAM AND DISCHARGE POINT DOWNSTREAM, OTHER FACTORS REMAINING CONSTANT. WISH A SKETCH WAS ATTACHED TO ENHANCE!

KEPHA

Dear Dr. Vasconcelos and Dr. Wright

Thank you very much for your attention and your useful comments.

At first I thought the alarm of negative pressure and discharge rate at the down-stream of program software is to illustrate the free-surface in some place of pipeline which has the negative pressure and it is behavior as air-pocket or vapor in the part of the pipeline and water flows completely to the down-stream but when I examined it, I saw this is the error of software.

To exam the software, I just increase the elevation of a point which is in the negative pressure limitation with add 300m and I see the software illustrated the fluid flow discharge as the initial model, equal 722 lit/s

Best Regards