System curve and pump flow

There are really several different conditions when you are discussing a pipeline with a high point. There is not a single system head curve.

The first condition is when the pipe is filling and the only pressurized pipe from the pump ends at the high point. You would create a system head curve and operating point for that case.

Then it depends on whether you get full pipe flow on the downstream side. You will initially have a partly full pipe and your system head curve will look like the one at startup. This will depend on the steepness and the flow. If the HGL is below the pipe, indicates that there will be partly full pipe. You won't have this problem is  the end point of the pipe is above the local high point.

Then it depends on whether you have an air release valve at the high point (you should).The valve will open when the HGL drops below the pipe. 

Before you run the system head curve, check out these cases in their own scenarios.

To add to Tom's response, the wiki on system head curves includes information what to expect when you are using an air valve at a local high point. 

For information on modeling air valves at high points, see this link.

Regards,

Scott

Thanks a lot for wikis,

I got the correct flow and system curve When I model an Air valve at a high point.

Now I want to know why HGL drop suddenly  after a valve as shown in attached

It looks as if you have multiple high points along the way. Each of them should have an air valve.

When the HGL is below the pipe, you are likely to have partly full pipes in that area. IF pressure drops below vapor pressure of water, the water will not be able to make it over the high point.

Another option is to place a pressure sustaining valve at the down end to keep the HGL above the pipe.

Sohail - to add to Tom's response - please see this article previously mentioned by Scott as it explains in more detail the use of air valves at high points and interpretation of the results.

Looking at the hydraulic gradient in your screenshot, it looks like the high point at station ~16,000 m and the one right near the end would likely also be open with some part-full flow on the downstream side. If you continue with the air valve approach, I recommend enabling the those two air valves, (treat as junction = false) then examine the profile and adjust as needed.