Negative Pressure in my Model

Question

Dear All, 
 I have designed a water supply system. In this system there are four pumping stations. (Fixed location). I have assigned my desired flow and head to each pump and demand at every junctions. When I have run my model, It's giving me negative pressure in few junctions in distribution lines where elevations are higher than reservoir's elevation as there are many high and low points in whole system. Please suggest me how to solve this negative pressure. Is this problem related to boundary conditions ?

Tom Walski · Accepted Answer

You mentioned that you will be serving a hilly area. If the range of elevations is large (difference between highest and lowest customer > about 50 m), it may be necessary to divide the system into multiple pressure zones. There would be a high zone for the highest elevation customers' a low zone for the lowest and possibly multiple intermediate zones. Depending on the elevation of the source, there will be pump stations to feed uphill, and PRV stations to feed downhill. 
 See the help for pressure zone manager.

Scott Kampa · Answer

Hello Syed, 
 Negative pressures can occur for a few reasons. They could be related to the elevation of the node, the elevation of the boundary, the head added by a pump, or excessive headlosses in pipes. Reviewing the input and results for some of these may identify the cause of the issue. This link has further information on other things you can look at to try to resolve the issue: Troubleshooting negative pressures at pumps, junctions, & other node elements . 
 If this doesn't help, a copy of the model files may be needed. 
 Regards, 
 Scott

Jesse Dringoli · Answer

To add to Scott's response: if these are high points where air valves would be located (with potentially partially-full flow) see the following article about the use of the air valve element to properly account for such cases so that the upstream pump "sees" the high point and knows to add enough head to overcome it: Why do I get a negative pressure at a high point in my system? Shouldn't the pump add enough head to push the water over the hill? 
 Viewing a profile of the area in question (going back to the boundary condition) may help you better understand what's going on.

Scott Kampa · Answer

Hello Syed, 
 Are the elevations in the system correct? The downstream reservoir has an elevation of 157 feet, but a nearby node (J-309) has an elevation of 176 feet. The pumps in your model will add enough head to help satisfy the demands and balance the hydraulic grade in the system. Because of this second item, increasing the elevation of the downstream reservoir has an impact on the pressure results. Similarly, the elevation of J-309 is quite high compared to both the nearby junctions and the reservoir. Reviewing this elevation recommended. 
 Regards, 
 Scott

Scott Kampa · Answer

Hello Syed, 
 If the elevations are correct, I'm not sure if adding more pump head will help, as the head will likely decrease as it nears the low elevation of the reservoir. 
 Are there air valves in the system now? If this is an existing system, do you have any details on field results to compare to the model data? 
 Regards, 
 Scott