Why do a I get a message that states "Pump X cannot deliver flow or head"?
This message can occur if the pump was trying to operate at the shutoff point (zero flow) or the pump would have to add more head to the system than is specified in the shutoff head, in order to elevate the water up to the downstream boundary condition. It will also be generated if the pump is operating beyond the defined pump curve. Basically, the pump cannot add enough head to be able to overcome the elevation and loss difference. WaterCAD, WaterGEMS, SewerCAD, SewerGEMS, and CivilStorm will extrapolate the pump characteristic curve past the maximum operating point if the curve does not go out to zero head. However this may not be an ideal operating point for the pump, so the user notification is generated.
To better understand this in your model, find the difference between the upstream and downstream boundary hydraulic grade and compare it with the shutoff point on the pump curve (maximum head value). For example if you are pumping from a reservoir to a downstream tank, find the difference between the tank elevation (min and max) and the reservoir Elevation. If the increase in elevation is greater than the pump shutoff head, then the pump will not be able to pass any flow in order to "lift" the water, and you'll encounter the notification in question. Looking at a profile view may also help in this case.
1) Check the pump curve to make sure it is defined correctly and as accurately as possible. A 3-point design curve or multiple point curve is preferable to have over a 1-point design curve because they are usually more accurate compared to the actual pump in the field.
2) Check your junctions or Demand Control Center to make sure your demands are correctly entered. Having too large a demand might cause your pump to operate off its defined curve.
3) Check your pipe for large reported headloss values. These could be an indication something is wrong with the pipe, such as an incorrect small pipe diameter, a pipe that is too long or short, a minor loss that is too large, or a roughness coefficient that is too.
4) Assuming that the flow and head data in the pump curve is correct, check the downstream elevations and hydraulic grade, as well as the elevation of the pump. If the difference in elevation between the pump and a downstream tank or reservoir is larger than the shutoff head of the pump, you may see that the pump will not operate and the user notification is generated. Also check for other pumps manifolded into the downstream system. If other pumps are on, they may be adding more head to the system than the pump in question can overcome.
5) If none of these appear to be the problem then it is possible that your controls are causing the notification to be generated. For instance, if the system demand is too large, it could turn the pump on and the wrong time, which could cause a pump that was running not to be able to deliver the flow or head at certain time steps. In order to fix this, you will have to review your controls to make sure they are set up correctly and functioning as designed. In this situation a user notification that you might want to look for is "Tank is Full" or "Tank is Empty" because in many situations pumps are designed to turn on and off based on tank levels. You typically do not want tanks to get full or empty as it triggers a built in altitude valve to close close the adjacent pipe. This is explained in more detail in the following wiki.
Pump exceeds maximum operating point user notification
What happens when a tank becomes full or empty?