What does it mean when a node reports a negative pressure?
A negative pressure occurs when the hydraulic grade is below the physical elevation of a node. If WaterCAD/GEMS says the pressures will be negative, then in all likelyhood you will have problems. Assuming all data input has been checked, there are usually two general causes of negative pressure: 1. Trying to serve a customer at too high of an elevation. This will show up as low/negative pressure at any demand. You need to increase pump head or adjust pressure zone boundaries. 2. Some restriction in the system. This will show up as good pressure during low demand and poor pressure at high demand. You need to look at the model results and see if the pipes are too small causing excess head loss or the pumps are inadequate such that they are running far off to the right of the curve. You need to upsize the pipes or pumps accordingly. You should also check your demands for errors. Since your demands are likely based on historic averages, a significant decrease in pressure may skew the resul
