When computing a transient simulation with a one way surge tank (has check valve = true), the hydraulic grade is sometimes higher than the top of the tank. How can this be?
In this situation, there are two hydraulic grades at the surge tank node - the hydraulic grade inside the tank (the water level) and also the hydraulic grade inside the pipeline connected to the tank. When you look at a profile path in the transient results viewer, or a time history of the pipe next to the surge tank, you are looking at the latter. This could certainly be higher than the tank top, as it represents the HGL pushing against the closed check valve. At the same time, the water level in the tank will be lower. To view both of these HGLs, enter a number in the "report period" field of the surge tank, re-compute the transient simulation, then look at the bottom of the transient analysis detailed report, available from Reports > Transient Analysis Reports. You will see a table of various extended results such as head of water, head of pipe, overflow rate, etc.
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Protective Equipment [FAQ]