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What do the following user notifications mean and how can I resolve them?
"Maximum calculated gas volume (____) exceeds the volume of the hydropneumatic tank (____). A larger tank should be used."
OR
"The bladder-type hydropneumatic tank fills its containing tank"
These message indicate that the tank has become empty during the transient simulation. The message about the "bladder-type" occurs when using a bladder in your tank (has bladder = true). In order to assure that the tank is providing adequate protection, such as making sure that the pressure doesn't drop too quickly due to the pressure/volume relationship and headloss, you will want to make sure the tank doesn't become fully empty. HAMMER assumes that you will design the system in this manner, so if the tank becomes empty and you see this notification, the results may be invalid as the calculated gas volume will exceed the size of the tank.
Preventing the tank from becoming empty can sometimes be a difficult task because there are many factors that impact the effectiveness of the tank, including the total tank size, initial pressure and gas volume or preset pressure (for bladder tanks, which determine the relationship between change in pressure and change in volume), headloss through the inlet, etc. It is therefore often a trial-and-error process to fix this, keeping in mind other constraints such as cost and space. The basic steps to take include adjusting the tank parameters, running the simulation, checking the transient envelope and max gas volume, adjusting as necessary, and repeating the steps again.
Try to make adjustments not just to the tank size, but the other parameters as well. Pay attention to other user notifications as well, as there may be some other issue unrelated to the hydropneumatic tank. For more information on modeling hydropneumatic tanks, please see this link. There is also a Hydropneumatic Tank example model located in the Samples folder within the HAMMER installation folder.
Modeling Reference - Hydropneumatic Tanks