The following attributes of the hydropneumatic tank influence the initial conditions calculation (steady state or EPS). You'll notice that they are all within the "Operating Range" or "Physical" section of the hydropneumatic tank properties.

• Elevation (base) - The elevation of the base of the tank. It is used as a reference when entering initial hydraulic grade in terms of "level" (i.e., if the "elevation (base)" is set to 20m and the operating range is set to "level", a "level (initial)" value of 1.0 represents an elevation of 21m).
• Operating Range Type - Specify whether the initial hydraulic grade of the tank is based on levels measured from the base elevation or as elevations measured from the global datum (zero). For example, if the base elevation is 20m, you want the initial hydraulic grade to be 70m., and you want to use levels, then select "level" for this field and enter 50m as the initial level.
• HGL (Initial) or Level (Initial) - Depending on the operating range type selected, this represents the known boundary hydraulic grade at the tank during steady state. It is the water surface elevation plus the pressure head of the compressed gas in the hydropneumatic tank. The transient simulation will begin with this head. However, if you've selected "true" for the "Treat as Junction" attribute, the transient simulation will ignore this value and instead use the computed steady state hydraulic grade
• Liquid Volume (Initial) - This represents the volume of liquid in the tank at the start of the initial conditions, corresponding to the initial HGL. This includes the inactive volume below the affective volume, when using the "constant area approximation" tank calculation model.
• Elevation - The elevation from which to calculate pressure in the hydropneumatic tank (typically the bottom of the tank.) It could be set to the estimated water surface, since the air pressure (used in the gas law equation) is above that point. However, the bottom elevation and water surface are typically very close, so this likely will not make a noticeable difference.
• Volume (Tank) - This represents the total volume of the tank. This is only used in an EPS simulation (to find the gas volume so that the gas law equation can be used) or when using the bladder option ("Has Bladder?" = "True") during a transient simulation. When using a bladder tank, WaterGEMS CONNECT assumes the bladder occupies this full tank volume at its "preset pressure,".
• Treat as Junction? - Selects whether or not the hydropneumatic tank is treated as a junction in steady state and EPS simulations. Note that if you wish to use the steady state/EPS results as input for a HAMMER transient analysis and you set this field to True, you will need to manually enter the Volume of Gas (Initial) for the tank for HAMMER
• Volume of Gas (Initial) - The initial volume of gas in the pressure vessel at the start of the simulation. During the transient event, the gas volume expands or compresses, depending on the transient pressures in the system. This value is not used in steady state or EPS analyses.
• Tank Calculation Model - Specifies whether to use the gas law or a constant area approximation method during steady state or EPS initial condition calculations. The constant area approximation uses a linear relationship; the user must specify minimum/maximum HGL and the corresponding volume between. The gas law model is non-linear and follows the gas law--as gas is compressed, it becomes harder to compress it further.
• Atmospheric Pressure Head - When using the gas law tank calculation model, this field represents atmospheric pressure at the location being modeled. This is required because the gas law equation works in absolute pressure, as opposed to gauge pressure.
• HGL on/HGL off - Exposed when using the constant area approximation method. The "HGL on" field is the lowest operational hydraulic grade desired, and the "HGL off" is the highest operational hydraulic grade desired. Corresponding controls should be entered to turn the pump on and off during an EPS simulation. Note that typically a transient simulation will use steady state initial conditions, so these fields are not considered; only the steady state HGL and user-entered gas volume are used to define the initial volume and head for the transient simulation.
• Volume (effective) - Exposed when using the constant area approximation method. Represents the volume between the HGL on and HGL off fields.