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Gas volume in hydro-pneumatic tank

Hello forum,

I'm having doubt (due to client "inquiries"), therefore I'm addressing this question to you all.

I'm using sealed, gas-law model, hydropneumatic tank element, to model the surge vessel. Total volume is 200m3 and initial gas volume is 20m3. Approx. 160s into the simulation time, the gas volume goes to ~120m3, after which the gas volume starts to decrease. After analyzing the "Water inflow" chart, I calculated that approx 100m3 of water flows out of the tank until 160th second, after which, water starts flowing back in.

Simple logic: at the beginning of simulation, vessel contains X kg of air which occupies 20m3 of volume, at starting pressure and temperature. At 160th second, we still have X kg of air, occupying ~120m3 of volume at minimum pressure and whatever temperature according to pv=krt.  Therefore, at 160th second of simulation, the vessel still contains about 80m3 of water.

Is this logic right or not?

The client finds it essential to retain some water within the vessel, during the transient event.

Thanks in advance.