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Suppose a closed tank does not have any pressure relief system and is set to drain from the bottom. It is said that because the headspace pressure drops as the liquid level goes down, the tank will not properly drain.

However, in real situations, I would guess air bubbles start to form in the bottom hole and rise to equilibrate pressure and allow the liquid to drain. I think this ends up depending on the rate of formation and motion of the bubbles which are affected by the geometry and fluid and so on. Is there a systematic way of looking at this? How do you work it out on paper?

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Good question. I suspect this will be very difficult to predict an accurate answer. Partly it could depend on the discharge line size or line geometry as you point out. Consider the example of emptying a 1 liter coca cola bottle up side down. It does come out, but slowly.

If you want to design the tank to be self draining I would provide some way to break the vapor lock or vacuum in the headspace of the tank if possible.

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