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Imagine a small, rigid steel container with two valves—one at the top and one at the bottom. The container is placed in a larger sealed room, where vacuum pumps reduce the pressure to 0.1 bar. A hose from the vacuum pumps is connected to the top valve of the steel container to equalize the pressure inside and outside the container. Then, a hose is connected to the bottom valve, and liquid is pumped into the steel container until it is completely filled. This procedure is called hermetic filling.

My question is: After removing the steel container from the vacuumed room and exposing it to atmospheric conditions, when the bottom valve is opened, the liquid begins to drain but stops after a small amount is drained. Why is this happening? Since the container is rigid and initially filled completely with liquid, I assume there’s no gas inside to exert pressure on the liquid. Therefore, the only internal pressure is the liquid’s hydrostatic pressure. Based on my calculations, I know that the hydrostatic pressure is way lower than the atmospheric pressure. So how can this pressure be sufficient to overcome the external atmospheric pressure and allow the liquid to drain? There must be another mechanism I'm missing that causes the liquid to drain. Simple representation of the system

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    $\begingroup$ "This procedure is called hermetic filling." What is your source for this statement $\endgroup$
    – Bob D
    Commented Sep 16 at 12:16
  • $\begingroup$ The example I provided comes from the manufacturing process of hermetically sealed transformers, specifically during the oil-filling stage of these products. $\endgroup$
    – Sam Malek
    Commented Sep 16 at 18:06

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If a container is actually filled entirely with liquid, the internal pressure will be strongly temperature-dependent, because the liquid contents will have a different coefficient of thermal expansion from the container.

Because the container is quite rigid, even a slight thermal expansion of the contents can produce a very large pressure (many atmospheres). This could certainly force liquid out an open valve. Similarly, a slight temperature decrease could produce an almost total vacuum inside the container (of course, the pressure will not drop below the oil's vapor pressure, but that is very low for transformer oil).

While I'm no transformer designer, this doesn't seem like healthy behavior. So I suspect that many transformers have some kind of expansion tank that keeps the pressure near atmospheric. (An expansion tank can have a piston or diaphragm or something like that which allows the pressure to equalize with outside air, or with an internal gas reservoir, without actually letting gas into the main tank.) If the expansion tank is on top of the transformer, that means the pressure at the top will be near atmospheric, while the pressure at the bottom is higher because of the hydrostatic pressure. So in this design, you would also have liquid flow out if you opened the bottom valve.

Alternatively, the main tank itself could act as the flexible component, if it is designed to be less than completely rigid. As with the expansion tank design, this would keep the pressure inside near atmospheric pressure, as the walls of the tank flex slightly to keep the inside and outside pressures close. And again, the pressure at the bottom would likely be consistently higher than atmospheric due to the added hydrostatic pressure.

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  • $\begingroup$ Thank you for your great response. You are correct. Typically, larger transformers are designed with an expansion tank to equalize the internal and external pressures. However, smaller units can also be designed with an expansion tank and be hermetically sealed from the outside. In my question, I modified the scenario to better understand the mechanics if the container were rigid. $\endgroup$
    – Sam Malek
    Commented Sep 17 at 7:49
  • $\begingroup$ However, there is still one point I don't quite understand. In the case of an expansion tank design, let’s assume the expandable tank is completely filled under the conditions mentioned, with a vacuum applied (as described around 0.1 bar), and that the temperature remains constant afterward. What factor would cause the liquid to drain when the valve is opened? Also let us assume that the hydrostatic pressure at the base is significantly lower than atmospheric pressure. $\endgroup$
    – Sam Malek
    Commented Sep 17 at 7:57
  • $\begingroup$ There are three things I can think of that would increase the interior pressure of the expandable tank above 0.1 bar: 1. Pressure exerted by the pump at filling time. This would remain even after the valves are closed, because the elasticity of the tank compresses the fluid inside. 2. Temperature increase after filling, because the fluid inside expands. 3. Compression of the flexible tank by the outside atmosphere, where the flexibility allows some or all of the atmospheric pressure to be transferred to the interior contents. $\endgroup$
    – anon
    Commented Sep 18 at 1:05

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