The condensed water is not flowing from the customer site back into the "external tank." That is what I imagined when I first read this.
This explains what the external tank is for. It is there to replenish the internal tank as needed. It is not part of the loop from tank, to reactor, to customer site condenser, back to the tank.
Nice work. I had the same imagination. Penon's language is obviously confused in places, and this can lead to other interpretive errors, even when he is not confused. I think you may be correct. I'm going to read it again.
Okay, done. It is possible that the operating loop is closed (he calls the system a "closed loop") and that the return pipe, where the flow meter is located, could be full, the issue is much more complex than if this were simply gravity flow back to the external tank, with, then, gravity to the internal tank.
With this description, the actual state of the return pipe is more difficult to anticipate. Is there air in the internal tank system? How does the pipe enter that tank, is it below the water level? The internal tank in what Paradigmnoia has shown is a large vertical pipe, apparently. The return pipe would theoretically enter below the water line, unless ....
Unless the pump rate out exceeded the return rate. This could empty the internal tank, and air would then flow back into the return pipe, and that pipe could become only partially full. This would only be a transient state, but it would create a pipe with air in it, back to the flow meter. What happens then depends, again, on flow rate. A possible issue could also be a water loss in the customer area. This would cause a temporary lowering of the apparent flow rate, unless the flow meter is now malfunctioning because of not being full of water.
If it is true that the meter was installed with no protective measures, such as a U-joint in the pipe to keep the meter unconditionally submerged (assuming no forced air to empty it), the system is vulnerable to artifact and that condition might even be set up by how the pump was operated, in addition to possible issues in the customer area.