is it intentionally designed to only get the liquid out via a pump ...

without a pump the lines show the liquid icon but no liquid ... I had to put a pump on the underground storage tank just to "extract" the liquid out ...

just checking if this is by design ...

Yes this is "by design".
Since the tank is underground fluids can easily "fall" into it. Getting them out again requires a pump to pump them up.

There should be a tool tip mentioning this on the entity (the box below the minimap) aswell as the item and tech research.

Sounds good, thanks for clarifying!

Ah I just mod and started using lol ... as my current game was so far along lol

Would be good to either set this as a togglable option and/or include it in the mod description. Guess Factoriants are trained to not read the tooltips in most cases.

Because of that, i feel like this reduces the usability - since i often have placement constraints, and setting up redundant tubing just to use it as a buffer - as one of its core functions.
Wouldn't mind it consuming power and have a dedicated liquid output.

Edit: Especially in the usecase of a buffer, it is bothersome - since you cant pump it back to the source.

I can extend the mod description to include this info. Disabling the behaviour would cause other problems with the tank not being completely fillable without using a pump etc, fluid systems are just janky. And I quite like the added logistical complexity that forcing pump usage for fluid output has.
I'll look into adding a setting but will not promise anything.

Thanks for that.
Noticed one flaw in that pump-requirement logic - hot steam would accumulate at top, and would be rather easy to get out.
And, the default storage tanks fill top bottom with gases.