Storage tank
The Storage Tank is a building that can store up to 2500 units of a fluid. It is a passive storage - it has no input and no output, depending on pressure to fill it, essentially being a large pipe.
A completely filled tank with 100 degrees hot water stores 212 Megajoules! This is the equivalent to 42 Basic accumulators, but the tanks needs only 3x3 tiles of space. See Liquids/Hot to learn more about using stored hot water.
Usage
Storage tank will only be filled to the same percentage of capacity as pipes that lead to it. Pipe that always carries about 5 water will fill the tank to about 50% of its capacity. Small Pumps can be used to fill the entire tank from a low pressure source (such as distant Pumpjacks).
Storage tank is typically used to store excess oils (Crude Oil, Light Oil and Heavy Oil) to ensure that Oil refinery works smoothly.
Storage tank can also be used as a water-based Accumulator. If hot water consumption by Steam engines changes a lot during the day-night cycle (for example because of Solar panels or Laser turrets), storage tanks can be filled with hot water during low power usage and then emptied during high.
See also