Liquid can enter or leave a tank by pumping, by gravity flow or by process pressure.
Vacuum can result from the outflow of liquid from a tank. Overpressure can result from the inflow of liquid into a
tank and from the vapourization, including flashing of the feed liquid, that occurs because of the inflow of the liquid.
Flashing of the feed liquid can be significant for feed that is near or above its boiling point at the pressure in the
tank. See 4.3 for calculation methods.