A fundamental principle in selecting the size of liquid vessels is to choose them large enough that during operation they never become completely full of liquid nor completely empty. As Fig. 10.1 shows, there must always be some vapor space above the highest liquid level to be experienced. A vessel completely filled with liquid may inadvertendly be valved off, and should the temperature of the liquid increase, which would increase the liquid volume, pressures could develop so enormous that the vessel could rupture. Also, the liquid should not be permitted to completely drain from the vessel, because this would result in carrying vapor along with the liquid to the next component. Vapor bubbles adversely affect the performance of control valves and liquid pumps, for example.