Pressure and/or vacuum relief valves  are used  on  liquid storage tanks and other process vessels  or systems to prevent structural damage due to  excess internal pressure or vacuum.
Storage tanks are pressurised when liquid is pumped in and compresses the existing vapour or  when increasing temperature causes increased  evaporation or expansion of existing vapour.
Conversely, vacuum may be created when pumping  out or decreasing temperature. To prevent  damage, vapour must be allowed to escape or enter  the tank at a specified pressure or vacuum. The  volume rate of venting depends upon the tank size,  volatility of the contents, the pumping rate, and the temperature. See  API Standard 2000  for the  procedures to determine venting requirements.

There are 3 types of  Pressure / Vacuum Relief Valves:
Pressure or Vacuum Relief Valves –  Weight-Loaded, Spring-Loaded

Pilot Operated

Pressure or Vacuum Relief Valves –  Air Operated