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 pressurized when liquid is pumped in and compresses the existing vapor or when increasing temperature causes increased evaporation or expansion of existing vapor. Conversely, vacuum may be created when pumping out or decreasing temperature. To prevent damage, vapor 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.
The pilot operated relief valve has a principal advantages over other types of relief valves:
• Seals in accordance with API 2000 requirements for Pilot Operated Relief Valves.
• It is fully open at 10% above set pressure.
These characteristics permit an operating pressure closer to the maximum allowable working pressure of the tank. High operating pressures reduce evaporation and total venting volume, thereby reducing product loss and cost of processing emissions.
A tank may also have provisions for emergency pressure relief due to fire exposure and/or an inert gas blanket in the vapor space.
A typical tank installation as shown in the drawing below, includes a pilot operated pressure/vacuum relief valve, a gas blanketing regulator and an emergency pressure relief valve.