What Is a Breathing Valve?
A breathing valve (also called a pressure/vacuum relief valve or conservation vent) is a device mounted on the roof of atmospheric storage tanks to control internal pressure and vacuum. It opens to allow air or inert gas to enter the tank during liquid withdrawal (preventing vacuum collapse) and allows vapor to exit during filling or thermal expansion (preventing overpressure). Breathing valves protect tank structural integrity while minimizing the release of flammable or hazardous vapors to the atmosphere.
| Parameter | Details |
|---|---|
| Function | Pressure and vacuum relief for atmospheric tanks |
| Pressure set point | Typically 1.5 to 15 mbar (0.6 to 6 in WC) |
| Vacuum set point | Typically -1.5 to -7 mbar (-0.6 to -2.8 in WC) |
| Tank design pressure | API 650 tanks: up to 17.2 mbar (7 in WC) |
| Valve type | Weight-loaded pallet or spring-loaded diaphragm |
| Sizes | 2 in to 24 in |
| Body materials | Carbon steel, 304/316 stainless steel, aluminum |
| Seat materials | Fluoroelastomer, PTFE, or metal-to-metal |
| Standards | API 2000 (sizing), API 2521 (testing), EN ISO 28300 |
| Typical installation | Flanged on tank roof nozzle, with or without flame arrestor |
How Breathing Valves Work
The valve contains two independent pallets (discs): one for pressure relief and one for vacuum relief. Under normal conditions, both pallets are seated by their own weight or by spring force, sealing the tank from the atmosphere.
Outbreathing (pressure relief): When the tank fills with liquid or vapor expands due to heating, internal pressure increases. Once the pressure exceeds the set point, the pressure pallet lifts, releasing vapor until pressure drops below the reseat value.
Inbreathing (vacuum relief): When liquid is pumped out of the tank or the contents cool, internal pressure drops. The vacuum pallet opens to admit air or blanketing gas, preventing the tank shell from collapsing under vacuum.
Key Design Considerations
- Sizing per API 2000: The valve must handle the worst-case inbreathing and outbreathing flows. The largest contributors are pump-out rate (inbreathing) and fire exposure (outbreathing).
- Vapor loss reduction: Breathing valves reduce evaporative emissions by 90-95% compared to open vents, satisfying EPA and local environmental regulations.
- Blanketing compatibility: In nitrogen-blanketed tanks, the breathing valve works in conjunction with a blanketing regulator that maintains a positive pad pressure.
- Freezing and fouling: In cold climates, condensation can freeze on the pallet seat. Heated or insulated designs prevent sticking.
Breathing valves differ from safety relief valves in that they operate at extremely low pressures (millibars, not bars) and protect low-pressure atmospheric tanks rather than pressurized vessels. The relief system design, including vent piping and disposal, must comply with the facility’s safety and environmental inspection requirements.
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