Every tank breathes. As the sun warms the shell and the day turns to night, the vapor space above the fuel expands and contracts, and the tank either vents hydrocarbon-rich vapor or draws in air to replace it. This daily cycle, called diurnal breathing, runs whether or not a single liter is dispensed, and it is one of the quietest sources of vapor losses on any site. A vapor recovery unit changes the story by giving that vapor somewhere useful to go.
Where breathing losses come from
During the warm part of the day, vapor in the ullage expands and its pressure rises. Without recovery, the pressure relief path vents that vapor to atmosphere, and what leaves is not air: it is the most volatile fraction of the stored product. At night the cycle reverses, the vapor contracts, and air is drawn in through the vent. That fresh air saturates with hydrocarbon again the next morning, and the loop repeats.
The loss compounds in a second way: fresh air entering the ullage carries oxygen and moisture, which is not what a well-run vapor space wants.
What a vapor recovery unit does with breathing vapor
Connected to the tank vent path, the recovery unit processes breathing vapor the same way it processes dispensing vapor: it draws the stream when pressure builds, condenses or separates the hydrocarbon, and returns product to storage. The tank no longer vents to atmosphere; it vents into the unit. The night-time intake is still air, but the next day’s expansion is captured instead of lost.
Because breathing follows the sun, its load profile is predictable: pressure build-ups in the warm hours, intake at night. A unit sized for dispensing duty usually finds breathing to be an easy additional load, but its control strategy must accept long quiet periods punctuated by gentle daily cycles.
Why capture pays back steadily
Breathing losses are small at any moment but constant, which makes them different from dispensing losses. They accumulate every single day, including weekends and quiet seasons. Capturing them turns a continuous emission and product loss into a modest but steady return, and it removes the most visible odor source around an otherwise well-run site.
Losses and capture at a glance
| Mechanism | When it happens | Without VRU | With VRU |
|---|---|---|---|
| Daytime expansion | Warm hours | Vented to atmosphere | Drawn into the unit and processed |
| Night-time contraction | Cool hours | Air drawn in through vent | Air intake still occurs, vapor captured next day |
| Vapor enrichment | Every warm-up cycle | Most volatile fraction lost | Returned as recovered product |
| Odor around the site | Persistent | Noticeable near vents | Strongly reduced |
| Storage of recovered liquid | Continuous | None | Returned to tank via the unit |
Setting up the unit for breathing duty
- Connect the tank vent path to the unit so expansion vapor is captured, not vented;
- Confirm the control strategy tolerates slow, low-flow cycles alongside dispensing peaks;
- Keep the vent valve settings compatible with the unit’s operating vacuum;
- Watch recovered liquid volumes for the daily breathing contribution;
- Review performance across seasons, since warm months carry the heaviest breathing load.
Frequently asked questions
Is breathing really significant compared with dispensing losses?
Per event it is small, but it happens every day without exception. Over a year the accumulated loss, and the odor it produces, is often enough to justify capture on its own, even before dispensing duty is counted.
Does the unit need to run all day for breathing?
No. The control strategy decides when to draw vapor, typically when ullage pressure builds. Modern units handle the intermittent breathing cycle alongside normal dispensing duty without special arrangements.
Why does captured breathing vapor return as liquid?
The unit condenses or separates the hydrocarbon fraction and drains the recovered product back to storage. The daily cycle therefore returns product rather than releasing it, which is the whole economic point of capture.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and manufactures vapor recovery units, membrane separation skids and condensers for fuel depots, terminals and retail sites. Contact us for duty assessment and equipment selection support.