Quick answer: the size of a vapor recovery unit is set by the site’s vapor generation, not by the nameplate of the tanks. Tank breathing driven by daily temperature swings, vapor displaced during unloading, and the turnover of the site together decide the flow the unit must handle. Size to those measured or estimated inputs, and the unit will run in its comfort zone; size from a brochure, and it will either gasp at peak or idle in short cycles.
Where the vapor actually comes from
Three sources feed a Stage III unit, and each behaves differently. Breathing loss is continuous and weather-driven: a tank inhales at night and exhales through the warm afternoon, so its contribution varies with season and tank fill level. Unloading pushes a large slug of vapor through the system in a short window, which sets the peak the unit must absorb or buffer. Dispensing returns vapor through the Stage II path at the rhythm of customer traffic. A unit sized only for average breathing will trip on unloading days; one sized only for unloading will short-cycle on quiet afternoons.
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The inputs to collect before choosing a capacity
Sizing starts with measurement, not catalogues. The list is short but each item matters:
- Tankage: number of tanks, nominal volumes and typical fill levels through the year;
- Throughput: monthly dispensing volume, with the busy months rather than the average;
- Unloading pattern: deliveries per week, tanker size, and whether deliveries stack at peak hours;
- Climate: the local temperature range, because breathing loss follows it closely;
- Emission limit: the concentration the site must demonstrate, which shapes the process route;
- Site constraints: available footprint, electrical capacity and distance to neighbours.
How capacity maps to equipment
Once the duty is known, capacity follows as a matter of engineering rather than guesswork. The unit must handle the sustained breathing load continuously and the unloading slug without tripping, which is why real installations include buffering: a unit that can tolerate a brief excursion above its continuous rating, or a vapor space that absorbs part of the slug, can be smaller than the raw peak suggests. This is also where the process route interacts with size, because a combination unit shares the load between stages and can therefore be more compact than a single-technology design facing the same duty.
| Sizing error | What it looks like on site | What it costs |
|---|---|---|
| Undersized for unloading | Trips or bypasses during deliveries | Emission excursions and restart delays |
| Undersized for breathing | Runs continuously at its limit | Shortened component life |
| Oversized for the site | Frequent start-stop short cycling | Wasted energy and accelerated wear |
| Right-sized with buffering | Steady runs, calm unloading windows | Lowest lifetime cost per recovered unit |
Why both extremes cost money
Undersizing saves on purchase and repays itself in trips, emission excursions and a unit that ages early. Oversizing is subtler: the purchase price climbs with capacity, and the unit spends its life starting and stopping, which is the operating regime that wears contactors, motors and seals fastest. Between the two sits a band where the unit runs long, steady cycles, and that band is where lifetime cost per recovered litre is lowest. Getting into the band is a measurement exercise, which is why suppliers who visit the site and read the tank data before quoting are worth their response time.
Practical points that refine the figure
Two refinements often change the outcome. First, seasonality: a unit comfortable in spring may be marginal in the hottest month, so the sizing check should use the worst month, not the median. Second, future plans: a site expecting throughput growth, added dispensers or changed fuel blends should carry that headroom into the capacity conversation now, because adding it later means replacing components rather than adjusting settings. Neither refinement is complicated, but both must be on the table before the order is placed.
Frequently asked questions
Can a unit be too large for the tanks it serves?
Yes, and it shows up as short cycling: the unit reaches its stop condition quickly, restarts soon after, and the frequent starts wear components while energy per recovered litre climbs. Sizing is a band, not a ceiling.
Does the emission limit change the required capacity?
Indirectly. A stricter limit usually pushes the design toward a combination route, and the process route affects how compactly a given duty can be handled. Capacity and route should be chosen together, not in sequence.
What single number should I bring to a supplier?
None. Bring the profile: tankage, throughput, delivery pattern and climate. A supplier who converts that profile into a capacity and shows the reasoning will deliver a unit that fits; a supplier who asks only for tank size is guessing.
More on unit behaviour after purchase is in our article on reading the signs that membrane modules need replacing, and the full technical articles library covers condensation and adsorption topics in depth. For the underground piping that feeds the vapor path, see our sister site Vohon composite piping.
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.