Sizing a Stage III vapor recovery unit for a small retail station is mostly a question of establishing the vapor load correctly before any equipment is selected. Throughput, the number of simultaneous operations and the product mix set the peak vapor rate, while the annual volume and operating pattern set the duty cycle. Getting those four inputs right is more important than comparing equipment options, because a unit selected against the wrong vapor rate will either be short of capacity on peak days or oversized and running inefficiently the rest of the year.
Start with the vapor rate, not the storage volume
The vapor generated during a fuel transfer is related to the volume transferred, so the peak vapor rate follows from the fastest transfer the site can actually perform. For a small station with one or two dispensers, that is often a single delivery into a tank while dispensing continues. The storage volume tells you how much ullage exists and therefore how the pressure behaves, but it does not by itself tell you the vapor rate the unit has to handle.
Inputs worth establishing before selection
Four inputs carry most of the decision: the maximum simultaneous transfer rate, the number of points that can operate together, the product mix and its vapor pressure characteristics, and the ambient temperature range the site sees in summer. Each of these has a larger effect on the required capacity than the equipment catalogue differences between suppliers.
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Where the site’s future plans are uncertain, it is usually cheaper to allow margin in the unit capacity than to replace the unit later. The margin should be justified by a plausible change in throughput, such as adding a dispenser or a second tank, rather than applied as a blanket factor.
How the load characteristics shape the selection
| Load characteristic | What it drives | What to establish |
|---|---|---|
| Peak vapor rate | Unit capacity and duct sizing | Highest simultaneous transfer rate |
| Operating pattern | Duty cycle and control strategy | Hours and frequency of operation |
| Product mix | Vapor composition and recovery route | Which fuels are handled and how often |
| Summer ambient | Condenser duty and cooling approach | Design temperature for the hottest month |
| Layout constraints | Footprint and installation method | Space available and access route |
| Future plans | Margin to design into the unit | Plausible throughput increase |
Sequence for a reliable sizing exercise
- Record the maximum transfer rate the site can physically achieve;
- Establish how many transfer points can operate at the same time;
- Note the product mix and the frequency of each product;
- Take the design ambient temperature for the hottest period of the year;
- Calculate the peak vapor rate and add margin only where a change is plausible;
- Check the resulting unit fits the available space and access before ordering.
Frequently asked questions
Is the tank size a good indicator of the unit size?
Only indirectly. Tank size affects ullage and pressure behaviour, while the vapor rate follows from the transfer rate. A site with large tanks and slow transfers can have a lower vapor rate than a site with smaller tanks and fast transfers.
Should the unit be sized for the busiest day of the year?
The unit has to cope with the highest rate the site can generate, but the control strategy can be designed so the unit does not run at full duty all year. Sizing for peak and operating to load are separate decisions.
How much margin is sensible?
Enough to cover a specific, plausible change in the site’s operation. Margin added without a reason usually results in a unit that runs far below its design point for most of its life.
Luoyang Wohong Petrochemical Equipment Co., Ltd. supplies double-wall composite fuel piping, fittings and electrofusion accessories for gas station and fuel depot projects. Contact us for pipeline layout review and product selection support.