The blower in a Stage III vapor recovery system is not simply a device that moves gas. It has to keep the tank ullage within a narrow pressure band while handling a flow that changes through the day and through the year. Blower selection therefore comes down to three numbers that have to be considered together: the flow the system must handle at peak, the vacuum it must hold without drawing liquid out of the tank, and the duty cycle it will actually experience in service.
Why the duty cycle decides more than the peak flow
A station that receives one delivery a week and dispenses modest volumes looks very different to a high-throughput site on a busy route. The first runs the blower for short periods and spends most of its time idle; the second accumulates vapor continuously. A blower selected only on peak flow will be oversized for the first site and may sit in a low-load region where it runs inefficiently, while the second site will reward selection based on continuous duty.
The three numbers to establish first
- Peak vapor flow the system can generate, from tank filling rate and dispenser throughput;
- The vacuum the system must hold without pulling liquid from the tank or the return line;
- The expected running hours and the proportion of time spent near peak.
Why oversizing is a common mistake
Oversizing feels safe because it appears to leave headroom. In practice an oversized blower pulls harder than the system needs, which increases liquid carryover risk from the tank and the return line, raises noise, and can push the unit outside the vacuum band it is trying to control. On a small site an oversized blower also runs well away from its efficient operating region, so the energy penalty is paid continuously.
Vacuum, liquid and protection
| Station profile | What it demands from the blower | Typical consequence if mismatched |
|---|---|---|
| Low throughput, occasional deliveries | Stable operation at low load, quiet running | Oversized unit cycles harshly and carries liquid |
| Busy urban site, frequent deliveries | Consistent flow across a long working day | Undersized unit cannot hold the ullage band |
| Depot or terminal duty | Continuous operation with high total volume | Insufficient duty rating shortens service life |
| Site with long vapor runs | Enough vacuum to overcome line pressure drop | Unit works continuously yet recovery stays low |
Whatever the profile, liquid carryover protection belongs in the selection rather than being added later. A knock-out arrangement ahead of the blower, a high-level trip on the liquid trap, and a clear drain routine keep liquid out of a component that is not designed to move it.
Checklist before ordering
Confirm the peak and average vapor flow, the control band for tank pressure, the running hours expected in a typical week, the pressure drop of the vapor route between tank and unit, and how the blower is protected against liquid. Where the station is expected to change, for example with added dispensers, it is better to select for the future duty cycle than to plan a blower replacement later.
Common questions
Can a larger blower be turned down instead of selecting a smaller one?
Turning down is possible where the drive allows speed control, but it does not remove the penalty of a unit that is fundamentally larger than the duty requires. Where speed control is available, selection should still start from the real duty rather than from a generous margin.
Does a longer vapor line change blower selection?
Yes. Line pressure drop adds directly to the vacuum the blower must produce, so a long or restricted route means the unit works harder for the same result. This is one of the reasons a low recovery rate is often traced to the vapor path rather than to the blower.
How often should blower performance be reviewed?
At least as part of the scheduled maintenance routine. Comparing the vacuum the unit holds and the running hours it accumulates against the commissioning record shows whether the vapor path is becoming restricted or whether the duty has grown.
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