A Stage III vapor recovery unit is not a machine that idles all day waiting for work. It follows the dispensers: it starts when fueling creates vapor worth handling, it pauses when the forecourt goes quiet, and it keeps running briefly after the last delivery to clear the vapor path. That follow-the-dispenser behavior is set by signals, delays and a run-on timer, and understanding the three makes most field behavior explainable.
The Signal Chain in Plain Language
When a dispenser authorizes a delivery, it also tells the Stage III unit that vapor is about to move. The unit runs its startup sequence and begins handling vapor as the nozzles work. When the last dispenser on that signal loop finishes, the unit does not stop instantly; it runs on for a set period, sweeping remaining vapor out of the line and into the tank or through the treatment stage. The same chain also carries fault states: a dispenser side interlock or a unit alarm can stop the sequence regardless of fueling activity. Every behavior on the forecourt maps back to this chain.
Why the Delays Exist
The delays are not waste; each one solves a real problem. A short start delay prevents the unit from chasing every micro-event on a busy forecourt. The run-on period handles the vapor that arrives after the nozzle closes, because vapor keeps drifting back through the return path for a while after liquid flow stops. Skipping or shortening these windows to make the unit feel more responsive is a classic commissioning mistake, and it shows up later as residual vapor smell at the dispensers or as excessive start-stop cycling that wears the drive components.
The Typical Sequence at a Glance
| Event | Unit behavior | Why it matters |
|---|---|---|
| Dispenser authorizes fueling | Start signal received; unit completes its startup sequence | Confirms vapor flow is coming before the blower commits |
| Fueling in progress | Unit runs and processes vapor from the return path | This is the main working period of the cycle |
| Last delivery ends | Unit keeps running for the run-on period | Clears vapor that arrives after the nozzles close |
| Run-on timer expires | Unit stops and returns to standby | Prevents pointless idling and saves the drive components |
What Installers Check During Setup
- Confirm the start signal source is wired from the correct dispenser group, so every fueling position triggers the unit.
- Verify the signal works from each dispenser in isolation, not just when the whole island runs.
- Set the run-on period to the value specified for the installation, and record it in the handover file.
- Watch one complete cycle from a real delivery and confirm startup, processing, run-on and stop all occur in order.
- Log the signal settings so future technicians can tell a deliberate setting from an accidental one.
Behavior Patterns That Point to Signal Trouble
Certain field symptoms trace straight back to the signal chain. A unit that never starts while fueling proceeds is looking at a broken or misrouted start signal. A unit that starts and stops in short bursts through a single delivery usually has its signal sourced too narrowly, so each dispenser handoff reads as an end of activity. A unit that runs far beyond any plausible run-on period may be seeing a stuck signal. All three are diagnosed with the sequence table above and a stopwatch, before any component is replaced.
Frequently Asked Questions
Can the run-on period be adjusted on site?
It is a setting, and it should only be changed to the value the installation specification calls for. Both extremes cause problems: too short leaves vapor in the path, too long wears the unit without benefit.
Why does the unit sometimes start when no one is fueling?
Most often a signal is being held or a diagnostic cycle is running. Compare the start time against dispenser activity; if nothing correlates, have the signal loop checked rather than assuming a unit fault.
Do all Stage III systems use the same signal logic?
The principle is common, but the signal type and wiring differ between installations. The handover documentation for the specific site is the reference, which is exactly why the settings should be recorded there.
Luoyang Wohong Petrochemical Equipment Co., Ltd. supplies Stage III vapor recovery equipment and works with installers to set up dispenser signal logic that keeps units running in step with the forecourt.