A vapor recovery unit that will not start is usually protecting itself or waiting for a condition that has not been met. The control system rarely hides the reason; it displays it in a list of permissives that most site staff learn only after an embarrassing hour with the manual. Knowing the common no-start causes turns that hour into ten minutes.
Start requests and what stands in their way
Between the start command and a running unit sits a chain of conditions: power available and phases healthy, no active alarms, isolation and service positions confirmed, pressures within starting limits, and any external signal the site interface expects. A unit refusing to start is telling you which link in that chain failed. The display usually names it; the skill is trusting the named cause over the urge to press start again.
The permissive conditions that trip people up
Field experience points at a short list. An isolation valve or breaker left in a service position after maintenance is the classic. A high-level signal from a condensate vessel that nobody drained after the last alarm. A low-pressure condition in the storage side that the start logic waits for. And the external enable: many units run on a site-wide signal that failed elsewhere, so the unit looks sick while being perfectly healthy.
Maintenance modes deserve their own caution. A unit left in local or maintenance mode after a service visit will ignore start commands politely and indefinitely. Finding the mode selector is the first check precisely because it is the easiest to forget and the last thing anyone suspects.
A checking order that finds the cause fast
- Read the active alarms and permissive list on the display before touching anything;
- Confirm the unit is not left in local or maintenance mode;
- Check breakers, isolations and valves returned to run positions after work;
- Look for level and condensate signals that block the start;
- Verify the external enable or site signal actually arrives at the unit;
- Only then suspect the unit itself and call the supplier with the display findings.
No-start causes and their signatures
| Cause | Typical signature | First action |
|---|---|---|
| Maintenance mode left on | Ignores start, no alarms | Check the mode selector |
| Valve or breaker in service position | Permissive list names the position | Return to run configuration |
| Condensate level high | Level alarm or silent permissive | Drain and reset the vessel |
| External enable missing | Unit healthy, site-wide symptom | Trace the site signal source |
| Genuine unit fault | Alarm with fault code | Record code, call supplier |
When to stop and call
The checking order above clears the causes that belong to site staff. When it ends at a fault code or a permissive that will not clear, the efficient move is to photograph the display, note the unit’s serial and recent activity, and hand the supplier a complete picture. Units get repaired faster from evidence than from descriptions, and the evidence is already on the screen.
Frequently asked questions
Is it safe to keep trying to start the unit?
Repeated start attempts against an active permissive achieve nothing and can extend a fault. The unit is not being stubborn; it is reporting a condition. Read the condition instead of retrying.
Where is the permissive list usually found?
On the main display behind the status or diagnostics page on most modern units. Older installations may need the manual, which is exactly why the permissive page deserves a bookmark during commissioning rather than a search during a no-start call.
Can a failed start damage the unit?
Not by itself. The risks come from workarounds: bypassing a permissive or forcing a contact to make a unit run. Whatever the permissive was protecting becomes the next call, usually with more damage attached.
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.