An overpressure trip is the vapor recovery unit protecting itself, and it usually means the unit saw conditions on its inlet side that the design did not expect. The trip itself is rarely the fault. The work is to find out what pushed the pressure up, and the fastest path is a checking order that starts with the cheap, common causes before touching the instrument list.
What overpressure actually means on a VRU
The unit is sized to pull a certain vapor stream at a certain flow. When the vapor arrives faster than the unit handles it, or when a restriction somewhere upstream raises the pressure at the inlet, the reading climbs until a protective trip stops the unit. So the question is always the same: more vapor than designed, or a restriction that makes normal vapor behave like too much.
The common causes, in checking order
Field experience puts a short list at the top. A blocked or partly closed vent path is the most frequent, followed by liquid collecting somewhere it should not, and then by upstream equipment changes that altered the vapor load. Instruments come late in the list, though a stuck sensing line can imitate all of the above, which is why it gets verified rather than assumed.
The value of the ordered list is that each check either clears a whole family of causes or points the search somewhere specific. A unit that trips only during unloading tells a different story from one that trips at random times, and the pattern is itself a diagnostic.
Check the pattern before the hardware
When did the trips start, and what else changed at the same time? A trip history aligned with a new product, a changed unloading schedule or recent maintenance points straight at the cause. Random trips with no pattern suggest something in the unit itself: fouling, a restriction that has grown gradually, or a sensing problem that comes and goes.
A checking sequence that works in the field
- Record when trips happen and what site activity coincides with them;
- Walk the vent and vapor paths from the unit backward, looking for closures and blockages;
- Check for liquid accumulation at low points and drains;
- Verify the pressure sensing line is open and the reading plausible;
- Compare current inlet conditions against the original duty assumptions;
- Only then consider instrument faults or internal restrictions.
Cause families and their signatures
| Cause family | Typical signature | First check |
|---|---|---|
| Blocked vent path | Trips correlate with specific operations | Walk the path, find the closure |
| Liquid accumulation | Trips after product transfers | Check low points and drains |
| Changed vapor load | Trips began after a site change | Compare against original duty |
| Sensing line problem | Readings implausible or jumpy | Verify the sensing line is open |
| Internal fouling | Gradual onset, worsening over weeks | Review maintenance history |
When to involve the supplier
If the ordered checks clear the obvious causes and the unit still trips, the remaining suspects sit inside the unit: heat exchanger fouling, pump performance or a control behavior that needs the manufacturer’s eyes. This is the point to send the trip log, the site activity record and the check results rather than a one-line complaint. A supplier who receives evidence diagnoses in one visit; a supplier who receives a symptom guesses across several.
Frequently asked questions
Can we just raise the trip setpoint to stop the trips?
That removes the alarm, not the cause. The trip exists to protect the unit from conditions that damage it or push it outside its duty. Find what drives the pressure up before touching the setpoint.
Do frequent trips damage the unit?
Repeated starts and stops add wear to rotating equipment and stress the process around the unit. Frequent trips are worth treating as a fault in their own right, regardless of the underlying cause.
Why does the trip pattern matter so much?
Because it splits the cause list in half before any hardware is touched. Trips tied to unloading point upstream at the vapor source; random trips point at the unit or its instruments.
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.