A vapour recovery unit that trips on high inlet pressure is telling you it cannot move the vapour arriving at its inlet. The instinct is to look at the control system or the trip setting, and occasionally that is right, but in most cases the restriction is physical: something in the vapour path is preventing flow, and the pressure rises until the protection operates.
Working through the physical causes first avoids the common outcome of a replaced sensor or a widened trip setting that leaves the underlying restriction in place.
What high inlet pressure actually means
The unit’s inlet pressure reflects the balance between the vapour arriving from the tank or line and the unit’s ability to move it. It rises when inflow increases, when the unit’s capacity falls, or when the path between them develops resistance. Since inflow is generally set by the site’s activity, the useful question is whether capacity or resistance has changed.
Capacity changes are usually gradual and show up as a slow increase in run time; resistance changes are often abrupt and correlate with a delivery, a maintenance activity or a weather event. That distinction alone narrows the search considerably.
| Pattern | Likely cause | First check |
|---|---|---|
| Trips during deliveries | Inflow spike exceeds capacity | Capacity against peak delivery rate |
| Trips in cold weather | Condensate blocking the inlet path | Line slope, low point drains, trap condition |
| Trips after maintenance | Valve left in wrong position or a restriction introduced | Valve positions and renewed components |
| Gradual rise over weeks | Fouled surface or degrading machine | Condenser condition and run-hour trend |
| Immediate trips at start-up | Control setpoint or sensor fault | Sensor reading against a local gauge |
The checks worth doing in order
Start at the inlet and work back towards the source. Confirm what the pressure actually is using an independent gauge rather than trusting the reading that produced the trip. Then check the path for obstruction: strainers, filters, traps and low point drains. Then check that valves are in the position the drawing says, because a valve left partly closed during earlier work is a common and easily missed cause.
Only after the physical path has been checked is it worth examining control behaviour. If the path is clear and the pressure is genuinely high, the question becomes whether the unit’s capacity has fallen, which brings in the condenser, the blower and the drive.
- Verify the pressure reading with a gauge independent of the trip circuit.
- Inspect and clean strainers or filters in the inlet path.
- Check every low point for liquid and confirm the trap is functioning.
- Confirm valve positions against the latest drawing.
- Compare run hours with throughput to see whether capacity has been degrading.
What to do before changing the trip setting
Raising a trip setpoint is the fastest way to make the symptom disappear and the slowest way to solve the problem. The setting exists to protect the machine and the upstream system, and if the pressure is genuinely high, moving the threshold transfers the problem to whichever component fails next.
If the trip setting is wrong, that should be established by measurement rather than by inference. Comparing the trip circuit’s reading with an independent gauge, at more than one operating condition, is the only reliable way to make that judgement.
Preventing the repeat trip
Once the cause of a high inlet pressure trip has been found and corrected, the useful next step is to make a repetition less likely. That usually means recording the conditions in which the trip occurred, adding a check to the routine schedule if the cause was something that a regular inspection would have caught, and confirming that the trip setting itself is correct for the installation.
Where the cause was an inflow spike during deliveries, the response may be operational rather than mechanical: sequencing deliveries differently, checking that the collection arrangement matches what the delivery process actually produces, or confirming that the unit’s capacity is genuinely adequate for the peak the site creates. Inflow that repeatedly exceeds capacity is a sizing question that no amount of maintenance will resolve.
Recording trips with their conditions also builds a pattern over time. A site that logs three trips a year under the same circumstances has a single problem to solve; a site that logs trips with no record has to reconstruct each one from memory.
Recording trips so the pattern becomes visible
A single trip is an event. Three trips with the same conditions recorded are a diagnosis waiting to be made. The record does not need to be elaborate: the time, the activity taking place on site, the ambient conditions and the pressure reading observed are enough to allow comparisons later.
Where trips recur during a particular operation, it is worth observing the system during that operation rather than only afterwards. Watching the inlet pressure while a delivery takes place, or during the first minutes of a cold morning, frequently identifies the mechanism immediately and removes the need for inference.
Frequently asked questions
Can high inlet pressure be caused by the tank side?
Yes. A blanketing system with an incorrect pressure band, a blocked vent or a tank valve in the wrong position can all raise the pressure the unit sees. The check should cover the tank side of the system, not only the unit.
Is it safe to run with the trip bypassed?
Bypassing protection to keep a station operating removes the safeguard while the underlying condition remains. It should not be treated as a solution, and where a temporary measure is unavoidable it needs a defined end point.
How quickly should a trip be investigated?
Promptly, and with the operating conditions at the time recorded. Trips that repeat under the same circumstances usually point to a single cause that can be corrected once.
Luoyang Wohong Petrochemical Equipment supplies three-stage vapour recovery equipment and supports operators with troubleshooting and commissioning. Describe the trip pattern and the conditions in which it occurs, and we will help you narrow the cause.