A vapor recovery unit that recovered well when commissioned rarely fails suddenly. Capacity disappears gradually, and because the change is slow, the operating team adapts to it without noticing. By the time recovery is visibly poor, the loss may be spread across several causes rather than concentrated in one. Separating those causes from the operating record is the fastest route back to the original performance.
The four usual routes to lost capacity
Capacity loss on these units comes from fouling of heat transfer surfaces, frost or ice formation in the cold section, mechanical wear in the machine, and control drift that quietly moves the unit away from its design point. Each leaves a different signature in the operating data.
Telling the causes apart
| Cause | Signature in the data | Typical response |
|---|---|---|
| Fouling of heat transfer surfaces | Approach temperature widens at the same duty | Clean or replace the affected surfaces |
| Frost or ice in the cold section | Progressive loss during long runs, recovery after a pause | Review defrost cycles and inlet moisture |
| Machine wear | Reduced flow at the same speed or higher current for the same flow | Inspect and overhaul the machine |
| Control drift | Operating temperature or setpoint slowly away from design | Re-calibrate and re-establish setpoints |
| Liquid carryover | Irregular behaviour, sometimes with alarms | Check separation upstream and the knock-out arrangement |
Reading the record before opening anything
- Pull three to six months of operating data at the same conditions where possible;
- Compare discharge and inlet temperatures, and the approach temperature across each stage;
- Check whether the loss is continuous or resets after a shutdown;
- Compare flow against machine speed or current to separate mechanical from thermal causes;
- Confirm the setpoints in the controller against the commissioning values;
- Only then open the unit, with a clear hypothesis to test.
Where moisture fits in
Moisture is the factor most often underestimated. Vapor streams carry water, and any moisture that reaches the cold section will eventually form frost, which blocks passages and reduces effective surface area. The loss then appears as a gradual decline that recovers after a shutdown, which is easy to misread as a machine problem. Reviewing the inlet moisture and the defrost arrangement explains most of these cases.
Rebuilding performance rather than restoring it
Once a unit has been brought back to condition, the opportunity exists to review whether the original design point still matches the duty. Where loading has changed since commissioning, restoring the unit exactly to its original settings may restore the original problem. The records collected while diagnosing the capacity loss are the same records needed to make that judgement.
Frequently asked questions
Does capacity loss always mean the unit needs overhaul?
No. Control drift and frosting account for a large share of reported losses and are addressed by adjustment and moisture management rather than by overhaul.
How often should performance be reviewed?
Reviewing the key operating parameters quarterly against commissioning values catches most drift before it becomes visible in recovery figures.
Can fouling be reversed by cleaning alone?
Often yes, if it is caught early. Where fouling has progressed to the point of restricting flow, cleaning may need to be combined with replacement of the affected section.
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.