A unit that runs, makes the expected noises and reports no fault, while producing almost no recovered liquid, is one of the more frustrating situations an operator can face. There is nothing obviously broken, so there is nothing obvious to fix. In practice these cases fall into three groups, and identifying which group applies is most of the work.
The three are: the vapour arriving is not what the unit was designed for, condensation is not happening as intended, or the liquid being produced is not reaching the collection point. Each has a different set of checks and a different fix.
Group one: the load is not what was assumed
Recovery depends on hydrocarbon reaching the unit in a condensable form. If the vapour stream has been diluted, whether by a blanketing gas, by air ingress, or by a change in how the tank is operated, the amount of hydrocarbon per unit of gas falls and liquid production falls with it. The unit appears to be running normally because it is.
Checks in this group are about the source: what is the inlet concentration, has the product changed, has the tank venting pattern changed, has any upstream work altered the way vapour is collected?
Group two: condensation is not happening
If hydrocarbon is arriving but not condensing, the problem lies in the cooling side. A condenser that is fouled, a refrigerant charge that is low, a control setpoint that has drifted, or a cooling medium whose temperature has risen all reduce the amount condensed. The symptoms are often a warm condenser outlet and a running time that has increased to compensate.
| Check | What it tells you | Where to look next |
|---|---|---|
| Inlet concentration and temperature | Whether the load matches the design basis | Source, blanketing and collection arrangement |
| Condenser outlet temperature | Whether cooling is achieving the intended condition | Refrigerant, control setpoint, cooling medium |
| Condenser surface condition | Whether heat transfer has degraded | Service history, cleaning records |
| Run hours against throughput | Whether the unit is working harder for less | Machine condition and control behaviour |
| Vessel and drain levels | Whether liquid is being produced but not collected | Level control, pump operation, drain routes |
Group three: liquid is produced but not collected
This group is the most easily overlooked, because everything upstream appears to be working. Liquid is generated, but it is not arriving where the measurement or the collection point expects it. Causes include a level instrument reading incorrectly, a pump that is not running when it should, a drain route that is blocked, or a vessel that is being drained continuously rather than accumulating.
The distinguishing feature of this group is that the process is working while the record says it is not. Checking the physical level in the vessel against the instrument reading is the fastest way to detect it.
A structured way to work through it
- Record the inlet conditions at the same time as the recovered volume, so the two can be compared.
- Check the condenser outlet temperature against its intended value, not against memory.
- Verify the vessel’s physical level against the instrument reading.
- Confirm the pump runs when the level rises, and that the drain route is clear.
- Compare current run hours with the throughput being handled, and with the commissioning record.
Setting expectations for what recovery should look like
Diagnosing low recovery is much easier when there is a documented expectation to compare against. Commissioning is the moment to record what normal looks like: recovered volume against throughput, inlet conditions, condenser outlet temperature and typical run hours. That record becomes the reference for every subsequent question about whether performance has changed.
Sites without such a record tend to rely on recollection, which is unreliable for a quantity that varies with weather and trading. The practical workaround is to establish a fresh baseline over a period of stable operation, note the conditions clearly, and treat subsequent readings as comparisons against that point rather than against an impression.
It is also worth distinguishing between recovery that has fallen and recovery that was never as high as assumed. The second case is a sizing or configuration question, and it is identified by comparing measured results against the design basis rather than against earlier readings from the same system.
Separating a measurement problem from a process problem
Before treating low recovery as a process fault, it is worth confirming that the measurement itself is sound. A level instrument that has drifted, a drain that bypasses the collection point or a totaliser that has been reset can all produce a record showing low recovery while the process is performing as intended. Checking the physical level in the vessel against the instrument reading takes minutes and resolves the question either way.
Once the measurement is confirmed, the comparison that matters is between the inlet conditions the unit is actually receiving and the conditions the unit was specified for. That single comparison usually identifies which of the three groups the problem belongs to.
Frequently asked questions
Could the unit be oversized and therefore working inefficiently?
Possible. A unit operating well below its intended duty range can behave differently from one at design point, particularly if the control strategy was set up for the higher load.
How do we tell a load problem from a mechanical one?
Measurement. If inlet conditions match the design basis and recovery is still low, the problem is likely downstream. If inlet conditions have changed, the unit may be behaving correctly for the vapour it is receiving.
Should we expect seasonal variation?
Yes, and it should be expected and allowed for. Cold weather changes both condensation behaviour and how much liquid accumulates in the vapour line, and normal seasonal variation should not be mistaken for a fault.
Luoyang Wohong Petrochemical Equipment builds three-stage vapour recovery units and supports operators with diagnosis and performance review. Share the inlet conditions and recovery figures you are seeing and we will help you establish which group the problem falls into.