Liquid is the most common cause of vapour recovery unit problems, and the failure usually starts in the condensate handling arrangement rather than in the machine. A unit that is properly sized, correctly installed and well maintained will still develop faults if condensate accumulates where it should not, or if the pump and level control cannot keep up with what the system actually produces.
Condensate handling deserves the same design attention as the vapour path, because it is the part of the system with moving parts, control logic and the highest exposure to seasonal variation.
Where condensate actually comes from
Two sources matter. Condensation in the vapour return line, which forms overnight and in cold weather as the line cools, and liquid carried over from the recovery process itself, where vapour is deliberately cooled to condense hydrocarbons. The first is intermittent and weather-dependent; the second is a designed part of the process.
Because the sources differ, they need different handling. Line condensate should be captured at low points and drained before it can travel further. Process condensate should be collected in a vessel sized for the expected production rate and removed at a rate the process can sustain.
| Consideration | If underestimated | Practical approach |
|---|---|---|
| Peak condensate rate | Vessel overflows or pump cycles excessively | Size the vessel and pump on the highest expected rate, not the average |
| Weather variation | Seasonal flooding of the trap or line | Base the design on cold and humid conditions, then verify online |
| Pump and level control | Short cycling, wear, nuisance alarms | Match control band to vessel size and expected inflow |
| Drain points | Liquid accumulates and reaches the machine | Identify and maintain every low point on the line |
| Disposal or return route | Bottled-up liquid with nowhere to go | Agree the destination before commissioning |
Traps, drains and low points
Every low point in the vapour path is a potential liquid trap, and every trap needs a way to be checked and emptied. The design objective is that liquid never reaches the machine inlet, which means either removing it before it gets there or ensuring the machine is protected if it does.
Slope is the cheapest control, and it is also the control most often compromised during installation. A line that is level rather than sloped may look acceptable at handover and behave badly within months, because the condensate that should have drained back to the tank stays in the line until it reaches a volume that finds its way somewhere else.
- Keep the vapour line continuously sloped with no local low points.
- Fit drains at any position where liquid can collect, with access for routine inspection.
- Ensure traps are sized for peak inflow rather than average conditions.
- Protect the machine inlet so that a liquid slug cannot cause mechanical damage.
- Record drain frequency, because a changing pattern is an early warning.
Pump selection and control
Pump selection follows the peak condensate rate, the head required to move liquid to its destination and the compatibility of the pump materials with the product being handled. Undersizing produces a vessel that fills faster than it empties; oversizing produces a pump that short cycles and wears out prematurely.
Level control is where most arrangements succeed or fail. A narrow control band on a small vessel means frequent starts and stops, which is hard on the pump and produces noisy operation. A wider band on a properly sized vessel produces fewer, longer cycles and far less wear. The control strategy should be chosen to suit the vessel it serves rather than copied between installations.
Designing the destination, not only the collection
Condensate handling is sometimes designed as far as the collection vessel and left there, with the question of where the liquid goes resolved once the system is running. That unresolved step is a common source of operational difficulty, because a vessel that fills with nowhere to send its contents will eventually overflow, alarm, or be manually drained at inconvenient times.
Deciding the destination early clarifies several other choices. If recovered liquid returns to storage, the route, the pumping head and any quality checks need to be defined. If it is collected for separate handling, the storage capacity, access for removal and the frequency of collection all need to be planned. Either way, the pump and level control arrangement follows from the destination rather than being selected independently of it.
It is also worth confirming how the arrangement behaves when the system is idle. Vessels that are drained continuously rather than accumulating will not reveal a gradual increase in condensate production, because there is no level to observe. Where the level is used as an indicator, it needs to be able to accumulate enough to show a trend.
Why condensate problems cluster in the first cold season
A newly commissioned system has often only been operating in mild weather. The first cold season is when line condensate production rises, when traps see volumes they have not seen before, and when any weakness in slope is exposed. Planning a set of checks for the first cold period, and recording what is found, is the most direct way to confirm the arrangement is adequate before a failure forces the question.
Frequently asked questions
Why does the VRU make an unsteady rumbling noise in cold weather?
That pattern usually indicates liquid reaching the machine, most often line condensate that has not drained away. Check the slope, the low point drains and the trap before considering mechanical work on the unit.
How often should traps be checked?
At the frequency of other routine checks, and more often in cold or humid seasons. Recording the drained volume each time turns the check into a trend rather than a task.
Can condensate be returned to the tank?
Where the product and the site arrangement allow, returning recovered condensate to the tank keeps the material in the system. The route should be agreed with the operator and confirmed against the product being handled.
Luoyang Wohong Petrochemical Equipment builds three-stage vapour recovery units with matched liquid handling arrangements, and works with operators on retrofits where condensate problems have already appeared. Tell us what the system is doing and we will help you work out where the liquid is going.