Condensate carryover is the quiet troublemaker of membrane vapor recovery units. The membrane stage is built for vapor; when liquid from upstream arrives as droplets or slugs, performance sags, alarms appear and the controls start cycling. Because the cause usually sits one or two stages upstream, crews often blame the membrane itself and miss the real fix. Learning the symptoms of carryover and working upstream from them solves the problem in one visit instead of several.
How Liquid Reaches the Membrane Stage
Vapor leaving the storage tanks carries hydrocarbon vapor and variable amounts of condensable material. The unit handles this by design: a knockout stage drops out the bulk liquid, piping slopes route the rest to drains, and only then does the stream reach the membrane. Carryover happens when one link in that chain fails: a drain plugged by debris, a knockout vessel overfilled because its pump stopped, a slope reversed by a shifted pipe support, or a sudden slug pushed by a fast valve opening after a long quiet period. The membrane then meets liquid it was never meant to process, and the symptoms follow quickly.
The Symptoms, in the Order They Appear
- First: small performance drift at the membrane stage, visible as a rising differential pressure or a lower recovery figure on similar deliveries.
- Then: intermittent alarms during or shortly after unloading, often dismissed as sensor fussiness.
- Next: visible liquid traces at membrane stage drains or sight glasses, sometimes with a milky look from entrained oil.
- Finally: control cycling that grows more frequent as liquid repeatedly reaches the sensing points.
Working the Problem Upstream, Not at the Membrane
Reading the symptoms against their usual upstream causes keeps the search short:
| Symptom | Likely upstream cause | Check first |
|---|---|---|
| Rising differential pressure after deliveries | Liquid reaching the stage | Knockout stage level and drain valve |
| Alarms shortly after unloading | Slug pushed out of a low point | Drain legs and line slopes |
| Milky traces at stage drains | Entrained oil in condensate | Knockout pump and float switches |
| Cycling grows frequent | Sensing points seeing liquid | Low point drainage after recent ground work |
The fix sequence that works is boring and effective: start where the liquid should have stopped and walk backwards toward the tanks.
- Empty and inspect the knockout stage: level, pump function, float switches and drain valves.
- Walk the vapor line slopes and drain legs, confirming low points drain freely and supports have not shifted.
- Check the events before the episode: a large offload, a cold snap, a valve opened quickly after idle, any of which can push a slug.
- Only then inspect the membrane stage itself: drains, differential pressure trend and condition of the elements.
- After the cause is fixed, log the event so the pattern becomes visible across seasons.
Separating Carryover From Other Faults
Carryover shares symptoms with other issues, so a short comparison keeps the diagnosis honest. Rising differential pressure at the membrane can also come from fouling that built over months, which shows in the trend history rather than after single events. Cycling can come from control settings or sensor drift, which shows in the alarm log pattern. The distinguishing mark of carryover is correlation with liquid events: heavy deliveries, cold mornings, drains that were known to be marginal. When symptoms and liquid events line up, the upstream chain is the place to look first.
Prevention Is a Short List
Stations that rarely see carryover do four simple things: they keep drain legs clear and heated where the climate needs it, they test the knockout stage pump and floats on a schedule, they inspect pipe supports after ground work near the vapor line, and they avoid fast openings of valves after long idle periods. None of these costs much; together they keep the liquid where the design put it, far from the membrane.
Frequently Asked Questions
Does carryover permanently damage the membrane?
Occasional droplets are within the design envelope; repeated slugs are not. Persistent carryover shortens element life, which is why the upstream cause should be fixed rather than the symptom tolerated.
Why do alarms appear after unloading rather than during it?
Liquid pushed during the delivery settles into low points and reaches sensing points as it drains down afterward, so the symptoms often lag the delivery by minutes to hours.
Can the membrane stage be flushed if liquid reached it?
Follow the unit maker service instructions. The first step is always stopping the inflow by fixing the upstream cause; flushing a stage while liquid keeps arriving solves nothing.
Luoyang Wohong Petrochemical Equipment Co., Ltd. supplies membrane vapor recovery units with serviceable knockout and drainage stages, and helps operators trace carryover to its source.