A Stage III vapor return line is designed to carry vapor, but real lines always carry some liquid with it: droplets entrained from the tank ullage, condensate forming in cool sections, and small amounts carried over from dispensing. The line works well when that liquid drains back to where it belongs. It works poorly when low points and siphons collect the liquid into pockets that restrict the vapor path. Slope planning and pocket control are therefore not finishing touches; they are the main design decisions for this line.
What a liquid pocket does
When vapor flows over a trapped liquid slug, the effective cross-section of the line shrinks and the pressure drop rises. The vacuum available at the dispensers falls, extraction becomes sluggish, and the processing unit works harder to reach the same recovery. In cold weather a trapped pocket can even freeze and block the line outright.
Pockets also hide diagnostics. A site that slowly develops a sag in the return line shows up first as a gradual loss of extraction performance, which is easy to misread as equipment wear.
Slope: consistent, not perfect
The practical rule is that the line should fall continuously back toward the tank or the designated liquid return point, with no local low points along the way. Perfect continuous slope over a long run with other services in the trench is not always achievable, so the design goal becomes: minimize the number and depth of low points, and make sure each one can drain.
Where a low point cannot be avoided, it should be deliberate, documented and, where the layout allows, provided with a drain leg rather than left as an accident of trench geometry.
Routing choices that create pockets
The classic offenders are dips to pass under other ducts, long horizontal runs with small settlement over time, and risers where the line climbs to enter the processing unit. Each riser is a potential siphon: liquid that reaches the bottom of a riser has to be pushed uphill by vapor pressure, and a full siphon can hold the line hostage until it is cleared.
Coordinating the return line route with the electrical and other services early in the design avoids most forced dips. Where the crossing is unavoidable, the dip should be as shallow and as short as the other services permit.
Design and verification checklist
- Agree the reference slope direction and the liquid return point before routing;
- Route to avoid dips, and keep unavoidable ones shallow and short;
- Fix the line supports so settlement cannot create new low points later;
- Check slope with a level during installation, section by section;
- Verify flow performance after installation, not just tightness;
- Document the final profile, including any deliberate low points.
Reading symptoms on an operating site
| Symptom | Line-related cause | First check |
|---|---|---|
| Extraction sluggish at some dispensers | Pocket near that dispenser branch | Profile of that branch |
| Vacuum at the unit is fine, sites are weak | Restriction in the return run | Low points and recent trenching |
| Trouble appears mainly in cold weather | Water pocket freezing | Drainage of low points |
| Performance fell after surface works | Settlement created a sag | Re-survey the affected run |
| Intermittent slugging noise | Liquid accumulating and releasing | Slope continuity and supports |
Treating the return line as part of the vapor recovery process, rather than as a piece of plumbing between the tank and the unit, keeps its design honest. A line with deliberate slope, few and documented low points and supports that hold the profile delivers the extraction performance the processing unit was sized for, year after year.
Frequently asked questions
How much slope is enough?
Designs typically specify a modest continuous fall back toward the tank, and the exact figure follows the project specification. The critical property is continuity: one unbroken fall matters more than a large average slope interrupted by sags.
Can a liquid pocket clear itself?
Small amounts can be pushed through at high vapor flow, but pockets tend to grow rather than shrink as condensate accumulates. A site whose performance improves on hot days and degrades otherwise is showing the classic pocket signature.
What if the trench geometry forces a dip?
Make it shallow, short and documented, confirm the line still meets performance after installation, and record the low point in the as-built profile so future troubleshooting starts in the right place.
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