While the tank truck pushes fuel into the underground tank, the vapor in that tank has to go somewhere, and the balance line is the road it takes back to the truck. One restriction in that road and the whole unloading slows, sprays or vents where it should not.
What the Balance Line Actually Does
Fuel entering the tank displaces vapor; the balance line returns it to the truck’s compartment instead of letting pressure build and escape elsewhere. When the line is clear, unloading runs at design speed and the station’s vapor stays in closed hardware. When it narrows, every downstream symptom appears at once: slow unloading, hissing at fittings, pressure relief where nobody planned it.
Hose and Connection Condition
The balance hose lives a harder life than its size suggests: dragged, driven near, and connected hastily in all weather. Inspect for kinked sections, crushed regions and cracked ends before each use. Camlock couplings need gaskets present and whole; a missing gasket is a vapor leak at the exact moment vapor flow is highest. Connect firmly and give the coupling a confirmation pull before opening valves.
Deck and Caps in the Off Season
Balance line deck fittings collect grit and ice when unused. Keep caps on, operate the valves periodically so they do not seize in one position, and check the vapor cap on the drop tube assembly for its gasket too. A cap that does not seal is a closed system that quietly is not.
Watching During the Unload
The first minute of unloading tells the truth: listen at the balance connection for continuous flow, watch the tank pressure indication if fitted, and glance at the fill point for seepage. If the truck’s meter crawls while its pump runs hard, suspect the balance path before anything else, stop and investigate rather than forcing it through.
| Check | When | Failure sign |
|---|---|---|
| Balance hose | Before each use | Kinks, crushing, cracked ends |
| Couplings | At connection | Missing or flat gaskets |
| Deck fittings | Periodic | Grit, seized valves |
| Unload watch | First minute | Slow meter, hissing fittings |
Frequently Asked Questions
Does a balance line need pressure testing?
Follow the site’s testing program; at minimum, leaks show up in the daily connection checks and the unloading watch.
Can a worn gasket wait for the next delivery?
No; it leaks at maximum vapor flow. Spares cost little and belong at the fill point.
Why does unloading slow in cold weather?
Partly vapor physics, partly stiffer hoses; a genuinely restricted line slows it far more than temperature does.
Does VOHON supply unloading and vapor balance equipment?
Yes. VOHON provides station vapor path equipment and supports unloading practice questions at lixiang@vohon.cn.