A vacuum decay check is the simplest way to prove a Stage III vapor path is tight before the system goes into service. The idea is plain: pull the isolated vapor line to a set vacuum, close it off, and watch how fast the reading falls. A sound path holds; a path with a loose connection, a damaged seal or an unsealed penetration bleeds down visibly. Doing this test before handover turns hidden installation defects into a punch list instead of a first-week breakdown.
Why Tightness Matters More on Stage III
The Stage III path works at low vacuum while the unit scours vapor from the tanker ullage during unloading. Small leaks that a pressure line would shrug off matter here, because air entering the path dilutes the vapor stream, skews the unit’s working point and makes recovery less effective. Leaks also let vapor escape during idle periods, which is exactly what the system exists to prevent. A decay test speaks to all of this in one number, which is why it belongs in every commissioning sequence.
Setting the Test Up
A useful test is deliberate rather than improvised. The sequence below covers the setup that produces readings worth recording.
- Isolate the section under test from the recovery unit and the tanker side so the result describes one defined path.
- Connect a calibrated vacuum gauge at a point specified in the system documentation, not wherever a port happens to be.
- Pull the section to the vacuum level the equipment documents call for, then close the isolation and start the clock.
- Watch the gauge for the stated observation period and write down the reading at each interval.
- Repeat any failing section after correction, and record both the failure and the passing repeat.
What the Numbers Mean
A perfect vacuum never holds still, so every specification allows some decay. What matters is the rate against the reference value in the documentation, adjusted for temperature and section volume. A fast initial drop that then settles usually points to a large single leak near the pump side; a steady linear fall suggests one or more small leaks distributed along the path. Neither pattern names the culprit, but both tell the crew whether to look at a fitting, a penetration or a component seal.
Common Failure Points the Test Exposes
| Failure point | Typical cause | Correction |
|---|---|---|
| Connection fittings | Under-torqued or misaligned during assembly | Re-seat and torque to the written values |
| Penetrations and risers | Sealing compound gaps at the slab | Re-seal and re-test the section |
| Coupler and adapter seals | Nicked or seated dry at installation | Replace the seal and lubricate as documented |
| Blanked branch points | Future-use stubs capped without rated caps | Fit proper blank fittings and mark them |
Recording Results the Handover Pack Can Use
A decay test only has value if the record survives. Note the section tested, the gauge used, the starting vacuum, the interval readings and the final verdict, and file it with the as-built drawings. Owners reviewing a handover should look for this page; its absence is a sign the tightness claim rests on someone’s memory. When the record exists, later service visits have a baseline to compare against, and a slow drift in holding performance becomes visible years before it becomes a complaint.
Making Decay Checks a Habit, Not a One-Off
Stations that repeat a shortened decay check during periodic service catch seal aging and fitting looseness long before a delivery problem surfaces. The equipment needed is modest, the procedure takes minutes per section, and the reading joins the maintenance file beside everything else. Tightness is not a feature a system keeps forever; it is a condition a station maintains, and the vacuum decay check is how that condition is measured.
What vacuum level should the test reach?
Use the level stated in the system documentation rather than a general rule, because the figure depends on section volume and component ratings. The documentation value is also the one the handover record should reference.
Can weather affect the decay reading?
Yes. Temperature changes during the observation period move the reading on their own, so note ambient conditions and avoid testing across sharp temperature swings such as direct sun on an exposed riser.
How often should the path be re-tested after handover?
A full check at commissioning, a repeat at the first annual service and then alongside periodic maintenance suits most sites. Increase frequency if readings drift or if the station records any vapor smell complaint.
Luoyang Wohong Petrochemical Equipment Co., Ltd. supplies Stage III vapor recovery equipment for fuel stations and supports contractors with commissioning and tightness testing practice.