Terminal · Storage
External Floating Roof
A large open top tank whose roof is a steel deck floating directly on the oil, sealed to the shell around its edge.
What it does
With almost no vapor space, it keeps evaporation low on large crude tanks at terminals and pipeline hubs. The deck is a pontoon or double deck type, with a primary rim seal and usually a secondary seal above it; wind across the open top drives most of its losses.
Basics
| Deck types | Pontoon or double deck |
| Rim seals | Primary, plus a secondary seal |
| Main losses | Rim seal and deck fittings, wind driven |
| Variant | Domed external floating roof, a dome added to block wind |
Common failures
- Rain or snow load not drained, so the deck tilts or sinks
Troubleshooting
- Symptom
- Water standing on the deck after a storm
- Likely cause
- Roof drain plugged or valve closed
- What to check
- Clear the roof drain and check it for oil, which means a leaking drain