Floating Roof Seal Loss — Engineering Fundamentals

1. The four components

An external floating roof tank emits VOC from four paths, each with its own AP-42 equation:

1. Rim seal — wind-driven vapor escape through the roof-shell rim gap. Largest single component for EFR. Scales with vⁿ.
2. Withdrawal — liquid film clinging to the shell as the roof descends with falling level. Small but nonzero. Scales with throughput.
3. Deck fittings — vapors leaking around each access hatch, gauge well, leg, vacuum breaker, etc. Per-fitting K from AP-42 Table 7.1-12.
4. Deck seam — bolted decks only; loss along the seam length. K_D · S_D · D² scaling.

2. The P* function

The dimensionless vapor function P* derived from Raoult's law + fugacity at the seal interface:

P* approaches x as x → 0 (low TVP) and approaches 1 as x → 1 (TVP → atm). Most refined-product and crude tanks operate at x < 0.3, where P* ranges 0.05–0.20.

3. Rim-seal physics

Wind blows across the tank roof, creating a pressure differential between windward and leeward rim. Vapor in the seal annulus is pulled toward the leeward side and escapes through the imperfect seal. The exponent n in vⁿ captures whether the seal is wind-dominated (n=2–3 for vapor-mounted, exposed) or laminar-leakage-dominated (n=1–1.5 for mechanical shoe with secondary).

Secondary seals cut rim loss by 60–80% vs primary-only. The "rim-mounted" secondary is most common; the "shoe-mounted" secondary is even better but mechanically more complex.

4. Deck fittings inventory

A single open slotted gauge well can equal 20 access hatches in loss. Quick win for emissions reduction: gasket every gauge well.

5. References

• EPA AP-42 §7.1.4 (Nov 2006 + updates). Tables 7.1-8 (seal K_R) and 7.1-12 (fitting K_F).
• API MPMS Ch. 19.2 — Evaporation Loss from Floating-Roof Tanks.
• 40 CFR 60 Subparts Ka / Kb — NSPS for floating-roof tanks.
• API 650 — Welded Tanks for Oil Storage (companion).