Tank Emissions

Floating Roof Seal Loss — Engineering Fundamentals

AP-42 §7.1.4 four-component breakdown: rim seal, withdrawal, deck fittings, and deck seam.

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Method

EPA AP-42 §7.1.4

Four-component loss model for external floating-roof tanks.

Vapor function

P* = x / (1+√(1−x))²

Dimensionless driver from Raoult's law at the seal.

Dominant fitting

Slotted gauge well

K_F = 14 lb-mol/yr; gasketing is the quick win.

Contents

Use this guide when you need to:

  • Break out floating-roof VOC loss into its four AP-42 paths.
  • Compute the P* vapor function from product TVP.
  • Inventory deck fittings and target high-loss sources.

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* = x / (1 + √(1 − x))² ; x = PVA / PA

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

FittingK_F (lb-mol/yr)Notes
Access hatch (gasketed)1.6per hatch
Roof leg0.53per leg; typical 30 for 100 ft tank
Vacuum breaker6.2per breaker
Rim vent0.71per vent
Gauge-float well, unbolted, ungasketed14biggest single fitting source
Sample well12cut by gasketed cover

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).

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Frequently Asked Questions

What are the four AP-42 loss components for a floating-roof tank?

AP-42 §7.1.4 breaks external floating-roof loss into rim-seal loss (wind-driven vapor through the rim gap, usually the largest), withdrawal loss (liquid film clinging to the shell as the roof descends), deck-fitting loss (vapors around hatches, legs, gauge wells, etc.), and deck-seam loss (bolted decks only).

What is the P* vapor function?

P* is the dimensionless vapor function derived from Raoult's law and fugacity at the seal, equal to x / (1 + √(1 − x))² where x is the ratio of product true vapor pressure to atmospheric pressure. Most refined-product and crude tanks run at x below 0.3, giving P* roughly 0.05–0.20.

Which deck fitting drives the most loss?

A slotted gauge well with no gasket is the largest single fitting source at K_F ≈ 14 lb-mol/yr — one open well can equal about 20 access hatches. Gasketing every gauge well is a quick emissions-reduction win, and secondary rim seals cut rim-seal loss by 60–80% versus primary-only.