› Floating Roof Tank Rim Seal VOC Emission Simulator Back
Tank / Air Emissions

Floating Roof Tank Rim Seal VOC Emission Simulator

A web tool that converts the operating parameters of an external or internal floating roof tank into an annual hydrocarbon VOC emission estimate using the API 2517 rim seal loss equation. Change diameter, seal type, vapour pressure and wind speed to see the impact on annual loss and regulatory compliance in real time.

Parameters
Tank diameter D
m
Roof type
External roofs are directly exposed to wind
Rim seal
Leak drops sharply from primary to liquid-mounted
Vapour pressure P
kPa
Summer gasoline RVP ≈ 60–90 kPa
Average molecular weight M_v
g/mol
Throughput Q
bbl/yr
Annual shipped volume drives withdrawal loss
Wind speed v
m/s
Service condition
Representative fluid preset (label only)
Results
—
Tank circumference (ft)
—
Rim seal loss (kg/yr)
—
Withdrawal loss (kg/yr)
—
Total VOC (t/yr)
—
Loss per barrel (g/bbl)
—
Compliance
—
Tank cross-section — rim seal vapour escape

The deck rides on the liquid surface; the rim seal bridges the gap to the shell. Strong wind creates a pressure difference across the seal and pulls VOC into the atmosphere.

Wind sensitivity — rim seal loss vs wind speed
Seal type comparison — same conditions, three seal styles
Theory & Key Formulas

$$L_{\text{rs}} = (K_{ra} + K_{rb}\,v^{n})\cdot \frac{P}{P_{\text{atm}}}\cdot \frac{M_v}{100}\quad\text{[lb/ft·yr]}$$

API 2517 rim seal loss per unit circumference. K_ra and K_rb depend on seal type, v is wind speed, n = 1.5, P is vapour pressure and M_v the molecular weight.

$$E_{\text{rim}} = L_{\text{rs}}\cdot \pi D\cdot 3.281\cdot 0.4536\quad\text{[kg/yr]}$$

Multiply by rim circumference πD (m→ft via 3.281) and 0.4536 to convert lb → kg per year.

$$E_{\text{wd}} = 0.0001\,Q\cdot \frac{P}{35},\qquad E_{\text{tot}} = E_{\text{rim}} + E_{\text{stand}} + E_{\text{wd}}$$

Withdrawal loss E_wd (Q in bbl/yr) and total emission. E_stand is 0 for an EFRT and a fixed 50 kg/yr for IFRT / dome roofs.

What is the Floating Roof Tank Rim Seal VOC Emission Simulator?

🙋
Some oil tanks have a roof that literally floats on the liquid, right? Why bother making the roof float?
🎓
Good question. Those are floating roof tanks, and they are standard practice for very volatile liquids like gasoline and crude. A fixed roof leaves a big vapour space above the liquid; when the sun heats it during the day, VOC vents out as the tank "breathes". That is breathing loss. Drop the roof onto the liquid and the vapour space disappears, cutting losses by roughly an order of magnitude.
🙋
So a floating roof means zero VOC? But the result changes a lot when I switch the "rim seal" selector on the left.
🎓
Zero is too optimistic. There is always a few-centimetre gap between the deck and the shell, and the rim seal bridges it. With an old primary-only mechanical seal, a wind gust drops the pressure on one side of the seal and VOC diffuses through the gap. API 2517 puts the still-air coefficient K_ra at 6.7 lb/ft·yr for that case, but only 0.4 for a liquid-mounted seal with a secondary. The wind exponent v^1.5 also matters: at a coastal terminal with 8–10 m/s gusts, emissions can multiply several fold.
🙋
Throughput is an input too. Where does annual barrels in/out actually show up?
🎓
That is the withdrawal loss term. Every time you pump out product, the level drops and a fresh strip of wetted shell is exposed. The thin oil film on that strip evaporates — typically tens of grams per barrel — and adds up over the year. Tanks with higher vapour pressure (summer gasoline) and higher turnover see larger withdrawal loss. The tool uses the linear approximation E_wd = 0.0001·Q·(P/35) and adds it to the rim seal term.
🙋
There is a "Compliance" badge at the top right. Where does the 25 t/yr threshold come from?
🎓
25 t/yr is just a reference value baked into the tool, rounded from typical large-tank thresholds in U.S. EPA NESHAP and Japanese air-pollution law. Real compliance depends on local rules — California's SCAQMD limits the physical rim gap, while Tokyo and Kanagawa use facility-wide VOC mass caps with seasonal factors. Use this tool to feel out sensitivities like "how much does a secondary seal reduce emissions?" or "how much worse is a windy site?", then confirm against your local regulation before reporting.

Frequently Asked Questions

Because the floating roof rides directly on the liquid surface, the breathing loss that dominates fixed roof tanks is essentially eliminated. What remains is the rim seal loss — diffusion and wind-driven flow through the narrow gap between the roof and shell — plus withdrawal loss as fresh shell area is exposed each time product is pumped out. Internal floating roof tanks (IFRT) also add small leaks from deck fittings such as deck legs and column splitters.
API 2517 / AP-42 expresses the rim seal loss as L_rs = (K_ra + K_rb · v^n) · P* · M_v / D. K_ra is the still-air coefficient for each seal type (6.7 for a mechanical primary seal, dropping to 0.4 for a liquid-mounted seal), K_rb is the wind-dependent coefficient, P* is a vapour pressure function (~P/P_atm), M_v is the average molecular weight and v is the ambient wind speed. This tool uses a simplified form that accepts kPa and m/s directly and returns values close to the API reference numbers.
An EFRT exposes the deck directly to the atmosphere, so solar heating and wind have a strong effect. An IFRT places the deck inside a fixed roof, so wind effects shrink but additional emission paths appear at deck fittings. This tool sets the EFRT standing loss to zero and adds a fixed 50 kg/yr fitting term for IFRT and geodesic domes. On real tanks the fitting term ranges from 100 to 500 kg/yr depending on fitting count and design.
The 25 t/yr value used here is a round figure chosen for quick reference, broadly aligned with the large-tank reporting thresholds in U.S. EPA NESHAP Subpart CC and Japanese air pollution control law. In practice, local rules differ. SCAQMD Rule 463 in California, for instance, limits the actual rim gap area to ≤3.8 cm² per metre of circumference, while Japanese prefectures use facility-wide VOC mass caps. Use the verdict as guidance and confirm against the local regulation that governs your site.

Real-World Applications

Refinery and terminal seal upgrade studies: Sites with dozens of 40–80 m diameter crude and gasoline tanks can typically cut 5–15 t/yr of VOC per tank by retrofitting from a mechanical primary-only seal to a liquid-mounted seal with a secondary. Switching the seal-type selector in this tool shows the before/after ratio and dollar loss instantly, making it useful for prioritising retrofit campaigns (high-RVP gasoline → crude → diesel).

Environmental impact assessment: Permit applications for new terminals must report annual VOC emissions estimated from AP-42 / API 2517, and relate them to ozone-formation potential and odour limits in the EIA report. This tool lets engineers swap diameter, roof type and fluid to compare a worst-case (peak summer vapour pressure plus high wind) with normal operation on a single screen, supporting concept-stage scoping.

EPA NESHAP and SCAQMD reporting packages: Oil majors and traders with US operations must submit EPA NESHAP Subpart CC declarations and SCAQMD Rule 463 gap inspection results every year. Generating wind-sensitivity charts here and overlaying site meteorology turns regulatory-risk quantification and corrective-action costing into a numerical input for internal ESG reporting.

Pre-scoping for detailed CFD studies: Large tanks are sometimes modelled in ANSYS Fluent or OpenFOAM to capture the diffusion and convection around the rim seal. The analytic estimate from this tool is useful as a mass-balance check for CFD (whether inflow/outflow agrees with API order of magnitude) and to size mesh resolution. A CFD value more than five times the analytic figure is a good prompt to revisit boundary conditions.

Common Misconceptions and Pitfalls

The biggest pitfall is assuming "floating roof means zero VOC". A floating roof eliminates breathing loss almost completely, but the rim seal and withdrawal terms always remain. A 60 m diameter gasoline tank with an old primary-only seal (K_ra = 6.7) running summer RVP 80 kPa easily exceeds 20 t/yr in this tool. "It is a floating roof" is not a regulatory defence — individual assessment that includes seal type and condition is required.

Next, relying only on annual-average wind speed. Rim seal loss scales as v^1.5, so a year of 4 m/s average wind can hide a few high-wind days (10 m/s) that contribute half of the actual emissions. The right workflow is to bin hourly wind data from a mesoscale model like AERMET and integrate this tool over the histogram. Watch the steep rise in the wind sensitivity chart and share the resulting envelope with operations.

Finally, treating API 2517 as a universal fluid model. The API equations were developed mainly for gasolines and naphthas that can be treated as quasi-pure hydrocarbons. Mixed crudes and chemicals such as methanol or MTBE need coefficient corrections. For MTBE in particular, ignoring the effective vapour pressure drop due to water absorption leads to a clear overestimate. The "service condition" selector here is for display only; for project work, adjust K_ra and K_rb based on the real fluid composition.

How to Use

  1. Enter tank diameter in meters (e.g., 40m for a crude oil storage tank)
  2. Input product vapor pressure in kPa at operating temperature (e.g., 8.5 kPa for naphtha)
  3. Specify molecular weight in g/mol (e.g., 92 g/mol for typical light naphtha)
  4. Enter annual throughput in barrels per year (e.g., 500,000 bbl/yr)
  5. Click Calculate to generate rim seal losses, withdrawal losses, and total VOC emissions in metric tons per year
  6. Compare total VOC and loss-per-barrel metrics against EPA 40 CFR Part 65 Subpart G compliance thresholds

Worked Example

A 50-meter diameter internal floating roof tank storing crude oil (MW = 120 g/mol, vapor pressure = 3.2 kPa at 40°C) with 600,000 bbl/yr throughput: Tank circumference = 157.1 m (515 ft). Rim seal loss ≈ 42 kg/yr. Withdrawal loss ≈ 18 kg/yr. Total VOC emission = 0.060 t/yr (60 kg/yr). Loss per barrel = 0.10 g/bbl. Status: Compliant with 40 CFR 65.84(b) exemption threshold of 6.5 t/yr annual emissions.

Practical Notes

  1. Rim seal losses dominate for volatile crude/condensate; withdrawal losses account for 15-30% of total for lower vapor pressure products like fuel oil
  2. Temperature-dependent vapor pressure changes ±5% per 10°C; update kPa seasonally for refined products with wide seasonal volatility swings
  3. External floating roof tanks emit 2–4 times higher rim seal losses than internal designs; factor weather exposure into compliance planning
  4. Tanks ≤300,000 bbl/yr throughput typically qualify for small tank exemptions regardless of emissions calculated