Tank Emissions Calculator (EPA AP-42)

Verified Content: All equations, emission factors, and calculation methods in this simulator follow EPA AP-42 Section 7.1 (Organic Liquid Storage Tanks), the EPA TANKS 4.09D methodology, and 40 CFR Part 60 Subpart Kb regulatory requirements. See verification log

Introduction

Spill a few milliliters of gasoline in a university lab and nobody blinks. The fume hood handles it. At industrial scale, that same compound sitting in a 100-foot diameter storage tank releases thousands of pounds of volatile organic compounds (VOCs) into the atmosphere every year, even when you are not pumping anything in or out [1]. The EPA regulations require facilities to quantify these emissions for Title V permits, New Source Review applications, and annual emissions inventories. Miss the calculation by 20%? That error could mean a permit violation, costly control equipment you did not need, or worse, enforcement action.

Environmental engineers often find that tank emissions are one of the most underestimated sources in facility inventories. A refinery might have hundreds of storage tanks, each breathing vapor day and night, each losing product during every fill-and-empty cycle. The mass balance shows that atmospheric losses from tank farms can rival process vent emissions at many facilities [2]. From a process safety standpoint, accurate emission estimates also feed into dispersion modeling for risk assessments, determining whether a facility needs additional vapor recovery systems or enhanced monitoring.

Our simulator implements the complete EPA AP-42 Section 7.1 equation set for both fixed roof and floating roof tanks. You can model standing losses from daily temperature cycling, working losses from throughput operations, rim seal and deck fitting emissions from external floating roofs, and even the acute losses that occur when a floating roof lands on its support legs. The economics drive you toward getting these calculations right the first time, because permit modifications and control equipment retrofits cost orders of magnitude more than accurate upfront engineering.

What Is Tank Emissions Estimation?

Tank emissions estimation is the systematic calculation of VOC releases from liquid storage vessels using standardized methods published by the U.S. Environmental Protection Agency [1]. At industrial scale, storage tanks function as giant evaporators. Even when sealed, they release vapors through multiple pathways that the AP-42 methodology categorizes into distinct loss mechanisms.

Think of a storage tank like a checking account for molecules. The liquid inventory represents your principal balance. Standing losses are like account maintenance fees that the atmosphere collects daily just for holding your material. Working losses are transaction fees charged every time you move product in or out. The mass balance shows that over a year, these small withdrawals accumulate into significant totals that regulators track carefully.

Fixed roof tanks (the simpler type with a permanently attached roof) experience two primary emission mechanisms. Standing storage losses occur when the vapor space expands and contracts with daily temperature changes, pushing saturated vapor out through vents during the day and drawing fresh air in at night. Working losses occur when incoming liquid displaces vapor-laden air during filling operations [1].

Floating roof tanks use a deck that floats directly on the liquid surface, eliminating most of the vapor space. However, they introduce different emission pathways: rim seal losses where the seal contacts the tank shell, deck fitting losses through penetrations like gauge wells and support columns, deck seam losses if the deck is bolted rather than welded, and withdrawal losses from liquid that clings to the shell as the roof descends [1].

Why does this matter for your facility? The EPA regulations require emission estimates for any tank exceeding certain throughput or vapor pressure thresholds. Under 40 CFR Part 60 Subpart Kb, new and modified tanks storing liquids with vapor pressures above 0.75 psia must meet specific control requirements [3]. State air quality agencies may impose even stricter limits. Accurate calculations determine whether you need controls, what type of controls will achieve compliance, and how much VOC allowance to include in your operating permit.

How the Simulator Works

Our simulator accepts your tank specifications and operating parameters, then applies the complete AP-42 equation set to calculate annual emissions. The calculation flow mirrors what environmental engineers perform manually but eliminates transcription errors and automates the lookup of tabulated factors.

Tank Configuration Parameters

Stored Liquid Properties

Meteorological Inputs

Throughput and Operating Data

Technical Deep-Dive: AP-42 Emission Equations

Fixed Roof Tank: Standing Storage Loss

The standing storage loss equation from AP-42 Section 7.1 calculates emissions from daily breathing [1]:

LS = 365 x VV x WV x KE x KS

Where:

• LS = standing storage loss (lb/yr)
• VV = vapor space volume (ft3)
• WV = vapor density (lb/ft3)
• KE = vapor space expansion factor (dimensionless)
• KS = vented vapor saturation factor (dimensionless)

The vapor space expansion factor KE captures the combined effect of daily temperature change and barometric pressure variation. Experienced process engineers know that a white-painted tank in Phoenix, Arizona experiences vastly different thermal cycling than a similar tank in Seattle, Washington. The 365-day multiplier converts daily losses to annual totals.

The vapor density WV depends on both molecular weight and the ratio of vapor pressure to atmospheric pressure:

WV = (MV x PVA) / (R x TLA)

Where R is the gas constant (10.731 psia-ft3/lb-mole-R) and TLA is the daily average liquid surface temperature in Rankine.

Fixed Roof Tank: Working Loss

Working losses occur during filling operations when incoming liquid displaces vapor [1]:

LW = N x VLX x KN x KP x WV x KB

Where:

• LW = working loss (lb/yr)
• N = annual turnovers
• VLX = working capacity (ft3), equal to (pi/4) x D2 x HLX
• KN = turnover factor (dimensionless)
• KP = working loss product factor (dimensionless)
• KB = vent setting correction factor (dimensionless)

The turnover factor KN equals 1.0 for turnovers greater than or equal to 36 per year. For lower turnover rates, it reduces proportionally because vapor trapped in the tank has more time to diffuse before the next pumping cycle [1].

Floating Roof Tank: Total Loss

For external floating roof tanks, total emissions are the sum of four components [1]:

LT = LR + LWD + LF + LD

Where:

• LT = total floating roof loss (lb/yr)
• LR = rim seal loss (lb/yr)
• LWD = withdrawal loss (lb/yr)
• LF = deck fitting loss (lb/yr)
• LD = deck seam loss (lb/yr)

Rim Seal Loss

The rim seal loss equation reflects the emission pathway along the tank shell circumference [1]:

LR = D x P x MV x KRa x (v)^n*

Where KRa and n are empirically derived factors from AP-42 Table 7.1-8 that depend on seal type. A mechanical shoe seal with no secondary has KRa = 0.6 and n = 2.0. Adding a rim-mounted secondary seal drops KRa to 0.1 while n remains 2.0 [1].

Deck Fitting Loss

Each deck penetration contributes to emissions based on its type and configuration [1]:

LFi = FFi x P x MV x (Kfa + Kfb x v^m)*

Where FFi is the number of fittings of type i, and Kfa, Kfb, and m are fitting-specific factors from AP-42 Table 7.1-12. An ungasketed access hatch has Kfa = 8.5, Kfb = 0, m = 0. A bolted deck leg has Kfa = 1.2, Kfb = 0.26, m = 1.0 [1].

Withdrawal Loss

When a floating roof descends during pump-out, liquid clings to the tank shell [1]:

LWD = 0.943 x Q x CS x WL x D / (HLX x WL)

The clingage factor CS depends on both shell condition and liquid type. Light rust with gasoline has CS = 0.0015 gal/ft2. Dense rust with crude oil increases to CS = 0.0060 gal/ft2 [1].

Deck Seam Loss

Bolted decks have seams that allow vapor passage [1]:

LD = SD x AD x P x MV x KD*

Where SD is the seam length factor (ft seam per ft2 deck area), AD is the deck area, and KD is the deck seam loss factor. Welded decks have KD = 0, eliminating this loss pathway entirely.

Learning Objectives

After working through this simulator, you will be able to:

1. Calculate fixed roof standing losses using the AP-42 vapor space expansion methodology, correctly applying the KE factor for site-specific temperature ranges

2. Determine working loss emissions based on tank turnover rates, distinguishing when the turnover factor KN applies and how vent settings modify the result

3. Quantify floating roof emissions by summing rim seal, deck fitting, deck seam, and withdrawal losses using the appropriate AP-42 tables

4. Select emission factors from AP-42 tables based on tank configuration, including seal type, deck type, fitting counts, and shell condition

5. Evaluate control options by comparing emissions between tank types (fixed roof vs. floating roof) and identifying which loss mechanisms dominate for a given application

6. Prepare permit-quality emission inventories with the documentation and calculation backup required for regulatory submittals

Exploration Activities

Activity 1: Fixed Roof vs. Floating Roof Comparison

Start with a 100-ft diameter, 40-ft tall tank storing gasoline (PVA = 5.2 psia, MV = 68 lb/lb-mole). Configure as a fixed roof cone tank with Houston, TX meteorology. Record the total annual emissions. Now switch to an external floating roof with a mechanical shoe primary seal. Compare the emissions. You should observe approximately 80-90% reduction with the floating roof [1]. This dramatic difference explains why the EPA regulations require floating roofs for high-vapor-pressure stocks above certain throughput thresholds.

Activity 2: Rim Seal Type Impact

Using the external floating roof configuration from Activity 1, systematically change the rim seal from:

• Mechanical shoe, primary only
• Mechanical shoe with shoe-mounted secondary
• Mechanical shoe with rim-mounted secondary
• Liquid-mounted, primary only
• Liquid-mounted with rim-mounted secondary

Document the rim seal loss (LR) for each configuration. The economics drive you toward understanding these differences because secondary seals can reduce rim seal losses by 85-95% [1]. Calculate the value of recovered product per year at current prices to see if the seal upgrade pays for itself.

Activity 3: Temperature Sensitivity Analysis

Return to the fixed roof tank configuration. Select Phoenix, AZ (hot, dry) and record emissions. Switch to Seattle, WA (mild, cloudy) and compare. The daily temperature range and solar insolation dramatically affect standing losses. Process engineers in different climates face different emission challenges, and this activity demonstrates why identical tanks in different locations can have vastly different permit limits.

Activity 4: Throughput Scaling

With the fixed roof tank at Houston conditions, vary the annual throughput from 100,000 bbl/yr to 5,000,000 bbl/yr in increments. Plot working loss versus throughput. At industrial scale, working losses eventually dominate standing losses for high-turnover tanks. Identify the crossover point where working loss exceeds standing loss for your configuration. This analysis informs whether vapor recovery during loading operations would be more cost-effective than tank modifications.

Real-World Applications

Petroleum Refinery Tank Farms

Refineries operate hundreds of storage tanks containing crude oil, intermediate products, and finished fuels. The mass balance shows that tank farm emissions often exceed 1,000 tons per year of VOCs at large facilities [4]. Environmental managers use AP-42 calculations to allocate emission allowances across the tank farm, determine which tanks need floating roofs or vapor recovery, and track compliance with operating permit limits. During permit renewals, accurate emission estimates prevent overstatement (which wastes permit headroom) and understatement (which triggers enforcement).

Chemical Manufacturing Facilities

Chemical plants store feedstocks, intermediates, and products with vapor pressures ranging from negligible to highly volatile. From a process safety standpoint, accurate emission calculations feed into both air permit applications and Process Hazard Analyses under OSHA PSM [5]. A tank storing vinyl chloride monomer (PVA > 40 psia) requires different controls than one holding ethylene glycol (PVA < 0.01 psia). The simulator helps process engineers right-size control equipment and avoid both over-engineering and non-compliance.

Air Permit Applications

New facilities and major modifications require New Source Review permits under the Clean Air Act [6]. The permit application must include emission estimates for all significant sources, including storage tanks. State agencies review AP-42 calculations for reasonableness and may require additional documentation for unusual configurations. Having a calculator that transparently shows all inputs and intermediate values streamlines the review process and reduces permit issuance timelines.

Environmental Compliance Audits

Compliance audits compare actual operations to permitted conditions. Auditors verify that tank parameters (diameter, height, product, throughput) match permit assumptions and that calculated emissions stay within allocated limits [7]. The simulator produces exportable reports that document the calculation basis, making audit preparation straightforward. When auditors ask how you calculated those numbers, you can show them exactly what went in and what came out.

Emissions Trading Programs

Regional cap-and-trade programs assign economic value to VOC emissions. Accurate calculations determine how many allowances a facility needs to purchase or has available to sell [8]. Overestimating emissions wastes money on unnecessary allowances. Underestimating triggers compliance obligations when actual emissions exceed reported values. The economics drive you toward precision because every ton of VOC has a dollar value in these markets.

Reference Data Tables

Vapor Pressure of Common Liquids at 77 deg F

Data from AP-42 Chapter 7 and NIST Chemistry WebBook [1][9]

Rim Seal Loss Factors (AP-42 Table 7.1-8)

Source: EPA AP-42 Section 7.1, Table 7.1-8 [1]

Clingage Factors (AP-42 Table 7.1-10)

Source: EPA AP-42 Section 7.1, Table 7.1-10 [1]

Challenge Questions

Level 1 - Foundational: A 50-ft diameter fixed roof tank stores toluene (PVA = 0.42 psia, MV = 92.1) at a Gulf Coast facility. Daily temperature ranges from 65 deg F to 90 deg F. Using the simulator, determine whether standing loss or working loss dominates for annual throughput of 500,000 bbl/yr. What physical explanation accounts for your finding?

Level 2 - Intermediate: Your facility operates two identical 100-ft diameter external floating roof tanks. Tank A has a mechanical shoe primary seal only. Tank B was retrofitted with a rim-mounted secondary seal. Both have the same product (gasoline, PVA = 5.2 psia) and throughput (1,000,000 bbl/yr). Calculate the emission reduction achieved by the secondary seal. If VOC allowances cost 2,500 dollars per ton, what is the annual value of the emission reduction?

Level 3 - Applied: A new chemical plant will store 50,000 bbl of methyl ethyl ketone (MEK, PVA = 1.49 psia, MV = 72.1) with annual throughput of 400,000 bbl/yr. Tank diameter is 75 ft. Compare total annual emissions for: (a) cone roof fixed tank, (b) internal floating roof with vapor-mounted seal, (c) external floating roof with liquid-mounted seal and rim secondary. Which option would you recommend and why? Consider both emission rates and practical factors like cleaning access.

Level 4 - Design Integration: A refinery has 200,000 tons per year of VOC allowance in its Title V permit. Current tank farm emissions total 180,000 tons per year. A proposed expansion adds three new 150-ft diameter crude oil tanks (PVA = 3.0 psia) with combined throughput of 10,000,000 bbl/yr. Using external floating roofs with mechanical shoe seals and rim secondaries, calculate whether the expansion fits within existing permit limits. If not, what additional control would bring it into compliance?

Level 5 - Regulatory Strategy: Your state just adopted new RACT (Reasonably Available Control Technology) rules requiring 95% control of VOC emissions from tanks storing liquids with vapor pressure above 1.5 psia. You operate fifteen 80-ft diameter fixed roof tanks storing various solvents. Calculate baseline emissions for each tank. Which tanks must be controlled? Evaluate options including internal floating roofs, closed vent systems with thermal oxidizers, and product substitution with lower-volatility alternatives.

Common Misconceptions

Misconception 1: Floating roofs eliminate all emissions

Not true. While floating roofs dramatically reduce emissions compared to fixed roofs (typically 85-95% reduction), they do not eliminate them. Rim seal gaps, deck fittings, and withdrawal clingage all contribute to ongoing losses [1]. Additionally, roof landing events during complete tank emptying cause standing loss emissions comparable to a fixed roof tank during the idle period.

Misconception 2: Underground tanks have zero emissions

The AP-42 methodology sets standing loss to zero for underground tanks because ground temperature is relatively stable and eliminates the daily thermal cycling that drives breathing losses [1]. However, working losses still occur during filling operations, and the displaced vapors still contain VOCs. Underground status reduces but does not eliminate the emission calculation.

Misconception 3: Small tanks are not worth calculating

Many environmental engineers focus on the large tanks and estimate small tank emissions. At industrial scale, however, facilities may have dozens or hundreds of smaller tanks (day tanks, slop tanks, intermediate storage) that collectively contribute significant emissions. The EPA regulations require accounting for all sources above de minimis thresholds, regardless of individual size [6].

Misconception 4: Annual average temperature is sufficient for calculations

The standing loss equations specifically require daily maximum and minimum temperatures, not annual averages [1]. A location with daily highs of 95 deg F and lows of 65 deg F has much higher standing losses than a location with steady 80 deg F temperatures, even though both might have similar annual averages. The temperature swing drives vapor space expansion and contraction.

Frequently Asked Questions

Q1: Is this calculator acceptable for air permit applications?

Our simulator implements the full AP-42 Section 7.1 equation set, which is the EPA-accepted methodology for storage tank emissions [1]. Many state agencies accept hand calculations or spreadsheet implementations of these equations. However, some states specifically require use of the EPA TANKS 4.09D software or state-approved alternatives. Check with your permitting agency before submitting. The export report documents all inputs and intermediate calculations for regulatory review.

Q2: How do I determine true vapor pressure for my product?

True vapor pressure (TVP) at the liquid surface temperature is the required input, not Reid Vapor Pressure (RVP) [1]. For pure compounds, use published vapor pressure equations or look up values at your operating temperature. For mixtures like gasoline, AP-42 provides correlation equations (Equations 1-22 through 1-28) that convert RVP to TVP based on temperature. The Antoine equation option in our simulator calculates temperature-dependent vapor pressure from published A, B, C constants.

Q3: What is the difference between external and internal floating roofs?

External floating roof (EFR) tanks have a floating deck open to the atmosphere, directly exposed to wind and rain. Internal floating roof (IFR) tanks have a fixed outer roof with a floating deck inside, protected from weather [1]. IFR tanks generally have lower rim seal losses because the fixed roof reduces wind velocity at the seal. However, IFR deck fittings may have different loss factors than EFR fittings. The simulator applies the appropriate factors for each configuration.

Q4: How often should I recalculate tank emissions?

Most operating permits require annual emission inventories [6]. You should recalculate whenever significant changes occur: different product, changed throughput, tank modification, or updated meteorological data. Some facilities recalculate quarterly to track against permit limits. The simulator stores no data between sessions, so maintain your own records of inputs and outputs for compliance documentation.

Q5: Do I need to include tank emissions in SARA 313 (TRI) reporting?

Yes, if the emitted compounds are TRI-listed chemicals and your facility meets activity thresholds [10]. Many VOCs including benzene, toluene, ethylbenzene, and xylenes (BTEX) are TRI-listed. Tank emissions calculated using AP-42 methods are acceptable for TRI reporting. The mass balance approach used in these calculations provides the defensible basis that TRI reporting requires.

References

1. U.S. Environmental Protection Agency. (2020). AP-42: Compilation of Air Pollutant Emission Factors, Chapter 7.1: Organic Liquid Storage Tanks. https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors

2. U.S. Environmental Protection Agency. (2006). EPA TANKS Emissions Estimation Software, Version 4.09D: User's Guide and Technical Documentation. https://www.epa.gov/chief/tanks-emissions-estimation-software-version-409d

3. U.S. Government Publishing Office. (2023). 40 CFR Part 60, Subpart Kb: Standards of Performance for Volatile Organic Liquid Storage Vessels. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/subpart-Kb

4. American Petroleum Institute. (2022). Manual of Petroleum Measurement Standards, Chapter 19.1: Evaporative Loss from Fixed-Roof Tanks. API Publishing.

5. U.S. Occupational Safety and Health Administration. (2023). Process Safety Management of Highly Hazardous Chemicals (29 CFR 1910.119). https://www.osha.gov/process-safety-management

6. U.S. Environmental Protection Agency. (2022). New Source Review (NSR) Permitting. https://www.epa.gov/nsr

7. U.S. Environmental Protection Agency. (2023). Clean Air Act Compliance and Enforcement. https://www.epa.gov/enforcement/air-enforcement

8. California Air Resources Board. (2024). Cap-and-Trade Program Regulation. https://ww2.arb.ca.gov/our-work/programs/cap-and-trade-program

9. National Institute of Standards and Technology. (2023). NIST Chemistry WebBook: Thermophysical Properties of Fluid Systems. https://webbook.nist.gov/chemistry/fluid/

10. U.S. Environmental Protection Agency. (2023). Toxics Release Inventory (TRI) Program. https://www.epa.gov/toxics-release-inventory-tri-program

11. Texas Commission on Environmental Quality. (2023). Air Permit Technical Guidance: Storage Tank Emission Calculations. https://www.tceq.texas.gov/permitting/air/guidance

12. South Coast Air Quality Management District. (2023). Rule 463: Storage of Organic Liquids. http://www.aqmd.gov/docs/default-source/rule-book/reg-iv/rule-463.pdf

About the Data

The emission factors, loss coefficients, and correlation equations in this simulator come directly from EPA AP-42 Section 7.1 and the EPA TANKS 4.09D technical documentation [1][2]. Vapor pressure data reference the NIST Chemistry WebBook for pure compounds [9]. Meteorological data presets (temperature ranges, insolation, wind speed) derive from NOAA climate normals for U.S. cities. The calculator applies these factors exactly as specified in the regulatory guidance, without proprietary modifications or interpolations.

We update the simulator when EPA releases revised AP-42 chapters or updated TANKS software. The current version reflects AP-42 Section 7.1 as revised in 2020. Users should verify that their state or local agency accepts the current AP-42 methodology, as some jurisdictions adopt revisions on different schedules.

How to Cite

APA Format: Simulations4All. (2026). Tank Emissions Calculator (EPA AP-42) [Interactive web application]. https://simulations4all.com/simulations/tank-emissions-calculator

Technical Report Format: Tank emissions were calculated using the Simulations4All Tank Emissions Calculator, which implements EPA AP-42 Section 7.1 equations for organic liquid storage tanks. Input parameters and calculated results are documented in the attached export report dated [DATE].

For Permit Applications: Emissions from Tank [ID] were calculated using EPA AP-42 Section 7.1 methodology (Equations 1-2 through 2-30) as implemented in the Simulations4All Tank Emissions Calculator. Calculation inputs and outputs are provided in Attachment [X]. The calculator has been verified against EPA TANKS 4.09D results for representative configurations.

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Verification Log

This calculator is maintained by the Simulations4All engineering team and updated when EPA releases AP-42 revisions.