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EPA AP-42 Tank Breathing Loss Calculator

Standing and Working Losses, Paint Absorptance, Antoine Equation Vapor Pressure, and Permitting Threshold Checks

Free tank breathing loss calculator for environmental engineers, tank farm operators, and air permit specialists who need to estimate VOC emissions from fixed-roof storage tanks. This calculator implements the EPA AP-42 Chapter 7.1 methodology to compute standing losses (daily breathing from temperature and pressure cycling) and working losses (displacement during filling operations). Enter the tank dimensions, liquid properties, paint condition, average ambient temperature, daily temperature range, latitude, and annual throughput, and get annual emissions in pounds and tons per year with a check against HAP permitting thresholds.

Standing losses occur because the vapor space above the liquid heats up during the day, expands, and vents through the tank's pressure/vacuum vent. At night, the vapor space cools, contracts, and draws in fresh air, which then picks up more vapor from the liquid surface. The magnitude depends on tank paint absorptance (how much solar heat the shell and roof absorb), daily temperature swing, vapor pressure of the stored liquid (calculated from the Antoine equation using the liquid surface temperature), and the size of the vapor space.

Working losses happen every time liquid is pumped into the tank, displacing vapor-laden air out through the vent. The amount depends on the fill rate, annual throughput, and whether the tank has a submerged fill pipe (which reduces turbulence and vapor generation) or a splash fill. Together, standing and working losses are the two components required for air permit applications and annual emission inventory reports under Title V and state-level programs.

Pro Tip: The single biggest factor in standing losses for most liquids is tank paint color. A white-painted tank with a solar absorptance of 0.17 can have 40-60% lower standing losses than an unpainted or dark-colored tank with an absorptance of 0.90. If you are close to a permitting threshold, repainting the tank white or aluminum is often cheaper than installing a vapor recovery system or switching to a floating roof.

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Tank Breathing Loss Calculations Guide →

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Tank Breathing Loss Calculator
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How It Works

  1. Enter Tank Geometry

    Input the tank diameter, shell height, average liquid level, and roof type (cone or dome). The calculator uses these to determine the vapor space volume, which directly affects the standing loss rate.

  2. Select Liquid and Vapor Properties

    Choose the stored liquid from the built-in library (gasoline, crude, benzene, toluene, etc.). You can override the molecular weight or vapor pressure if you have site-specific data. The calculator computes the true vapor pressure at the liquid surface temperature using built-in Antoine equation coefficients.

  3. Enter Paint and Climate Data

    Select the tank paint color/condition to set the solar absorptance factor (values from AP-42 Table 7.1-6). Enter the site's average ambient temperature, daily temperature range, and latitude (used to look up solar insolation from AP-42 Table 7.1-7).

  4. Enter Throughput and Review Results

    Input the annual throughput in barrels per year and the vent pressure setting. The calculator computes annual turnovers from throughput and tank capacity, then shows standing losses, working losses, and total annual emissions in lb/yr and ton/yr with a check against HAP permitting thresholds (10/25 ton/yr).

Built For

  • Environmental engineers preparing emission calculations for a Title V air permit application for a tank farm with 20 fixed-roof tanks
  • Tank farm operators estimating annual VOC emissions for their facility's emission inventory report
  • Air permit specialists checking whether a new gasoline storage tank will trigger major source permitting thresholds
  • Plant managers evaluating the emission reduction benefit of repainting tanks white vs. installing internal floating roofs

Features & Capabilities

EPA AP-42 Chapter 7.1 Methodology

Implements the standing loss (Eq 7.1-11) and working loss (Eq 7.1-28) equations from AP-42 for vertical fixed-roof tanks, including the vapor space expansion factor (K_E), vented vapor saturation factor (K_S), and working loss turnover factor (K_N).

Antoine Equation Vapor Pressure

Calculates true vapor pressure at the liquid surface temperature using the Antoine equation with built-in coefficients for common petroleum liquids, solvents, and chemicals. Vapor pressure and molecular weight can be overridden with site-specific values.

Paint Absorptance Library

Includes solar absorptance values for common tank paint conditions: white (good/poor), aluminum, light gray, medium gray, and dark/unpainted surfaces. Absorptance values are from AP-42 Table 7.1-6.

Permitting Threshold Checks

Compares the calculated annual emissions against HAP thresholds: 10 ton/yr single HAP and 25 ton/yr total HAP. Flags results that approach or exceed these limits, with a note that PSD and Title V thresholds vary by pollutant and attainment area.

Assumptions

  • Standing loss and working loss equations follow EPA AP-42 Chapter 7.1 (5th edition with supplements) for vertical fixed-roof tanks.
  • Vapor pressure is calculated from the Antoine equation at the liquid surface temperature, which is approximated as the average ambient temperature plus a paint-dependent solar heating offset.
  • The tank vent is assumed to operate at atmospheric pressure (no significant pressure or vacuum setting on the conservation vent).

Limitations

  • Does not cover floating roof tanks (internal or external), pressure tanks, or underground tanks, which have different loss mechanisms and AP-42 equations.
  • Does not model flashing losses that occur when hot liquid is transferred into a cooler tank, which can produce emissions above the standard working loss estimate.
  • Assumes a single-component liquid. For multi-component mixtures like gasoline, the true vapor pressure should be the Reid Vapor Pressure (RVP) adjusted to liquid surface temperature, not a simple Antoine calculation for a single component.

References

  • EPA AP-42, 5th Edition, Chapter 7.1 - Organic Liquid Storage Tanks (Fixed Roof Tanks).
  • EPA TANKS 4.09d Software Documentation - Standing Storage Loss and Working Loss Methodology.
  • API Manual of Petroleum Measurement Standards, Chapter 19.1 - Evaporative Loss from Fixed-Roof Tanks.

Frequently Asked Questions

Standing losses are continuous emissions caused by the daily heating and cooling of the vapor space above the liquid (breathing). They happen even if the tank is not being filled or emptied. Working losses are emissions displaced out of the tank vent during liquid filling operations. For high-throughput tanks, working losses often exceed standing losses. For low-throughput tanks (like seasonal storage), standing losses dominate.
Paint color determines how much solar radiation the tank absorbs, which directly affects the daily temperature swing of the vapor space. A white-painted tank (absorptance 0.17) heats up much less than an unpainted or dark tank (absorptance 0.90), resulting in less vapor expansion and lower breathing losses. AP-42 Table 7.1-6 provides the specific absorptance values. Switching from dark paint to white can reduce standing losses by 40-60% depending on the climate.
The Antoine equation is an empirical formula that relates vapor pressure to temperature: log(P) = A - B/(C+T), where A, B, and C are constants specific to each chemical and T is the temperature. For tank emission calculations, the vapor pressure at the liquid surface temperature determines how much vapor is present in the tank headspace. Higher vapor pressure liquids (like gasoline) produce much higher breathing losses than lower vapor pressure liquids (like diesel).
No. This calculator covers fixed-roof tanks only. Floating roof tanks (both internal and external floating roofs) have a different set of AP-42 equations (Chapter 7.1, sections on floating roof losses) that calculate rim seal losses, deck fitting losses, and deck seam losses instead of standing and working losses. Floating roofs eliminate most of the breathing loss by sitting directly on the liquid surface and eliminating the vapor space.
Use the average liquid height over the year, not the maximum or minimum. For tanks that cycle regularly between full and nearly empty, the average is typically about 50% of the shell height. For tanks kept mostly full with small drawdowns, the average might be 80-90%. The liquid level affects the vapor space volume, which directly scales the standing loss. A tank kept 90% full has half the vapor space of a tank kept at 80%, and therefore roughly half the standing losses.
Disclaimer: This calculator implements the EPA AP-42 Chapter 7.1 methodology for screening-level emission estimates. Actual emissions depend on site-specific conditions including tank condition, vent settings, and liquid composition that may differ from the AP-42 default assumptions. Permit applications should reference the specific AP-42 edition accepted by your regulatory agency.

Learn More

Emissions

Tank Breathing Loss Calculations (EPA AP-42)

How to calculate standing and working VOC emissions from fixed-roof storage tanks per EPA AP-42 Chapter 7.1, including expansion factors, turnover factors, and permitting thresholds.

Read Guide →

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