Every above-ground storage tank holding volatile organic liquids loses some product to the atmosphere through vapor emissions. The EPA quantifies these losses using the methods in AP-42, Chapter 7 (Liquid Storage Tanks), and these calculated emissions directly determine your facility's VOC inventory, Title V permit thresholds, emission fees, and required controls. Underestimating tank losses can trigger enforcement actions; overestimating them can force you into unnecessary controls or higher permit tiers.
The AP-42 methods divide tank losses into two categories: standing losses (also called breathing losses) caused by daily temperature and pressure cycling, and working losses caused by filling and emptying the tank. This guide walks through both formulas for fixed-roof tanks, identifies the key factors that drive emissions, and explains how paint color, insulation, vent settings, and tank geometry affect the numbers.
Two Types of Losses
Standing losses (breathing losses) occur even when a tank is not being filled or emptied. During the day, solar heating warms the tank shell and the vapor space, increasing the vapor pressure of the liquid and expanding the gas. This pushes vapor out through the tank vent. At night, the tank cools, the vapor contracts, and ambient air is drawn in through the vent. Each daily cycle expels a small amount of VOC-laden vapor. The magnitude depends on the vapor pressure of the liquid, the temperature swing, the volume of the vapor space, the tank paint color and condition, and the vent pressure/vacuum settings.
Working losses occur during tank filling operations. As liquid enters the tank, it displaces the vapor space above it. If the tank is not equipped with vapor recovery, this displaced vapor (saturated with VOC at the liquid surface temperature) is expelled through the vent. The emission rate depends on the throughput volume, the vapor pressure at the liquid surface, the saturation factor (how close to saturation the displaced vapor is), and the number of turnovers per year.
For fixed-roof tanks, total annual losses equal standing losses plus working losses. For external floating-roof tanks, the AP-42 methodology is different: losses are driven by seal gaps, fitting losses, and deck seam losses rather than the thermal breathing mechanism. Internal floating roofs also have their own methodology. This guide focuses on fixed-roof tank calculations, which are the most common for smaller storage applications.
Tank Breathing Loss Calculator
Calculate standing and working VOC emissions from fixed-roof storage tanks per EPA AP-42 Chapter 7.1. Includes paint absorptance, Antoine equation, and permitting thresholds.
Standing Loss Formula
The AP-42 standing loss equation for a vertical fixed-roof tank is: L_S = 365 × V_V × W_V × K_E × K_S, where L_S is the annual standing loss (lb/yr), V_V is the vapor space volume (ft³), W_V is the vapor density (lb/ft³), K_E is the vapor space expansion factor (dimensionless), and K_S is the vented vapor saturation factor (dimensionless).
The vapor space volume V_V depends on the tank diameter, the outage (distance from the liquid surface to the roof), and the roof geometry (cone, dome, or flat). For a cone-roof tank with a typical roof slope, V_V = (π/4) × D² × H_VO, where D is the tank diameter and H_VO is the effective vapor space outage height, adjusted for the roof cone volume.
The vapor space expansion factor K_E accounts for the daily temperature and pressure cycling that drives breathing. It depends on the daily ambient temperature range, the solar heating factor (determined by paint color and condition), the average atmospheric pressure, and the vent pressure/vacuum settings. A tank with pressure/vacuum vents set at ±1 oz/in² will have lower K_E than a freely vented tank because the vents only open when the pressure differential exceeds the setpoint, reducing the number of effective breathing cycles.
The vented vapor saturation factor K_S ranges from about 0.5 for very volatile liquids (where the expelled vapor is fully saturated) to nearly 1.0 for low-volatility liquids (where the expelled vapor is more dilute). AP-42 provides equations and charts for K_S based on the vapor pressure at the liquid surface temperature.
Tank Breathing Loss Calculator
Calculate standing and working VOC emissions from fixed-roof storage tanks per EPA AP-42 Chapter 7.1. Includes paint absorptance, Antoine equation, and permitting thresholds.
Working Loss Formula
The AP-42 working loss equation for a fixed-roof tank is: L_W = Q × K_N × K_P × W_V × K_B, where L_W is the annual working loss (lb/yr), Q is the annual net throughput (ft³/yr), K_N is the turnover factor (dimensionless), K_P is the working loss product factor (dimensionless), W_V is the vapor density (lb/ft³), and K_B is the vent setting correction factor.
The turnover factor K_N adjusts for how many times the tank is filled and emptied per year. For turnovers per year (N = Q / V_tank) greater than 36, K_N = (180 + N) / (6 × N). For N ≤ 36, K_N = 1.0. High turnover rates actually reduce per-unit losses because the vapor space has less time to reach saturation between fills. This is counterintuitive but reflects the physical reality that rapid cycling expels less saturated vapor per unit of throughput.
The product factor K_P is 1.0 for most volatile organic liquids. For crude oil and similar complex mixtures, AP-42 provides adjustment factors. The vent setting correction K_B is similar to the standing loss vent factor and reflects the pressure/vacuum relief settings.
Working losses are typically the dominant emission source for tanks with high throughput. A terminal loading rack that fills and empties a gasoline tank several times per month will have working losses that far exceed standing losses. Conversely, a tank that sits with the same product for months (intermediate storage in a refinery, for example) may have standing losses as the dominant source.
Key Factors: Paint, Solar Heating, and Vent Settings
Tank paint color and condition have a direct impact on standing losses through the solar absorptance factor in the K_E calculation. A white or aluminum-painted tank in good condition has a solar absorptance of about 0.17, while a dark-colored tank (black, dark brown) can reach 0.97. The difference in standing losses between a well-maintained white tank and a weathered dark tank can be a factor of 2 to 3. Keeping tanks painted white or aluminum and in good condition is one of the cheapest emission reduction measures available.
Vent settings (pressure/vacuum relief valves) control when the tank breathes. A freely vented tank (no pressure/vacuum control) breathes with every temperature cycle. Setting the vents to ±0.5 to ±1.0 oz/in² gauge prevents the tank from breathing during small temperature swings, reducing standing losses by 10-40% depending on the settings and climate. Higher vent settings reduce emissions further but require the tank to be designed for the resulting internal pressure, which adds structural cost.
Insulation reduces the temperature swing in the vapor space by damping the solar heating cycle. An insulated tank has a smaller daily temperature range, reducing K_E and standing losses. Insulation is most cost-effective in hot climates with large diurnal temperature swings and for tanks storing high-vapor-pressure liquids where the emission reduction justifies the insulation cost. Some facilities insulate just the roof (where solar heating is most direct) rather than the entire shell.
Liquid vapor pressure is the dominant physical property. True vapor pressure at the liquid surface temperature determines both the vapor density and the saturation factor. Gasoline at 60°F has a TVP of about 4-8 psia, while diesel at the same temperature has a TVP well below 1 psia. Gasoline tank losses are typically 10-50 times higher than diesel tank losses for the same tank geometry, which is why gasoline storage is subject to much more stringent control requirements.
Permitting Thresholds and Compliance
AP-42 tank emission calculations feed directly into air permit applications. For federal Title V permitting, a facility's potential to emit (PTE) above 100 tons per year of any criteria pollutant (or 10/25 tons per year of HAPs) triggers the requirement for a Title V operating permit. VOC emissions from tank farms can easily push a facility over these thresholds, particularly for gasoline, crude oil, and chemical feedstock storage.
Many state programs have lower thresholds. Some states require permits for any facility emitting more than 5-25 tons per year of VOC. In nonattainment areas for ozone, the major source thresholds drop to 50, 25, or even 10 tons per year of VOC depending on the severity of the nonattainment classification. Tank losses that seem minor in isolation can become significant when aggregated across all tanks at a facility.
Control requirements triggered by emissions include vapor recovery units (VRUs, 90-98% control efficiency), internal floating roofs (85-99% reduction in standing losses), external floating roofs, and closed-vent systems tied to flares or thermal oxidizers. EPA Method 21 leak detection (LDAR) may also apply to tank fittings, hatches, and sample ports. The cost-effectiveness of controls is evaluated in best available control technology (BACT) or lowest achievable emission rate (LAER) analyses depending on the attainment status.
For emission inventory reporting (annual or semi-annual), most state agencies accept AP-42 calculations using facility-specific tank dimensions, throughput records, and TANKS 4.09 software (or its successor methodology). Some states require meteorological data specific to your county rather than the generic AP-42 defaults. Always check your permit conditions for the required calculation methodology.
Tank Breathing Loss Calculator
Calculate standing and working VOC emissions from fixed-roof storage tanks per EPA AP-42 Chapter 7.1. Includes paint absorptance, Antoine equation, and permitting thresholds.