Storage tanks are used in many industries and manufacturers ranging from adhesive manufacturers to aerospace manufacturers. Other facilities also store fuel in tanks for their personal industry vehicles. 

    All these facilities are prone to emissions from tanks that result from regular tank usage and the volatility of the stored liquid. The scope of tank usage is broad, and if you are a professional in any of these industries, this article is for you. This article gives you everything you need to know about EPA storage tank emissions, including:

    • The AP-42 chapter 7 Regulations for Liquid Storage Tanks.
    • Origins and background API Tank calculations and AP-42 Emission factors.
    • The different types of tanks and processes that produce air emissions.
    • How to Start Managing your Tank Emissions for Federal Reporting.
    • How to improve your tank emission management strategy using a software solution.

    AP-42 Chapter 7 Liquid Storage Tanks Overview

    This rule explores methodologies for estimating air emissions from liquid storage tanks. AP 42 emission factors and emission inventories are used to manage air quality properly. Emission factors are the best method to estimate air emissions for formulating emission control strategies. The EPA, federal agencies, consultants, and industry use air emissions estimates to determine permitting and control programs and to gauge the effects of certain sources on the overall air quality. Models were used to get these estimates as accurate as possible.

    The American Petroleum Institute (API) has worked tirelessly to conduct countless studies of emissions calculations. They have studied various scenarios, official documentation, and actual tank emissions to formulate the best fitting equations. They retain the legal rights to these equations, and multiple software companies have used them to simplify the emissions calculation process. They have carried out studies that cover a wide range of scenarios, including emissions from the thin film of oil of the walls of the tank and different shaped tank roofs. These calculations also cover different vapour pressures and VOC content for various liquids.

    Although other speciated organic compounds may be hazardous, the primary pollutant of concern is VOCs. AP42 Chapter 7.1 for Organic Liquid Storage Tanks addresses storage tanks in normal working conditions, and the calculations cover all estimates for these conditions.


    Air Emissions Factors 

    Emission factors and inventories are essential tools for air quality management. Accurately estimating emissions allows you to:

    • Determine if you are subject to specific permits and control programs.

    • Develop your emission control strategies.

    • Develop mitigation strategies and explore the effects of sources.

    • Help federal organizations plan for emission reduction programs.


    The data for these estimates is provided by source-specific emission tests and continuous monitoring systems. Continuous emission monitors are the preferred method of data collection. However, it is not always possible to accurately measure the emissions from your sources, so the API’s formulated methodologies help calculate emissions. This also helps your facility to stay in compliance with the Clean Air Act Amendments Of 1990 (CAAA) and the Emergency Planning and Community Right-To-Know Act (EPCRA) of 1986.


    What Is An AP-42 Emission Factor?

    This is a representative value that relates the emissions produced to the activity that causes the emission. This value can be derived by using a simple equation:


    AP-42 emission factor equation

    For this equation to be the most effective, it needs to consider variables such as tank diameter, liquid temperature, and wind velocity. These factors affect the amount of emissions produced by a given tank and are essential variables in managing your tank emissions.

    Factors Affecting the Quantity of Tank Emissions

    You may notice that certain tanks emit a more considerable amount of emissions than others based on various factors. To properly evaluate your emissions and for accurate estimations, it would be best if you considered:

    • Tank parameters
    • The chemical makeup of the liquid being stored/processed
    • Atmospheric conditions of the tank’s location
    • Amount of liquid moving through the storage tank
    • Timeframe of the liquid’s standing time

    Tank parameters include the size, shape, color, finishing, roof type, and other variables that should be considered during emissions calculations. For example, dark-colored tanks will absorb more heat than lighter-colored ones, affecting the amount of emissions produced. This means that you should take note of the type of tanks that you have in your facility. Below is a table that outlines the types of tanks.

    Tank Parameters for Tank Emission Management: Types of Tanks 

    Tank Type

    Description and Emission Sources

    Fixed Roof Tanks

    This is the least expensive tank type. It operates with a slight internal pressure to prevent the release of vapors during small temperature changes. They are either freely vented or equipped with a pressure vacuum vent. Emissions from this tank type are mainly from the level of the liquid in the tank.

    Horizontal Fixed Roof

    These are generally less than 40 000 gallons for both above-ground and underground services. Losses from changes in temperature or atmospheric pressure are minimal for underground tanks.

    External Floating Roof Tanks

    This tank has a roof that sits on the liquid and goes down as the liquid reduces. Standing losses from this tank type are from the rim seal. The liquid exposed when the roof is lowered is subject to evaporation, which is part of working losses.

    Internal Floating Roof Tanks

    These tanks are vented by circulation vents at the top of the fixed roof. Emissions from these tank types may come from the space between the deck and tank wall.

    Domed External Floating Roof Tanks

    The roof for this tank type functions to block the wind. The difference between emissions from domed external roof tanks and external roof tanks is because of differences in deck fittings when estimating emission. This tank type can be reported as an internal floating roof tank unless specified by the report.

    Variable Vapor Space Tanks

    This tank type can accommodate vapor changes from temperature and pressure by having expandable vapor reservoirs. The standing loss emissions from this tank type depend on the permeability of the diaphragm and if there is leakage through the seam where the diaphragm is attached to the tank wall. The primary emissions from this tank type happen during tank filling when there is vapor displacement.

    Pressure Tanks

    These tanks can be operated in a way that minimizes evaporative and working losses. Vapor losses can sometimes occur during filling operations. The estimation method for non-boiling liquids in low-pressure tanks is similar to the one for fixed roof tanks. Remember to account for the vent set pressure in both working and standing loss calculations.


    The Chemical Makeup of Storage Tank Contents

    The chemical makeup of the stored liquid is fundamental because it determines the VOC content of the chemicals and the potential to emit. This is the essential aspect that determines the amount and type of emissions produced. Various liquids can be stored in tanks, including diesel fuel, gasoline, acids, and other chemicals. The volatility of the liquid determines how much of it will be emitted. Some industries also use tanks to store adhesives or paints, which have a different viscosity and emission potential. Industries that carry out combustion will have oils and natural gas that they burn during their industry process. Some chemicals, such as petroleum distillates, are volatile, and this quality contributes to how much they are emitted. The volatility of these chemicals is shown as Tank True Vapor pressure.

    Atmospheric Conditions of Tank Surroundings

    Atmospheric conditions play a significant role in liquid vaporization. Therefore, you must know the temperature ranges of your geographical location because this affects the Vapour Pressure of your stored liquids. Hence your calculations should account for the weather and altitude.

    • Definition: Vapour pressure is the force that vaporizes volatile liquids, and it is related to the composition of the liquid. Smaller molecules have lower boiling points which means they are more active; that is why vapor pressure is inversely proportional to molecular size and boiling point. Vapour pressure also increases with temperature.

    You can learn more about how to measure atmospheric temperature in your calculations and receive more detailed calculations from ERA’s the Science of Storage Tank Emission Calculations eBook.



    How Do Tanks Cause Air Emissions?

    Tanks emit during various tank processes and when they are standing within the facility and these are routine emissions according to the EPA.

    Working Losses

    Tank loading emissions are released when liquid is being pumped in and out of the tank. In some industries, tanks have to be drained and refilled. These processes release many vapors depending on the loading and unloading method. When you fill the tank, the liquid rises and reduces the vapor space, releasing it into the atmosphere. If a tank is only filled up once a month, then the emissions will be less than if a tank is filled up weekly. In floating roof tanks, the roof of the tank lowers with the liquid, which leaves the walls of the tank exposed and prone to emitting.

    Paint and adhesive manufacturers fill the tanks with all the ingredients then use a mechanical arm to mix everything. During the mixing, the liquid heats up, and this increases the emission potential of VOCs. This is a form of working loss that should be accounted for during your storage tank emissions calculations.

    Standing Losses

    Breathing and heat expansion allows for some emissions to occur. The emptier the tank, the more room for vapors to build up. Breathing loss is the that is expelled from the tank due to thermal expansion. During the day, when the tank is heated, the gas expands, and it is released to the atmosphere, and at night the gas cools and is absorbed from the atmosphere.

    Tip: To avoid standing losses, keep your fixed roof tanks as full as possible.

    Cleaning and Degassing

    When a tank is cleaned, some vapors are released into the atmosphere. Emissions can occur during unloading or sludge removal. Degassing is commonly done to tanks with crude oil and gasoline. When vapors are forced out of a tank during cleaning, it produces emissions that should be included in your estimates. Below is a table of the emissions that are made from the cleaning process.

    Emissions from Tank Cleaning Process

    Tank Cleaning Procedure


    Normal Pump out

    Removing as much liquid as possible from the tank. Air flows into the tank which means no emissions are assumed.

    Standing Idle

    Residual material evaporates and generates vapours without forced ventilation.

    Vapour Space purge

    The remaining vapours are forced out of the tank.

    Sludge Removal

    Other volatile materials are removed from the tank while vapours are being forced out of the tank.

    Remain Clean

    There are no emissions when the tank is empty and clean.


    When the tank is refilled vapours are displaced and released and vapours from the incoming liquid cause emissions.


    Mobile Tanks

    These tanks are loaded and unloaded more frequently than any other tank type and are prone to emissions. It would be best to account for emissions from mobile sources and loading losses from when the liquid is pumped into the tank.

    How to Start Managing Your Tank Emissions for Federal Reporting

    Tanks emission management is the process that allows you to track, measure, and estimate your tank emissions accurately. This is useful to stay in compliance with air quality management rules and for federal reports such as TRI reporting. To correctly manage your emissions, you must make sure the process is specific to you and your tank types. Below is a walkthrough of the questions you must ask during your tank management strategy formulation.

    The process outlined above is industry and facility-specific, and you must address these questions accurately to use storage tank emissions calculation software effectively. After answering these questions, you are well equipped to start using them in your calculations or reporting process.

    How Can I Reduce My Tank Emissions?

    During tank emissions management, it is also essential to control your emissions to produce minimal air pollution. These control methods depend on the tank type and liquid stored in your tanks. The following is an example of how you can reduce emissions from tanks.

    • Controlling emissions from fixed roof tanks can be done in several ways. To minimize evaporation from the stored chemical, you can install an internal floating roof and seals to reduce evaporation of the chemical being stored. This method has a control efficiency ranging from 60 to 99 percent, depending on the type of roof and seals you choose to install, and the type of organic liquid stored.
    • Increasing the vent set pressure for fixed roof tanks may eliminate routine emissions if the vent set pressure is higher than the pressure that develops in the vapor space during regular tank operation.
    • Additional evaporative loss control may be provided by installing a secondary seal.

    These examples barely scratch the surface of how to control tank emissions. The EPA provides more information on emission control for each tank type.

    The Best Tanks Emission Management Strategy

    To improve your tank emission management, you need to carefully track all your emissions and accurately deduce the effects of your CMSs. You must also note when you have exceeded the federal emission threshold for TRI or Tier II reports and have impeccable record keeping.

    Improve Your Data Collection

    Part of having impeccable data collection. As mentioned above, you must know the liquid's chemical composition that is stored in your tanks. This is achieved by having updated safety data sheets (SDSs) and obtaining material composition claims for your blends.

    Use a Tank Emissions Software

    Using Tanks Emissions Estimation Software for your data collection and tracking is a sure way to reduce errors when carrying out calculations. The calculations for tanks span multiple spreadsheets and are prone to human error. You must look out for software that uses the AP 42 Chapter 7 calculation methodology. You must also ensure that the software account for your tank types and varying atmospheric conditions in your region. This will simplify your Tank emission management greatly. Also, ensure that you have threshold approach notifications; this will help you properly choose CMS devices and emission prevention strategies.


    How to Choose Storage Tank Emissions Calculation Software

    Choosing emissions management software requires you to have knowledge about your facility and operations. You should consider your tank types, chemicals, your tank processes, and the types of calculations that the software uses. This will assist you in selecting the right software for your reporting requirements.

    The software must use verified API Tank equations and API tank calculations. It should encompass reports for working losses and standing losses along with maintenance Start-up and Shut down (MSS events).

    If you have blends in your tanks, the selected software must have the capacity to calculate petroleum distillates with EPA standard and mixed organic liquids, among many other liquids.

    Your software should also have these key features:

    • Automated data collection. This includes the ability to upload data directly from your vendors. Details on product specifications such as vapor pressure, density, and VOC content are easily uploaded with minimal effort from your EHS professionals.
    • Regulatory updates built-in. Regulations are constantly changing, and having a system that automatically updates the specific regulations for your operations is extremely useful.
    • Capacity for different tank types. There are various types of tanks and having a system that has calculations for all the different tank types in your facility is essential for tank emission management.

    ERA’s tank emissions platform encompasses all of these requirements and many more. It is equipped to handle domed external roof tanks, horizontal tanks, vertical fixed roof tanks, internal floating roof tanks, external floating roof tanks, open-top tanks, and mobile tanks. It also combines API Tank equations, AP-42 chapter 7, and constant scientific research to give you accurate calculations for your tank emissions.



    This blog was co-authored by:

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    Kundai Mufara
    Post by Kundai Mufara
    December 9, 2021
    Kundai Mufara is a Science Content and Technical Writer at ERA.