AP-42 Chapter 7: TANKS Software - Frequent Questions

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HOW DO I GET A COPY OF THE USER'S MANUAL?

The TANKS 5.1 User Guide (pdf) (870.8 KB)  is available here and on the TANKS webpage.

HOW DO I ESTIMATE EMISSIONS FROM A HEATED TANK?

If a fixed roof tank is heated, the vapor space can be assumed to be at a constant temperature and no breathing losses will occur. The tank must be heated to a uniform temperature and be well insulated, thus isolating it from heat gains and losses due to insolation and variations in ambient temperature. Changes in atmospheric pressure are assumed to have only negligible contributions to breathing losses and are not considered.

Under Characteristics and Tank Characteristics on the Tank Data screen, you should answer the question “Is Tank Insulated?” with “Fully Insulated” for a vertical fixed roof tank or “Fully Insulated or Underground” for a horizontal fixed roof tank. Then, answer the question “Is Tank Heated?” with "Yes" which will allow you to enter temperature data for the Maximum, Average, and Minimum Liquid Bulk Temperature (degrees Rankine) and the number of heating cycles per year.

Note: to determine degrees Rankine, add 459.67 to the temperature in Fahrenheit.

You also should check the information in Table 7.1-3 of Chapter 7 of EPA's Compilation Of Air Pollutant Emission Factors (AP-42) to ensure that the program accurately estimates vapor pressure at the desired temperature for the chosen organic liquid.

HOW DOES THE TANKS MODEL GENERATE SPECIATED EMISSIONS?

The model offers several options for speciating emissions into their component parts. When the user is able to specify the properties and amounts of the liquids stored, the program relies upon Raoult's Law to estimate component emissions. The user must supply this information for custom mixtures and custom petroleum liquids.

HOW DO I OBTAIN ANTOINE'S COEFFICIENTS FOR OTHER CHEMICAL COMPOUNDS?

Two of the best resources are the Handbook of Chemistry and Physics and Perry's Chemical Engineering Handbook. These references should be available at most technical libraries. Other references are:

  • Reid, Prausnitz and Sherwood, Properties of Liquids and Gases, McGraw Hill, 1987.
  • Yaws and Yang, Property Data: To Estimate Vapor Pressure Easily, Hydrocarbon Processing, October 1989.
  • Boublík, Fried and Hála, The Vapor Pressures of Pure Substances, Selected Values of the Temperature Dependent Vapor Pressure in the Normal and Low Pressure Regions, Elsevier Science Publishers, Amsterdam, 1984.

Antoine's Coefficients are also in the NIST Chemistry workbook, which is available online at: http://webbook.nist.gov/chemistry/

**Please note that in some resources, such as the NIST Chemistry Workbook, the Antoine's equation constants in bar, K must be converted to mmHg, degrees C for use in TANKS 4.0. To do this, add 2.8751 to A, keep B the same, and add 273.15 to C.

I HAVE A TANK THAT IS OPERATED WITH LITTLE OR NO CHANGE IN THE LIQUID LEVEL (i.e. a "constant level" tank). I BELIEVE THE MODEL IS OVERESTIMATING THE WORKING LOSSES FROM THE TANK. WHAT SHOULD I DO?

TANKS 5.0 has the option of estimating emissions using monthly/annual throughput, the monthly/annual sum of decreases in liquid level (for floating roof tanks) or the monthly/annual sum of increases in liquid level (for fixed roof tanks). Note, that values must be greater than or equal to zero. This information is entered under Contents on the “Tank Data” page. Choose the method for determining the “Sum of Increases in Liquid Level Method” or “Sum of Decreases in Liquid Level Method” as “User Input” to enter these values or “AP-42 Calculation” to enter throughput values and have TANKS 5.0 calculate the sum of decreases in liquid level (for floating roof tanks) or the sum of increases in liquid level (for fixed roof tanks).

MY STORAGE TANK HAS A NITROGEN BLANKET. HOW SHOULD I ESTIMATE EMISSIONS?

Whether the tank is of fixed roof or internal floating roof design, no modifications to the equations are required. The gas blanket will not affect emissions if the tank is operated near atmospheric pressure.

CAN I USE TANKS TO ESTIMATE EMISSIONS FROM A FIXED ROOF TANK OPERATED UNDER "LOW PRESSURE" CONDITIONS?

Low pressure conditions are defined by API as an operating range of just above atmospheric to 15 psig. The TANKS application allows the user to input vapor space pressure values of 0 to 15 psig.

I VENT MY ROOF TO A CONTROL DEVICE. HOW SHOULD I ESTIMATE EMISSIONS?

When entering characteristics for your tank on the “Tank Data” page under “Characteristics” and “Control Device Settings”, please select "Control Device" in response to "Is Tank Equipped with a Control Device". You will then be prompted to enter the efficiency of your control device.

WHAT DO I DO IF MY TANK COLOR IS NOT FOUND IN AP-42?

You should select the most similar color available to represent your tank. If you have a paint with high reflectivity, use white.

THE METEOROLOGICAL DATA IS NOT AVAILABLE IN TANKS FOR MY STORAGE TANK LOCATION.  WHERE CAN I GET THIS INFORMATION?

Sometimes it is possible to substitute another city from TANKS that has similar meteorological data.

Meteorological data for cities that cannot be substituted can be found online at one of the following web sites and entered as a Custom Meteorological Location on the “Customize” page of TANKS 5.0:

United States:  National Solar Radiation Data Base (https://nsrdb.nrel.gov/about/u-s-data.html)

International:  NASA Surface Meteorology and Solar Energy Data Set (https://power.larc.nasa.gov/) 

CAN I USE THE TANKS MODEL TO ESTIMATE EMISSIONS FROM A TANK STORING AN INORGANIC LIQUID?

Although the equations used in the model were developed to estimate evaporative losses from storage of organic liquids, it is possible to use the model if the inorganic liquid has a measurable vapor pressure and the Antoine’s constants (A, B, and C) or vapor pressure equation constants (A and B) for the liquid are available. Emission estimates produced by the model will be of unknown accuracy due to the absence of confirmatory testing for emissions from inorganic liquid storage tanks.

MY TANK FITTINGS ARE NOT IN AP-42. HOW CAN I CALCULATE EMISSIONS?

Loss factors for the most common fitting types and control configurations have been developed through testing conducted by the American Petroleum Institute (API) and reviewed by EPA. The fittings tested were generic designs that approximated evaporative loss mechanisms common to most vendor components. In most cases, if the exact design is not found, one that is of similar geometry will be available and should be selected.

API, with the support of EPA, has developed the Tank Seals and Fittings (TSF) Program which can be used to develop loss factors for vendor specific component designs. You should contact API for further information if you want to conduct testing to establish a loss factor for your component.

HOW CAN I ESTIMATE EMISSIONS FROM DEGASSING AND CLEANING OPERATIONS DURING A TANK TURNAROUND?

TANKS 5.0 allows for the estimation of cleaning emissions from fixed and floating roof tanks. Enter the tank information on the “Tank Data” page and then enter additional information related to the cleaning event on the “Emissions Events” page. Emissions estimates will appear on the screen.

HOW CAN I ESTIMATE EMISSIONS FROM ROOF LANDING LOSSES in the TANKS program? Updated February 2010.

TANKS 5.0 allows for the estimation of landing loss emissions from floating roof tanks. Enter the tank information on the “Tank Data” page and then enter additional information related to the roof landing event on the “Emissions Events” page. Emissions estimates will appear on the screen.

MY TANK CONTAINS A TWO-PHASE LIQUID. WHAT DO I DO?

Use the properties (P, Mv, etc.) of the top phase to estimate emissions. Calculate throughput using the overall liquid.

IF MY HORIZONTAL FIXED ROOF TANK IS UNDERGROUND, HOW DO I ESTIMATE EMISSIONS?

You should select Horizontal Fixed Roof Tank as your tank type on the “Tank Data” page. Then, under “Characteristics” and “Tank Characteristics”, you should answer the question “Is Tank Insulated or Underground?” with “Fully Insulated or Underground” Since the surrounding earth limits the diurnal temperature change, the model assumes that there are no breathing losses from the tank (breathing losses from atmospheric pressure fluctuations are assumed to be negligible). Only working losses will be calculated.

I HAVE A GASOLINE SERVICE STATION WITH UNDERGROUND FIXED ROOF TANKS THAT ARE VAPOR BALANCED. WHAT EQUATIONS SHOULD I USE TO ESTIMATE EMISSIONS?

Use the equations presented in Section 7.1.3.6 of AP-42 for variable vapor space tanks.

MY TANK STORES DIFFERENT LIQUIDS DURING THE YEAR. HOW DO I ACCOUNT FOR THIS VARIABILITY?

TANKS 5.0 allows you to enter different chemicals in one tank on a monthly basis. On the “Tank Data” page, under “Contents”, choose “Enter Monthly Values” as the “Input Type” and choose the appropriate chemical name and throughput or the sum of decreases in liquid level (for floating roof tanks) or the sum of increases in liquid level (for fixed roof tanks) for each month. Note that all chemicals must be of the same type, as determined by the chosen “Chemical Category of Liquid”:

  • AP-42 Organic Liquids (Table 7.1-3)
  • Custom Organic Liquids 
  • AP-42 Petroleum Liquids (Table 7.1-2)
  • Custom Petroleum Liquids 
  • Custom Mixture

CAN TANKS BE USED TO ESTIMATE EMISSIONS FROM INDOOR TANKS? WHAT ABOUT HEATED INDOOR TANKS?

Yes, it can be used for indoor fixed roof tanks. Under “Characteristics” and “Tank Characteristics” on the Tank Data page, you should answer the question “Is Tank Insulated?” with “Fully Insulated” and then answer the question “Is Tank Heated?” with "Yes" to estimate emissions from heated vertical and horizontal fixed roof tanks. This will allow you to enter temperature data for the Maximum, Average, and Minimum Liquid Surface Temperature (degrees Rankine) and the number of heating cycles per year. Use the temperature of the building for all three entries. The application does not calculate breathing losses because the temperatures are constant. TANKS 5.0 will calculate emissions whether the tank is at room temperature or is heated above that. Note: to determine degrees Rankine, add 459.67 to the temperature in Fahrenheit.  

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Last updated on February 29, 2024