AP-42 Chapter 7: TANKS Software - Frequent Questions
- How do I get a copy of the user's manual?
- How do I estimate emissions from a heated tank?
- How does the TANKS model generate speciated emissions?
- How do I calculate emissions from a fixed roof tank with a roof geometry other than a cone or dome?
- How can I obtain Antoine's coefficients for other chemical compounds?
- 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?
- My storage tank has a nitrogen blanket. How should I estimate emissions?
- Can I use TANKS to estimate emissions from a fixed roof tank operated under "low pressure" conditions?
- I vent my fixed roof tank to a control device. How should I estimate emissions?
- What do I do if my tank color is not found in AP-42?
- The meteorological data is not available in TANKS for my storage tank location. Where can I get this information?
- Can I use the TANKS model to estimate emissions from a tank storing an inorganic liquid?
- My tank fittings are not in AP-42. How can I calculate emissions?
- How can I estimate emissions from degassing and cleaning operations during a tank turnaround?
- How can I estimate emissions from roof landing losses in the TANKS program? Updated February 2010.
- My tank contains a two-phase liquid. What do I do?
- If my horizontal fixed roof tank is underground, how do I estimate emissions?
- I have a gasoline service station with underground fixed roof tanks that are vapor balanced. What equations should I use to estimate emissions?
- My tank stores different liquids during the year. How do I account for this variability?
- Can TANKS be used to calculate emissions from indoor tanks? What about heated indoor tanks?
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If a 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 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.
The menu selection "Is Tank Heated" on the Physical Characterics screen should be answered "Yes" to estimate emissions from vertical and horizontal fixed roof tanks. This will allow you to enter the temperature data directly on the Tank Contents screen. There are temperature data input fields for Average, Minimum and Maximum Liquid Surface Temperature and for Bulk Liquid Temperature (degrees Fahrenheit). The temperature at which the tank is heated should be entered in all four data fields. If the tank is not well insulated, daily variations in liquid surface temperatures may be observed. The Minimum and Maximum Liquid Surface Temperatures, if available, should be input to estimate breathing losses.
You also should check the information in the chemical database to ensure that appropriate data are available for the program to estimate vapor pressure at the desired temperature. For example, if Option 1 is used in the chemical database for estimating vapor pressure (valid in the temperature range from 40 to 100 F) and the desired temperature is over 100 F, the program will calculate a vapor pressure at 100 F (it will not extrapolate to higher temperatures). Options 2, 3 or 4 in the chemical database should be used for estimating vapor pressures at temperatures greater than 100 F.
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 and Dalton's Law to estimate component emissions. If liquid properties are not known, Version 4.0 of TANKS has default liquid speciation profiles for crude oil and selected petroleum products. The last option uses tank headspace vapor speciation data for fixed roof tanks. The user must supply this data.
The model is programmed to handle cone and dome roofs which are the most common geometries. Other geometries must use one of these options to calculate the "Roof Outage" (the additional tank shell height in feet that would produce a volume of space equivalent to the space under the roof alone). The Roof Outage is one component of the Vapor Space Outage used in the breathing loss calculation.
You must calculate the Roof Outage external to the program based on the geometry of the roof. Once a value of the outage is obtained, modify the program using either the dome or cone roof options by varying either the dome roof radius or the cone roof slope until an equivalent outage is calculated. As an example, if the tank has a flat roof, the "Cone Roof" option can be selected and the slope set to "0".
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.
Baublik, Fried and Hola, 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.
**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.
The inputs to the working loss equation should be modified for constant level tanks since the working losses are based on changes in the height of the liquid. First, determine the annual tank turnover rate by dividing the tank throughput by the tank volume. Next, determine the average change in liquid height and divide it by the maximum liquid height to establish a ratio. Multiply the annual tank turnover rate by this ratio to obtain the adjusted annual turnover rate for input to the model.
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.
Low pressure conditions are defined by API as an operating range of just above atmospheric to 15 psig. The TANKS model uses equations developed under atmospheric conditions and is not programmed to estimate emissions from low pressure tanks. However, you may use API Bulletin 2516, Evaporation Loss From Low-Pressure Tanks, to determine if your storage pressure is sufficient to prevent standing storage losses from the material stored. Working losses can be calculated as a percentage of the throughput.
If you have test data from the outlet of the control device, you can use it to estimate emissions rather than using the model. If you have a limited amount of data, but have conducted inlet and outlet testing, you can calculate the efficiency of the control device and apply it to emissions estimated by the model.
You should select the most similar color available to represent your tank. If you have a paint with high reflectivity, use white.
Sometimes it is possible to substitute another city from TANKS that has similar meteorological data. The substitutions can be found in Appendix C of the TANKS User's Manual.
Meteorological data for cities that cannot be substituted can be found online at the following web sites:
International: NASA Surface Meteorology and Solar Energy Data Set (https://power.larc.nasa.gov/)
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 data are available for one of the vapor pressure options in the chemical database. 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.
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.
The following procedure can be used to approximate emissions from each step of the operation:
- For a fixed roof tank, calculate emissions from one turnover with the turnover factor (KN) = 1 to account for vapors displaced during filling and then add the emissions from 1 turnover calculated as if the tank had a floating roof to account for clingage.
- For a floating roof tank, calculate emissions for one turnover then add the emissions from the tank assuming it has a fixed roof with a height equal to the height of the legs (about 6 or 7 ft.) to approximate the vapor displaced from the space under the floating roof.
Cleaning (sludge handling)
- Most wet sludges are about 80% to 90% liquid by weight. A conservative approach for estimating emissions is to assume the sludge is 80% liquid. The remainder is assumed to be VOC and emitted. As an alternative, the actual sludge moisture content can be determined.
In November 2006, Section 7.1 of AP-42 was updated with subsection 184.108.40.206.2 Roof Landings. The TANKS program has not been updated with these new algorithms for internal floating roof tanks. It is based on the 1997 version of section 7.1.
It is possible to estimate these losses in TANKS by using a portion of the guidance developed for degassing and cleaning a tank by modeling the vapor space under the roof as a fixed roof tank and calculating the emissions from one turnover. This is less accurate than using section 220.127.116.11.2 of AP-42.
Use the properties (P, Mv, etc.) of the top phase to estimate emissions. Calculate throughput using the overall liquid.
You should select the Horizontal Tank option and from the Physical Characteristics screen and answer "Yes" to the question, "Is the tank 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.
Use the equations presented in Section 18.104.22.168 of AP-42 for variable vapor space tanks.
Estimate emissions for the time period over which each liquid was stored, and sum the emissions to obtain the annual emissions.
Yes, it can be used for indoor tanks. It should be entered as a heated tank and input all 4 temperatures with the temperature of the building. The program knows that the temperatures are constant so no breathing losses are generated. TANKS doesn't care if the tank is at room temperature or is heated above that.