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EQ Winter 2016 Tanks Article Fig 2In 2016, EPA proposed its potential enforcement initiatives for the years 2017 through 2019, targeting what it viewed as the most important environmental problems where non-compliance is a significant contributing factor and where enhanced enforcement could make a difference.1 First on the list of new, potential initiatives was protecting communities from exposure to toxic air emissions by focusing on emissions from organic liquid storage tanks.  According to EPA, storage tanks are potentially significant sources of excess air emissions at many sites, including terminals, refineries, and chemical plants.  Some of these tanks are located in ozone nonattainment areas, communities of environmental justice concern, or other areas with sensitive populations. 

On July 25, 2018, the US EPA proposed changes to AP-42 Chapter 7. The primary driver for this round of revisions was the need to address emissions from tank cleaning events. Other revisions included proposed changes to the temperature equations, which would impact estimated emissions for all tanks regardless of tank type, and guidance for estimating emissions from tanks storing hot stocks, taking into account different configurations of insulation. 
Finally, on November 20, 2019, EPA released the final revisions to AP-42 Chapter 7. A few of the updates include new calculation methodology for tank cleaning and degassing losses, updated meteorological data, and the upward revision to the default vapor pressure for No. 6 Oil. The impact of these final AP-42 revisions on estimated emissions may vary greatly, depending upon the tank scenario. For those who had not been accounting for tank cleaning emissions previously, then beginning to account for them will have a significant impact. The changes to the temperature equations will typically result in only single-digit percent differences, unless the tank stores hot stock and the elevated temperature had not been previously accounted for. In fact, the impact on tanks storing hot stocks will vary from virtually no change at all to an increase of 100 times or greater, depending upon how you had been previously modeling these tanks.

With the release of the final revisions to AP-42 Chapter 7, TANKS 4.09d is no longer a valid tool for tank emission calculations.

Fortunately, there is a better option available for calculating tank emissions. 

TankESP Software Product Suite Version 5.0 Incorporates Proposed AP-42 Changes

In early 2019, BREEZE Software released the BREEZE TankESP Software Product Suite Version 5.0. TankESP is a tank emissions calculation software product suite that uses the emission estimation procedures from Chapter 7. It is widely regarded as the most accurate tank emission calculation tool on the market because it was designed by Rob Ferry, one of the authors of AP-42 Chapter 7.1 and American Petroleum Institute (API) documents 19.1, 19.2 and 19.4.

The primary driver for this new version of TankESP was to incorporate the changes to AP-42, which were working their way through the EPA review process. Version 5.0 enabled TankESP users to estimate emissions using emission methods from either the latest draft version of AP-42 or the pre-revision version.

With the very recent release of final revisions from the EPA, the BREEZE Development Team is working on updating BREEZE TankESP and expects to have a new version available in January 2020. This new version of BREEZE TankESP (5.1) will implement the final revisions across all three TankESP editions: TankESP PRO (the successor to TankESP_d), TankESP FX (for fixed-roof tanks only), and TankESP LT (a basic version similar to TANKS 4.09D). These editions range in capability and price to best satisfy the needs of our clients. For example, if you have no floating-roof tanks, then you can purchase the FX edition since you don't need the floating-roof tank functionality. Or, if you only want to estimate working and standing losses as you did with TANKS 4.09D, then you can purchase the LT edition rather than paying for the floating-roof landing and tank cleaning functionality of the PRO edition.

BREEZE TankESP also includes several additional technical enhancements, including the inclusion of three short-term emission calculation methods based on technical guidance from the American Petroleum Institute (technical report 2576) and the Texas Commission on Environmental Quality (guidance documents APGD 6250 and APGD 6419), and resolves the technical deficiencies associated with the US EPA's TANKS 4.09d software, which is now no longer supported by the US EPA. In fact, the website for TANKS 4.09D has a warning in bold red font which states that the program is “outdated [and] not reliably functional,” and that they no longer provide assistance in its use and it is to be used “at your own risk.”

Deficiencies of TANKS 4.09d & their Resolution in BREEZE TankESP

The following are only some of the many technical deficiencies of TANKS 4.09D that are resolved in TankESP:

TANKS 4.09D Issue

Resolution in BREEZE TankESP

Guidepole. TANKS allows defaulting the guidepole configuration (with the default assumption being an uncontrolled, unslotted guidepole). The potential emissions from a guidepole in an EFRT, however, are such that any default for this fitting may result in a large error in the overall estimate of emissions.Both TANKS and TankESP require the user to enter the rim seal configuration, rather than offering a default entry, due to the potential magnitude of the resulting emissions. TankESP similarly requires the user to make entries for the guidepole quantity and status (while allowing defaults for all other deck fittings).
Recalculate Deck Fittings. When a change has been made to the tank diameter, TANKS only recalculates deck fitting quantities if the deck fittings are shown as “Typical”. Furthermore, if a change is made to the type of deck, TANKS only adjusts the deck fitting selections if “Typical” is shown. If a change has been made to any deck fitting (such as changing the control status from Ungasketed to Gasketed), then none of the deck fitting quantities will recalculate with a change in tank diameter, and none of the deck fitting selections will adjust with a change in the deck type.When a change is made to the tank diameter, TankESP recalculates the quantity of each deck fitting that has not had an override entered for the quantity of that particular deck fitting. When a change is made to the type of deck, TankESP adjusts each deck fitting selection and quantity that does not have an override entry.
IFRT Deck Support Legs. TANKS allows changing the IFR deck support legs from “Roof Leg or Hanger Well” to “Roof Leg (3-in. Diameter)” without explaining that the latter is only appropriate for EFR-type deck legs {the 3-inch diameter is not the critical parameter, it is the 30-inch long or longer leg housing of an EFR-type deck (versus the 12-inch long housing typical of an IFR-type deck leg) that matters}.TankESP does not allow combining EFR-type deck legs with an IFR-type deck. In order to model the deck legs as the taller EFR-type, the user must select an EFR-type deck (i.e., a steel pontoon or double-deck type, which are available options for a tank with a fixed roof, resulting in the configuration that TANKS labels “Domed External Floating-Roof Tank”).
EFRT Deck Support Legs. TANKS does not explain that deck legs for double-deck EFRs are similar to the center-area legs of pontoon EFRs (i.e., an assumed 48-inch housing length), and thus the factors for “center area, sock” may be used when a double-deck EFR is equipped with leg socks (cf. API MPMS 19.2 Table 6).TankESP allows selecting leg socks as a control level for a double-deck EFR, and appropriately applies the same emission factor as for a center-area leg of a pontoon-type EFR.
Change of Tank Type. If an alteration to a tank changes the tank type (e.g., installing an IFR in a Fixed-Roof Tank, or installing a dome on an EFRT), TANKS requires the creation of a new tank record.TankESP allows changing the type of fixed roof or floating roof within a tank record, and automatically recognizes the change in all calculations of future emissions (while retaining records for prior emissions based on the prior tank details).
Saving changes. TANKS prompts the user for whether changes are to be saved when closing a tank record. This allows the user to run “what-if” scenarios without overwriting the existing tank record, as long as the user does not save the changes - or so the user may think. In actuality, if the user runs a report of the what-if scenario, then TANKS automatically saves the changes to that tank record. The user may then click “No” to the “Save changes?” prompt and think that the original tank record has been preserved, when in fact it has been overwritten.Any manner of what-if scenarios can be run in TankESP, and the original record will be maintained if the user simply closes the file without saving it. Should the user want to retain both the original and the edited versions, then the edited version simply needs to be saved to a different file name.
Monthly emissions. TANKS does not account for the actual number of days in each month, but rather simply divides the annual period by 12. This causes emissions for February to be overestimated by about 10%, with smaller errors in the other months.TankESP bases monthly emission estimates on the actual number of days in the month.
Vent settings. TANKS does not account for fixed-roof tank vent settings in the calculation of working loss. API, in cooperation with EPA, revised the working loss equation to account for the vent settings. This revision is included in the Third Edition of API 19.1 and in the 2006 revisions to AP-42 Section 7.1, but it has not been incorporated into EPA's TANKS program.TankESP includes the vent setting term in the calculation of fixed-roof tank working loss.
Riveted tanks.TANKS does not have Tank Construction as a variable for IFRTs, thereby not accommodating a selection of “Riveted” with a Mechanical-shoe seal.TankESP allows the selection of the Riveted factor for Mechanical-shoe seals for all types of floating-roof tanks.
Sample Pipe or Well. This Fitting Type in TANKS is not found in AP-42, and it poses unnecessary confusion. Two of the options refer to “Slotted Pipe,” and are duplications of “Slotted Guide-Pole” options. The other option, “Slit Fabric Seal,” should be an option under “Gauge-Hatch/Sample Well.”TankESP lists deck fitting options that clearly correspond to the selections provided in AP-42.
Horizontal Tanks. TANKS incorrectly calculates the height of the vapor space, Hvo, in a horizontal tank as (D/2), rather than as (He/2) per AP-42 Equation 1-15.TankESP correctly calculates Hvo as (He/2) for horizontal tanks.


EQ Winter 2016 Tanks Article Fig 1Why Switch to TankESP?

With the AP-42 Chapter 7 revisions finalized and TANKS 4.09D no longer supported by the US EPA, now is the time to upgrade to BREEZE TankESP. In addition to all of the benefits of TankESP discussed above, the software also includes many time-saving features for environmental professionals, such as:

  • Accommodating mid-month changes in service
  • Allowing for changes in tank construction, such as adding a dome roof to an open-top external floating-roof tank
  • Accounting for control efficiency when vapors are routed to a control device
  • Allowing for reporting customization to meet the requirements of regulatory agency


Visit the TankESP product page to learn more and contact the BREEZE Team at or (972) 661-8881 for any questions you may have.

Challenges in Calculating Tank Emissions

One key challenge (and potential source of error) in tank emission calculations is outdated or incorrect tank design and liquid product characterization data.  Facilities need to:

  • Update product chemical/physical properties with reasonable HAP speciation,
  • Record and report tank turnaround inspection and maintenance activities, and
  • Update tank seal and design parameters.

Additionally, it is important to check regulatory requirements and operating permit limits (e.g., NSPS Subpart Ka, Kb, and BWON requirements).  Finally, and perhaps most significantly, due to the technical issues with TANKS 4.09d, owner/operators may need to revisit historical emission limits or permitting analyses when updating emission calculation methodologies.  Many facilities have converted site tank emission data into TankESP and are using this calculation tool to address these challenges.

Trinity has extensive experience implementing and using BREEZE for permit applications and emissions inventories, and has addressed each of these issues.  Trinity can provide complete permit application support for tank projects from strategy to emission calculations, through permit application preparation and support with state and local agencies.

1 Public Comment on EPA's National Enforcement Initiatives for Fiscal Years 2017-2019, published in the Federal Register on September 15, 2015, 80 FR 55352.
2 AP 42, Fifth Edition, Volume I Chapter 7:  Liquid Storage Tanks, November 2006, .
3 Storage Tank Emissions Determination Challenges, Russ Nettles TCEQ Air Quality Division, presented to the 2015 Emissions Inventory Workshop, January 14, 2015.