Calculating Tank Emissions with TankESP

EQ Winter 2016 Tanks Article Fig 2EPA recently proposed its potential enforcement initiatives for the years 2017 through 2019, targeting what it views 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 is 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.  With increased scrutiny by EPA on tank emissions, facilities should review their current tank emission compliance demonstration methodologies, including emissions calculated for permitting and emissions inventories, to ensure they are accurate, up-to-date, and defensible in case of EPA review.

In addition to proposing this potential enforcement initiative, EPA has stopped supporting its TANKS Database (TANKS 4.09d).  TANKS 4.09d has long been the standard emission calculation tool for tank emissions.  While TANKS 4.09d is still available on EPA's website, EPA states that the model should be "used at your own risk."  Further, EPA recommends use of the equations and algorithms specified in AP-42 Chapter 72 for estimating VOC emissions from storage tanks.

Fortunately, there is a better option available for calculating tank emissions.  TankESP is a tank emissions calculation program (available in both database and spreadsheet versions) which follows the AP-42 Chapter 7 methodology for estimating VOC emissions from storage tanks.  This article details the known issues with TANKS 4.09d and describes why many companies are switching to TankESP.  With an EPA enforcement initiative imminent and TANKS 4.09d no longer supported by EPA, it is critical for facilities operating large storage tanks to ensure their emission calculation methodologies are accurate and conform to EPA's recommended methodology.

Deficiencies of TANKS 4.09d

EPA's decision to stop supporting TANKS 4.09d is related to a number of documented technical issues with the program.  It was developed and maintained by EPA's Office of Air Quality Planning and Standards (OAQPS) as a tool to calculate emissions based on the methodologies in AP-42 Chapter 7.  Both AP-42 Chapter 7 and TANKS 4.09d were last updated in November 2006.  However, TANKS 4.09d has a number of documented errors in how the program incorporates the AP-42 Chapter 7 equations, including those described below.

1.  Annual bulk liquid temperature affecting all tanks (except heated tanks)
In TANKS 4.09d, the liquid bulk temperature, for all tanks, is calculated using the annual average ambient temperature, which does not change from month to month even if the user selects the monthly report.  As a result, the liquid surface temperature is underestimated in summer and overestimated in winter.  For tanks with greater throughput during the summer months, TANKS 4.09d will underestimate monthly emissions.

2.  Heated vertical fixed roof tanks
There are two issues with the emission calculation in TANKS 4.09d for heated tanks.  First, TANKS 4.09d caps the stock true vapor pressure at the value corresponding to 100°F when the storage temperature exceeds 100°F, if using Option 1 (interpolation method) to determine vapor pressures.  In Option 1, vapor pressures at a given temperature are determined by interpolation method using vapor pressures from 40°F to 100°F provided by the user.  The storage temperatures of many heated tanks exceed 100°F; thus TANKS 4.09d typically underestimates emissions from these tanks.

Secondly, TANKS 4.09d assumes that all heated tanks are fully insulated.  For fully insulated tanks, the vapor space temperature range is the same as the liquid surface temperature range.  As a result, standing losses for insulated tanks are not taken into account.  When a heated tank is not insulated or is only partially insulated, standing losses still occur for the fixed roof heated tanks.  This scenario is not accurately modeled in TANKS 4.09d.

3.  Floating roof tanks with warm products
TANKS 4.09d calculates bulk liquid temperatures based solely on ambient temperatures and does not let users input the temperature of warm products (i.e., products stored at temperatures above ambient).  In this case, TANKS 4.09d does not correctly compute the vapor pressure at the actual product storage temperature.  As a result, TANKS 4.09d underestimates3 the emissions from the tank.

In addition, TANK 4.09d does not calculate emissions from the following source operations:

  • Tank landing emissions (included in AP-42 Chapter 7);
  • Tank cleaning emissions;
  • Short-term emissions;
  • Flashing emissions; and
  • Tanks that are mixed (i.e., mechanical mixing or air sparging).

Even though these significant issues have been recognized by EPA, no updates to the software have been made in nine years!  As a result, TANKS 4.09d should not be used for the following sources:

  • Heated tanks;
  • Tanks storing warm product;
  • Tanks with significant variations in throughput month by month; and
  • Monthly/ozone season emissions for all tanks.

As a result of these technical issues, the Texas Commission on Environmental Quality (TCEQ) has not accepted TANKS 4.09d emission calculations from the above source categories since 2011.  In addition, TCEQ has performed differential absorption lidar (DIAL) studies on tanks which have shown that actual emissions from certain tanks have been underestimated. Recently, Arizona's Pinal and Pima Counties are requiring emissions calculations based on AP-42 Chapter 7 equations for cases where TANKS 4.09d would calculate emissions incorrectly.  An EPA tank enforcement initiative has the potential to further raise the visibility of the technical issues with TANKS 4.09d within state regulatory agencies and EPA.

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

AP-42 Chapter 7 includes more than 70 equations and over 100 variables, and does not lend itself to simple emission calculations.  TankESP, developed by well-known tank expert Robert Ferry at TGB Partnership, calculates tank emissions following the methodologies in AP-42 Chapter 7 and resolving the technical issues with TANKS 4.09d.  In addition, TankESP can calculate tank landing emissions (following the AP-42 Chapter 7 methodology), tank cleaning emissions, short-term emissions, and 12-month rolling totals.  TankESP is available in spreadsheet (Microsoft Excel) or database (Microsoft Access) format.  Both TankESP formats allow for easy block copy and paste functions to quickly populate throughput and product data for routine emission calculations.  Figure 1 provides a side-by-side comparison of TANKS 4.09d and TankESP.

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.  Some facilities have converted site tank emission data into TankESP and are using this calculation tool to address these challenges.

Through our association with Tank ESP developer TGB Partnership, Trinity has extensive experience implementing and using TankESP 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.