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API MPMS 19.1 (2012): Evaporative Loss from Fixed-Roof Tanks – A Technical Overview
Comprehensive Analysis of the API Manual of Petroleum Measurement Standards Chapter 19.1 for Quantifying Evaporative Losses from Fixed-Roof Storage Tanks
Scope and Applicability
API MPMS 19.1 (2012), part of the American Petroleum Institute’s Manual of Petroleum Measurement Standards (MPMS), provides a rigorous methodology for estimating evaporative losses from fixed-roof storage tanks (FRTs). These losses, primarily caused by vapor displacement during filling (working loss) and diurnal temperature changes (standing storage loss or breathing loss), are a significant source of volatile organic compound (VOC) emissions in the upstream, midstream, and downstream petroleum industries.
The standard applies to atmospheric fixed-roof tanks storing organic liquids with true vapor pressures between 0.1 psia and 14.7 psia. It covers both standing storage and working losses for tanks operating under normal conditions, including those with internal floating roofs that are not closely fitted (if applicable per special cases). Excluded from its scope are pressure-vacuum vented tanks, refrigerated storage, and floating-roof tanks (covered under API MPMS 19.2). The method is widely adopted for regulatory emission inventories, engineering design evaluations, and compliance reporting under agencies such as the U.S. EPA, Environment Canada, and regional air quality boards.
Tip: API MPMS 19.1 (2012) is harmonized with EPA’s AP-42 Chapter 7.1 methods but provides additional guidance on liquid surface temperature estimation and saturation factors, offering greater accuracy for site-specific conditions.
Technical Requirements
The calculation methodology in API MPMS 19.1 (2012) is based on fundamental mass transfer principles. The two primary loss components—standing storage loss (LS) and working loss (LW)—are computed separately and summed to obtain total evaporative loss.
Standing Storage Loss (Breathing Loss)
Standing storage loss results from thermal expansion and contraction of the vapor space inside the tank. The standard provides equation 1-1:
LS = VV × WV × KE × KS
Where:
VV = vapor space volume (ft³)
WV = vapor density (lb/ft³) at average liquid surface temperature and vapor pressure
KE = vapor space expansion factor (dimensionless), dependent on diurnal temperature range, vapor pressure, and relief settings
KS = vented vapor saturation factor (dimensionless), accounting for the degree of saturation of the exiting vapor
Working Loss
Working loss occurs when vapor is displaced during filling and, to a lesser extent, when air is drawn in during emptying. The equation is:
LW = Q × WV × KN × KP
Where:
Q = annual net throughput (bbl)*
WV = vapor density (lb/ft³)
KN = turnover factor (dimensionless)
KP = working loss product factor (dimensionless), typically 1.0 for volatile organic liquids
*The standard recommends converting throughput to consistent mass or volumetric units per the measurement practices of API MPMS Chapter 12.
Key input parameters include tank geometry (diameter, height, shell color, roof type), liquid characteristics (vapor pressure curve, molecular weight, liquid surface temperature), vapor space conditions (temperature, composition), and meteorological data (ambient temperature range, solar insolation). Accurate determination of the liquid surface temperature is critical, as it directly affects vapor pressure and density. The standard includes detailed procedures for calculating liquid surface temperature using the ambient temperature, tank color, and liquid properties.
Warning: Incorrect estimation of liquid surface temperature is one of the most common sources of error in emission calculations. Always use the recommended heat transfer correlations from API MPMS 19.1 and validate against field temperature measurements when available.
Table 1: Typical Input Parameters for Fixed-Roof Tank Emission Calculation
| Parameter | Symbol | Units | Example Value |
|---|---|---|---|
| Tank diameter | D | ft | 120 |
| Tank height (shell) | HS | ft | 48 |
| Average liquid height | HL | ft | 30 |
| Liquid surface temperature | TLS | °F | 65 |
| True vapor pressure of liquid at TLS | PV | psia | 5.5 |
| Molecular weight of vapor | MW | lb/lb-mol | 66 |
| Annual net throughput | Q | bbl/yr | 2,000,000 |
| Ambient temperature range | ΔTA | °F | 30 |
| Paint solar absorptance | α | — | 0.45 (white) |
Implementation Highlights
Implementing API MPMS 19.1 (2012) requires systematic data collection and processing. Key steps include:
- Tank characterization: Gather shell course dimensions, roof type (cone, dome, or umbrella), paint condition, and vent settings (pressure/vacuum relief valve set points and capacities).
- Liquid property analysis: Obtain true vapor pressure curves as a function of temperature, liquid molecular weight, and average liquid composition. Use Reid vapor pressure (RVP) correlations per API MPMS 19.1 appendices if direct vapor pressure measurement is unavailable.
- Meteorological data: Obtain local ambient temperature profiles, solar insolation, and wind speed to compute vapor space temperature and expansion factors.
- Throughput data: Record annual receipts and deliveries; working loss is proportional to throughput, so accurate metering per API MPMS standards is essential.
- Calculation: Apply equations (1-1) through (1-7) sequentially. The standard provides example calculations in Appendix C to verify proper use.
Best practice: Use the liquid surface temperature calculation method that includes daily temperature cycling and paint absorptance effects (Section 4.5 of the standard). This approach yields more accurate results than simple arithmetic mean temperature assumptions.
Many operators integrate the methodology into emission inventory software such as the EPA’s TANKS program or proprietary platforms. It is important, however, to verify that the software implements the 2012 version correctly, particularly with respect to saturation factor (KS) tables and the vapor space expansion factor (KE) derived from the standard’s integral method.
Compliance Notes
API MPMS 19.1 (2012) serves as the reference method for emissions estimation in many regulatory programs, including Title V permit applications, state implementation plans (SIPs), and greenhouse gas reporting rules. The standard is also cited in 40 CFR Part 98 (Mandatory Greenhouse Gas Reporting) for petroleum and natural gas systems.
Key compliance considerations include:
- Documentation: Maintain records of all input parameters, data sources, and calculation outputs. Auditors routinely request tank data sheets, liquid analyses, and meteorological records.
- Material balance reconciliation: Compare estimated losses against reconciled inventory gains/losses from tank gauging per API MPMS Chapter 18.2. Discrepancies exceeding ±20% may indicate erroneous inputs or meter inaccuracies.
- Updates: When a newer version of API MPMS 19.1 is published (e.g., 2020 edition), regulatory agencies may require transition within a defined period. Check the effective date for your jurisdiction.
Critical: Failure to properly account for vapor saturation at low throughput and high turnover can lead to significant underestimation of working losses. The standard includes special provisions for cases where the turnover factor (KN) is less than 1.0; such scenarios must be carefully evaluated.
Frequently Asked Questions
Q: What is the difference between API MPMS 19.1 (2012) and the earlier 2002 version?
A: The 2012 edition introduced significant revisions to the vapor space expansion factor calculation and the saturation factor method for standing storage loss, improving accuracy for tanks operating in warm climates. It also aligned more closely with AP-42 updates and provided new guidance for estimating liquid surface temperature using hourly meteorological data.
A: The 2012 edition introduced significant revisions to the vapor space expansion factor calculation and the saturation factor method for standing storage loss, improving accuracy for tanks operating in warm climates. It also aligned more closely with AP-42 updates and provided new guidance for estimating liquid surface temperature using hourly meteorological data.
Q: Can API MPMS 19.1 be used for insulated or heated tanks?
A: The standard is primarily intended for uninsulated, ambient-temperature fixed-roof tanks. For heated tanks, the liquid surface temperature must be determined directly from process conditions, and the expansion factor equations should be applied with caution. Insulated tanks may require site-specific heat transfer analysis not fully covered in the standard.
A: The standard is primarily intended for uninsulated, ambient-temperature fixed-roof tanks. For heated tanks, the liquid surface temperature must be determined directly from process conditions, and the expansion factor equations should be applied with caution. Insulated tanks may require site-specific heat transfer analysis not fully covered in the standard.
Q: How do I handle tanks with internal floating roofs when using API MPMS 19.1?
A: For fixed-roof tanks equipped with internal floating roofs that are not liquid-mounted, API MPMS 19.1 permits using the fixed-roof methodology if the floating roof is not considered “closely fitted.” However, for well-fitted internal floating roofs, API MPMS 19.2 (Evaporative Loss from Floating-Roof Tanks) is the appropriate standard. The 2012 edition provides a decision tree in Appendix A to help users determine the correct method.
A: For fixed-roof tanks equipped with internal floating roofs that are not liquid-mounted, API MPMS 19.1 permits using the fixed-roof methodology if the floating roof is not considered “closely fitted.” However, for well-fitted internal floating roofs, API MPMS 19.2 (Evaporative Loss from Floating-Roof Tanks) is the appropriate standard. The 2012 edition provides a decision tree in Appendix A to help users determine the correct method.
Q: Are there software tools that implement API MPMS 19.1 (2012)?
A: Yes, the U.S. EPA’s TANKS program (version 5.0 and later) incorporates the 2012 methodology. Several commercial emission inventory systems also offer modules based on this standard. Users should verify that the version and algorithms are current, as software updates sometimes lag behind standard revisions.
A: Yes, the U.S. EPA’s TANKS program (version 5.0 and later) incorporates the 2012 methodology. Several commercial emission inventory systems also offer modules based on this standard. Users should verify that the version and algorithms are current, as software updates sometimes lag behind standard revisions.