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API MPMS 19.3A 1997 (2007): Comprehensive Guide to Evaporative Loss Measurement from Floating Roof Tanks
Understanding the Methodology, Technical Requirements, and Compliance Implications of the Manual of Petroleum Measurement Standards Chapter 19.3A
In the petroleum and petrochemical industries, accurate measurement of evaporative losses from storage tanks is essential for environmental compliance, product accounting, and operational efficiency. The American Petroleum Institute (API) Manual of Petroleum Measurement Standards (MPMS) provides industry-recognized methodologies for this purpose. Specifically, API MPMS 19.3A 1997 (2007) outlines procedures to estimate vapor losses from floating roof storage tanks. This article explores the scope, technical requirements, implementation considerations, and compliance aspects of this important standard.
Standard Reaffirmation: API MPMS 19.3A was originally published in 1997 and reaffirmed in 2007. This reaffirmation confirms that the technical content remains current without revision. Users should continue to reference the 1997 edition while verifying that any applicable addenda or corrigenda are included.
Scope of API MPMS 19.3A 1997 (2007)
This standard presents methods for estimating evaporative losses from external and internal floating roof tanks storing volatile liquid hydrocarbons, such as crude oil, gasoline, and other volatile organic compounds (VOCs). It covers three types of losses:
- Standing Storage Loss: Losses occurring when the tank is not undergoing a filling or emptying operation due to vapor space expansion caused by daily temperature variations and wind effects.
- Withdrawal Loss: Losses occurring when liquid is withdrawn, leaving a film on the tank shell that subsequently evaporates.
- Roof Landing Loss: Losses that occur when the floating roof lands on its support legs, exposing the liquid surface to the atmosphere.
The standard applies to tanks with external floating roofs (open top with a seal) and internal floating roofs (inside a fixed roof tank). It does not apply to fixed-roof tanks, pressurized vessels, or non-hydrocarbon liquids. The vapor losses accounted for in the standard include both the hydrocarbon loss from the stored liquid and any vapors that escape through the seal system, deck fittings, and deck seams.
Technical Requirements and Calculation Methodology
API MPMS 19.3A employs empirical formulas derived from industry test data. The general framework calculates total emissions as the sum of three components:
Ltotal = Ls + Lw + Li
where Ls = standing storage loss, Lw = withdrawal loss, and Li = roof landing loss.
Key variables and factors are specified for each loss component. For example, standing storage loss from an external floating roof tank is expressed as:
Ls = Ks · D · (Mv · Pvap / R · T) · (Ew / Ew0)
where parameters are fully defined in the standard: Ks = seal loss factor, D = tank diameter, Mv = vapor molecular weight, Pvap = true vapor pressure, R = ideal gas constant, T = liquid temperature, and Ew / Ew0 is a wind speed function.
The standard provides tabulated loss factors for different rim seal configurations, deck fitting types, and deck seam conditions. These factors are crucial for accurate estimation and are embedded in the equations.
| Category | Required Data | Typical Sources |
|---|---|---|
| Tank Geometry | Diameter, height, roof type (external/internal), leg height | Tank construction drawings / API 650 data sheets |
| Roof Characteristics | Rim seal type (primary, secondary), deck fittings (ladder, gauge hatch, sample well, etc.), deck seam construction | Field inspection, maintenance records, seal manufacturer |
| Liquid Properties | True vapor pressure, molecular weight, liquid temperature, density | Lab analysis, product specification sheets, API MPMS Chapter 20 |
| Meteorological Data | Ambient temperature range, average wind speed (for external roofs), solar irradiation (if used) | Local weather station, historical climatological data |
| Operating Data | Annual throughput, maximum and average liquid levels, number of roof landings per year | Operations logs, inventory records, SCADA |
Implementation Highlights and Data Considerations
Applying API MPMS 19.3A requires careful assembly of tank-specific data. Below are key implementation highlights:
Rim Seal Type Verification
The choice of loss coefficient (Ks or rim seal wiper factor) depends on the seal type: mechanical shoe, wiper, tube seal, or other. Inspection of seal condition often reveals degradation that increases losses beyond standard factors. Facility owners should perform routine inspections and document the exact seal arrangement and condition.
Common Pitfall: Using a generic seal factor without verifying the actual seal type (e.g., assuming a secondary wiper when only a primary seal is present) can lead to significant underestimation of standing storage losses. Always ground‑truth seal configurations during tank inspections.
Liquid Vapor Pressure
True vapor pressure (TVP) at the storage temperature is a dominant driver of emissions; using appropriate API methods (e.g., API MPMS Chapter 20 or ASTM D6377) is recommended for accurate determination. Seasonal changes in temperature can markedly affect TVP and should be accounted for in annual loss estimates.
Roof Landing Activity
If the roof lands multiple times per year due to frequent emptying or operational practices, the roof landing loss can become a significant component. Accurate logs of landing events are essential.
Meteorological Factors
For external floating roof tanks, wind speed directly influences standing loss. Using site‑specific data rather than a broad regional average improves accuracy. For internal floating roofs, the fixed roof above reduces wind effects, and the standard adjusts the calculations accordingly.
Tip: When wind data is not available at the tank site, consider using nearby airport wind records or interpolated gridded data (e.g., NCDC/NOAA) with appropriate adjustments for altitude and terrain.
Compliance Notes and Industry Relevance
While API MPMS 19.3A is a voluntary consensus standard, it has become a key reference in regulatory frameworks. The U.S. Environmental Protection Agency (EPA) references the standard for calculating emissions from petroleum storage tanks under the Clean Air Act (e.g., AP‑42 guidance). States and local air quality agencies often require use of this method for emission inventories, new source review permits, and annual reporting.
Regulatory Risk: Inaccurate emissions estimates—due to poor application of the standard or missing data—can result in non‑compliance with local, state, or federal limits. This may lead to penalties, revised permit conditions, or mandated installation of add‑on controls.
Key compliance considerations include:
- Accurate emission estimates are critical for determining VOC emission fees, offsets, and applicability of control requirements (e.g., BACT, RACT).
- Inconsistencies in data quality (e.g., assumed vs. measured seal type) can lead to under‑ or over‑estimation, resulting in regulatory risk.
- The standard is widely accepted for product loss calculations and reconciliation in custody transfer.
- Reviewing API MPMS 19.3A alongside API MPMS 19.3B (fixed roof tanks) and API 650/653 provides a comprehensive approach to storage tank emissions management.
While the 2007 reaffirmation does not introduce technical changes, users must ensure they are employing the correct edition and any subsequent corrigenda. Industry professionals are encouraged to participate in API standing committees to stay current with updates.
Q: Is API MPMS 19.3A applicable to both external and internal floating roof tanks?
A: Yes, the standard covers both external floating roofs (open tanks with a seal) and internal floating roofs (those with a fixed roof above the floating deck). The equations include separate terms and factors for each configuration.
A: Yes, the standard covers both external floating roofs (open tanks with a seal) and internal floating roofs (those with a fixed roof above the floating deck). The equations include separate terms and factors for each configuration.
Q: What does “reaffirmed 2007” mean for this 1997 standard?
A: Reaffirmation indicates that the standard has been reviewed and deemed current and technically sound; no revisions were required. The 1997 edition remains the official document, though any published errata or addenda should be followed.
A: Reaffirmation indicates that the standard has been reviewed and deemed current and technically sound; no revisions were required. The 1997 edition remains the official document, though any published errata or addenda should be followed.
Q: What are the main differences between API MPMS 19.3A and 19.3B?
A: Chapter 19.3A addresses evaporative losses from floating roof tanks, while 19.3B covers fixed roof tanks. They use different equations and loss factors because the physical mechanisms differ (e.g., wind effects, vapor space expansion).
A: Chapter 19.3A addresses evaporative losses from floating roof tanks, while 19.3B covers fixed roof tanks. They use different equations and loss factors because the physical mechanisms differ (e.g., wind effects, vapor space expansion).
Q: Do regulatory agencies require the use of API MPMS 19.3A?
A: Many regulatory programs accept the standard as an approved methodology for estimating storage tank emissions. The U.S. EPA frequently references it in AP‑42 and state implementation plans (SIPs). It is often mandatory for Title V and NSR permit applications.
A: Many regulatory programs accept the standard as an approved methodology for estimating storage tank emissions. The U.S. EPA frequently references it in AP‑42 and state implementation plans (SIPs). It is often mandatory for Title V and NSR permit applications.
Disclaimer: This article is for informational purposes only and does not substitute for the original API MPMS document. For detailed calculations and compliance, refer to the full text of API MPMS 19.3A 1997 (2007).
Last reviewed: 2026