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API MPMS 19.3B (1997, Reaffirmed 2002) – Evaporation Loss from Internal Floating-Roof Tanks
A Technical Overview of Measurement and Calculation Standards for Evaporative Hydrocarbon Losses
The API Manual of Petroleum Measurement Standards (MPMS) Chapter 19.3B, originally published in 1997 and reaffirmed in 2002, provides industry-recognized methodologies for estimating evaporative hydrocarbon losses from internal floating-roof (IFR) storage tanks. This standard is a critical reference for emissions inventory reporting, regulatory compliance, and loss control in petroleum storage terminals and refineries. The following article examines the scope, technical requirements, implementation highlights, and compliance aspects of this important standard.
Scope and Application
API MPMS 19.3B applies to vertical, cylindrical, fixed-roof tanks with an internal floating roof (including both liquid-mounted and vapor-mounted roofs). The standard covers tanks storing volatile hydrocarbon liquids, such as crude oil, gasoline, and naphtha, at ambient or near-ambient temperatures. It provides calculation procedures for:
- Evaporative loss from rim seals (primary and secondary)
- Losses from deck fittings (access hatches, gauge wells, column wells, etc.)
- Losses from deck seams (for bolted or welded decks)
- Withdrawal losses caused by roof movement during emptying and filling operations
The standard explicitly excludes losses from external floating roofs (covered in Chapter 19.3A) and fixed-roof tanks (Chapter 19.2). It is applicable worldwide for both inventory control and emissions reporting.
Tip: When applying API MPMS 19.3B, always verify that your tank configuration matches one of the internal floating roof types described in the standard (liquid-mounted or vapor-mounted). Misclassification can lead to significant calculation errors.
Note on reaffirmation: Although reaffirmed in 2002, many practitioners consider API MPMS 19.3B as a mature standard. However, users should check for later revisions or addenda that may have superseded parts of this edition, especially if used for regulatory submissions.
Technical Requirements and Calculation Methodology
Key Emission Sources
The standard categorizes evaporative loss into three primary sources:
- Standing storage losses (Ls) – Continuous emissions through rim seals, deck fittings, and deck seams due to vapor concentration gradients and wind-induced air circulation in the vapor space.
- Withdrawal losses (Lw) – Additional emissions caused by the wetting of the roof and shell as the liquid level drops.
- Filling losses – Addressed separately; often considered part of withdrawal losses.
Calculation Framework
The total loss from an internal floating-roof tank is:
LT = Ls + Lw
Where each term is determined using a combination of emission factors (provided in the standard) and tank-specific parameters (vapor pressure, liquid properties, tank diameter, number of fittings, roof type, etc.). The standard relies on empirical emission factors derived from extensive field testing conducted by API and industry consortia.
| Emission Source | Component / Example | Emission Factor (lb per unit per day) | Remarks |
|---|---|---|---|
| Rim Seal – Primary | Liquid-mounted primary seal only | 0.2 – 0.5 per ft of seal length | Depends on seal type and condition |
| Rim Seal – Secondary | Primary plus secondary seal | 0.1 – 0.3 per ft of seal length | Reduction due to secondary seal |
| Deck Fittings | Access hatch, gauge hatch, sample well | 1.0 – 5.0 per fitting (varies) | Detailed factors per fitting type |
| Deck Seams | Bolted deck (per ft of seam) | 0.02 – 0.09 per ft of seam | Welded decks have negligible loss |
Best practice: To achieve higher accuracy, use tank-specific emission factors whenever available (e.g., through vapor balance or direct measurement). The default factors in the standard are conservative and may overestimate losses for well-maintained tanks.
Implementation and Operational Considerations
Data Collection Requirements
To apply API MPMS 19.3B, operators must collect:
- Tank dimensions (diameter, height, roof type)
- Number and type of deck fittings and rim seals
- Liquid properties (true vapor pressure, molecular weight, density)
- Average ambient temperature and vapor space temperature
- Throughput information (annual turnover rate)
The standard provides correlations for vapor pressure estimation if laboratory data are not available, but direct measurement is preferred for high-accuracy applications.
Software and Tools
Many commercial emissions management systems (e.g., those used for EPA’s Greenhouse Gas Reporting Program) include built-in calculators based on API MPMS 19.3B. The standard also forms the basis for emission factors used in the US EPA’s AP-42 compendium. Operators should validate that their software matches the calculation logic of the standard, particularly for vapor pressure determination.
Common mistake: Using incorrect vapor pressure (e.g., RVP instead of true vapor pressure at storage temperature) can introduce errors exceeding 50%. Always convert RVP to true vapor pressure using the methods in API MPMS 19.3B or ASTM D2879.
Compliance and Regulatory Alignment
Regulatory Use
API MPMS 19.3B is referenced by numerous environmental regulatory bodies worldwide. In the United States, the EPA accepts this standard for estimating storage tank emissions under 40 CFR Part 98 (GHG Reporting) and for state air quality permits. Similarly, the European Union’s EMEP/EEA Guidebook and national agencies in Canada, Australia, and the Middle East recognize the API methodology.
Audit and Documentation
To demonstrate compliance, operators should maintain:
- Tank inspection records confirming roof type, seal condition, and fitting dimensions
- Copies of the emission factor tables used (with standard edition noted)
- Calculations showing all input variables and intermediate steps
- Justification for any deviations from default factors
Regular inspection of rim seals and deck fittings is essential; emissions increase significantly with seal gap and fitting deterioration. The standard does not mandate specific maintenance intervals but provides guidance on evaluating seal effectiveness.
Limitations of the 1997/2002 Edition
Users should be aware that the emission factors in this edition were developed based on test data from the 1980s and early 1990s. Newer seal designs (e.g., metallic shoe seals with improved secondary seals, liquid-mounted resilient foam seals) may achieve lower losses than predicted. In such cases, operators may apply site-specific factors following the standard’s guidelines for developing factors.
Regulatory tip: If you are required to use a specific edition of API MPMS 19.3B by a regulator, confirm the exact title. Some jurisdictions adopt the standard by reference, and the 1997/2002 edition may have been superseded by later editions (e.g., 2018). Using an outdated edition could affect compliance.
FAQs
Q: What is the difference between API MPMS 19.3A and 19.3B?
A: API MPMS 19.3A covers evaporation loss from external floating-roof tanks (where the roof is exposed to the atmosphere), whereas 19.3B covers internal floating-roof tanks housed inside a fixed roof. The vapor space conditions and emission factors differ significantly between the two configurations.
A: API MPMS 19.3A covers evaporation loss from external floating-roof tanks (where the roof is exposed to the atmosphere), whereas 19.3B covers internal floating-roof tanks housed inside a fixed roof. The vapor space conditions and emission factors differ significantly between the two configurations.
Q: Can I use API MPMS 19.3B for tanks storing refrigerated or heated products?
A: The standard is intended for near-ambient temperature storage. For refrigerated (e.g., LNG) or heated products, the vapor-liquid equilibrium assumptions in the standard may not hold. Refer to other chapters of MPMS or specialized standards (e.g., API 2000 for low-temperature storage).
A: The standard is intended for near-ambient temperature storage. For refrigerated (e.g., LNG) or heated products, the vapor-liquid equilibrium assumptions in the standard may not hold. Refer to other chapters of MPMS or specialized standards (e.g., API 2000 for low-temperature storage).
Q: Does the standard require periodic re‑calculation?
A: The standard does not specify a recalculation frequency, but industry best practice is to update loss estimates whenever there is a significant change in tank configuration (e.g., seal replacement, fitting change) or annually for emissions reporting. The standard’s emission factors are designed for steady-state conditions; transient events (e.g., cleaning, vapor balancing) are not covered.
A: The standard does not specify a recalculation frequency, but industry best practice is to update loss estimates whenever there is a significant change in tank configuration (e.g., seal replacement, fitting change) or annually for emissions reporting. The standard’s emission factors are designed for steady-state conditions; transient events (e.g., cleaning, vapor balancing) are not covered.
Last updated: 2026. This article is for informational purposes and should not replace the official API standard. Always consult the latest edition of API MPMS 19.3B for regulatory compliance and engineering calculations.