1. Scope and Historical Context

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1. Scope and Historical Context

The API Manual of Petroleum Measurement Standards (MPMS) Chapter 19.5, published in August 2009, represents the authoritative industry consensus for quantifying evaporative losses from atmospheric storage tanks. Officially titled Atmospheric Storage Tanks, Floating Roofs, this first edition standard supersedes the historic API Publication 2514A (1974) and addresses the modern requirements for environmental compliance, product loss minimization, and accurate vapor recovery system (VRS) performance verification.

The standard unifies two primary operational scenarios under a single framework:

• Floating Roof Tanks: Loss measurement for internal floating roof (IFR) and external floating roof (EFR) tanks.
• Vapor Recovery Systems: Testing and performance measurement for fixed roof tanks equipped with a closed VRS or vapor balancing system.

The 2009 edition was a direct response to increasingly stringent air quality regulations and the growing economic value of recovered hydrocarbon vapors. By replacing the default factor approach of API Publ 2514A with a structured two-tier methodology, API MPMS 19.5 provides operators with a defensible and auditable path for emissions reporting.

2. Technical Requirements and Calculation Methodology

The standard formalizes a dual-tier approach to loss quantification, balancing simplicity with the need for precision in high-stakes regulatory environments.

2.1 Tier 1 Methodology

Tier 1 relies on predetermined emission factors derived from broad empirical datasets collected across standard industry tank configurations. Inputs required include tank dimensions (diameter, height), roof type, seal configuration (liquid-mounted, vapor-mounted, double seal), shell color and condition, and annual throughput. The results are considered screening-level estimates suitable for preliminary emissions inventories or when site-specific measurement data is unavailable or impractical.

2.2 Tier 2 Methodology

Tier 2 represents a significant advancement in measurement rigor. It replaces default factors with site-specific, directly measured parameters. This method is mandatory for performance testing of vapor recovery systems and is the standard for proving compliance under regulations such as US EPA NSPS Subpart Kb. Core components include:

• Continuous or periodic vapor flow measurement at the VRS inlet using calibrated flow meters.
• In-line gas composition analysis via gas chromatography or calibrated hydrocarbon analyzers.
• Temperature and pressure compensation to convert measured volumes to standard conditions.
• Integration of measured data with tank level gauging and throughput data for mass balance reconciliation.

2.3 Key Loss Components

The standard provides detailed equations for calculating the following primary loss categories:

• Standing Storage Loss: Loss from rim seals, deck fittings, and deck seams.
• Withdrawal Loss: Vapors generated during roof settling and product extraction.
• Vapor Recovery System Loss: Net emission from the VRS, calculated as the mass of vapors recovered vs. vapors introduced.

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