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Scope and Purpose
Scope and Purpose
API Publication 2517, first issued in 1989 and amended by the 1994 Addendum, provides a standardized methodology for estimating evaporation losses from external floating-roof tanks storing volatile liquids. The document supersedes earlier API bulletins and remains a key reference for tank emission calculations in the petroleum industry.
The scope covers standing storage losses and withdrawal losses from external floating-roof tanks. It accounts for losses through rim seals, deck fittings (including hatches, gauging ports, and vacuum breakers), and deck seams. The publication also addresses the effect of deck-mounted equipment and seal systems on overall emission rates.
Technical Requirements and Calculation Methodology
API Publ 2517 presents a comprehensive set of equations to estimate total evaporation loss from external floating-roof tanks. The loss per year (LT) is the sum of standing storage loss (LS) and withdrawal loss (LW):
- Standing storage loss (LS): losses occurring due to vapor breathing and permeation when the tank is not being filled or emptied. It includes rim seal losses, deck fitting losses, and deck seam losses.
- Withdrawal loss (LW): loss of vapors when the tank is being emptied, caused by liquid draining from the roof and wetting the tank walls.
Key Parameters
| Parameter | Symbol | Typical Units | Description |
|---|---|---|---|
| Tank diameter | D | ft (m) | Nominal diameter of tank shell |
| Wind speed | U | mph (m/s) | Average wind speed at tank site |
| Vapor pressure | PVA | psia (kPa) | True vapor pressure of stored liquid at storage temperature |
| Seal factor | KS | lb-mol/ft·yr (kg-mol/m·yr) | Empirical factor depending on seal system |
| Fitting loss factor | KF | lb-mol/yr (kg-mol/yr) | Loss factor per fitting type |
Rim Seal Losses
The rim seal loss is calculated using the equation:
LR = KS · D · PVA · MV · KC
where MV is the molecular weight of the vapor and KC is the product factor for custom sealing systems.
Deck Fitting Losses
Deck fitting losses depend on the number and type of fittings. The total loss from deck fittings is the sum over all fittings of the product of fitting loss factor (KF) and vapor pressure adjustment.
Tip: When using API Publ 2517 for regulatory compliance, always verify that the latest addendum or errata is applied. The 1994 Addendum includes corrections to several loss factors for deck fittings and clarifies the treatment of rim-mounted secondary seals.
Implementation Highlights
To properly implement the API Publ 2517 methodology, the following steps are essential:
- Characterize the tank and stored liquid: Gather data on tank dimensions, roof type, seal configuration, number and type of fittings, average wind speed, and liquid vapor pressure at storage conditions.
- Select appropriate loss factors: Use the tables provided in the publication to determine KS and KF for each seal and fitting type. For non-standard configurations, consult the manufacturer or perform a custom calculation.
- Calculate seasonal or annualised losses: The standard allows for monthly or annual average conditions. For regulatory reporting, annual