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Exhaust Emissions Databank (for civil aircraft engines only).
H2.5.2.5 Engine Type
Potential sources of site-specific data on engine types being tested include airline maintenance,
base operations section, and sampling. If site-specific engine data for commercial airlines is not
available but the aircraft operator is known, then an appropriate default engine can be chosen
based on the operator’s national fleet. Airline fleet data, including aircraft engine model, is
published in Bucher & Co.’s JP Airline-Fleets International (Reference 2) and Jet Information
Services’ World Jet Inventory (Reference 24). If the aircraft operator is not known, default,
typical aircraft-engine data is provided in EPA’s Procedures for Emission Inventory Preparation,
Volume IV, Chapter 5. For military aircraft, site-specific information on engine types being
H-18
tested may be obtained from the maintenance section of the air base where testing occurs.
Default aircraft engine data is also listed in the EPA document listed above.
H3 NON-COMBUSTION SOURCES
Each type of non-combustion stationary and area source requires a different methodology to be
used for calculating the rate of emissions, as described in the following paragraphs.
H3.1 Fuel Storage Tanks
H3.1.1 Methodology
Table H-4 shows the “pathways” of HC emissions from the three types of fuel storage tanks
commonly found at airports.
As Table H-4 shows, there are a number of “pathways” for evaporated HC to escape from fuel
storage tanks and enter the atmosphere. Accurate calculation of emissions escaping through each
of these pathways requires information on the tank structure, fuel type, meteorology, and
operating practices. In general, fixed-roof tanks tend to be older and result in the greatest
atmospheric emissions. The presence of a volume of vapor space above the level of liquid in the
tank promotes evaporation of the fuel hydrocarbons and their subsequent release to the
atmosphere through the breather valve. Tanks equipped with a floating roof are able to reduce
evaporative emissions by eliminating the vapor space between the liquid level in the tank and the
tank roof. However, some emissions do occur through various seals and openings and because
fuel clings to the tank walls as the liquid level and roof are lowered.
H-19
Tank type Standing Storage Emissions Working Emissions
Fixed roof Breathing emissions:
Changes in temperature or pressure
cause imbalance between internal
and external vapor pressure.
Breather valves are opened to
equalize pressure, allowing emission
of evaporated hydrocarbons.
Displacement emissions:
During filling of tank, liquid displaces gas inside tank,
forcing it to be expelled through the breather valve.
Air saturation emissions:
During fuel removal, air drawn into the tank becomes
saturated with hydrocarbons and expands, thus causing
an imbalance of vapor pressure with the atmosphere. This
imbalance is relieved by venting to the atmosphere.
External
floating roof
Rim seal, roof fitting emissions:
Emissions occur from rim seals and
roof fittings due to slight imbalances
in internal and external vapor
pressure. Exposure of floating roof
to wind increases emission rate.
Clingage emissions:
As roof is lowered during withdrawal, fuel clings to the
tank walls and evaporates when exposed to the
atmosphere. Evaporation rate increases with wind speed.
Internal
floating roof
Rim seal, deck fitting,
deck seam emissions:
Emissions occur from rim seal, deck
fitting, and deck seam due to slight
imbalances in internal and external
pressure. Lower emissions because
roof is protected from wind.
Clingage emissions:
As roof is lowered during withdrawal, fuel clings to the
tank wall and evaporates. Wind does not increase the
evaporation rate.
Table H-4: Pathways of Hydrocarbon Emissions To
Atmosphere From Fuel Storage Tanks4
A methodology for calculating the sum of hydrocarbon emissions from above-ground and belowground
fuel storage tanks is given in Section 7.1, Volume I of EPA’s Compilation of Air
Pollutant Emission Factors. A wide range of storage tanks is covered in this document, including
fixed roof, internal floating roof, external floating roof, variable vapor space, and pressure tanks.
The general methodology is to identify each of the major pathways for hydrocarbons to escape
from storage tanks to the atmosphere, and use available information to estimate emissions
through each pathway. An overview of information requirements is given below. The two main
categories of emissions are standing storage emissions, which result from changes in the
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