6/08 Petroleum Industry 5.2-1

5.2 Transportation And Marketing Of Petroleum Liquids1-3

5.2.1 General

The transportation and marketing of petroleum liquids involve many distinct operations, each of

which represents a potential source of evaporation loss. Crude oil is transported from production operations

to a refinery by tankers, barges, rail tank cars, tank trucks, and pipelines. Refined petroleum products are

conveyed to fuel marketing terminals and petrochemical industries by these same modes. From the fuel

marketing terminals, the fuels are delivered by tank trucks to service stations, commercial accounts, and local

bulk storage plants. The final destination for gasoline is usually a motor vehicle gasoline tank. Similar

distribution paths exist for fuel oils and other petroleum products. A general depiction of these activities is

shown in Figure 5.2-1.

5.2.2 Emissions And Controls

Evaporative emissions from the transportation and marketing of petroleum liquids may be

considered, by storage equipment and mode of transportation used, in four categories:

1. Rail tank cars, tank trucks, and marine vessels: loading, transit, and ballasting losses.

2. Service stations: bulk fuel drop losses and underground tank breathing losses.

3. Motor vehicle tanks: refueling losses.

4. Large storage tanks: breathing, working, and standing storage losses. (See Chapter 7, "Liquid

Storage Tanks".)

Evaporative and exhaust emissions are also associated with motor vehicle operation, and these topics

are discussed in AP-42 Volume II: Mobile Sources. 

5.2.2.1 Rail Tank Cars, Tank Trucks, And Marine Vessels -

Emissions from these sources are from loading losses, ballasting losses, and transit losses. 

5.2.2.1.1 Loading Losses - 

Loading losses are the primary source of evaporative emissions from rail tank car, tank truck, and

marine vessel operations. Loading losses occur as organic vapors in "empty" cargo tanks are displaced to the

atmosphere by the liquid being loaded into the tanks. These vapors are a composite of (1) vapors formed in

the empty tank by evaporation of residual product from previous loads, (2) vapors transferred to the tank in

vapor balance systems as product is being unloaded, and (3) vapors generated in the tank as the new product

is being loaded. The quantity of evaporative losses from loading operations is, therefore, a function of the

following parameters: 

- Physical and chemical characteristics of the previous cargo;

- Method of unloading the previous cargo;

- Operations to transport the empty carrier to a loading terminal;

- Method of loading the new cargo; and

- Physical and chemical characteristics of the new cargo.

The principal methods of cargo carrier loading are illustrated in Figure 5.2-2, Figure 5.2-3, and Figure 5.2-4. 

In the splash loading method, the fill pipe dispensing the cargo is lowered only part way into the cargo tank. 

Significant turbulence and vapor/liquid contact occur during the splash 
5.2-2 EMISSION FACTORS 6/08 Figure 5.2-1. Flow sheet of petroleum production, refining, and distribution systems. (Points of organic emissions are indicated by vertical arrows.)
6/08 Petroleum Industry 5.2-3

Figure 5.2-2. Splash loading method.

Figure 5.2-3. Submerged fill pipe.

Figure 5.2-4. Bottom loading.
5.2-4 EMISSION FACTORS 6/08

(1)

loading operation, resulting in high levels of vapor generation and loss. If the turbulence is great enough,

liquid droplets will be entrained in the vented vapors.

 A second method of loading is submerged loading. Two types are the submerged fill pipe method

and the bottom loading method. In the submerged fill pipe method, the fill pipe extends almost to the bottom

of the cargo tank. In the bottom loading method, a permanent fill pipe is attached to the cargo tank bottom. 

During most of submerged loading by both methods, the fill pipe opening is below the liquid surface level. 

Liquid turbulence is controlled significantly during submerged loading, resulting in much lower vapor

generation than encountered during splash loading. 

The recent loading history of a cargo carrier is just as important a factor in loading losses as the

method of loading. If the carrier has carried a nonvolatile liquid such as fuel oil, or has just been cleaned, it

will contain vapor-free air. If it has just carried gasoline and has not been vented, the air in the carrier tank

will contain volatile organic vapors, which will be expelled during the loading operation along with newly

generated vapors. 

Cargo carriers are sometimes designated to transport only one product, and in such cases are

practicing "dedicated service". Dedicated gasoline cargo tanks return to a loading terminal containing air

fully or partially saturated with vapor from the previous load. Cargo tanks may also be "switch loaded" with

various products, so that a nonvolatile product being loaded may expel the vapors remaining from a previous

load of a volatile product such as gasoline. These circumstances vary with the type of cargo tank and with

the ownership of the carrier, the petroleum liquids being transported, geographic location, and season of the

year. 

One control measure for vapors displaced during liquid loading is called "vapor balance service", in

which the cargo tank retrieves the vapors displaced during product unloading at bulk plants or service

stations and transports the vapors back to the loading terminal. Figure 5.2-5 shows a tank truck in vapor

balance service filling a service station underground tank and taking on displaced gasoline vapors for return

to the terminal. A cargo tank returning to a bulk terminal in vapor balance service normally is saturated with

organic vapors, and the presence of these vapors at the start of submerged loading of the tanker truck results

in greater loading losses than encountered during nonvapor balance, or "normal", service. Vapor balance

service is usually not practiced with marine vessels, although some vessels practice emission control by

means of vapor transfer within their own cargo tanks during ballasting operations, discussed below.

Emissions from loading petroleum liquid can be estimated (with a probable error of ±30 percent)4

using the following expression:

where: 

LL = loading loss, pounds per 1000 gallons (lb/103

 gal) of liquid loaded 

S = a saturation factor (see Table 5.2-1)

P = true vapor pressure of liquid loaded, pounds per square inch absolute (psia) 

(see Section 7.1, "Organic Liquid Storage Tanks") 

 M = molecular weight of vapors, pounds per pound-mole (lb/lb-mole) (see Section 7.1, "Organic

Liquid Storage Tanks")

T = temperature of bulk liquid loaded, °R (°F + 460)
6/08 Petroleum Industry 5.2-5

Figure 5.2-5. Tank truck unloading into a service station underground storage tank and practicing

 "vapor balance" form of emission control.

Table 5.2-1. SATURATION (S) FACTORS FOR CALCULATING PETROLEUM LIQUID

LOADING LOSSES

Cargo Carrier Mode Of Operation S Factor

Tank trucks and rail tank cars Submerged loading of a clean cargo tank 0.50

Submerged loading: dedicated normal service 0.60

Submerged loading: dedicated vapor balance

 service 1.00

Splash loading of a clean cargo tank 1.45

Splash loading: dedicated normal service 1.45

Splash loading: dedicated vapor balance service 1.00

Marine vesselsa Submerged loading: ships 0.2

Submerged loading: barges 0.5

a For products other than gasoline and crude oil. For marine loading of gasoline, use factors from Table 5.2-

2. For marine loading of crude oil, use Equations 2 and 3 and Table 5.2-3.
5.2-6 EMISSION FACTORS 6/08

Figure 5.2-6. Tank truck loading with vapor recovery.

The saturation factor, S, represents the expelled vapor's fractional approach to saturation, and it accounts for the

variations observed in emission rates from the different unloading and loading methods. Table 5.2-1 lists

suggested saturation factors. 

Emissions from controlled loading operations can be calculated by multiplying the uncontrolled emission

rate calculated in Equation 1 by an overall reduction efficiency term:

The overall reduction efficiency should account for the capture efficiency of the collection system as

well as both the control efficiency and any downtime of the control device. Measures to reduce loading

emissions include selection of alternate loading methods and application of vapor recovery equipment. The latter

captures organic vapors displaced during loading operations and recovers the vapors by the use of refrigeration,

absorption, adsorption, and/or compression. The recovered product is piped back to storage. Vapors can also

be controlled through combustion in a thermal oxidation unit, with no product recovery. Figure 5.2-6

demonstrates the recovery of gasoline vapors from tank trucks during loading operations at bulk terminals.

Control efficiencies for the recovery units range from 90 to over 99 percent, depending on both the nature of the

vapors and the type of control equipment used.5-6 However, not all of the displaced vapors reach the control

device, because of leakage from both the tank truck and collection system. The collection efficiency should be

assumed to be 99.2 percent for tanker trucks passing the MACT-level annual leak test (not more than 1 inch

water column pressure change in 5 minutes after pressurizing to 18 inches water followed by pulling a vacuum

of 6 inches water).7 A collection efficiency of 98.7 percent (a 1.3 percent leakage rate) should be assumed for

trucks passing the NSPS-level annual test (3 inches pressure change) A collection efficiency of 70 percent

should be assumed for trucks not passing one of these annual leak tests6.
6/08 Petroleum Industry 5.2-7

Sample Calculation - 

Loading losses (LL) from a gasoline tank truck in dedicated vapor balance service and practicing vapor

recovery would be calculated as follows, using Equation 1:

Design basis -

Cargo tank volume is 8000 gal

Gasoline Reid vapor pressure (RVP) is 9 psia

Product temperature is 80°F

Vapor recovery efficiency is 95 percent

Vapor collection efficiency is 98.7 percent (NSPS-level annual leak test)

Loading loss equation - 

where:

S = saturation factor (see Table 5.2-1) - 1.00

P = true vapor pressure of gasoline = 6.6 psia

M = molecular weight of gasoline vapors = 66

T = temperature of gasoline = 540°R

eff = overall reduction efficiency (95 percent control x 98.7 percent collection) = 94 percent

Total loading losses are:

Measurements of gasoline loading losses from ships and barges have led to the development of emission

factors for these specific loading operations.8

 These factors are presented in Table 5.2-2 

and should be used instead of Equation 1 for gasoline loading operations at marine terminals. Factors are

expressed in units of milligrams per liter (mg/L) and pounds per 1000 gallons (lb/103

 gal).
5.2-8 EMISSION FACTORS 6/08

Table 5.2-2 (Metric And English Units). VOLATILE ORGANIC COMPOUND (VOC) EMISSION

FACTORS FOR GASOLINE LOADING OPERATIONS AT MARINE TERMINALSa

Vessel Tank

Condition Previous Cargo

Ships/Ocean Bargesb Bargesb

mg/L

Transferred

lb/103 gal

Transferred

mg/L

Transferred

lb/103 gal

Transferred

Uncleaned Volatilec 315 2.6 465 3.9

Ballasted Volatile 205 1.7 —d —d

Cleaned Volatile 180 1.5 ND ND

Gas-freed Volatile 85 0.7 ND ND

Any condition Nonvolatile 85 0.7 ND ND

Gas-freed Any cargo ND ND 245 2.0

Typical overall 

 situatione Any cargo 215 1.8 410 3.4

a References 2,9. Factors are for both VOC emissions (which excludes methane and ethane) and total

organic emissions, because methane and ethane have been found to constitute a negligible weight fraction

of the evaporative emissions from gasoline. ND = no data. b Ocean barges (tank compartment depth about 12.2 m [40 ft]) exhibit emission levels similar to tank ships. 

Shallow draft barges (compartment depth 3.0 to 3.7 m [10 to 12 ft]) exhibit higher emission levels. c Volatile cargoes are those with a true vapor pressure greater than 10 kilopascals (kPa) (1.5 psia). d Barges are usually not ballasted. e Based on observation that 41% of tested ship compartments were uncleaned, 11% ballasted, 24% cleaned,

and 24% gas-freed. For barges, 76% were uncleaned.

In addition to Equation 1, which estimates emissions from the loading of petroleum liquids, Equation

2 has been developed specifically for estimating emissions from the loading of crude oil into ships and ocean

barges: 

CL = CA + CG (2)

where: 

CL = total loading loss, lb/103 gal of crude oil loaded

CA = arrival emission factor, contributed by vapors in the empty tank compartment before loading,

lb/103

 gal loaded (see Note below)

CG = generated emission factor, contributed by evaporation during loading, lb/103

 gal loaded

Note: Values of CA for various cargo tank conditions are listed in Table 5.2-3.
6/08 Petroleum Industry 5.2-9

(3)

5.2-3 (English Units). AVERAGE ARRIVAL EMISSION FACTORS, CA, FOR CRUDE OIL LOADING

EMISSION EQUATIONa

Ship/Ocean Barge Tank Condition Previous Cargo Arrival Emission Factor, lb/103 gal

Uncleaned Volatileb 0.86

Ballasted Volatile 0.46

Cleaned or gas-freed Volatile 0.33

Any condition Nonvolatile 0.33

a Arrival emission factors (CA) to be added to generated emission factors (CG) calculated in Equation 3 to

produce total crude oil loading loss (CL). Factors are for total organic compounds; VOC emission factors

average about 15% lower, because VOC does not include methane or ethane. b Volatile cargoes are those with a true vapor pressure greater than 10 kPa (1.5 psia).

This equation was developed empirically from test measurements of several vessel compartments.8 The

quantity CG can be calculated using Equation 3:

where:

P = true vapor pressure of loaded crude oil, psia

M = molecular weight of vapors, lb/lb-mole

G = vapor growth factor = 1.02 (dimensionless)

T = temperature of vapors, °R (°F + 460)

Emission factors derived from Equation 3 and Table 5.2-3 represent total organic compounds. 

Volatile organic compound (VOC) emission factors (which exclude methane and ethane because they are

exempted from the regulatory definition of "VOC") for crude oil vapors have been found to range from

approximately 55 to 100 weight percent of these total organic factors. When specific vapor composition

information is not available, the VOC emission factor can be estimated by taking 

85 percent of the total organic factor.3

5.2.2.1.2 Ballasting Losses - 

Ballasting operations are a major source of evaporative emissions associated with the unloading of

petroleum liquids at marine terminals. It is common practice to load several cargo tank compartments with

sea water after the cargo has been unloaded. This water, termed "ballast", 

improves the stability of the empty tanker during the subsequent voyage. Although ballasting practices vary,

individual cargo tanks are ballasted typically about 80 percent, and the total vessel 15 to 40 percent, of

capacity. Ballasting emissions occur as vapor-laden air in the "empty" cargo tank is displaced to the

atmosphere by ballast water being pumped into the tank. Upon arrival at a loading port, the ballast water is

pumped from the cargo tanks before the new cargo is loaded. The ballasting of cargo tanks reduces the

quantity of vapors returning in the empty tank, thereby reducing the quantity of vapors emitted during

subsequent tanker loading. Regulations administered by the U. S. Coast Guard require that, at marine

terminals located in ozone nonattainment areas, large tankers with crude oil washing systems contain the

organic vapors from ballasting.10 This is accomplished principally by displacing the vapors during ballasting

into a cargo tank being simultaneously unloaded. In other areas, marine vessels emit organic vapors directly

to the atmosphere. 

Equation 4 has been developed from test data to calculate the ballasting emissions from crude oil

ships and ocean barges8

: 

LB = 0.31 + 0.20 P + 0.01 PUA (4)
5.2-10 EMISSION FACTORS 6/08

where:

LB = ballasting emission factor, lb/103 gal of ballast water

 P = true vapor pressure of discharged crude oil, psia

UA = arrival cargo true ullage, before dockside discharge, measured from the deck, feet;

(the term "ullage" here refers to the distance between the cargo surface level and the deck

level)

Table 5.2-4 lists average total organic emission factors for ballasting into uncleaned crude oil

cargo compartments. The first category applies to "full" compartments wherein the crude oil true ullage

just before cargo discharge is less than 1.5 meters (m) (5 ft). The second category applies to lightered, or

short-loaded, compartments (part of cargo previously discharged, or original load a partial fill), with an

arrival true ullage greater than 1.5 m (5 ft). It should be remembered that these tabulated emission factors

are examples only, based on average conditions, to be used when crude oil vapor pressure is unknown. 

Equation 4 should be used when information about crude oil vapor pressure and cargo compartment

condition is available. The following sample calculation illustrates the use of Equation 4. 

5.2-4 (Metric And English Units). TOTAL ORGANIC EMISSION FACTORS 

FOR CRUDE OIL BALLASTINGa

Compartment Condition

Before Cargo Discharge

Average Emission Factors

By Category Typical Overallb

mg/L Ballast

Water

lb/103 gal

Ballast Water

mg/L Ballast

Water

lb/103 gal

Ballast Water

Fully loadedc 111 0.9

A

129 1.1 Lightered or previously

 short loadedd 171 1.4

a Assumes crude oil temperature of 16°C (60°F) and RVP of 34 kPa (5 psia). VOC emission factors

average about 85% of these total organic factors, because VOCs do not include methane or ethane.

b Based on observation that 70% of tested compartments had been fully loaded before ballasting. May

not represent average vessel practices. c Assumed typical arrival ullage of 0.6 m (2 ft). d Assumed typical arrival ullage of 6.1 m (20 ft).
6/08 Petroleum Industry 5.2-11

Sample Calculation - 

Ballasting emissions from a crude oil cargo ship would be calculated as follows, using Equation 4:

Design basis -

Vessel and cargo description: 80,000 dead-weight-ton tanker, crude oil capacity 500,000 barrels

(bbl); 20 percent of the cargo capacity is filled with ballast water

after cargo discharge. The crude oil has an RVP of 6 psia and is

discharged at 75°F.

Compartment conditions: 70 percent of the ballast water is loaded into compartments

that had been fully loaded to 2 ft ullage, and 30 percent is

loaded into compartments that had been lightered to 15 ft

ullage before arrival at dockside.

Ballasting emission equation -

where:

 P = true vapor pressure of crude oil

= 4.6 psia

UA = true cargo ullage for the full compartments = 2 ft, and true cargo ullage for the lightered

compartments = 15 ft

LB = 0.70 [0.31 + (0.20) (4.6) + (0.01) (4.6) (2)]

+ 0.30 [0.31 + (0.20) (4.6) + (0.01) (4.6) (15)]

= 1.5 lb/103 gal

Total ballasting emissions are:

(1.5 lb/103

 gal) (0.20) (500,000 bbl) (42 gal/bbl) = 6,300 lb

Since VOC emissions average about 85 percent of these total organic emissions, emissions of VOCs are

about: (0.85)(6,300 lb) = 5,360 lb 

5.2.2.1.3 Transit Losses - 

In addition to loading and ballasting losses, losses occur while the cargo is in transit. Transit losses

are similar in many ways to breathing losses associated with petroleum storage (see Section 7.1, "Organic

Liquid Storage Tanks"). Experimental tests on ships and barges4

 have indicated that transit losses can be

calculated using Equation 5: 

LT = 0.1 PW (5)
5.2-12 EMISSION FACTORS 6/08

where:

 LT = transit loss from ships and barges, lb/week-103 gal transported

P = true vapor pressure of the transported liquid, psia

W = density of the condensed vapors, lb/gal 

Emissions from gasoline truck cargo tanks during transit have been studied by a combination of theoretical

and experimental techniques, and typical emission values are presented in Table 5.2-5.11-12 Emissions depend

on the extent of venting from the cargo tank during transit, which in turn depends on the vapor tightness of

the tank, the pressure relief valve settings, the pressure in the tank at the start of the trip, the vapor pressure of

the fuel being transported, and the degree of fuel vapor saturation of the space in the tank. The emissions are

not directly proportional to the time spent in transit. If the vapor leakage rate of the tank increases, emissions

increase up to a point, and then the rate changes as other determining factors take over. Truck tanks in

dedicated vapor balance service usually contain saturated vapors, and this leads to lower emissions during

transit because no additional fuel evaporates to raise the pressure in the tank to cause venting. Table 5.2-5

lists "typical" values for transit emissions and "extreme" values that could occur in the unlikely event that all

determining factors combined to cause maximum emissions. 

Table 5.2-5 (Metric And English Units). TOTAL UNCONTROLLED ORGANIC EMISSION

FACTORS FOR PETROLEUM LIQUID RAIL TANK CARS AND TANK TRUCKS

Emission Source Gasolinea

Crude

Oilb

Jet

Naphtha

(JP-4)

Jet 

Kerosene

Distillate

Oil No. 2

Residual

Oil No. 6

Loading operationsc

 Submerged loading - 

 Dedicated normal serviced

 mg/L transferred 590 240 180 1.9 1.7 0.01

 lb/103

 gal transferred 5 2 1.5 0.016 0.014 0.0001

 Submerged loading - 

 Vapor balance serviced

 mg/L transferred 980 400 300 —e —e —e

 lb/103 gal transferred 8 3 2.5 —e —e —e

 Splash loading - 

 Dedicated normal service

 mg/L transferred 1,430 580 430 5 4 0.03

 lb/103

 gal transferred 12 5 4 0.04 0.03 0.0003

 Splash loading - 

 Vapor balance service

 mg/L transferred 980 400 300 —e —e —e

 lb/103 gal transferred 8 3 2.5 —e —e —e
Table 5.2-5 (cont.).

Emission Source Gasolinea

Crude

Oilb

Jet

Naphtha

(JP-4)

Jet 

Kerosene

Distillate

Oil No. 2

Residual

Oil No. 6

6/08 Petroleum Industry 5.2-13

Transit losses

 

 Loaded with product

 mg/L transported

 Typical 0 - 1.0 ND ND ND ND ND

 Extreme 0 - 9.0 ND ND ND ND ND

 lb/103 gal transported

 Typical 0 - 0.01 ND ND ND ND ND

 Extreme 0 - 0.08 ND ND ND ND ND

 Return with vapor

 mg/L transported

 Typical 0 - 13.0 ND ND ND ND ND

 Extreme 0 - 44.0 ND ND ND ND ND

 lb/103

 gal transported

 Typical 0 - 0.11 ND ND ND ND ND

 Extreme 0 - 0.37 ND ND ND ND ND

a Reference 2. Gasoline factors represent emissions of VOC as well as total organics, because methane and

ethane constitute a negligible weight fraction of the evaporative emissions from gasoline. VOC factors for

crude oil can be assumed to be 15% lower than the total organic factors, to account for the methane and

ethane content of crude oil evaporative emissions. All other products should be assumed to have VOC

factors equal to total organics. The example gasoline has an RVP of 69 kPa (10 psia). ND = no data. b The example crude oil has an RVP of 34 kPa (5 psia). c Loading emission factors are calculated using Equation 1 for a dispensed product temperature of 16°C

(60°F). d Reference 2. e Not normally used.

In the absence of specific inputs for Equations 1 through 5, the typical evaporative emission factors

presented in Tables 5.2-5 and 5.2-6 should be used. It should be noted that, although the crude oil used to

calculate the emission values presented in these tables has an RVP of 5, the RVP of

crude oils can range from less than 1 up to 10. Similarly, the RVP of gasolines ranges from 7 to 13. In areas

where loading and transportation sources are major factors affecting air quality, it is advisable to obtain the

necessary parameters and to calculate emission estimates using Equations 1 through 5. 

5.2.2.2 Service Stations -

Another major source of evaporative emissions is the filling of underground gasoline storage tanks at

service stations. Gasoline is usually delivered to service stations in 30,000-liter (8,000-gal) tank trucks or

smaller account trucks. Emissions are generated when gasoline vapors in the underground storage tank are

displaced to the atmosphere by the gasoline being loaded into the tank. As with other loading losses, the

quantity of loss in service station tank filling depends on several variables, including the method and rate of

filling, the tank configuration, and the gasoline temperature, vapor pressure and composition. An average

emission rate for submerged filling is 880 mg/L (7.3 lb/1000 gal) of transferred gasoline, and the rate for

splash filling is 1380 mg/L (11.5 lb/1000 gal) transferred gasoline (see Table 5.2-7).5
5.2-14 EMISSION FACTORS 6/08

Table 5.2-6 (Metric And English Units). TOTAL ORGANIC EMISSION FACTORS

FOR PETROLEUM MARINE VESSEL SOURCESa

Emission Source Gasolineb CrudeOilc

Jet

Naphtha

(JP-4) Jet Kerosene

Distillate Oil

No. 2

Residual Oil

No. 6

Loading operations

 Ships/ocean barges

 mg/L transferred —d 73 60 0.63 0.55 0.004

 lb/103 gal transferred —d 0.61 0.50 0.005 0.005 0.00004

 Barges

 mg/L transferred —d 120 150 1.60 1.40 0.011

 lb/103 gal transferred —d 1.0 1.2 0.013 0.012 0.00009

Tanker ballasting

 mg/L ballast water 100 —e ND ND ND ND

 lb/103 gal ballast

 water 0.8 —eND ND ND ND

Transit

 mg/week-L transported 320 150 84 0.60 0.54 0.003

 lb/week-103 gal

 transported 2.7 1.3 0.7 0.005 0.005 0.00003

a Factors are for a dispensed product of 16°C (60°F). ND = no data. b Factors represent VOC as well as total organic emissions, because methane and ethane constitute a

negligible fraction of gasoline evaporative emissions. All products other than crude oil can be assumed to

have VOC factors equal to total organic factors. The example gasoline has an RVP of 69 kPa (10 psia). c VOC emission factors for a typical crude oil are 15% lower than the total organic factors shown, in order to

account for methane and ethane. The example crude oil has an RVP of 34 kPa (5 psia). d See Table 5.2-2 for these factors. e See Table 5.2-4 for these factors.

Emissions from underground tank filling operations at service stations can be reduced by the use of a

vapor balance system such as in Figure 5.2-5 (termed Stage I vapor control). The vapor balance system

employs a hose that returns gasoline vapors displaced from the underground tank to the tank truck cargo

compartments being emptied. The control efficiency of the balance system ranges from 93 to 100 percent. 

Organic emissions from underground tank filling operations at a service station employing a vapor balance

system and submerged filling are not expected to exceed 40 mg/L (0.3 lb/1000 gal) of transferred gasoline. 
6/08 Petroleum Industry 5.2-15

Table 5.2-7 (Metric And English Units). EVAPORATIVE EMISSIONS FROM GASOLINE SERVICE

STATION OPERATIONSa

Emission Source

Emission Rate

mg/L

Throughput lb/103 gal Throughput

Filling underground tank (Stage I)

 Submerged filling 880 7.3

 Splash filling 1,380 11.5

 Balanced submerged filling 40 0.3

Underground tank breathing and emptyingb 120 1.0

Vehicle refueling operations (Stage II)

 Displacement losses (uncontrolled)c 1,320 11.0

 Displacement losses (controlled) 132 1.1

 Spillage 80 0.7

a Factors are for VOC as well as total organic emissions, because of the methane and ethane content of

gasoline evaporative emissions is negligible. b Includes any vapor loss between underground tank and gas pump. c Based on Equation 6, using average conditions.

A second source of vapor emissions from service stations is underground tank breathing. Breathing

losses occur daily and are attributable to gasoline evaporation and barometric pressure changes. The

frequency with which gasoline is withdrawn from the tank, allowing fresh air to enter 

to enhance evaporation, also has a major effect on the quantity of these emissions. An average breathing

emission rate is 120 mg/L (1.0 lb/1000 gal) of throughput. 

5.2.2.3 Motor Vehicle Refueling -

Service station vehicle refueling activity also produces evaporative emissions. Vehicle refueling

emissions come from vapors displaced from the automobile tank by dispensed gasoline and from spillage. 

The quantity of displaced vapors depends on gasoline temperature, auto tank temperature, gasoline RVP, and

dispensing rate. Equation 6 can be used to estimate uncontrolled displacement losses from vehicle refueling

for a particular set of conditions.14 

(6)

where:

ER = refueling emissions, mg/L

)T = difference between temperature of fuel in vehicle tank and temperature of dispensed fuel, °F

TD = temperature of dispensed fuel, °F

RVP = Reid vapor pressure, psia

Note that this equation and the spillage loss factor are incorporated into the MOBILE model. The MOBILE

model allows for disabling of this calculation if it is desired to include these emissions in the stationary area

source portion of an inventory rather than in the mobile source portion. It is estimated that the uncontrolled

emissions from vapors displaced during vehicle refueling average 1320 mg/L (11.0 lb/1000 gal) of dispensed

gasoline.5,13

Spillage loss is made up of contributions from prefill and postfill nozzle drip and from spit-back and
5.2-16 EMISSION FACTORS 6/08

Figure 5.2-7. Automobile refueling vapor recovery system.

 overflow from the vehicles's fuel tank filler pipe during filling. The amount of spillage loss can depend on

several variables, including service station business characteristics, tank configuration, and operator

techniques. An average spillage loss is 80 mg/L (0.7 lb/1000 gal) of dispensed gasoline.5,13

Control methods for vehicle refueling emissions are based on conveying the vapors displaced from

the vehicle fuel tank to the underground storage tank vapor space through the use of a special hose and

nozzle, as depicted in Figure 5.2-7 (termed Stage II vapor control). In "balance" vapor control systems, the

vapors are conveyed by natural pressure differentials established during refueling. In "vacuum assist"

systems, the conveyance of vapors from the auto fuel tank to the underground storage tank is assisted by a

vacuum pump. Tests on a few systems have indicated overall systems control efficiencies in the range of 88

to 92 percent.5,13 When inventorying these emissions as an area source, rule penetration and rule

effectiveness should also be taken into account. Procedures For Emission Inventory Preparation, Volume

IV: Mobile Sources, EPA-450/4-81-026d, provides more detail on this.

References For Section 5.2

1. C. E. Burklin and R. L. Honercamp, Revision Of Evaporative Hydrocarbon Emission Factors,

EPA-450/3-76-039, U. S. Environmental Protection Agency, Research Triangle Park, NC, August

1976.

2. G. A. LaFlam, et al., Revision Of Tank Truck Loading Hydrocarbon Emission Factors, Pacific

Environmental Services, Inc., Durham, NC, May 1982.

3. G. A. LaFlam, Revision Of Marine Vessel Evaporative Emission Factors, Pacific Environmental

Services, Inc., Durham, NC, November 1984.

4. Evaporation Loss From Tank Cars, Tank Trucks And Marine Vessels, Bulletin No. 2514,

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