BULK GASOLINE STORAGE TANKS

 

Date Initiated:

August 10, 1998 

Dates Modified / Updated: 

April 25, 2008 

PROCESS DESCRIPTION:

Gasoline bulk storage tanks release ROG vapors containing listed substances into the 

atmosphere. The majority of emissions can be categorized as bulk storage tank fitting 

losses, rim seal losses, working losses, deck seam losses, degassing releases, and refilling 

losses. Emission rates are highly dependent upon the storage tank size, construction, and 

design as well as annual fuel throughput. Most bulk gasoline storage tanks in San Diego 

County are equipped with either internal or external floating roofs. Fixed roof tanks are 

best quantified with the standard gasoline dispensing and storage procedures which can 

be modified to apply to any individual site's equipment and procedures. 

The primary components of the gasoline emissions are benzene, hexane, toluene, xylenes, 

and a mixture of other nonmethane hydrocarbons. Additional emissions occur at the bulk 

terminal loading racks and vapor processing units which are quantified separately from 

the bulk tanks. 

Emission estimation techniques for the bulk storage tanks are based upon procedures 

published by the EPA (AP-42, Section 7.1, (9/97). Additional equipment and emission 

speciation information was obtained from the San Diego APCD permit files and 

CAPCOA publications. District estimation techniques for emission inventory purposes 

are as follows: 

 

Overall Emissions: Overall bulk storage tank emissions are the sum of rim seal, deck 

fitting, working, deck seam, degassing, and refilling losses. Individual compounds are 

quantified using a standard vapor speciation profile for reformulated / oxygenated 

gasoline; 

Ea = (Lr + Lwd + Lf + Ld + Ldg + Lrf) x Ci

Eh = Ea / H

Where: 

Ea = Annual gasoline emissions, lbs/year
Eh = Maximum hourly gasoline emissions, lbs/hour 

Lr = Rim seal losses, lbs/year 

Lwd = Withdrawal losses, lbs/year 

Lf = Deck fitting losses, lbs/year 

Ld = Deck seam losses, lbs/year 

Ldg = Degassing emissions, lbs/year 

Lrf = Refilling emissions, lbs/year 

Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

 

Calculation of emissions from each of the contributing processes listed above depends 

upon several somewhat complex estimation procedures which must be performed for 

each individual storage tank. These specific calculation procedures are also from AP-42; 

Rim Seal Losses: Emissions from the rim seals around the edges of the bulk storage tank 

floating roof depend primarily upon the diameter of the tank (i.e.; length of seal), the type 

of seal, and the type of roof (internal or external). An empirical procedure is provided in 

AP-42 to calculate these emissions along with 'seal factors' necessary to fit the data. 

Lr = (KRa + (KRb x V^n)) x D x Mv x Kc x P* x Ci

Where: 

Lr = Rim seal losses, lbs/year 

KRa = Zero wind speed rim seal loss factor, lb-mole/ft yr, (Table 7.1-8) 

KRb = Wind speed dependent rim seal loss factor, lb-mole/(mph)^n ft yr, (Table 7.1-8) 

V = Average ambient wind speed at tank site, mph, (Table 7.1-9) 

 = 6.9 mph for San Diego County 

n = Seal related wind speed exponent, dimensionless, (Table 7.1-8) 

D = Tank diameter, ft 

Mv = Average vapor molecular weight, lbs/lb-mole 

 = 66.0 for gasoline vapor RVP 10 
Kc = Product factor 

 = 0.4 for crude oil 

 = 1.0 for all other organic liquids 

P* = Vapor pressure function, dimensionless 

 = (Pva / Pa) / [1 + (1 - (Pva / Pa)^0.5)^2] 

 = 0.122 for gasoline bulk storage in San Diego County 

Pva = Vapor pressure at daily average liquid surface temperature, psia, (Table 7.1-2) 

 = 5.7 for Gasoline RVP 10 at 65 F 

Pa = Atmospheric pressure, psia 

 = 14.7 psia at sea level 

Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

 

Withdrawal Losses: Emissions caused by the withdrawal of gasoline from the bulk 

storage tanks consist primarily of liquid fuel remaining on the exposed interior wall 

surface after the lowering of the floating roof. The emission rate depends upon the 

storage tank dimensions (exposed surface area per gallon of fuel withdrawn) and the 

interior wall condition (shell clinkage factor). An estimation procedure is provided in AP42 to calculate these emissions along with representative 'clinkage factors' necessary to 

predict the amount of fuel retained on the wall surface. 

Lwd = [(0.943 x Q x C x Wl) / D] x [1 + (Nc x Fc / D)] x Ci

Where: 

Lwd = Withdrawal losses, lbs/year 

0.943 = Units conversion factor (constant), 1000 ft3 - gal/bbl2

Q = Annual gasoline throughput, bbl/yr, (Note 1 barrel = 42 gallons) 

C = Shell clinkage factor, bbl/1000 ft2, (Table 7.1-10) 

Wl = Average organic liquid density, lbs/gal 

 = 6.1 lbs/gal for gasoline 

D = Tank diameter, ft 
Nc = Number of fixed roof support columns, dimensionless 

Fc = Effective column diameter, ft 

Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

Deck Fitting Losses: Emissions of gasoline vapor from bulk storage tank deck fittings 

depend upon the number and type of fittings on the floating roof of each individual tank. 

An empirical procedure is provided in AP-42 to predict these emissions along with the 

'deck fitting loss factors' necessary to estimate vapor emissions from each type of fixture. 

Lf = Ff x P* x Mv x Kc x Ci

Where: 

Lf = Deck fitting losses, lbs/year 

Ff = Total deck fitting loss factor, lb mole/year 

 = [(NF1 x KF1) + (NF2 x KF2) + ... + (NFnf x KFnf)] 

NFi = Number of deck fittings of a particular type (i = 0, 1, 2,..., nf), 

dimensionless 

nf = Total number of fitting types, dimensionless, (Tables 7.1-11 to 7.1-15) 

KFi = Deck fitting loss factor for a particular type of fitting, lb mole/year 

 = [KFai + (KFbi x (Kv x V)^mi)] 

 KFai = Zero wind speed loss factor for fitting type 'i', lb mole/year, (Table 

7.1-12) 

 KFbi = Wind speed dependent loss factor for fitting type 'i', lb mole/year, 

(Table 7.1-12) 

 Kv = Fitting wind speed correction factor, dimensionless 

 = 0.7 for external floating roof tanks 

 = 0.0 for internal and domed external floating roof tanks 

 V = Average ambient wind speed, mph 

 = 6.9 mph for San Diego County 

 mi = Loss factor for a particular type of deck fitting, dimensionless, (Table 

7.1-12) 
P* = Vapor pressure function, dimensionless 

 = (Pva / Pa) / [1 + (1 - (Pva / Pa)^0.5)^2] 

 = 0.122 for gasoline bulk storage in San Diego County 

 Pva = Vapor pressure at daily average liquid surface temperature, psia, (Table 

7.1-2) 

 = 5.7 for Gasoline RVP 10 at 65 F 

 Pa = Atmospheric pressure, psia 

 = 14.7 psia at sea level 

Mv = Average vapor molecular weight, lbs/lb-mole 

 = 66.0 for gasoline vapor RVP 10 

Kc = Product factor 

 = 0.4 for crude oil 

 = 1.0 for all other organic liquids 

Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

Deck Seam Losses: Internal floating roof tanks with bolted decks may have deck seam 

losses from vapors escaping through gaps. An empirical procedure is provided in AP-42 

to predict these emissions along with the 'deck seam loss per unit seam length factor' 

necessary to estimate emissions from bolted decks. Since there are no remaining bolted 

floating roof decks in San Diego County, the EASIER inventory data forms and 

calculation procedures omit this calculation. 

Ld = KD x SD x D^2 x P* x Mv x Kc x Ci

Where: 

Ld = Deck seam losses, lbs/year 

KD = Deck seam loss per unit seam length factor, lb mole/ft year 

 = 0.0 for welded floating roof decks 

 = 0.14 for bolted floating roof decks 

SD = Deck seam length factor, ft/ft2 
 = Lseam / Adeck 

Lseam = Total length of deck seams, ft 

Adeck = Total deck surface area, ft2 

D = Tank diameter, ft 

P* = Vapor pressure function, dimensionless 

 = (Pva / Pa) / [1 + (1 - (Pva / Pa)^0.5)^2] 

 = 0.122 for gasoline bulk storage in San Diego County 

Pva = Vapor pressure at daily average liquid surface temperature, psia, (Table 7.1-2) 

 = 5.7 for Gasoline RVP 10 at 65 F 

Pa = Atmospheric pressure, psia 

 = 14.7 psia at sea level 

Mv = Average vapor molecular weight, lbs/lb-mole 

 = 66.0 for gasoline vapor RVP 10 

Kc = Product factor 

 = 0.4 for crude oil 

 = 1.0 for all other organic liquids 

Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

Degassing Emissions: Occasionally, bulk gasoline storage tanks must be emptied and 

degassed to allow for repair activities both inside the tank and on the floating decks. 

Degassing bulk storage tanks results in organic emissions from both the evacuated vapor 

space and any evaporated liquid remaining on the walls, floor, or deck. The San Diego 

County APCD Vapor Recovery staff has developed the following calculation procedure 

to estimate these emissions; 

Ldg = (Lvapor space + Lwalls + Lroof + Lfloor) x Ci

Where: 

Ldg = Degassing emissions, lbs/year 
 = [(0.462 x D^2) + (0.707 x D)] x Ci

 D = Bulk storage tank diameter, ft 

 Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

Lvapor space = Vapor space evacuation losses, lbs/year 

 = 0.315 x D^2 (assuming the following information) 

 = (3.14 x D^2 / 4) x H x (1/385) x Mv x 0.39 where; 

 3.14 = Pi, (dimensionless mathematical function used to calculate interior 

tank volume) 

 D = Bulk storage tank diameter, ft 

 H = Average height of roof when tank is vented, ft, (Assume 6 ft) 

 (1/385) = Conversion factor, lb moles gas/ft3 

 Mv = Molecular weight of gasoline vapor, lbs/lb mole, (Use 66.0 for 

gasoline vapor) 

 0.39 = Mole fraction of gasoline vapor in vented space assuming a liquid 

temperature 

 of 65 F and a partial pressure of 5.7, (i.e.; 5.7 psia / 14.7 psia) 

Lwalls = Liquid evaporation losses from the side walls of the vented tank, lbs/year 

 = 0.707 x D (assuming the following information) 

 = 3.14 x D x H x 0.25 x 0.15 where; 

 3.14 = Pi, (dimensionless mathematical function used to calculate wall surface 

area) 

 D = Bulk storage tank diameter (also twice the radius), ft 

 H = Height of roof when tank is vented, ft, (Use 6 ft if unknown) 

 0.25 = Assumed average thickness of liquid on tank walls after draining, 

millimeters 

 0.15 = Conversion factor for gasoline liquid, lbs/mm ft2 

Lroof = Liquid evaporation losses from the inside roof of the vented tank, lbs/year 
 = 0.029 x D^2 (assuming the following information) 

 = (3.14 x D^2 / 4) x 0.25 x 0.15 where; 

 3.14 = Pi, (dimensionless mathematical function used to calculate roof surface 

area) 

 D = Bulk storage tank diameter, ft 

 H = Average height of roof when tank is vented, ft, (Assume 6 ft) 

 0.25 = Assumed average thickness of liquid on interior roof after draining, 

millimeters 

 0.15 = Conversion factor for gasoline liquid, lbs/mm ft2 

Lfloor = Liquid evaporation losses from the floor of the vented tank, lbs/year 

 = 0.118 x D^2 (assuming the following information) 

 = (3.14 x D^2 / 4) x 0.1 x 1.5 where; 

 3.14 = Pi, (dimensionless mathematical function used to calculate floor surface 

area) 

 D = Bulk storage tank diameter, ft 

 H = Average height of roof when tank is vented, ft, (Assume 6 ft) 

 0.1 = Assumed average thickness of liquid on floor after draining, centimeters 

 1.5 = Conversion factor for gasoline liquid, lbs/cm ft2 

Refilling Emissions: After degassing and repairing the bulk gasoline storage tanks, a 

refilling operation must occur which generates gasoline vapor emissions. Liquid fuel 

surges into the tank from below and forces vapors out the vents until the liquid surface 

meets the floating roof and begins to raise the structure. The San Diego County APCD 

Vapor Recovery staff has developed the following calculation procedure to estimate these 

emissions; 

 

Lrf = (3.14 x D^2 / 4) x H x (1/385) x Mv x 0.39 x Ci

 = 0.315 x D^2 x Ci

Where: 
Lrf = Refilling emissions (after degassing operations), lbs/year 

3.14 = Pi, (dimensionless mathematical function used to calculate interior tank 

volume) 

D = Bulk storage tank diameter, ft 

H = Average height of roof when tank is vented, ft, (Assume 6 ft) 

(1/385) = Conversion factor, lb moles gas/ft3 

Mv = Molecular weight of gasoline vapor, lbs/lb mole, (Use 66.0 for gasoline 

vapor) 

0.39 = Mole fraction of gasoline vapor in vented space assuming a liquid temperature

 of 65 F and a partial pressure of 5.7, (i.e.; 5.7 psia / 14.7 psia) 

Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

Hourly Emission Estimates: Average hourly emissions are estimated by dividing annual 

emissions by the hours of operation (Note: gasoline vapor contains no compounds 

currently deemed acutely hazardous). 

Eh = (Ea x Ci) / Hrs

where: 

Eh = Hourly emission estimate, lbs/hour 

Ea = Annual emission estimate, lbs gasoline vapor/year 

Ci = Concentration of each listed substance in the gasoline vapor, lbs/lb 

Hrs = Hours of bulk storage tank operation, hours/year 

 

EMISSIONS INFORMATION:

Emission estimation procedures and factors are based on information from EPA AP-42 

(Section 7.1). Gasoline speciation data is from both the EPA (NESHAPS Document 

453/R-94-002a) and CAPCOA (Air Toxics Hot Spots Industry wide Risk Assessment 

Guidelines). 

 
 

ASSUMPTIONS / LIMITATIONS:

- Emission estimation procedures are based upon AP-42 methods for floating roof 

storage tanks containing organic liquids. Fixed roof storage tanks may not be 

accurately quantified with these procedures. 

- Gasoline vapor speciation is based on information in the EPA NESHAPS 

Document for the Gasoline Distribution Industry (vapor speciation for 

reformulated / oxygenated fuel) and the CAPCOA Industry wide Risk Assessment 

Guidelines (liquid speciation). 

- Emissions of organic vapors containing listed substances from tanks storing 

diesel, distillate oil, kerosene, jet fuel, motor oil, lubricants, machining oils, and 

residual oil are assumed to be below reporting levels. Aviation gas, however, 

should be considered equivalent to gasoline unless site specific information 

indicates otherwise. 

- Retail gasoline storage and dispensing equipment should not be evaluated with 

these estimation techniques. Separate calculation procedures have been developed 

for these processes and equipment. 

- Facilities tend to misreport the annual "tank refilling" activities needed to 

complete the above equations. Refilling emissions are calculated by the District 

only after a degassing operation when the tank vents are left open to properly raise 

the floating roof. Emissions from typical refilling operations with closed vents are 

included in the rim seal and withdrawal estimates. To avoid overestimating 

refilling emissions, the database calculation procedure uses the "number of 

degassing events" to estimate refilling emissions. 

FORMS:

Use the reporting form for all sizes of bulk gasoline storage tanks. Verify the accuracy of 

the permit equipment description. Do not report diesel, distillate, or jet fuel fuel storage. 

Due to the complexity of rim seal and fitting losses, emission rates for these releases are 

Gasoline Speciation Data Weight % Vapor Weight % Liquid

Benzene 0.4 % 1.0 % 

Ethyl Benzene 0.1 % 1.6 % 

Hexane, Isomers of 1.4 % 1.8 % 

Toluene 1.1 % 8.0 % 

Xylene, Isomers of 0.4 % 2.4 % 

2,2,4-Trimethylpentane 0.7 % 0.8 % 
 

typically calculated separately and entered into the database as constants (lbs released / 

year) and (lbs released / hr) . Since the emission rates are only dependent upon tank size 

and design, the emissions are unaffected by throughput. The District database is not 

currently programmed to perform these tank specific calculations.