ATTACHMENT D-4 

Emission Reduction Calculations for 

Gasoline Storage Tank Pressure-Vacuum Vent Caps 

R.C.S.A. Section 22a-174-30 

Back2round 

The amendment to R.C.S .A. Section 22a-174-30 reduces emissions of volatile organic 

compounds (VOCs) by requiring the use of "pressure-vacuum vent caps" (P-V vent caps) 

on stationary gasoline storage tanks at facilities that are subject to the Stage II vapor 

control regulation. The amendment also requires the use of a two-point closed system for 

the transfer of gasoline from a gasoline tanker truck to an stationary storage tank; 

improves Stage II system maintenance; clarifies testing requirements and increases 

testing frequency. The P-V vent cap requirement becomes effective as of May 10, 2005 . 

Emission Calculations 

P-V vent caps act to limit the amount of breathing losses that occur daily due to 

barometric pressure changes and gasoline evaporation enhanced by fresh air replacement 

when gasoline is drawn from the stationary tank. The Department's emission 

calculations focus on the reductions resulting from the new requirement for P-V vent 

caps at all Stage II gasoline service stations . Additional reductions expected from the 

requirements for two-point closed system are not quantified at this time. The 

requirements for more comprehensive performance testing and improved maintenance 

procedures are expected to result in greater rule effectiveness . 

2007 Uncontrolled Emissions 

Uncontrolled emissions at Stage II equipped operations in 2007 due to stationary tank 

breathing are estimated to be: 

Eunc = EFUnc X Q1999 X Fr x Gr x CF 

Where: 

E, = 2007 uncontrolled emissions (tons/day) 

EFunc = 1 .0 lbs VOC/1000 gallon throughput 

Q1999 = gasoline sold in CT 1999 (thousands of gallons/year) 

= 1,431,153.91 (from CT's 1999 Periodic Inventory) 

Fr = fraction of gasoline sold at Stage II service stations 

= 0.95 (based on data in the Department's 1996 & 

1999 Periodic Inventories) 

Gr = growth in gasoline sales from 1999 to 2007 

= 1 .10 (surrogate, developed from the Connecticut 

Department of Transportation 

VMT estimates for 1999 and 2007) 

CF = (1 ton/2000 lbs) x (1 year/365 days) 

Application of this formula yields : 

2007 Uncontrolled Emissions = 2.0 tons/day 
2 

D-4 

2007 Emissions with Connecticut's P- V Vent Cap Requirement 

The California Air Resources Board (CARB) estimates that P-V vent caps reduce 

breathing losses from stationary storage tanks by 90%.1 Connecticut's R.C.S.A. Section 

22a-174-30 requires vent caps on all Stage 11-equipped operations dispensing 10,000 

gallons or more of gasoline in any calendar month. The Department has previously 

determined that more than 95% of the gasoline sold in Connecticut is dispensed through 

Stage 11-equipped pumps. Resulting controlled emissions in 2007 are: 

" Assuming 100% rule effectiveness: 

2007 Emissions with EPA Rule and the 

Connecticut Rule = 2 .0 tons/day (1 - (0.9 * 1 .0)) 

= 0.2 tons/day 

" Assuming 80% rule effectiveness: 

2007 Emissions with EPA Rule and the 

Connecticut Rule = 2.0 tons/day (1 - (0.9 * 0.8)) 

= 0.6 tons/day 

Incremental Shortfall Credit Due to Connecticut's Regulation 

Reductions creditable toward the shortfall are the difference between uncontrolled and 

controlled emissions in 2007. Accounting for a range of possible rule effectiveness: 

" Assuming 100% rule effectiveness: 

Shortfall Credit = 2.0 tons/day - 0.2 tons/day 

= 1 .8 tons/day 

" Assuming 80% rule effectiveness : 

Shortfall Credit = 2.0 tons/day - 0.6 tons/day 

= 1 .4 tons/day 

Based on the above analysis, the P-V vent valve requirement of R.C.S.A. Section 22a174-30 will result in between 1 .4 and 1 .8 tons/day additional VOC reduction beyond 

those incorporated into the Department's ozone attainment plan and EPA's calculation of 

Connecticut's emission reduction shortfall. By the end of 2004, the Department plans to 

complete a mid-course review evaluating progress toward attainment of the 1-hour ozone 

standard in Connecticut. That document will include a discussion of how the combined 

reductions from the P-V vent valve regulation and other adopted control measures are 

sufficient to meet EPA's shortfall requirement for Connecticut. 

1 CARB's estimate of P-V vent cap control efficiency was cited in an attachment to a July 19, 2000 letter 

from David Conroy (EPA Region 1) to David Wackter (CTDEP).