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OAR Box 1168 

Prepped by Ollie Stewart 

Document Number: 

92) ll-C-21 

Docket Number: 

A-89-10 

Printed 4/29/2008 11:23:20 AM Sheet 92 of 131 
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UNITED STATES ENVIRONMENTAL PROTECTIC 

Office of Air Quality Planning and Stand 

Research Triangle Park, North Carolina 2 

A-89-10 

II-C-2 1 

April 10, 1990 FEB I f. 1991 

1 EPA AIR DOCKET 

Dear Reg Neg Committee Member: 

At the last Reg Neg meeting, it was requested that we provide a write-up 

describing the revisions that we have made to the database as presented by 

Robert Pahel-Short. His presentation is Attachment 7 of the Meeting Minutes. 

This letter describes the two refinements we have made to the previous 

version (2/14/90) of the ethylene oxide (EO) and butadiene (BD) databases. 

First, we have identified several screening values that should have been 

marked as "pegged", but were not. We identified these by review of the raw 

data, and comparing them with the computerized databases. These corrections 

had 1ittle impact. 

The second and more significant change is related to the emission 

factors used with screening values greater than 10,000 ppm. In the original 

CMA reporting of the E0 and BD data and in the 2/14/90 database, the SOCMI 

leak factors were used for all screening values greater than or equal to 

10,000 ppm. This was done because the data at a majority of the plants 

included values where the screening instrument was "pegged" at 10,000 ppm. 

Since the SOCMI leak factors were derived from plants with higher leak 

frequencies, this factor very likely overstates the emissions from components 

at the EO and BD plants which, as a group, have much lower leak frequencies. 

Therefore, for the plants where screening values were equal to or greater than 

10,000 ppm and were not pegged, we recalculated emissions from the correlation 

curve. Also, we used these data to derive a new leak factor and applied it, 

instead of the SOCMI leak factor, to those components screened as being 

pegged. This results in leak factors that are lower for all components, 

except flanges. 

In addition, we considered how data would be generated at plants that 

will be subject to the standard in the future. It is reasonable to expect, at 

plants that are running LDAR programs, that the screening data collected in 

the LDAR programs will be used to estimate emissions. However, it appears 

that this approach, along with currently available emission factors, may 

overstate emissions. The reason is that screening values in LDAR programs are 

obtained before repairs are made. Since repairs are made immediately after 

the screening, the before-repair readings overstate average emissions for the 

period. For example, if readings and repairs are made at the end of the 

quarter, at day 90, the emission estimates will represent the emissions dn day 

90. On day 91, emissions will be lower due to the repairs. Emissions will 
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increase over time until the next round of screenings and repairs, which would 

occur on day 180 in a quarterly program. Average emissions will depend on the 

progression of leaks during the quarter. However, it seems reasonable to 

assume that the emissions correspond approximately to the average of before 

and after-repair screening values. 

To examine the effect of this, we looked at the EO and BD data. Since 

we have no after-repair data, we had to estimate the effect of repair. 

Several options were possible. One is to assume repaired components have no 

detectable emissions. Another is to assume that components are repaired to 

just below the leak definition. A third, and the one we used, is to assume 

that the distribution of emission rates among repaired components mirrors the 

distribution among non-leaking components. Robert's last 2 overheads 

illustrated this result. This seems reasonable, given that repaired 

components would exhibit variation in their degree of repair depending on the 

component's physical condition. 

Based on our assumption that the repaired components emit at the same 

rate as the average non-leaking components, we can calculate an after-repair 

emission factor. We could then average the before-repair factors that were 

calculated directly from the EO and BD data, with the after-repair factors to 

determine an estimate of what average emissions over the quarter would be. We 

believe this is a reasonable approach for estimating the average emissions for 

a unit that is meeting a specified percent leak level. 

If you have any questions, or would like a copy of the updated database 

please call me at (919) 541-5262. 

Sincerely, 

AC^tJZ LdZy£-— 

Rick Colyer 

Standards Development Branch 

cc: Interested parties