Technical Memorandum 

TO: EPA Docket No. EPA-HQ-OAR-2010-0682 

 

FROM: Brenda Shine, Environmental Engineer 

Refining and Chemicals Group, SPPD (E143-01)

DATE: March 15, 2012

SUBJECT: Derivation of Hourly Emission Rates for Petroleum Refinery Emission Sources 

Used in the Acute Risk Analysis

I. Purpose: 

This memorandum documents the assumptions used to derive the hourly emission rates used to 

estimate acute risks from emissions of petroleum refinery emission sources. In past Risk and 

Technology (RTR) rulemakings, when we have lacked hourly emissions data or other specific 

information about the source category processes, we assumed that the 1-hour emission rate for 

any emission point could be 10 times higher than its average hourly emissions, calculated by 

dividing annual emissions by 8760 hours per year. The basis for this assumption was derived 

from an analysis of short term release information collected from facilities in the HoustonGalveston area and a comparison against routine emission rates for an entire facility. The 

conclusions of this analysis were that the hourly emissions from any single release event to the 

average annual VOC release rate for an entire facility was seldom greater than a factor of 10. 

For the petroleum refinery source category, we have additional knowledge of the emission point 

release characteristics that enable us to refine the default factor of 10 to something more realistic 

for this source category. 

 

II. Acute Factors for the Petroleum Refinery Source Category: 

Instead of using the default factor of 10 described above, we estimated variability of hourly 

emissions based on the operating characteristics of refinery emission sources in the following 

manner: 

a. For many sources that are associated with continuous operations and are essentially 

steady state, such as Fluid Catalytic Cracking Units (FCCU), and sulfur recovery plants 

(SRU), we would not expect significant variability in hourly emissions. Typical source 

test variability is typically ±30 to 50%. We have applied an hourly multiplier of 2 to 

estimate maximum hourly rates as an upper range of the expected variability. This 

assumption was used for the following types of sources: FCCU, SRU, Chemical 

Processes, miscellaneous process vents, refinery processes (not specified) and 

incinerators. 

b. For delayed cokers, we considered the average venting cycle time in calculating our 

hourly emission rate. The process cycle for delayed cokers is 1 day (24 hours), with the 
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depressurization vent having an average venting cycle time of, on average, 2 to 3 hours 

per cycle. Since the depressurization vent opens for 730 to 1,095 hours per year, a factor 

of 8 to 12 times the annual average emission rate could be used to account for this 

emission process. The emissions are expected to be somewhat variable, with higher 

emissions at the start of the venting cycle. We therefore applied an hourly multiplier of 

20 to estimate maximum hourly rates for delayed coker emissions. 

c. There are three types of catalytic reformers: Continuous, cyclic, and semi-regenerative. 

Continuous reformers (SCC 30601602) are expected to have stable operations (much like 

the FCCU and SRU), so a factor of 2 is appropriate for estimating the maximum hourly 

emissions rate. Cyclic reformers (SCC 30601603) typically vent for 2,000 to 4,000 hours 

per year, suggesting a factor of 2 to 4 for these units. However, their emissions over the 

venting period are expected to be somewhat variable, so a factor of 10 is applied as a 

conservative estimate. Semi-regenerative reformers (SCC 30601604) are expected to 

vent approximately 10-15 days per year, suggesting a factor of 30 or 40. The venting is 

not expected to be uniform over this period, so a factor of 60 is applied for these units. 

d. For evaporative loss sources such as wastewater, we expect moderate variability in 

maximum hourly emissions from annual average emissions. Emissions from wastewater 

are dependent on organic loading to the unit and on the mass transfer (volatilization) rate 

from the wastewater surface (which is dependent on the wastewater temperature and air 

wind speed). The loading rates are expected to vary by a factor of 2 during normal 

operations and the mass transfer rates are expected to vary by a factor of 2 due to 

seasonal/meteorological variations. We therefore applied an hourly multiplier of 4 to 

estimate maximum hourly rates from wastewater sources. 

e. For fugitives from equipment leaks, we note that the current methods of estimating 

emissions make use of correlation equations in conjunction with Method 21 readings that 

ultimately provide an hourly emission rate for the monitoring period in question. The 

actual emission rates estimated based on the direct Method 21 readings are commonly 

divided by two to estimate the average emission rate between monitoring intervals (i.e., 

assuming the leak started mid-way between monitoring intervals as described in the 

emissions protocol). We expect the emission estimates from the direct Method 21 

readings to provide a direct measure of the maximum hourly emissions (as leak repair 

will be applied to reduce hourly emissions immediately after a monitoring cycle). 

Therefore, we applied an hourly multiplier of 2 to estimate maximum hourly rates from 

the annual average emission rates. 

f. For storage vessels, higher hourly rates from loading operations can occur; however, the 

majority of tanks are floating roof tanks; emissions from these tanks are associated with 

wind-driven losses from fitting controls and clingage from exposure of external floating 

roof tank shells. These emissions would not be expected to vary as significantly as fixed 

roof tanks, but the emissions from floating roof tanks are expected to be dependent on 

temperature (seasonal variability) and, in the case of external floating roofs, wind speed. 
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There may also be variability in the crude oil processed or the intermediates and products 

stored in a given tank. As with wastewater sources, each of these factors is expected to 

cause a factor of 2 variability. We therefore applied an hourly multiplier of 4 to estimate 

maximum hourly rates from storage tanks. 

g. For other transfer and loading operations, maximum hourly rates can be approximated 

from pumping rates; a factor of 2 for these sources would not adequately account for 

higher maximum hourly emissions in most cases. Considering the hours of actual 

product loading during the year (commonly 1,000 to 2,000 hours per year), we applied a 

factor of 10 to these emission sources.