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TECHNICAL MEMORANDUM

DATE: December 2013

TO: Docket EPA-HQ-OAR-2010-0707

FROM: Eastern Research Group

SUBJECT: Determination of Permanent Storage Capacity Equivalents for Temporary 

Storage Facilities

1.0 Introduction

Subpart DD regulates each affected facility (e.g., loading and unloading stations, grain 

dryers, grain handling operations) at any grain terminal elevator or any grain storage elevator. 

Grain terminal elevators and grain storage elevators are defined by their permanent grain storage 

capacity. Grain terminal elevator means any grain elevator which has a permanent storage 

capacity of more than 2.5 million bushels (with exceptions). Grain storage elevator means any 

grain elevator located at any wheat flour mill, wet corn mill, dry corn mill used for human 

consumption, rice mill, or soybean extraction plant which has a permanent grain storage capacity 

of 1 million bushels. 

Temporary storage facilities (TSFs) started to be used in the 1980’s as another means to 

store grain at grain elevators. After reviewing information on TSFs, EPA concluded that the 

amount of grain stored in TSFs is not equivalent to the same amount of grain stored at permanent 

storage units, due to differences in turnovers and operation.

This memorandum documents the methodology developed to determine the permanent 

storage capacity equivalent to the TSF capacity at a grain elevator. Section 2.0 presents the 

differences between TSFs and other grain storage units. Section 3.0 presents the equivalency 

methodology.

2.0 Background

Under the U.S. Warehouse Act, TSFs are licensed and are defined by the following 

criteria:1

• Use of asphalt, concrete, or other approved base material;

• Use of rigid self-supporting sidewalls;

• Use of aeration;

• Use of an acceptable covering (e.g, tarp).

Information collected in responses to surveys that EPA sent to grain elevators, shows that 

although TSFs are a type of grain storage unit, their purpose and use are significantly different 

than other types of storage units.

2 TSFs are intended for bulk storage of grain on a temporary 
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basis, i.e., they are intended to handle intermittent surges and surpluses, and are not used 

necessarily every year, even though the structure may be in place for several years. The survey 

responses show that, on average, TSFs have one turnover per year. They are filled one time in a 

year and emptied one time in a year. Other types of storage units (buildings, bins [not including 

TSFs], and silos) have, on average, nine turnovers per year, and a significantly higher throughput

of grain in a year than TSFs. The same amount of grain stored in TSFs could be stored in 

smaller-sized permanent storage units that are turned over more frequently. Due to uncertainties 

in crop forecasts and fluctuations in crop yields and economics, TSFs are used rather than 

constructing other types of structures that are more costly and may not be warranted in the 

future. 

Emissions from affected facilities at grain elevators are proportional to the amount of 

grain throughput. Consequently, affected facilities associated with TSFs have significantly less 

emissions than affected facilities associated with other types of storage units, and the capacity of 

TSFs, as an indicator of emissions, is not a one-to-one equivalency to the capacity of other types 

of storage units. 

3.0 Determination of Permanent Storage Capacity Equivalent

Based on the information collected in the surveys and the different uses between TSFs 

and other types of storage units, EPA concluded that the capacity of TSFs, as an indicator of 

emissions, is not a one-to-one equivalency to the capacity of other types of storage units. As a 

result, EPA analyzed the survey information and developed a method for calculating an adjusted 

TSF storage capacity that would be equivalent to the storage capacity of other types of grain 

storage units. This adjusted storage capacity for TSFs would then be used to calculate 

“permanent storage capacity” by the summing the adjusted TSF capacity with the capacity for all 

other types of structures. 

The primary difference between TSF and permanent storage is the throughput of grain. 

An equation was developed to factor the permanent storage capacity by a ratio relating the grain 

throughput for TSFs and the grain throughput for permanent storage units. Equation 1 shows this 

relationship:

Ctp = Cp + ( TT

TP

∗ Cp) (1)

Where:

Ctp = Total permanent grain storage capacity for applicability determination, of all 

buildings, bins (including TSFs), and silos used to store grain (bushels).

Cp = Total storage capacity of all buildings, bins (excluding TSFs), and silos used 

to store grain (bushels).

TT = Maximum annual throughput of grain for all TSFs used to store grain 

(bushels per year).

Tp = Maximum annual throughput of grain for all buildings, bins, and silos 

(excluding TSFs) used to store grain (bushels per year).
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Generally, grain elevators will know the grain capacity of the permanent units, and have a 

historical record of the throughputs associated with the permanent units. Grain elevators that are 

considering using TSFs would not have a historical record of throughput. However, they will 

know the design capacity of the TSF. As discussed in section 2.0, the average TSF turnover

calculated from the survey responses is almost 1, meaning that the capacity is almost equal to the 

throughput. Equation 1 may be simplified using this assumption, and rearranged to be written as:

Ctp = Cp + (

Cp

Tp

∗ Ct) (2)

Where:

Ct = Total storage capacity of all TSFs used to store grain (bushels).

For greenfield facilities or grain elevators that are adding a new permanent storage unit, 

design capacities of permanent and TSFs will be available but historical throughput data is not 

available to use equations 1 or 2. For this situation, and other cases when historical throughput is 

not available, a default factor was developed to be used instead of the ratio in equation 2. The 

default factor was developed based on the survey responses provided by 121 grain elevators. The 

survey requested capacity and throughput information for permanent storage units and TSFs, as 

well as their date of first use. Information was provided for the 3-year period from 2005-2007. In 

some cases, permanent storage units were added in 2006 or 2007, or a later date. 

The default factor was calculated from grain elevators that had both TSFs and permanent 

storage units, and had grain throughputs in at least one of the three years requested. Data were 

not used if grain throughput was not provided in any of the three years or if the permanent 

storage unit was indicated to be constructed after 2007. The ratio of permanent storage capacity 

to permanent storage throughput was calculated for each year (for the years 2005-2007) for each 

facility. The average ratio for each year was then calculated for all the facilities analyzed. The 

lowest average ratio calculated over the 3-year period was then used as the default factor. Using 

the lowest value provides the most conservative estimate of equivalency. The default factor was 

determined to be 0.34 and is used to calculate equivalency, as shown in equation 3: 

Ctp = Cp + (0. 34 ∗ Ct) (3)

Where:

Ctp = Total permanent grain storage capacity for applicability determination, of all 

buildings, bins (including TSFs), and silos used to store grain (bushels).

Cp = Total storage capacity of all buildings, bins (excluding TSFs), and silos used 

to store grain (bushels).

0.34 = Default ratio of permanent storage capacity to annual throughput.

Ct = Total storage capacity of all TSFs used to store grain (bushels).
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Appendix A shows the calculation of the default factor based on the survey responses. Appendix 

B shows the derivation of the permanent storage capacity equivalency equation.

4.0 References

1. United States Warehouse Act. 

http://www.fsa.usda.gov/FSA/webapp?area=home&subject=coop&topic=was-ua

2. Database of Grain Elevator Survey Responses. Docket ID Number EPA-HQ-OAR-2010-0706.
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Appendix A. Calculation of Default Equivalency Factor Based On Grain Elevator Survey Responses

Facility

Permanent Storage Capacity Permanent Storage Throughput Equivalency Ratio

2005 2006 2007 2005 2006 2007 2005 2006 2007

A 400,000 400,000 400,000 1,665,000 1,865,000 1,865,000 0.24 0.21 0.21 

B 3,600,000 3,600,000 3,600,000 6,878,000 9,805,000 10,186,000 0.52 0.37 0.35 

C 7,500,000 7,500,000 7,500,000 11,500,000 14,080,000 16,550,000 0.65 0.53 0.45 

D 1,586,344 1,586,344 1,586,344 7,410,587 7,482,084 6,707,766 0.21 0.21 0.24 

E 1,216,746 1,216,746 2,387,340 8,702,918 9,487,918 10,297,512 0.14 0.13 0.23 

F 500,000 1,804,000 1,804,000 12,500,000 13,561,000 15,522,000 0.04 0.13 0.12 

G 900,000 900,000 900,000 6,139,000 4,762,000 6,082,000 0.15 0.19 0.15 

H 4,600,000 4,600,000 4,600,000 13,500,000 15,000,000 15,000,000 0.34 0.31 0.31 

I 1,510,901 1,510,901 1,510,901 5,186,738 6,776,801 8,259,652 0.29 0.22 0.18 

J 7,711,985 7,711,985 7,711,985 13,000,000 15,000,000 14,000,000 0.59 0.51 0.55 

K 2,977,000 2,977,000 2,977,000 4,671,712 2,656,980 2,886,889 0.64 1.12 1.03 

L 1,533,385 1,533,385 1,533,385 11,521,316 10,985,000 9,216,700 0.13 0.14 0.17 

M 1,520,000 1,520,000 1,520,000 1,520,000 1,400,000 1,400,000 1.00 1.09 1.09 

N 485,000 485,000 485,000 21,355,000 15,406,739 18,250,536 0.02 0.03 0.03 

O 1,272,732 1,272,732 1,272,732 450,600 3,430,000 8,650,000 2.82 0.37 0.15 

P 1,209,027 1,812,882 1,812,882 10,938,818 12,244,807 12,479,365 0.11 0.15 0.15 

Q 9,271,105 9,271,105 9,271,105 95,031,230 121,750,464 110,578,665 0.10 0.08 0.08 

R No data No data 2,000,000 No data No data 3,152,828 

Not 

calculated

Not 

calculated

 

0.63 

S 4,201,515 4,201,515 4,201,515 6,876,000 4,729,000 11,000,000 0.61 0.89 0.38 

T 4,025,000 4,025,000 4,025,000 12,000,000 10,500,000 10,000,000 0.34 0.38 0.40 

Average

 

0.47 

 

0.37 

 

0.34 
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Appendix B

Derivation of Permanent Grain Storage Capacity Equivalency Equation

(1) Calculate number of permanent units that would be needed to store temporary grain:

# Permanent units = TSF throughput

avg throughput of permanent storage per unit

(2) Determine equivalent permanent storage capacity:

Equivalent permanent storage capacity = (# permanent units)(average capacity of each permanent unit)

(3) Combine equations (1) and (2):

Equiv. perm. storage capacity = ( TSF throughput

avg throughput of perm.storage per unit)(avg capacity of each perm. unit)

(4) The average permanent storage throughput and capacity per unit is calculated from:

Avg permanent storage throughput = 

Total Permanent Storage Throughput

# permanent storage units

Avg permanent storage capacity = 

Total Permanent Storage Capacity

#permanent storage units

(5) Rewrite equivalent permanent storage capacity:

Equiv. perm. storage capacity = ( TSF throughput

Total permanent storage throughput

# permanent storage units

)(Total permanent storage capacity

#permanent storage units

)

(6) Cancel terms:

Equiv. perm. storage capacity = ( TSF throughput

Total permanent storage throughput)( Total permanent storage capacity)

(7) Rearrange equation to:

Equivalent permanent storage capacity = ( Total permanent storage capacity

Total permanent storage throughput)( TSF throughput)

(8) If one turnover is assumed for TSFs, the equation may be re-written in terms of 

temporary storage capacity:

(9) Equivalent permanent storage capacity = ( Total permanent storage capacity

Total permanent storage throughput)( TSF capacity)