The screening procedure can be used to determine emissions limits at sites where the nearest meteorological (STAR) station is not representative of the meteorology at the site. If the screen shows that emissions from the site are adequately protective, then the need to collect site-specific meteorological data can be eliminated.
The screening procedure is generally most helpful for facilities meeting one or more of the following conditions:
• Multiple stacks with substantially different release specifications (e.g., stack heights differ by >50%, exit temperatures differ by >50 °K, or the exit flow rates differ by more than a factor of 2),
• Terrain located between 1 km and 5 km from the site increases in elevation by more than the physical height of the shortest stack (i.e., the facility is located in complex terrain), or
• Significant distance between the facility's stacks and the site boundary [guidance on determining whether a distance is “significant" is provided in Step 6(B) of the procedure].
Steps 1 through 9 of the screening procedure present a simplified method for determining emissions based on the use of the “worst-case" stack. If the simplified method shows that desired feed rates result in emissions that exceed allowable limits for one or more pollutants, a refined analysis to examine the emissions from each stack can be conducted. This multiple-stack method is presented in Step 10.
The steps involved in screening methodology are as follows:
Step 1. Define Source Characteristics
Step 2. Determine the Applicability of the Screening Procedure
Step 3. Select the Worst-Case Stack
Step 4. Verify Good Engineering Practice (GEP) Criteria
Step 5. Determine the Effective Stack Height and Terrain-Adjusted Effective Stack Height
Step 6. Classify the Site as Urban or Rural
Step 7. Determine Maximum Dispersion Coefficients
Step 8. Estimate Maximum Ambient Air Concentrations
Step 9. Determine Compliance With Regulatory Limits
Step 10. Multiple Stack Method
Step 1: Define Source Characteristics
Note:
3 Worksheet space is provided for three stacks. If the facility has additional stacks, copy the form and revise stack identification numbers for 4, 5, etc.
Nearby Building Dimensions
Consider all buildings within 5 building heights or 5 maximum projected widths of the stack(s). For the building with the greatest height, fill in the spaces below. Building Height (m) _____ Maximum projected building width (m) _____
Nearby Terrain Data
Determine maximum terrain rise for the following 3 distance ranges from the facility (not required if the highest stack is less than 10 m in height):
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Distance from facility to nearest shoreline (km) _____
Valley width (km) _____
Step 2: Determine the Applicability
of the Screening Procedure
If the answer is “no" to all the preceding questions, then the HWCAQSP is acceptable. If the answer to any question is “yes", the procedure is not acceptable.
Step 3: Select the Worst-Case Stack
If the facility has several stacks, a worst-case stack shall be chosen to conservatively represent release conditions at the facility. Follow the steps below to identify the worst-case stack.
Apply the following equation to each stack:
K = HVT
where:
K=an arbitrary parameter accounting for the relative influence of the stack height and plume rise.
H=Physical stack height (m)
V=Flow rate (m 3/sec)
T=Exhaust temperature (°K)
Complete the following table to compute
the “K" value for each stack:
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Select the stack with the lowest “K" value. This is the worst-case stack that will be used for Steps 4 through 9.
Worst-Case Stack is identified as Stack No. ___
Step 4: Verify Good Engineering
Practice (GEP) Criteria
Confirm that the selected worst-case stack meets Good Engineering Practice (GEP) criteria. The stack height to be used in the subsequent steps of this procedure may not be greater than the maximum GEP. Maximum and minimum GEP stack heights are defined as follows:
CEP (minimum)=H+(1.5×L)
GEP (maximum)=greater of 65 m or H+(1.5×L)
where:
H=height of the building selected in Step 1 measured from ground level elevation at the base of the stack
L=the lesser dimension of the height or projected width of the building selected in Step 1
Record the following data for the worst-case stack:
Stack height (m) =_____
H(m) =_____
L(m) =_____
Then compute the following:
GEP (minimum) (m)=_____
GEP (maximum) (m)=_____
• If the physical height of the worst-case stack exceeds the maximum GEP, then use the maximum GEP stack height for the subsequent steps of this analysis;
• If the physical height of the worst-case stack is less than the minimum GEP, then use generic source number 11 as the selected source for further analysis and proceed directly to Step 6;
• If the physical height of the worst-case stack is between the minimum and maximum GEP, then use the actual physical stack height for the subsequent steps of this analysis.
Step 5: Determine the Effective Stack Height and the Terrain-Adjusted Effective Stack Height (TAESH)
The effective stack height is an important factor in dispersion modeling. The effective stack height is the physical height of the stack plus plume rise. As specified in Step 4, the stack height used to estimate the effective stack height may not exceed GEP requirements. Plume rise is a function of the stack exit gas temperature and flow rate.
In this analysis, the effective stack height is used to select the generic source that represents the dispersion characteristics of the facility. For facilities located in flat terrain and for all facilities with worst-case stacks less than or equal to 10 meters in height, generic source numbers are selected strictly on the basis of effective stack height. In all other cases, the effective stack height is further adjusted to take into account the terrain rise near the facility. This “terrain-adjusted effective stack height" (TAESH) is then used to select the generic source number that represents the dispersion characteristics of the facility. Follow the steps below to identify the effective stack height, the TAESH (where applicable), and the corresponding generic source number.
(A) Go to Table 5.0-1 and find the plume rise value corresponding to the stack temperature and exit flow rate for the worst-case stack determined in Step 3.
Plume rise =____(m)
(B) Add the plume rise to the GEP stack height of the worst-case stack determined in Steps 3 and 4.
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(C) Go to the first column of Table 5.0-2 and identify the range of effective stack heights that includes the effective stack height estimated in Step 5(B). Record the generic source number that corresponds to this range.
Generic source number = _____
(D) If the source is located in flat terrain4, or if the generic source number identified in Step 5(C) above is 1 or 11 (regardless of terrain classification), use the generic source number determined in Step 5(C) and proceed directly to Step 6. Otherwise, continue to Step 5(E).
Note: 4 The terrain is considered flat and terrain adjustment factors are not used if the maximum terrain rise within 5 km of the facility (see Step 1) is less than 10 % of the physical stack height of the worst-case stack.
(E) For those situations where the conditions in Step 5(D) do not apply, the effective stack height shall be adjusted for terrain. The TAESH for each distance range is computed by subtracting the terrain rise within the distance range from the effective stack height.5
Note: 5 Refer to Step 1 for terrain adjustment data. Note that the distance from the source to the outer radii of each range is used. For example, for the range >0.5-2.5 km, the maximum terrain rise in the range 0.0-2.5 km is used.
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1EPA, Guideline on Air Quality Models (Revised), EPA-450/2-78-027R, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina, July, 1986, incorporated by reference in s.
NR 660.11.
2Auer, August H. Jr., ``Correlation of Land Use and Cover with meteorological Anomalies,'' Journal of Applied Meteorology, pp. 636-643, 1978.
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If the terrain rise for any of the distance ranges is greater than the effective stack height, set the TAESH equal to 0 and use generic source number one for that distance range.
Record the generic source numbers from Table 5.0-2 based on each of the TAESH values.
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Step 6: Classify the Site as Urban or Rural
(A) Classify the land use near the facility as either urban or rural by determining the percentage of urban land use types (as defined in Table 3; for further guidance see the footnoted references) that fall within 3 km of the facility.
6
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If the urban land use percentage is less than or equal to 30% based on a visual estimate, or 50% based on a planimeter, the local land use is considered rural. Otherwise, the local land use is considered urban.
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Note:
6 The delineation of urban and rural areas, can be difficult for the residential-type areas listed in Table 5.0-3. The degree of resolution in Table 5.0-3 for residential areas often cannot be identified without conducting site area inspections. This process can require extensive analysis, which, for many applications, can be greatly streamlined without sacrificing confidence in selecting the appropriate urban or rural classification. The fundamental simplifying assumption is based on the premise that many applications will have clear-cut urban/rural designations, i.e., most will be in rural settings that can be definitively characterized through a review of aerial photographs, zoning maps, or U.S. Geological Survey topographical maps.
(B) Based on the TAESH and the urban/rural classification of surrounding land use, use the following table to determine the threshold distance between any stack and the nearest facility boundary.
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Record the following information:
Threshold distance from the table (m): ___
Minimum distance from any stack to property boundary (m): ___
If the minimum distance between any stack and the nearest facility boundary is greater than the threshold distance, the surrounding buffer distance is considered significant and the facility is likely to benefit from use of the HWCAQSP relative to the Tier I and II limits (see discussion of benefits from using HWCAQSP in Introduction section).
Step 7: Determine Maximum Dispersion Coefficients
(A) Determine maximum average hourly dispersion coefficients. Based on the results of Step 6(A), select either Table 5.0-4 (urban) or Table 5.0-5 (rural) to determine the maximum average hourly dispersion coefficient.7 For flat terrain [defined in Step 5(D)] and for all sites with generic source numbers 1 or 11, use Step 7(A) (1). For rolling or complex terrain (excluding generic sources numbers 1 and 11), use Step 7(A) (2).
Note: 7 For the distance range 6 to 20 kilometers, generic source number 1 is used to conservatively represent the maximum dispersion coefficient.
(1) Search down the appropriate generic source number column [based on Step 5(C)], beginning at the minimum fenceline distance listed in Step 6(B).8 Record the maximum average hourly dispersion coefficient encountered.
Maximum Average Hourly Dispersion Coefficient = _____ (mg/m 3/g/sec)
Note: 8 Exclude all distances that are closer to the facility than the property boundary. For example, if the actual distance to the nearest property boundary is 265 meters, begin at the 300 meter distance in Tables 5.0-4 and 5.0-5.
