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| Back to Section 1 Index |
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| Procedure for Sizing Oil-Water
Separators |
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| We are including a formula, with
an example, to aid in selecting a separator model to handle stormwater
run-off. These procedures do not take into account separators sized
based upon spill capacity. If local codes require different sizing
criteria, they shall be deemed to govern. |
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| 1. Step One: |
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| Select the rainfall intensity,
from Figure 1. below, based upon the separator installation location
and number of years you wish to base the flow calculation upon.
The values on the chart in Figure 1. are in inches per hour (in/hr).
Divide by 12 (in/ft) to convert inches to feet. |
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| 2. Step Two: |
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| Determine the total drainage area,
in square feet (ft2), that the separator will service. |
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| 3. Step Three: |
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| Multiply rainfall intensity in
feet per hour (ft/hr) by the drainage area in square feet (ft2)
to obtain flowrate in cubic feet per hour (ft3/hr). |
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| 4. Step Four: |
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| Divide flowrate by sixty to obtain
flowrate in cubic feet per minute (ft3/min). |
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| 5. Step Five: |
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| Multiply flowrate in cubic feet
per minute (ft3/min) by 7.48 (gal/ft3) to convert flowrate to U.S.
gallons per minute (gal/min). This value is the flowrate to use
in selecting a separator model. |
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| 6. Step Six: |
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| Select a separator model (OWS,
POWS, CO, PCO, SOLO-10) based upon the installation and performance
requirements. Refer to catalog drawings based upon the model selected.
Look for the Flow Rate column on catalog drawings and select a separator
model that will meet or exceed the flowrate calculated in Step Five. |
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Example: An oil-water separator
handling stormwater run-off from a bulk plant. The length and
width of the drainage area is 75 feet and 85 feet. The plant is
located in Portland, Maine and the period of rainfall to be considered
is ten years. The free oil discharge from the oil-water separator
must be 15 parts per million (ppm) or less. The separator is going
to be directly buried underground.
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| 1. |
From
Figure 1.C, rainfall intensity in Portland, Maine once in
ten years is 1.5 inches per hour (in/hr). Divide by 12 (in/ft)
to get 0.125 (ft/hr). |
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The area to
be considered is: 75 (ft) * 85 (ft) = 6375 (ft2) |
| 3. |
Determine flowrate
in cubic feet per hour: 0.125 (ft/hr) * 6375 (ft2) = 796.9
(ft3/hr) |
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Convert to cubic
feet per minute: 796.9 (ft3/hr) / 60 (min/hr) = 13.3 (ft3/min) |
| 5. |
Multiply by
7.48 to obtain U.S. gpm: 13.3 (ft3/min) * 7.48 (gal/ft3) =
99.3 (gal/min) |
| 6. |
Since the separator
is buried, choose a cylindrical separator. Our OWS model separator
is cylindrical and will produce a free oil effluent quality
of 15 ppm. From catalog Drawing No. OWS-1-0, our separator
model OWS-1000 is rated for a 100 gallon per minute (gpm)
flowrate which is greater than the 99.3 gpm flowrate calculated |
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