18.4 Design Criteria Applicable to Both API and CP Separators
The following are design criteria applicable to API and CP oil/water separators:
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
Locate the separator offline and bypass the incremental portion of flows that exceed the adjusted water quality design flowrate.
The separator shall be installed upstream of any other stormwater treatment facility.
The separator shall be installed upstream of any pumps to prevent oil from emulsifying.
Separators may be installed upstream or downstream of flow control facilities.
Separator vaults shall be watertight. Pipes entering and exiting a vault below the water quality design water surface shall be sealed using a non-porous, non-shrinking vault.
Separator vaults shall have a shutoff mechanism on the outlet pipe to prevent oil discharges during maintenance and to provide emergency shutoff capability in the event of a spill.
The inlet shall be submerged at least 2 feet from the vault bottom.
The submerged inlet helps to dissipate flows. The minimum distance from the bottom helps to minimize resuspension of sediments.
The inlet shall be extended above the water quality design water surface.
Extending the tee to the surface allows air to escape reducing turbulence.
The inlet and outlet pipe shall be a minimum 8" installed with a tee.
The outlet shall be sized to pass the modified water quality design flow.
The outlet shall be a tee extending 1 foot below the outlet the water quality design water surface.
The outlet tee shall extend above the water quality design water surface.
Vault baffles shall be concrete or stainless steel and be securely attached to the vault.
Provide access to each separator bay. If the length of width of a bay exceeds 50 feet, additional access points are necessary.
Access points for the forebay and afterbay shall be positioned over the inlet and outlet tee to allow for visual inspection and access.
Use absorbents in the afterbay if necessary.
For areas likely to produce larger amounts of oil, install a bleed-off pipe which discharges to a separate waste oil tank.
18.4.7 Sizing - Stoke’s Law
Gravity oil water separators are designed using Stoke's law principles. The principles define the rise velocity of an oil droplet based upon the density of the oil, the size of the oil droplet, and properties of the water. Stoke's Law relies on laminar flow and spherical oil droplets.
Stokes Law equation for rise rate, Vt (cm/sec):
Vt = the rise rate of the oil droplet (cm/s or ft/s)
g = acceleration due to gravity (cm/s² or ft/s2)
ρw = density of water at the design temperature (g/cm³ or lbm/ft3)
ρo = density of oil at the design temperature (g/cm³ or lbm/ft3)
d = oil droplet diameter (cm or ft)