Consider designing a flow separator that, while functioning under average flow rate conditions, also has to work when a once-in-10- or-25-year storm hits. And depending on the region of the country under consideration, that storm's downpour can vary considerably as well.

In March 2003, that was the task facing Daniel Cobb and Vaikko Allen II of Vortechnics, Inc. (Scarborough, ME; http://rbi.ims.ca/3846-556) in coming up with a flow separator configuration to meet such wide criteria without allowing floating debris and sediments to wash out into the environment in the outflow. While such devices existed, the challenge to Cobb and Allen was to come up with one having minimal footprint with low cost to install and easy to maintain to minimize lifetime expenses.

Starting out to save acquisition dollars, they decided to base the separator on existing, widely available products from concrete precasters-essentially cylinders with standard manhole cover access on top. The cylinders are relatively lightweight for the volume enclosed, making shipping and handling easier, and their size could be scaled up or down to meet local flow requirements and regulations. A vertical cylinder installation also takes up a comparatively small ground area.

Because of the cylindrical shape, Allen decided on spinning the flow through the separator to provide vortex separation-having sediments fall to the bottom as the flow slows in the cylinder center. The design team then turned to CFD, using programs from Fluent (http://rbi.ims.ca/3846-557) and Flow Science (Flow-3D) (http://rbi.ims.ca/3846-558) to cross check each other's code's results as the design evolved. "CFD was used to target debris and sediment-removal performance and cost, evaluating ideas before building a full-scale prototype in concrete," Cobb says. The prototype was then evaluated in the company's full-scale lab capable of introducing 4ft³ /sec (1,760 gal/min) into the system-the equivalent rainfall on a quarter-acre parking lot of 4 inches/hr (a 10-year storm in Maine).

The resulting VortSentry^(TM) design, available in October 2003, has no moving parts and no baffles or components in the center that could interfere with access through the manhole cover for easily scooping or vacuuming out debris. Runoff with debris and sediments enters through the inlet pipe (see figure). At lower flow rates, all flow enters the lower treatment chamber tangentially, which swirls the flow, dropping sinking pollutants in the center in a conical pile over time.

When the water rises above the outlet, the curved baffle wall in the upper chamber extends below the water surface, trapping trash, hydrocarbons and other floating debris. During high flow conditions, a portion of the flow comes out from under the debris-retaining baffle and over the flow partition or bypass weir, while settleable material is still directed into the treatment chamber. The outlet flow control port and head equalizer baffle at the outlet tend to limit and stabilize flow rates through the chamber, promoting settling-even at high inflows. "The flow control port, in combination with the weir height, acts as a 'hydraulic shock absorber,' decelerating flow energy," notes Cobb. That facilitates separation and eases any stirring up of already-settled material.

The VortSentry configuration can be used "anywhere separation of debris or particles from a fluid stream by gravity or flotation is needed," says Cobb. He cites specifics in rubber processing to plant water discharges and nuclear plant emergency water-recirculation systems.

Daniel P. Cobb, P.E. is R&D Manager for Vortechnics, Inc., and holds a BS in Mechanical Engineering from the University of Maine. He is a certified Project Management Professional with nine years of experience bringing new products to market and leading cross-functional design teams. Vaikko Allen II joined Vortechnics in 1995 while completing work towards a bachelor's degree from the University of Southern Maine in Environmental Science (Water Resources). As the Technical Manager, his current duties include developing products and strategies to respond to the regulatory and technical challenges facing stormwater design engineers.