ENHANCED INTERCEPTING STRUCTURE FLOW REGULATION FOR CONVEYANCE OPTIMIZATION
Abstract:The Milwaukee Metropolitan Sewerage District (District) has recently completed the flow-regulation evaluation phase of its Central Metropolitan Interceptor Sewer (MIS) System Improvement Project. The goal of this project is to upgrade the system to provide an additional 50 years of service life while optimizing its hydraulic performance over this period. Flow regulation is a key component of this optimization, which maximizes the flow conveyed through the MIS system while providing protection from potential basement flooding through direct connections to the system. As part of the system improvement alternatives, the project team explored several options to regulate flow from the tributary combined sewers into the Central MIS.
In the combined sewer service area, flows are routed to the Central MIS from City of Milwaukee combined sewers through a network of 137 intercepting structures (IS). These intercepting structures, commonly called IS structures, serve to intercept and divert all dry-weather flow plus a small portion of wet-weather flow into the Central MIS System. IS structures are characteristically comprised of two chambers. The combined sewer chamber commonly has a sump placed in the flow line of the combined sewer. The sump discharges through an existing regulator in a common wall shared with the downstream MIS chamber connecting to the Central MIS system. Typically, weirs are situated on the downstream sides of the sumps of sufficient height to direct all dryweather flow and some additional wet-weather flow to the Central MIS system through the flow control devices. Many of these existing control devices have been removed due to excessive maintenance or are currently inoperable due to corrosion, effectively functioning as fixed orifices.
Design flows through each IS structure were determined by proportioning the total combined sewage flows at the wastewater treatment plant as a percentage of each IS structure's respective service area to the total combined sewer area. The Central MIS system was modeled to determine suitable maximum hydraulic grade lines for the entire system. Regulation devices for each entry point to the interceptor sewer were then chosen based on respective flowrates into the interceptor sewer at each of these points. The project team developed design criteria for this selection process and applied this criteria to each of the IS structures in the system. Design flows were categorized into three ranges to facilitate the selection process. The lowest flow range included flows of less than 2 cubic feet per second. Middle range flows ranged from 2 to less than 10 cubic feet per second. The highest flow range accounted for flows of greater than 10 cubic feet per second. Selection criteria for flow-regulating devices in each of these flow ranges follows:
IS structures with design flows in the lowest range were selected to be regulated by fixed vertical orifice flow regulation devices. The size of the orifice is selected to pass the design flow when the upstream head on the orifice is equal to the weir height in the combined sewer. The minimum orifice diameter used is six inches to reduce the risk of clogging. The rehabilitation effort in the IS structure generally includes removal of the existing flow control device and sufficient concrete to install an appropriately sized wall thimble in the opening. An orifice plate is mounted on the wall thimble in the MIS chamber and an isolation gate is also mounted over the orifice plate to improve the maintainability of the MIS System. The new flow control devices should normally fit within the existing chambers with no structural revisions necessary, allowing the least expensive rehabilitation.
IS structures with design flows in the middle range were analyzed hydraulically to determine whether fixed vertical orifices could be used. If a fixed vertical orifice is not hydraulically acceptable, the second device selected for use in Central MIS intercepting structures is the Hydroslide® type float operated gate distributed by Grande, Novac & Associates, Inc. (GNA) of Montreal, Quebec. The Hydroslide® type gate provides accurate and precise flow control over a wide range of flow and head conditions. Providing that the Hydroslide® regulator is required, the rehabilitation effort in the IS structure depends upon the size of the existing MIS chamber. This regulator has a float arm with a length determined by the relative head over which control is accomplished. In many cases, the MIS chamber must be reconstructed to accommodate the arm swing of the float. The Hydroslide® regulator is placed on the downstream wall in the chamber over the outgoing MIS connecting sewer. The existing flow control device in the common wall is removed and a wall thimble and isolation gate is provided in the common wall opening in the MIS chamber to facilitate maintenance. Structural requirements necessary for this type of regulation can be potentially expensive.
IS structures with design flows in the highest range were analyzed hydraulically to determine whether a Hydroslide® regulator could be used. In most cases to date, IS structures in this flow range require a power operated, modulating sluice gate. The rehabilitation effort and structure size requirements and configuration are predicated on existing constraints at the specific IS structure locations. Typically, the existing flow control device(s) in the common wall should be removed. A self-contained stainless steel modulating sluice gate is mounted on a wall thimble cast into the opening of the common wall. The gate is operated by a hydraulic (oil) power unit located nearby and controlled by an ultrasonic flow-measuring device installed downstream in the mainline MIS. An isolation gate or stop planks are provided to facilitate maintenance. Although this device requires an outside power source to operate, modulating sluice gates were selected by RUST/HARZA because of their ability to regulate downstream hydraulic grade line and their ability to function without human operator intervention. These installations are costly but provide the“strict”flow regulation required for the larger flows.
The proposed flow control strategy for the Central MIS utilizes devices that control the hydraulic grade line in the interceptor sewers, wherever practical, rather than regulating the actual flowrates through each IS structure. As such, the amount of flow that enters the Central MIS depends upon available downstream capacity. The regulating devices chosen will provide a reliable and effective means to achieve the desired optimization.
The paper will present relevant details and case histories of IS structures in each of the defined flow ranges. It will also present an evaluation of alternatives, recommended cost-cutting measures, rehabilitation costs and benefits. The paper should offer valuable insights to owners, engineers and operators with capacity concerns in similar combined sewer systems.
Document Type: Research Article
Publication date: January 1, 2002
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