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Session 04A: Collection, Pump Stations and Conveyance: Pump Stations
9:45am - 12:00pm
Session Chair: Eddie Speer, CDM Smith;
9:45am - 10:30am
Holistic Design Approach for Seattle Public Utility's Tunnel Effluent Pump Station
Eric Bergstrom1, Kelsey Whittaker2
1HDR Engineering, United States of America; 2Seattle Public Utilities;
Seattle Public Utilities (SPU) will be constructing a 56 mgd pump station at the end of the 2.7-mile, 18-foot 10-inch-diameter combined sewage storage tunnel as part of the Ship Canal Water Quality Project. The Tunnel Effluent Pump Station (TEPS) will drain the tunnel after flows in the regional wastewater conveyance system have receded following a wet weather event. This project will reduce the frequency of CSO events to less than one CSO event per year at each of the tributary basin combined sewer outfalls, meeting the US Environmental Protection Agency (USEPA) and Washington State Department of Ecology consent decree requirements.
Rather than approaching the design of the pump station to first meet 1) HI standards and then adapt the design to consider 2) constructability, and finally 3) operations and maintenance (O&M), the design team took a holistic approach that considered all three factors simultaneously. This approach result in a wet well different than a standard HI configuration but a physical model study was used to establish conformity with HI standards.
Key design objectives included:
· A wet well/dry well style pump station,
· Features that facilitate removal of solids that cannot be pumped from the 100 foot deep wet well, and
· Minimize groundwater infiltration leakage into the drywell.
SPU is proceeding with a “donut” pump station concept that is a dry well shaft constructed within the center of a larger tunneling shaft. The annular space between the two shafts is the wet well. This concept evolved from meeting SPU’s criteria and satisfying the constructability and O&M objectives, in addition to the HI standards. Key features of the design include an efficient structural configuration to resist 75 feet of wet well hydrostatic head, minimal coordination required between the tunneling contractor and the pump station contractor.
10:30am - 11:15am
Pump, Motor or Mounting as Source of Vibration
HDR, United States of America;
“Pump designs are being optimized to reduce weight and manufacturing costs, thus the new lightweight designs are more susceptible to vibration…” Finite element analysis (FEA) is a sophisticated tool to determine natural frequency and vibration issues of a pump prior to its manufacturing. This paper will review several installations where the cause of the vibration was the motor mounting.
Mounting of motors to pump discharge heads cannot be overlooked as a source of vibration. In a recent installation, the pump discharge head was not stiff enough. Additional stiffeners were added to the pump as determined by a FEA analysis. When reinstalled, the remanufactured discharge head did not meet the required 120% separation.
The source of the nonconforming vibration was blamed on the structure and wet well hydraulics. The manufacturer’s claim was their sophisticated FEA analysis demonstrated the equipment had its natural frequency >120% above the operating speed.
After it was determined that the pumps were operating within HI Standards, the manufacturers decided to perform additional vibration analysis to determine the discrepancy between field measured natural frequency and FEA calculated values. The assumptions in the FEA calculations that the bolted connection of the pump and motor is a rigid connection, was found to be incorrect.
At another installation, the connection between the discharge head and motor were determined not to be rigid. The “fix” was to add bolted clamps to the head to secure the motor.
FEA is a powerful and sophisticated tool for manufactures to employ to confirm their equipment meets minimum criteria. Measuring the natural frequency of the entire assembly should be part of the owner’s and engineers contract/purchasing documents. The ultimate test of whether equipment meets contract requirement is rigorous field testing by a Level III vibration analysis technician.
11:15am - 12:00pm
Pumping Up to 450 MGD Downhill: Lessons Learned
Jim Brown, Roland Chadburn
City of Portland, Environmental Services;
One alternative to increase the capacity of a conveyance system is addition of pumps and controls to intermittently use a gravity pipeline as a pressure conduit when system gravity conveyance capacity is inadequate. The author will present some of the problems encountered and solutions developed for the City of Portland BES Columbia Boulevard Wastewater Treatment Plant effluent conveyance system.
The CBWTP effluent conveyance system includes a 450 MGD peak capacity pump station, two pipelines that are each approximately 10,000 feet, and five gate structures. The system operates as a gravity conveyance during dry weather and when flows to the plant from CSO events are less than the gravity system conveyance capacity.
When CSO flows to the plant exceed system gravity capacity the effluent pump station is operated to pump flow to the Columbia River. The final project required to provide the full 450 MGD system capacity was completed in 2006.
When operated as a pumped system: pipeline failures, hydraulic structure damage, junction structure overflows, unstable pump controls, and other operational issues occurred.
Starting in 2013 BES Engineering and Operations and Maintenance staff collaborated to investigate changes to system operations and automation to reduce or eliminate operational problems.
Following analysis of the system and constraints it was determined that most of the system operational issues were caused by the pump system automated controls during and immediately after the transition from a gravity system to a pumped system.
A proposed control scheme was developed and tested prior to implementation of programming changes.
The programming changes were completed in 2017 and have successfully eliminated the majority of problems experienced with flow regime transition from gravity to pumped flow.