How Bypass Projects Have Changed During the 21st Century
Over the past 20 years or so, sewer lift stations across Southern California have been working tirelessly to keep up with the ever-increasing demand as more and more housing is being built. Today, the older and smaller neighborhood pump stations that have long been neglected are being replaced or updated to handle the increased effluent that is being pumped and to repair the damage that has occurred over the past 20 plus years.
During the past 20-plus years, the sewer bypass equipment and methods of installation have undergone a steady evolution; today, automatically controlled pumps are the trend. This technology has been with us for quite a while; however, it is very costly. With most bypass work being of rather short duration, past projects have not warranted the expense of this type of technology. Olsson Construction Inc., has two such projects that are each nearly a full year in duration. The requirements for automatic controlled pumps and 100 percent backup make these projects very interesting, as studies of where the future of bypass pumping is heading.
The College Avenue, Costa Mesa, Calif., jobsite is an excellent example. The district requires that the bypass operates in the same way that the existing pump station runs. Griffin Dewatering has installed two electrically powered, vacuum-assisted, 10-in. non-clog pumps that are controlled by Variable Frequency Drives (VFDs) operating off of pressure transducers and installed in an adjacent manhole. The original telemetry from the pump station is also tied in to the VFDs. The district’s control room receives alarms and warning signals, as well as running times and fluid levels, the same as when the pump station was on line.
These two primary bypass pumps are 100 percent backed up by two diesel-powered pumps that are equipped with auto start controls that are activated by on/off floats. Each one of the pumps on this project is capable of pumping more than 50 percent of the required flow of 8 mgd, making one pump able to control the flow most of the day. This gives the bypass additional capacity to handle extreme situations, such as storm water or broken water lines that could add to the total flow. Once the pumps were installed and tested, and the floats and transducer levels were set, the district’s telemetry was changed over, and the pump station was taken out of service. The old station is being demolished, and a larger, modern pump station is being built to replace it.
The selection of the correct pump for the job is the most critical part of any bypass project. Many factors need to be taken into consideration when sizing the pump:
- Rate of flow that needs to be controlled. This is important to ensure that adequate capacity for peak flow is available. Often we use more than one pump to meet the requirement. Using multiple pumps allows for control when the flow is high, but not so large that one pump is turning on and off too frequently. This can occur during lower flows, which can damage the motor.
- Available power source. It is obvious that enough power must be available to run the pumps or alternatively diesel-powered pumps will need to be utilized.
- Often neglected, suction lift can mean the difference between meeting the flow requirements or not. As the suction lift increases, the pumping capacity decreases, therefore, a pump that is installed at the bottom of a wet well can pump to its full potential, while one that is installed 20 ft higher may only be able to pump a fraction of its potential.
- The use of the “non-clog” impeller design. This not only increases efficiency, but it is also more forgiving as it typically uses wear rings instead of a wear plate. Wear plates require very close tolerance, which, when worn, greatly affect the efficiency of the pump, thus lowering the capacity. Wear rings are less affected by minor wear.
- Another major consideration is Total Dynamic Head. This includes factors like difference in elevation and friction loss within the discharge piping system.
A second project for Olsson Construction Inc., has been installed in the City of Los Alamitos, Calif., at the Yellowtail Drive, Westside Pump Station. This project consists of a total of six vacuum-assisted non-clog pumps at two locations in order to accomplish the total bypass of the station. Two 8-in. model 8NCRD pumps, one electric-powered primary pump and one diesel-powered back-up pump, capable of controlling 3 mgd, were installed inside the pump station fence near the wet well. Discharge is directed to a force main, that flows into a manhole located in the Old Ranch Road, which is adjacent to the Pump Station property.
Two electric-powered, 16-in. model 16NHG primary pumps and two diesel-powered, 10-in. model 10NCRD backup pumps, capable of controlling 11 mgd, are installed in Old Ranch Road. The electric pumps at both locations are controlled by soft start control panels, which allow the motors to start automatically. As the name implies, they ramp slowly up to speed and run at a set RPM until a signal is received to shut off; they then ramp down slowly and turn off. All of the diesel-powered pumps are being controlled by on/off floats and run at a fixed RPM.
The pump station is being completely bypassed so that the wet well can be cleaned, inspected and repaired, if necessary. After completion of this phase, the plugs will be deflated and flow will be allowed to go into the wet well. Because of the proximity to neighboring houses and issues with odor and noise, the replacement of the two 8-in. pumps is being investigated. As an alternative, an electric submersible could be installed in the wet well and connected to the existing discharge. The second phase of work on this pump station involves moving the pump motors from an above-ground mounting down to the pump frames at the lower level of the pump station. Also, there will be replacement of valves, fittings and piping to modernize the station.
Although all of these bypasses are set to work automatically and the pumps are capable of handling solids up to 4.5-in., the human factor is still necessary to monitor the oil levels in the vacuum-assisted pumps and to clean out the rags and debris from the pump impellers on a regular basis. Most of this maintenance can be done during the normal work day by any of the labor force already on the job.
Both pump stations are reporting data telemetrically to the Orange County Sanitation District control room, where district personnel are monitoring the pumps. Olsson Construction onsite workers are doing the daily maintenance as needed. The only difference between Griffin Dewatering pumps and the original station pumps is that Griffin’s are operating with greater suction lift, and there is much less storage capacity in the manholes from which we are pumping.
The design and submittal of equipment and engineering data for use on these projects took a great deal of time. Having been through it, we now have a better understanding of the expectations of the sanitation district on these types of projects. What has been learned on these jobs will be invaluable as we move forward into the future. Expectations are high that the funding for infrastructure repair will be a priority during the next decade.
Tim Boicourt is Griffin Dewatering Farwest Superintendant, based in Ontario, Calif.