April 2021 Vol. 76 No. 4
Features
Collaboration Secures the Success of Rehab Bypass Project
By Justin Childers, regional strategic account manager – pump solutions, Sunbelt Rentals
Rehabilitation bypass projects are no easy feat. They require careful planning, a thorough risk assessment and most importantly, solid partnerships to get the job done safely and effectively.
At the Cross Creek Water Reclamation Facility (WRF) Influent Pump Station in Fayetteville, NC, each of these factors came together to ensure the success of rehabilitating deteriorating portions.
The rehabilitation required bypassing a total of 48 million gallons per day (MGD) of sanitary sewage to upgrade and rehabilitate the bar screens, wet well and screw pumps at the station influent structure.
As part of this rehabilitation, it was also necessary to replace a deteriorating 60-inch RS Reinforced Concrete Pipe that spanned up to the headworks. This required taking flow off the 60-inch line, as well as off separate 24-, 30- and 36-inch lines entering the station. Bypassing these lines would allow for running the station with external pumps.
In short, it wasn’t a simple project.
Owned by the Public Works Commission (PWC) in Fayetteville, the project required the insight and innovation of a number of engineers, contractors and pump experts. Sunbelt Rentals has a long-standing relationship with PWC, having performed a number of large- and medium-diameter bypasses during other rehabilitations and cured-in-place pipe (CIPP) projects. At the recommendation of PWC Water Resource Engineer John Allen, Crowder Construction of Charlotte brought on Sunbelt Rentals to work with their Senior Project Manager Tony Nash. Engineers from Hazen and Sawyer also joined the team. Together, these team members worked closely to determine the best approach to the job ahead of them.
Rehab challenges, risks
As with any sanitary sewer pump bypass project, the teams encountered challenges and potential risks when assessing the required rehabilitation at the Cross Creek WRF Influent Pump Station, both logistical and environmental.
The main challenges were the submergence of a suction pipe south of the pump station and the depth of the manholes. The suction manhole in one location was 30 feet deep. Essentially, it was too deep for the pumps to properly bypass; if the pumps were set above the grade at the top of the nearby berm, suction lift would not be achievable due to net positive suction pump (NPSH) limitations.
At the discharge location, the team was faced with the challenge of diverting flow from one channel to another as work progressed, without shutting down the bypass. This was important for keeping the plant in operation.
Another challenge was the need to maintain a certain amount of submergence level. Submergence became an issue to upstream manholes being at lower elevation than the suction location at the plant. This was due to the plant being at a higher elevation to protect it from flooding, as it sits along the Cape Fear River. The lower elevation took surcharging out of the equation, as it could very likely cause a sanitary sewer overflow.
Lastly, the teams needed to consider environmental factors, since the area is in a low line and
prone to hurricanes that could lead to flooding. Should a hurricane hit during the bypass, losing the pump station was simply not an option. Therefore, it required the teams to account for the highest amount of flows as possible to protect against potential sewage overflow into the nearby Cape Fear River. This was taken into account by ensuring that the pumps could handle the average, peak and up to two and a half times the peak flow.
Pumping solutions
The goal of the bypass solution was to simultaneously activate three pump locations with 100 percent redundancy. These systems connected via level transducers that monitored the sewer levels and would automatically turn on the pumps if they sensed fluid levels above a set height, as well as control RPM levels during low flow periods. The RPM control also assisted with fuel efficiency. The systems would solely operate the station, meaning there was no room for failure. The teams kept a backup pump on each pump location site, along with additional discharge lines, as added protection.
Overcoming the submergence of the suction pipe required multiple meetings between PWC, Hazen and Sawyer, Crowder Construction and Sunbelt Rentals — with the final determination being made to bench down the pumps to achieve suction lift. Due to the upstream manhole elevation, this was imperative in order to not surcharge and overflow.
To implement this portion of the project, the teams brought in surveyors to determine the correct elevation heights, and PWC evaluated and confirmed them. Subsequently, Crowder Construction excavated the dirt down to 17 feet where the pumps would be placed.
The three pump locations each had a specific number of pumps and pump sizes to allow the bypass to achieve the correct flow rate. Each pump was set in containment berms to lower the risk of environmental impact, and they were connected via an auto dialer capable of calling PWC, Crowder Construction and Sunbelt Rentals in the event of an emergency.
Pump Location One included six, 18-inch Quiet Flow Diesel pumps, three of which were primary and three backups, to accomplish the required flow rate of 18,750 gpm. There was also one standby pump. Each pump was self-contained with environmental skids to contain 120 percent of the on-board fluids. Combined, the primary pumps were capable of producing 25,000 GPM, above the GPM required.
Pump Location Two was set up similar to location one, with the exception of smaller pumps and fewer backup pumps. It included four 18-inch Quiet Flow Diesel pumps designed to provide a flow rate of 12,500 gpm, with two backups. A standby pump was also added.
Both pump locations one and two included a suction location with independent HDPE SDR 17 suction tubes, 24-inch HDPE SDR 17 for location one and an 18-inch HDPE SDR 17 for location two, placed in the manholes. Each pump utilized independent HDPE SDR 26 discharge piping, which will be connected through a 36-inch barrel manifold with 18-inch flanges in and 24-inch flanges out, exiting into the 24-inch HDPE SDR 26 main discharge line.
Each manifold connection included an 18-inch knife gate valve to allow for independent isolation of each of the pumps. In case of pump failure, the pumps could be removed while under bypass and a backup one set up. At the suction location, the pumps were discharged into 24-inch HDPE SRD 26 discharge piping.
Pump Location Three was located at a manhole, where it was necessary to install a 12- to 24-inch sewer plug in the downstream invert to create a wet well. There were also two, 6-inch diesel trash pumps installed to achieve 2,083 gpm, with one as the primary and the other as backup.
The teams also installed a backflush line, so the pump station could back flush waste that was not collected back into the sewer line. This allowed it to be regenerated to clean drinking water.
The path to success with the Cross Creek WRF Influent Pump Station rehabilitation was thoroughly a group effort. It required ongoing communications between stakeholders and plan revisions to ensure every scenario was covered to complete a safe bypass. The result was a relatively short timeline for completing the project. Planning began in February 2019 and the project was completed, including breakdown, in October 2019.
With these improvements, the station is now functioning effectively and safely. •
FOR MORE INFORMATION:
Crowder Construction, (800) 849-2966, crowderusa.com/
Sunbelt Rentals, (800) 667-9328, sunbeltrentals.com
ABOVE: The rehabilitation required bypassing a total of 48 million gallons per day of sanitary sewage to upgrade and rehabilitate the bar screens, wet well, and screw pumps at the station influent structure.
LEFT: The rehabilitation of the Cross Creek Water Reclamation Facility Influent Pump Station required careful planning, a thorough risk assessment – and most importantly, solid partnerships to get the job done safely and effectively.
Comments