Canadian treatment plant employs bypass system during pump motor replacement
The city of Ottawa is the capital of Canada, and the Robert O. Pickard Environmental Center (ROPEC) is the city’s wastewater treatment plant. Originally built in 1962, the plant underwent a series of expansions in the 1970s, ‘80s and ‘90s, and now handles more than 111 million gal per day (mgd) of raw sewage. ROPEC provides secondary treatment of residential, commercial and industrial wastewater before the treated effluent is returned to the Ottawa River.
The plant’s raw sewage pumping station has six dry-pit pumps with large 950-hp motors. In 2013, after a condition assessment, the city was informed that the conditions of three of the six motors were outside acceptable limits. An electrical testing firm confirmed that the insulation in the windings on three of the pump motors had deteriorated to the point where these motors could fail catastrophically. Motor failure would reduce the station’s total pumping capacity in the midst of the high-flow season: the spring melt.
These motors dated back to the 1980s. The city had installed variable frequency drives (VFDs) to provide better control of the inflow to the treatment plant. VFDs can provide energy savings, but they also create more heat in the stator windings and can cause accelerated deterioration of the insulation, especially with standard motors that are not rated for inverter duty. Such was the case in Ottawa.
While the procurement of new motors started, the least affected of the three motors facing failure had its stator rewound, after which it was reinstalled. This was enough to increase its reliability, but not enough to comfortably weather the next high-flow season.
Imminent Given that three pumps had to be taken offline, the city needed additional temporary pumping capacity installed and operational before the high flow period during the spring runoff. According to the city’s consultant, CH2M, additional capacity of 56 mgd at 72.2 ft of total dynamic head was required. The only viable option to quickly mitigate this impending problem was to install temporary bypass pumps. Working with the consultant; Doran, the general contractor; and Xylem’s Water Solutions team, four pumps were proposed: two Flygt NS-3531 pumps and two Flygt CS-3400 pumps. Each pump required 100 ft of power and monitoring cables as well as VFD controllers with the Flygt MAS 711 pump monitoring system. The pumps have Class H trickle-impregnated insulation in their stator windings for use with VFDs, eliminating problems with degraded stator windings.
Because of the new motor installation’s forecasted lead time, the bypass system was planned to stay in place for eight to 12 months, or until the two new motors were received. It was early January, and less than 70 days remained to complete the design, construction, installation and commissioning of the temporary pumping system.
The pump manufacturer had the necessary equipment in its rental fleet and quickly provided its pump selections along with performance curves and dimensional drawings to the consultant for review. For the bypass project, the city chose to sole-source the manufacturer and contractor. Sole-sourcing also expedited the completion of the work given the tight timeline. The pumps also were being installed 50 ft below grade and electrical power was readily available at the plant, which made submersible electric pumps the most feasible option.
Four bypass pumps were installed to complete the necessary motor replacements.
Unique Pump Stands
The pump station has one common intake that splits into two screening channels and two wet wells. CS-3400 pumps were mounted in each screening channel and NS-3531 pumps were mounted in each wet well. The pumps were mounted on custom stands above the permanent pump intakes to prevent any flow interference with the remaining operational pumps. Pump stands were designed with hold-down clamps to tightly anchor the pumps without damaging or modifying the galvanized stands.
The pump stand legs for the NS-3531 pumps were designed so larger pipes can roll around smaller stationary pipes. The smaller internal pipe is welded to the stand, whereas the larger pipe is loose and free to turn around the smaller pipe. This prevents solids from getting caught in the stand and clogging the pump by allowing for passage of long, stringy solids without clogging the stand and running the risk of starving or clogging the pump suction. With the larger pipes rolling with the flow of liquid, it prevents any significant accumulation of rags or solids from building up on the intake screen.
The piping contained inside the pumping station was all steel and had to be designed and installed to go around the existing equipment and concrete structure of the station. The piping was designed to minimize the friction losses while fitting between the existing infrastructure. Steel piping was chosen due to accessibility constraints. The contractor was able to secure welders in the construction area, but it would not have been able to get a fusion machine for high-density polyethylene thermoplastic piping. The radius of the elbows was kept to a minimum to reduce the losses in the piping as much as possible.
Each pump was carefully lowered into place with the discharge elbows and the first length of the discharge pipe already installed. The liquid level in the wet well was above the pump, which is 11 ft tall with the stand. The pumps were installed 10 ft above the wet well floor. There are two discharge pipes per channel as each pump has its own discharge piping. There are no valves in the piping; when the pumps stop, the piping empties itself back in the wet well, which helps keep the pump suction and stands free of rags and debris.
Pump controls were an important part of the design because Class 1, Division 2-rated pumps were being installed in a Class 1, Division 1-rated environment. For this to be acceptable, the pumps could never be energized when any portion of the pumps, other than the cables, were exposed to the atmosphere in the wet well. A low-level float switch was installed with each pump controller to automatically turn off the pumps and ensure that the pumps would remain fully submerged.
The contractor also verified the routing of the pump cables. Each pump required a minimum of 100 ft of power and monitoring cable just to reach an electrical room located on the north side of the plant. From that point, standard junction boxes and cable extensions were then used to connect to the pump controllers. The four VFD controllers were installed outside the raw sewage pump station against a steel sea shipping container that housed the step-down transformer and the rest of the electrical components required for this bypass project.
Once the piping information was gathered, the proprietary Xylem selection tool was used to generate an accurate system curve and determine where the pumps would operate on their performance curves. These pumps had to operate as closely as possible to their best efficiency point to prevent internal cavitation or high radial loading on the bearings and mechanical seals.
Based on the system curve analysis, a capacity of 66 mgd was expected with all four pumps operating. However, after the bypass pumps were commissioned in March 2014, a capacity of 72 mgd was achieved with all four temporary pumps operating. Considering that the consultant had originally requested 56 mgd, all parties were pleased to get 28% more capacity from the pumping system than initially expected.
There were many challenges that had to be overcome to make this bypass project a success, up until the day the pumps were commissioned. When the pumps were being brought online, one of the CS-3400s would not start. Initially, it was believed that the pump was clogged because it was tripping on high current. However, the culprit turned out to be a defective VFD, which was replaced within two days.
After numerous emails, phone calls and site meetings, this project was completed in a span of 70 days. Close cooperation among all stakeholders coupled with innovative design in a narrow timeframe enabled one of Canada’s largest cities to provide the reliable level of service customers expect of their wastewater utility.