Progressive cavity pump system solves WWTP’s transfer issues
The City of San Mateo Wastewater Treatment Plant (WWTP) operated in northern California handles a capacity of up to 15.7 million gal per day. The original plant was constructed to serve the surrounding community in 1930. It has gone through several upgrades over the years, with the latest expansion completed in 2009.
Part of the recent expansion included two new centrifuges to assist in dewatering the treated cake produced by the plant on a daily basis along with shafted screw conveyors for transporting the cake from the centrifuges to the storage hopper, where it is then trucked off site for land application. The conveyor system spanned some 60 ft at a 30-degree incline and proved to be troublesome from day one. Main issues involved hanger bearings and premature failure of the conveyor shafts. Several equipment repairs were made to no avail.
In March 2009, City of San Mateo WWTP maintenance staff was dealing with the major material handling issue and sought a long-term solution to this cake transfer problem. Preliminary evaluation of cake transport options revealed that a piston pump used for concrete pumping might be a suitable technology. This system included a dual piston pump powered by a high-pressure hydraulic pump and associated mechanical, hydraulic and electronic controls.
The WWTP decided to fully evaluate this technology by renting a system for a period of eight months before procuring permanent setup. The pump proved to be fully capable of transporting the cake, although 300- to 500-psi discharge pressure was needed to achieve the required pumping rate of 5 tons per hour due to the pulsating nature of this pump design. Upon review, the combination of annual preventative maintenance costs, overall power consumption and operational noise generated by this style of system was deemed to be an unfavorable solution to this application.
Staff concluded that the concrete pump was not a viable long-term solution and engaged the services of Flo-Line Technology Inc. to further evaluate the application and recommend pumping alternatives. The company has experience in slurry and filter/centrifuge cake transport applications from years of dealing with global mining operations using a variety of transport technologies.
Flo-Line Technology reviewed application specifics and consulted with progressive cavity (PC) pump manufacturer Netzsch Pumps North America LLC to see if it had used its pumps effectively on the dewatered filter cake applications. Netzsch Pumps President Thomas Streubel cited several cases in which the company’s PC pump technology was used effectively to transfer filter cake (with up to 40% solids content) in pump systems around the world.
Upon review of the application, the cake was determined to have a solids content ranging from 18% to 25% by weight. The city makes great efforts to produce cake with the least possible amount of water, as the added weight results in higher transport costs. Conversely, the drier the cake, the more difficult it becomes for rotating equipment to handle it. Application specifics such as system layout, typical flow rate requirements, friction analysis and piping detail were analyzed. Extensive research among existing filter cake systems was reviewed, and a formal proposal was created to review with the city staff.
The filter cake pump system is designed to transfer the cake by rotation of a metal rotor within a compressible stator. The interference fit between the two components form a series of cavities that continuously trap and move media in a smooth linear fashion based on rotation speed. This positive-displacement style pump commonly is known as a PC pump based on its internal design of progressing cavities. Due to the viscous nature of the filter cake media, an additional patented auger screw was supplied within the pump inlet hopper to ensure positive feeding of the rotor/stator components.
Part of the system supplied was the Netzsch Friction Loss Reduction (FLR) unit. The friction loss between dewatered sludge cake and the inner surface of a pipeline can be reduced by adding lubricants (e.g., water or polymer solutions). As a result, the operating pressure of the system can be reduced significantly with minimal addition of lubrication media. Reduction in pressure means reduced drive power requirements and lower schedule piping, which add up to cost savings.
The FLR system helps achieve considerable reductions in pressure, cutting back on operating costs and improving the lifetime of the entire system. Within the system is the Netzsch Ring Nozzle, which provides a continuous 360-degree even layer around the entire pipe surface. This lubrication media is delivered to the ring nozzle via a small PC pump, and flow is controlled with the use of a variable-frequency drive located in the system’s common control panel. This optional lubrication device is programmable and based on the discharge line pressure sensed by a pressure transmitter located directly off the pump outlet.
The equipment was delivered to the site within about two months and needed to be reviewed by the city, local contractors and Flo-Line Technology in detail to allow for quick installation. The city had a very short time frame because it could not retain cake without processing for longer than 14 days. Flo-Line Technology President John Novoselac provided a detailed review of the challenges required to implement this system in a short installation window of seven to 10 days and discussed streamlining methods for minimizing the time frame.
The local contractor required three days to remove the existing rental equipment, pour the new concrete base and install the new cake pump. The electrical team required another two days to run new conduit and make necessary control connections. With all parties following the plan set forth for the quick-install timetable, the result left plenty of time for Flo-Line Technology to test the system fully without the added pressure of a short time frame.
This project was an excellent example of team effort by all parties involved going beyond contractual norms to achieve a common objective for the benefit of the city of San Mateo, plant officials said.
After the installation in September 2010, Flo-Line Technology fine-tuned the system to ensure successful performance. This dynamic application requires a thorough understanding of the system components and control details. The automation required control system customization and fine-tuning in order achieve smooth system interaction. The result proved to deliver steady cake transfer to the storage hopper at a moderate speed, resulting in optimal expected wear life for the main pump components.
Through several days of continued evaluation, the system was determined to be tuned properly for operation and system training commenced. Plant operaters quickly acquired basics on automatic operation and readily accepted the new filter cake transportation system. The maintenance team already was familiar with PC pumps, as they had been using them readily on thin sludge and polymer applications for years.
Routine maintenance on PC pump wear components was a known routine throughout the plant, and the system required no special tools or training that the city did not already possess. A simple shift in thinking allowed PC pump technology to be incorporated into a new application that traditionally had been overlooked.
Today the system operates automatically, requiring zero staff oversight. Once the centrifuges feed the pump hopper, the system senses the incoming feed and responds by transferring it to the storage hopper. If the cake is particularly dry and the system pressure spikes due to higher friction, the automatic lubrication portion adds a small amount of water or polymer to the discharge pipe to reduce friction, bringing the system pressure back down to ensure longevity of the pumping components.
Safety devices such as dry-run protection and high-pressure switches were installed to remove all known possibilities of potential equipment damage. With the small cost of these safety devices, the system becomes nearly bulletproof and can save a plant thousands of dollars by reducing potential “learner’s damage” while adjusting to new pieces of equipment.
While the City of San Mateo WWTP found itself in a serious jam due to existing filter cake transport equipment issues, a fresh approach by the maintenance staff created a novel solution.
According to plant officials, this project is an excellent example of bringing together a knowledgeable local equipment supplier, an innovative global equipment manufacturer and plant staff to jointly solve a major challenge.
Challenge: Hanger bearings and premature failure of conveyor shafts formed the basis of a cake handling problem at the City of San Mateo WWTP.
Solution: The plant explored cake transport options and opted for a filter cake pump system featuring a positive-displacement style pump and friction-loss reduction unit.
Conclusion: The system operates automatically and requires zero staff oversight, proving that this oft-overlooked pump technology is viable. In addition, dry-run protection and high-pressure switches were installed to remove all known possibilities of potential equipment damage.