As consumer demand for convenient, disposable wipes continues to grow to over $2 billion annually, nearly double the rate of retail purchasing since the early 2000s, pump manufacturers are forced to develop new designs to take on the growing challenge of handling more difficult to pump materials at increasing volumes. The increased maintenance costs directly related to higher volumes of larger, non-biological solids has shifted the focus from system energy efficiency costs, to system operating and maintenance costs related to equipment failure due to ragging. Currently, many pumps require weekly, even daily, maintenance to unplug blockages, which increases labor and parts costs well beyond those in the past and also creates greater health and safety risks to operators and residents in areas where pumps stations fail.
While original pump designs performed well pumping soft biological solids with more consistent, manageable solids sizes, current pump designs must contend with the presence of much larger, stringy materials. These stringy masses carry increasing amounts of grease and grit; the fibers in the wipes pick up the fats, oil and grease in the waste stream and offer a congealing medium. The rags get heavier with these coagulants; necessitating that fluid velocity in the piping system to properly carry these heavier solids has increased from recommendation of 2 to 4 feet per second in a pre-wipe period, to a range of 5 to 7 feet per second now, in order to avoid settling issues. This increased velocity is critical in piping design and pump operating speed considerations—the increased velocity contributes to greater pipe abrasion. The potential increase in abrasive materials attached to rags will play a part in the selection of proper materials in the pump wet end to maximize its operating life.
In 2011, one manufacturer developed a cutter pump design to address the increase in rag material in not only wastewater applications, but other solids-handling applications in the agricultural and industrial markets. The original design incorporated a hardened rotating cutter ring affixed to the impeller, serving as both a cutter ring with teeth and as the pump wear ring and a stationary cutting element fixed in the suction inlet. As solids pass into the pump the stationary cutter ring works with the rotating cutter shearing the materials and reducing them to a manageable size to pass through the impeller vanes.
Figure 1. Three generations of cutters designed to meet the need for handling a steadily increasing volume of non-biological solids. (Courtesy of Cornell Pump Company)Use in municipalities across North America have proven this to be an effective solution to reducing and, in some cases, eliminating the need for weekly maintenance/re-ragging visits, lowering labor costs and freeing up staff to attend to other issues. The design modification from a standard solids handling pump slightly reduces the pump efficiency, but the increase in energy cost is much lower when compared to the maintenance cost required to unclog pumps on a regular basis.
Since 2011, the further increasing volume of non-biological solids (rags) led to a second-generation design of the cutter, a more aggressive design, to meet requirements of the increasing solids present in the waste stream. The combination of greater volumes of solids and low-fluid velocity in the piping system at times causes the rag material to settle in the inlet pipe or suction elbow of the pump at the eye of the impeller.
The need to grab and draw the material into the pump cutting area inspired several design considerations, including the addition of a separable auger element fitted to the impeller to increase the cutting surface area and improve the cutting efficiency of the pump. This second generation design proved to be effective in field tests. In one southwestern Washington water authority, the design eliminated bi-weekly service calls, reducing maintenance costs by more than $30,000 annually. The additional second generation design modification to the inlet of the pump reduced pump efficiency by a few points. However, when the additional energy costs were weighed against the greatly reduced maintenance costs the overall operating cost was markedly reduced. The ability to retrofit the separable auger into existing installations is an added feature.
While the second generation of cutter design proved to be very effective in taking on the more difficult applications, the manufacturer continued to look for a more robust design in anticipation that the presence of disposable products would only get worse over time. One new cutter pump design incorporates a solids handling impeller with vanes cast all the way through the eye of the impeller, which is fitted with a heat treated hardened cutting surface that extends across the entire suction inlet of the pump. This removable cutting element also serves as the impeller wear ring and works in conjunction with the updated stationary hardened blade to increase the cutting efficiency of the pump with minimal degradation to pump efficiency. The one piece impeller design is more robust than the second generation, able to be retrofitted into existing designs and offers superior cutting performance.
Recommendations & Observations
- When designing pumping systems to handle the presence of rag material, select a pump design that can handle the volume of material anticipated.
- Fluid velocity in the inlet piping system must be able to carry the solids into the pump inlet for proper performance. Pipe size and pump operating speed are critical to proper velocity.
- Pump operating speed is important to cutting efficiency. Consult with your pump supplier for the minimum recommended operating speed.
- Select the proper construction materials for the pump to reduce wear from abrasion. Harder materials or coatings can increase the operating life of the pump system.
- Whether upgrading an existing system or designing new, be sure to compare the system operating and maintenance cost to energy efficiency costs. Most of the time, running the system to best handle the pumped material with minimal downtime will be a less expensive station to operate and create a safer environment for the maintenance staff.