Pumps – Processes
Reexamination Certificate
2000-09-01
2002-12-10
Walberg, Teresa (Department: 3742)
Pumps
Processes
C048S048000
Reexamination Certificate
active
06491501
ABSTRACT:
BACKGROUND
The present invention relates to a system for transporting high-viscosity materials; and more specifically, an efficient, progressing cavity pump system for transporting high-solids, high-viscosity, dewatered materials, such as dewatered sludge.
Sludge dewatering is one of the fastest growing segments of the municipal wastewater treatment industry. Municipal wastewater treatment plants who have previously placed their waste activated sludge in sludge lagoons or drying beds, or who have previously directly land-applied their waste activated sludge, are now being forced by EPA Section 503 regulations and economics to further process the sludge. Such further processing includes dewatering of the sludge.
EPA Section 503 regulations went into effect in 1993 to establish requirements for the final use and disposal of sewage sludge. These regulations escalated the costs of final disposal of sewage sludge, which in turn gave strong incentive to municipal wastewater treatment plants to reduce the amount of sludge being disposed. The removal of water from the sludge (dewatering) is one of the more practical means to reduce sludge volumes and waste. Therefore, municipal wastewater treatment plants have been using increasing amounts of resources to install or create more efficient dewatering equipment.
Presently, dewatered sewage sludge is transported away from dewatering devices by four different methods: belt conveyors, screw conveyors, piston pumps, and progressing cavity pumps. Pumps have inherent advantages over conveyors. Specifically, the sludge is transported through a pipeline, rather than being exposed to the atmosphere, which significantly reduces odor; sludge can fall off, or be blown off, a conveyor belt, causing a safety and house-cleaning problem; and a dewatered sludge pipeline is easy to heat-trace and insulate as opposed to conveyors, which are not possible to heat-trace or insulate.
In North America, piston pumps comprise a majority of the market share for dewatered sludge pumps. The most common piston pumps utilize a pair of material cylinders in which a corresponding pair of material pistons reciprocate. The sludge material is received at the inlets of each of the material cylinders, the feed into which is controlled by inlet gate tube or poppet valves. Additionally, the flow of the sludge from the material cylinders to an outlet is controlled by outlet poppet valves, respectively. The inlet valves are controlled by hydraulic inlet valve cylinders, and the outlet valves are, likewise, controlled by hydraulic outlet valve cylinders. The material pistons are coupled to hydraulic drive pistons, which are in turn operated according to a hydraulic control system. As the drive pistons and their associated material pistons come to an end of their stroke, one of the material cylinders is discharging material to the outlet, while the other material sender is loading material from the inlet. Accordingly, the inlet and outlet valves are controlled to allow the material to be discharged from the first cylinder and new material to be loaded into the second cylinder. At the next pump cycle, when the second material piston is at the end of its stroke, the inlet and outlet valves will be controlled such that the material is permitted to be discharged from the second cylinder and new material is permitted to be loaded into the first material cylinder. As a result of this design, the material in each cylinder will come to a stop each time a piston reaches the end of its stroke, allowing the inlet and outlet valves to change positions. This material must be then accelerated from a rest condition by a piston on the next stroke. Accordingly, significantly high pressure levels are generated in each of these cylinders during the stroke. Also, significantly high pressure levels are generated in the pipeline to overcome the resulting acceleration losses. Additionally, the resulting flow of materials from the outlet is a pulsating flow. A further disadvantage of the piston-type pumps is that the pumps must be powered by hydraulics and corresponding hydraulic and valve controls, which significantly increase the costs of the pump. Such complexity also increases the costs in maintenance and repairs for the pump systems.
Furthermore, according to federal regulations Section 503, municipal wastewater treatment plants are also required to measure and document the mass flow rate for sludge transport applications. In regard to incinerators, control efficiencies and sludge feed rates have to be reported in mass flow for the proper calculations in determining pollutant limits. A significant disadvantage with the use of the piston-type pump is that the determination of mass flow rate based on volume is complex due to the number of parameters needed for such calculations. For example, a hydraulically-driven piston pump requires two position switches in the hydraulic cylinder to sense the start and stop positions of the piston and to determine the stroke length. A third proximity switch on the discharge valve senses when the valve opens and closes. The piston pump must calculate the volumetric efficiency for each stroke of the pump. The stroke volume is large and even when being fed by a twin screw feeder, the volumetric efficiency could vary a significant amount between strokes, since the inlet valves are an obstruction to suction flow.
The volumetric efficiency is calculated by timing from a startup of the piston stroke to the opening of the valve, and timing from the opening of the valve to the end of the piston stroke when the valve closes. Using time instead of stroke position to determine volumetric efficiency does not compensate for fluctuations in velocity of the piston (i.e., the point where the piston actually goes from no-load to load). Accordingly, typical accuracy for such flow-rate calculation has been found to have a relatively high variance.
Progressing cavity pumps provide an alternative to piston pumps. A progressing cavity pump includes an elongated, externally-threaded rotor rotating within an elongated, internal helical-threaded stator, where the stator has one more lead or start than the rotor. Pumps of this general type are typically built with a rigid metallic rotor and a stator, which is formed from a flexible or resilient material such as rubber. The rotor is made to fit within the stator bore with a compressive fit, which results in seal lines where the rotor and stator contact. These seal lines define or seal off definite cavities bounded by the rotor and stator surfaces. As the rotor turns within the stator, the cavities formed by the seal lines progress from the suction end of the rotor/stator pair to the discharge end of the rotor/stator pair. During one revolution of the rotor, one set of cavities is opened at exactly the same rate that the second set of cavities is closing. This results in a predictable, pulsationless flow.
While such progressing cavity pumps are less expensive and less complicated than the piston pumps, conventional progressing cavity pump systems also have several characteristics that may make them less attractive for use in transporting the high-solids dewatered sludge. Specifically, the volumetric efficiency (filling efficiency) for conventional progressing cavity pumps in such applications can be approximately 50%. Additionally, the footprint of conventional progressing cavity pumps and associated feeders are relatively long and narrow, making it substantially difficult for most municipal wastewater treatment plants to be retrofitted with such systems.
Accordingly, there is a need for a pump system for transporting high-solids, high-viscosity, dewatered materials that is relatively inexpensive and uncomplicated, that has a compact design (footprint), that produces a non-pulsating flow, that has a relatively high volumetric efficiency, that allows for accurate and uncomplicated calculation of mass flow-rate, and that does not necessitate relatively high pressure levels within the system.
SUMMARY
The present invention provides a system and me
Brown Todd E.
Sliwinski Richard A.
Snyder Charles L.
Wild Alan G.
Fastovsky Leonid M
Moyno, Inc.
Thompson Hine LLP
Walberg Teresa
LandOfFree
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