Patent Application
20060251808
The present invention relates to pumps and more specifically, high-capacity pumps used to pump water for irrigation and other purposes.
In mankind's development of agricultural and urban areas, water has been a critical element. Water is used for agricultural purposes as well as human and animal needs. Nowhere is this more evident than in China where vast areas must be developed using irrigation systems much like the development in the United States in the last century. One of the key rivers in China is the Yellow River. It is so named because of its color, caused by a significant sand content. The silt content of this river is the highest in the world of the major rivers as shown in the following table:
Sand is a remarkably abrasive material and is used to remove material from underlying structure. When the Yellow River material is pumped, the abrasive sand particles continue to act on the pumping surface in much the same way that sand is purposely used to remove surface material such as paint and oxidation particles. In the Yellow River valley, the microstructure of the Yellow River sand has an average grain size of 0.29 microns. This particle size is significantly smaller than average sand. Instead of being less abrasive, it is much more so and, compared to other major rivers in the world, the Yellow River provides one of the most highly abrasive environments.
When irrigation pumps are designed to pump massive amounts of liquid water, they are, generally speaking, centrifugal pumps. A typical capacity of such a pump would be roughly 2000 usgpm to 180,000 usgpm. Also of importance is the flow velocity of the water containing the fine sand or silt particles. Pumps used to transfer such large volumes of water typically have a water flow velocity of not less than 50 meters/sec. The combined speed and volume of this water containing abrasive particles can erode stainless steel castings away in less than 2 months. Without any special form of treatment, a pump operating under these conditions generally lasts for no more than 2 months.
In order to extend the life of pumps of this type, the previous approach has been to apply hard materials, such as tungsten carbide, to the surfaces. This material, which in many cases is used in cutting tools, has an extraordinarily tough surface. Nonetheless, the tungsten carbide generally lasts for no more than eight months in this extreme environment.
Although the tungsten carbide, because of its hardness, resists abrasion, it is susceptible to cavitation. Cavitation is a phenomena where localized pressure in a hydrodynamic flow falls below the vapor pressure of the liquid being pumped. The result is the creation of vapor and air bubbles along the surface of the flow channel or passages. When these bubbles collapse, water rushes in to fill the void and subjects the passage wall to significant high frequency pressure fluctuations causing cavitation to occur. The resulting impact upon the passage surface causes deterioration in a manner that equals the effects of abrasion from the sand content.
When the tungsten carbide has been eroded, new tungsten carbide must be applied to the pump surface. Since the application of tungsten carbide is done by either wire arc spraying, flame spraying and fusion, or High Velocity Oxygen Fuel (HVOF) spray, the restoration of the surfaces becomes extremely complicated and expensive. In either case, the pump must be taken to a factory or techniques developed to reapply the tungsten carbide in situ. In situ reapplication of the coating becomes more complicated because the material thus applied must be directed generally toward the surface at right angles. When the impellor for the pump is a shrouded impellor having closed passages, the application of the tungsten carbide traditionally applied at right angles has to be done in extremely confined circumstances. Of the methods used for application of tungsten carbide, methods that do not allow a tool to be positioned at right angles to the surface such as needed for passages provide less than optimal application of the tungsten carbide for such passages.
Thus, there exists a need in the art for a material that is at least as long-lived as tungsten carbide and other hard materials, but at the same time is easy to apply.
The present invention relates to a method and apparatus for protecting pumps used to deliver highly abrasive liquids. The pump liquid flow surfaces are prepared and the surfaces so prepared are coated with a polyurea coating.
In a preferred embodiment of the invention, the coating is applied at moderate temperature so as to enable in situ application.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated herein and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described processes, systems or devices, and any further applications of the principles of the invention as described herein, are contemplated as would normally occur to one skilled in the art to which the invention relates.
Referring to
As discussed previously, the pump 10 operates in a challenging environment for two reasons. The first is that there is a significant flow of liquid for pumping purposes. The second is that the abrasive content of the liquid so being pumped is extraordinarily high. With this set of extremely adverse circumstances, the flow channels 18 and/or flow passages 34 and 36 are coated in accordance with the present invention to provide surprising durability that rivals previous attempts at coating these channels and passages using hardened material like tungsten carbide.
In order to practice the invention, flow channels 18, and passages 34 and 36 are appropriately prepared. The surface is prepared by making sure all grease or oils are removed. Once the surface is clean and dry, it is grit blasted using 60 grit or coarser sand at 100 to 125--psig air pressure. It is important that all contaminants be removed from the surface to be coated, including previous remnants of epoxy primer and polyurea, if the surfaces are to be re-coated. For steel, iron, or galvanized material, the surfaces are abrasive blasted with 40 to 70 micron size clean Al2O3 grit at a minimum 100-psi compressed air pressure. The surface should be blasted to expose clean metal surface. It should be noted that if the pump and flow surfaces are to be re-coated that appropriate solvents are employed to remove traces of the previously coated material.
The surfaces so cleaned are then prepared by using a primer. As herein illustrated, the bonding primer is a two-part epoxy primer manufactured by Reltek, LLC. It consists of a proprietary bisphenol-A resin and cycloaliphatic amine hardener which are mixed thoroughly until the resultant mix appears to be of smooth and uniform consistency. This can be done either by hand or through a commercially available static mixing nozzle. Application of the epoxy primer is achieved by commercially available adhesive coating systems available by manufacturers such as TAH Industries. The primer can also be applied by brush or roller using short to medium nap roller. The epoxy primer is cured according to the following table:
Full cure of the epoxy primer is achieved after five days.
This primer is applied to the metal surfaces and it creates a surface that promotes chemical cross-linking of the molecules between the primer and the material to be applied to the primer. For the maximum cross-linking to be achieved, the subsequent material is applied before the primer is fully cured. Total curing of the primer before application of the polyurea will not enable significant cross-linking to occur between the primer and protective polyurea finish coat. The primer can be applied by brushing or spraying (previously noted) as needed to obtain a relatively thin coat of approximately 0.1 mm or less.
Alternatively, the surface could be primed with a solvent-based primer that ensures all available surface of the metal is available for cross-linking. Such a primer would contain an alcohol, butanol, and toluene solvent base. Solvent based primers can be procured from a large number of manufacturers such as Sherwin Williams and PPG Industries.
Once the primer is applied, the passages so covered are coated with a 100% solids plural component polyurea. Polyurea is a class of polymers containing an amine group, however pre-polymer components may include both pure polyureas or hybrid formulations that may consist of a polyether, polytetramethyleneglycol, polyurethane or similar pre-polymer composition.
The type of polyurea that yields the best performance results on internal pump parts experiencing the noted severe particle erosion and cavitation is available from Adiabatics, Inc. The polyurea can be applied in spray form with a low pressure of around 30 to 80 psi at a range of temperatures from 20° to 150° F. Higher temperatures permit a shorter curing time. Preferably, the epoxy primer is applied with a thickness of approximately 0.1 mm and the polyurea is applied with a coating thickness of approximately 0.25 mm to 10 mm and more preferable approximately 1.5 mm to 3 mm. The typical property of polyurea elastomer is a tensile strength of up to 5,000 psi and a durometer hardness ranging from a Shore A hardness of 30 to Shore D hardness 80.
Satisfactory polyurea based polymer coatings can also be applied by commercially available high pressure plural component application equipment that can be purchased from Graco, Incorporated or Glas Craft, Incorporated. This type of equipment controls both application pressure and temperature to provide a very consistent polyurea coating that cures more rapidly than those typically applied by the low pressure equipment. The lowest achievable durometer hardness of polyurea polymer coatings applied by the high pressure equipment is typically not as soft in durometer hardness as that which can be achieved using low pressure application equipment. From test results, it is apparent that mid to upper range Shore A hardeness (60 to 70) durometer coatings outwear both the softer and harder durometer coatings for the water or saltwater pump applications. However it is not to say, satisfactory coatings cannot be achieved using the high pressure application equipment.
In
Such an arrangement is illustrated in
The type of pump illustrated in
Polyurea is not a waterproof material. Polyurea is not specified for long-term liquid application since water will eventually permeate and migrate to the substrate and may cause the coating to separate from the base material. In the extreme environment illustrated above where there is a high degree of abrasive material in the liquid being pumped, the polyurea protects the base material, but mechanically erodes before migration of the liquid causes the polyurea to separate from the base material. Thus, a synergism is achieved by the use of the material that would not normally be considered for such an abrasive environment.
It is also noted that the application of the resilient polyurea reduces cavitation. Cavitation is a phenomenon where because of hydrodynamic conditions, the localized vapor pressure falls to the point where the liquid is in a vapor form adjacent to the surface of the flow channels. Collapse of the bubbles thus induced causes liquid to impinge with great force in a very concentrated fashion. Cavitation additionally causes erosion of the surface of the flow channels. When the liquid being pumped has abrasive material, the cavitation effects are exacerbated. By providing a resilient polyurea material, cavitation is minimized. The relatively low temperature application of polyurea in situ restoration of the coating is extremely feasible, thus avoiding the need for removal of the pump from the work site and transport to and from a manufacturing site for application of the coating. Thus it is seen that the utilization of the polyurea offers synergistic and highly unexpected benefits in protecting a pump from the rigors from pumping an extremely abrasive content liquid, such as the Yellow River.
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.
