Patent Application
20230258187
This application claims priority under 35 U.S.C. §119 from German Patent Application No. 102020003855.7, filed Jun. 26, 2020, the entire disclosure of which is herein expressly incorporated by reference.
The invention relates to a centrifugal pump for conveying of solids-containing media having an arrangement for reduction of backflow from a first space into a second space.
Centrifugal pumps have gaps through which fluid is possible at various points, for example between the impeller and the housing, where a pressure difference causes a leakage flow which results in very high loss in some cases. The seal here must be chosen with regard to the order of size of the gap such that it is neither too large, such that the efficiency of the centrifugal pump falls as a result of high losses through this gap, nor may the gap be too small, because there is otherwise the risk of encroachment, i.e. contact between the rotating component and the stationary component.
Such a seal may, for example, be a split ring seal arrangement. Split ring seal arrangements in centrifugal pumps serve to seal spaces at different pressures. The arrangement comprises a nonrotating element and a rotating element. The nonrotating element may, for example, be a split ring disposed on the housing, or the housing itself or a housing part. The rotating element may, for example, be a race disposed on the impeller, or the impeller itself or part of the impeller, for example the cover plate of the impeller in the case of a closed impeller.
The gap formed between the rotating element and the nonrotating element acts as a throttle between the spaces at different pressures and prevents an excessive flow from the space at higher pressure to the space at lower pressure. The smaller the gap between the two elements, the smaller the losses of efficiency of the centrifugal pump. However, this effort is opposed by the fact that too small a gap is very difficult to reconcile with the manufacturing tolerances and the operating influences. The aim is to avoid contact between the elements in order to prevent rubbing of the rotating element against the nonrotating element and hence to preclude wear. On account of the necessary tolerances in the production of the individual components, there is a minimum gap width which just prevents contact between elements, thus resulting in friction and wear. However, in operation, especially in the startup or shutdown of the pump, situations occur time and again in which there is contact and then occurrence of compression or material wear.
In the case of conveying of solids-containing media, moreover, widening of the gap caused by the abrasive action of the soil particles has to be expected. Thus, particularly in the case of centrifugal foul water pumps, a rising loss of efficiency has to be expected.
One example of a solids-containing medium is wastewater, especially communal and industrial wastewater. This generally includes untreated wastewater (e.g. foul water, feces), wastewater (mechanically cleaned water from sedimentation tanks), sludge (e.g. activated sludge, fresh sludge, effluent and seeding sludge) and rainwater. Industrial wastewater can under some circumstances be very corrosive or abrasive on the centrifugal pumps used, especially the media-contacting components of the centrifugal pump.
In order to take account of constant wear to the gap seal in centrifugal pumps for conveying of abrasive fluids, there has already been a proposal to provide a means of readjusting the gap by means of adjustable sealing elements. DE 35 13 116 A1 describes such a gap seal. A manually adjustable gap seal is comparatively complex in its production and requires a lot of experience from the operating personnel using it. The adjustment, monitoring and readjustment of the sealing elements in good time requires a comparatively high level of time and effort.
It is generally the case that cast components are frequently used in centrifugal pumps. Casting forms a solid body in the desired shape from a liquid material after solidification. Thus, it is specifically possible to produce the desired housing structures or impellers or other components of the centrifugal pump. Cast materials in centrifugal pump construction are generally iron-carbon alloys.
DE 10 2017 223 602 A1 specifies a split ring-race pair in a centrifugal pump based on silicon carbide. The hardness of the material is supposed to protect the centrifugal pump from abrasive wear. For this purpose, a ceramic element made of silicon carbide is inserted into a casting mold and then cast with a metallic casting material.
DE 10 2018 214 650 A1 describes a split ring seal of a centrifugal pump based on calcium carbonate in the aragonite modification, which, with a high hardness, is more wear-resistant to abrasive substances.
Especially in the case of centrifugal pumps that are used for conveying of solids-containing media, there are corrosion and wear phenomena in the region of the split ring seal. Owing to the high brittleness of most abrasion-resistant ceramic materials, the ceramic solutions proposed, in the case of particular component geometries, are generally very costly and inconvenient in their implementation, and can under some circumstances (for example as a result of parts breaking away) lead to disrupted operation.
It is an object of the invention to specify a centrifugal pump for conveying of solids-containing media. Damage to split rings by abrasive wear is to be effectively reduced. Furthermore, the pump should maintain its efficiency in operation for long periods. The centrifugal pump is to feature high reliability and a long lifetime. It is additionally to assure simple assembly. Furthermore, the centrifugal pump is to be a convincing solution by virtue of very low production costs.
This object is achieved in accordance with the invention by a centrifugal pump for conveying of solids-containing media having the features of the independent claim. Preferred variants can be inferred from the dependent claims, the description and the figures.
According to the invention, a centrifugal pump for conveying of solids-containing media has at least one arrangement for reduction of backflow through a nonrotating element that at least partly has a layer of carbon.
According to the invention, such an arrangement for reduction of backflow can be configured as a split seal which may be formed by a split ring and a race or by a split ring and an impeller. This arrangement serves to seal spaces at different pressures and acts as a throttle between the spaces. In this arrangement, a first space is understood to mean a space at higher pressure and a second space to mean a space at lower pressure. In the centrifugal pump, accordingly, the space at higher pressure is the space in the pressure port and the spiral housing. The space at lower pressure is the space in the suction region upstream of the impeller.
The split ring is arranged on the pump housing by means of a press fit and is correspondingly fixed and nonrotating. The split ring as such is disposed directly on the pump housing. In addition, it forms a gap with a rotating counterpart element. The rotating element may, for example, be a race on which the impeller is disposed, or the impeller itself or part of the impeller, for example a radial and/or axial surface of the top cover of the impeller in the case of a closed impeller.
Advantageously, the split ring has a carbon layer on a radial surface, for example the inside of the split ring, and/or on an axial surface, for example the end face of the split ring. This results in an enormous increase in the hardness of a standard split ring made of a cast material and/or a stainless steel material. The split ring thus receives effective protection against the abrasive action of solid particles in the conveying medium.
The carbon layer is particularly advantageous with regard to contact with or encroachment of the counterpart element. Owing to the particularly smooth surface of the carbon layer and its unusual hardness, the split ring is insensitive with respect to rubbing action by a counterpart element.
In one variant of the invention, a second split ring is used for sealing of the impeller against the bearing support cap. This split ring also has a carbon layer that protects the split ring particularly against the abrasive action of solids-containing media and unwanted contact with the impeller.
According to the invention, the split ring interacts with a counterpart element in order to prevent a particularly small gap for reduction of backflow from a space at higher pressure to a space of lower pressure in the pump. This counterpart element may be configured in the form of a race disposed on a prepared surface of the cover plate of the impeller. In an alternative variant, the counterpart element may take the form of a worked radial and/or axial surface of the cover plate of the impeller. In both variants, according to the invention, a carbon layer has been applied to the gap-forming surfaces. Ideally, this protects the rotating counterpart element from the abrasive action of the solids-containing medium.
A particularly advantageous configuration of split rings is from standard metallic materials, especially cast materials and/or stainless steel materials, which are then coated with a particularly hard carbon layer that provides protection from abrasion. In this way, it is possible to produce split rings from an inexpensive raw material that can simultaneously be worked by standard manufacturing methods.
Carbon layers are understood to mean layers in which carbon is the predominant constituent. The carbon layer may be applied, for example, by a PVD (Physical Vapor Deposition), a physical gas phase deposition, for instance by evaporation or sputtering) or a CVD (Chemical Vapor Deposition) method.
The carbon layer is preferably an amorphous carbon layer, especially a tetrahedral hydrogen-free amorphous carbon layer, which is also referred to as a ta-C layer. The atomic bonds belonging to the crystal lattice of graphite (a total of 3 in each case) are identified by the “sp2” designation. The hybridization here is sp2 hybridization.
In the case of a diamond layer, each carbon atom forms a tetrahedral arrangement with four adjacent atoms. In this spatial arrangement, all the atomic distances are equally short. Therefore, very high bonding forces act between the atoms, and in all spatial directions. This results in the high strength and extreme hardness of diamond. The atomic bonds belonging to the crystal lattice of diamonds, a total of four in each case, are identified by the designation “sp3”. The hybridization is thus sp3 hybridization.
In a particularly favorable variant of the invention, the carbon layer consists of a mixture of sp3- and sp2-hybridized carbon. This layer is characterized by an amorphous structure. It is also possible for extraneous atoms such as hydrogen, silicon, tungsten or fluorine to be incorporated into this carbon network.
The inventive arrangement of a carbon layer on a split ring and on a counterpart element, for example a race, leads to a considerable reduction in abrasive wear.
The arrangement of a carbon layer on a split ring creates an extremely smooth axial surface with antistick properties without any need for complex mechanical reworking of the impeller. In addition, it is possible for multiple split rings to be introduced in a coating reactor, preferably executed as a vacuum chamber, where the ta-C coating is applied under moderate thermal stress. Thus, the centrifugal pump of the invention with at least one split ring is notable for comparatively low production costs.
In a particularly favorable variant of the invention, the carbon layer is applied as a coating of a split ring. The thickness of the layer is advantageously more than 0.5 µm, preferably more than 1.0 µm, especially more than 1.5 µm. In addition, it is found to be favorable when the carbon layer is less than 18 µm, preferably less than 16 µm, especially less than 14 µm.
Ideally, the coating of carbon has an extremely smooth axial surface with antistick properties, in which the average roughness volume Ra of the carbon layer is less than 0.7 µm, preferably less than 0.5 µm, especially less than 0.3 µm.
The ta-C coating has a very low coefficient of friction coupled with very good chemical stability. The hardness of the coating comes close to the hardness of diamonds, with the hardness preferably being more than 20 GPa, preferably more than 30 GPa, especially more than 40 GPa, and less than 120 GPa, preferably less than 110 GPa, especially less than 100 GPa.
With an average of 40 to 75 GPa, ta-C coatings are harder than a-C:H layers. In addition, ta-C does not contain any hydrogen. Therefore, it can be assumed that ta-C on contact with water (at temperatures above 80° C.) will be more stable than a-C:H. In contact with other liquids - especially polar liquids - containing molecules incorporating hydrogen, ta-C could likewise have better stability than a-C:H.
Preferably, the carbon layer is not applied directly to a split ring; instead, an adhesion promoter layer is provided first. This preferably consists of a material that both has good adhesion to steel and prevents diffusion of carbon, for example through the formation of stable carbides. Adhesion promoter layers used that meet these demands appropriately include thin layers of chromium, titanium or silicon. In particular, chromium carbide and tungsten carbide have been found to be useful adhesion promoters.
In an advantageous variant of the invention, the coating has an adhesion promoter layer that preferably includes a chromium material. The adhesion promoter layer preferably consists to an extent of more than 30% by weight, preferably more than 60% by weight, especially more than 90% by weight, of chromium.
The ta-C coating of the invention is a simple, readily achievable and economically viable coating for split rings in centrifugal pumps. The coating of the invention, as well as very high hardness, also has excellent sliding properties and good chemical stability. In particular, most metallic materials feature higher ductility by direct comparison with a ceramic material.
The reason for the advantage of the higher hardness resulting from the ta-C coating is that small and large solid particles that are often present in the solids-containing media can now have a significantly reduced abrasive effect on the split seal, i.e. the split ring and a counterpart element. The flow causes these solid particles normally to act like an abrasive. Split ring, race, impellers and/or housing parts of the suction side that are coated with ta-C have an extremely hard protective layer against abrasion, which distinctly increases the period of use thereof in the conveying of solids-containing media.
For the coating, PECVD/PACVD methods may be used with preference. Plasma excitation of the gas phase is effected here by the introduction of pulsed DC, or mid-frequency (kHz range) or high-frequency (MHz range) power. For reasons of maximized process variability with different workpiece geometries and loading densities, the introduction of pulsed DC has additionally been found to be useful.
Ideally, PVD methods are used for the coating. These methods are particularly simple and have a low process temperature. This technology leads to layers into which extraneous atoms may also be incorporated if required. The process regime is preferably effected in such a way that changes in microstructure and dimensions of the materials to be coated (metallic, gray iron, etc.) are ruled out.
The ta-C coating has the advantage over a CVD diamond layer that the coating temperature for CVD diamond layers is 600 to 1000° C., and that for amorphous carbon layers such as ta-C is well below 500° C. This is of high technical relevance particularly for the coating of metallic materials. The production of PVD diamond layers is impossible.
Further features and advantages of the invention will be apparent from the description of working examples with reference to the drawings, and from the drawings themselves.
In the region of the suction mouth 1, a fixed, nonrotating element 2, here in the form of a split ring, in the interior of the housing portion 10 is connected to the housing portion 10 by means of a press fit. The element 2 and the impeller 4 are spaced apart from one another, such that a gap is formed between the element 2 and the impeller 4, which functions as a sealing gap with geometrically identical faces.
In the execution according to the diagram in
The rotating element 22 and the nonrotating element 21 in the working example are made from a stainless steel material, coated in accordance with the invention with ta-C. The two mutually axially aligned end faces of the rotating element 22 and of the nonrotating element 21 are pushed against one another by the force-generating element 17. The result is a minimal gap. Friction is minimized by the ta-C coating. A lubricant film of conveying medium is formed in the gap between the faces of the rotating element 22 and of the nonrotating element 21 that are in contact. The arrangement 20 together with the device 13 prevents backflow from a pressure space 5 of the pump into a suction space 1 of the centrifugal pump.
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 003 855.7 | Jun 2020 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/067138 | 6/23/2021 | WO |
