Abstract:
In a method for producing a water jet ( 45 ) that contains abrasive and emerges under high pressure from a nozzle ( 44 ), uninterrupted operation with, at the same time, greater working performance and lower costs is made possible in that, in a first step, an abrasive suspension ( 34 ) containing abrasive and water is provided at normal pressure, in a second step the provided abrasive suspension ( 34 ) is brought to a working pressure that is greatly above normal pressure, and in a third step a water jet ( 45 ) containing abrasive is produced, with a nozzle ( 44 ), from the abrasive suspension ( 34 ) that is under the working pressure.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to German application number No. 10 2009 043 697.9, filed 1 Oct. 2009, the entirety of which is incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Field of Endeavor 
         [0003]    The present invention relates to the field of working on workpieces with water jets. It concerns a method for working on, in particular cleaning, a workpiece with a water jet that contains abrasive and emerges from a nozzle under high pressure, and to a water jet installation useful for executing the method. Moreover, the invention relates to a method for application of the water jet method. 
         [0004]    2. Brief Description of the Related Art 
         [0005]    Components of power plant installations are subject to high mechanical and thermal load during their operation. This applies particularly to gas turbine components exposed to the flow of hot gas, whose surfaces, in addition to being exposed to the extreme mechanical and thermal loads, are additionally exposed to unwanted thermal and chemical reactions with the formation of non-metallic layers, such as scale or corrosion coverings, with negative effects on the operating behavior. This necessitates regular service intervals for checking the state of these components and removing and/or cleaning, repairing or, if necessary, replacing them. 
         [0006]    Methods for cleaning gas turbine components such as, for example, blades, are known in a multiplicity of realizations. The methods that are known and have been introduced in this field include that of sand blasting. Air that is compressed to a plurality of bars and to which an abrasive material is added, is directed on to the surface to be treated. The particles of the abrasive material impacting with high energy on the surface produce a cleaning effect. Disadvantages of this method, however, are an imprecise scattering and a relatively coarse removal of material, with disadvantageous alterations of the surface quality of the workpiece. 
         [0007]    Another type of cleaning method is based on the high-pressure water jet technique, wherein pure water jets or water jets mixed with an abrasive are applied to the surface to be cleaned. The high-pressure water jet technique uses water pressures of up to 600 MPa, in order to produce a high-power water jet. Such a high-power water jet can be used as a tool for cutting or cleaning applications that acts in all directions. 
         [0008]    Depending on the respective application, water jets operating according to three differing principles are used, namely: 
         [0009]    (1) pure water jets (see  FIG. 1 ), 
         [0010]    (2) water jets containing abrasive that are generated through injection of an abrasive into a previously produced pure water jet (abrasive injection water jets, AIWJ; see  FIG. 2 ), and 
         [0011]    (3) water jets containing abrasive in which the jet is produced through a pressurized suspension of the abrasive emerging from a nozzle (abrasive suspension water jets, ASWJ; see  FIG. 3 ). 
         [0012]    In the case of the first principle, represented in simplified form in  FIG. 1 , in a water jet installation  10  water is fed to a pressure pump  12  via a water feed line  11  and pumped at high pressure into a pressure line  13 , which leads to a suitable nozzle  14 . The water under high pressure in the pressure line  13  then emerges from the nozzle  14  as required, forming a high-energy water jet. Such pure water jets can be used to cut soft materials such as, for example, fabrics, leather, solidified foams, foodstuffs, etc. 
         [0013]    For cleaning applications, it is mainly systems operating with a pure water jet that are used. Typical parameters for cleaning with a pure water jet are working pressures of up to 300 MPa and volumetric flow rates of approximately 30 liters/min, which result in a high energy consumption (up to 150 kW). Corresponding high-pressure pumps are likewise very expensive. 
         [0014]    In the case of the second principle, represented in  FIG. 2 , in a water jet installation  20  water is again fed to a pressure pump  12  via a water feed line  11  and pumped at high pressure into a pressure line  13 , which leads to a suitable nozzle  14 . The water under high pressure in the pressure line  13  then emerges from the nozzle  14  as required, forming a high-energy water jet. In a succeeding mixing tube  16 , an abrasive is then admixed to the pure water jet, in an injection device  17 , which abrasive has been brought via an abrasive feed  18 . A high-energy water jet  19  containing abrasive then emerges at the end of the mixing tube  16 . Such an installation is described, for example, in the printed publication WO-A1-2005/051598. Such AIWJ jets (abrasive injection water jets) are used mainly in stationary cutting applications. They can be used to cut all technical materials, such as: 
         [0015]    all metals (steel, aluminum, copper, titanium, etc) 
         [0016]    glass 
         [0017]    synthetic materials 
         [0018]    composite materials, and 
         [0019]    concrete. 
         [0020]    The ASWJ jets (abrasive suspension water jets) produced according to the third principle are generally used for mobile and special applications. The advantages of the ASWJ jets, as compared with the AIWJ jets produced according to the second principle, are a higher efficiency (higher by a factor of up to 4-5) and the possibility of being able to use these jets in all positions and environments. 
         [0021]    In the case of the third principle, represented in  FIG. 3 , in a water jet installation  30  water is again fed to a pressure pump  12  via a water feed line  11  and pumped at high pressure (up to 200 MPa) into a pressure line  13 , which leads to a suitable nozzle  14 . At a T-piece  21 , the flow of water is divided. One portion flows directly to the nozzle  14 , via a first choke valve  27  and a mixing piece  28 . A second, smaller portion flows in a bypass line  23 , via a second choke valve  22 , into a pressure tank  24  that is filled with abrasive and that is refillable after removal of a blind plug  25 , and from there flows, via a shutoff valve  26 , to the mixing piece  28 . As the water flows through the pressure tank  24 , it carries the abrasive particles along with it. Then, in the mixing piece  28 , the resultant water/abrasive mixture is put into the main water flow. The proportion of abrasive in the water jet  29  that contains abrasive and emerges from the nozzle  14  can be controlled by the choke valves  22  and  27 . Such a system is described, for example, in the printed publication DE-A1-199 09 377. 
         [0022]    The main disadvantages of the currently known systems operating, according to the third principle, with pressures of between 50 MPa and 200 MPa are: 
         [0023]    the imprecise control of the proportion of abrasive in the suspension; 
         [0024]    the lack of possibility of continuous operation, since after a certain period of time it is necessary to interrupt operation and refill the pressure tank with abrasive; and 
         [0025]    the high working pressures require correspondingly dimensioned components of the water jet installation, with the consequence of more difficult handling and a limited scope of application in respect of confined spatial conditions. 
       SUMMARY 
       [0026]    One of numerous aspects of the present invention includes a method, in particular suitable for cleaning applications, for treating workpieces with a water jet that contains abrasive and emerges under high pressure from a nozzle, which method can be operated continuously and avoids the disadvantages of known methods described above, and a water jet installation for executing the method. Another aspect includes providing such a method and such an installation that meet the requirements of use for power plant installations, for example turbines. This domain of application requires effective use in confined spatial conditions, such as in narrow gaps, and moreover is highly demanding with respect to the surface quality following the working operation. 
         [0027]    Another aspect of the present invention includes, in a first step, an abrasive suspension containing abrasive and water is provided at normal pressure, in a second step the provided abrasive suspension is brought to a working pressure that is above normal pressure, and in a third step a water jet containing abrasive is produced, with a nozzle, from the abrasive suspension that is at the working pressure. 
         [0028]    Owing to the preparation of the suspension being effected at normal pressure, suspension can be provided continuously, without the need to interrupt the production and application of the jet. In a manner known per se, in this case the abrasive contained in the water very greatly augments the cleaning effect of the jet. 
         [0029]    According to one exemplary embodiment of the invention, a mixture of water and the abrasive is produced in an open mixing vessel, for the purpose of providing the abrasive suspension that is at normal pressure. It is thereby ensured that the suspension in the mixing vessel can be replenished without difficulty at any time. 
         [0030]    Preferably, the mixture in the mixing vessel is kept continuously in motion, in particular by an agitator. 
         [0031]    Another exemplary embodiment of the method is distinguished in that a working pressure of a plurality of MPa, in particular of approximately 15 MPa to 25 MPa, is used. The comparatively low working pressure makes it possible to use less expensive components (e.g., pumps) and reduces the energy consumption. In addition, another advantage of the invention includes that the low working pressure allows the use of small-dimension and flexible components of the water jet installation, such as pressure lines and cleaning heads, as a result of which even those surfaces that are difficult to access can be treated effectively. As a result, in certain cases, it is possible to dispense with the resource-intensive removal of the workpieces to be cleaned. In the power plant industry, above all, this constitutes an advantage not to be underestimated, resulting in considerable cost savings for the power plant operator. 
         [0032]    According to a further preferred embodiment, an abrasive having a hardness of at least 7 according to the Mohs scale is added to the water. The particles of the abrasive have a diameter in the range from 0.1 mm to 0.3 mm. 
         [0033]    Preferably, the abrasive suspension is brought to the working pressure by a pump, and the abrasive suspension brought to working pressure is routed from the output of the pump directly to the nozzle, via a pressure line, a diaphragm pump, in particular, being used as a pump. 
         [0034]    An embodiment of the water jet installation according to principles of the present invention is characterized in that the pump is a diaphragm pump, the diaphragm pump has a pump chamber that is delimited by a diaphragm and connected to the intake line via an inlet valve and connected to the pressure line via an outlet valve, and the valves each have a valve sleeve, which constitutes a central valve passage and which is closed, at the downstream end, by a closing element that rests on a valve seat and that is spring-biased contrary to the direction of flow. In comparison with other pump types, such as piston pumps, the use of a diaphragm pump has the advantage of low wear. 
         [0035]    A preferred further development is characterized in that the valve sleeve and the closing element of the valves are produced from a hard metal, in particular tungsten carbide, and the valve seats are ground-in. 
         [0036]    In particular, the closing element is ball-shaped in the region corresponding to the valve seat, and is biased in the closing direction by a pressure spring. 
         [0037]    Another embodiment of the installation according to principles of the present invention is characterized in that a pressure relief valve is arranged in the pressure line. 
         [0038]    Preferably, the mixing vessel has an agitator equipped with a motor, and is realized as an open vessel. 
         [0039]    Methods embodying principles of the present invention can be used, advantageously, for cutting and/or cleaning tasks in the case of power plant components, in particular boilers, heat exchangers and turbines. 
         [0040]    Through application of features defined more fully herein, it has become possible, for the first time, to combine in an advantageous manner the advantages of various known methods of the water jet technique and thereby to open up new application possibilities for this technique. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    The invention is to be explained more fully in the following with reference to exemplary embodiments in conjunction with the drawing, wherein 
           [0042]      FIG. 1  shows the simplified diagram of a water jet installation, according to the prior art, operating with pure water; 
           [0043]      FIG. 2  shows the simplified diagram of a water jet installation, according to the prior art, operating with added abrasive, according to the injection principle; 
           [0044]      FIG. 3  shows the simplified diagram of a water jet installation, according to the prior art, operating with abrasive suspension; 
           [0045]      FIG. 4  shows the simplified diagram of a water jet installation, according to an exemplary embodiment of the invention, operating with abrasive suspension; 
           [0046]      FIG. 5  shows the longitudinal section through a conventional inlet or outlet valve of a diaphragm pump suitable for the installation according to  FIG. 4 ; and 
           [0047]      FIG. 6  shows the longitudinal section through an inlet or outlet valve modified with respect to  FIG. 5  and optimized for the installation according to  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0048]    The simplified diagram of a water jet installation operating with abrasive suspension, according to an exemplary embodiment of the invention, is reproduced in  FIG. 4 . The water jet installation  40  includes a mixing vessel  31 , a diaphragm pump  36  that on its input side is connected, via an intake line  35 , to the mixing vessel  31 , and a nozzle  44  connected to the output of the diaphragm pump  36  via a pressure line  39 . 
         [0049]    An abrasive suspension  34  is mixed and held ready under normal pressure in the mixing vessel  31 . An agitator  33 , which is driven by a motor  32 , is provided to mix and maintain the abrasive suspension. The mixing vessel  31  can be open at the top, such that the components of the abrasive suspension can be replenished if required and without interruption of operation. The operation under normal pressure facilitates considerably the controlled addition of water and abrasive to the mixing vessel  31  for the purpose of maintaining a constant mix ratio. Variants of an automated loading of the mixing vessel  31  are preferred in this case, and can be realized with comparatively simple technical systems. Continuous operation of the water jet installation is therefore ensured with a small amount of equipment. 
         [0050]    The diaphragm pump  36 , which has a pump chamber  38  delimited by a diaphragm  37 , draws in suspension from the mixing vessel  31 , via an inlet valve  41 , during an intake stroke (movement to the left in  FIG. 4 ) and, in a working stroke (movement to the right in  FIG. 4 ), forces it at high pressure into the pressure line  39 , via an outlet valve  42 . The suspension flows via the pressure line  39  (in which a pressure relief valve is arranged, in order to prevent damage to the pump  36  as a result of excess pressure) directly to the nozzle  44 , which is composed of hard metal (tungsten carbide). There, a water jet  45  is realized, which contains abrasive and which, depending on the requirement of the application, can be of a concentrated, spread or other form. 
         [0051]    Owing to the abrasive component in the water jet, the pressure in the pressure line  39  can be reduced, as compared with the technique operating with pure water ( FIG. 1 ), from 200 MPa to 15 MPa to 25 MPa, preferably 20 MPa, without impairing the cleaning effect. This allows the use of small-dimension pressure lines in the form of flexible tubes having diameters of less than 12 mm. Such flexible tubes have a high flexibility (radius of bend less than 50 mm) and are therefore also suitable for use in confined spatial conditions such as, for example, those prevailing within the blading of turbines. 
         [0052]    Owing to the abrasive component in the pumped suspension, a diaphragm pump  36 , the structure and function of which are described, for example, in the printed publication U.S. Pat. No. 6,899,530, can be used instead of a conventional piston pump. These pumps are normally used for pumping corrosive and abrasive media, but at comparatively low pressures. In the present application, the drawn-in suspension is brought to pressures of approximately 15 MPa to 25 MPa by such a pump. Operation at these pressures is achieved in that the inlet and outlet valves  41 ,  42 , which are subject to particular wear, have been modified according to  FIGS. 5 and 6 . 
         [0053]    Diaphragm pumps are pumps that operate volumetrically, which produce pressure through the mechanical displacement of synthetic diaphragms. In order to achieve a constant pressure and flow, each pump chamber ( 38  in  FIG. 4 ) is equipped with two valves ( 41 ,  42  in  FIG. 4 ). A pump contains mostly three to five such pump chambers. Owing to the high flow velocity of the abrasive suspension upon opening of the valves, it is mainly the latter that are subject to wear (the erosion is very largely dependent on the velocity of the eroding particles). 
         [0054]    The standard design of the valves of the pump chamber of a diaphragm pump of the type described is reproduced in  FIG. 5 : the valve  42 ′ of  FIG. 5  includes an (annular) valve sleeve  46 , which delimits a central valve passage  50 . At the downstream end of the valve sleeve  46 , a disc-shaped closing element  48 ′ is pressed against a valve seat  47 ′ by a pressure spring  49 , and thus closes off the valve passage  50  and therefore the adjoining pump chamber. If pressure is generated in the pump chamber  38  by the diaphragm  37 , the closing element  48 ′ lifts away from the valve seat  47 ′, against the pressure of the spring  49 , and a volumetric flow leaves the pump chamber  38  through the associated valve passage. 
         [0055]    In the case of the valve  42 ′, a main problem consists in that, if the valve does not close properly, or no longer closes properly, high local flow velocities occur at the site of the leakage, and erode the closing element  48 ′ and the valve sleeve  46  to a very great extent. Even tungsten carbide valves become thus eroded in less than half an hour. The reason for the lack of tightness in the case of such standard valves is the lack of centering of the disk-shaped closing element  48 ′ in the valve sleeve  46 : the closing element  48 ′ does not have sufficient guidance and, owing to the (flat) shape of the standard closing element  48 ′ (ground-in radius of the valve seat  47 ′), there are some regions in which there is no surface contact between the closing element  48 ′ and the valve seat  47 ′ if the closing element  48 ′ is not perfectly centered. 
         [0056]    In order to remedy this, the valve geometry has been altered, according to  FIG. 6 . The closing element  48  of the valve  42  now has the shape of a ball, or portion of a ball. The result of this is that, even if the closing element  48  is not perfectly centered, there is nevertheless surface contact on the entire circumference of the valve seat  47 , and tightness is achieved. At the same time, the contact surface on the valve seat  47  has been enlarged considerably. Moreover, all sealing surfaces have been ground-in, in order to achieve a good seal. Tungsten carbide is used as a material for the closing element  48  and the valve sleeve  46 . It has been found that, because of these measures, the required service intervals can be extended considerably. Intervals of 50 operating hours and more have been found to be sufficient. 
         [0057]    With an installation according to  FIG. 4 , a water jet containing abrasive can now be produced with a pressure of approximately 15 MPa up to 25 MPa in continuous operation, which water jet can be used, particularly advantageously, in the domain of power plant engineering. In particular, the following cleaning tasks can be performed: 
         [0058]    In the case of steam boilers, the tubes of the tube bundle can be cleaned. 
         [0059]    In the case of turbines, the blading or other components can be cleaned, it being possible, frequently, to dispense with removal of the same since, according to principles of the present invention, even spaces between the blades can be cleaned in an effective manner when in the mounted state. This allows considerable cost savings as compared with conventional methods of cleaning. 
         [0060]    Furthermore, advantageously, according to principles of the present invention, surfaces in power plants can be worked on: 
         [0061]    Water jet honing: the central bores of steam turbine rotors are worked on. This enables the machine times to be reduced considerably, as compared with conventional methods. 
         [0062]    Blade reconditioning: the blade surfaces of gas turbines are worked on, in order to remove surface cracks. 
         [0063]    As compared with the systems based on pure water jets, the following advantages can be achieved in this case: 
         [0064]    Lesser energy consumption; 
         [0065]    Improved cleaning performance; 
         [0066]    Settable surface characteristics in the case of the surfaces to be worked on; 
         [0067]    Superior surface quality; 
         [0068]    Settable material removal rates; 
         [0069]    Improved handling capability, owing to the reduced pressure; 
         [0070]    Smaller dimensions of feed lines (for example, having a flexible tube diameter of less than 12 mm) and nozzles; 
         [0071]    Smaller radius of bend of the feed line, of less than 50 mm, allows use in confined spatial conditions, even in narrow gaps; and 
         [0072]    Lower costs of the installation. 
       LIST OF REFERENCES 
       [0000]    
       
         
           
               10 , 20 , 30 , 40  water jet installation 
               11  water feed line 
               12  pressure pump 
               13 , 39  pressure line 
               14 , 44  nozzle 
               15  water jet 
               16  mixing tube 
               17  injection device 
               18  abrasive feed 
               19 , 29 , 45  water jet (containing abrasive) 
               21  T-piece 
               22 , 27  choke valve 
               23  bypass line 
               24  pressure tank (with abrasive) 
               25  blind plug 
               26  shutoff valve 
               28  mixing piece 
               31  mixing vessel 
               32  motor 
               33  agitator 
               34  abrasive suspension 
               35  intake line 
               36  diaphragm pump 
               37  diaphragm 
               38  pump chamber 
               41  inlet valve 
               42 , 42 ′ outlet valve 
               43  pressure relief valve 
               46  valve sleeve 
               47 , 47 ′ valve seat 
               48 , 48 ′ closing element 
               49  pressure spring 
               50  valve passage 
           
         
       
     
         [0106]    While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.