Patent Publication Number: US-2011062119-A1

Title: Underwater marking with a plasma arc torch

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 61/242,175, filed Sep. 14, 2009 which is hereby incorporated herein in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present application relates to plasma arc torches configured to operate underwater, and associated methods. 
     2. Description of Related Art 
     Cutting with plasma arc torches is sometimes conducted underwater to reduce the noise associated with plasma cutting and minimize the adverse environmental impact of the cutting process. The water traps the plasma generated emissions and particulates produced by the cutting that otherwise would be discharged into the air. Additionally, underwater cutting reduces the amount of harmful glare, ultraviolet radiation, and noise to which workers may otherwise be exposed. 
     SUMMARY OF VARIOUS EMBODIMENTS 
     However, thus far the benefits of underwater operation of plasma arc torches have not been realized for marking. 
     The present disclosure in one aspect describes a method of operating a plasma arc torch on a workpiece. The method comprises submerging a surface of the workpiece underwater, producing a plasma arc with the plasma arc torch, and substantially surrounding the plasma arc with a flow of gas. The surface of the workpiece may be submerged at least two (2) inches underwater in some embodiments. The method further includes submerging at least a portion of the plasma arc torch underwater, and directing the plasma arc substantially surrounded by the flow of gas at the surface of the workpiece which is submerged underwater. The method also includes marking the surface of the workpiece which is submerged underwater with the plasma arc, whereby the plasma arc penetrates through only a portion of the thickness of the workpiece. The current used to produce the plasma arc during the operation of marking the workpiece may be between eight (8) and thirty-five (35) amperes 
     In some embodiments the method may further comprise directing the flow of gas at least one of around and along a body of the plasma arc torch to thereby generate a swirling protective air curtain which substantially surrounds the plasma arc, such as by using an air curtain attachment mounted on the body of the plasma arc torch. Thereby the method may further comprise directing the flow of gas between a nozzle of the plasma arc torch and the air curtain attachment and out of an outlet defined between the nozzle and the air curtain attachment. 
     In additional embodiments, the method may further comprise cutting completely through the thickness of the workpiece with the plasma arc produced by the plasma arc torch, and this may be conducted underwater after the marking operation. The current used to produce the plasma arc during the operation of cutting the workpiece may be between thirty (30) and seven-hundred and fifty (750) amperes. The method may further comprise maintaining the flow of gas at a substantially constant rate of flow at least throughout the operations of marking the workpiece and cutting the workpiece. Additionally, the nozzle of the plasma arc torch may not need to be replaced with an alternate nozzle between the operations of marking the workpiece and cutting the workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Having thus described the embodiments in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1A  illustrates a top view of a water table according to an example embodiment; 
         FIG. 1B  illustrates a side view of the water table of  FIG. 1A  according to an example embodiment; 
         FIG. 2  illustrates an air curtain attachment according to an example embodiment; 
         FIG. 3  illustrates a dry table according to an example embodiment; 
         FIG. 4  illustrates an alternate example embodiment of an air curtain attachment; and 
         FIG. 5  illustrates a method of operating a plasma arc torch on a workpiece according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Apparatuses and methods for marking a workpiece underwater now will be described more fully hereinafter with reference to the accompanying drawings in which some but not all embodiments are shown. Indeed, the present development may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     One operation for which plasma arc torches are commonly used is cutting, wherein the plasma arc produced by the plasma arc torch cuts completely through the workpiece. Previously, one method of plasma arc cutting was to cut the workpiece underwater, using a water table. Water tables, such as the embodiment of a water table  10  illustrated in  FIG. 1A-1B , may comprise an elevated tub  12  with a grate  14  comprising a plurality of metal bars  16  positioned therein. The grate  14  supports the workpiece which is to be operated upon. Prior to operation, the water table  10  fills with water or the grate  14  descends such that the workpiece is submerged under the water. In embodiments of water tables wherein the water level rises, this may occur via pumping water into the water table, or flowing compressed air into a chamber which displaces the water, and thereby causes the water level to rise. After the workpiece is submerged, the head of a plasma arc torch is also submerged into the water. 
     During cutting an “air curtain” or “gas bubble” is formed by a flow of gas near the cutting zone. This protects the plasma arc from being extinguished by the water. Using such a configuration, the gaseous emissions produced by the cutting may be captured by the water. Additionally, noise and ultraviolet light emissions produced by the cutting operation may be reduced. Although the air curtain may be produced by many different types of structures, one embodiment of an air curtain attachment  120  is illustrated in  FIG. 2  for exemplary purposes. Further, although the attachment will be described as an additional structure which is coupled to the plasma arc torch, the attachment may also be manufactured so as to be integral with the plasma arc torch. 
     As illustrated in  FIG. 2 , the example attachment  120  includes a cylindrical support body  122  formed from a material such as chromium plated brass. The cylindrical support body  122  includes a split upper portion  124 , which forms a clamping collar. A socket head cap screw (not shown) joins both sides of the clamp together to secure the cylindrical support body  122  to the outer surface of a plasma arc torch  100  (shown in phantom). The cylindrical support body  122  extends in spaced relation from a torch body  110  defined by the plasma arc torch  100 , and forms an annular opening  130 . 
     An insulating sleeve  132  is positioned between the cylindrical support body  122  and the plasma arc torch  100  for insulating the cylindrical support body from the torch body  110 . In this regard, the insulating sleeve  132  may be formed of a low grade phenolic. The insulating sleeve  132  may be secured to the inside surface of the cylindrical support body  122 . An O-ring  134  is secured within an internal groove of the insulating sleeve  132  and helps secure the insulating sleeve to the torch body  110 . During installation the cylindrical support body  122  and the insulating sleeve  132  may be slid onto the torch body  110  and positioned as shown in  FIG. 2 . 
     A cylindrical sleeve  140  is received into the annular opening  130  of the cylindrical support body  122 . The cylindrical sleeve  140  may be formed of anodized aluminum to form a light-weight, but strong structure that is resistant to corrosion. The cylindrical sleeve  140  extends in spaced relation along the front end of the torch body  110  to define an annular air chamber  142  extending along the front end and forming an annular outlet opening  144  positioned adjacent a nozzle  112  of the plasma arc torch  100 . The rear portion of the cylindrical sleeve  140  is received in the annular opening  130 . O-rings  146  are secured within annular grooves  148 , and help retain the cylindrical sleeve  140  to the cylindrical support body  122 . The outlet opening  144  defined by the cylindrical sleeve  140  may be between about 1/32 inch to about 1/16 inch. 
     As further illustrated in  FIG. 2 , the lower portion of the cylindrical support body  122  is diametrically enlarged to allow enough room to create the annular grooves  148  in which the O-rings  146  are positioned. At least one air channel orifice  150  also extends from the diametrically enlarged portion through the cylindrical support body  122  and the cylindrical sleeve  140 . The air channel orifice  150  terminates at the annular air chamber  142  and allows a high velocity gas to be injected into the annular air chamber  142  in swirling relation downward around and/or along the front end of the torch body  110  and through the outlet opening  144  for generating an evenly formed protective air curtain. An air fitting  152  is mounted on the diametrically enlarged portion of the cylindrical support body  122  and communicates with the air channel orifice  150 . Standard hoses (not shown) screw into the air fitting  152  and provide a source of high velocity gas. An enlarged air plenum  154  is defined between the cylindrical sleeve  140  and the inner surface of the cylindrical support body  122 . Thus, high velocity gas is first injected into the air plenum  154  before passing into the annular air chamber  142 . 
     The annular air chamber  142  also includes an enlarged air plenum  156  into which air is injected before passing downward through the annular air channel  142 . The torch body  110  has an annular groove  158  which forms the enlarged air plenum  156 . During operation, the high velocity gas is discharged into the air channel orifice  150  and into the first plenum chamber  154  as mentioned above. In one embodiment, the gas is distributed in the plenum chamber  154  and then moves through a plurality of evenly spaced orifices  150  that extend tangentially into the second plenum chamber  156 . The tangentially inclined orifices  150  provide a swirling gas flow within the plenum chamber  156 . The high velocity gas swirls downward through the annular air channel  142  around and along the torch body  110  and is discharged through the outlet  144  to form a protective air curtain for the plasma arc. The swirling high velocity gas forms an evenly distributed air curtain which helps prevent water flowing into the cutting zone. Additionally, the swirling high velocity gas expands outward after exiting the outlet  144  and forms a larger diameter air curtain than may be accomplished with other constructions. Thus, the water may be less prone to flow into the cutting zone than with other constructions. 
     Accordingly, underwater cutting may conducted using embodiments of an air curtain attachment as described above. However, the expense and effort required to dispose of the used water and clean the water table resulted in an industry shift to use of dry tables. Dry tables, such as the embodiment of a dry table  210  illustrated in  FIG. 3  typically rely on a downdraft system whereby a grate  214  comprising a plurality of metal bars  216  is positioned on top of a plenum  276  configured to suck the fumes emitting from the cutting operation down and through an exhaust  222  away from the workpiece  218  which is being operated on. The fumes may thereafter be filtered or otherwise treated before being exhausted to the environment. However, use of dry tables may be less effective at treating the emitted fumes. Additionally, dry table fume removal systems are also expensive, and they may not reduce the noise produced during cutting or the ultraviolet emissions from the plasma arc. Accordingly, there has been a trend to return to use of water tables for underwater cutting, particularly in Europe where some locations have stricter pollution limitations than in the United States. However, marking, which is another common operation conducted with a plasma arc torch, has thus far complicated the use of water tables by being conducted above water, as will be explained below. 
     Marking is an operation in which the plasma arc penetrates into the thickness of a workpiece only superficially. In order to accomplish this, marking uses a current which is relatively low as compared to a current used for cutting. For example, cutting with a plasma torch may involve use of currents in the range of thirty (30) to seven-hundred and fifty (750) amperes, whereas marking may involve currents in the range of eight (8) to thirty-five (35) amperes. Due to use of a much lower current, the fume, noise, and light emissions produced during marking may be significantly less than those produced by cutting. Accordingly, there has not been a motivation to conduct marking underwater. 
     Further, it was not expected that a plasma arc with a marking current would be able to operate underwater. In this regard, even the inventors of the present application were skeptical that a plasma arc would function underwater with a marking current. The inventors feared that a low current arc would be extinguished by the water. These fears were confirmed when the inventors attempted to mark underwater with a plasma arc torch lacking an air curtain attachment, and the plasma arc was found to be unstable and had a tendency to extinguish. The inventors suspected that the low current plasma arc would similarly extinguish when used in conjunction with an air curtain. This expectation was based on the inventors&#39; knowledge that when an air curtain is used, there is still some water splashing around inside the air curtain, and the surface of the workpiece remains wet. 
     Despite the skepticism of the inventors, an experiment was performed using a plasma arc torch having an air curtain attachment. An embodiment of a plasma arc torch  300  with an air curtain attachment  320  used in the experiment is illustrated in  FIG. 4 . Although the air curtain attachment  320  differs from the air curtain attachment  120  shown in  FIG. 2  and described above, the functionality and principles of operation are substantially the same. For example, a flow of gas enters the air curtain attachment  320  through an air fitting  352  and is directed around and/or along the torch body  310  to thereby generate a swirling protective air curtain which substantially surrounds a plasma arc produced by the plasma arc torch  300 . Thereafter, the gas is directed between a nozzle  312  of the plasma arc torch  300  and a sleeve  340  of the air curtain attachment  320 . Finally, the flow of gas exits through an annular outlet opening  344  to produce the swirling protective air curtain. 
     To the surprise of the inventors, a stable plasma arc was produced within the air curtain despite using a current configured for marking. Thus, the plasma arc torch was able to mark a workpiece. Accordingly, a method of operating a plasma arc torch on a workpiece was developed, as illustrated in  FIG. 5 . The method comprises an operation  402  of submerging a surface of the workpiece underwater. Further, as indicated at operation  404 , the method may comprise submerging the workpiece at least 2 inches underwater. Additionally, the method includes an operation  406  of producing a plasma arc with the plasma arc torch, and an operation  408  of substantially surrounding the plasma arc with a flow of gas. The method further comprises submerging at least a portion of the plasma arc torch underwater at operation  410 . For example, at least the nozzle may be submerged underwater. Also, the method may include directing the plasma arc substantially surrounded by the flow of gas at the surface of the workpiece which is submerged underwater at operation  412 . Further, the method comprises marking the surface of the workpiece which is submerged underwater with the plasma arc, whereby the plasma arc penetrates through only a portion of the thickness of the workpiece at operation  414 . A first current used to produce the plasma arc during the operation  414  of marking the workpiece may be between eight (8) and thirty-five (35) amperes in one embodiment. 
     With regard to the operation  408  of substantially surrounding the plasma arc with a flow of gas, the method may further comprise an operation  418  of directing the flow of gas at least one of around and along a body of the plasma arc torch to thereby generate a swirling protective air curtain which substantially surrounds the plasma arc. Further, an air curtain attachment mounted on the body of the plasma arc torch may direct the flow of gas around and/or along the body of the plasma torch at operation  420 . For example, either of air curtain attachments  120  and  320  illustrated in  FIGS. 2 and 4  may be used. Additionally, at operation  422 , the flow of gas may be directed between a nozzle (for example, nozzle  112  or  312 ), and the air curtain attachment. Thereafter, the flow of gas may be directed out of an outlet defined between the nozzle and the air curtain attachment (for example, the annular outlet opening  144 ,  344 ) at operation  424 . 
     Further, the method may comprise cutting completely through the thickness of the workpiece with the plasma arc produced by the plasma arc torch at operation  426 . The cutting operation  426  may be conducted after the marking operation  414  because the workpiece might shift positions after being cut, although other orders of operation are possible. The cutting operation  426  may use a current of between thirty (30) and seven-hundred and fifty (750) amperes to produce the plasma arc. Further, the cutting operation  426  may be conducted underwater, as noted at operation  430 . As shown at operation  434 , the flow of gas may be maintained at a substantially constant rate of flow at least throughout the marking operation  414  and the cutting operation  426 . Additionally, the nozzle need not be replaced with an alternate nozzle between the operations  414 ,  426  of marking the workpiece and cutting the workpiece. 
     Accordingly a method of marking and a method of marking in conjunction with cutting is provided. The method of marking underwater provides great efficiency benefits which have hereto been unrealized for the various reasons discussed above. Now, as a result of marking and cutting both being conducted underwater, there is no need to raise or lower the level of the water with respect to the workpiece between the marking and cutting steps. Previously, since methods of marking underwater using a plasma arc torch were not available, it was necessary to mark above water, which involved raising or lowering the water level in between cutting and marking, depending on the order of operation. Further, as a result of the use of a single nozzle and the same gas flow rate for the air curtain for both the marking and cutting steps, rapid changes from marking to cutting and vice versa may occur. Therefore, the methods presented herein achieve the unexpected result of being able to both cut and mark underwater, which may provide significant cost savings as a result of not requiring lowering or raising the water level. Further, the methods achieve the advantages of reduced fume, light, and noise pollution, as discussed above. 
     Many modifications and other embodiments will come to mind to one skilled in the art to which these embodiments pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.