Abstract:
A plug assembly includes a plug stem having a plug stem base, a plug head, and a fastening mechanism for fastening the plug stem base to the plug head, the fastening mechanism comprising at least two retainer clamps that surround and provide an interference fit between the plug stem base and the plug head. A method of replacing or attaching a plug head to a plug stem includes providing a plug stem having a plug stem base, providing a plug head, providing a fastening mechanism comprising at least two retainer clamps, and coupling the fastening mechanism around the plug stem base and the plug head to provide an interference fit between the plug stem base and the plug head.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a utility conversion of U.S. Provisional Patent Application Ser. No. 61/059,682, filed Jun. 6, 2008, for “PLUG ASSEMBLY WITH PLUG HEAD ATTACHMENT.” 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to industrial valves. More specifically, this invention relates to devices for attaching valve plug heads to valve plug stems. 
       BACKGROUND OF THE INVENTION 
       [0003]    Valves and valve plugs are well known in the art. Typically, valve plug heads are positioned within the valve to control the volume of flow passing through the valve. By modifying the position of the plug head relative to the valve seat, control of the flow volume is achieved, thus allowing diversion and restriction of fluid flow. Plug heads are subjected to fluid forces, chemical attack, thermal stresses, impact from particulates and debris, as well as the forces used to attach it to the plug stem and seat loading forces that can occur when the plug head comes into contact with the valve seat. The valve head is typically attached to a plug stem, which in turn is connected to an actuating device. This actuating device is controlled to move the plug stem, which acts to change the position of the plug head to control the flow passing through the valve. 
         [0004]    The plug stem is subjected to axial forces as the actuator moves it, mounting forces relating to the actuator attachment, and the long cylindrical section is subjected to bending forces. The plug head and the plug stem perform distinctly different purposes and are subjected to very different forces. The plug head, sitting in the middle of the flow stream, diverts and/or restricts flow, and is subjected to fluid and seat loading forces and to forces related to attaching the plug head to the plug stem. In contrast, the plug stem is moved by an actuating device to provide a sealing surface and is subject to axial and bending forces. In industrial, high volume/flow rate valves, the forces placed on plug stems and plug heads are typically significant contributors to valve failure. 
         [0005]    Traditionally, valve plug heads are either composed of one monolithic material or make use of more than one material. Plug heads employing more than one type of material have particular advantages, in particular, better erosion and corrosion resistance, improved shock absorption, working life, and thermal expansion qualities. However, the use of a plurality of material types has been limited by the ability to effectively join the materials together economically and without creating stress points that limit the life of the plug head. 
         [0006]    The most common methods of fixing dissimilar materials together in a valve plug are taper fitting or interference fitting, both of which employ a retaining ring that is fixed around the plug head. Taper fittings have been shown typically to subject the plug head to undesirable stresses, contribute to thermal expansion problems, and are difficult to repair. The typical taper fitting design requires a mating of two conical surfaces, one on the plug head, and the other on the retaining ring. Since neither the plug head nor the retaining ring can be manufactured with ideal cone shapes, the plug and seat may not mate perfectly. As such, loading between the two mated structures may not be uniform. Additionally, the force of the retaining ring on the plug head is exerted close to the edge of the plug head and is generally perpendicular to the angle of the conical surface. The location and angle of the force can introduce undesirable tensile forces into the portion of the plug head that bears the force. Often the desired plug head material may demonstrate weak tensile strength, thus introducing additional tensile forces that can either limit the selection of plug head materials or that can cause breaking of the edge of the plug head, separating the plug head from the plug stem and causing valve failure. Also, as the retaining ring wears away through normal corrosion and erosion, the shape of the contact area can change, typically moving closer to the edge of the plug head. This contact area change tends to concentrate forces on the edge of the plug head and increases the likelihood that the edge of the plug head will fracture, thereby causing the plug head to separate from the plug stem. The stresses induced with the taper fit are difficult to quantify and, therefore, can detract from a valve plug&#39;s performance. Variables in the welding process, such as weld shrinkage, inter-pass temperature, amperage of weld, inert gas environment, and the amount of initial burn-in, can change the amount of stress in the plug head. 
         [0007]    As noted above, typical prior taper fit designs attach the taper fit ring to the plug stem via welding. This approach results in the retaining ring and the plug stem becoming permanently joined into one component. If the plug head wears away or breaks and the plug stem is still usable, the typical taper fit design does not lend itself to achieving the proper concentricity between the plug head and the plug stem after the plug head has been replaced. When a taper fit valve plug is repaired, the plug stem has already been machined, so it is not possible to make adjustments in the plug stem to ensure concentricity with the plug head. If the plug head is misaligned, adjustments cannot be made without cutting the taper fit ring off. For at least these reasons, taper fit valve plugs are usually discarded (as opposed to being repaired) when the plug head has broken or worn away. During assembly, the taper fit ring is typically fit tightly around the ceramic plug and the taper fit ring is welded to the plug stem. At elevated operating temperatures, the taper fit ring increases in size more than the plug head, and the plug head becomes somewhat loose in the taper fit ring, which thereby leads to early failure of the fit in operating conditions. 
         [0008]    Interference fittings typically require a bulkier retaining ring, contributing to the load on the plug head. Interference fittings also require more complex procedures to replace plug heads and are generally limited in their service temperature ranges. An interference fit achieves more uniform loading of the plug head than does the taper fit. However, the typical interference fit uses a one-piece retaining ring that not only holds the plug head but also attaches the plug head/retaining ring assembly to the plug stem. The interference fit also must have sufficient material to allow for the wear due to erosion and corrosion without causing the plug head to separate from the plug stem. These requirements result in a bulkier retaining ring than is required to hold the plug head in place, which contributes to an additional load on the plug head. This additional load introduces tensile stresses, which tend to contribute to plug head breaking and separation, which can result in valve failure. 
         [0009]    Even with interference fit designs, the task of replacing the plug head is quite complex. To replace the plug head, the interference fit ring must be cut, separating the plug head and ring assembly from the plug stem. This process is usually performed on a lathe or mill. If the ring is to be used again, it is necessary to separate the ring from the plug head. Certain combinations of plug head and interference fit ring materials can be separated by heating the assembly in an industrial oven. If the coefficient of thermal expansion of the retaining ring is sufficiently higher than the plug head, the retaining ring will expand more quickly and the interference fit will be negated as a space forms between the two surfaces. This approach is somewhat destructive and requires that the interference-fit ring be carefully checked before reuse. Also, this heating method only works with certain combinations of materials. Moreover, even when it may work, the plug head replacement process requires specialized manufacturing facilities that are generally unavailable to users in remote locations. Therefore, replacing plug heads for valve plugs is not a typical industry practice for certain combinations of materials or user locations. 
         [0010]    Another problem with interference fittings is that service temperature ranges are limited because of differential thermal expansion between the plug head and ring materials. The amount of interference between the plug head and the ring is directly related to the amount of stress in a plug head. The amount of interference at ambient temperature becomes a concern when it places large amounts of stress on the plug head. Thus, when the valve plug is installed and is warming to operating temperature, the plug head is more highly stressed and is more vulnerable to failure. It has also been observed that because of these stresses, certain valve plugs, head and rings, could not be used because the ambient temperatures, or below ambient storage temperatures, could cause the plug head to fail before they could placed into service. 
         [0011]    Also, both taper fittings and interference fittings suffer from the impracticalities of stress relieving heat affected weld zones with heat treatments. For highly corrosive fluid applications and with certain materials, it is important to stress relieve heat affected weld zones with heat treatments. With both prior existing taper fit and interference fit designs, this has not been considered practical because stress-relieving typically is performed at temperatures high enough to allow the plug head to be excessively loose in the ring, and it is not possible to assure that the plug head would return to its proper position upon cooling. Therefore, even though heat treatments might be beneficial, they have generally been avoided. 
         [0012]    In view of the foregoing shortcomings, it would be desirable to provide a valve plug design that uses a clamping system to attach the valve plug head to its valve plug stem, and to thereby provide a means of assembling and replacing worn plug heads in the field, while allowing use of different materials for the plug head and the plug stem, where the different materials are selected specifically to address the different function of the plug head and the plug stem. This type of plug design is particularly desirable for use in flow streams that are erosive or corrosive in nature, because plug heads in these kinds of streams typically suffer material loss due to the erosion and/or corrosion and require regular replacement. Often the plug head wears out before other valve components. Therefore, minimizing the occurrences when the plug head fails and must be replaced is very desirable in improving the life cycle and efficiency of the valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  illustrates a side view of a plug head and a plug stem according to an embodiment of the present invention; 
           [0014]      FIG. 2  illustrates an enlarged view of clamps, plug stem, and plug head according to an embodiment of the invention; 
           [0015]      FIG. 3  illustrates an enlarged side view of a plug head and a plug stem according to an embodiment of the present invention; 
           [0016]      FIG. 4  illustrates a representative system of a valve plug according to an embodiment of the present invention in a valve assembly; and 
           [0017]      FIG. 5  illustrates a section view of a valve plug having a plug head mounted to plug stem base of a plug stem according to an embodiment of the invention. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0018]    One embodiment of the present invention includes attaching a ceramic (or other sacrificial material) plug head to a plug stem. The embodiment uses two retainer half rings (clamps) and two or more bolts/nuts to hold the head onto the plug stem. In this fashion, replacement of the plug head can be easily and quickly accomplished in the field. The two clamps are configured such that a gap is left therebetween on both sides. The bolts/nuts are sacrificial bolts which are inexpensive. Rather than undo the bolting, these bolts can easily be cut off and thrown away. The bolting can then be replaced and a new plug head installed, reusing all of the major components. Other embodiments may use three or more retainer rings (clamps) to hold the plug head onto the plug stem. It is understood that all modifications and embodiments discussed herein may also be adapted to include three or more retainer rings (clamps). 
         [0019]    When ceramic plugs are used, it is common to replace field worn plug heads with new ones. These applications often involve scaling, erosion and high temperatures. This design has several advantages over current designs. This design is easier to work on in the field and the factory. Assembly of this design is simpler than previous designs. Some old designs even required shrink fitting and welding of the parts to hold them together. This precluded field assembly of plugs. This design allows simple field assembly. Assembly in the factory is also simplified. The gap left between the two clamps is important as many of the services which require erosion-resistant plug heads have solids in the fluid stream. Solids tend to build up on parts. When threaded parts are used it can make undoing threads impractical or impossible, which would make reusing the stem impossible. 
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    An embodiment of the present invention includes a plug assembly that includes a plug head made of a sacrificial material that is attached to a plug stem with two retainer half rings. The plug assemblies of this invention are adapted for use in industries such as, for example, mining, chemical processing, and oil and gas refining, where the flow is abrasive and/or corrosive and which may contain substantial quantities of sediment, debris or scale. Valves in certain erosive and/or corrosive flow streams encounter a significant amount of sediment, debris or scale which comes through the pipe line. This invention provides plug heads and stems being made of dissimilar materials having different properties that optimize the performance of the plug head, the plug stem and the fastening band. Also, this invention is adapted to ease the process of repair and replacement of valve plug components, permitting maintenance to be accomplished in the field without requiring specialized manufacturing equipment or highly skilled personnel. The invention is also adapted to provide a tight shutoff by permitting improved alignment of the plug head and seat ring. 
         [0021]    In a particular embodiment of the invention, the plug assembly allows the plug head to shift from side to side. When control valves are produced, there is always a certain amount of variation in the parts and assembly. However, in order to provide tight shutoff, the plug head and seat ring must line up perfectly. Since manufacturers cannot make the parts perfectly, plug assemblies need a certain amount of adjustability. 
         [0022]    In most valves, the seat ring can shift a little from side to side to allow the seat ring to center up on the plug. This is typically done by keeping the bonnet a little loose while stroking the plug into the seat repeatedly. Once the seat ring has moved to center itself, the bonnet is then tightened, locking the seat into place. This process is described in detail in the Mark One User Manual (VLENIM0001) steps 7.9 to 7.9.2, the contents of which are incorporated by reference herein. Some valve designs allow the bonnet to shift a little from side to side instead of shifting the seat ring, as when a screwed in seat is used. In the case of survivor, the seat ring is pinched between the valve outlet and the valve body. Performing a seat centering procedure is more difficult and is described in the Survivor User Manual (VLENIM0036), the contents of which are incorporated by reference herein. 
         [0023]    Because this process is so difficult to perform, one particular embodiment of the present assembly is configured to allow the plug head to float (instead of the bonnet or seat ring). This particular embodiment allows the plug head to move slightly from side to side. This action allows the plug head to find a natural center on the seat ring, providing the best shutoff possible. 
         [0024]    Referring to  FIGS. 1 and 3 , a particular embodiment of the plug head and plug stem of the present invention is illustrated. A valve plug  100  is shown having a plug head  101  held in a plug stem base  103  which in turn is mounted on the plug stem  102 . As shown in  FIG. 2 , the plug head  101  includes a base portion with a beveled edge  110 . A distal portion of the plug stem  102  includes a beveled edge  112 . The plug head  101  is held in the plug stem base  103  by two clamps (half rings)  104  which provide an interference fit between the plug head  101  and the plug stem  102 . The two clamps  104  are fitted over the plug head  101  and the plug stem base  103  and are tightly held in place by four bolts  120 . In a particular embodiment of the invention, the bolts  120  are fixed in place by nuts  107   a,    107   b.  Alternatively, two clamps can be held in place by pins, screws, welds, brazing, clamps or the equivalent. Additionally, the clamps  104  can include a hinge mechanism or tongue-and-groove mechanism to hold the two clamps  104  together on one end, while relying on two bolts  120  to secure the two clamps  104  together at an opposing end. The two clamps  104  can provide shock absorbing capabilities and stress relief to the plug head  101  during use. 
         [0025]    The preferred plug head  101  can be composed of structural ceramics because of its resistance to wear and degradation in flow streams that are erosive (having fine-grit particles) and corrosive (due to the chemical composition of the flow). Structural ceramics are a class of materials that includes, but is not limited to silicon carbide, silicon nitride, aluminum oxide, zirconium oxide, tungsten carbide, whisker-reinforced blends of ceramics, two-phase ceramics, and the like. Alternative materials which may be substituted for structural ceramics for the plug head  101 , include, but are not necessarily limited to, cermets, which are compounds that are combinations of ceramics and metals, cast iron, silicon iron, white iron, heat treated martensitic steels (such as 440 or 416 grade steel), CrCoFe alloys (such as STELLITE® M alloy 3, STELLITE® alloy 6, and STELLITE® 12), or other metals. Alternative materials with similar properties can be substituted without departing from the concept of this invention. 
         [0026]    The plug stem  102 , plug stem base  103  and clamps  104  can be composed of materials selected for ease of machining to a smooth surface, having good tensile strength, reasonable ductility and cost effectiveness. Included within this class of materials are titanium and its alloys, zirconium and its alloys, niobium and its alloys, titanium-niobium alloys, alloy steels, carbon steels,—iron-based superalloys, stainless steels, nickel and its alloys,—nickel-based superalloys, copper-based alloys, cobalt alloys,—cobalt-based superalloys, aluminum and its alloys, magnesium alloys, tantalum, and the like. Alternative materials with similar properties can be substituted without departing from the concept of this invention. 
         [0027]    The clamps  104  can be composed of metal alloys, including but not limited to titanium and its alloys, zirconium and its alloys, niobium and its alloys, titanium-niobium allows, alloy steels, carbon steels,—iron-based superalloys, stainless steels, nickel and its alloys, —nickel-based superalloys, copper-based alloys, cobalt alloys,—cobalt-based superalloys, aluminum and its alloys, magnesium alloys, tantalum and metals of similar properties. Alternative materials with similar properties can be substituted without departing from the concept of this invention. 
         [0028]      FIG. 4  shows a representative system of a valve plug  100  of this invention in a valve assembly  300 . The valve plug  100  is shown in a substantially closed position with the plug head  101  closing a first flow path  303  from the valve chamber  301  and a second flow path  302 . The plug stem  102  is shown connected to the actuator  304  and sealed with the shaft  305  in close, preferably fluid tight proximity, with valve stem support (or shaft support) packing (not shown).  FIG. 4  shows an embodiment of the valve plug  100  in its working environment in a typical valve assembly  300 . The actuator  304  functions to position the valve plug  100  either in the shown closed position or retracted to permit fluid flow from the first flow path  303  to the second flow path  302 . Alternatively, the flow can, as is common in some valves, flow in the opposite direction. 
         [0029]      FIG. 5  shows a section view of an alternative embodiment of a valve plug  200  having a plug head  201  mounted to plug stem base  203  of a plug stem  202 , this embodiment having additional compliance structure provided. In contrast to the embodiment described in  FIG. 1 , the present embodiment includes a plug stem base  203  having a base portion with a non-beveled end or edge  214 . Additionally, the plug stem base  203  can have a smaller diameter than the diameter of the plug head  201 . Plug head  201  can include a base portion with a beveled edge  210 . This design permits the plug head  201  and clamps  204  to move and adjust relative to the plug stem  202 , which allows the plug head  201  to self center when in use in a valve assembly, such as the valve assembly  300  of  FIG. 4 . An embodiment of the valve plug  200  can include washers or springs  220  within or adjacent to plug stem base  203  to provide cushioning and separation between the plug stem base  203  and the plug head  201 . Representative spacers include, but are not limited to Bellville washers and spring mechanisms. 
         [0030]    Alternative embodiments of the invention include spacers (not shown) on the bolt sections between sections of the clamps  204  that are adjacent to plug head  201  in order to prevent deflection of the clamps  204 . 
         [0031]    The described embodiments, including the various materials, specific components, and dimensions, are to be considered in all respects only as illustrative and not as restrictive. The invention should not be considered limited to the particular preferred and alternative embodiments, rather the scope of the invention is indicated by the appended claims. All changes, modifications and alternatives which come within the meaning and range of equivalency of the claims are to be embraced as within their scope.