Patent Publication Number: US-2013248751-A1

Title: Control valve seal assembly energized by shape memory alloys and fluid valves comprising same

Description:
BACKGROUND 
     1. Field of the Disclosure 
     The invention generally relates to control valve seals and more specifically to control valve seals that are energized by, or comprise, shape memory alloys. 
     2. Related Technology 
     Fluid valves control the flow of fluid from one location to another. When the fluid valve is in a closed position, high pressure fluid on one side of the valve is prevented from flowing to a lower pressure location on the other side of the valve. Often fluid valves contain a movable fluid control member and a seat of some sort that cooperates with the fluid control member to control or stop fluid flow through the valve. While many different types of fluid valves exist, the general principle of physically separating a higher pressure fluid region from a lower pressure fluid region applies to all fluid valves. Because of this pressure difference, fluid from the high pressure side will naturally try to migrate to the lower pressure side by any means possible. Often space between the movable control member and a valve housing may provide an avenue by which higher pressure fluid can migrate (or leak) to the lower pressure region. In order to prevent leaks, most fluid valves include one or more seals between valve parts to prevent fluid leaks. 
     In less severe temperature environments, the seals of fluid valves may be made of relatively pliable materials such as elastomeric materials. Elastomeric seals are relatively easy to install due to their pliable nature. More particularly, elastomeric seals can be stretched or otherwise manipulated during installation. Because of this flexible nature, elastomeric seals also adapt to minor structural variations between valve parts. However, elastomeric seals are temperature limited to environments less than about 450° F. Above about 450° F., elastomeric materials begin to break down, which can lead to fluid leaks. Another drawback to elastomeric seals is that elastomeric seals tend to lose the ability to apply a load to another member at higher temperatures. 
     Seals in fluid valves used in high temperature environments are generally made from more robust materials, such as graphite. While graphite seals are relatively temperature tolerant for most high temperature operations, graphite seals are relatively rigid. This rigidity of graphite seals makes graphite seals more difficult to place in the valve during assembly. Additionally, once placed, graphite seals require a relatively constant load or pressure between valve parts, which may not be desirable in environments having large changes in operating temperature. 
     SUMMARY OF THE DISCLOSURE 
     A fluid valve includes a valve body having a fluid inlet and a fluid outlet connected by a fluid passageway. A valve seat is disposed within the fluid passageway. A fluid control member is movably disposed within the fluid passageway, the fluid control member cooperating with the valve seat to control fluid flow through the fluid passageway. A seal is disposed within the valve body, the seal preventing fluid from leaking through the valve body when the fluid control member is in a closed position. The seal is made from a shape-memory alloy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a fluid control valve including a shape-memory alloy seal assembly; 
         FIG. 2  is a close up cross-sectional view of circle  2  in  FIG. 1 ; 
         FIG. 3  is a close up cross-sectional view of circle  3  in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of one embodiment of a seal in the seal assembly of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of another embodiment of the seal in the seal assembly of  FIG. 1 ; 
         FIG. 6  is an alternate embodiment of a seal assembly; 
         FIG. 7  is yet another alternate embodiment of a seal assembly. 
     
    
    
     DETAILED DESCRIPTION 
     A seal assembly constructed in accordance with the disclosure advantageously has a relatively low stress at low temperatures and a higher stress at higher temperatures. The lower stress state facilitates assembly of a control valve, including installing the seal assembly, because the seal assembly is more pliable and manipulatable at low stress. The higher stress state, on the other hand, promotes better sealing at high temperatures. The higher stress state counteracts or offsets different thermal expansion rates of valve parts, and higher fluid pressures at higher temperatures. In some cases, the seal provides lower frictional forces for dynamic seals that move with a fluid control member. Additionally, the seal assembly is useful over a very large range of temperatures. For example, the seal assembly may be used in control valves experiencing temperatures from between 0° F. to over 1000° F. The seal assembly solves the problems of prior art seals discussed above by forming at least part of the seal assembly from a shape-memory alloy material, 
     Shape memory alloys have unique properties that permit them to undergo a solid state phase change when heated (e.g., from a deformed martensite phase to an austenite phase). When in an austenite phase, a ring-shaped seal assembly formed from shape-memory alloy material may have a diameter (or inner dimension) that is slightly smaller than an inner diameter of a valve part to facilitate assembly of the seal assembly into the fluid control valve. At a transition temperature, the ring-shaped seal assembly transforms into an austenitic phase, which causes the ring-shaped seal assembly to attempt to expand. This attempted expansion causes the ring-shaped seal assembly to press against certain valve components, thereby creating a stronger seal and/or compensating for different rates of thermal expansion between certain valve components. 
     Examples of a shape memory alloy materials that may be suitable for use in forming the seal assembly include Nickel Titanium, also known as NiTi or Nitinol (near-equiatomic titanium-nickel alloy). Other shape-memory and superelastic alloys, or high temperature shape-memory alloys, such as NiTi X alloys, wherein X is Hf or Zr substituted for Ti and/or X is Cu, Pd, Pt and/or Au substituted for Ni, e.g., NiTiCu or TiNiPd. 
     Shape-memory alloys, such as NiTi, exhibit two remarkable strain recovery properties in wrought form, i.e., the shape memory effect and superelasticity. The first property refers to an ability of an shape-memory alloy to recover from large mechanically induced strains (i.e., up to 8%, e.g., in extant shape-memory alloy structures) by moderate increases in temperature. The latter property refers to the rubber-like, hysteretic strain recovery in relatively high temperature regimes. In each case, the underlying mechanism is a reversible martensitic transformation between solid-state phases that can be induced by changes in temperature or stress. Some shape-memory alloys also have excellent structural properties and excellent corrosion resistance, which are particularly useful properties in fluid control valve components. 
     Turning now to  FIG. 1 , an exemplary fluid control valve  10  may include a valve body  12  having a fluid inlet  14  and a fluid outlet  16  connected by a fluid passageway  18 . A valve seat  20  may be positioned within the fluid passageway  18 . A fluid control member, such as a valve plug  22  may be slidably mounted within the valve body  12 , the valve plug  22  interacting with the valve seat  20  to control fluid flow between the fluid inlet  14  and the fluid outlet  16 . In this embodiment, a cage  24  may also be mounted within the valve body  12 . The cage  24  acts as a guide for the valve plug  22  as the valve plug moves within the valve body  12 . Additionally, the cage  24  may include a plurality of perforations or widows designed to characterize fluid flow through the fluid control valve  10 . The valve plug  22  may be attached to a valve stem  26  that is moved by an actuator connected to a valve bonnet  30 . The actuator operates the valve stem  26  in a reciprocating fashion to move the valve plug  22  towards, or away from, the valve seat  20  to control fluid flow through the valve body  12 . 
     When the valve plug  22  is in a closed position, contacting the valve seat  20 , to prevent fluid flow through the valve body  12 , fluid pressure builds up on the inlet side of the valve body  12 . As a result, fluid will attempt to flow from the high pressure inlet side to the low pressure outlet side by any means available. For example, fluid may attempt to flow through any gaps created between the valve seat  20  and the valve body  12 , through any gaps between the cage  24  and the valve body  12 , or through any gaps between the valve plug  22  and the cage  24 . Other gaps may exist through which fluid may attempt to flow. One or more seal assemblies  32  may be placed in the gaps described above (or in any other gaps) to stop fluid flow through the gaps. 
     While a sliding stem valve is disclosed herein as an exemplary embodiment of a fluid control valve, the seal assemblies described herein may be used in virtually any type of fluid valve that includes a seal. For example, the disclosed seal assemblies may be used in various types of valves, e.g., ball valves, globe valves, butterfly valves, or eccentric plug valves. 
       FIG. 2  is a close up of a portion of the seat ring  20  and the valve body  12 . The valve body  12  may include a shoulder  40  sized and shaped to support an annular ledge  42  formed in an outer surface of the seat ring  20 . In the embodiment of  FIG. 2 , an annular channel  44  may be formed in an outer surface of the seat ring  20 , below the annular ledge  42 , for receiving the seal assembly  32 . In other embodiments, the seal assembly may be received in an annular channel formed in the valve body  12 . In yet other embodiments, the seal assembly  32  may simply be located between an outer surface of the seat ring  20  and an inner surface of the valve body  12 . Regardless, the seal assembly  32  prevents fluid from flowing between the seat ring  20  and the valve body  12 . 
       FIG. 3  is a close up portion of the valve body  12 , the cage  24 , and the valve plug  22 .  FIG. 3  illustrates some alternate locations for the seal assembly  32 . For example, the seal assembly  32  may be located in an annular channel  46  in the cage  24 , to prevent fluid flow between the cage and the valve body  12 . In other embodiments, the seal assembly  32  may be located in an annular channel formed in the valve body  12 . Alternatively, the seal assembly  32  may be located in an annular channel  48  in the valve plug  22  to prevent fluid flow between the cage  24  and the valve plug  22 . In other embodiments, the seal assembly  32  may be located in an annular channel formed in the cage  24 . The seal assembly  32  locations illustrated in  FIGS. 2 and 3  are only some examples of possible seal assembly locations. The disclosed seal assemblies may be located at virtually any location within the valve body  12 . 
       FIG. 4  illustrates one embodiment of a seal assembly  32 . In this embodiment, the seal assembly comprises a ring-shaped element made from a shape-memory alloy material. The seal assembly  32  has a w-shaped cross-section, comprising an opening  60  on one side. Two free ends  62 ,  64  may be connected by a convoluted portion  66  including a first convex portion  68  and a second convex portion  70  connected by a concave portion  72 . The free ends  62 ,  64  may angle outwardly, away from one another so that when the shape-memory alloy material changes from a martinsitic phase to an austenitic phase increased material stress will be directed in an outward direction, causing the free ends  62 ,  64  to apply pressure to valve elements located on either side of the free ends  62 ,  64  (e.g., the valve body  12  and the valve seat  20  in  FIG. 2 ). This increased stress will improve sealing capacity between the two valve elements. 
       FIG. 5  illustrates another embodiment of a seal assembly  32 . The seal assembly  32  has a U-shaped cross-section including two free ends  82 ,  84  connected by a convex portion  86 . An opening  80  may be formed in one side of the seal assembly  32 . Similar to the embodiment of  FIG. 4 , when the shape-memory alloy material changes from a martinsitic phase to an austenitic phase, increased material stress will be directed in an outward direction, causing the free ends  82 ,  84  to apply pressure to valve components located on either side of the free ends  82 ,  84 . 
       FIG. 6  illustrates yet another embodiment of a seal assembly  32 . The seal assembly  32  in  FIG. 6  may be used in environments where the use of graphite seal material is desirable. The seal assembly  32  has a U-shaped cross-sectional element including two free ends  92 ,  94  connected by a convex portion  96 , similar to the embodiment of  FIG. 5 . An opening  90  may be formed between the two free ends  92 ,  94 . A first graphite ring  97  may be located proximate the first free end  92  and a second graphite ring  99  may be located proximate the second free end  94 . The first graphite ring  97  may be seated in an annular channel  50  formed in a first valve component, such as the valve body  12 . The second graphite ring  99  may be seated in an annular channel  44  formed in a second valve component, such as the valve seat  20 . In the embodiment of  FIG. 6 , the U-shaped element biases the first and second graphite rings  97 ,  99  outward to produce a tight seal between the valve body  12  and the valve seat  20 . 
       FIG. 7  illustrates yet another embodiment of a seal assembly  32 . The seal assembly  32  may include a U-shaped element made of a shape memory alloy material, similar to the seal assembly  32  of  FIG. 5 . The U-shaped element may be located between the valve seat  20  and the valve body  12 . A seal retention mechanism, such as a spring clip  101 , may be located upstream of the U-shaped element to restrain movement of the U-shaped element within the space between the valve seat  20  and the valve body  12 . The spring clip  101  may be located at least partially within an annular channel  103  in the valve seat. Because the opening  80  of the U-shaped element is oriented towards inlet fluid flow, the fluid pressures the free ends  82 ,  84  outward, thus enhancing the fluid seal between the valve seat  20  and the valve body  12 . 
     In each of the embodiments described above, the openings  60 ,  80 ,  90 , may be oriented towards higher pressure fluid (i.e., towards the inlet in the direction of fluid flow) to further enhance sealing capacity by mechanically pressuring the free ends  62 ,  64 ,  82 ,  84 ,  92 ,  94  outward. This mechanical pressure may augment the increased material stress of the shape-memory alloy when changing from the martinsitic phase to the austenitic phase or the mechanical pressure may provide an increased sealing force before the shape-memory alloy material reaches the transition point. 
     The seal assemblies described herein provide increased sealing capacity at high temperatures. The seal assemblies also facilitate assembly of fluid control valves by being flexible at low temperatures. Other benefits include a more uniform geometry, which also simplifies the manufacturing process. 
     Although certain seal assemblies and fluid control valves have been described herein in accordance with the teachings of the present disclosure, the scope of the appended claims is not limited thereto. On the contrary, the claims cover all embodiments of the teachings of this disclosure that fairly fall within the scope of permissible equivalents.