Abstract:
A packing spring for a steam turbine comprising a flat, elongated spring body of specified length, width and thickness dimensions; a flange at one end of the main spring body; and at least one bend in the main spring body, located substantially midway along the length dimension of the spring body.

Description:
[0001]    This invention relates to seals used in steam turbines and, specifically, to a spring configuration for a packing seal used between a stationary turbine component and the turbine rotor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Turbomachines use numerous sealing techniques to establish seals between stationary and rotating parts of the turbine. In some instances, the seals are designed to provide radial movement away from the rotating components to prevent rubs during operation. In other instances, the springs bias the seal segments toward the rotor, while fluid pressure applied during operation counters the spring force so as to move the seal teeth out of engagement with, but in close proximity to the rotating component to achieve the desired sealing function.  
           [0003]    In steam turbines, for example, it is customary to employ a plurality of arcuate seal ring segments to form a labyrinth seal about and between the stationary and rotating components. Typically, the arcuate seal ring segments are disposed in an annular groove in the stationary component (or casing), and are designed to be concentric about the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. The seal faces typically carry a radially-directed array of axially spaced teeth that are radially spaced from an array of axially spaced annular grooves forming the sealing surfaces of the rotating component. Alternatively, the rotating component may have a smooth surface in radial opposition to the array of teeth on the seal faces. In any event, the sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface(s) of the rotating component.  
           [0004]    The annular groove in the stationary component is generally dovetail-shaped, having locating flanges directed axially toward one another and defining a slot therebetween. The stationary component is split lengthwise such that the semi-annular dovetail grooves may receive correspondingly-shaped arcuate seal ring segments. More particularly, the arcuate segments are similarly dovetail-shaped with a pair of flanges directed axially away from one another for disposition within the dovetail groove, with a narrow neck joining the seal face and the flanges of the segment and passing through the slot defined by the locating flanges of the grooves. The neck carries the arcuate seal face radially inwardly of the groove when installed, i.e., the arcuate seal face is radially adjacent the rotor.  
           [0005]    Many designs utilize springs to return and hold the seals against a stop to a designed radial clearance. While numerous spring designs have been used over the years, each has significant disadvantages. For example, flat springs used with turbine packing seals require a large amount of radial space behind the packing ring to meet spring loads and to avoid overstressing the spring during large displacements. Another disadvantage is the fact that each packing ring segment is contacted in only one location, i.e., at the mid point of the seal segment. In this condition, the packing segment can rotate about that pivot point with relatively small input forces. If a cyclic force were applied to the packing ring, it would be possible to create a vibratory mode which could lead to high cycle fatigue of the packing ring and/or spring. Finally, the current flat spring design allows for the application of a single spring constant only.  
           [0006]    An alternative approach to flat springs is the use of coil springs applied at two or more locations in each segment. Coil springs decrease the amount of free space behind each packing segment to a minimum, equal to the desired segment travel, and prevent pivoting about the spring contact points, since each segment is supported at multiple points. However, the coil spring design requires multiple cylindrical pockets milled into the back of each seal segment. These milled pockets may interfere with other hardware that may be installed into the segment. In addition, variable spring rates are not easy to attain without nesting multiple coil springs. It may be desirable to control the spring rate, but that may unload one of the nested springs, allowing it to vibrate and become damaged.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0007]    This invention provides additional space in the existing or new seal location. This space can be used to include additional devices or added material for reducing stress and increasing product life. The invention also provides a seal support that prevents pivoting about the support point, thus reducing the risk of high cycle fatigue or improper seal operation, and also permits a spring rate that may vary throughout the stroke of the seal.  
           [0008]    The spring configuration in accordance with this invention is similar to a flat or leaf spring design, but contains either a simple single or compound bend near the center of the spring length. This configuration alters the support points of the seal, thus resisting pivoting of the segment about the traditional single contact point near the center of the seal. It also reduces the amount of radial space required by the spring, and permits compound spring constants which may be tuned to meet design requirements.  
           [0009]    Thus, in one aspect, the invention relates to a packing spring for a steam turbine comprising a flat, elongated spring body of specified length, width and thickness dimensions; a flange at one end of the main spring body; and at least one bend in the main spring body, located substantially midway along the length dimension of the spring body.  
           [0010]    In another aspect, the invention relates to a seal ring and packing ring assembly comprising at least one arcuate seal ring segment located in a stationary turbine casing, and at least one spring located radially between the casing and the seal ring segment, the at least one spring comprising a flat, elongated spring body of specified length, width and thickness dimensions; a locating flange at one end of the spring body; and at least one bend in the spring body, located substantially midway along the length dimension of the spring body.  
           [0011]    The invention will now be described in detail in connection with the drawings identified below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a fragmentary cross-sectional view of a portion of a steam turbine looking transversely of the axis thereof and illustrating a known labyrinth seal arrangement;  
         [0013]    [0013]FIG. 2 shows a conventional flat seal spring of length L, width W and thickness T;  
         [0014]    [0014]FIG. 3 shows a simple angled seal spring in accordance with the invention with spring lengths L, Width W, thickness T, and a center bend at an angle α;  
         [0015]    [0015]FIG. 4 is a simplified schematic showing one support arrangement for the spring shown in FIG. 3; and  
         [0016]    [0016]FIG. 5 is a simplified schematic similar to FIG. 4 but showing another support arrangement. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Referring now to FIG. 1, there is illustrated a portion of a steam turbine, generally designated  10 , including a turbine rotor or shaft  12 , disposed in a turbine housing  14 . The shaft  12  is supported for rotation by conventional means, not shown, within the turbine housing. A multiple-stage labyrinth seal  16  includes a plurality of seal rings  18 ,  20  and  22  disposed about the turbine shaft  12  separating high and low pressure regions  28  and  30 , respectively. Each seal ring is formed of an annular array of a plurality of arcuate seal segments  32 . In general, the labyrinth seal  16  functions by placing a relatively large number of partial barriers to the flow of steam from the high pressure region  28  to the low pressure region  30 . Each barrier forces steam, attempting to flow parallel to the axis of turbine shaft  12 , to follow a tortuous path whereby a pressure drop is created. The sum of all the pressure drops in the labyrinth seal  16  is, by definition, the pressure difference between the high and low pressure regions  28  and  30 .  
         [0018]    The arcuate seal ring segments  32  have sealing faces  34  and radial projecting teeth  36 , each sealing face  34  being formed by a pair of flanges  38  extending axially away from one another. The radially outer portions of the seal ring segments  32  include locating flanges or hooks  40  which also extend axially away from each other and the segment  32  in opposite directions. As illustrated in FIG. 1, the turbine housing  14  has a generally dovetail-shaped annular groove  42  defined along its radially innermost portions by a pair of locating flanges  44  which extend axially toward one another defining a slot  46  therebetween. Flat springs  48  are located between the segments  32  of seal rings  18 ,  20   22  and housing  14  and serve to bias the segments  32  radially inwardly toward the rotor  12 .  
         [0019]    [0019]FIG. 2 illustrates a conventional flat spring  48  of specified length L and width W, with a main spring body  50  and a locating flange  52  at one end thereof. The flange extends away from the body  50  at an angle of 90° relative to the body  50 .  
         [0020]    [0020]FIG. 3 illustrates a spring  54  in accordance with an exemplary embodiment of this invention. The spring  54  (made of Inconel X- 750  or other suitable metal/alloy) has a length equal to L 1 +L 1  and a width W. The spring  54  also includes a main spring body  56  and a locating flange  58  at one end thereof. In this design, however, there is a simple, single bend  60  near the center of the length of the main spring body  56 . It will be understood that the bend may also be of a compound nature. In one example, the spring has a length (L 1  +L 1 ) of 12 inches, a width W of 1½ inches and a thickness T of about ⅛ inch. The bend angle α is about 11°. It will be appreciated that the dimensions may vary depending primarily on the diameter of the seal. The spring length may thus vary between about 6 inches and about 18 inches, and the bend angle may vary between about 5°-15°.  
         [0021]    Significantly, the new design changes the support points vis-a-vis the seal, as best seen in FIGS. 4 and 5. These are simplified representations of the various components for the sake of clarity and ease of understanding. FIG. 4 illustrates one arrangement where a spring  62  in accordance with the invention includes a spring body  64  formed with a center bend  66 . The spring is located radially between a seal segment  68  and a stationary turbine component or casing  70 . The locating flange (see  58  in FIG. 3) has been omitted. In this arrangement, the spring is supported by the casing  70  at the opposite ends  72 ,  74  of the spring, and contacts the segment  68  at a pair of locations midway between the opposite ends  72 ,  74  and the center bend  66 . Note that no contact is made at the center bend. This arrangement resists any potential pivoting motion of the seal segment  68  that might otherwise occur with a single point of contact at the center of the seal as with prior spring designs.  
         [0022]    [0022]FIG. 5 illustrates a variation where similar reference numerals but with the prefix “ 1 ” added, are used to designate corresponding components. Thus, spring  162  is located radially between seal segment  168  and casing  170 . The spring has a main body  164  and, again, the locating flange has been omitted. As in the case of FIG. 4, the opposite ends engage the casing  170 , and the spring body  164  engages the seal segment  168  at locations midway between the opposite ends  172 ,  174  and center bend  166 . In this case, however, the design is such that the center bend  166  of the spring also engages the casing  170 . In other words, the spring  162  “bottoms out” against the casing  170  at the center bend  166  when the seal segment  168  moves to its radially outermost position.  
         [0023]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.