Patent Application
20070272687
The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:
Referring to
The control valve stem 42 slidably engages with a bushing 46 pressed into a bore 50 through a cover 54. The control valve actuator is positioned relative to the cover 54 by an actuator yoke 58 that abuts a yoke surface 62, on the cover 54, thereby aligning the control valve stem 42 with the bushing 46 and the bore 50. The yoke surface 62 may be flat to further facilitate aligning the yoke 58 with the bushing 46 and the bore 50. The yoke surface 62 is substantially perpendicular to the bore 50 and bushing 46. The perpendicularity provides alignment to the actuator such that the stem 42 moves through the cover 54 in a perpendicular manner to the yoke surface 62. The control valve stem 42 is in operable communication with the head 30 such that movement of the stem 42 results in movement of the head 30, which directly controls the annular flow area between the head 30 and the valve seat 26. The annular flow area controls the flow rate of the steam through the valve 10. Thus control of the flow rate of the steam is directly controllable by controlling the control valve actuator.
The control valve may completely stop flow of the steam by extending the stem 42 and moving the control valve head 30 into contact with the seat 26. An accurate alignment of the head 30 with the seat 26 is necessary to assure valve closure as the head 30 contacts the seat 26. Such alignment within the pressure vessel is provided to the head through a balance chamber 66. The balance chamber 66 abuts a third surface, also referred to herein as chamber surface 70, which may be flat, on the interior of the cover 54 that is perpendicular to the bore 50. An exterior cylindrical wall 80 of the head 34 slidably engages an internal cylindrical wall 76 of the balance chamber 66, thereby guiding the head 30 in a direction perpendicular to the flat surface 70. Accordingly, by making the flat surfaces 70, 62 perpendicular to the bore 50, the actuator (not shown), the actuator yoke 58, the control valve stem 42, the balance chamber 66 and the head 30 are all kept in alignment relative to the cover 54. Consequently, the cover 54 is an important component in maintaining accurate alignment of the head 30 with the seat 26. The valve body 14 also provides alignment of the head 30 with the seat 26 since both the seat 26 and the cover 54 are attached directly to the body 14.
Referring to
Maintaining a seal against the high pressure of the steam requires uniform distribution of the forces generated by the bolts 82 on the cover 54. A second surface, referred to herein as surface 94, which may be flat, of the cover 54, is substantially perpendicular to the holes 78 and provides a surface for the heads of the bolts 82 to seat against. The flat surface 94 is substantially parallel to the flat surface 86 but is on an opposite side of the periphery 74 from the seal flat surface 86. All four of the flat surfaces 62, 70, 86 and 94 are substantially parallel to one another thereby positioning the bore 50 perpendicular to the surface of the cover 54 as well as perpendicular to the seal surface 90 of the body 14 providing alignment of the head 30 with the seat 26 as described above.
In addition to providing alignment of the valve components, the cover also must withstand stresses that arise from the high temperatures and pressures, to which the internal surfaces of the cover are subjected. One embodiment of the invention distributes these stresses by doming the cover 54. Specifically, a central portion 100 of the cover 54, containing among other things the flat surfaces 62, 70 and bore 50, is displaced axially from the periphery 74. A generously curved internal surface 104 connects the displaced flat surface 70, of the central portion 100, with the flat surface 86, of the periphery 74. Similarly, a generously curved external surface 108 connects a flat surface 62, also referred to herein as a fourth surface, portion of the surface 108, of the central portion 100, with the flat surface 94, of the periphery 74. The thickness between the flat surfaces 86 and 94, the curved surfaces 104 and 108 and the flat surfaces 70 and 62 are substantially equal to one another. Stated another way, the cover 54 has a substantially constant wall thickness throughout.
The domed shape of the cover 54 reduces the stresses throughout the cover 54, as well as the stresses that act on the bolts 82 when compared to a cover that is flat and not dome shaped. During heat up, for example, when a cold assembly is exposed to hot steam, the internal surfaces 70 and 104, which are in direct contact with the steam, heat up first and thus expand first. Thermal bending, which may cause a dishing in a flat cover as described in the background above, does not cause dishing with the domed cover. Thermal bending does not occur in the domed cover 54 due to the geometry of the domed shape. Specifically, the expanding internal flat surface 70, for example, is positioned substantially midway between the flat surfaces 86 and 94 of the periphery 74, thereby distributing this expansive load symmetrically into the full thickness of the periphery 74, instead of into the surface 86 alone. By distributing the stress symmetrically there is little force acting to deform the cover 54 into a dished shape. Additionally by distributing the load into the center of the periphery 74 the full thickness of the periphery 74 is resisting the load resulting in a reduction in the overall stress of the periphery 74.
Loading applied to the bolts 82 from the cover 54 is also reduced by the domed shape of the cover 54. Since, as described above, the periphery 74 is under less total stress, the periphery 74 does not expand as much. Consequently, bending forces applied to the bolts 82 by the cover 54 are reduced since the holes 78 remain parallel to and in alignment with the holes 84, in the body 14.
The domed shape of the cover 54 also reduces the stresses generated by pressure acting on the cover 54. This is due to the increase in stiffness of the cover 54 that results from the domed shape, which is similar to the stiffening that results when ribbing is added to any flat shape.
The improved load distributing characteristics of the cover 54 permit the cover 54 to use a thinner overall thickness and less total material to fabricate than a flat cover for the same application. Additionally, since the flat surfaces that may need to be machined on the domed cover 54 include less total area than a comparable flat cover, for example, the costs for material, time and labor to perform such machining operations should also be less.
A feature referred to as “leakoff,” which allows for the porting off of high pressure/high temperature steam, is sometimes desirable during throttling of control valves. Therefore, the domed cover 54 may include a port 112 in fluidic communication with the bore 50 and an exterior circumference 116 of the periphery 74. Port 112 also may allow for leakoff of the steam to atmosphere or to a steam recovery unit through piping (not shown) connected to a flange 120 bolted to the circumference 116. In the domed cover 54, the port 112 may be formed at an angle relative to the flat surface 86 in order to avoid fluidically connecting the port 112 to any of the surfaces 86, 94, 62, 70, 194 and 108.
Some embodiments of the invention may include some of the following advantages: less cost to manufacture, less total mass, less time to fabricate, more durable, longer life, lower stresses internally and on fastening bolts, internal and external mounting surfaces for valve component and reduced dishing and bowing.
While the embodiments of the disclosed method and apparatus have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the embodiments of the disclosed method and apparatus. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments of the disclosed method and apparatus without departing from the essential scope thereof. Therefore, it is intended that the embodiments of the disclosed method and apparatus not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the embodiments of the disclosed method and apparatus, but that the embodiments of the disclosed method and apparatus will include all embodiments falling within the scope of the appended claims.
