In-Place Tensioning Washer

Pressure vessels, such as those found in power plants, are provided with a variety of access openings ranging from small hand holes to large reactor vessel covers. The closures must withstand high pressures and usually high temperatures. Optimum closure integrity is achieved through uniformly distributing the load about the closure by uniformly tensioning all of the studs used to support the closure about the access opening.

Off-the-shelf stud tensioning devices include multiple stud tensioning rings which tension all or some of the studs simultaneously. These devices are brought in when needed and removed after use. In some applications, a load cell can be installed permanently about each stud for use when needed.

However, many studs within power plants are subjected to temperatures of up to 1000.degree. F. (538.degree. C.) during operation of the plant. Therefore, permanent load cells are ruled out as they employ rubber or polymer materials for sealing purposes. Also, many of the studs in fossil power plants are of a large bolt diameter. Use of a multiple stud tensioning device or a portable load cell is not always feasible since tensioning of large diameter bolts requires relatively high forces and the spatial constraints associated with power plants may deny access to the large equipment required to deliver these large forces.

An exemplary large bolt diameter which may reach 1000.degree. F. (538.degree.0 C.) during operation is employed on the high pressure/intermediate pressure (HP-IP) turbine cylinder. These cylinders must be opened and resealed once or twice in a five year period. The studs on the cylinder closure must be prestressed to 45,000 psi (3.17.times.107 kg/m.sup.2) as the nuts are installed. The hydraulic force on each stud is then relaxed, resulting in a uniform stress level in all studs about the closure. Stud tensioning is not again required until the next opening of the cylinder. Due to the large bolt diameter of the studs used on the HP/IP cylinder closure, no commercial load cells or multiple stud tensioning devices are available. Stud tensioning is presently performed from above by pulling with a hoist. This method of tensioning the studs is extremely time consuming and labor intensive.

What is needed is a device which can be permanently mounted about each stud for prestressing of the studs about the closure and for subsequent stressing of the studs for removal of the closure when necessary. Such a device must be able to tension large diameter bolts and survive an environment which reaches 1000.degree. F. (538.degree. C.).

The invention is an in-place tensioning washer apparatus for stressing a stud to a predetermined stress level and for maintaining the stud at the predetermined stress level. The stud has a first end in engagement with the workpiece. The apparatus includes an annular ledge, which annular ledge is attached proximate a second end of the stud, and a fluid driven tensioning device for applying force against the annular ledge and against the workpiece to tension the portion of the stud between the annular ledge and the workpiece to the predetermined stress level. The tensioning device includes an upper portion, a lower portion and a torus. The torus is adapted to expand under pressurized fluid to force the upper portion a predetermined distance away from the lower portion to create a gap between the upper and lower portions, movement of the upper portion apart from the lower portion by the predetermined distance acting to force the annular ledge away from the first end of the stud in engagement with the workpiece and to tension the portion of the stud between the annular ledge and the workpiece to the predetermined stress level. The device further includes shims for maintaining the gap between the upper portion and the lower portion after depressurization of the torus. The shims are adapted for insertion into the gap when the torus is pressurized. By maintaining the gap, the shims maintain the upper portion apart from the lower portion by the first predetermined distance and act to maintain the tension in the stud at the predetermined stress level.

The torus is adapted to be repressurized by pressurized fluid to force the upper and lower portions a second predetermined distance away from each other with this second distance being at least as great as the first distance to permit removal of the shims. Depressurization of the torus after removal of the shims will relax the stress level in the stud.

The upper portion of the tensioning washer is removable from the lower portion to enable replacement of the torus. The torus used in the tensioning washer is preferably hollow and comprised of a metallic material to enable it to withstand high temperatures and high internal pressurization. The preferred material for high temperature, high pressure applications is molybdenum or a molybdenum based alloy. For low temperature, low stress applications, a rubber or polymer torus may be employed.

The invention is also directed to a stud tensioning system for tensioning the studs by which a closure is mounted for sealing an opening in a pressure chamber. Each stud has a first end in engagement with the workpiece and an annular ledge is attached proximate a second end of each stud, the second end being opposite the end of the stud in engagement with the workpiece. The system further includes a fluid driven tensioning device for applying force against the annular ledge and against the workpiece to tension the portion of each stud between the annular ledge and the workpiece to a predetermined stress level, a tensioning device being disposed about each stud. Each tensioning device consists of an upper portion, a lower portion, and an annular torus. Each torus is adapted to expand under pressurized fluid to force the upper portion a predetermined distance away from the lower portion to create a gap between the upper portion and the lower portion and to tension each stud to the predetermined stress level. The system further includes shims maintaining the gap between the upper portion and the lower portion of each tensioning device after depressurization of each torus. Maintaining of the gap acting to maintain each upper portion apart from each lower portion by the first predetermined distance to maintain the tension in each stud at the predetermined stress level.

Each torus of the stud tensioning system is adapted to be repressurized by pressurized fluid to force the upper and lower portions a second predetermined distance away from each other with this second distance being at least as great as the first predetermined distance to permit removal of the shims to enable relaxation of the stress level in each of the studs by depressurizing the tori.

The tori are preferably made of metal in order to withstand high temperatures and high stresses. Each tensioning device includes a pressurized fluid connecting fixture for connecting a source of pressurized fluid to each torus. All of the pressurized fluid connecting fixtures are preferably connected by a single pressurized fluid line to permit simultaneous stressing of each of the mounting studs about the closure.

FIG. 1 is a sectional view in elevation of the tensioning washer as mounted about a stud taken along the lines I--I in FIG. 2;

FIG. 2 is a perspective view of a portion of the tensioning washer system according to the invention with parts broken away and with tensioning of only four studs shown for clarity;

FIG. 3 is a sectional view in elevation of the tensioning washer of FIG. 1 with the torus in the expanded condition; and

FIG. 4 is a sectional view in elevation of an alternative embodiment of the tensioning washer having an annular bellows instead of a torus.

Referring now to the drawings, FIG. 1 depicts the in-place tensioning washer apparatus 10 disposed about a stud 12. Stud 12 is provided with threads 13 about its exterior at one end and with threads 14 about its exterior at the opposite end. Threads 14 engage threads 16 lining cavity 18 within vessel 20. While useful for the many applications requiring prestressing of bolts, it will be clear from the following description that the tensioning washer apparatus of the present invention is particularly well suited for tensioning studs used to retain access covers in the high temperature, high pressure environment associated with power plant operations. Vessel 20 may be any vessel requiring a prestressed stud including nuclear reactor vessels, steam generators, and turbine chambers. Stud 12 cooperates with other studs as observable in FIG. 2 to secure a closure 22 on vessel 20. Due to the high pressures associated with the vessels upon which closure 22 is mounted, the load must be uniformly distributed among the studs 12 so that no stud experiences loads that could lead to failure.

Proximate the end of stud 12, opposite the end in engagement with vessel 20, is a nut 24 having interior threads 25. Nut 24 holds the closure 22 onto vessel 20. Nut 24 also serves as a ledge against which pressure is applied to tension the stud. Nut 24 may be attached to an upper segment of tensioning washer 10 but is preferably separate. Threads 25 of nut 24 engage threads 13 of stud 12 provided at the end of stud 12 opposite the end in engagement with vessel 20. Some closure applications employ a bolt instead of a stud. The tensioning washer can also be used to tension bolts with the bolt shaft acting as the stud and the bolt head acting as the ledge. Through-bolts, which have a threaded shaft with a nut on each end, can also be tensioned using this apparatus. Disposed between closure 22 and nut 24 are lower washer portion 26 and upper washer portion 28. A spacer 30, which may be required for relatively long studs, is preferably positioned between nut 24 and upper washer portion 28. The bottom face 32 of the lower washer portion 26 contacts closure 22. The bottom annular ledge 33 of nut 24 or of the bolt head of the alternative embodiment described above is contacted by the upper surface of upper washer portion 28 or, when spacer 30 is used, by the upper surface of spacer 30. In the upper surface 34 of lower washer portion 26 is formed an annular groove 36. Upper washer portion 28 is provided with an annular protrusion 38 which extends into annular groove 36. Annular protrusion is preferably provided with a depression 39. In the preferred embodiment, annular groove 36 and depression 39 of protrusion 38 cooperate to form a doughnut-shaped cavity 40. Within cavity 40 is positioned a toroid 42 which is preferably of a doughnut-shaped cross-section in the expanded condition and is hereafter referred to as a torus, the torus 42 having a hollow central cavity 43. The toroidal shape of torus 42 is preferred since there are no stress concentration points and the entire torus undergoes tensile hoop stressing during expansion. Conversely, a rectangular or other shaped toroid could be used but would have stress concentration points, limiting the maximum stress levels achievable. Torus 42 is coupled by pressurized fluid supply fixture 44 to a pressurized fluid supply line 46. Only four studs 12, four stud tensioning washers 10, and a portion of closure 22 are depicted in FIG. 2 for clarity.

In operation, torus 42 is positioned between lower washer portion 26 and upper washer portion 28. Lower washer portion 26, upper washer portion 28, and spacer 30 are then slid onto stud 12. Nut 24 is threaded hand-tight onto stud 12 so that annular ledge 33 contacts the upper surface of spacer 30. Pressurized fluid line 46 is then coupled to fluid supply fixture 44. Pressurized fluid is supplied to the hollow central cavity 43 of torus 42 through fluid supply line 46 and fluid fixture 44, causing torus 42 to expand. The torus 42 expands into contact with the bottom surface 39 of protrusion 38, exerting a force on protrusion 38 which forces upper washer portion 28 away from lower washer portion 26. As depicted in FIG. 3, a gap 48 between upper washer portion 28 and lower washer portion 26 results. Since stud 12 is threaded into vessel 20 and nut 24 is initially threaded onto stud 12 hand-tight to place annular ledge 33 into contact with spacer 30, eliminating any gaps between nut 24, spacer 30, upper washer portion 28, lower washer portion 26, and closure 22, movement of upper washer portion 28 with respect to lower washer 26 by expansion of torus 42 also moves nut 24 (and spacer 30 when used) and causes stud 12 to be put into tension between threads 25 of nut 24 and threads 16 of vessel 20. By forcing upper washer portion 28 a predetermined distance away from lower washer portion 26, a predetermined stress level within stud 12 is developed. Shims 50, which are preferably half-moon shaped and are manufactured to a predetermined thickness, can then be inserted into gap 48. The shims are substantially undeformable and act to maintain gap 48 between upper and lower washer portions 28 and 26 after release of the internal pressurization of torus 42, thereby perpetuating the predetermined stress level within stud 12.

When closure 22 is next to be removed, torus 42 is pressurized to the original pressure or to a slightly higher value, creating a gap the same as or slightly larger than gap 48, enabling the shims 50 to be easily removed. Depressurization of torus 42 now results in relaxation of the stress level in stud 12, making possible hand removal of nut 24 and washer 10 and freeing the closure for removal.

A commercially available high pressure pump and pressure regulator (not shown) supplies pressurized fluid at a predetermined pressure level through fluid supply line 46 to fluid fixture 44 and thereby to torus 42. The torus is expanded to a predetermined pressure, forcing the upper washer portion 28 a predetermined distance away from the lower washer portion 26. This predetermined axial movement causes nut 24 to move a predetermined distance, imparting tension to stud 12 at a predetermined level. This predetermined stress level can then be maintained by shims 50, which are sized according to the predetermined gap 48 formed by the relative movement of upper washer portion 28 with respect to lower washer portion 26. While the pressure parameters are dependent on the diameter and length of the stud as well as on the size of the torus and washer assembly, the three inch (0.0762 m) diameter bolts on a HP/IP turbine are to be tensioned to approximately 45,000 psi (3.17.times.10.sup.7 kg/m.sup.2).

Through use of interconnecting fluid supply line 46 depicted in FIG. 2, in-place tensioning washers 10 mounted about each closure mounting stud 12 can be pressurized in unison, thereby simultaneously and uniformly prestressing each stud 12. Insertion of shims 50 and depressurization of the fluid supply line is then all that is required for uniformly stressing all studs about a closure.

In order for the system to function as an inplace tensioner in many power plant applications, the apparatus 10 must survive an environment which reaches 1000.degree. F. (538.degree. C.). The tensioning apparatus must be able to function after ten years in service and must be able to subsequently stress the stud or bolt to a great enough extension that shims 50 can be removed. In order to achieve these criteria, a metallic material is preferred for the torus 42. The preferred metallic material for the torus is molybdenum, which has a yield strength of about 80,000 psi (5.63.times.10.sup.7 kg/m.sup.2). Molybdenum based alloys could also be employed. The preferred shape for the torus is as depicted in FIG. 1. The torus is internally pressurized to up to 80,000 psi (5.63.times.10.sup.7) kg/m.sup.2) in order to achieve the 45,000 psi (3.17.times.10.sup.7 kg/m.sup.2) stress level desired in HP-IP turbine studs. This high internal pressure acts to plastically deform the torus. However, the torus retains sufficient strength and elasticity to be repressurized to slightly more than 80,000 psi (5.63.times.10.sup.7 kg/m.sup.2) for shim removal. The used torus is then replaced with a new one for the next cycle by separating upper and lower washer portions 28 and 26. Alternative embodiments of the torus such as the embodiment depicted in FIG. 4 are also possible. In the embodiment of FIG. 4, the torus 52, shown in the expanded condition, is formed like an annular bellows. Circumferential folds 54 open and close in response to pressurization and depressurization respectively to permit the torus to expand and contract. FIG. 4 also illustrates tensioning of a bolt 60 rather than a stud.

The torus may be manufactured from a material such as rubber or a polymer for stressing relatively small diameter bolts which do not experience high temperatures. However, for the system to function as an in-place tensioner in many power plant applications, the apparatus 10 must be able to survive an environment which may reach up to 1000.degree. F. (538.degree. C.). The tensioning apparatus must be able to retain a closure on a vessel which experiences high internal pressures. With stud 12 engaging workpiece 20, the closure is retained on workpiece 20 with force passing through lower washer portion 26, upper washer portion 28, and, for relatively long studs, spacer 30, The torus is used only during installation and removal of closure 22 and is otherwise depressurized. In such applications, the tensioning apparatus must be able to stress the bolt to a great enough extension that shims 50 can be removed, even after as long as 20 years in service. In order to achieve these criteria, the torus 42 must be manufactured from a strong yet elastic temperature-resistant material. Since current polymer technology does not include a polymer which can withstand such temperatures, a metallic material is presently preferred. The preferred metallic material is molybdenum and the preferred shape is the toroidal shape depicted in FIG. 1 since a toroidal structure is the annular structure which can tolerate the greatest internal pressure. The torus is pressurized up to 80,000 psi (5.63.times.10.sup.7 kg/m.sup. 2) in order to achieve the 45,000 psi (3.17.times.10.sup.7 kg/m.sup.2) stress level desired in the three-inch (0.0762 m) diameter HP/IP turbine studs. Smaller bolt diameters would require significantly lower pressures. The high internal pressure used to stress the HP/IP studs acts to plastically deform the metallic torus. However, the torus retains sufficient strength and elasticity to be repressurized to the same or slightly higher than the original level of pressurization for shim removal. The used torus is then preferably replaced with a new one for the next cycle by separating upper and lower washer portions 28 and 26 after annular ledge 33 has been removed by unthreading nut 24 in the preferred embodiment. As previously described, alternative embodiments of the torus, such as that depicted in FIG. 4, are possible. In the embodiment of FIG. 4, the torus 52, shown in the expanded condition, is shaped and performs like a bellows. Folds 54 extend circumferentially about the outer surface 56 and the inner surface 58 and act as hinges to allow torus 52 to collapse upon depressurization.

Therefore, the present invention contemplates the use of an in-place tensioning washer system particularly useful for tensioning large studs which experience high temperatures. The system preferably employs a metallic torus which stresses a stud from beneath the nut which holds the closure against the vessel. The invention contemplates the insertion of shims into the gap which is formed between the upper and lower portions of the tensioning apparatus when the torus expands under pressurization. The shims act to maintain the gap and thereby perpetuate the predetermined stress level within the stud. The system is capable of withstanding long exposure at elevated temperatures.

While there has been described what at present is considered to be the preferred embodiment of the invention, it will be understood that various modifications and alterations may be made therein without departing from the true spirit and scope of the invention and it is intended to cover all such modifications and alterations in the claims appended hereto.