METHOD OF ASSEMBLY OF A BLADE ARRANGEMENT FOR A STEAM TURBINE

Abstract: A steam turbine of the type employing axial entry blades (12) is provided with a plurality of generally tangentially oriented grooves (34) in the rotor steeples. Resilient, deformable pins (32) are driven into the grooves (34) under the blade platforms (14) so as to urge the blade roots (16) radially outward.

Description: The present invention generally relates to a method of assembly of a blade arrangement for a steam turbine having a rotor with a plurality of generally axially extending grooves disposed therearound, a section of rotor between adjacent grooves defining steeples, a plurality of blades, each blade having a root in registration with one of the grooves and a platform juxtaposed the steeples that are adjacent the groove with which the root registers, and means for locking each root in the groove with which it registers to prevent axial movement of the blade. Steam turbines of the type employing "axial entry" blades comprise a rotor having a plurality of generally fir tree shaped generally axially extending grooves, with the blades circularly disposed therearound. Each blade has a generally fir tree-shaped root at a proximal end thereof for registration with one of the grooves and a shroud integral with the blade at a distal end thereof. During normal operation, under loead, centrifugal force holds the blade roots tights in the grooves with which they register. During turning gear operation however, the centrifugal force is insufficient to hold the blade roots tight in their grooves and hence the blades "flop", i. e., the blade roots rock circumferentially and axially in the grooves. This relative motion between the blade root and the rotor grooves may cause fretting of the root. Blade "flop" may aggravate other problems. For example, "axial entry, integral shroud" blades are often used in the first rows of some turbines because they are more reliable than riveted shrouds. Usually, the shrouds are tightly butted, but in certain rows a small gap is intentionally provided between adjacent shrouds to allow for thermal expansion. Faces of the shrouds may wear from snubbing that occurs as a result of blade "flop". It is known that certain steam turbines manufactured by Brown Boveri Corporation incorporate a means intended to solve this problem. That means comprises coil springs disposed in a plurality of radially oriented holes in the bottom of each root. It is believed that the intent of the coil spring is to urge the root radially outward against the edges of the groove to prevent relative motion between the root and groove. This solution, however, is considered undesirable because it is mechanically complicated and further because the radially oriented holes weaken the root. Further, the amount of force that can be exerted by the coil springs is limited by the size of the hole that can be machined in the root, and also by the size of the spring. The assignee of the present application has implemented two other prior art means for solving this problem. The first is to drive shims between the bottom-most portion of each root and the bottom of the groove with which that root registers. The second is to cement each root in its respective groove with an adhesive such as Loctite®. The problem with these solutions is that disassembly of the turbine is made difficult and time consuming. Further, US-A-3 037 741 describes a turbine blade mounting arrangement wherein resilient members are retained in provided openings between the platform of a blade, so that these members frictionally engage the lower surface of the blade platforms. Thus, vibration damping is provided by frictional forces in order to reduce vibrational stress. US-A-4 022 545 discloses a damper construction for turbine blades having roots with shoulders engaging the complimentary retaining surfaces of the rotor by insertion of elastic split metallic roll pins disposed at the bottom of the roots in order to also dampen vibration of the plates during high speed turbine operation. It is therefore desirable to provide a method of assembly of a blade mounting arrangement which results in reduced blade "flop" which is simple, inexenspive, reliable and easy to implement. With this object in view, the present invention resides in a turbine blade mounting arrangement as defined in the claim. The invention will become more readily apparent from the following description of preferred embodiments thereof shown, by way of example only, in the accompanying drawings, wherein: Figure 1 is a perspective view of a section of a turbine. Figure 2 is a side plan view of an axial entry blade of the type having a generally fir tree shaped root. Figure 3 is a detailed side plan view illustrating the cooperation between one of the motion restraint pins of the present invention and a blade platform. Figure 4 illustrates various cross-sectional shapes that the motion restraint pins of the present invention may assume. Figure 5 illustrates a portion of a turbine equipped with the motion restraint pins of the present invention. Referring to the drawings, wherein like numerals represent like elements, there is illustrated in Figure 1 a portion of a turbine labeled generally 10 comprising a rotor 20 and a plurality of blades 12. Each blade 12 comprises a platform 14 and a root 16 disposed at the proximal end thereof. The blade 12 may also comprise an integral shroud 26 at the distal end thereof, although the present invention is not limited in application to steam turbines of the type employing integral shroud blades. Thus, the present invention has application to steam turbines of the type employing free standing blades. The rotor 20 comprises a plurality of generally axially extending grooves 18 disposed therearound. As illustrated, each root 16 and each groove 18 have a generally fir tree shape and each root 16 is in registration with one of the grooves 18. Disposed between adjacent grooves 18 are steeples 36. As shown, the platform 14 of each blade is juxtaposed a pair of steeples. As also illustrated in Figure 1, there may be a small clearance 22 between adjacent platforms 14 which may open further under hot rotor conditions. There may also be a small clearance 24 between each root 16 and the blade 18 with which it registers. Still further, there may be a small clearance 25 between adjacent shrouds 26 which may also open under hot rotor conditions. The existence of these clearances may result in blade "flop" during turning gear operation. Still further, if the blades 12 are of the type that employ integral shrouds 26, opposing faces of the shrouds 26 may wear from snubbing. Referring now to Figure 2, there are illustrated further details of the turbine. As is common in steam turbines employing axial entry blades, each blade 12 is provided with a lock pin 30 that prevents the blade from moving in the axial direction of the rotor. The lock pin 30 is disposed between the platform 14 and one of the steeples 36 with which the platform 14 is juxtaposed. The lock pin 30 resides in a generally tangentially extending slot in the steeple and an aligned slot in the platform. As illustrated, the lock pin 30 is generally centrally disposed between front and rear ends of the platform. What has been described thus far is well known in the art. According to the invention, blade "flop" is reduced by wedge means 32 disposed between the base 15 of each platform 14 and the steeples 36 with which the platform 14 is juxtaposed. (See Figure 2). Preferably, there is an interference fit between the wedge means 32 and the base 15 of platform 14 so that the wedge means 32 urge bearing lands 28 of the root 16 radially outward against edges of the groove 16. It is to be understood that the wedge means 32 should be provided on each side of the platform, as best illustrated in Figure 5. Moreover, it should be understood that although Figure 2 illustrates wedge means disposed on each of the front and rear ends of the platform (i.e., forward and rearward of the lock pin 30), the invention is not limited thereto. Thus, the wedge means 32 need only be provided on one end of the platform. According to the preferred practice of the invention, the wedge means 32 comprise resilient and deformable pins ("motion restraint pins") constructed of stainless steel. Preferably, the motion restraint pins 32 are constructed of ASTM A565 grade 616 material. As shown, the motion restraint pins 32 reside in generally tangentially extending slots 34 disposed in opposing ones of the steeples 36 with which each platform 14 is juxtaposed. See Figures 2 and 5. If only a single pair of motion restraint pins is utilized, the slots 34 and motion restraint pins 32 may be disposed either forward or rearward of the lock pin 30, one motion restraint pin 32 being disposed on each side of the platform as shown in Figure 5. If it is desired to utilize two pair of motion restraint pins 32, they may be disposed both forward and rearward of the lock pin 30 as illustrated in Figure 2. Figure 4 illustrates various shapes that the motion restraint pins 32 may assume. Thus, the motion restraint pin may have a generally C-shaped cross section, a square cross section, a rectangular cross section, or a semi-circular cross section. It is preferred that the cross sectional shape of the slot 34 be substantially that of the cross sectional shape chosen for the motion restraint pin 32. It is also preferred that the cross sectional dimension (width or diameter) of the motion restraint pin 32 be substantially that of the slot 34 so that there is an interference fit between the motion restraint pins 32 and the base 15 of the platform 14. See Figure 3. It will be appreciated that the motion restraint pins urge the roots 16 radially outward. Thus, the motion restraint pins 32 urge bearing lands 28 of roots 16 against the edges of grooves 18, whereby blade flop is eliminated. It will also be appreciated that, if blades 12 employ integral shrouds 26, the motion restraint pins 32 will urge opposing faces of adjacent shrouds into abutment, whereby wear from snubbing is eliminated. According to the invention, a method of assembling a turbine utilizing the motion restraint pins 32 comprises the following steps. Reference is made to Figure 5. A generally tangentially extending slot 34 is machined in each steeple at a location other than where the lock pin 30 is to be located. The root 16 of a blade 12 is then inserted into a groove 18 and the lock pin for the blade 12 is inserted. A motion restraint pin 32 is then inserted in the slot 34 and driven beneath the platform 14. Another motion restraint pin 32′ is then inserted in the slot 34 next to the motion restraint pin 32 previously driven under the platform 14. Then, the root 16′ of a blade 12′ is inserted in the next groove 18 in succession and its platform 14′ is driven over the motion restraint pin 32′. The lock pin 30 for the blade 12′ is then inserted. A motion restraint pin 32″ is inserted in the slot 34′ and driven beneath the platform 14′. A motion restraint pin 32‴ is inserted in the slot 34′, and so on until all blades in the row have been assembled. It is preferred that a curved tool having a blunt end be utilized to drive the motion restraint pins beneath the platforms. Thus, the blunt end of the curved tool should be placed against the motion restraint pin and the motion restraint pin should be driven under the platform by hammering the other end of the tool.

Claims: A method of assembly of a blade arrangement for a steam turbine (10) having a rotor (20) with a plurality of generally axially extending grooves (18) disposed therearound, a section of rotor between adjacent grooves (18) defining steeples (36), a plurality of blades (12), each blade (12) having a root (16) in registration with one of the grooves (18) and a platform (14) juxtaposed the steeples (36) that are adjacent the groove (18) with which the root (16) registers, and means (30) for locking each root (16) in the groove with which it registers to prevent axial movement of the blade (12), the method comprising: machining a generally tangentially extending slot (34) in each steeple at a location other than where the lock pin (30) is to be located, inserting the root (16) of a blade (12) into a groove (18) and the lock pin (30) for the blade (12), inserting a motion restraint pin (32) into the slot (34) driving it beneath the platform (14), inserting another motion restraint pin (32') into the slot (34) next to the motion restraint pin (32) previously driven under the platform 14, and inserting the root (16') of a blade (12') into the next groove (18') in succession and driving its platform (14') over the motion restraint pin (32'), the pins driven under the platforms urging in cooperation with the pins previously inserted, before insertion of the blade roots, the roots radially outward against the edges of the grooves (18) for preventing the roots (16) from rocking in the grooves (18).

Inventors: HODGSON, ANTHONY, PARTINGTON, ALBERT JOSEPH
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1988
Main classification: F01D5
IPC codes: F01D5