Patent Application
20090022591
The present invention relates to a rotating blade and rotor for a steam turbine and, more particularly, to an attachment arrangement for attaching a blade of a steam turbine to a rotor that minimizes local and average stresses.
The steam flow path of a steam turbine is formed by a stationary cylinder and a rotor. A number of stationary vanes are attached to the cylinder in a circumferential array and extend inward into the steam flow path. Similarly, a number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades form a stage. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. The blade airfoils extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached to it.
The blade airfoils extend from a blade root used to secure the blade to the rotor disc. Conventionally, this is accomplished by imparting a fir tree shape to the root by forming approximately axially extending alternating tangs and grooves along the sides of the blade root. Slots having mating tangs and grooves are formed in the rotor disc. When the blade root is slid into the disc slot, the centrifugal load on the blade, which is very high due to the high rotational speed of the rotor—typically 3600 rpm for a steam turbine employed in electrical power generation, is distributed along portions of the tangs over which the root and disc are in contact. Because of the high centrifugal loading, the stresses in the blade root and disc slot are very high. It is desirable, therefore, to minimize the stress concentrations formed by the tangs and grooves and maximize the bearing areas over which the contact forces between the blade root and disc slot occur. This is especially desirable in the latter rows of a low pressure steam turbine due to the large size and weight of the blades in these rows and the presence of stress corrosion due to moisture in the steam flow. The latter stages experience higher local stresses that may lead to lower fatigue life of the rotor and the rotating blades. There is also an increasing demand for longer rotating blades, which requires the rotor and blades to operate under even higher loads.
In addition to the steady centrifugal loading, the blades are also subject to vibration.
It is therefore desirable to provide a rotor and blade attachment configuration that has centrifugal load carrying capability, reduced local stresses on the rotor (wheel) and the fillets of the rotating blades, while maintaining average and shear stresses low.
In one embodiment of the invention, a blade mountable to a disc comprises a blade platform and a blade root extending from the blade platform. The blade root comprises first, second, and third hooks and first, second and third necks. Each hook comprises a contact surface and a non-contact surface, and an angle between each contact surface and each non-contact surface is optimized to reduce local and average stresses.
In another embodiment of the invention, a blade mountable to a disc comprises a blade platform and a blade root extending from the blade platform. The blade root comprises first, second, and third hooks and first, second and third necks. Each hook comprises a contact surface and a non-contact surface, and an angle between each contact surface and each non-contact surface is about 70.6°.
In a further embodiment of the invention, a turbine comprises a blade root extending from the blade platform. The blade root comprises first, second, and third blade hooks and first, second and third blade necks. Each blade hook comprises a contact surface and a non-contact surface, and an angle between each contact surface and each non-contact surface. The turbine further comprises a rotor disc comprising a slot. The slot comprises first, second and third rotor hooks and first, second and third rotor necks. Each rotor hook comprises a contact surface in contact with a corresponding contact surface of the blade and a non-contact surface spaced from a corresponding non-contact surface of the blade. The rotor contact surfaces are angled from the rotor non-contact surface at the same angle as the angle between the blade contact surfaces and the blade non-contact surfaces. The angle is optimized to reduce local and average stresses between the contact surfaces.
Referring to
The top hook 8 includes a top slanted contact, or crush, surface 18. The top hook 8 also comprises a top non-contact surface 20. The middle hook 12 comprises a middle slanted contact, or crush, surface 22 and a middle non-contact surface 24. The bottom hook 16 comprises a bottom slanted contact, or crush, surface 26 and a bottom non-contact surface 28.
As shown in
The top hook 48 comprises a top slanted, or crush, surface 72 and a top non-contact surface 74. The middle hook 52 comprises a middle slanted contact, or crush, surface 76 and a middle non-contact surface 78. The bottom hook 56 comprises a bottom slanted contact, or crush, surface 80 and a bottom non-contact surface 82.
Referring to
Referring to
The crush surfaces are rotated, or oriented, such that the transition angle between the crush surfaces and the non-contact surfaces is about 70.6°. The slant angle is generally substantially symmetrical about the axis X. Concentrated stresses result when load paths are forced to change direction. By providing the slanted crush surfaces, the change in direction is less severe and the stress concentration is lower. The slanted crush surfaces also permit a larger fillet radius in the transition distance. The larger fillet radius also results in a lower concentrated stress, while increasing the crush contact area.
Referring to
The top hook 8 of the bucket dovetail 4 comprises a top hook fillet 32. The top hook fillet 32 comprises two radii 32r1, 32r2 and a flat surface 32f. The middle hook 12 of the bucket dovetail 4 also comprises a middle hook fillet 36 that comprises a first radius 36r1 a second radius 36r2 joined by a flat surface 36f. The bottom hook 16 of the bucket dovetail 4 comprises a bottom hook fillet 40 that comprises a compound radius 40r ending with a flat 40f at the bottom of the bucket dovetail 4.
Referring to
The top hook 48 of the rotor dovetail 46 comprises a top hook fillet 60. The top hook fillet 60 comprises a single radius 60r. The middle hook 52 comprises a middle hook fillet 64 and the bottom hook 56 comprises a bottom hook fillet 68. The middle hook fillet 64 comprises two radii 64r1, 64r2. As shown in
As shown in
The top hook fillet 60, on one hand, and the middle and bottom hook fillets 64 and 68, on the other hand, are different and optimized to carry loads equally. The top hook fillet 60 has a larger radius 60r than the middle hook fillet 64 and the bottom hook fillet 68 to provide a smooth transition with the top rotor surface 84.
The hook thickness and neck length controls the load sharing between hooks as well as the bending and shear stiffness/stresses in the hook. All of this contributes to the degree of concentrated stress and strain. The hook thickness and neck length are optimized to minimize local and average stresses. As shown in the drawing figures, the hook thickness is the difference between the dimensions from the X axis along the dovetail centerline Y. For example, the top hook 24 has a thickness of 14.466−8.817=7.643.
As described herein, the location of the radii, the values of the radii, and the other aspects of the shape of the bucket dovetail and rotor dovetail, including, but not limited to, the hook thicknesses and neck lengths, are optimized to minimize the local and average stresses. As shown in the drawing figures, the values of the location of the radii, the values of the radii, the hook thicknesses and neck lengths are shown in millimeters, and the corresponding dimensions in inches are shown in square brackets. However, it should be appreciated that the bucket dovetail and rotor dovetail may be scaled to greater or lesser sizes provided that the shapes remain the same. The values shown in the drawing figures may thus be considered non-dimensional.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
