Necessity truly is the mother of invention for the Tennessee Valley Authority (TVA). With as many as six major outages scheduled each year for its Hydro Modernization program, which began in 1992, TVA needed to reduce outage durations as much as possible to minimize lost generation. The goal of the modernization program, to be completed in 2018, is to increase capacity and efficiency at most of TVA’s 31 hydro plants. In total, these plants have a capacity of more than 5,400 MW.
Alignment of each unit before returning it to service took as long as a week. TVA saw an opportunity to shorten this time and consequently reduce associated outage costs, if an innovative solution could be reached. The goal was to reduce the duration of alignment activities — including plumbing the shaft center of rotation, measuring shaft run out, and equalizing the thrust bearing load — to one to two days, while maintaining or even improving accuracy of the alignment process. To accomplish this goal, TVA developed a Vertical Shaft Alignment Tool (VSAT) that includes multiple instruments that supply data to a customized software program. VSAT paid for itself in just one major outage.
The old way of aligning vertical shafts
Typically, alignment of vertical shafts in hydro generating units and other machinery, such as pumps, requires equalizing the load on the thrust bearing shoes or thrust member, plumbing the shafts to gravity, and plumbing the center line of the shaft throw circle (called run out or throw). Many of the tools and methods available to accomplish such alignments date back 100 years and tend to be slow and difficult to use.
The most common method (the industry standard), called the four plumb wire method, has been used since vertical hydro units were developed. To check the plumb or verticality of a shaft using this method, four music wires (or piano wires) are connected to a bracket at an upper elevation — such as at the thrust bearing bridge — and are connected at four locations 90 degrees apart. The bottom ends of these wires are connected to large plumb bobs (usually 50-pound weights) that are immersed in high-viscosity oil or other fluid for damping.
Using electric micrometers, plant personnel take readings of distances from the wires to the shaft at multiple elevations. The micrometer gives an audible click in the operator’s earphones when it makes contact between the wire and shaft. The micrometer acts as a switch to allow current to flow through the earphones when the circuit is completed through the wire and shaft, which are electrically connected.
By measuring the distances from the wires to the shaft at top and bottom elevations, personnel can determine the amount of deviation from plumb. By measuring the distances from the wires at elevations on each side of the generator-to-turbine shaft coupling, personnel can determine kink (deviation from parallel centerlines, also called dogleg) and offset (non-concentric centerlines). And by measuring the plumb of the shaft at various rotational positions (i.e. 0 degrees, 90 degrees, 180 degrees, and 270 degrees), personnel can determine shaft run out. This method, although workable and reasonably accurate, is very time-consuming, taking as long as a week.
Developing an alignment tool
TVA developed VSAT as a research and development project.1 In essence, VSAT involves mounting one pair of inclinometers on the turbine shaft and one pair on the generator shaft to measure deviation from plumb. TVA personnel then obtain shaft run out using multiple proximity sensor measurements from a stationary point relative to shaft movement, including associated offset and kink results from calculations.
To perform the calculations needed to determine deviation from plumb of the shaft, TVA developed a custom alignment software program. The Windows-based program operates on a laptop computer specially designed for rugged working conditions. The VSAT software performs these calculations while factoring out any horizontal movement, known as skate. This movement or skate can occur due to movement of the shaft in the clearances of the radial guide bearings as the shaft is rotated. VSAT can use any type of proximity sensor, such as eddy current (TVA’s preferred type of sensor), capacitive, inductive, laser, or mechanical (like common mechanical dial indicators).
The purpose of VSAT is to assist personnel in aligning a vertical shaft’s axis or center of rotation with the earth’s gravitational pull and to measure run out as the shaft is rotated. Although VSAT was designed primarily to assist in aligning the vertical turbine-generator shafts in hydroelectric units, it can be used to align vertical shafts in pumps and similar equipment. It is important to align a vertical shaft with the direction of the earth’s gravity to reduce and/or equalize the bearing load and, thus, to reduce wear on the bearings.
Applying the tool
To take the place of the standard four plumb wire method, VSAT had to meet the following six requirements:
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— Initially measure plumb at one position without rotating the shaft (called “rough” plumb);
— Measure the plumb of the center of rotation as the shaft is rotated;
— Measure run out or throw of the shaft as it is turned;
— Measure the shaft kink and coupling offset;
— Provide a method of adjusting the thrust bearing shoes to tilt the shaft toward the plumb position without changing load on the shoes; and
— Provide easier and faster use.
VSAT meets all the above requirements. Many of the technologies used — such as electronic digital inclinometers, proximity sensors, and micro computers — have only become available in the past ten years. The digital inclinometers provide more accurate plumb measurements than the four plumb wire method and are less subject to vibration and error.
A significant advantage of VSAT’s design is its ability to measure the static “as found” plumb of the shaft and to make adjustments to bring the shaft to a “rough” plumb position. In this rough plumb position, personnel can then rotate the shaft to find the plumb of the center of rotation. This is important because it is difficult to plumb a shaft without the load on the thrust bearing shoes equalized, and it is difficult to equalize the load on the shoes until the shaft is plumbed. With the shaft rough plumbed and the thrust bearing shoe load equalized, the unit will rotate readily on its hydrostatic bearing so the more accurate plumb of center of rotation can be checked and corrected as necessary. TVA plumbs its vertical hydro units to the center of rotation while holding industry standard tolerances.
Once VSAT measures deviation from plumb, a precision method is required to adjust the elevation of the thrust shoes to swing the shaft to the corrected plumb position (or plumb of center of rotation). There are several different types of thrust bearings — such as the jack screw, compression tube, and various spring-loaded types. The first two types have screws under each thrust shoe that can be adjusted up or down to load or unload a shoe or swing the shaft to a different position. The spring-loaded types are self-loading, but shimming of the thrust bridge support legs is required to swing the shaft.
The jack screw type is the most common design in TVA’s hydro plants. In the past, TVA used the slug-arc method to measure jack screw movement. This involved using a wrench and arc measuring scale to convert jack screw rotation and the screw thread pitch into axial screw movement. The method was called “slug” arc because originally the jack screw was slugged loose with a wrench and hammer and then its hand tightness was measured against the arc scale versus the original slugged tight (load carrying) position. This arc difference was a measure of the amount of spring in the jack screw, which translated into the amount of load on that thrust shoe.
Improvements in the method included using the generator brake jacks to unload the thrust shoes so that they could be turned using a hand wrench.
TVA further improved the method by replacing the wrench and arc measurement scale with a sheave (a grooved wheel), pull potentiometer (pot), and digital indicator. This new method was called the electronic slug arc method. Using this method, the sheave is attached to the jack screw and the cable from the pull pot wraps around the outside diameter of the groove in the sheave. As the screw and sheave turn, they extract or retract the cable from the pull pot, which results in an electrical signal to the digital indicator. The indicator displays a value in inches of sheave arc that is proportional to the rotation. The custom alignment software uses this value, along with the jack screw’s pitch, to calculate the axial movement of each thrust shoe screw required to bring the shaft to plumb. The software also is used along with the slug arc method to check the thrust shoe load and calculate the movements required to equalize load.
Results
TVA has used VSAT and the electronic slug arc method exclusively for the past seven years, as part of its Hydro Modernization and maintenance programs, to align more than 40 units. With combined major outage and lost generation costs equaling tens of thousands of dollars per day for conventional units and much more for a large pump-turbine unit, trimming a few days off the outage through alignment improvements is very lucrative. TVA recouped all the research and development costs for VSAT in just one outage of a major pump-turbine.
— By Jeffery C. Jones, principal system engineer, Tennessee Valley Authority, 1101 Market Street, LP3P-C, Chattanooga, TN 37402; (1) 423-751-8933; E-mail: [email protected].
Note
- VSAT is covered by U.S. Patent 7,111,407.
