Traditionally, plumbness measurements on a vertical hydro-turbine/generator shaft involved stringing a series of wires along the length of the shaft, attaching a weight to the end of the wires and then measuring the space from the wire to the shaft using an electronic micrometer. Although this method was inexpensive and has been used for many years, it did require access to a long length of the shaft to achieve an accurate resolution. Also, measurements involve physically measuring the distance between the wire and the shaft at various elevations on the shaft, increasing the amount of time and personnel requirement for the measurement.
Ludeca Inc. presents its experience with a laser-based system which replaces the time-consuming wire method. Measurements now can be performed in a fraction of the time it would otherwise take with the wire measurement method. Ludeca’s measurement system, known as the PERMAPLUMB, uses a self-adjusting mechanical mirror, always plumb to earth, that reflects a Class 1 laser beam into a detector. It requires only 14 inches of axial space along the shaft. The mirror and transducer are attached by a bracket that uses magnets on the turbine shaft. From a single 270-degree shaft rotation, the system calculates and displays angularity and corrective moves and provides a statistical quality measurement of the data. A “move” function allows monitoring of corrections as they are being made. The resolution is better than 0.00002 inch per foot, which is more accurate than required by NEMA. Adjacent turbines also can continue to operate since the system is insensitive to vibration.
Introduction
Plumbness is the relationship of a rotating centerline to gravity. It can be thought of as the verticality of a centerline. In practice, with vertical hydro shaft measurement, it is actually “un-plumbness” that is being measured, since the factor quantified is the difference from perfectly vertical as shown in Figure
Figure 1. Shaft Centerline of Rotation in Relationship to the Plumb Line
Plumbness is expressed as an angle. Since the angle is small, an angular slope, or rate of change, is a more appropriate form of expression than degrees or radians. The most common unit of expression for this parameter in relation to vertical hydro shafts is thousandths of an inch per foot or thousandths of an inch per inch. Plumbness is measured in two planes. If you were looking down on the shaft, the two planes of measurements would be the zero-to-180-degree plane and the 90-to-270-degree plane.
Achieving plumbness in vertical hydro shaft applications is essential for proper operation. Precision plumbness helps reduce bearing temperatures, reduces shaft movement, reduces vibration and improves efficiency. From a scheduling perspective, achieving this accurately in the least amount of time is essential in saving money.
Measuring Plumbness
In measuring plumbness, various methods exist, including the use of tight-wires, lasers and optics. The tight-wire is the most commonly used method and it is also the least expensive method.
Four tight-wires are strung vertically along the length of the vertical hydro shaft. Typically, a non-magnetic stainless steel variety is used that is 0.020 to 0.030 inch in diameter. To ensure a minimum required range on the micrometer, the wires are placed approximately equidistant from the shaft. Each wire is spaced at 90-degree intervals from each other around the hydro shaft. A finned weight of 20 to 30 pounds is suspended at the end of each wire where it is submerged in an oil bath to help dampen movement and vibration in the wire.
An electric micrometer measures the distance between the hydro shaft and the wire. The wire and hydro shaft are electrically connected to each other in such a way as to complete an electrical circuit when the micrometer makes contact. This, in turn, activates an electronic auditory tone that lets the user know when to stop advancing the micrometer. Readings are taken at points along the shaft to establish the relative position between the tight-wire and the hydro shaft as shown in Figure 2.
Figure 2. Tight-wire Plumbness Measurement Concept
The tight-wire method excels in being low cost and having a rather intuitive setup. However, it is a measurement where “attention to detail” is paramount in reducing the error involved in the measurement process.
It is important to ensure that vibration be virtually non-existent during the measurement process. Disturbances of the tight-wire during measurement will distort the accuracy of the readings. In some cases, neighboring hydro units are kept in operation while the measurement process is being performed. If the units were forced to be shut down for the sake of the measurement process, lost revenue from the operation of those turbines would result. Additionally, chances increase for the wire to be physically disturbed when other projects are being performed near the vicinity of the measurement, particularly during a repair or overhaul.
The technician must be careful that the measurement is performed consistently and accurately. It is assumed that the surface being measured is of high quality and that it is representative of concentricity of the shaft or coupling. Shaft out-of-roundness, pits, rust or grime can affect the accuracy of the reading. Variations of the micrometer reading can vary from person to person depending on when they stop advancing the micrometer to touch the tight-wire.
Accuracy in tight-wire readings increases when a greater axial length is measured. This means it is desirable to string the tight-wire to the longest distance that can be achieved. This also means that more of the shaft is dedicated to the measurement procedure. Consequently, the micrometer operator must travel farther distances to measurement points up and down the length of the shaft, or additional operators must be employed. Once the readings are taken, data needs to be recorded accurately by the operator or the person in charge of evaluating the data.
It is apparent that there are many factors involved in tight-wire measurements that can induce error strictly from the measurement process as well as in prolonging the amount of time and space needed to take the measurements. To achieve an increase in productivity, it is necessary to reduce the human element from the measurement process, reduce the amount of area needed to be measured on the shaft, and reduce the amount of time required to obtain measurements and perform corrections.
Laser-Based Plumbness Measurement Method
The PERMAPLUMB system is a laser-based plumbness measurement tool. It circumvents a great number of the limitations of the tight-wire method by removing the human element from the measurement process through the use of a laser and a computer for data acquisition. This helps to ensure that the accuracy of the measurements is not user dependent. In addition, the PERMAPLUMB system offers a measurement resolution an entire order of magnitude better than that obtainable with the tightwire/micrometer method. The system also sets up easily and gives alignment readings quickly on demand, reducing the amount of time dedicated to the measurement process.
The system consists of a laser and mirror mounted on a compact magnetic bracket that is only 14 inches long, as shown in Figure 3.
Figure 3. The PERMAPLUMB and 14-inch Bracket
The laser transducer is mounted upon the bracket as shown in Figure 4.
Figure 4. Laser Transducer Mounted upon the Permaplumb Bracket
A beam is reflected off a self-leveling mirror on the bottom of the bracket back into a 1-micron-resolution detector located inside the laser transducer. The mirror’s two-axis pivot points ensure that the mirror’s surface will always maintain relative plumbness to the horizon, as shown in Figure 5.
Figure 5. Self-adjusting Mirror Assembly
Plumbness is measured by simply attaching the system to the vertical shaft by means of its integrated magnetic feet and rotating the shaft to four positions 90 degrees apart as shown in Figure 6. A measurement is taken at each of the 90-degree positions. After the last measurement is taken, the plumbness results can be displayed through the computer. Correcting plumbness is usually done in one of two ways: axially moving (shimming) the thrust bearing or translating (horizontally moving) the thrust bearing. PERMAPLUMB will provide you with correction values and a live “move” function with which to monitor corrections in real time for either (or both) of these two approaches.
Figure 6. Attaching the PERMAPLUMB to the Hydro Shaft
Using PERMAPLUMB for Vertical Hydro Shaft Alignment
The PERMAPLUMB system can readily be integrated into procedures that require vertical shaft plumbness measurements. Because the shaft will be rotated, it is necessary to take into account factors that will ensure no obstructions or interferences during the rotation. This includes backing off any adjustable bearing shoes so that the shaft may be rotated without any rubbing.
Standard checks and safety procedures always should be performed prior to the measurement process. Such checks include ensuring that no “dog leg” or run-out exists in the shaft. Dial indicators or proximity probes can be mounted at various positions along the shaft to quickly determine if such problems exist and if they need to be addressed prior to and/or after the plumbness measurements and corrections. Proximity probes achieve high accuracies and are less susceptible to shaft surface finish inaccuracies than dial indicators. Data can be acquired in one turn of the shaft, thereby speeding up the process of correcting and detecting “dog leg” and run-out issues.
The equal loading of all of the pads of the thrust bearing needs to be ensured. Various procedures exist depending upon the type of thrust bearing that is being used. Methods range from the “slugged arc” method to the more advanced and time-saving method of integrating load cells into the adjustable shoes. In most cases, plumbing the shaft and equalizing the loads will go hand-in-hand between corrections.
Measurements begin by mounting the PERMAPLUMB system onto any area along the length of the hydro shaft. The easiest area to access would typically be the deck above the turbine, as shown in Figure 7.
Figure 7. The PERMAPLUMB System Installed on the Hydro Shaft
The system is then connected to a laptop computer and power supply.
Dimensions and parameters are entered into the WinPLUMB software package. Such dimensions include the thrust bearing data to make corrections on the thrust bearing, translation data to monitor the shaft position at the upper and lower guide bearings, and tolerances to help determine when the shaft is plumbed to tolerance, as shown in Figure 8.
Figure 8. Entering Dimensions, Tolerances and Targets into the WinPLUMB Software
Once the dimensions are entered, it is time to take the measurements. This begins by turning on the high-pressure lubrication system and having the mounted PERMAPLUMB system on the shaft rotated to the designated “0” degree position. The high-pressure lubrication system is then deactivated to let the shaft settle down. This is where the first measurement point will be taken. The high-pressure lubrication system is then reactivated and the process is repeated for the next three points over 270 degrees of shaft rotation.
The results achieved (see Figure 9) will display the plumbness results in mils per inch. The NEMA tolerance of 0.25 mils per foot is entered (as 0.0208 mils per inch) into the tolerance function so that the system can indicate if the tolerance was achieved. The measurement resolution is better than 0.00002 inch per foot, which is far better than the tolerance required by NEMA.
Should corrections be required on the thrust bearing pads, PERMAPLUMB provides the amount each pad should be moved up (or down) to achieve tolerance, as shown in Figure 10. A special feature in the software also allows for “Pad Up & Down”, which yields optimized corrections for adding and subtracting to the pad height in order to achieve plumbness without changing the shaft elevation.
Figure 10. Thrust Bearing Corrections for Each Shoe
Corrections on the pads are performed and the shaft is remeasured to verify plumbness. The whole process can be measured “live”, if needed. A special “live move” mode is activated to continuously update the hydro shaft’s plumbness condition as corrections are being made, as shown in Figure 11. This not only updates the actual plumbness value, but continuously updates the predicted thrust bearing pad corrections as well as the shaft position change at the upper and lower guide bearings. Plumbness is able to be continuously monitored for situations in which the thrust bearing itself may need to be translated.
The “live move” mode is particularly useful in ensuring that corrections are made without interference during the move. Should the shaft contact the guide bearings or become obstructed during the repositioning, this would become evident in this mode.
Assurance of measurement accuracy during the measurement process is vital. Data collected needs to be relied upon to perform corrections and verify results. During the data collection process, a sample of 32 readings per second can be collected per measurement point. The user can select how many seconds of data are to be collected per measurement point, up to 204 seconds. This is extremely useful when vibration could become an issue during measurement, which is usually the case when adjacent turbines are running during the measurement process.
The “standard deviation” display ensures that the measurement duration selected creates stable data to overcome vibration issues. This is useful in finding the optimum amount of time required for data collection to create a stable measurement while reducing measurement time to the minimum possible.
Measurement accuracy also is ensured through rotation to four points. A special feature known as “circular completion” ensures that the hydro shaft is rotated about an axis with zero interference. It ensures that readings at the four points follow the equation that the sum of the zero-degree and 180-degree readings should equal the sum of the 90- and 270-degree readings. If that equation is violated during measurement, a circular completion error will result showing the degree to which this violation occurs. A value of 0.2 in the raw detector data or less would be considered a good reading. This feature indicates if shaft interference issues have occurred during measurement
Repeatability of the measurement is essential as a standard measurement process. PERMAPLUMB features a repeatability function to allow for previous measurements to be compared to the current measurements. Repeatable measurements ensure that “what you see is what it really is.”
Repeatability checks with the PERMAPLUMB also can be used to verify shaft rigidity. A perfectly rigid shaft will create identical plumbness readings with the PERMAPLUMB no matter where the system is mounted on the hydro shaft. Rigidity can be inspected between two shafts that are connected by a solid coupling.
Next, we describe how this is done: Plumbness is measured below the coupling at least twice to establish repeatability. Plumbness is then measured above the coupling, again twice to ensure repeatability. The results between the measurements taken above and below the coupling are then compared to establish rigidity. Any difference in the results of greater than 0.004 mils per inch would be considered a lack of rigidity.
Conclusion
The ability to perform a plumbness measurement in the least amount of time with a high degree of accuracy benefits in both the short term, with improved time savings, and the long term, with improved service life and efficiency. Quick setup, reliability, immunity to vibration, accuracy and time savings are all factors that make the PERMAPLUMB system ideal for plumbness measurements.
About the author
Daus Studenberg is an applications engineer for Ludeca Inc. in Miami, Fla. He holds a bachelor’s degree in mechanical engineering from the University of Florida. His responsibilities at Ludeca include field service, technical support, training and product development. Questions regarding this article can be sent to Daus.Studenberg@ludeca.com.
More information about the PERMAPLUMB system can be found at:
http://www.ludeca.com/prod_permaplumb.php
http://www.ludeca.com/prod_winplumb.php
