By Enes Zulovic
Shaft alignment of a vertical hydro turbine-generator unit is a crucial yet highly specialized technical skill.
Accurate shaft alignment requires precise measurements and calculations. In theory, alignment is a straightforward process. However, in actual applications, it is often compounded by out-of-plumb shafts, guide bearing misalignment, thrust bracket deflections, or alkali-aggregate reaction just to name a few of the challenges. These challenges can produce considerable delays and unsatisfactory results in alignment work.
Based on experiences at Hydro Tasmania, I offer several ideas and guidelines for improving the process of aligning vertical shafts in hydro turbine-generator units.
Improving the alignment process
The process of aligning a shaft on a turbine-generator unit involves several steps. This includes determining an alignment procedure; establishing alignment tolerances; calculating and visualizing alignment data; and deciding when to check the accuracy of the alignment.
In the following sections, I offer ideas for improving each step in the process.
Determining the alignment procedure
For most project owners, alignment of a vertical shaft of a hydro unit involves:
- Leveling the generator thrust bracket (sometimes referred to as the spider or bridge) and shimming the thrust bracket legs to plumb the unit;
- Checking and correcting the shaft offset and dogleg at the shaft couplings;
- Checking the static runout and correcting errors caused by thrust block fit; and
- Making the bearings, stator, and turbine seal rings (labyrinths) concentric to a common shaft centerline.
- Checking and correcting for “soft foot,” which occurs when one of the thrust bracket feet does not sit flat on the base;
- Removing all shaft seals or contact points that would prevent the shaft from rotating in its natural position;
- Making the center of shaft throw plumb at the turbine bearing journal; and
- Adjusting the height of the shaft so that the turbine runner inlet edges closely align with the wicket gate cheek plate edges, which ensures a clean hydraulic profile.
- Shaft system runout, measured with dial gages or proximity probes during slow manual rotation;
- Shaft system plumb, measured with a precision level;
- Generator rotor and stator shape and concentricity, calculated from air-gap measurements;
- Concentricity of bearing housings to the shaft system, measured using micrometers; and
- Concentricity of runner stationary wear rings to the center of runner rotation, using feeler gages.
However, each owner may approach this work differently. Variations in alignment procedure depend primarily on the thrust bearing arrangement of the unit.1 Common thrust bearing arrangements are rigid, self-leveling, spring, and adjustable pad (shoe).
In addition to the standard items listed above, Hydro Tasmania performs four additional alignment steps. This extra work helps ensure precise alignment, which improves machine life and reliability by reducing vibration levels. These steps are:
Establishing alignment tolerances
Another part of the alignment process is determining the degree of accuracy required for shaft alignment. Several industry standards and guidelines are available to help provide realistic objectives for shaft alignment.1,2,3,4,5 However, engineering judgment must be used to determine if they are appropriate for a given situation.
Based on Hydro Tasmania’s experiences, I conclude that the tolerances listed in Table 1 can be achieved with reasonable effort and budget.
Click here to enlarge image
Existing guidelines and standards do not specify tolerances with regard to runout at turbine seal clearances (wear rings and labyrinths). Variations in turbine design mean that manufacturers’ requirements for runout at these locations vary. I believe that turbine seal (wear ring) runout of 20 percent of the minimum wear ring diametral clearance is an achievable criteria.
Calculating and visualizing alignment data
Performing and recording alignment measurements at the elevations of the thrust bearing, guide bearings, and shaft couplings can ensure quality alignment. At Hydro Tasmania, I take several measurements during the shaft alignment process. These measurements include, but are not limited to:
It may be impossible to use laser alignment equipment on vertical shaft hydro machines because the construction of the machine and foundation prevents line-of-sight access from top to bottom of the shaft. Other equipment can be used for vertical shaft alignment, such as electronic level, plumb 4 wires (piano), and tight wires.
Taking these measurements can generate significant amounts of data, and it can be challenging to present the information in a coherent manner for review. The use of spreadsheets and computer-generated plots has replaced manual graphical methods to present this information because the plots are easy to generate, modify, and interpret.
At Hydro Tasmania, I have developed spreadsheets for interpretation of level, shaft plumb, shaft runout, concentricity and shaft profile, generator air gaps, and wear ring alignment data. From these spreadsheets, plots can be developed to visually summarize data.
Deciding when to check the accuracy of the alignment
How often should shaft alignment be checked? Inspection and checking of the shaft alignment system can be time-consuming and expensive. Some guidelines recommend an initial major inspection at the end of the first year of operation, then an inspection every five to ten years.3 At a minimum, Hydro Tasmania recommends taking measurements every six years of shaft runout, horizontal level, verticality, and straightness.
Ongoing monitoring of shaft orbits can be used to check the shaft and guide bearing alignment. This can be achieved using proximity probes permanently installed on the unit, combined with vibration records taken during slow roll rotation during machine shutdown. Synchronous shaft orbit measurements taken during machine run-down can give a good indication of static runout. The presence of a twice synchronous vibration, measured using a vibration analyzer, can indicate misalignment of stationary components to the rotating element.
Correcting excessive shaft throw
One particularly challenging aspect of shaft alignment is correcting excessive shaft throw. Shaft throw is the radius of rotation of the shaft axis at any level about a true line normal to the thrust pad plane and extended downward from the bearing center. Shaft throw is measured with the shaft rotating, using a dial indicator between a fixed point and the rotating part. This diametric measurement (i.e., twice the radial measurement) is also known as the total indicator rearing, which is determined by taking the difference between the minimum and maximum readings for one complete shaft revolution.
Determining when shaft throw is excessive can be challenging. Hydro Tasmania uses set alignment tolerances. Typically, if these tolerances are exceeded, shaft throw is excessive and correction is required. However, correction may not be performed if a unit has a history of running successfully with tolerances outside the guidelines.
Four methods are used to correct excessive shaft throw. The challenge is to determine which is the most suitable method for the specific situation.
One method is to scrape or grind a taper on the two circular key rings in a direction 180 degrees opposite to the high point of shaft throw. Hydro Tasmania has successfully used this method on several machines. However, if the interference fit between the bore of the thrust block and the shaft is too tight, the forces resulting from the interference fit are predominant and scraping the key rings has minimal to no effect.
If a check shows that shaft runout has not responded to scraping the ring keys, it is likely that the shrink fit between the shaft and thrust block is too tight. Some manufacturers recommend scraping the bore of the thrust block to increase the diameter slightly. The goal of the scraping should be to provide a minimum permitted interference fit of 0.01 to 0.02 millimeter.
Accurately grinding the thrust block face at an angle can correct the shaft throw. This requires accurate measurements, calculations, and precise machining in the workshop.
If the thrust runner is bolted to the thrust block, installation of shims can be used to form a taper to prevent distortion of the thrust runner.
Other variables may affect efforts to correct shaft throw. For example, the design of some hydro units features a layer of insulation in the thrust block to prevent circulating currents. This insulation normally is fixed to the inner part of the thrust block (hub), and the main outer part of the thrust block has a high interference fit on the insulating material. Deterioration of the insulating material or installation and removal techniques that temporarily allow relaxation of this internal interference fit can allow movement between the thrust block and hub.
Any relative movement in this area affects the alignment of the thrust block to the shaft axis and results in changes in shaft runout. If the movement is large enough, a measurable gap may appear at the outer edge of the insulating material. The size of this gap can be measured and monitored using feeler gages. I recommend that this insulation air gap be checked before and after heating to ensure that there is no change. If a change is discovered, the thrust block must be reheated and reclamped to ensure the gap is uniform and consistent.
It also can be difficult to correct shaft throw if components of the shaft system possess machining errors. Workshop machining tolerances such as perpendicularity, parallel, concentricity, runout, and circularity need to be defined in relation to precision alignment acceptance limits. Any remedial machining work performed must comply with these tolerances. Shaft runout and plumb limits for an assembled machine cannot be specified above what is possible to achieve when machining individual components in a workshop environment. Similarly, it is unrealistic to specify machining tolerances that are smaller than the accuracy of the measuring equipment being used.
Machining tolerances for the shaft and thrust block, when checked at the factory, are satisfactory if they comply with requirements of the Institute of Electrical and Electronics Engineers Inc. (IEEE).6 Factory tests should include inspection of the shaft and thrust block, and any deviation should be corrected.
Training personnel to conduct shaft alignment
At Hydro Tasmania, I believe the time and money spent to qualify and certify personnel to conduct shaft alignment is justifiable. Such training can result in more efficient, higher quality work achieved in a timely manner. It is my opinion that managers, engineers, technicians, front line supervisors, and tradespeople should receive shaft alignment training. The objective of the training is to give personnel a working knowledge of what’s involved in the alignment process and the skills needed to achieve accurate alignment.
Although most of us do not like to be tested, appraising our skill level is beneficial. I recommend utilities institute a procedure to certify that their personnel conducting shaft alignments are sufficiently qualified.
Improving standards; sharing good practices
In my opinion, existing standards and procedures for alignment of vertical hydro units need to be improved. For example, I am not aware of an international standard that provides details on shaft throw correction methods. If owners of hydro projects throughout the world collaborated to develop detailed international standards and guidelines, the hydro industry could improve the long-term safe and reliable operation of hydro turbine-generator units.
I also recommend that hydro project owners share details of their alignment procedures with one another. This sharing can lead to improved knowledge and enhanced skills throughout the industry. s
Mr. Zulovic may be reached at Hydro Tasmania, 4 Elizabeth Street, Hobart, Tasmania 7000 Australia; (61) 3-62305319; E-mail: enes.zulovic@ hydro.com.au.
1 Duncan, B., Alignment of Vertical Shaft Hydro Units, U.S. Department of the Interior’s Bureau of Reclamation, Denver, Colo., United States, 2000.
2 “Installation of Vertical Hydraulic-Turbine-Driven Generators and Reversible Generator/Motors for Pumped Storage Installations,” Standard NEMA MG. 5.2, National Electrical Manufacturers Association, Rosslyn, Va., United States, 1972.
3 Hydroelectric Turbine Generator Units – Guide for Erection Tolerances and Shaft System Alignment, CEATI International, Montreal, Quebec, Canada, 2008.
4 “Guidelines on Methodology for Hydroelectric Francis Turbine Upgrading by Runner Replacement,” IEA Technical Report, International Energy Agency, Paris, 2001.
5 “Guide for the Operation and Maintenance of Hydro Generators,” Standard IEEE 492, Institute of Electrical and Electronics Engineers Inc., New York, United States, 1999.
6 “IEEE Standard for Hydraulic Turbine and Generator Integrally Forged Shaft Couplings and Shaft Run Out Tolerances,” Standard ANSI/IEEE Std 810-1987, Institute of Electrical and Electronics Engineers Inc., New York, United States, 1987.
Enes Zulovic, senior mechanical engineer, is responsible for unit alignment at Hydro Tasmania’s plants.
This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.