shaft alignment manual

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Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.

Shaft Alignment GuideTR-112449

Final Report, September 1999

EPRI Project Manager R. Knipschield

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA 800.313.3774 650.855.2121 [email protected] www.epri.com

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIESTHIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Rota-Tech, Inc.

ORDERING INFORMATIONRequests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box 23205, Pleasant Hill, CA 94523, (800) 313-3774. Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc. Copyright 1999 Electric Power Research Institute, Inc. All rights reserved.

CITATIONSThis report was prepared by Rota-Tech, Inc. 4104 Cindy Lane Denver, NC 28037 Principal Investigator J. Campbell Nuclear Maintenance Applications Center (NMAC) 1300 W.T. Harris Blvd. Charlotte, NC 28262 This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Shaft Alignment Guide, EPRI, Palo Alto, CA: 1999. Report TR-112449.

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REPORT SUMMARY

Shaft misalignment has long been recognized as a source of problems for machinery, operators, and owners. Experts agree that correct shaft alignment is one of the most important elements in improving machinery reliability. In an effort to reduce the frequency of machinery misalignment and improve machinery reliability, this guide provides users with an understanding of the concept of proper shaft alignment through a discussion of the fundamentals of alignment. This guide also provides a limited discussion of machine problems, the impact of misalignment on the machine, and the consequences of misalignment on machine reliability. Background Within a nuclear power station, machinery shaft misalignment is responsible for major expenditures in the form of labor, machinery parts, and lost generation capacity. In response, large amounts of time and money are continually invested in state-of-the-art alignment systems, equipment, and training. Most of the training, however, focuses on operating the new systems with little or no training on the principles of proper shaft alignment. Consequently, fundamental problems and causes of misalignment continue to be overlooked, misalignment continues to occur, and machinery reliability is not improvedeven after deployment of these costly systems. Objective To provide maintenance personnel with a thorough understanding of the fundamentals of proper shaft alignment in order to enhance the use of all shaft alignment systems. Approach This guide provides users with a tool to assist in decision making for improvement of reliability associated with rotating machinery, specifically improvements in shaft alignment practices. The document is geared to the whys rather than the hows of shaft alignment. With the age of most nuclear power plants and the training programs in place, the procedure for the actual task of performing shaft alignments is well documented. The guide could not be written without some specifics of certain alignment tasks; however, a blend of technical information has been the goal in the preparation of this document. Results The resulting guide provides a thorough discussion of the fundamental causes and effects of misalignment on machinery and how knowledge of the fundamentals of shaft alignment is crucial to performing consistent, correct shaft alignment.

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EPRI Perspective Within the power station, shaft misalignment is responsible for major expenditures in the form of lost generation capacity, as well as labor and machinery parts. This guide provides plant maintenance personnel with fundamental information that will enhance their ability to achieve proper alignment using any shaft alignment system, thus increasing mean time between failures (MTBFs) and improving the reliability of all machinery with each alignment. TR-112449 Keywords Maintenance Shaft alignment Reliability

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EPRI Licensed Material

ACKNOWLEDGMENTSEPRI would like to recognize the contributions of the following individuals in the development and review of this guide: William Bramlett Deyton Brunson Michael Calistrat Alistair Campbell Pedro Cassanova Galen Evans Bob Fulbright Jerry Garner Frank Hale Charlie Jackson Darron Jones Randy Kerr Jon Mancuso Richard Massey Larry Pope Kyle Russell Tony Scheetz Deiter Seidenthal Dale Smith Watson Tomlinson Randy VanSurDam Oconee Station/Duke Energy Corp. Brunson Instrument Company Michael Calistrat & Associates Bently Nevada Corp. Ludeca, Inc. Ludeca, Inc. McGuire Station/Duke Energy Corp. Commanche Peak/Texas Utilities Electric Co. Catawba Station/Duke Energy Corp. Consultant Commanche Peak/Texas Utilities Electric Co. PECO Energy Co. Kop-Flex Couplings A-Line Mfg. Commanche Peak/Texas Utilities Electric Co. Duke Engineering & Services Commanche Peak/Texas Utilities Electric Co. Ludeca, Inc. Smith Services Duke Energy Corp. Oconee Station/Duke Energy Corp.

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PREFACEShaft misalignment has long been recognized as a source of problems for machinery, operators, and owners. Within the power station, shaft misalignment is responsible for major expenditures in the form of labor, machinery parts, and lost generation. Because experts agree that maintaining correct shaft alignment is essential to improving machinery reliability, large amounts of time and money are continually invested in state-of-theart laser alignment systems, equipment, and training. Unfortunately, this solution is much akin to the golfer who thinks If only I could afford expensive clubsthey would make my game better, while completely disregarding the fundamentals. There is no argument that more accurate alignment results can be obtained through the use of these systems; however, the problems that caused the misalignment often continue to be overlooked, just as they were before the systems were deployed. In light of this, the objective of this guide is to provide a thorough explanation of the fundamentals of proper shaft alignment and to give examples of how this knowledge can improve all shaft alignment practices. Because misalignment is a function of the behavior of the total machine train and system, this guide also includes a limited discussion of machine problems, the effects of misalignment, and the consequences of misalignment on machine reliability. The premise of this guide is that practicing shaft alignment with a thorough understanding of the fundamentals can enhance the effectiveness of all shaft alignment systems, resulting in improved machine reliability.

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CONTENTS

1 INTRODUCTION.................................................................................................................. 1-1 2 FUNDAMENTALS OF A PROPER SHAFT ALIGNMENT ................................................... 2-1Thorough Understanding of the Fundamentals of Shaft Alignment ..................................... 2-2 Complete Analysis of the Machine From the Ground Up .................................................... 2-3 Thorough Understanding of the Behavior of the Machine As Part of the System ................ 2-3 Thorough Understanding of the Impact of Misalignment on the Machine............................ 2-3 Proper Sequence of Alignment Analysis............................................................................. 2-4

3 EFFECTS OF MISALIGNMENT .......................................................................................... 3-1Types of Machines Aligned in Nuclear Stations .................................................................. 3-1 Bearings ............................................................................................................................. 3-1 Alignment Tolerances......................................................................................................... 3-6

4 MACHINE FRAME DISTORTION - SOFT FOOT................................................................. 4-1Soft Foot............................................................................................................................. 4-1 Measuring Soft Foot Index.................................................................................................. 4-2 Distinguishing Types of Soft Foot, Possible Causes, and Proper Correction Techniques......................................................................................................................... 4-2 Parallel Air Gap.............................................................................................................. 4-3 Bent Foot or Angled Base.............................................................................................. 4-4 Taper Shims to Remove Soft Foot...................................................................................... 4-5 Baseplate and Foundation Irregularities ............................................................................. 4-6 Deterioration .................................................................................................................. 4-6 Machinery Vibration ....................................................................................................... 4-6 Induced Soft Foot and Nozzle Loads.................................................................................. 4-7 Nozzle Loads ..................................................................................................................... 4-9 Jacking Bolts or Taper Pins .............................................................................................. 4-10 Gaps Without Soft Foot .................................................................................................... 4-12

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5 MACHINERY POSITION CHANGES................................................................................... 5-1Infrared Thermography....................................................................................................... 5-7 Alignment and Preloads ..................................................................................................... 5-8

6 SHAFT COUPLINGS AND POWER TRANSMISSION ........................................................ 6-1Flexible Couplings .............................................................................................................. 6-1 Restoring Forces and Moments.......................................................................................... 6-5 Misalignment ...................................................................................................................... 6-6 Advantages and Disadvantages of Coupling Types............................................................ 6-9

7 VERTICAL MACHINES ....................................................................................................... 7-1Vertical Machines with Rigid Couplings .............................................................................. 7-1 Vertical Machine Behavior ............................................................................................. 7-1 Causes of Misalignment ..................................................................................................... 7-2 Alignment Procedure (Steps 19) .................................................................................. 7-6 Pump Coupling Procedure (Steps AG) .................................................................... 7-7 Alignment Procedure (continued)................................................................................... 7-8 Conclusions........................................................................................................................ 7-9

8 REFERENCES .................................................................................................................... 8-1

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LIST OF FIGURESFigure 1-1 Reverse Dial Setup ................................................................................................ 1-2 Figure 1-2 Typical Laser Alignment System ............................................................................ 1-2 Figure 2-1 Colinear Alignment................................................................................................. 2-1 Figure 2-2 Offset Misalignment ............................................................................................... 2-1 Figure 2-3 Angular Misalignment............................................................................................. 2-2 Figure 2-4 Offset and Angularity.............................................................................................. 2-2 Figure 3-1 Ball Bearing (Anti-Friction Bearing) ........................................................................ 3-2 Figure 3-2 Laser Alignment System ........................................................................................ 3-4 Figure 3-3 Two Misaligned Shafts Using a Spacer Coupling ................................................... 3-5 Figure 4-1 Parallel Air Gap...................................................................................................... 4-3 Figure 4-2 Bent Foot or Angled Base ...................................................................................... 4-4 Figure 4-3 Step Shim .............................................................................................................. 4-5 Figure 4-4 Pump Base Degradation Resulting From Transmitted Forces................................ 4-7 Figure 4-5 Examples of Induced Soft Foot .............................................................................. 4-8 Figure 4-6 Pump Outboard Replacement Keys ..................................................................... 4-10 Figure 4-7 Key Adjusting Bolts .............................................................................................. 4-11 Figure 5-1 Alignment of Shaft Centerline Heights.................................................................... 5-2 Figure 5-2 Expansion Chart .................................................................................................... 5-3 Figure 5-3 Acculign Bars Measuring Movement of a Steam Generator Feed Pump ................ 5-4 Figure 5-4 Laser Monitoring Movement Between Steam Generator Feed Pump and Turbine ............................................................................................................................ 5-5 Figure 5-5 Dynalign (Dodd Bars) Being Used to Monitor Alignment Changes ......................... 5-5 Figure 5-6 Precision Sight Level Used for Optical Alignment Checks ...................................... 5-6 Figure 5-7 Jig Transit Used for Measuring Alignment.............................................................. 5-6 Figure 5-8 Scales Used With Jig Transits and Precision Levels .............................................. 5-7 Figure 5-9 Shaft Orbits Acquired From Eddy Current Probes on a Sleeve Bearing Machine........................................................................................................................... 5-8 Figure 6-1 Gear Coupling........................................................................................................ 6-2 Figure 6-2 Grid Coupling ......................................................................................................... 6-3 Figure 6-3 Diaphragm Coupling .............................................................................................. 6-4 Figure 6-4 Flexible Disk Coupling............................................................................................ 6-5 Figure 6-5 Stub Shaft Replacement ........................................................................................ 6-6

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Figure 6-6 Point of Moment..................................................................................................... 6-7 Figure 6-7 Angular Misalignment............................................................................................. 6-7 Figure 6-8 Angular Misalignment and Offset at P2 .................................................................. 6-8 Figure 6-9 Angular Misalignment and Offset at P1 and P2 ...................................................... 6-8 Figure 7-1 Typical Stuffing Box, Noting Four Points Where Dimensional Runouts and Concentricities Are To Be Measured ............................................................................... 7-2 Figure 7-2 Typical Discharge Head (Motor Stand)................................................................... 7-3 Figure 7-3 Alignment Fixture Aligning Motor to Stuffing Box.................................................... 7-4 Figure 7-4 Adjustable Coupling Spacer and Nut in a Typical Pump Coupling.......................... 7-5 Figure 7-5 Pump Coupling Indicator Positions......................................................................... 7-6

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LIST OF TABLESTable 3-1 Alignment Tolerances.............................................................................................. 3-6 Table 6-1 Coupling Advantages and Disadvantages ............................................................... 6-9 Table 7-1 TIR Measurements From Installing the Pump Coupling........................................... 7-7 Table 7-2 TIR Measurements From Installing the Pump Coupling........................................... 7-8 Table 7-3 TIR Readings From the Vertical Pump Shaft Alignment Procedure ......................... 7-8 Table 7-4 TIR Readings From the Final Pump Shaft Alignment .............................................. 7-9

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1INTRODUCTIONSimply put, shaft alignment is placing two or more shafts along one axis or centerline. Although proper shaft alignment that increases machine reliability is much more complex, this simplistic approach is how alignment is often performedwith total disregard for the fundamental problems and causes of shaft misalignment. The widespread use of this approach is shown by the increasing use of laser shaft alignment systems. Management personnel, acting on input from supervisors, technical personnel, and end users, often purchase these systems with the erroneous assumption that the laser shaft alignment system will eliminate their misalignment problems. In fact, it seems that anyone who can align machines relatively easily or quickly using these systems instead of other methods is now considered to have achieved total proficiency in performing shaft alignment, which greatly improves reliability. In some cases, reliability is improved. This is normal where improper practices were previously performed with methods that have been used for a number of years. An example would be someone performing alignment using the rim and face method and not accounting for sag in the indicator. In reality, however, this new-found knowledge gives the user and management the ability to perceive that good alignment has been performed by the fact that the given tolerances are quickly and sometimes easily reached. The result is that fundamental problems and causes of misalignment continue to be overlooked, just as they were before these costly systems were deployed. The most widely used methods for aligning machines with accuracy are the laser systems and the reverse dial methods. See the reverse dial method in Figure 1-1 and a typical laser alignment system in Figure 1-2.

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EPRI Licensed Material Introduction

Figure 1-1 Reverse Dial Setup

Figure 1-2 Typical Laser Alignment System

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It is easy to understand the widespread use of laser alignment systems for these reasons: They are easy to set up. They record data accurately (when correct data are input). They give fast and accurate alignment corrections. They assist in making machinery adjustments to achieve alignment.

But, if the goal of proper shaft alignment is to improve machinery reliability, then the success of these systems should be measured by whether the mean time between failures (MTBF) increases with each alignment and, if so, by what percentage as compared to before the implementation of the system. Misaligned machines can be divided into two categories: Machines with minor alignment problems Machines with major alignment problems

Machines with minor alignment problems exhibit satisfactory run time with a long mean time between failures. These are the machines that, when checked for shaft alignment deviations from specified targets and tolerances, are fairly close to being aligned from the last time they were aligned. These are also the machines that do not show signs of stress, that is, the foundation is in satisfactory condition and has been for some time. Removal of the machine does not reveal piping strain or loads. The bearing temperatures are within range, and vibration typically remains within acceptable ranges for long periods of time. These machines have several advantages. In many cases, the machinery is not far from ambient temperature. Nozzle loads are small, and soft foot has been corrected if there was a problem. The foundation and grouting were installed properly, and periodic lubrication is performed at regular intervals and in a prescribed manner. These machines have a long run time between failures. The machines with major alignment problems are the ones that, when checked for alignment, are always far out of specifications when as found shaft alignment data are taken. If accurate records have been kept, the misalignment is always in the same direction perpendicular to the centerline of the shaft. The offsets and angularities are always somewhat close to the last data that were taken, if conditions were the same. These machines typically operate under high load conditions, at elevated temperatures, and with piping loads. The foundation reveals signs of deterioration, and the mean time between failures is short. Most of the time used in alignment is spent to put the machine where the correct position is thought to be. Great energy and expenses are expended to correct the obvious problems, but the root cause of misalignment is ignored and the cycle continues. These are the machines that generally have high profits associated with them and the ones that this guide focuses on.

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A failure is defined as the following: The condition or fact of not achieving the desired end or ends A cessation of proper functioning or performance Nonperformance of what is requested or expected

All of the above definitions can be applied to failure of machinery at a power station. As related to shaft alignment, a failure can imply a degradation of any of the components or subcomponents within a machine or piece of equipment. These failures can damage or destroy couplings, rotors, mechanical seals, or bearings. With proper shaft alignment, machinery reliability is improved, MTBF is increased, and the cost and quantities of replacement parts are reduced. The new buzz word is mean time between repairs (MTBR), which is essentially a MTBF because, if a repair is made, in a sense it is still a failure. Only preventive maintenance can be performed as a repair without a failure. Most maintenance, other than corrective, now being employed at power stations is either predictive maintenance (PDM) or reliability-centered maintenance (RCM). Remember that, in order to have complete and satisfactory alignment, there must also be satisfactory coupling alignment. The couplings joining two or more shafts are a part of the rotor systems; however, based on coupling manufacturers allowable misalignment, satisfactory coupling alignment does not mean satisfactory shaft alignment.

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2FUNDAMENTALS OF A PROPER SHAFT ALIGNMENTProper shaft alignment is the aligning of two or more shafts to a colinear position at operating conditions, while ensuring that the shafts are operating with a minimum of forces applied to the individual shafts and across the coupling. See Figure 2-1 for an example of colinear alignment.

Figure 2-1 Colinear Alignment

How is shaft misalignment defined? There are three types of misalignment conditions: Offset - when two shafts are not coincidental to the same axis or centerline (see Figure 2-2) Angularity - when one or two shafts move away or toward the centerline as they approach or distance themselves from one or more machines (see Figure 2-3) A combination of the two (see Figure 2-4)

Figure 2-2 Offset Misalignment

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EPRI Licensed Material Fundamentals of a Proper Shaft Alignment

Figure 2-3 Angular Misalignment

Figure 2-4 Offset and Angularity

Very seldom is one type of misalignment (offset or angularity) present without the other. Since this is the case, the term offset better illustrates misalignment not on the centerlines. Some authors prefer the term parallel misalignment, but this implies that the offsets are always parallel to each other. For the purposes of this document, misalignment not on the centerlines is referred to as offset misalignment. The essential elements for performing proper shaft alignment include: Thorough understanding of the fundamentals of shaft alignment Complete analysis of the machine from the ground up Thorough understanding of the behavior of the machine as part of the system Thorough understanding of the impact of misalignment on the machine

Thorough Understanding of the Fundamentals of Shaft AlignmentAlthough proper training in the fundamentals of alignment is the first step to achieving satisfactory results, it is by no means the only step. Additional training should include: The basic mathematics involved with alignment The concept of soft foot and all of its variables Proper training on the alignment system being used A thorough understanding of thermal growth and machine running positions

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A basic understanding of coupling behavior for the various designs of couplings used An understanding how alignment impacts machines of different designs

Complete Analysis of the Machine From the Ground UpWhile analysis in this sense does not necessarily mean the use of vibration analysis or infrared thermography, these and other types of data collection techniques do need to be used to gather data to analyze machine problems. Visual inspection plays a very important part in machine analysis as it relates to shaft alignment. The condition of the baseplate and anchor bolts should be one of the first things inspected when approaching a machine for alignment or analyzing a problem machine where misalignment is the suspected cause of problems. Look closely at the condition of the grout under the base of a machine if this type of base is used. Look for leaks of water, oil, or other fluids, noting any corrosion to the base that these fluids might have caused. If the machine train consists of a drive steam turbine, look for leaks around the gland steam sealing area. If the baseplate and grout are damaged or deteriorating, this will hinder satisfactory alignment and ensure that the alignment will have to be done again sooner than expected. If the coupling is a lubricated coupling, look closely for a pattern of lubricant spraying or leaking from the coupling. Depending on the type of guard, sometimes machines using a gear-type coupling can be analyzed for coupling or alignment problems by placing a piece of white cloth or paper on or under the coupling guard. If a pattern of lubricant is visible, you should suspect a coupling problem or misalignment. Testing shaft runout is an essential part of pre-alignment checks. With the widespread use of laser systems, sometimes shaft and coupling/coupling hub runouts are overlooked. The shafts should be rotated and all runouts taken before aligning the shafts.

Thorough Understanding of the Behavior of the Machine As Part of the SystemLook closely at the way piping is routed to and from the machine. Visually inspect the piping support apparatus. Are they adjustable struts, rigid struts, or spring hangers? Getting a feel for the piping route and what the designer had in mind when calculating piping expansion and the direction of expansion can be very important. Is the machine being operated as it was intended?

Thorough Understanding of the Impact of Misalignment on the MachineStated differently, this also refers to the behavior of the machine. First, determine if the machine displays symptoms of misalignment. Has a thorough coupling inspection been performed? Has the vibration data taken on the machine told you that alignment is the most probable cause of the 2-3


machine behavior? Most of the time, misalignment will show up at half speed, but it can also be seen at full speed. Additionally, misalignment can be seen in the axial direction. Vibration is not proportional to misalignment. (For more information, see Alignment and Preloads in Section 5.) Forcing functions acting on a machine with misalignment can improve vibration levels. Knowing the history of a machine is vitally important. What maintenance was performed during the last inspection or rebuild? Was the rotor balanced? Has there been a trend of increased vibration, or was it a sudden change? Has the machine characteristically been a problem? If so, is the machine being operated at or near its design limits? Have conditions changed?

Proper Sequence of Alignment AnalysisAll of these questions and many more require answers when diagnosing the cause of machinery problems. A good predictive maintenance program in conjunction with a good root cause analysis program goes a long way toward resolving alignment and machine problems. Using the proper sequence in alignment is crucial to ensure that if any problems arise from the machine after alignment that they are not due to misalignment or improper alignment, or associated with areas that should have been checked during the alignment process.

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3EFFECTS OF MISALIGNMENTMost machines used in the nuclear industry, such as turbines, pumps, fans, compressors and other equipment, are similar to those used in other applications or other industries. This section details how shaft misalignment affects these machines and the consequences of these machines failing for any reason, but especially due to misalignment. Special consideration must be given to the equipment in the nuclear industry due to the circumstances under which it operates.

Types of Machines Aligned in Nuclear StationsThe machines in a nuclear station can be divided into three categories: power production systems, safety systems, and support systems. Nuclear power production machines do not differ greatly from their counterparts in other power stations. They include: Feedwater pumps Booster pumps Condensate pumps Heater drain pumps Drive turbines Electric motors Steam turbines/generators Safety pumps Fans

It is obvious why failure in these machines is costly; lost generation capacity is the single largest cause of lost income at a power station. Misalignment can and does affect the support system of machines. This support system consists of the components of the machine itself and the structure that supports the machine.

BearingsBearing failure or degradation is a factor that impairs reliability. Misalignment places forces on the bearings that reduce the life of the bearing. This can be noted visually when looking at shaft orbits on sleeve bearing machines. The results of this problem on anti-friction bearing machines typically require vibration analysis and time for the problem to reveal itself. The loads placed on 3-1

EPRI Licensed Material Effects of Misalignment

anti-friction bearings are shown during analysis at some frequency associated with the bearing design, such as ball pass frequency or inner race frequency. For this to reveal itself, typically some damage has to occur to the bearing. Over time, the damage will continue to escalate to the detectable point. Bearing temperature is a good indication that a problem exists. Unfortunately, direct reading temperature instrumentation is typically not installed on anti-friction bearing machines unless they are in a critical application. Even then you are getting only part of the picture because the probe is usually contacting the outer race, and temperature equalization and heat transfer can give some misleading information. If the area of concern or damage is directly on the temperature probe, indications of a problem are easily seen.

Figure 3-1 Ball Bearing (Anti-Friction Bearing)

The bearing in Figure 3-1 is typical of the radial bearings installed in a majority of pumps that use anti-friction bearings. This bearing and the associated data are for a 6312 bearing. The amount of clearance in a bearing has a relationship to the amount of misalignment present and the amount of external preload on the bearing and shaft, which is also impacted as the misalignment adds a bending moment to the shaft. Depending on the amount of shaft deflection and the position along the shaft where this deflection occurs, the bearing acts accordingly. Using the above bearing as an example, if shaft deflection or bowing from misalignment or other sources results in the deflection occurring close to the bearing, the bearing must flex a certain amount with the shaft. The 6312 bearing above, utilizing a C3 fit, has an internal clearance of 2343 micrometers (m), or .0009.0017 inches. This internal clearance is the total distance through which one bearing ring can be moved relative to the other. Dividing this clearance in half gives the radial clearance between the ball and the inner or outer race. This not only includes the radial direction of clearance, but also the axial direction of clearance. This says that if shaft deflection occurs that causes the inner race to try to skew within the outer race, then on one side of the bearing, the

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clearance is closed, and diagonally across the bearing, the clearance is closed. This generates higher temperatures, the ability to lubricate the bearing has been diminished, and wear occurs at an accelerated rate. Mechanical seals fail for several reasons that can be associated with misalignment. First, the bearings begin to fail, and then the shaft is allowed to move in relation to the stationary seal face. Exactly how this happens depends on the type of misalignment present and the state of the bearings. Nozzle loads typically affect the stationary seal faces and their concentricity and perpendicularity to the shaft centerline. Nozzle loads can affect the bearing housing, causing deflection of the shaft and the rotating portion of the seal. Trying to discover which seal component is being affected is very difficult. Therefore, when the loads need to be reduced anyway, removing nozzle loads on the machine is very important and is the best method of solving the problem without major analysis and time expenditure. In theory, the hysteresis the failure of a property to return to its original value once an applied external force is no longer appliedof the seal component, particularly a pusher or multiplespring rotating face, is evident as it tries to maintain contact with the stationary face at a given machine speed. This contact and flatness are of utmost importance. Seals are lapped flat, using a measurement of light bands. If seals did not need to be flat to seal, then shaft movement would be irrelevant. The shaft could bend, move axially a great amount, move radially a great amount, and the seal still would not leak. But shaft deflection in the range of .00001 of an inch (0.254 micron) is far greater than the light band range to which the seal is lapped. Surface speed (surface feet per minute or SFPM) is a factor that, when added to the equation, reduces seal life. This is why smaller shafts at higher speeds can use a seal without leakage better than a machine with a larger shaft. Most alignments performed on horizontal machines use some sort of laser system. Figure 3-2 below shows a typical laser alignment tool attached to a shaft.

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Figure 3-2 Laser Alignment System

Methods of measuring misalignment and correcting it are varied. Shaft alignment technology has progressed over the years, and systems are now very sophisticated. This does not imply that some of the older methods are inadequate or unacceptable under certain situations. Some tools commonly used to align shafts are the following: Straight edge Feeler gage Parallel blocks (plain or adjustable) Micrometers Calipers Dial indicators Lasers

All of these methods are good, based on the following criteria: 3-4 Tolerances Speed of machines


Criticality of equipment Time involved in completing the job The resolution required to achieve satisfactory results

To determine how much time should be spent on shaft alignment in order to achieve satisfactory results, the following questions must be answered: How important is the machine to the overall operations and profit of the plant? Has the machine been performing satisfactorily in the past, and are records available to prove the reliability of the machine? Has the machine been thoroughly analyzed to determine what has an effect on misalignment? What is the cost of maintenance for the machine? Are parts expensive, and is the machine labor intensive? Is the machine presently off-line for maintenance? Is the repair or alignment on the critical path?

Figure 3-3, showing two misaligned shafts using a spacer coupling, is indicative of the type of problems encountered in the field.

Figure 3-3 Two Misaligned Shafts Using a Spacer Coupling

NOTE: This diagram is exaggerated for the purposes of illustration. It is worth noting that the spacer coupling, as shown in Figure 3-3, can have two different angles at the points of power transmission, as denoted by alpha and beta. The angle theta is the angular misalignment between shafts; the importance of this cannot be overemphasized. However, the

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coupling angle is just as important because the behavior of the rotors of machines is probably impacted more by the coupling angle than by the shaft misalignment angle.

Alignment TolerancesMuch of the discussion about shaft alignment centers on alignment tolerances. What is close enough to achieve the desired results? Because of the age of most nuclear power plants, the manufacturers literature is sometimes sketchy. Depending on the date of publication, the Operations & Maintenance (O&M) manuals provide alignment targets or tolerances are based on a zero offset and a zero angularity alignment. Many O&M manuals state something similar to the following: The machine shall be aligned within .002 on the rim (offset), and .001 on the face (angularity). Usually, no information on the length of spacer is included. The alignment tolerance information in Table 3-1 applies to both vertical and horizontal shafts. These suggested tolerances are the maximum allowable deviations from desired values (targets), whether such values are zero or nonzero. Use them in the absence of in-house specifications or tighter tolerances from the machinery manufacturer. Normally, the columns labeled Excellent apply to all alignment work. The exception is rough machinery designed to vibrate, such as ball mills, shaker screens, hammer mills, and so on. For such machinery, the information in the Acceptable columns can be used.Table 3-1 Alignment Tolerances Courtesy of Ludeca, Inc.

Tolerances for Shaft Alignment Short Couplings Spacer ShaftsExcellent Offset Angularity RPM 600 900 1200 1800 3600 7200mils mils per mils per inch 10"

Acceptable Offset Angularitymils mils per inch

Exc.

Accpt.

mils per mils per mils per 10" inch inch

5.0 3.0 2.5 2.0 1.0 0.5

1.0 0.7 0.5 0.3 0.2 0.1

10.0 7.0 5.0 3.0 2.0 1.0

9.0 6.0 4.0 3.0 1.5 1.0

1.5 1.0 0.8 0.5 0.3 0.2

15.0 10.0 8.0 5.0 3.0 2.0

1.8 1.2 0.9 0.6 0.3 0.15

3.0 2.0 1.5 1.0 0.5 0.25

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4MACHINE FRAME DISTORTION - SOFT FOOTMachine frame distortion or casing deflection is a topic that deserves considerable discussion and is the most overlooked area during the process of shaft alignment. It is also the area that can pay the highest returns for reliability of machinery when associated with proper shaft alignment. Frame distortion can be divided into three categories: Soft foot Baseplate and foundation irregularities Induced soft foot and nozzle loads

Soft FootSoft foot in the classic sense is the flexing or bending of the frame foot when it is tightened to the base. Any gap that exists is reduced with this tightening, and forces are applied to the frame or casing. These forces result in casing deformation and in some interaction between the stationary and rotating parts of the machine. In most cases, a complete check for soft foot is not done. The machine to be moved (MTBM) is usually checked, but the stationary machine is seldom checked for soft foot. This, in turn, continues to result in problems with the stationary machine. For example, consider the comparison of turbine-driven feedwater pumps to motor-driven pumps at the power station. When performing alignment on turbine-driven pumps, typically above 10,000 HP, there is a tendency to move the pump. In doing so, it quickly becomes evident just how much soft foot is present due to classic soft foot problems or piping-induced soft foot. When aligning motordriven machines and the motor is the MTBM, the machine is seldom checked for soft foot. Rarely is the machine ever unbolted from the pedestal or base. This is an area where much progress could be made in improving the reliability of the machines. Instead, often the distortion due to piping-induced soft foot on the driven machine is mistaken for other problems. One such problem is the contact of wear rings in overhung horizontal pumps, such as American National Standards Institute (ANSI) or American Petroleum Institute (API) pumps. Often radial reaction of the rotor and impeller are blamed for this when it might be due to distortion, particularly at higher temperatures. In comparison, on motor-driven machinery, when the motor is the MTBM, there are seldom any influences present other than the classic soft foot examples. The exception to this is rigid conduit, which imposes forces on the motor, or on larger motors, the water cooling lines, which can also induce soft foot due to piping strain. 4-1

EPRI Licensed Material Machine Frame Distortion - Soft Foot

Measuring Soft Foot IndexIt is common to measure soft foot by mounting an indicator on the machine base so that the stem is on top of the foot, but this method has numerous drawbacks. Depending on how the foot is bent, the indicator can under indicate the soft foot or even read zero when there is significant soft foot. This will leave the machinery installer with the mistaken idea that the bolt and foot are okay, when, in fact, harmful distortion exists in the machine frame. The indicator can also indicate soft foot that does not move the shaft centerline and, therefore, can be ignored. This is usually the case when the shims and feet are very large compared to the size and the load area of the bolt. If the load area is correctly supported, the rest of the foot can move a considerable distance vertically without any distortion passed to the machine frame and bearings. The success of the indicator-on-top-of-the-foot method is highly dependent on machine geometry. For example, a foot movement of 0.002 inches (0.05 mm) has entirely different consequences for machines with only 6 inches (15.24 cm) between the feet than for those with 40 inches (101.6 cm) between the feet. In short, the indicator-on-top-of-the-foot method misses significant soft feet, gives false alarms, is highly affected by machine size, and is, in general, a poor method of measuring machine frame distortion. A much more indicative value can be obtained from measurements taken at the shaft. Because soft foot really means machine frame distortion, any system that purports to measure it must some how quantify distortion in the machine frame. One simple, yet effective, way to do this is to determine if the bearings moved when the hold-down bolts were tightened. If the bearings did not move when the hold-down bolts were tightened, then there was little or no machine frame distortion and certainly not enough to influence the rotor. On the other hand, if the bearings did move due to the hold-down bolts being tightened (or loosened), then the frame is sufficiently distorted to affect the running position and, subsequently, the condition of the rotor. For most, if not all, industrial machines, it is geometrically impossible for a single bolt to distort a machine frame in such a way that both bearings move, resulting in a shaft movement that is pure parallel displacement. Furthermore, if only one bolt is inspected at a time, any motion in the nearest bearing will be largely vertical. In fact, purely horizontal movement of a bearing due to tightening a single base bolt is geometrically impossible. To summarize this: Machine frame distortion from tightening (or loosening) a single bolt always induces a change in the vertical shaft angle. Machine frame distortion can be measured by quantifying change in the vertical shaft angle.

Distinguishing Types of Soft Foot, Possible Causes, and Proper Correction TechniquesNot all soft feet are the same. They are caused by a variety of conditions, some of which might not even be related to the machine itself. However, as a rule, all soft feet are bad and should be eliminated. The method of elimination depends upon the source or cause of the soft foot. There is 4-2


no general purpose, one-kind-cures-all soft foot correction. If your soft foot check indicates a soft foot, fix it. However, be careful to analyze the type and cause of the soft foot before doing anything. Indiscriminate shimming or trial and error will most often makes things worse. Although a high-resolution laser system can be used for gauging the amount and effect of the soft feet, it cannot determine the cause nor the corrective action. The soft foot mode of a highresolution laser system senses shaft deflection caused by soft foot, accurately and reliably. These systems display a soft foot index that is an absolutely reliable indication of machine frame distortion (soft foot). They do not display the correction for the amount of machine frame distortion detected. There is no device yet made that can measure soft foot at the shaft or on top of the foot and accurately analyze the necessary correction. Shaft-mounted devices, even the best laser systems with soft foot functions, are not gap meters, nor are they shim meters. This is a limitation of all measuring systems that are mounted anywhere except between the bottom of the foot and the top of the base. In other words, if you are not measuring in the gap under the foot, you are not measuring the gap under the foot. Cause and corrective action can be determined by using feeler gages, which are essential for removing soft foot. Proper feeler gage technique for a single foot is to measure the gap under all four corners of the same foot. From these four readings, an excellent idea of the shape of the gap can be developed. The maximum allowable soft foot typically is 0.002 inch (50 m), although a real effort should be made to obtain zeros.

Parallel Air GapCondition: This is the mental picture most often associated with the word soft foot. It is where three feet sit solid and flat, but one foot does not touch (see Figure 4-1). A feeler gage will find an equal gap at all four corners of the foot. Contrary to common assumption, this type of soft foot is quite rare. Note that the foot diagonally opposite will show soft foot, but a smaller amount. It is impossible to have three parallel air gaps on one four-footed machine. Likewise, it is impossible to have two air gap feet side by side. They must be diagonally opposite each other.Soft Foot

Figure 4-1 Parallel Air Gap

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Causes: One leg is too short. One baseplate mounting pad is not coplanar with the other three. Inadequate shims are under one foot.

Correction: Add shims to equal the amount shown on the feeler gage. Do not fall into the often unproductive trap of attempting to divide the shims with the diagonally opposite foot. The laser system readings of the four feet indicate relative coplanarity of the feet. This, in accordance with the feeler gage results, will often show that three of the feet are largely coplanar, and the fourth foot is clearly the one to be shimmed. If the laser system shows two diagonal soft feet with the same value and the feeler gage gaps are the same across the diagonal, then shim both feet. With experience, both diagonally opposed soft feet can sometimes be shimmed according to the coplanarity of the four feet, but this is not recommended. It is far better to shim one foot and take the readings again. You will often find that the problem is solved.

Bent Foot or Angled BaseCondition: The bottom of the foot is not coplanar with the base. It has feeler gage readings that clearly show a slope from one corner of the foot to another. Often, but not always, one corner or one side of the foot is touching the base, and the foot acts as a lever when bolted down (see Figure 4-2). Because of this, the bent foot usually induces soft foot in the two opposing feet and sometimes in the fourth foot as well. This gives the machine the appearance of having three or four soft feet, but they will all go away when the bent foot is corrected.

Figure 4-2 Bent Foot or Angled Base

Causes: Machinery that has been dropped or handled roughly Bent or poorly machined baseplates Severe angularity misalignment Feet that have been welded Foundation settling

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Correction: Re-machine the feet, the base, or both; or build a step shim. Step shims are a fieldproven method that is both safe and effective. The idea is to build a set of steps that match the slope of the foot (see Figure 4-3).

Figure 4-3 Step Shim

The procedure to build a step shim is as follows: 1. Fill any gap that exists under the entire foot so that one corner or edge of the foot is touching the shim. 2. Measure the largest remaining gap. 3. Divide this gap by 4, 5, or 6 (the number of steps) to obtain the step thickness. 4. Select 4, 5, or 6 shims of the step thickness and insert them one step at a time. Without lifting the machine, insert them by hand only until they are snug. Some adapting of the method is required for feet that are bent diagonally or skewed. Each vertical shim correction to the foot requires the steps to be rebuilt. Do not expect the steps to fit back in exactly the same way after shimming because vertical angularity corrections affect the slope of the feet. After the final alignment, trim and discard the excess portion of the step shims, as shown in Figure 4-3.

Taper Shims to Remove Soft FootA shim fabricated with a taper or compound taper can be used in place of step shims. By mapping or measuring the gap and angles between the base and the subject foot, a shim can be machined to remove the taper. This provides the advantage of not having gaps or steps associated 4-5


with the step shimming method. However, there are also disadvantages associated with this method. First, if you are trying to keep the number of shims to a minimum, the taper shim adds to the total shim thickness being used and must be accounted for on all feet. A minimum shim thickness of 1/16 inch (1.6 mm) is usually required to fabricate a step shim. Second, the shim typically requires fabrication from carbon steel in order to be clamped to a magnetic chuck on a surface grinder.

Baseplate and Foundation IrregularitiesBaseplate or pedestal condition and the foundation play an integral part in the ability of machines to remain in an aligned condition. The baseplate to foundation interface, which is typically grout, is an area that deserves scrutiny. Because many problems can occur within baseplates and foundations that affect the operability of the machine, baseplate conditions play an integral role in shaft alignment.

DeteriorationThe baseplate and foundation might be subject to certain environmental conditions that cause them to become unstable or deteriorate over time. The grout can begin to flake, erode, or crumble due to the environment to which the foundation is subjected. Deterioration is most often found in chemical plants, but it can occur in almost any situation. Continuous cleaning with large quantities of water can also have an adverse effect on the grout and metal portions of baseplates. Over time, rust and erosion of the grout can weaken the baseplate or foundation. If forces are present from piping-induced loads, the baseplate reacts to these forces and can move accordingly. Piping-induced or nozzle loads can cause the grout to crack.

Machinery VibrationThe problems associated with vibration are twofold: The first problem deals with the vibration caused by shaft misalignment. Vibration does not always reveal misalignment. Several factors are involved, such as stiffness of the bearings and the machine support structure. The external and internal preloads imposed on the machine can dampen the vibration. Soft foot, regardless of the type, can cause vibration. This is easily revealed when a soft foot bolt is loosened and the vibration decreases. The second problem involving vibration is in the baseplate and supporting structure. Baseplate looseness and grout cracking or dusting is evidence of high nozzle loads or high cycle fatigue. An example of high cycle fatigue would be predominantly high vane passing frequencies on a centrifugal pump. This high frequency vibration for an extended period of time begins to fatigue the grout and baseplate. Shaft alignment is hard to maintain or correct without going to the root cause of the problem.

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The fatigue can manifest itself in the form of broken parts on the machine or even destruction of the baseplate over time. The grout is usually one of the first areas to show damage (see Figure 4-4), such as dusting or crumbling under vibratory loads.

Figure 4-4 Pump Base Degradation Resulting From Transmitted Forces

Induced Soft Foot and Nozzle LoadsFor the purpose of this guide, nozzle loads are divided into two categories: induced soft foot which creates nozzle loads in the vertical direction, and nozzle loads for forces and moments in the horizontal direction. Condition: A high-resolution laser alignment system shows soft foot, usually two feet on same side or same end of machine, and a feeler gage finds a gap, usually parallel or nearly parallel. A secondary symptom is that the foot does not get better (it becomes worse) or another foot becomes much worse after shimming the gap amount (See Figure 4-5).

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Figure 4-5 Examples of Induced Soft Foot

Causes: External forces on the machine. Coupling strain and pipe strain are the two most common. If the coupling is difficult to assemble because of misalignment, expect an induced soft foot until the alignment is improved. Other sources of external forces are: 4-8


Overhung machines or attachments Belt or chain loads Gears Any overhung or extended shafts Hoses Flex conduit Structural bracing attached to the machine Jacking bolts inadvertently left tight

Correction: Remove the source of the force. Note that a high-resolution laser alignment system makes a good tool for testing for pipe strain during construction. Attach the laser alignment system and enter soft foot mode before attaching any pipe, but with all base bolts tight. Remove any soft foot from the machine and leave the laser alignment system set up to read one foot (any foot). Tighten the bolts on the piping. If the laser alignment system records more than 1.5 mils (37 m) movement while the flange bolts are being tightened, the piping is straining the machine. As further proof, test again for soft foot while the piping is tight, and compare the data to pre-piped soft foot conditions.

Nozzle LoadsTypically, nozzle loads are on the driven machine, unless the driver is a steam turbine (forces from the piping are transmitted into the pump casing or other type machines and continue on into the foundation or baseplate). Remembering that for every action there is a reaction, it is easy to see why the large forces applied through the piping can destroy a baseplate quickly. As the baseplate gives up some of its strength, the loads imposed on the machine tend to move the machine more freely. The result is misalignment and the possibility of catastrophic failure of all machines in the train. Large, high-energy pumps, such as feedwater pumps, can use keys to limit movement of the casing. This does not reduce the stresses in the casing or reduce distortion from nozzle loads or piping strain. It changes only the location where the forces and stresses enter (and leave) the casing. The forces and moments in the horizontal aligning direction are a very important area of concern associated with nozzle loads. These forces tend to move the machine in the horizontal direction in much the same manner as the induced soft foot moves the machine in the vertical direction. In most cases, a moment is also associated with these forces. When trying to align a machine with this problem, moving the machine requires a great deal of force to bring the machine into alignment. By applying this force against the opposing forces, casing distortion is added to the problem. As temperatures increase in the machine, the machine may move in a direction that is not anticipated in relation to the hot alignment analysis performed.

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Over time, these forces manifest themselves in the form of rubs internal to the machine, pedestal and baseplate damage, and grout failure. These horizontal forces and moments are best detected by loosening all hold-down bolts, removing casing keys if they are present on the subject machine, and loosening on all jacking bolts if they have been tightened in an effort to maintain alignment. By observing where the machine moves and the forces required to move the pump back into position, information can be gained about the forces present. Large pumps, such as feedwater pumps, often use keys to limit movement of the pump and direct thermal growth in a particular direction. The typical design uses a pin on the coupling end of the pump and a rectangular key on the thrust end of the pump. The pin on the coupling end provides for very limited movement in the horizontal and axial direction. The key on the opposite end of the pump allows for thermal growth in the axial direction and limited movement, usually less than 5 mils (0.1mm) in the horizontal lateral direction. When pumps such as these are loosened from the baseplate in an effort to detect lateral forces and moments about the axis of the nozzles, these keys must be loosened or removed. Many times, this requires grinding welds that hold the keys or key blocks in place. Figure 4-6 shows one stations resolution to achieving horizontal moves and remedying lateral or horizontal forces and a twist in the pump. Figure 4-6 shows the modified key block used with adjustments, to be made after alignment, and the welded jacking brackets.

Figure 4-6 Pump Outboard Replacement Keys

Jacking Bolts or Taper PinsMachine frame distortion can also be caused by dowel or taper pins used to limit the movement of the machine. Jacking bolts are used occasionally in alignment to limit the movement of machines that move around on the base. Care must be taken to ensure that casing distortion is not caused by these jacking bolts (see Figure 4-7).

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Figure 4-7 Key Adjusting Bolts

In many instances, the taper pin (or in some cases straight type fitted dowels) is used improperly to limit the movement of machines. Analysis of the total axial growth of machines should be performed to determine the fixed end of the machine, and the taper pins should be installed on that end of the machine. Installing taper pins diagonally from one end of the machine to the other increases the probability of casing distortion at elevated temperatures. The manufacturer or a reliable consultant should be consulted if there are questions concerning doweling the feet of a machine.

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Gaps Without Soft FootCondition: Sometimes, a foot that does not show soft foot by high-resolution laser alignment system readings will have a rather large gap under it when the bolt is loose and no gap when the bolt is tight. Another version of this same phenomenon is that the laser alignment system indicates a relatively small soft foot, but the feeler gages find a much larger gap. Causes: The base is moving. The foot is bending without bending the machine (weak or flimsy feet). The base or machine is cracked, loose, or otherwise defective.

Correction: Even given the absence of soft foot in this condition, most people choose to shim the gap, although it rarely improves the running condition of the machine. The base or machine must be repaired or rebuilt to eliminate this condition. Note that this condition often has the side effect of making vertical alignment corrections very unpredictable or even impossible. If the machine can be aligned (it responds to vertical corrections) and is not loose or broken, then the gap can often be ignored. If the tightening and loosening of the foots base bolt does not affect the shaft centerline (no soft foot reading), then the bearings are not being moved or distorted, so no harm to the rotor, bearings, or coupling occurs.

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5MACHINERY POSITION CHANGESThe running alignment position of machines is the aligned position of two or more machines relative to each other at running conditions, which differs from the cold or shutdown aligned position. Often, machines are misaligned in a shutdown condition in hopes that the alignment during operation (or under running conditions) is within the acceptable tolerances for that machine. While this is often referred to as hot alignment, there are others who define hot alignment as mounting alignment equipment and capturing data immediately after machines are taken off line. Although this method is probably better than nothing if certain rules are followed, it is not very accurate and is about the same as guessing where the alignment of the machines should be. There are several methods that provide alignment criteria for machines to be misaligned in the cold condition and achieve alignment during operation. These methods are listed below in order of least to most accurate: Guess where the machines should be aligned Shut down and perform the alignment Use the manufacturers recommendations Monitor the machines from one condition to the other

The manufacturer of a boiler feedwater pump and the manufacturer of a drive turbine tend to give information based on their respective machines. This information is based on either monitored or calculated data, and this data is typical for thermal growth considerations of the respective machines. Most often, you will get numbers from the manufacturers that represent some vertical change in the machines relative to ground or to another machine. Some horizontal change might also be provided. More often than not, however, the horizontal misalignment targets are far from being what the manufacturer of the machines provides or what can be calculated at the power station. Monitoring alignment changes allows you to determine targets in the horizontal direction. Horizontal types of misalignment are generally due to piping forces (either static or dynamic) that prevent the pump from being aligned where desired or move the pump after startup. Large pumps present problems in this area. Many large pumps have keys or a combination of pin and key along the bottom of the pump casing. Some pumps even have keys radially projecting from the sides of the casing to either limit the twist in the pump casing or, in most cases, deliberately try to make the pump casing grow in a particular direction.

5-1

EPRI Licensed Material Machinery Position Changes

When targets reveal that a pump must have a given amount of horizontal angularity or horizontal offset, provisions must be made to modify these keys from their as-built configuration (See Section 4). Figure 5-1 is a graphical representation supplied by one pump manufacturer of the calculated thermal growth of a motor-driven pump with a gear box.

Figure 5-1 Alignment of Shaft Centerline Heights

These alignment targets are calculated based on an expansion chart, as shown in Figure 5-2.

5-2


Figure 5-2 Expansion Chart

Typically, the horizontal changes supplied by the manufacturer are not close to what is encountered in the field when the machines in question are acting together with the entire system. Although the vertical changes may occasionally be within the range provided by the manufacturer, the horizontal changes seldom are. The terms hot alignment and thermal growth do not disclose the complete story of running position alignment. The most accurate terminology is transient alignment monitoring because it best describes what running position alignment is about. You must monitor the alignment changes under all conditions to establish an understanding of the behavior of the machines. Capturing alignment changes within all operating parameters gives you an opportunity to explore these changes. Machines can be monitored starting with the machine cold (at shutdown) and monitoring the changes as the machine reaches its operating condition. Monitoring can also be performed starting with the machine at operating conditions and monitoring to shutdown. Monitoring can also take place anywhere in between, if certain parameters are to be observed without determining the full amount of changes of the alignment. The preferred method is to monitor the machine from operating conditions to shutdown. This enables you to analyze and implement the alignment data during shutdown and observe the results on startup. This saves an additional shutdown to align if the opposite method is used.

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There are two concepts involved in monitoring machine movement and arriving at the alignment targets. For the purpose of clarity and convenience, these methods can be referred to as absolute and relative monitoring. Absolute monitoring involves the technique of measuring one machine from a fixed reference point from the ground, such as a column, wall, or floor. The types of monitoring equipment that do this include precision sight levels and jig transits, Acculign bars, and Jackson cold water stands. The relative alignment methods monitor the changes in alignment between two or more machines. The equipment used for relative monitoring includes Dynalign or Dodd Bars, Permalign lasers, and rotating Vernier gages. When absolute measurements are compared between two or more machines, these measurements can also be referred to as relative. See Figures 5-3 through 5-8 for pictures of various types of monitoring equipment.

Figure 5-3 Acculign Bars Measuring Movement of a Steam Generator Feed Pump

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Figure 5-4 Laser Monitoring Movement Between Steam Generator Feed Pump and Turbine

Figure 5-5 Dynalign (Dodd Bars) Being Used to Monitor Alignment Changes

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Figure 5-6 Precision Sight Level Used for Optical Alignment Checks

Source: Brunson Instrument Company

Figure 5-7 Jig Transit Used for Measuring Alignment

Source: Brunson Instrument Company 5-6


Figure 5-8 Scales Used With Jig Transits and Precision Levels

Source: Brunson Instrument Company

Infrared ThermographyInfrared thermography can play a very important part in analyzing misalignment. Thermography can detect problems through temperature rises in the couplings or bearings. It cannot distinguish the amount of misalignment, only the results of misalignment. In many cases, this is just as important as measuring the amount of misalignment. Used together, data from both infrared thermography and transient alignment monitoring systems can be very informative to the technician, as well as to management personnel who may need to see more evidence of problems in order to allocate resources necessary to resolve the problems. Past studies involving alignment analysis have determined that some previously held beliefs about misalignment are not necessarily true, including the following: Misalignment is easily detected by high vibration levels. Misalignment increases cost of operations due to larger energy consumption. 5-7


Both of these have been used in the past as major selling points for alignment hardware companies.

Alignment and PreloadsA preload is a directional load or force applied to the rotating shaft. Two categories of preloads are internal and external. Internal preloads deal with forces generated from within the machine and go far beyond the scope of this guide. Only the external type of preload exists for shaft misalignment. There are other types of external preloads that interact with or impact the structure or casing of the machine, including piping loads (forces and moments) and soft foot. The immediate effect of a preload due to misalignment is to force the shaft into one sector of a bearing. A strong indication of preloads, both magnitude and direction, can be determined with the use of proximity probes (x and y) close to the bearing. These preload data are in the form of shaft orbits. The use of bearing metal thermocouples in conjunction with shaft orbits and infrared thermography can yield excellent results in determining if misalignment exists. This is easily detected and brought to light in a machine, such as turbine generator train, or a high-energy pump, such as a feedwater pump. The vibration might be low on one bearing accompanied by a high temperature, while the adjacent bearing will have a higher vibration and lower bearing temperature. The amount of preload can be related directly to the amount of misalignment. Spring-type couplings, such as a diaphragm coupling, exhibit the least amount of preloads on a bearing and its supporting structures, while a rigid-type coupling will impose the most preload. In Figure 5-9, a circle or ellipse, as shown in the first two orbits, is the norm when no unidirectional loading or preloads are present. As you move across the page, greater preloads are encountered. The last orbit is where the shaft is located in the bottom of the bearing due to a large amount of misalignment, and the results can show up as twice the shaft speed. An elevation in bearing temperatures can also accompany this scenario. Remember, there are other things that can cause increased vibration. Misalignment occurs perpendicular to the shaft orbit and forces the orbit to flatten; thus, the sensors perceive this as twice the running speed.

Figure 5-9 Shaft Orbits Acquired From Eddy Current Probes on a Sleeve Bearing Machine

A phase difference of 90 degrees between x and y probes should be theoretically true; however, two probes can show a phase difference of 180 degrees. A steady-state preload will cause the shaft to move eccentric to a position within the bearing. This type of orbit is seen most often in machines with gear-type couplings. These preloads can also unload a bearing, resulting in a 5-8


lower temperature in one bearing and creating an opportunity for an unstable shaft, which may result in oil whirl. These external preloads can also be due to the misalignment itself. Piping strain and soft foot pose another problem with casing deformation. You have a choice of where you want these forces to enter the pump. They can enter through the piping or through the keys and supporting structures of the pump casing. These forces can also be transmitted into the structure supporting the pump, such as the base and grout of the machine (see Figure 4-4). Smaller machines with anti-friction bearings pose special problems with preloads. Detection might need to be performed with infrared thermography, as well as vibration analysis, to detect preloads that have an effect on alignment and reliability of the machines. Machines that use a gearbox for speed changes will have preloads associated internally with the machine, which act upon alignment while the machine is in operation. While piping strain impacts alignment, it also impacts the wear of parts. Piping strain as it relates to misalignment is often overlooked. This is particularly true in the horizontal direction. What appears as a minor amount of piping growth to the piping designer can be a major amount to rotating machinery personnel. Piping growth due to heat can have a severe impact on the misalignment of machines. Some of this misalignment can be accounted for with transient alignment monitoring and corrections. Cold piping strain in the horizontal direction must be accounted for and remedied as stated in the Induced Loads section. Radial and axial pump keys under the casing do not eliminate these loads; they just enter the casing from another location. The forces against the keys constrain the pump, but they add to the loads on the casing just the same. These keys may require modification from the original installed position.

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6SHAFT COUPLINGS AND POWER TRANSMISSIONShaft couplings and power transmission go together with shaft alignment. Shaft alignment is seldom performed without the opportunity at least to look at the flexible shaft coupling that connects the machines being aligned. During shaft alignment, machines with lubricated couplings are generally inspected or preventive maintenance is performed. Dry-type couplings also require inspection for damage such as fatigue or cracking. This guide does not attempt to discuss all the details of shaft couplings and how they are designed, applied, and used. Instead, they are briefly discussed in this section in the areas where they play an important role in the process of aligning shafts and in the behavior of machines due to shaft and coupling misalignment.

Flexible CouplingsA flexible coupling transfers or transmits power from one machine to another and makes accommodation for some shaft misalignment. There are two types of flexible couplings: one allows for misalignment by sliding, the other by flexing. Typical couplings that allow for misalignment through sliding are gear-type couplings and flexible grid-type couplings. The misalignment of two shafts is accommodated during rotation of the shafts by the sliding of gear meshes (see Figure 6-1) or, in the case of the grid coupling, the grid to the grooves in the hubs. The grid coupling also has some bending involved, but this is used for torsional loading.

6-1

EPRI Licensed Material Shaft Couplings and Power Transmission

Figure 6-1 Gear Coupling

Courtesy of Falk Corp. Figure 6-2 shows a grid-type coupling.

6-2


Figure 6-2 Grid Coupling

Courtesy of Falk Corp Couplings that allow for misalignment due to bending are flexible diaphragm couplings and flexible disk couplings. The diaphragm coupling can use a single steel diaphragm or a convoluted diaphragm made up of several layers of thin flexible steel plates. Figure 6-3 shows a diaphragm coupling.

6-3


Figure 6-3 Diaphragm Coupling

A multiple disk or disk pack coupling is shown in Figure 6-4.

6-4


Figure 6-4 Flexible Disk Coupling

Restoring forces are very important because all couplings resist being misaligned. The coupling tends to try to run in a straight direction, and this imposes preloads on the shaft, trying to force the shaft into a particular sector of the bearing.

Restoring Forces and MomentsYou should be aware of coupling behavior in misalignment. All couplings resist being misaligned and try to operate in a non-misaligned condition (hence the term restoring forces). These forces act on the shafts in the form of a moment arm trying to bend the shaft and, in doing so, adding stresses to the shaft. Misalignment under these conditions can fatigue a shaft (and/or coupling) and eventually result in failure (see Figure 6-5). Resistance to being misaligned occurs only under conditions where torque is transmitted and not in a standstill condition. When machines are borderline aligned and shafts move into a region or area of misalignment while torque is being transmitted, coupling lockup can occur on gear-type couplings.

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Figure 6-5 Stub Shaft Replacement

MisalignmentCoupling misalignment differs from shaft misalignment. Coupling alignment or misalignment is the angle in degrees from the axis of one shaft to the axis of another shaft. The coupling manufacturer usually provides allowable coupling misalignment in terms of degrees of misalignment. If the manufacturer gives a number in thousandths of an inch (0.001=25 m) for offset, it is the measurement of the distance between flex planes of the coupling times the tangent of the allowable angle of misalignment. Taking into consideration the restoring forces of the coupling and the bending moments acting on the shafts, a rule of thumb can be proposed. If the misaligned shafts are graphed on paper with the proper scaling and a line is extended from the centerline of one shaft to a point of intersection on the opposing shaft, this is the point where the moment occurs (see Figure 6-6).

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P1

P2

Figure 6-6 Point of Moment

Figures 6-7, 6-8, and 6-9 illustrate three types of misalignment. These illustrations can be in the form of vertical or horizontal misalignment. For the purposes of illustration, the angles of shaft misalignment are constant in all examples. Because couplings are the concern in this section, the angles will be given in degrees and mils per inch (25 m) (mrad). A shaft coupling spacer of 12 inches (30.4 cm) is assumed. The first example (see Figure 6-7) is of a machine to be moved with only angular misalignment between shafts or across the coupling.

Figure 6-7 Angular Misalignment

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In Figure 6-7, the angle of the shaft (MTBM) is 15 mils per foot (375 m per 30 cm). The angle at the coupling flex plane at P1 is 0. The coupling misalignment at P2 is .072 degrees.

P1

P2

Figure 6-8 Angular Misalignment and Offset at P2

Figure 6-8 illustrates an offset at P2 of 15 mils (375 m) and the same angle (15 mils per foot) (375 m per 30 cm) or .072 degrees. The misalignment of the coupling will be .072 degrees at P1 and 0 degrees at P2. Note: The actual offset of the shaft (MTBM) at the appropriate measuring point (P1) will be 0.

P1

P2

Figure 6-9 Angular Misalignment and Offset at P1 and P2

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Figure 6-9 shows the same offset and angle to the other side of the shaft. Note how the shaft (MTBM) crosses the centerline of the stationary machine shaft. The misalignment of the coupling at P1 is the same .072 degrees. The coupling misalignment at P2 is .143 degrees or twice the coupling misalignment at P1. Note: The actual shaft misalignment as measured at P1 would be 30 mils (750 m). With the above referenced misalignment, a bending moment would be introduced into the shaft (MTBM) at approximately 25 inches (62.5 cm) from P1.

Advantages and Disadvantages of Coupling TypesThe advantages and disadvantages of various couplings are shown in Table 6-1.Table 6-1 Coupling Advantages and Disadvantages Coupling Type Gear-type couplings Advantages Transmit more power have a greater power to weight ratio than other coupling types. Accommodate for axial shaft movements due to rotor movement by design, or rotor movement due to thermal growths. Disadvantages Require lubrication - Must be stopped to lubricated. The exception to this is a continuous lube coupling that provides lubrication with the use of a pressurized oil system. If the oil is kept clean, this can add to the life of a gear coupling. Coupling lock up This can occur under certain operating conditions. This phenomenon can limit the travel or movements of shafts in the axial direction or limit the movement of the coupling to allow for misalignment. Failure of disks and life is proportional to misalignment Corrosion and fretting

Disk-type-couplings

Disk pack couplings can transmit more power per given size or weight than other types of non-lubricated couplings. Inspection can be performed while running. Failed disks can be replaced relatively easily. Simple design No lubrication required Will tolerate greater angular misalignment Smaller than other diaphragmtype couplings Can accommodate more axial movement Torsionally soft

Diaphragm-type couplings

Limited axial travel Larger diameter Heat generation due to windage Complicated in design Heavier than other diaphragm-type couplings Accommodate small shaft separation Very little damping

Convoluted diaphragm couplings

Grid-type couplings

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EPRI Licensed Material Shaft Couplings and Power Transmission Table 6-1 (continued) Coupling Advantages and Disadvantages Coupling Type Elastomeric-type couplings Tire couplings Geared rubber Spider couplings Advantages Disadvantages

Impose small radial forces on bearings due to offset misalignment Cost is low Requires no bolting Transmit large torque

Centrifugal force Thrust loads One hub must be moved for installation. Accommodate little offset

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7VERTICAL MACHINESVertical Machines with Rigid CouplingsVertical Machine BehaviorPower plant pumps in services such as heater drain and/or condensate pump applications tend to pose special problems with shaft alignment. These problems are of consequence in nuclear power stations due to the use of mechanical seals and the possible premature failure of these seals. Many of the seal failures are due to improperly aligned shafts in the pumps. When investigating the root cause of these problems, shaft alignment is typically at the top of the list. Of major concern, along with the seal failures, is the accelerated wear of the pressure breakdown device found in most stuffing boxes on this type of pump. This wear may be attributable to problems with shaft misalignment as wear escalates in the pressure breakdown area due to misalignment, causing a pressure increase in the stuffing box area and subsequent seal failures: On the surface where the motor adjoins the discharge head On the stuffing box mating surface On the base mounting plate At the last stage bowl joint and surface

Without all of these surfaces being true, coupling alignment is all that can be performed. Shaft alignment is directly related to the accuracy of these mating surfaces and/or fits. Problems of this nature might not have been evident in packed pumps, and if leakage did occur, it was not a major concern. Note: The discharge head from many manufacturers is supplied with a rabbet fit or registration for the motor to fit precisely onto for proper alignment. In some cases, this fit does not accurately align the motor shaft to the pump shaft, and the fit must be machined to achieve proper alignment. The adjoining stuffing box and related fits must also be true to the discharge head because this is the area where alignment data will be taken. Figure 7-1 is a diagram of a typical stuffing box, noting areas where runouts and concentricities are to be taken.

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EPRI Licensed Material Vertical Machines1

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2 3

Figure 7-1 Typical Stuffing Box, Noting Four Points Where Dimensional Runouts and Concentricities Are To Be Measured

Causes of MisalignmentParallelism and angularity of shafts are influenced by the amount that the baseplate and subsequent motor rotor support system are out of level. This support system includes the thrust bearing and housing, motor frame, and motor stand or discharge head. All of these items play an important role in misalignment that will eventually manifest itself in component failures, in the mechanical seal in particular. True shaft alignment cannot be performed on vertical machines with rigid coupling without alignment of all components that are stacked and/or suspended from the base (floor). Figure 7-2 shows a typical discharge head, also called a motor stand. The discharge head and base mounting are the starting points for all misalignment problems experienced in this type of pump. Some manufacturers require a baseplate that is level within .002 inches per foot (50 m per 30 cm). For example, if the bolt circle is 48 inches (1.2 m), the possible out-of-level is .008 inches (200 m). This might not sound like much, but using the linear approach of alignment, with a the suspended pump that is 15 feet (3 m) long, then .030 inch (650 m) deflection is at the bottom of the pump. This can cause a moment on the shaft at one of the line bearings, at the mounting, or at the bolted joint interface.

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EPRI Licensed Material Vert

shaft alignment manual

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