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Case study: Pump vibration at an electrical utility Tags: vibration analysis A Reliable Plant reader, a maintenance planner at a major U.S utility, recently submitted this case study It offers invaluable information to all of you who work on pumps During monthly vibration data collection rounds, a power station’s 500-horsepower vertical turbine circulating water pump experienced a large increase in vibration compared to the previous month readings The running speed vibration trend showed the November 2004 reading went from a normal 0.257 inches/second velocity to 0.468 inches/second in February 2005, and then increased again to 0.637 inches/second in March Analyst review of the data found the largest response at the top of the motor, which is normal for most vertical pumps as they tend to pivot at the pump discharge head mounting to the floor Resonance is common with this design of pump, however, the natural frequency of this pump/motor system was found to be well above the 500 rpm operating speed of the pump through the use of a simple bump test The waveform pattern showing a predominant 1x (running speed) cycle supported an imbalance/wear condition Knowing this, shaft displacement readings were taken in an effort to judge the severity of the problem After using a strobe light on the exposed pump shaft below the coupling to ensure no keyway or protrusion would interfere, a shaft stick (fish tail) with the vibration sensor mounted on it was safely held against the shaft In the past, the shaft displacement had generally been around three to five mils Now the vibration exceeded 16 mils, all at running speed A spring outage was previously planned in three weeks for this unit Due to the rapid change in vibration and the severity, recommendations were made to pull the pump during this outage The pump was monitored on a bi-weekly basis until the outage to ensure it would not catastrophically fail Preparations were made for impeller replacement as this was likely the source of the high running speed vibration due to imbalance, possibly from a broken vane due to foreign object damage or excessive wear ring clearances This type of damage was suspected because this pump is in a pit fed by a nearby river and has a history of damage due to debris River water is screened into the intake house, however, during the spring, high river levels tend to bypass debris and repetitive failures of the screen wash system have allowed debris past the screens which travels to the pit at the plant However, plant management still needed convincing that this was the problem, so prior to the outage, divers were brought in to inspect the circulating water pit for debris and to look at the pump impeller The pit was surprisingly clean, but the diver found the pump impeller tips damaged and an obvious offset to the fit of the impeller to the bowl This likely meant that the impeller wear ring was worn excessively and/or the lower shaft was bent Armed with this information, the pump was then scheduled for repair during the outage Upon inspection, the impeller wear ring was found worn, blade/vane tips were damaged/bent and the shaft was bent Parts were available for repair, but additional time was required to order a new shaft The condenser was found with excessive debris, so the hypothesis is that the pump had passed large debris that likely bent the shaft Repairs were completed within the timeframe of the scheduled outage and the pump was successfully returned to service After the outage, an inspection of the remotely located river water intake house found damaged intake screens and inefficient screen washing In an effort to prevent any future failures to the circulating water pumps, repairs were made to the screen and a new screen wash pump with a stand-by unit were installed A new control system was added to sense differential pressure on the screen improving the overall efficiency of this system Regular preventive maintenance inspections of the intake house system were developed and scheduled in the computerized maintenance management system to ensure this often neglected system is kept in good condition To date, less debris has been found in the condenser water boxes and the circulating water pumps have been running fine, but we are still fighting the high water levels during the spring rains Acoustic Emission: The Next Generation of Vibration Techniques Martin Lucas, Kittiwake Tags: vibration analysis, condition monitoring History, experience and familiarity count for a lot where conditioning monitoring is concerned, but that doesn’t negate the need for change, innovation and the advancement of tried, tested and trusted techniques The late Steve Jobs commented: “Innovation is the ability to see change as an opportunity, not a threat.” Condition monitoring (CM) is transforming rapidly and so too must the mindset of CM practitioners and users It’s not good enough to simply disregard a disruptive technology in an effort to protect the “old guard.” When combating downtime, there’s no place for historical sentiment Steadily disrupting traditional vibration techniques is acoustic emission (AE) AE technology spawned from the aviation industry where vibration analysis simply couldn’t be easily applied, short of a suicidal maintenance technician hanging off the wings AE technique is based on frequencies much higher than are monitored in the repetitive, synchronous movement of vibration These frequencies are the result of shock, impact, friction and cracking, for example By this means, it is possible to detect impending failure before damage occurs, as well as monitoring its progress thereafter With well-defined ISO standards, traditional vibration techniques including vibration monitoring and vibration analysis have provided a trusted approach to condition monitoring for the past 30 years Yet, it remains a complex science and requires sophisticated knowledge and understanding from a seasoned expert In contrast, AE technology extends and simplifies the science, placing the power of vibration techniques directly into the hands of every engineer Signals can be processed at the AE sensor into an easily understandable form Of course, vibration analysis (VA) as a technique will have a place for many for years to come for many end users However, there is no escaping the fact that there is often a requirement for a costly and unsustainable level of knowledge required to affect a good diagnosis For VA, the defect repetition frequencies are critically dependent upon the machine component design and geometry, as well as the precise running speed Vibration can occur independently in the X, Y or Z axis, and so orientation of the sensor is as important as location For a detailed interpretation, it is also necessary to know internal machine geometries, shaft speeds, meshing frequencies, etc., and to analyze the data before making a diagnosis So in summary, VA is valuable but too often overly complicated With acoustic emission (AE), signal processing is undertaken automatically at the sensor level With vibration analysis (VA), the signal is processed downstream manually or semiautomatically The areas in which vibration and AE both apply can be illustrated as overlapping circles However, AE provides an earlier warning, detecting wear and small defects, whereas with vibration, damage must have occurred to detect a signal AE will pick up a lack of lubrication, friction and cracking, which vibration will not, although it must be acknowledged that the totality of information obtained from AE will be more limited than that derived from vibration The signal processing required by AE is not something that can be performed by just anyone; it’s a high-frequency signal, so the user must have the knowledge to interpret the squiggly lines on a stethoscope However, recent developments have enabled this processing at the sensor level The sensor output can now provide pre-characterized numbers that tell you about the condition of the machine AE technology has been effectively de-skilled, enabling much wider application use Suitable for continuously running machinery as well as machinery operating intermittently, slowly or for short durations, AE allows the user to diagnose problems with machinery at an early stage, carry out maintenance procedures and then monitor the improvement It provides real-time information with early sensitivity to faults and applicability to a wide range of rotational speeds As awareness of the unique capabilities of AE increases, so too does the number of applications that it is suited to, many of which have proven difficult for other forms of condition monitoring to address For example, the analysis of signals, whether from AE sensors or accelerometers, requires a sufficiently long period of machine running at constant speed so that a statistically meaningful signal characterization can be made But that is where the similarity stops AE can be effective after around 10 seconds of measurements For example, the algorithm used to derive the widely used acoustic emission parameters of Distress and dB Level in the MHC range of products from Kittiwake Holroyd requires a 10second period of running at an approximately constant speed This compares favorably to Fast Fourier Transform (FFT) based vibration analysis, which typically needs 60 to 120 seconds of measurement time and tight tolerances on machine speed for an effective signal interpretation In cases where a hand-held instrument is used for periodic CM, it may be possible to interrupt normal machine operation and put it into a special continuously running mode for the duration of CM measurements However, such disruption is not always possible and never convenient Furthermore, it is not compatible with the current trend toward CM automation, which requires continuous online monitoring with permanently installed sensors inputting CM data or status into supervisory control and data acquisition (SCADA) systems or programmable logic controllers (PLCs) So why are many CM practitioners being so resistant to the benefits that AE brings to the table? It may be because many people have invested a lifetime in vibration and are understandably wary of losing power and status After all, if you “dumb down” vibration, surely this reduces the perceived value that they bring to the organization Actually, it doesn’t Just because AE is disruptive as a technology, it in no way invalidates traditional vibration techniques but simply extends the impact way beyond what has been able to be achieved to date For vibration techniques to be effective, you need equipment that’s far from cheap coupled with clever people to get the best from it Every result must be analyzed to understand what’s good and what’s bad For those who cannot afford the luxury of in-house vibration experts, there are many vibration specialists who offer a contract monitoring service, which is not an insignificant investment While for some, the criticality of certain applications coupled with the scale of some companies might justify this cost, others could still benefit from the efficiencies realized by similar CM techniques Furthermore, the “we don’t buy into one-month wonders/we’ve all been bitten by the latest whizz-bang technology” argument no longer rings true Indeed, AE techniques are only deemed disruptive because they are now mature with a proven track record Ultimately, maintenance personnel are responsible for keeping machinery running If they are empowered to monitor condition themselves, identify where action is needed and then check that the action taken has solved the problem, then AE has significant advantages of cost, speed, flexibility and ease of field application in comparison to traditional vibration analysis techniques It is the efficient and effective approach to CM To nurture the technology of a new era, a broader, longer-term view is required Surely it makes sense to embrace CM techniques that provide for the greatest protection or longest period of warning for potential damage and eventual failure By “de-skilling” technology, all maintenance professionals are empowered to make informed decisions quickly and with confidence, ultimately enabling them to positively and significantly impact a company’s bottom line Of course, there is room for sentiment in business but not at the expense of progress Using Orbits for Condition Monitoring Gary James, Ludeca, Inc Tags: condition monitoring, vibration analysis Orbits have historically been used to measure relative shaft movement within a journal-type bearing The shape of the orbit told the analyst how the shaft was behaving within the bearing as well as the probable cause of the movement This was accomplished using proximity probes usually mounted through the bearings with a 90-degree separation and a tip clearance set to around 0.050 inches With today’s modern analyzers, it is possible to also collect an orbit using case-mounted velocity probes or accelerometers to see how the machine housing is moving Another way of putting it would be the orbit represents the absolute path in space that the machine housing moves through (see Figure 1) Figure Figure This is accomplished utilizing a two-channel instrument and collecting an orbit with the sensor of choice being a velocity probe or accelerometer This is what’s referred to as a poor man’s operating deflection shape or ODS (see Figure 2) The analyst can interpret the data to determine machine movement at a particular measurement location or a section of the machine if a tachometer trigger is used during orbit collection as a phase reference Analysts must keep in mind the exact location of each sensor so that when they look at the shape of the orbit it is possible to tell the movement in relationship to the sensor’s location The sensors should be placed 90 degrees apart or at least as close as possible to 90 degrees Keep in mind that a properly wired sensor shows motion toward the sensor as a positive signal and motion away from the sensor as a negative signal An orbit is usually collected while the machine is at its normal operating state or speed, but it can also be collected while the machine is increasing or decreasing in speed, such as during a coast-down or startup The data can be collected in a steady state, in what is known as an unfiltered orbit, requiring no tachometer (see Figures and 3A), or at multiples of running speeds such as first, second or third order to look for issues relating to that or another specific frequency (see Figures 4, 5, and 7) Figure - Unfiltered displacement orbit Figure 3A - Unfiltered velocity orbit Figure – Second order setup Figure – Third order setup Figure – Second order results Figure – Third order results How to identify, correct a resonance condition Alain Pellegrino, Laurentide Controls Ltd Tags: vibration analysis, condition monitoring, predictive maintenance, maintenance and reliability Many experts working in the field of vibration analysis will agree that resonance is a very common cause of excessive machine vibration Resonance is the result of an external force vibrating at the same frequency as the natural frequency of a system Natural frequency is a characteristic of every machine, structure and even animals Often, resonance can be confused with the natural frequency or critical frequency If equipment is operating in a state of resonance, the vibration levels will be amplified significantly, which can cause equipment failure and plant downtime It is, therefore, important that the running speed of equipment be out of the resonance range How to identify a resonance frequency Many techniques can be used to identify and/or confirm a high vibration level caused by a resonance frequency It is very important to confirm a resonance phenomenon by at least two different types of tests before trying to correct it We will look at a few techniques commonly used in the industry Techniques used to confirm a resonance Impact test:One of the most commonly used methods for measuring a system’s natural frequency is to strike it with a mass and measure the response This method is effective because the impact inputs a small amount of force in the equipment over a large frequency range When performing this technique, it is important to try impacting different locations on the structure since all of a structure’s resonant frequencies will always be measurable by impacting at one location and measuring at the same location Both drive point and transfer point measurements should be taken when attempting to identify machine resonances This type of test must be performed with the equipment off This way you can easily identify the natural frequencies of the equipment (see Figure 1) Figure Impact Test, Equipment Off Impact test using an instrumented hammer:This test is basically the same as a regular impact test, except that an instrumented hammer is used to excite the system This hammer, Figure Time Domain Format Figure FFT Format Demodulated Current Spectrum In recent years, one of the most exciting advancements in PdM technologies is the demodulated current spectrum In order to better understand demodulation, the concept of modulation should briefly be addressed Modulation is when lower frequencies are merged on top of a higher frequency In other words, lower frequencies ride on the higher frequency signal This makes the carrier frequency the dominant peak in the FFT spectrum, and most of the information is lost in the noise floor of the spectrum Although they have always been present in the current spectrum, the repetitive load variation frequencies have been difficult to identify and trend Demodulation is simply the process of taking the carrier frequency out of the spectrum In this case, the carrier frequency is the fundamental electrical frequency being used The fundamental frequency in the United States is 60 Hz In many other countries, it is 50 Hz After removing the carrier frequency, the frequencies related to repetitive load variations are left behind and shown on the demodulated current spectrum Frequencies such as speed, pole pass, belt pass, vane pass, gears and bearing frequencies can be identified and trended in the demodulated current spectrum In effect, the motor is acting like a permanently installed transducer Due to the technology being relatively new, trending remains the most accurate method of identifying a problem in a machine The ability to have baseline data when the machine is in good health is ideal, while comparing data to similar machines is also very effective In the future, as historical and statistical data is compiled, there will be more alarming guidelines established for different types of equipment Locating Belt Frequencies The basic current signature of a belt-driven machine is shown in Figure It shows the fundamental 60 Hz frequency being the dominant peak Note the sideband peaks on each side of 60 Hz that are labeled with their frequencies This type of signature has the potential of being misconstrued as a potential rotor bar problem In this example, the sidebands are actually related to belt pass frequency The mechanical frequency in rotations per minute (RPM) is calculated using the following equation: Mechanical frequency = (the change between line frequency and peak of interest) x 60 The mechanical frequency of interest in this case would then be calculated by multiplying 6.5 Hz by 60 The result would be 390 RPM, which is the belt pass frequency of this machine Rotor bar problems show up at pole pass frequency, with the peaks much closer to the fundamental frequency Pole pass frequency is calculated with the following equation: PolePass= synchronous speed – (slip frequency / # of poles) Figure Belted machine current spectrum Figure Demodulated spectrum Although the mechanical peaks in Figure are prevalent, this is often not the case Usually, the mechanical peaks will be lost in the noise floor of this type of spectrum This is where the demodulated signal becomes so important Figure shows the demodulated spectrum derived from the current signature in Figure Notice how much cleaner and easier to read this example appears Without the 60 Hz electrical frequency present, the remaining mechanical frequencies become much more prevalent The sideband frequencies shown in Figure now show up in Figure at 6.5 Hz along with a 2x and 3x belt pass frequency in the demodulated spectrum Looking closely at the current spectrum, the 2x and 3x frequencies are present but not as easily identified Belt pass peaks in this type of spectrum are good early indicators of belt alignment, wear and sheave problems Running Speed Frequencies Both drive and driven speeds will also be found in the spectrum if there is a problem As in vibration, a 1x rotational speed will signify an unbalanced effect on the machine In Figure 4, the fan speed shows up at just over 25 Hz In Figure 5, the running speed of an 1,800-RPM pump motor can be identified at just less than 30 Hz In both cases, trending the amplitude of these peaks can tell us many things about the condition of the machine Normally, this peak will be at or near the noise floor of the spectrum The two most common reasons that the amplitude will climb on direct-drive pump assemblies are due to misalignment or a damaged coupling The amplitude response on the spectrum is remarkably sensitive A noticeable amplitude increase will occur with the incorrect key length in the hub The common flexible-type couplings used will also show up when they fatigue, crack or become twisted An example of a brittle flexible coupling with stress cracking is shown in Figure Figure shows the follow-up test completed after a new coupler and laser alignment was performed Notice the amplitude of the rotational speed peak at 30 Hz relative to the amplitude of the low-frequency product flow noise in each spectrum Figure Pump Assembly with Problem Figure Pump Assembly Repaired Other Demodulation Opportunities Another common frequency found in a demodulated current spectrum is the pump vane pass and fan blade pass frequency This will help to trend and locate problems with the impeller or flow restrictions In a demodulated current spectrum, it is calculated by using the following equation: Vane pass = pole pass frequency x # of vanes With the growth of the technology, knowledge and software advancements, locating bearing faults, gear mesh and other mechanical frequencies with MCSA will also become more common Incorporating into a PdM program With all that demodulated MCSA can do, there are still many questions as to how it will fit into and benefit a PdM program Frequently asked questions might include: Why I care about finding mechanical faults with a demodulated current signal if I can find them with technologies like vibration analysis or infrared thermography? How reliable is the data that is generated and can it take the place of vibration? How often should MCSA be completed on equipment? With any condition monitoring technology, there are strengths and weaknesses Each technology applied will give a more complete view of the health of the equipment For best results, it is recommended to complete MCSA at least quarterly If a program is testing less frequently than this, the overall results of the motor testing program will be compromised As with any technology, it is critical to have enough data to accurately trend the history of the machine As for MCSA technology looking for mechanical faults, there are many reasons why this can benefit a PdM program For example, when it comes to belt and coupler problems, demodulation will give an earlier and often more accurate fault indication than vibration analysis The amount of energy created by the early stages of this type of fault is relatively low When belts or couplers begin to wear, it is often not noticed in a vibration spectrum until the fault is nearing catastrophic failure A demodulated current spectrum has the ability to detect the fault early enough to provide plenty of time to plan and schedule the repairs However, demodulated MCSA is not intended to take the place of a vibration program It is best used as a complimentary technology to a good vibration program An added benefit of this technology would be in remote equipment locations or areas where equipment is not accessible during normal operations On this type of equipment, visual inspections can be difficult, and the ability to perform vibration analysis is limited Depending on the risk assessment, remote wiring transducers for vibration may be too costly In this case, MCSA would work well due to the ability of the equipment to be tested from the motor control center Part of any strong PdM program is having the ability to verify a fault with more than one technology This not only ensures the validity of the fault but also helps make a more accurate and precise repair recommendation The importance of verification with a second technology is never more evident than on a critical piece of equipment that requires plant outages for repair Demodulated MCSA brings an added dimension to this effort and has proven itself to be an invaluable tool for any PdM program Field Balancing Rigid Rotors "If you want to find the secrets of the universe, think in terms of energy, frequency and vibration." ― Nikola Tesla Could Tesla's secret be the energy wasted due to vibration at a frequency equal to shaft speed all caused by rotor unbalance? Balanced rotors are critical for achieving production and profit goals Unbalance creates high vibration, which leads to other faults resulting in decreased machine life, wasted energy and reduced efficiency Smooth-running machines are required for producing products that meet customer specifications The IOSR Journal of Mechanical and Civil Engineering states that rotor unbalance is the major cause of vibration problems A good balancing process is essential for successful physical asset management What is a Rigid Rotor? A rotor that operates at a rotational speed below 70 percent of its critical speed is considered to be a rigid rotor The critical speed is the speed at which resonance occurs by exciting its natural frequency Ninety percent or more of rotors are rigid What is Unbalance? Rotor unbalance can be defined as a condition that exists when the geometric center and the center of mass (also known as the center of gravity) not coincide In reality, these centers never coincide exactly, but the goal of rotor balancing is to reduce the unbalance to the point that machine life is not negatively impacted by the residual unbalance The balancing technician attempts to bring the center of mass and the geometric center to the same point or close enough to meet a predetermined balance standard There will always be some residual unbalance Balancing Standards Several standards organizations have developed balancing standards for use on various machines ISO and API are a couple of examples Some companies develop their own standards Which standard should you use? Choose standards that will allow you to achieve the required machine life and preserve equipment functions to the levels required by your processes This may sound like a political answer, but in reality, the standards may vary depending on your plant's requirements The requirements will already be known if you are using a well-defined physical asset management process Unbalance is measured in units of ounce/inches or gram/millimeters However, when you balance in the field, you usually balance to a vibration standard because it is easier to determine machine vibration levels than to determine residual unbalance The vibration due to unbalance is directionally proportional to the amount of unbalance If the vibration levels due to unbalance are lowered to an acceptable level, the amount of unbalance is also brought to within an acceptable standard Forces Incurred by Unbalance • • • Unbalance produces rotor vibration Double the amount of unbalance, and the forces due to unbalance double Double the rotor speed, and the forces quadruple Types of Unbalance • • • Static or force Couple Dynamic or a combination of force and couple Causes of Rotor Unbalance • • • Blowholes in cast parts Eccentricity Improper key length Stress relief distortion • Thermal distortion • Corrosion or uneven wear • Deposit buildup and component shift • Asymmetrical design and assembly errors • Rotor damage • Repair work errors such as use of wrong coupling bolts or washers Balancing is performed after acquiring vibration and phase measurements There are three requirements for balancing: steady vibration, steady phase, and the vibration and phase measured must be due to rotor unbalance Before attempting to field balance a rotor, follow these basic steps: • Perform a vibration analysis on the machine • Take radial and axial vibration readings • Ensure the rotor is clean • Verify there are no loose parts on the rotor • Always correct other problems before attempting to balance • Use a dial indicator to check total indicated runout (TIR) • Remove previous balance correction weights Data collectors/analyzers such as the Vibxpert can be used to perform the vibration analysis necessary to identify unbalance • Unbalance is a radial force and produces a vibration frequency equal to shaft speed The radial force can sometimes produce a vibration in an axial direction, especially in overhung or cantilevered rotors In these rotors, the axial vibration may be even greater than the radial vibration The radial force causes the shaft to deflect, producing an axial direction vibration in the bearings Normally, unbalance will produce a 1× or shaft speed vibration that is 80 percent or more of the total vibration If 1× vibration is less than 80 percent, suspect other problems in addition to the unbalance This is the same as stating that the vibration at other frequencies should not exceed 20 percent of the overall vibration It may be necessary to correct the other problems before the rotor can be properly balanced Unbalance always exerts an equal force in all radial directions, but the vibration due to the unbalance is almost never equal in all directions The horizontal vibration is usually the highest in amplitude because most machines are less stiff in that direction Be careful because there are exceptions such as looseness in the bearings and vertical resonance If vibration amplitudes in the radial positions differ by 5:1 or more, other problems usually exist If more than three harmonics of shaft speed are visible in the spectrum, other problems could be present, with the most likely being looseness All other problems should be corrected before attempting to balance the rotor The image below shows the corresponding measurement and weight correction planes Phase Indications of Rotor Unbalance Phase is a very important tool when diagnosing unbalance because 1× vibration can be generated by several other problems Phase numbers shown below may vary by 15 degrees Force unbalance phase difference: degrees when comparing the same radial positions on two bearings Couple unbalance phase difference: 180 degrees when comparing the same radial positions on two bearings Dynamic unbalance phase difference: to 180 degrees when comparing the same radial positions on two bearings The phase shift from horizontal to vertical should be approximately 90 degrees for all types of rotor unbalance when measured on the same bearing The phase should be steady and repeatable when attempting to balance (It may become unstable after a trim balance because other problems tend to become more dominant.) A good indication of unbalance is that the phase difference between inboard horizontal (IBH) and outboard horizontal (OBH) should equal the phase difference between inboard vertical (IBV) and outboard vertical (OBV) Phase errors drastically affect balancing For example, • A phase error of 7.5 degrees may produce an 8:1 vibration reduction • A phase error of 15 degrees may produce a 4:1 vibration reduction • A phase error of 30 degrees may produce a 2:1 vibration reduction • A phase error of 60 degrees or more may produce no reduction in vibration Balancing Rules All rotors regardless of their diameter/width (D/W) ratio should be balanced in two planes However, on overhung rotors, if vibration in the bearing nearest the overhung mass has an amplitude of approximately four times the vibration in the other bearing, single-plane balancing may work Overhung rotors with a D/W ratio of 4:1 or more may sometimes be balanced within the tolerance by a single-plane balance In addition, sometimes narrow overhung rotors don't respond well to two-plane balancing On overhung narrow rotors, it becomes more difficult to separate the influence of the two weight planes on the two measurement planes This is simply because narrow rotor weight planes are in close proximity to each other To determine if a two-plane balance job is needed, set up and take four measurements but enter only one correction plane (single plane) Ask the program for estimated reductions If the estimates are within tolerance, continue with a single-plane balance If the estimated reduction is out of tolerance, change the setup to two correction planes and continue Other Indications of Rotor Unbalance The time waveform should contain a strong sine wave component Acceleration measurements distort the time waveform because higher frequencies are amplified Steady phase and steady amplitude are also requirements for balancing If either is not steady, don't diagnose the problem as unbalance Sometimes it may be wise to make vibration measurements and then shut down the machine Start the machine again and see if the vibration and phase measurements can be duplicated Selecting a Proper Trial Weight Proper trial weight (TW) selection is important because a weight that is too light may not provide an adequate response for calculating correction weights and placement A trial weight that is too large may wreck the machine Trial weights should adhere to the 30/30 rule: change the phase by 30 degrees or the amplitude by at least 30 percent Calculating Trial Weights F = Rotor weight × 10 percent F = (1.77) × (TW) × (R) × (rpm ÷ 1,000)2 Where: F = Force in pounds, TW = Trial weight in ounces, R = Radius in inches Example: 5,400-pound rotor, 1,800 rpm, 13.5-inch radius F = 5,400 × 10 percent; F = 540 540 = (1.77) x (TW) x (13.5) x (3.24) 540 = 77.4 TW TW = 6.98 ounces Typical Single-Plane Procedure Set up equipment on the rotor as outlined by the balance equipment manufacturer Make "reference run" or "calibration run," measuring vibration and phase Add trial weight Make trial run Remove trial weight Add correction weight Make trim run Add trim weight if needed Make no more than two trim runs If more than two trim runs are needed to achieve the standard, leave all weights in place, erase the data collected on this balance job and begin the process anew Typical Two-Plane Procedure Set up equipment on the rotor as outlined by the balance equipment manufacturer Make "reference run" or "calibration run," measuring vibration and phase Add trial weight to plane #1 Make trial run #1 Remove trial weight from plane #1 and place on plane #2 Make trial run #2 Remove trial weight from plane #2 Add correction weights to planes #1 and #2 Make trim run and add trim weights if needed Make no more than two trim runs If balance job is not within specs after two trim runs, leave weights in place, erase the data collected on this balance job and begin the process anew When rotors won't stay in balance, the following problems should be suspected: • Thermal sensitivity • Operating near a resonance • Rotor erosion • Material buildup • Speed changes bringing the machine into a resonance condition • Loose rotor part The mindset of "It's a fan and it shakes so it must need balancing," is referred to as the balancing syndrome and can get you into trouble Here are some tips that will help you avoid such pitfalls • The time signal should not be too distorted from sinusoidal • The shaft speed vibration amplitude should be steady and repeatable • The phase should be steady and repeatable • No more than three harmonics of shaft speed should be present in the spectrum (Plain bearings may be an exception.) • There should be no raised noise floor • No sub-harmonics should be present • The rotor should be clean and free of buildup • Balancing should be the last corrective measure (correct obvious problems first) Rotors not become unbalanced without a cause If a rotor has been acceptably balanced and becomes unbalanced, always try to detect and correct the root cause of the unbalance Sometimes rotor-reinforcing gussets may crack, allowing the rotor to deform and resulting in a sudden unbalance If the rotor is rebalanced without repairing the cracks, sudden and catastrophic machine failure may occur if the gussets fail completely Such problems can usually be avoided by performing a thorough cleaning and inspection 10 11 Vibration is measured in displacement, velocity and acceleration Velocity and acceleration are generally the most often used units, but when you balance rotors, you usually use displacement measured in mils This is because displacement gives the best indication at low frequencies, and you are dealing with 1× shaft speed when balancing The technician needs to be aware that mil of vibration on a machine running at 900 rpm is not comparable to mil of vibration on a machine operating at 3,600 rpm There is considerably more force exerted on the 3,600 rpm machine Why a Balance Job Goes Wrong The top causes for failing to achieve the required standard when balancing include: A mistake in setting up the data collector/analyzer Failing to remove a trial weight A problem that is not unbalance (usually looseness) Entering the trial weight location incorrectly into the balancing instrument Overhung load requiring two-plane balance Weight placement not in the proper direction with relation to the rotor direction Existing resonant condition Becoming proficient at rotor balancing requires having good tools and being knowledgeable in their use A fundamental knowledge of balancing theory and balancing procedures is also critical for success Having a good grasp of vibration analysis will help to ensure that the technician performs balancing only when the problem is unbalance Because of many variables, there remains some art to balancing With better tools and an improved knowledge, base balancing has become less of an art and more of an applied science Using Dynamic Electric Motor Monitoring to Identify Mechanical Issues Dynamic electric motor testing is often called on-line testing because it requires the motor to be running and generally assumes the motor is in its natural environment Dynamic testing involves the connection of voltage probes and current transformers Connecting dynamic test equipment is safe, quick and non-intrusive Data is acquired and results are displayed in a summary format The collected data is compared to the user-entered nameplate information and is presented in a pass/fail format with both current test data and trending logs displayed after each successive test The Need for Motor Testing Every reliability technician knows that costs associated with motor failures can be devastating to any business operation Finding that a motor is operating with conditions that create excessive heat or stress is a guide to the technician to make changes in the motor’s operation and to monitor its insulation Knowing that a motor is in imminent danger of failing provides the technician with time to schedule repairs at his convenience rather than having the motor dictate to him due to a catastrophic failure Reducing unscheduled downtime while increasing efficiency and profitability are common goals of all reliability technicians Dynamic motor testing and monitoring is a relatively new concept aiding and advancing the capabilities of those responsible for the safe and continuous operation of electric motors and related equipment What Dynamic Testing Tells You A motor is one part of a complete system that includes incoming power quality, the motor and the driven load Many motor problems are created by poor incoming power quality, and many more problems can be attributed to the load and load-related issues State-of-the-art dynamic motor test equipment is capable of separating electrical issues from mechanical issues as well as defining power-related problem areas Good test equipment will provide an enormous amount of information regarding the incoming power, including voltage levels, imbalances and harmonic content A small amount of voltage imbalance will result in a much larger amount of current imbalance and increase losses within the motor Harmonic distortion also results in wasted energy causing overheating due mainly to non-sinusoidal sine waves These issues directly affect a motor’s performance and its ability to handle its load Overall, poor power quality manifests itself as higher heat within the stator and rotor, reducing efficiency and eventually resulting in premature motor failures Monitoring power quality and making necessary adjustments are essential in maintaining motor longevity Besides power condition, dynamic testing provides extensive information about the motor’s behavior and offers evidence of potential mechanical problem areas The test equipment tracks current levels and unbalances, load levels and torque-related information Combining this data with the power quality information, the equipment can predict de-rating factors that indicate potential problem areas Torque and torque ripple add another piece of the puzzle that is required for consistent and accurate diagnosis of the motor’s health Torque ripple is defined as the division of maximal torque divided by average torque during the acquisition period The torque ripple itself is a measure of how small the torque band is that surrounds a steady state average torque Torque ripple is independent of power condition and current level It provides a visual look at how the driven load is performing and is an indicator of rotor stress Dynamic testing can identify rotor bar problems with a high degree of accuracy, and trending logs make tracking them over time easy and predictable Rotor bar and cage defects result in lost efficiency and higher heat culminating in premature motor failure Measuring and tracking efficiency is a very difficult task The operating efficiency of a motor cannot be easily measured in a field application Many standards have a number of requirements that commonly can only be fulfilled in a laboratory environment These standards also usually concentrate on ensuring a proper description of a motor's capabilities under good operating voltage conditions In the field, however, there is little room for requirements like uncoupling a motor or regulating the voltage level for a saturation run Questions regarding a particular motor's capabilities are found to be secondary in importance when compared to the operating efficiency under the given conditions in the field The result of such an environment is that true efficiencies are unrealistic to obtain Operating efficiencies, however, are of crucial importance to an energy-conscious management The requirements for a true measurement of operating efficiency in a field environment are ample and unrealistic (like installing torque transducers on the shaft of the motor and measuring the input power to the motor at the motor terminals, frequently at high voltage levels) Instead of true efficiency measurement, efficiency estimation becomes the only field-friendly approach for energy management The difference between operating efficiency measurement and operating efficiency estimation is that the former attempts to find the true operating efficiency via direct measurement, while the latter accepts a small measure of inaccuracy for severely increased user friendliness Case Studies In a controlled laboratory experiment, vibration analysis and dynamic data were acquired on a new 5-horsepower, 460-volt motor The motor was disassembled, and the outer race of the drive-end bearing was intentionally damaged (Figure 1) Figure The motor was reassembled, and new data collected The common formula used in vibration analysis was applied to the data acquired, and the results were posted on both the vibration spectra and the torque spectra The resulting calculations concluded that the outer race defect should appear at 107 hz with sidebands related to the motor speed in the vibration spectra and twice the fundamental frequency in the torque spectra (Figures and 3) The outer race defect with its sidebands was much easier to determine in the dynamically acquired data than the vibration spectra Figure Vibration spectra Figure Demodulated torque spectra Rotor Bar Problems In another laboratory-controlled study, a 1-horsepower motor running under full load on a small dynamometer was tested thoroughly with the dynamic tester The results were then saved and analyzed The rotor was removed, and a 5/8-inch hole was drilled through one rotor bar, severing it completely The motor was assembled and retested under identical conditions Again, the results were saved and then compared to the original data (Figures and 5) Figure Figure The broken rotor bar was clearly defined in the current signature analysis without any difficulty or intensive diagnosis Cavitation At a large electric power plant in North Carolina, mechanics noticed that one of three 15,000horsepower pumps developed a lower flow level than the other two The mechanics blamed the motor, while the electricians maintained the pump was at fault Dynamic electric data was acquired and analyzed Figure shows the resultant torque ripples Figure One pump displayed the large variations in the torque ripple, while the two pumps that were operating normally had the smaller torque signature As a result of this testing, a diver was sent into the pit and found that the bolts on one end-bell had rusted off, allowing the flute that directed water into the pump to fall off This situation caused the pump to circulate water outside the pump and created cavitation Bolts on the other pumps were also in need of replacement and would have failed in the near future The downtime required to make the repairs cost the facility several million dollars, but the increased productivity after the repairs easily compensated for those costs In conclusion, dynamic motor testing and the state-of-the-art equipment available today are rapidly becoming the tools of choice for reliability technicians worldwide The technology is quite young, and new innovations are continuously expanding its capabilities and horizons