VIBRATIONS WITH VARIABLE SPEED PUMPS

Transcription

1 VIBRATIONS WITH VARIABLE SPEED PUMPS May 2006 Prepared for Public Interest Energy Research Program Energy in Agriculture Program California Energy Commission th St. Sacramento, CA by Dr. Charles M. Burt, Franklin Gaudi and Dr. Xianshu Piao Irrigation Training and Research Center (ITRC) California Polytechnic State University (Cal Poly) San Luis Obispo, CA cburt@calpoly.edu

2 EXECUTIVE SUMMARY At certain pump rotation speeds (RPMs), damaging vibrations can occur. This is particularly important with Variable Frequency Drive (VFD) installations. Key points are: 1. Guidelines exist for the magnitude of acceptable and damaging vibrations with pumping plants. 2. Equipment is available to diagnose vibration problems. 3. Critical speeds and magnitudes of problematic vibrations are different in every installation. Therefore, it is impossible to generalize whether there will be no vibration problems with VFD applications. 4. For a new pump purchase, options can be specified that can reduce vibration problems. 5. Vibration problems in the field are not only caused by the pump they can be magnified by the pump piping, supports, etc. 6. Before an existing single speed pump is converted to a VFD application, the existence of, and magnitude of, vibrations at various speeds can be determined before the VFD is installed. 7. Options exist to reduce vibrations that occur with installed pumps (either new or converted VFD systems). The recommendations for proposed VFD pumping applications can be summarized as follows: 1. Check with the pump manufacturer when purchasing a new pump assembly to determine if they would recommend either of the following: a. A stiffer discharge head, which will help to move the structural resonance above the operating RPM. b. Reduced shaft vibration, by installing more frequent bearings (such as at 5 spacing instead of 10 ). 2. Perform the two vibration analysis tests (resonance and excitation) once the pump/motor/platform assembly has been completed in the field. 3. Predict if there will be vibration problems in the anticipated RPM range of the pump. 4. If possible, minimize the pump (excitation) vibrations by improving alignment or changing bearings. 5. If damaging vibration problems are still anticipated, make modifications to the structure. This is generally done by stiffening various components (discharge head, piping supports, etc.) to increase the resonant frequency so that it is above the anticipated pump RPMs. 6. Re-analyze the resonant frequency using the vibration test equipment, to determine if the structure modifications achieved the desired results. This ITRC Report provides an overview of vibration diagnosis, and a discussion of how to remedy certain types of vibration problems. Irrigation Training and Research Center -i- Vibration Summary for VFDs

3 TABLE OF CONTENTS Executive Summary... i Pump Vibrations A Quick Overview... 1 General...1 Acceptable Vibration Limits...2 Excitation Vibration... 3 Excitation Vibration General Discussion...3 Excitation Vibrations Measurement...4 Resonance... 6 Resonance General...6 Resonance Critical Speeds of Pump Shafts...6 Resonance Structural...8 Structural Resonance Measurement...8 Putting Two and Two Together Reducing Vibration Problems Avoiding the Critical RPM Range...13 Order Special Pump Options...13 Repair Causes of Excitation...13 Change the Resonance Frequency...13 Recommendations Acknowledgements Irrigation Training and Research Center -ii- Vibration Summary for VFDs

4 LIST OF FIGURES Figure 1. Acceptable field vibration limits for pumps (Courtesy Goulds Pumps, Inc.)... 2 Figure 2. Excitations at multiples of the RPM... 4 Figure 3. Accelerometer located on the motor housing... 5 Figure 4. Applied force (excitation) using the Dytran Dynapulse hammer... 8 Figure 5. Vibration of structure (response) created by the applied force... 9 Figure 6. Structural resonance (natural frequency) pinging the shaft... 9 Figure 7. Structural resonance (natural frequency) pinging the motor housing... 9 LIST OF TABLES Table 1. Vibration causes (courtesy Goulds Pumps, Inc.)... 3 Table 2. Critical Shaft Speed (RPM)... 7 Irrigation Training and Research Center -iii- Vibration Summary for VFDs

5 PUMP VIBRATIONS A QUICK OVERVIEW General Two phenomena must be considered when evaluating the potential for vibration problems with VFD applications: a. Structural Resonance. This is the natural vibration frequency of the complete pump structure, and will include all combinations of the following components: i. Motor ii. Column iii. Discharge piping iv. Pump platform v. Any other component in the pump structure The structural resonance is measured by hitting a pump (that is not operating) with a special hammer. Electronic instrumentation attached to the frame of the pump then measures the magnitudes and frequencies of the vibrations that the hit produced. What it means: Assume the structural resonance of a pumping plant is measured as 879 RPM as determined by the hammer test. If a pump on the structure is operated at 879 RPM, whatever normal vibrations the pump has at that speed will be amplified by the structure. b. Excitation Vibration. This refers to the patterns of vibrations caused by the rotating parts of the pump and motor without considering the structural resonance. This is measured by attaching vibration measurement equipment to a pump, turning the pump on, and taking measurements. Irrigation Training and Research Center -1- Vibration Summary for VFDs

6 Acceptable Vibration Limits Figure 1 provides guidelines for allowable vibrations in field installations. Figure 1. Acceptable field vibration limits for pumps (Courtesy Goulds Pumps, Inc.) Irrigation Training and Research Center -2- Vibration Summary for VFDs

7 EXCITATION VIBRATION Excitation Vibration General Discussion The excitation vibration is the source of vibration problems. With no rotation of a pump/motor assembly, there is no problem. Every piece of rotating equipment will emit a vibration signature (also known as excitation resonance ). When this signature pattern changes, failure is often imminent. Therefore, in large or critical installations it is not unusual to install a vibration meter on the pump to monitor the pump status; if hooked into a SCADA system it can be a valuable tool. What is less well understood by most engineers and managers is how this signature can be used to predict potential problems in planned VFD installations. The table below indicates common causes of vibration in pump assemblies. Table 1. Vibration causes (courtesy Goulds Pumps, Inc.) Table 1 indicates that machines will generate mechanical vibration or excitations at multiples (harmonics) of their running speeds. This type of vibration is called "synchronous" vibration. For example, unbalance causes a force that moves the bearing (causes vibration) in any direction (plane) at a rate of exactly once per revolution (1 RPM). In addition, a pump with 5 vanes on the impeller can generate hydraulic pulses (which can be measured as mechanical vibration) at exactly 5 times per revolution (5 RPM). Similarly, Irrigation Training and Research Center -3- Vibration Summary for VFDs

8 different mechanical problems (unbalance, misalignment, etc.) tend to generate their own characteristic vibration signatures. Table 1 indicates that because of the effect each problem has on the vibration signal we measure, they each tend to generate vibration at specific (RPM related) frequencies. Figure 2 illustrates this point. Figure 2. Excitations at multiples of the RPM Other vibration generators may not be tied specifically to the machine's rotational speed. Bearing problems and electrical problems, for example, tend to generate vibrations at specific frequencies other than exact multiples (harmonics) of running speed. This type of vibration is referred to as "non-synchronous" or "sub-synchronous" (below 1 RPM) vibration. To reiterate previous and future statements: Excitation is NOT to be confused with Resonance. An excitation is a vibration force that only occurs when the motor/pump is in operation. A resonance is a natural frequency at which a mass vibrates while at rest when a force is applied. The relationship between the two is that vibration problems are amplified when the excitation frequency moves into the same range as the resonance frequency (such as by increasing or decreasing the pump RPM with a VFD). Excitation Vibrations Measurement Because objects move back and forth when vibrating, vibration is defined as being "cyclical", or "sinusoidal". The measurement of that movement is known as the "amplitude". 1. The amplitude is a measure of the amount of movement. 2. This amount of movement is related to the severity of the vibration. There are three methods of describing amplitude: 1. Displacement measures the total distance the transducer (bearing) travels back and forth during one 'cycle' of movement (a 'cycle' is the process of moving from one extreme to the other and back again to the starting point). 2. Velocity measures the maximum speed the transducer achieves during a cycle. 3. Acceleration measures the force(s) that are causing the back and forth movement. Irrigation Training and Research Center -4- Vibration Summary for VFDs

9 Physical measurement of the excitation vibration is done by ITRC using commercial hardware and software. The key components are: 1. The LDS Dactron Photon II data analyzer/collector 2. RT Pro 6.05 software to analyze and manipulate the data. 3. Accelerometer. This is the device that is connected to the pump structure (see Figure 3). Figure 3. Accelerometer located on the motor housing This hardware/software will provide a graph similar to Figure 2. Irrigation Training and Research Center -5- Vibration Summary for VFDs

10 Resonance General RESONANCE Resonance is the natural frequency of a component or combination of components (assembly). All structures have a resonant frequency. If you impact the structure with enough force to make it move, it will vibrate briefly at its natural frequency. A structure will have a resonant frequency in each of its 3 directional planes (x, y and z, or horizontal, vertical and axial). Resonance serves to amplify the vibration due to whatever vibration force (or excitation) is present at (or near) that resonant frequency. Resonance does not cause vibration it amplifies it. Resonance problems occur in two primary forms, which are: 1. Critical Speeds, and 2. Structural Resonances (a.k.a. the natural frequency of a structure) Resonance Critical Speeds of Pump Shafts Critical speeds occur when a component rotates at its own natural frequency (for a simple mass). For example, a shaft (or rotor) at a given length and a given diameter will begin to move in the shape of a two-person jump rope at its critical speed. This is created when the rotational speed (RPM) coincides with the natural frequency of the shaft (RPM). The tiniest amount of unbalance (something that is always present) is enough to cause vibration when an object rotates at its critical speed. Shafts that are sped up or slowed down slowly are susceptible to this because the mechanism may pass through its critical speed (also known as the 1 st critical ). In addition, 2 nd and 3 rd criticals may also occur if the shaft speed gets high enough. For example, imagine that the shaft has not one but two imbalances. At higher speeds the shaft will begin to act as though a person grabbed the center of a jump rope, creating two halves that are rotating opposite to one another. Another way to imagine this is to picture a sine wave. The process could continue for a 3 rd critical and so on. In the case of vertical pumps, the frequency (or RPM) at which typical pumps are run is relatively low. For this reason, the 1 st critical is usually the only one of concern. Table 2 below can be used to determine the 1 st critical shaft speed (in RPM) based on: Shaft diameter, and Shaft length (or the spacing between bearings) Irrigation Training and Research Center -6- Vibration Summary for VFDs

11 Table 2. Critical Shaft Speed (RPM) Critical Shaft Speed, rpm Shaft Dia (in) Shaft Length (in) Because shafts are so commonly used and so well understood, the shaft behavior is often examined at the pump factory to determine if there will be vibration problems due to shaft rotation only. Irrigation Training and Research Center -7- Vibration Summary for VFDs

12 Resonance Structural Structural resonance problems tend to be more common than a critical shaft speed problem. The structural resonance (or natural frequency) is the vibration that occurs in a structure when a force is applied. For pumps, the structure includes, among other things: 1. Motor 2. Discharge head 3. Shaft 4. Column 5. Bowl/impeller assembly 6. Pump support structure and structural components Structural Resonance Measurement ITRC uses the same equipment and software for structural resonance measurement as for measuring excitation vibrations, plus an impact hammer. ITRC uses the LDS Dactron Photon II data analyzer and RT Pro 6.05 software to measure, analyze and manipulate the field data. The devices connected to the Photon II used for taking the structural resonance measurements are the Dytran Dynapulse hammer and accelerometer. The Dynapulse hammer (which is literally a hammer that a person hits against the structure) is used to create a vibration within the structure. The force that is induced into the structure is measured by a pressure transducer that is built into the tip of the hammer (Figure 4). The force is measured in pounds. Figure 4. Applied force (excitation) using the Dytran Dynapulse hammer Irrigation Training and Research Center -8- Vibration Summary for VFDs

13 The subsequent response from the structure is also measured using an accelerometer (Figure 5) which is measured in gn (1 gn = ft/sec 2 ). Figure 5. Vibration of structure (response) created by the applied force The data sets above (Figures 4 and 5) are then combined in vibration analysis software to develop the Frequency Response Function (FRF) that characterizes the relationship between an excitation signal and a response signal. Figure 6 illustrates the magnitude of the vibration per pound force at a various frequencies. This data was collected by ITRC, by pinging (hitting) a vertical pump shaft. Figure 6. Structural resonance (natural frequency) pinging the shaft Figure 7 illustrates the results, using the same equipment, obtained by pinging the motor housing. Figure 7. Structural resonance (natural frequency) pinging the motor housing Irrigation Training and Research Center -9- Vibration Summary for VFDs

14 Please note that the vertical scales of Figures 6 and 7 are different. The peaks also occur at slightly different RPMs (labeled as CPM on the figures). The red vertical lines indicate the relative vibration problem if the structure were excited at a frequency of 1780 RPM. The green vertical lines indicate a peak resonance frequency of the structure that would occur somewhere around 1538 RPM (measured by pinging the shaft) or 1494 RPM (measured by pinging the motor housing). Both measurements were done when the pump was turned off. It appears that the pinging of the motor housing gives the best estimation of where the structure will have the greatest resonance. Using the same pump, the actual vibrations were measured on the motor housing at various operation RPMs. These values, seen in the table below, indicate that the maximum displacement occurred at 1494 RPM rather than at the 1538 RPM value that had been determined by pinging the shaft. Table 3. Vibrations measured on the motor housing. RPM of the pump Displacement measured by the accelerometer on the motor base (inches) The structural resonances of interest in a VFD application are only those in the range of the VFD-controlled pump operation RPMs. Irrigation Training and Research Center -10- Vibration Summary for VFDs

15 PUTTING TWO AND TWO TOGETHER (Resonance plus Excitation) The structural resonance becomes a problem when some forcing frequency (or excitation) comes close to the resonant (natural) frequency of the structure. Fortunately, the excitation force ratio decreases as a square of the RPM ratio, so if the two frequencies overlap at low RPMs, the driving force (excitation) can be relatively small. The steps below outline the procedure that is used to predict the existence and magnitude of vibrations at critical resonant frequencies. While the prediction is not precise, it gives a good indication of the general magnitude of problems that will exist at new RPMs. Step 1 Structural Resonance This information is taken from the graph showing the structural resonance signature that was determined by pinging the motor base (Figure 7). Two values are of interest - Value A B The displacement measured at the frequency that the pump operates at 100% RPM - in other words, when a single speed motor is used. The largest displacement measured at a lower frequency that is in the anticipated operation range after a VFD is installed In this case, when the motor base was "pinged", there was a peak displacement at a frequency of about 1494 RPM. Frequency Displacement Value (RPM) (gn/lbf) A B Amplification Factor (AF) = Displacement B / Displacement A AF =.055/.001 AF = 5.50 times as strong at the peak frequency point of interest (B) 2 Excitation (1x RPM) This information is taken from the 1x RPM excitation graph. Use the displacement measured at the RPM at which the single speed pump is operating Frequency Displacement (RPM) (inches) Reduction Factor As the RPM changes, the 1xRPM vibration (i.e., the vibration that occurs at the same frequency as shaft RPM) changes by the square of the RPMs. In general for prediction of what will happen to a pump installation if an existing pump is converted to a VFD operation, the operation RPMs will be lower than the RPM when the pump was only operating at one speed. Therefore, the displacement measured in step (2) above will be reduced at the lower RPM - not considering any interaction with the structural resonance that might happen to amplify the vibrations. Irrigation Training and Research Center -11- Vibration Summary for VFDs

16 Reduction Factor (RF) = (RPM new )² / (RPM original )² At an RPM of 1494, the RF = (1494/1780) 2 RF = Vibration Prediction To predict what the vibration will be when the pump operates at the same RPM as the structural resonance, perform the following computation that decreases the vibration due to a lower pump RPM, but increases it due the amplifying effect of the structural resonance Vibration Prediction (VP) = (Original displacement) x RF x AF VP = (.0001 inches) x 5.5 x.7 VP = inches of displacement VP = 0.4 mils of displacement Note** - In this particular case, the measured displacement at ITRC was 1.1 mils. But the resolution of the measurement of the displacement in step (2) was such that the difference between 0.4 mils (predicted) and 1.1 mils (measured) falls within the expected level of accuracy. What is also of significance is that: a. Field tests showed that the maximum vibration did occur at the RPM predicted by the structural resonance measurements, and b. The maximum vibration was almost 10 times the vibration that occurred at the "nominal" RPM of the pump before the VFD was employed. 5 Check the Vibration Level The table shown here can be applied to a large number of rotating machines with reasonable confidence. It is a distillation of data from a wide range of industrial machinery, and is considered up to date. Vibration Level Extreme Excessive Tolerable Acceptable Below 30 Hz (1800 RPM) 10 mils p-p 4.2 mils p-p 1.5 mils p-p 0.6 mils p-p p-p = peak to peak Irrigation Training and Research Center -12- Vibration Summary for VFDs

17 REDUCING VIBRATION PROBLEMS Steps to reduce potential or existing vibration problems fall into four categories: 1. Never operate in the critical RPM range. 2. Order pump options that can be expected to minimize certain types of vibration. 3. Repair causes of large magnitudes of excitation. 4. Change the resonance frequency so that critical values are outside the window of operating RPMs. Avoiding the Critical RPM Range This is commonly done with engine/gear drive/vertical pump installations. It is an unsatisfactory alternative when a pump must be capable of being adjusted in very small speed increments to maintain a precise flow rate or water level. Order Special Pump Options The most common options are: 1. Stiff discharge heads 2. Large diameter shafts 3. Close pump shaft bearing spacings (e.g., 5 instead of 10 ; 5 is typical for oil lubricated, and 5-10 is typical for water lubricated) Repair Causes of Excitation The most common options are: 1. Improve alignments 2. Replace unevenly worn rotating parts Change the Resonance Frequency There are 3 methods available: 1. Stiffen the structure thereby raising the resonant frequency of the structure. 2. Add mass to the structure thereby lowering the resonant frequency. 3. Add a dynamic absorber to the structure. This method attaches the equivalent of a tuning fork to the structure. This attachment is tuned to the same resonant frequency as the structure and sets up an out-of-phase signal that effectively cancels out (reduces) the signal being generated by the structure. The dynamic absorber must be properly sized to handle the forces being generated. This option is almost never seen in agricultural installations. In the case of vertical pumps the most common method is to stiffen the discharge head. Careful attention must be taken to prevent deforming of the discharge head if the stiffening is done in the field. Irrigation Training and Research Center -13- Vibration Summary for VFDs

18 RECOMMENDATIONS The recommendations for proposed VFD applications can be summarized as follows: 1. Check with the pump manufacturer when purchasing a new pump assembly to determine if they would recommend: a. A stiffer discharge head, which will help to move the structural resonance above the operating RPM. b. Reduced shaft vibration, by installing more frequent bearings (such as at 5 spacing instead of 10 ). 2. Perform the two vibration analysis tests (resonance and excitation) once the pump/motor/platform assembly has been completed in the field. 3. Predict if there will be vibration problems in the anticipated RPM range of the pump. 4. If possible, minimize the pump (excitation) vibrations by improving alignment or changing bearings. 5. If damaging vibration problems are still anticipated, make modifications to the structure. This is generally done by stiffening various components (discharge head, piping supports, etc.) to increase the resonance frequency so that it is above the anticipated pump RPMs. 6. Re-analyze the resonant frequency using the vibration test equipment, to determine if the structure modifications achieved the desired results. Irrigation Training and Research Center -14- Vibration Summary for VFDs

19 ACKNOWLEDGEMENTS Funding for this work was provided by the Public Interest Energy Research (PIER), which is administered by the California Energy Commission Contact: Ricardo Amón The following web page provides excellent technical information regarding vibrations: The following pump manufacturers provided valuable advice and information: Cascade Pump Contact: Clark Wilson; Peerless Pump Contacts: Jeff Michaels, or Michael Grant; Goulds Pump Sage Technologies HQ 1601 N. Sepulveda Blvd. #501 Manhattan Beach, CA Marilyn Cooper Sales Engineer Phone: Fax: Cell: Evro T. Wee Sit Technical Specialist Phone: Renee Walter Customer Support Engineer Ph: Fax: Irrigation Training and Research Center -15- Vibration Summary for VFDs