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1 Vibration Measurement Systems and Guidelines for Centrifugal Fans A Field Perspective Robert A Shannon PE Design Engineering Manager Howden Buffalo Inc
Vibration Measurement Systems and Guidelines for Centrifugal Fans A. Field Perspective,Robert A Shannon PE,Design Engineering Manager. Howden Buffalo Inc, This paper presents an overview and discussion on vibration measurement systems. transducer types advantages and disadvantages and usage guidelines for centrifugal fan. applications The emphasis is on vibration measurement systems from a field perspective. considering transducer types and configurations vibration transmissibility and typical. diagnostic considerations used to quantify vibration severity and fan mechanical. INTRODUCTION, Regarding the current industrial specifications and codes prescribed for vibration. measurement and qualifications for rotating equipment specifically centrifugal fans. much discussion has ensued as to the most effective and optimal vibration measurement. systems for confident and accurate measurement assessment and mechanical diagnosis. Comparatively speaking centrifugal fans typically operate at lower rotative speeds than. their rotating equipment counterparts such as industrial steam and gas turbines and. centrifugal compressors Centrifugal fans however represent a classification of rotating. equipment that embody unique mechanical designs operating applications and. engineering features which are susceptible to a wide variety of vibration problems in the. VIBRATION THRESHOLDS, When a fan vibration monitoring and analysis program uses overall vibration readings for. periodic measurements it is important to identify what normal levels of vibration are and. what levels indicate significant problems or impending failure thresholds It is a fact that. the vast majority of centrifugal fans and other classifications of rotating machinery have. generally similar overall vibration levels that indicate their mechanical condition. Unfortunately it is also true that there are so many exceptions to the general rule and so. many variables affecting how failure develops that is it not possible to provide a single set of. limits that apply to all machines, There are six possible sources of information concerning acceptable and unacceptable. vibration levels for particular fans and applications These are. Fan Manufacturer s specifications,Bearing clearance limitations. Industry standards,General vibration severity guidelines. Comparison of similar fan applications,Long term observation. Manufacturer s vibration specifications are an excellent source of information but are not. always available Industry standards generally apply to types of similar machines or. components and are less specific than manufacturer s data General vibration severity. guidelines are the broadest and most general reference but they provide a valid starting. point if nothing more specific is available A typical set of general vibration severity. guidelines is shown in Figure 1,VIBRATION VELOCITY. Inches Second Peak,0 005 In Sec EXTREMELY SMOOTH,005 01 In Sec VERY SMOOTH. 01 02 In Sec SMOOTH,02 04 In Sec VERY GOOD,04 08 In Sec GOOD. 08 16 In Sec FAIR,16 32 In Sec SLIGHTLY ROUGH,32 64 In Sec ROUGH. Above 64 In Sec VERY ROUGH,Figure 1 Vibration Severity Guidelines. Comparing vibration levels of several identical fans provides a very good indication of. normal vibration levels, Long term observation of the performance of each fan is the best basis for establishing. acceptable and unacceptable vibration levels No matter what method is used initially to. establish these thresholds experience and observation will ultimately determine the most. accurate and useful threshold levels, Once normal operation vibration levels are established the next question is How high. should the alarm levels be set As a general and practical rule of thumb a vibration level of. twice the nominal established level or that which exceeds 40 45 of the allowable. diametral bearing clearance sleeve bearings should be considered the warning threshold. for further investigation and problem diagnosis Depending on the manufacturer operation. conditions bearing clearances and mechanical configuration two to three times the normal. level might be considered the threshold for immediate shutdown to avoid severe damage. While the preceding discussion has covered overall vibration levels the same approach can. be applied to vibrations at specific frequencies If manufacturer s specifications or other. initial guidance are not available there are general vibration severity charts that include. frequency Examples are shown in Figures 2 and 3 Some include converted vibration. measurements between displacement velocity and acceleration for specific frequency. ranges and severity classifications Here again experience and observation are the ultimate. indicators of the best values to use,Figure 2 Vibration Severity Chart DLI. Figure 3 Vibration Severity Chart IRD, Many of these charts are highly generalized and should be used carefully Specifically for. fans AMCA Standard 204 05 Balance Quality and Vibration Levels for Fans. provides rotational speed vibration limit values for factory tests as below in Figure 4. seismic readings,Fan Rigidly Mounted Flexibly Mounted. Application Support System Support System,Category mm s in s mm s in s. BV 1 12 7 0 50 15 2 0 60,BV 2 5 1 0 20 7 6 0 30,BV 3 3 8 0 15 5 1 0 20. BV 4 2 5 0 10 3 8 0 15,BV 5 2 0 0 08 2 5 0 10, Application Categories for Balance and Vibration Most industrial power generation and complex process fans are. Category BV 3 and BV 4,Figure 4 AMCA 204 05 Vibration Levels for Fans. A seismic vibration level of 0 10 in s peak is generally regarded as an acceptable level of. running speed vibration amplitude in the fan industry This should be tempered however. with the specific application in which the fan is used. VIBRATION MEASUREMENT CONVENTIONS, The most basic consideration associated with vibration measurements is the direction of. measurement and vibration they represent There are three axes for measurement as. shown in Figure 5 They are horizontal vertical and axial The horizontal and vertical. measurement axes are sometimes referred to as radial It is important to note that each of. these axes or measurement directions is always designated relative to the base plane. foundation of the component being examined and not to the floor. Routine vibration measurements for any one fan are usually but not always taken in all. same directions This is because vibration readings from specific axes provide the best. indications of particular problems For example unbalance is usually indicated by high. readings in the horizontal or vertical directions Misalignment however is usually indicated. by high readings in the axial direction, Figure 5 Vibration Measured in Three Different Planes. It is a good practice to take vibration readings on bearing housings because it is through. these that the primary vibration forces are transmitted If for any reason it is not possible to. take particular readings in these locations care must be taken to select measurement points. that are on a solid part of the machine s structure Sheet metal housings panels covers. access doors and guards are not reliable measurement points Measurement locations that. require removal of protective guards and covers or present risk of contacting rotating or. moving parts should be avoided, The keys to accurate repeatable vibration measurement and data recording are not. complicated They are based on common sense care organization and consistency. Accurate vibration measurement requires selection of the best measurement locations on. the fan assembly It also requires choosing the best type of vibration transducer for the. particular frequency or frequency ranges of primary interest In recent years. piezoelectric accelerometers have become by far the most widely used transducer type. due to their overall frequency characteristics size reliability and general sturdiness. Unless measurement of unusually low amplitude low frequency vibrations is required an. accelerometer is typically the best choice for overall data accuracy Most modern vibration. measurement instruments provide a choice of displacement velocity or acceleration as. vibration measurement parameters independent of the type of transducer being used It is. desirable for accuracy to use the parameter that gives the best flattest response across the. frequency range of interest It turns out that in general velocity is the best choice as the. parameter for machine vibration measurement The most common exception to this rule is. use of acceleration at higher frequencies to detect problems with specific components such. as roller bearings, How the vibration transducer is applied to the measurement point is another critical aspect. of vibration measurement accuracy The more firmly the transducer is coupled to the. machine the better its response will be Figure 6 gives an example of the frequency. limitations associated with various transducer mounting methods for a particular. accelerometer From a practical standpoint the best attachment method a threaded stud is. not often used because setting up and using it can be expensive and time consuming When. using magnetic or hand held without probe mountings the mounting surface should be flat. and smooth with a light coating of silicone grease at the contact area to ensure accuracy. Accelerometer Upper Frequency Unit,Threaded Stud or Bolted On 600 000 CPM. Magnetic Holder 120 000 CPM,Hand held without Probe 90 000 CPM. Figure 6 Frequency Limitations of Accelerometers By Mounting Method. TRANSDUCER TYPES, Accelerometers The compression type piezoelectric accelerometer shown in Figure 7. was one of the first type of piezoelectric accelerometers to be developed. Figure 7 Compression Type Piezoelectric Accelerometer. There are also other designs for accelerometers Shear type accelerometers have an active. piezoelectric crystal oriented to deform in a shear plane and are often preferred for. higher sensitivity acceleration measurements where significant base distortion is. anticipated from large thermal transients or where they are mounted on flexible. structures The piezoelectric accelerometer has been widely considered the standard. vibration transducer for machine vibration measurement The configuration of the. piezoelectric crystal and seismic mass depend on the transducer frequency range output. The seismic mass is clamped to the base by an axial bolt bearing down on a circular. spring The piezoelectric crystal element is squeezed between the mass and the base. When a piezoelectric material experiences a force it generates an electric charge between. its surfaces There are many such materials with quartz being one of the most commonly. used Synthetic ceramic piezo materials have recently been developed for applications at. high temperatures If the temperature of a piezo material is increased to its curie. temperature the piezoelectric property is lost and the transducer will be defective and not. repairable, When the accelerometer is displaced in primary measurement direction axis the force. required to move the seismic mass is born by the active piezoelectric element Per. Newton s second law this force is proportional to the acceleration of the mass The force. on the crystal produces the output signal which is therefore proportional to the. acceleration of the transducer Because of the linear response of the piezoelectric crystal. accelerometers have a very large dynamic range The smallest acceleration levels they. can sense are determined only by the electrical noise of the electronics and the highest. levels are limited only by the destruction of the piezoelectric element itself. Accelerometers are thus favored for their superior performance. The piezoelectric accelerometer is constructed to be quite stable over long periods of. time and will maintain its calibration if it is not abused Accelerometers can be damaged. by excessive temperatures and shock loading To insure that the internal crystal is not. cracked or damaged accelerometers should be calibrated annually when used in service. with temporary or portable data acquisition systems A small crack will cause the. sensitivity to be reduced and also will greatly affect the resonance and thus the frequency. response Accelerometers that have damaged crystals cannot generally be repaired or. refurbished, The frequency range of the accelerometer is very wide extending from very low. frequencies in some units to several tens of kilohertz The high frequency response is. limited by the resonance of the seismic mass coupled to the springiness of the piezo. element This resonance produces a very high peak in the response at the natural. frequency of the transducer and this is usually somewhere near 30 kHz for commonly. used accelerometers A rule of thumb is that an accelerometer is usable up to about 1 3 of. its natural frequency Data above this frequency will be accentuated by the resonant. response but may be used if the effect is taken into consideration The typical frequency. response of a piezoelectric accelerometer is shown in Figure 8. Figure 8 Typical Accelerometer Frequency Response Curve. When mounting an accelerometer it is important that the vibration path from the source. to the accelerometer is as short as possible especially if rolling element bearing vibration. is being measured, Figure 9 Accelerometer Phase Lag Roll off vs Frequency. Figure 10 Accelerometer Temperature Sensitivity, Other parameters for specifying accelerometers are phase lag roll off and temperature. sensitivity shown in Figures 9 and 10 Certain grades of piezoelectric material and. associated amplifier circuitry will have measurement phase lag drift due to the unique. response time of the circuit The phase drift will typically match the instability associated. with the accelerometer natural frequency and low frequency nonlinearities Finally it is. very important to select the proper accelerometer for the temperature application Most. accelerometers have averaged sensitivities mV g for normal use in the range of 40 F. 180 F Today high temperature accelerometer models have been developed for use up to. Velocity Transducers Standard application velocity transducers are made with a moving. coil outside a stationary magnet as depicted in Figure 11. Figure 11 Velocity Seismic Transducer, The velocity transducer was one of the first vibration measurement transducers to be. built It consists of a coil of wire and a magnet so arranged that if the housing is moved. the magnet tends to remain stationary due to its inertia The relative motion between the. magnetic field and the coil induces a current that is proportional to the velocity dx dt of. motion The measuring unit thus produces a signal directly proportional to vibration. velocity It is advantageous in that the vibration measurement is self generating and. needs no conditioning electronics in order to operate and it has relatively low electrical. output impedance making it fairly insensitive to noise induction Another type of velocity. transducer consists of an accelerometer with a built in electronic integrator which is. superior to the classic mechanical style seismic velocity probe. In spite of these advantages the velocity transducer has many disadvantages that make it. nearly obsolete for new installations although there are many of them still in use today It. is relatively heavy and complex and thus expensive and it has poor frequency response. extending normally from about 10 Hz to 1000 Hz Figure 12 shows a standard coil mass. velocity transducer frequency response curve, Figure 12 Velocity Transducer Frequency Response Curve. The spring and the magnet make up a low frequency resonant system with a natural. frequency of about 10 Hz This resonance needs to be highly damped to avoid a large. peak in the response at this frequency Coil mass velocity transducers because of their. construction are orientation and temperature dependent It is important that the seismic. transducer be mounted within its prescribed directional range and at the proper. application temperature, Eddy Current Proximity Probes The Eddy Current Probe is a permanently mounted. displacement transducer and requires a signal conditioning amplifier to generate an. output voltage proportional to the distance between the transducer end probe tip and the. shaft See Figure 13,Figure 13 Eddy Current Proximity Probe. It operates on a magnetic principle and is thus sensitive to magnetic anomalies in the. shaft care should be taken that the shaft is not magnetized or has mechanical. imperfections scratches burrs debris at the probe observation area to assure the output. signal is not distorted It is important to realize that the transducer measures relative. displacement between the bearing and the journal and does not measure total vibration. level of the shaft or the housing The displacement transducer is very commonly installed. in large machines with journal bearings where it is used to detect bearing failure and. other mechanical malfunctions and to shut the machine down before catastrophic failure. occurs Usually an orthogonally mounted pair of probes is threaded through the bearing. casing to an exposed area of the journal in the horizontal and vertical planes The gap. voltage change measured between the horizontal and vertical planes will produce a shaft. orbit on an oscilloscope displaying the path of the journal as it migrates around in the. bearing Various tip diameters and thread sizes configurations are offered to allow. ranges as small as 200 micro inches to upwards of 1 1 inches of displacement See Figure. Figure 14 Proximity Probe Voltage vs Gap Relationship. VIBRATION TRANSMISSIBILITY, When one determines the actual vibration that a fan produces it is critically important to. understand the difference between the values of casing readings compared to direct shaft. proximity readings A single measurement on a bearing casing with an accelerometer for. example is NOT equal to the actual shaft motion when the casing acceleration. measurement is converted to displacement Rather an assessment of vibration energy. transmissibility between the shaft and bearing housing must be made to determine the. actual severity of vibration This principle is exhibited in Figure 15 By understanding. the differences between bearing absolute motion casing motion relative to earth zero. reference shaft relative motion shaft motion within the bearing and shaft absolute. motion shaft motion referenced to casing motion a transmissibility ratio can be. determined Typically for sleeve bearings assuming rigidly mounted bearing supports. the relative shaft vibration amplitude may be 30 50 higher than the bearing cap seismic. reading Once a transmissibility ratio is determined a single acceleration measurement. taken on a bearing housing can then be compared to actual shaft displacement using the. ratio It is therefore very useful to measure casing and proximity measurements. simultaneously to determine the compliance or rigidity of the shaft bearing system and. the affect on vibration transmission A flexible or overly compliant bearing support can. add or subtract vibration amplitude read with an accelerometer depending on the casing. vibration vector phase angle it produces, Figure 15 Vibration Transmissibility between Shaft and Bearing Casing. CASE HISTORY 1 SEISMIC vs PROXIMITY PROBE TRANSMISSIBILITY. This case history reveals the importance of assessing transmissibility ratio between a. support bearing and shaft Figure 16 displays data from a power plant distributed control. system DCS on a forced draft fan The time trend from a peak operation day last year. shows an increase in inboard horizontal vibration yellow as load green and fan speed. white are increased reaching approximately 6 0 mils pp The inboard bearing X Y. horizontal vertical vibration channels from proximity probes are 8AFDFIHN PV and. 8AFDFIVS PV respectively Note there is a minimal change in outboard bearing.