the service magazine of the PRÜFTECHNIK Group

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1 No. 11 Focus: Fans & blowers the service magazine of the PRÜFTECHNIK Group PRÜFTECHNIK News TTS the new service tool Temporary Telediagnosis Service, TTS for short, is a new service tool from PRÜFTECHNIK for manufacturers and operators of machines and systems. When systems are subject to repeated failure or frequent shutdowns, temporary measurement equipment can be installed to measure machine vibrations over a certain period to pinpoint the possible causes. Telediagnosis means that diagnoses can be performed by Condition Monitoring Service When the kiln fan trips Dr. Edwin Becker specialists remotely, avoiding the cost of travel expenses to the site. This edition of telediagnose.com is dedicated to fans and blowers, and to building awareness for TTS, the new tool for availability-oriented maintenance. We hope you will find the information in this edition interesting and will be happy to answer any questions you may have. In this edition: TTS the new service tool When the kiln fan trips Acceptance measurements on cooling tower fans Using field balancing to reduce vibrations Monitoring bearing lubrication using vibration measuring equipment Measuring straightness and flatness Calculating natural belt frequencies Measuring load resonance curves without shutting off the exhaust fan News Rotary kilns must be in motion 24 hours a day. When the kiln fan trips because of excessive vibration, this interrupts operation a situation that is especially aggravating when the cause of the fault cannot be found. The following example of a typical incident in a cement factory demonstrates the flexibility of TTS (Temporary Telediagnosis Service) and its immediate benefit. It s a Friday with the weekend just around the corner. Suddenly the vibration monitoring system registers a limit value violation at a kiln fan. The shift manager asks himself: Would it be OK to raise the limit values yet again, or is it time for specialists to have a look? Let s be on the safe side, he thinks, and calls the PRÜFTECHNIK Hotline in Ismaning, Germany. For the diagnosis specialists manning the phones, situations like these are an everyday occurrence. After a brief discussion of the problem, it is decided to install a temporary telediagnosis service (TTS) with the VIBNODE online monitoring system in the near future. In addition to a certain amount of curiosity in the VIBNODE system, the shift manager was convinced by the argument that even the best specialist can only identify vibration causes if they actually arise during measurement. A VIBNODE system with 6 accelerometers was installed only a few days later. It was set up so that the measurement results could be automatically ed to the Monitoring Center via the Internet. Figure 3 shows the arrangement of the sensors on the fan, while Figure 4 depicts the VIBNODE system installed onsite with an integrated telephone modem. After only two weeks it was apparent that the vibrations in the axial direction were far stronger than originally assumed. In certain situations excitations increased to as much as 18 mm/s. >> 1 Fig.1: Rotary kiln Fig.2: View of the inside of a rotary kiln

2 Experience has shown that axial vibrations have great damage potential. Figure 6 shows a diagram recorded just before a kiln stoppage. For approx. 30 minutes vibration severity increased to extremely high levels. The cause for these excitations was a natural axial vibration that only occurred within a very narrow rotational speed range. The measurement and diagnosis report therefore recommended that this critical and relatively narrow rotational speed range of the kiln fan be blocked. Fig.3: Arrangement of sensors on the fan Fig.4: TTS mounted on the kiln fan Fig.5: Remote monitoring concept The Service & Diagnostic Center can provide this type of TTS and diagnosis service anywhere in the world. After all, thanks to the Internet, measurement data can easily be sent anywhere on the globe in an instant. And when GPRS modems are used with a flat rate, measurement results can even be sent out without a telephone connection and at an affordable price. 2 Preview Our next issue will focus on wind turbines. Diagnosis experience: Balancing of rotor blades Application: Determining alignment targets to reduce vibrations Application: Correctly monitoring lowspeed drive train components Technology: Re-Sampling What is it? New: Particle counter for wind turbine gearboxes. Fig.6: Vibration velocity signals recorded over one and a half months. The marked area is zoomed.

3 Condition Monitoring Service Acceptance measurements on cooling tower fans Sven Fleischer Cooling towers are complex machines of various designs that are increasingly being built of lightweight construction. Cooling tower fans are required to undergo acceptance testing for vibration because, as in other machines, experience has shown that low vibration levels lead to a longer service life. Therefore, operators and system and component vendors often agree on vibration severities according to ISO (see Figure 1). Fig.2: Installation of a gearbox fan unit in a power plant Fig.1: Allowable vibration severities Torsional and axial natural vibrations Imbalance and eccentricity in a fan impeller Cooling towers are subject not only to machine vibrations, but also to a variety of other vibration forms, including natural vibration (see Figure 3). Thus, vibrations in cooling tower fans can be differentiated into aerodynamic vibrations of the fan impeller, locally acting vibrations from drive components, vibrations from the base frame and basic structure, natural vibrations, and vibrations and secondary vibrations from structureborne sound. Foundation vibrations usually lie between 0.1 and 10 Hz and can cause cracks in the construction. Vibration levels over 10 Hz cause machine vibrations and, when in a frequency range of 10 to 300 Hz, can have an impact on the functioning of the machine. Housing resonance and noise only become noticeable in the high frequency range causing high noise levels but having less damage potential. Acceptance testing, therefore, needs to differentiate between machine vibration and noise measurement. Electrical disturbance frequencies Defects in bearings Foundation vibrations Fig.3: Potential vibration exciters in a cooling tower fan Fig.4: Typical measurement locations on a cooling tower motor Bending vibrations in coupling Alignment errors Gear mesh vibrations Gear mesh frequencies 3 Fig. 5: Machine spectra measured on 7 gearbox units

4 Condition Monitoring Service Using field balancing to reduce vibrations Marcel Kenzler While vibration analysis does not always lead to immediate improvements, vibrations can be reduced significantly by balancing the machine under operation conditions. Field balancing has many advantages The machine component that is to be balanced does not need to be removed and transported to the balancing bench Changes in balance condition that arise during operation can be corrected Rotors of virtually any weight and size can be balanced Field balancing takes roller bearing influences into consideration as shown below. Fig.1: Industrial fan Allowablidual imbalance Calculation examples e all g mm/kg Fig.2: The balancing run was successful Balancing on the balancing bench Field balancing takes bearing influences into consideration. 4 Balancing reduces vibration Initially, the axial fan in Figure 2 made noise like a compressor and had vibration values of 20 mm/s. After field balancing with a 155 g balance mass, it ran like clockwork with only 3 mm/s. Balancing protects roller bearings In light of the damaging effect of secondary vibrations, it is worth taking into consideration the functional and damage behavior of roller bearings. Practitioners know that service life is shortened when vibration levels rise above 10 mm/s. The application leaflet on Field balancing under difficult conditions can be downloaded here: downloads/an17_balancing.pdf 1) Industrial fan Rotor mass m = 4000 kg RPM n = 1500 rpm Balancing radius r = 500 mm Grade G 6.3 (as per DIN ISO 1940) Question: Is 21 g of residual imbalance allowable? e all = 38 g mm kg as per diagram u = Rest r m 21 g 500 mm = 4000 kg g mm = 2.6 kg => 21 g of residual imbalance is allowable! 2) Precision fan Rotor mass m = 220 kg RPM n = 6000 rpm Balancing radius r = 200 mm Grade G 2.5 (as per DIN ISO 1940) Question: Is 5 g of residual imbalance allowable? e all = 4 g mm kg RPM as per diagram u = Rest r m 5 g 200 mm = 220 kg g mm = 4.5 kg => 5 g of residual imbalance is not allowable!

5 5 Condition Monitoring Application Monitoring bearing lubrication using vibration measurement equipment Florian Buder When do grease-lubricated roller bearings need to be relubricated? While large roller bearings are increasingly being equipped with automatic continuous lubrication systems, this is not cost-effective in fans such as the one shown in Figure 1. These units are lubricated by hand at specific intervals. But how long should these intervals be? Online Condition Monitoring Systems (online CMS) can provide an answer. When a roller bearing runs dry, a rough running sound arises that can easily be detected in the high frequency Fig.1: Fan system Fig.2: Broadband disturbance frequency Fig.3: After greasing, the amplitude of the acceleration decreases acceleration spectra, for example. At that point, it is high time to relubricate the bearing to lower vibration strain. With advanced frequency-selective CMS, the broadband disturbance frequencies shown in Figure 2 can be selected as characteristic lubrication values and the associated amplitudes can be monitored as trend values. Figure 3 illustrates that lubrication intervals are very easy to determine: After lubrication, the amplitude of the acceleration decreased markedly. Of course, to use this analytical method, you must first identify the frequency band on which the lubrication condition has an impact. Glossary of terms Did you know? Balancing A procedure in which the mass distribution on a rotor is checked and, if necessary, corrected by adding a balancing mass to ensure that rotational frequency vibrations and/or bearing forces remain within specified limits at the operating speed of the machine. Rigid rotor A rotor whose center axis stays straight all the way up to its operating speed (it does not bend). Elastic rotor A rotor whose center axis bends in the running speed range and whose imbalance therefore must be corrected at high speeds and in multiple planes. One-plane balancing (Static balancing) A procedure in which the mass distribution of a rigid rotor is corrected in only one plane. Two-plane balancing (Dynamic balancing) A procedure similar to static balancing, except that the correction takes place in two planes. Multi-plane balancing A procedure that is used with bendingelastic rotors and that requires an imbalance correction in more than two balancing planes. Static imbalance A static imbalance occurs when a rotating shaft does not pass through the center of gravity of the rotating body. A typical property of static imbalance is that the plane in which the imbalance lies coincides with the radial plane of the center of gravity. This generates circular mechanical vibrations at a right angle to the rotating shaft. Dynamic imbalance Dynamic imbalance arises when the rotating shaft does not coincide with one of the stable main axes of inertia of the component. The rotating axis is tilted in its center of gravity. The center of gravity of the rotating body remains stationary while the axis wobbles because of the circular motion in the opposite direction. Quality grade (Allowablidual imbalance as per ISO 1940) Imbalance/total weight ratio at defined rotational speeds see also the examples on page 4. Characteristic imbalance values Bearing vibrations, shaft vibration and performance values Phase angle A measure of the rotational movement from activation of the trigger pulse to the maximum vibration amplitude. Bending-critical speed The rotational speed at which a rotor bends most strongly into a certain natural shape. Each rotor has several bending-critical speeds.

6 Alignment Application Measuring straightness and flatness Michael Stachelhaus When rotational vibration levels are raised, the search for a geometric cause of the vibration should be an integral part of proactive maintenance. In addition to dimensional tolerances, geometric tolerances also grow in importance in high speed fan and blower systems. Examples of geometric deviations are irregularities in the straightness and flatness of foundations and base frames. Inaccuracies in the foundation can be measured with laser-based testing equipment to a resolution of 0.02 mm. The LEVALIGN measuring system can be used to measure the straightness and flatness of surfaces at distances of up to 40 m. Thults are then displayed in a diagram. Figure 1 shows the straightness of the base frame of a large blower being measured. The operator of the blower contracted PRÜFTECHNIK to measure the straightness of the motor foundation in the course of overhauling the motor. Thults showed that the base plate had an offset of 1.2 mm at one foot. This degree of offset influences motor behavior and, in the case of this particular fan drive, was detrimental to the vibration behavior of the fan motor. After the LEVALIGN measurements were made, the error was corrected before the overhauled motor was remounted. This was accomplished by adding shims of the calculated correction height beneath the other three feet. A control measurement confirmed that the offset was now at a tolerable level of approx mm. The motor was remounted, realigned and put back into operation. A final vibration measurement showed that the motor was now running within the allowable vibration tolerances. Fig.1: Preparations before measurement and view of measurement components 6 Fig.2: The LEVALIGN report shows a deviation of the left front motor foot

7 Condition Monitoring Basics Calculating natural belt frequencies Dr. Edwin Becker In small to medium power ranges, belt drives can be used to adapt drive speeds to machine speeds. Because belts only transfer a limited level of power, machines usually use several belts of the same length or timing belts. High vibration levels occur when individual belt strands are unequally loaded because the lengths are out of tolerance. The unloaded strand of the belt should be slightly slack when under the largest load. If the slack strand remains too taught when under a large load, is will slip, causing high noise levels. If the belts flutter, the cause of the vibration can be found by means of frequency analysis, for example. Belt vibrations that are due solely to belt flaws occur at the belt frequency d r L w D r and have a large number of multiples, where the amplitudes are larger in the direction of the force transfer. Usually the calculated belt frequencies range up to 100 Hz The belt frequency can be calculated as follows: D f B = r n D d 60 L = r n π π d w 60 L w D r in mm = Effective pulley diameter n D in rpm = Pulley speed L w in mm = Effective belt length In addition to the belt frequency, the rotational frequency of the drive pulley can appear up to the 5th harmonic. At times the vibration level may rise and fall periodically. Yet another special characteristic of belt drives is that you should avoid 1:1 translations. These will often result in unusual vibrations. 7 Condition Monitoring Technology Measuring load resonance curves without shutting off the exhaust fan Christian Schlumpf Frequency converters with direct torque control (DTC) have a typical torque control time of approx. 1 to 2 ms. Vector-controlled or field-oriented converters with pulse width modulation (PWM) achieve 10 to 20 ms, while converters with U/f control without signal regeneration have a control time that lies significantly above 100 ms. In fans, a high torque control time is desirable to be able to accelerate the moment of inertia of the fan within a short period of time. A high torque control time, however, has the disadvantage that it can excite mechanical natural vibrations in the fan system. At the control times specified above, DTC and PWM converters can excite natural vibrations up to 1000 Hz and 100 Hz, respectively, and simple U/f converters can excite mechanical natural vibrations up to 10 Hz. To limit thonance risk, fans are often equipped with protection systems such as VIBREX that switch off the fan abruptly when the allowable Fig.1: Load-resonance curve of the 1st order measured at the motor vibration velocity is exceeded. The risk of resonance due to the converter can be assessed by means of TTS or with temporary vibration and speed measurements. Vibration and speed are measured simultaneously and displayed in a vibration-speed diagram. Figure 1 shows thonance curve of an exhaust fan that is at risk of vibration. The curve was measured during Resonance range normal operation with fluctuations in the operating speed. After only three hours of measurement under load conditions, it was concluded that the rotational speed range of rpm leads to increased levels of vibration load on the motor and that this range should be dampened in the frequency converter.

8 News CPK for operators of blower systems Condition-oriented maintenance has become standard practice in blower systems. While operation-critical blowers are increasingly being equipped with online measurement equipment, maintenance staff and service providers usually perform mobile vibration analyses. The use of coded measurement points is very convenient. The figure shows a directory tree of measurements on blowers. PRÜFTECHNIK performs the diagnosis service on the basis of the measurement results provided by the operator and makes the reports available. (CPK = Condition Monitoring Partner Concept) 8 Foundation measurements Often blowers need to be erected flexibly to keep vibration loads on buildings and foundations low. With VIBXPERT, an accelerometer with a low lower limit frequency and a suitable vibration foot, the vibrations emitted in the foundation can be recorded with high resolution. Dates Information on all trade fairs, seminars and other important events of the PRÜFTECHNIK Group can be found on our website at Monitoring Center now certified Germanische Lloyd Industrial Services GmbH has certified the Monitoring Center of the PRÜFTECHNIK AG and confirmed that its process flows are in conformity with GL Directive IV Part 4 of GL Wind. Thus, PRÜFTECHNIK customers now have the opportunity not only to purchase hardware and software from PRÜFTECHNIK but also to take advantage of the diagnosis services offered by this newly certified Monitoring Center. Certification involved adapting the procedures followed in the Monitoring Center to the ISO 9001 system. Contact PRÜFTECHNIK Condition Monitoring GmbH Ismaning, Germany Fax: Fax: info@pruftechnik.com PRÜFTECHNIK Alignment Systems GmbH Ismaning, Germany Tel: Fax: info@pruftechnik.com