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1 Biblid: (2016) 20; 1; p 4-8 UDK: 662.6:63 Expert Paper Stručni rad PROCESS EQUIPMENT CONDITION MONITORING SYSTEMS BASED ON VIBRATION ACQUISITION AN OVERVIEW AND A CASE STUDY SISTEMI ZA PRAĆENJE STANJA PROCESNE OPREME BAZIRANI NA MERENJU VIBRACIJA PREGLED I PRIMER IZ PRAKSE Željko KANOVIĆ*, Milena PETKOVIĆ*, Zoran JELIČIĆ*, Davor ANDRAŠIĆ** * University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Trg Dositeja Obradovića 6, Serbia **Subotica DH PUC, Segedinski put 22, Subotica kanovic@uns.ac.rs ABSTRACT Condition monitoring has aroused a considerable interest in modern control theory and practice. Nowadays, it is widely used to monitor industrial machinery in order to facilitate predicting failures, enhance schedule maintenance and reduce the probability of downtime in process industry plants, thus saving time and money. In this paper, we will present the basics of condition monitoring based on vibration acquisition, along with an example of such a system implemented in a thermal power plant. Key words: condition monitoring, vibration acquisition, preventive maintenance, ISO REZIME Praćenje stanja procesne opreme podrazumeva praćenje vrednosti radnih parametara, kao što su vibracije, temperatura, radni pritisak i slično, u cilju identifikovanja značajne promene koja može predstavljati indikaciju pojave kvara. Upotreba ove tehnike prediktivnog održavanja sve je rasprostranjenija i praktično je postala standard u modernim upravljačkim sistemima, jer omogućava predviđanje pojave kvara, kreiranje pouzdanijeg, adaptivnog plana održavanja opreme, produžavanje ekploatacionog perioda opreme i smanjenje verovatnoće vanrednog zaustavljanja proizvodnog procesa, koje može imati veoma velike negativne ekonomske efekte. Jedna od najčešće korišćenih tehnika za praćenje stanja opreme je bazirana na merenju vibracija. Ova tehnika je po prirodi neinvazivna i zahteva mala ulaganja, a obezbeđuje veoma pouzdanu detekciju i dijagnostiku kvarova, pogotovo kod rotacionih mašina (elektromotori, generatori, pumpe, ventilatori i slično). Upotreba ove tehnike definisana je međunarodnim standardom ISO U okviru ovog rada predstaviće se osnove tehnike praćenja stanja opreme merenjem vibracija, definisati osnovne veličine koje se prate i dati okvire u kojima se njihove vrednosti moraju kretati po standardu ISO Prikazaće se i jedan primer implementacije ovakvog sistema u pogonu za proizvodnju toplotne energije, koji je integrisan sa postojećim nadzorno-upravljačkim sistemom, a omogućava praćenje i arhiviranje podataka o vibracijama u vremenskom, i po potrebi, frekvencijskom domenu. Na kraju će biti istaknute prednosti uvođenja ovakvog sistema i navedeni neki od problema koji su rešeni njegovim uvođenjem. Ključne reči: praćenje stanja procesne opreme, merenje vibracija, preventivno održavanje, ISO INTRODUCTION The issue of robustness and reliability is very important to guarantee the good operational condition of the entire industrial system. Therefore, the condition monitoring of system components has received considerable attention in recent years (Kanović et al., 2011; Kanović et al., 2012). Early fault diagnosis and condition monitoring can reduce the consequential damage, breakdown maintenance costs and the amount of spare parts needed on stock. Moreover, it can prolong the machine lifespan and increase its performance and availability (Mobley, 1999). The equipment condition is mostly examined on the basis of the signals acquired through the sensors and ing instrumentation methods. Signals can manifest fault signature by means of an appropriate signal processing technique. A number of condition monitoring techniques have been developed, which monitor certain parameters of the equipment and determine its health. In this paper, we will present the basics of condition monitoring based on vibration acquisition along with the characteristic parameters used for evaluation. Vibration monitoring is a very efficient technique, especially for rotary machines such as electric motors, generators, compressors, turbines, blowers or fans (Kanović et al., 2013). The materials and methods presented in the paper are in accordance with the international standard ISO 10816, which deals with the issue of vibration-based condition monitoring systems. An example of such a system implemented in a thermal power plant will also be presented, emphasizing the benefits acquired by its installation and exploitation. MATERIAL Vibration Measurement and Analysis Fundamentals Vibration is the physical movement or oscillation of a mechanical part about a reference position. It can be measured by displacement, velocity, and acceleration. Displacement is usually measured with inductive proximity sensors and it is used when evaluating the condition of slow-rotating machines such as hydraulic turbines. It is expressed in millimeters or micrometers. Velocity is expressed in millimeters per second (mm/s) and measured with a velocity transducer that has a relatively flat frequency response ranging from 10 to 2,000 Hz. The velocity transducer measures the vibration by using a fixed permanent 4 Journal on Processing and Energy in Agriculture 20 (2016) 1

2 magnet and a coil of wire mounted on a spring. Such sensors are usually robust and unsuitable for mobile in-situ measurements, thus they are most often utilized for permanent measurements. Acceleration is measured with an accelerometer and expressed in millimeters per second squared (mm/s 2 ), or, more often, in gravitational acceleration g (1g=9.81m/s 2 ). An accelerometer measures the vibration generated by using piezoelectric crystals. A piezoelectric crystal produces a voltage signal when the crystal is deflected. A change in speed will cause the crystal to deflect and display a change in vibration. Accelerometers can be of very small dimensions, so they are suitable for all kinds of measurements, mobile or permanent, and for all mounting modes, which is the reason why these sensors are widely used. However, it is possible to measure displacement with an accelerometer, acceleration with a velocity transducer, etc., with the differentiation or integration of signals. In practice, it is most common to use accelerometers for vibration measurement, then integrate the obtained signal and turn it into velocity, which is most suitable for condition monitoring since most recommendations on vibration levels are expressed in velocity units, i.e. mm/s. The total energy of all vibration components generated by a machine is reflected by broadband, or overall, amplitude measurements. The normal convention for expressing the frequency range of broadband energy is a filtered range between 10 and 10,000 Hz. Because most vibration-severity charts are based on this filtered broadband, caution should be exercised to ensure that the collected data are consistent with the charts. Since vibration is transmitted as an AC signal, there are four signal parameters that may be used to condition the signal. These parameters have a direct impact on the measurement value. If the wrong parameter is used, the measurement could be either too high or too low, thus causing possible maintenance action to be, or not to be, accomplished erroneously. These parameters are peak-to-peak, zero-to-peak, average and root-mean-square (Taylor, 2003), each of which is illustrated in Fig. 1. The peak-to-peak amplitude reflects the total amplitude generated by a machine, a group of components, or one of its components. The unit of measurement is useful when the analyst needs to know the total displacement or maximum energy produced by the machine s vibration profile. Technically, peakto-peak values should be used in conjunction with actual shaftdisplacement data, which are measured with a proximity or displacement transducer. Peak-to-peak terms should not be used for vibration data acquired by using either relative vibration data from bearing caps or when using a velocity or acceleration transducer. The only exception is when vibration levels must be compared to vibration severity charts based on peak-to-peak values. Fig. 1. Vibration signal parameters Zero-to-peak, or simply peak, values are equal to one-half of the peak-to-peak value. In general, relative vibration data acquired using the velocity transducers are expressed in terms of peak. Average value is, in fact, the arithmetic mean value, i.e. the height of the rectangle whose surface is equal to the surface below the signal curve. It is rarely used as signal parameter since it does not consider the shape of the curve. Root-mean-square (RMS) is the statistical average value of the amplitude generated by a machine, one of its components, or a group of components. RMS vibration velocity, for example, is defined as 1 T 2 vrms v ( t) dt T 0 = (1) This parameter is most often used for vibration-based condition monitoring, since all recommendations stipulated in the relevant standard ISO are expressed by using this parameter. Measurement Locations and Conditions Measurements should be performed on exposed parts of the machine that are normally accessible. Care should be taken to ensure that measurements reasonably represent the vibration of the bearing housing and do not include any local resonance or amplification. The locations and directions of vibration measurements should be such that they provide adequate sensitivity to the machine dynamic forces. Typically, this requires two orthogonal radial measurement locations on each bearing cap or pedestal, as shown in Fig. 2. Care should also be taken to ensure that the measuring system is not influenced by environmental factors such as temperature variations, magnetic fields, sound fields, power source variations, transducer cable length and transducer orientation. Measurements should be carried out when the rotor and the main bearings have reached their normal steady-state operating temperatures and with the machine running under specified conditions, for example at rated speed, voltage, flow, pressure and load. On machines with varying speeds or loads, measurements should be made at all conditions at which the machine would be expected to operate for prolonged periods. The maximum measured value under these conditions should be considered representative of vibration severity. If the measured vibration is greater than the acceptance criteria allowed and the excessive background vibration is suspected, measurements should be made with the machine shut down to determine the degree of external influence. If the vibration with the machine stationary exceeds 25 % of the value measured when the machine is running, corrective actions may be necessary to reduce the effect of background vibration. Machine Classification The vibration severity is classified according to the machine type, rated power or shaft height, and system flexibility. Significant differences in design, type or bearings and structures require a separation into two different machine groups (the shaft height, H, is in accordance with ISO 496). Machines of these two groups may have horizontal, vertical or inclined shafts and can be mounted on rigid or flexible s. Journal on Processing and Energy in Agriculture 20 (2016) 1 5

3 Group 1 involves large machines with rated power above 300 kw and electrical machines with the shaft height H higher than 315 mm. These machines normally have sleeve bearings. The range of operating (or nominal speeds) is relatively broad and ranges from 120 r/min to r/min. Group 2 involves medium-sized machines with a rated power above 15 kw up to and including 300 kw, and electrical machines with the shaft height H between 160 and 315 mm. These machines normally have rolling element bearings and operating speeds above 600 r/min. According to system flexibility, machines can be classified as rigid or flexible ed. As typical examples, large and medium-sized electric motors, mainly with low speeds, would normally have rigid s, whereas turbo-generators or compressors with power greater than 10 MW and vertical machine sets would usually have flexible s. In some cases, a assembly may be rigid in one measuring direction and flexible in the other. In such cases, the vibration should be evaluated in accordance with the classification which corresponds to the measurement direction. Evaluation Criteria ISO provides a general description of the two evaluation criteria used to assess vibration severity on various classes of machines. One criterion considers the magnitude of observed broad-band vibration; the second one considers the changes in magnitude, irrespective of whether they increase or decrease. Criterion I: Vibration magnitude This criterion is concerned with defining limits for vibration magnitude consistent with acceptable dynamic loads on the bearings and acceptable vibration transmission into the environment through the structure and foundation. The maximum vibration magnitude observed at each bearing or pedestal is assessed against the evaluation zones for the class. The evaluation zones have been established from the international experience. The following evaluation zones are defined to permit a qualitative assessment of the vibration of a given machine and provide guidelines on possible actions. a) - Zone A: The vibration of newly commissioned machines normally falls within this zone. - Zone B: Machines with vibration within this zone are normally considered acceptable for unrestricted long term operation. - Zone C: Machines with vibration within this zone are normally considered unsatisfactory for long-term continuous operation. Generally, the machine may be operated for a limited period in this condition until a suitable opportunity arises for remedial actions. - Zone D: Vibration values within this zone are normally considered to be of sufficient severity to cause damage to the machine. The values for the zone boundaries which are given in Table 1 are based on the maximum broad-band values of vibration velocity measured from two orthogonally oriented radial transducers. Therefore, when using these tables, the higher of each of the values measured from the two transducers in each measurement plane should be used. When the maximum measured values of vibration velocity are compared to the corresponding values in Table 1, the severity zone which is most restrictive shall apply. b) Fig. 2. Measuring points for a) horizontally and b) vertically mounted machines Criterion II: Change in vibration magnitude This criterion provides an assessment of a change in vibration magnitude from a previously established reference value. A significant change in broad-band vibration magnitude may occur, which requires some action even though zone C of Criterion I has not been reached. Such changes can be instantaneous or progressive over time and may indicate incipient damage or some other irregularity. Criterion II is 6 Journal on Processing and Energy in Agriculture 20 (2016) 1

4 specified on the basis of the change in broad-band vibration magnitude occurring under steady-state operating conditions. Table 1. Classification of vibration severity zones Vibration Velocity (mm/s RMS) rigid Zone D Zone C Zone B flexible Steady-state operating conditions should be interpreted to include small changes in the machine power or operational conditions. When Criterion II is applied, the vibration measurements being compared shall be taken at the same transducer location and orientation, and under approximately the same machine operating conditions. Obvious changes in the normal vibration magnitudes, regardless of their total amount, should be investigated so that a dangerous situation may be avoided. When an increase or decrease in vibration magnitude exceeds 25 % of the upper value of zone B, as defined in Table 1, such changes should be considered significant, particularly if they are sudden. Diagnostic investigations should then be initiated to ascertain the reason for the change and to determine what further actions are appropriate. Operational limits For long-term operation, it is common practice to establish operational vibration limits. These limits take the form of alarms and trips. Alarm provides a warning that a defined value of vibration has been reached or a significant change has occurred, at which remedial action may be necessary. In general, if an alarm situation occurs, operation can continue for a period whilst investigations are carried out to identify the reason for the change in vibration and define any remedial action. Trips specify the magnitude of vibration beyond which further operation of the machine may cause damage. If the trip limit is exceeded, Zone A rigid flexible 15 kw<p<300 kw 300 kw<p<50 MW motors 160 mm<h<315 mm motors 315 mm<h Group 2 Group 1 immediate action should be taken to reduce the vibration or the machine should be shut down. Different operational limits, reflecting differences in dynamic loading and stiffness, may be specified for different measurement positions and directions. The alarm limits may vary considerably, up or down, for different machines. The values chosen will normally be set relative to a baseline value determined from experience for the measurement position or direction for that particular machine. It is recommended that the alarm limit is set higher than the baseline by an amount equal to 25 % of the upper limit of zone B. If the baseline is low, the alarm may be below zone C. It is recommended that the alarm limit not normally exceed 1.25 times the upper limit of zone B. If the steady-state baseline changes (for example after a machine overhaul), the alarm setting should be revised accordingly. The trip limits will generally relate to the mechanical integrity of the machine and be dependent on any specific design features which have been introduced to enable the machine to withstand abnormal dynamic forces. The values used will, therefore, generally be the same for all machines of similar design and would not normally be related to the steady-state baseline value used for setting alarms. DISCUSSION In this section, we will present an example of vibration-based condition monitoring system. This system was implemented in a thermal power plant which provides thermal energy for heating purposes of the town of Subotica. The system was built according to the latest specifications of ISO 10816, listed also in this paper. It is incorporated as a part of the existing Supervisory, Control and Data Acquisition (SCADA) system of the thermal power plant facility. Vibrations were measured on all recommended locations of motors driving pumps and fans in the facility. High sensitivity vibration acceleration sensors (accelerometers) were used for the measurements. These sensors provide standard 4-20 ma current signal, which improves the ability of integration with the existing SCADA. All measurements are visible in the main operator screen, shown in Fig. 3, which also provides the detailed insight to the measurements on the specified machines, their history, trends, activated alarms and trips and so on, as shown for the circulation pump in Fig. 4. Fig. 3. Main operator screen Journal on Processing and Energy in Agriculture 20 (2016) 1 7

Since the introduction of this condition monitoring system, the significant improvement of the entire process system reliability has been achieved.

5 Since the introduction of this condition monitoring system, the significant improvement of the entire process system reliability has been achieved. A number of unpredicted situation and faults have been prevented and the overall maintenance process has been improved. The most common faults, regarding bearing malfunctions, can now be efficiently and precisely predicted and prevented, which decreases the number and duration of maintenance interventions and improves the heating quality in the town. CONCLUSION Fig. 4. Operator screen showing circulation pump vibration measurements In this paper, the basics of condition monitoring based on vibration acquisition are presented. The overview of some of the most important vibration parameters, measuring techniques, recommendations for measurement location, evaluation criteria and vibration severity zone boundaries is given, all in accordance with the international standard ISO An example of a condition monitoring system based on vibration measurement, implemented in a thermal power plant, is also displayed. The evaluation considered in this paper is limited to broadband vibration without reference to frequency components or phases. This will, in most cases, be adequate for the acceptance testing and operational monitoring purposes. However, for longterm condition monitoring purposes and for diagnostics, the use of vibration vector information is particularly useful for detecting and defining changes in the dynamic state of the machine. In some cases, these changes would go undetected when using only broad-band vibration measurements. Phase- and frequency-related vibration information is increasingly being used for monitoring and diagnostic purposes (Bano et al., 2015). Thus, it should be combined with all of the above-mentioned facts in order to obtain an efficient and reliable fault detection and diagnosis system, which would improve the overall process system performance. ACKNOWLEDGMENT: This paper is a result of the research within the project TR32018, ed by the Ministry of Education, Science and Technology, Republic of Serbia. REFERENCES Bano, A.S., Pineda-Sanchez, M., Puche-Panadero, R., Martinez- Roman, J., Kanović, Ž. (2015). Low-Cost Diagnosis of Rotor Asymmetriesin Induction Machines Working at a Very Low Slip Using the Reduced Envelope of the Stator Current. IEEE Transactions on Energy Conversion, 30, (4), Kanović, Ž., Rapaić, M.R., Jeličić, Z.D. (2011). Generalized Particle Swarm Optimization Algorithm - Theoretical and Empirical Analysis with Application in Fault Detection. Applied Mathematics and Computation, 217, Kanović, Ž., Jakovljević, B., Rapaić, M., Jeličić, Z. (2012). Expert System for Induction Motor Fault Detection Based on Vibration Analisys. Journal on Processing and Energy in Agriculture, 16, (1), Kanović, Ž., Matić, D., Jeličić, Z., Petković, Milena (2013). Induction Motor Fault Diagnosis Based on Vibration Analysis a Case Study. Journal on Processing and Energy in Agriculture, 17 (1), Mobley, R. K. (1999). Root cause failure analysis. Newness, Boston, MA, USA. Taylor, J. L. (2003). The Vibration Analysis Handbook, Vibration Consultants Inc, Tampa, FL, USA. Received: Accepted: Journal on Processing and Energy in Agriculture 20 (2016) 1

INTERNATIONAL STANDARD

INTERNATIONAL STANDARD ISO 7919 2 Second edition 2001 11 15 Mechanical vibration Evaluation of machine vibration by measurements on rotating shafts Part 2: Land based steam turbines and generators in excess