CONDITION MONITORING THROUGH NON DESTRUCTIVE TECHNIQUE full report
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ABSTRACT
Condition monitoring of components and plants are of great importance for safe and reliable operation and for increasing productivity of plants. The challenges towards the condition monitoring can be successfully met by employing non-destructive evaluation (NDE) techniques. Vibration monitoring techniques are applied for periodic / continuous assessment of machinery parts and plants. Acoustic emission technique is used for leak detection and for structural integrity monitoring applications. Infrared thermographs are employed for condition monitoring applications in steel, electrical and petrochemical industries. Lubricant analysis by ferrography, and filed signature mapping are also used for condition monitoring applications. Here, applications of these NDE techniques could help to properly diagnose faults in plants components, enables taking timely decision about repair / replacement of components / plants, thus ensuring increased safety, reliability and productivity.
INTRODUCTION
The successful operation of structures / components during their entire life requires the implementation of a dedicated programme for condition assessment through in-service inspection (ISI) of all critical components of the plants / structures. The condition assessment through ISI and life prediction approaches enable uninterrupted operation, avoidance of unplanned shutdowns and taking decision on repair, up-gradation, modernization and replacement of necessary components for extension of the overall life of plants beyond their design lives. This is achieved through meticulous planning and incorporation of non-destructive evaluation (NDE) techniques which aims at detection and characterization of defects, stresses, corrosion micro structural degradations and dimensional changes that occur in components during service life, due to exposure to high temperature, pressure, static and dynamic loads, hostile environment etc.
1. SIGNATURE ANALYSIS
Condition monitoring implies determination of the condition of a machine or device and itâ„¢s change with time in order to determine itâ„¢s condition at any given time. The condition of the machines may be determined by physical parameters like vibration, noise, temperature, oil contamination, wear debris etc... A change in any of these parameters, called signatures, would dictate a change in the condition poor health of the machine. If properly analyzed this thus becomes a valuable tool to determine when the machine needs maintenance and in the prevention of machinery failure which can be catastrophic and result in unscheduled break downs.
2. CONDITION BASED MAINTENANCE
Apart from condition monitoring of plants during their operations, the task of conditions based maintenance is also important for increasing plant economy. The different methods of maintenance being practices in various industries are break down maintenance, regular preventive maintenance and condition based maintenance or
predictive maintenance. The break down maintenance is very expensive in terms of maintenance cost, safety hazards, unplanned production loss and non-availability of spares. The improved method of maintenance that can reduce down time is known as preventive maintenance. A regular preventive maintenance can definitely bring down break downs but failures can still occur. The implementation of condition based maintenance or predictive maintenance can significantly improve the results obtained from preventive maintenance. This is aimed at achieving reliability based maintenance which is balanced approach including the good aspects of preventive maintenance, predictive maintenance and pro -active maintenance.

3. NON-DESTRUCTIVE EVALUATIONS (NDE)
NDE techniques, such as, vibration analysis, acoustic emission, and infra red thermography, which have capabilities for online monitoring applications, can be used for the conditioning assessment applications. When a large number of machines are involved, the one characteristic that is common to practically all machines is vibration. When ever, machinery vibration increases beyond safe limits, the common reason is some mechanical trouble-unbalance, misalignment, worn out gears or gearings, loose ness etc . . . . Condition assessment of plants through vibration analysis is a very important method in spite of relatively higher initial cost of instruments. Acoustic Emission Technique (AET) is an advanced technique for real time monitoring application. An AE transducer or sensor acoustically coupled to a sample detects elastic (Acoustic) energy emitted by the sample and gives information about the dynamic changes taking place in the sample. AET is widely used for assessing structural integrity of critical components, such as pressure vessels, pipe line, storage vessels and gas cylinders. Infra red thermography (IR) is the mapping of IR radiations arising from the natural or stimulated thermal radiations of an object and can be used for online monitoring applications. Lubrication monitoring is carried out at periodic intervals to identify the condition for the lubricant and to access the likely damages to the machinery parts, through debris analysis by ferrography and quality assessment of lubricants. More recently for condition monitoring and life extension problems, a new dimension has been added to the existing NDE approaches with the availability and adoption of procedure like field signature mapping.
1. VIBRATION MONITORING TECHNIQUES

Vibration is referred to as oscillation of an object about some equilibrium point. VB monitoring technique has gained wide interest and acceptance for condition monitoring applications. This is based on exciting vibrations in component by local external impact or recording the vibration generated in a components under operating conditions the most common source of vibration are gear gear-mesh, vane passing , rotor imbalance, misalignment, eccentricity, damaged bearings or gears, loose components, rubbing components ,bend shafts, cavitations.

Vibration of a machinery is accessed with a help of transducers by measuring the amplitude of vibration in terms of their parameters i.e. displacement velocity and acceleration.
1.1 VIBRATION MONITORING INSTUMENTATION
Following figure shows a measuring and analysis system that may be used for monitoring the vibration signals from machines.
Following table gives general guide lines for identifying the causes of vibrations. The relation between the fault and frequency, amplitude and direction of vibration, are given. This is a useful guide for pin-pointing the cause in case vibration levels at certain frequency are seen to increase.
Sl. No FAULT FREQUENCY DIRECTION OF VIBRATION
1. ROTATING UNBALANCE SAME AS RUNNING SPEED RADIAL
2. MISALIGNMENT OF
BEARINGS 2*SPEED RADIAL AND AXIAL
3. ROLLER BEARING DEFECT AT BALL OR ROLLER SPEEED ULTRA SONIC FREQUENCIES (20-60KHZ) RADIAL AND AXIAL
4. OIL FILM WHIRL IN HIGHSPEED TURBO MACHINES 0.5*SPEED RADIAL
5. DAMAGED OR WORN GEARS NO: OF TEETH* RPM RADIAL
TABLE 1.1 (a): VIBRATION CAUSES IDENTIFICATION
1.2 APPLICATION OF VIBRATION MONITORING TECHNIQUES
Figure 1.2(b) shows one typical example of the failure rate of components of different machineries in plants which are maximum for rolling bearings as compared to other components. One of the reasons why machinery problems are caused by failure of rolling bearings is that the number of rolling bearings assembled into machinery is a few orders of magnitude larger than any other machine elements. Among various NDE techniques, vibration technique is the most commonly used method for the detection of failure of rolling bearings as shown in figure 1.2 (a)

1.3 CONDITION MONITORING THROUGH VIBRATION ANALYSIS IN STEEL INDUSTRY
In steel industry, maintenance cost accounts for nearly 10%-15% of the production cost. Maintenance affects the target, quality and profitability of the plant. Implementation of modern concepts of condition based maintenance (CBM) can appreciably reduce the maintenance costs and enhance reliability of machine performance and quality of the output.
The effectiveness of CBM through vibration analysis can be understood by the example of the Rourkela Steel Plant (RSP). With implementation of CBM activities at RSP, there has been substantial growth in all aspects encompassing the maintenance system Figure 1.3 (a) was responded that the programme for condition monitoring in RSP has grown from 40 to 140 critical equipments in a span of last three years. There has been significant increase in the number of major breakdown prevention cases from 10 to 102, which is more than 10 times, during the previous five years, resulting in substantial financial savings.
2. ACOUSTIC EMISSION TECHNIQUE
Acoustic emission is the class of phenomenon whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material. The energy released travels as spherical wave front and can be picked up from the surface of a material using highly sensitive transducers, usually electro-mechanical in nature, placed on the surface of the material.
Figure 2 (a): A E Technique
The wave thus picked up is converted into electrical signal which on suitable processing and analysis can reveal valuable information about the source causing the energy release. In metals the different sources are generation and-propagation of cracks, movement of dislocations, formation and growth of twins, decohesion and fracture of brittle inclusions, phase transformations etc. In composites, the sources of AE are matrix cracking, debonding and fracture of fibers.
2.1 ACOUSTIC EMISSION SET UP
Fig: 2.1(a). TESTING SET UP
The diagnostics can be performed without the product pumping-over interruption.
Among the existent non-destructive control methods, the acoustic-emission method is the only one that provides to exclude completely the sudden damage of constructions, pipelines, and vessels. Originally conceived as an NDT tool for pressure vessels, Acoustic Emission testing (AE) has become much wider in scope. We now apply it to all types of process monitoring as well as for its original purposes of flaw detection and structural integrity inspection. AE sensors respond with amazing sensitivity to motion in the low ultrasonic frequency range (10kHz - 2000kHz). Motions as small as 10-12 inches and less can be detected. These sensors can hear the breaking of a single grain in a metal, a single fiber in a fiber-reinforced composite, and a tiny gas bubble from a pinhole leak as it arrives at the liquid surface. By detecting sources as small as these, or as large as brittle crack advance, AE technology warns of danger, informs about structural health and watches over costly and critical processes.
2.2. A E DURING HYDRO TESTING OF A HORTON SPHERE
AE monitoring during hydro testing of a 17 m Horton sphere was carried out. Figure 2.2 (a) shows typical locations of AE sensors (150 kHz resonant frequency each) mounted on the Horton sphere, along with a typical AE source location map. The hydro test of the vessel was carried out to a maximum pressure of 22 kg/ cm2, with periodic holds at different pressures. A reloading cycle from 20 kg/cm2 to 22 kg/cm2 was immediately carried out. During the hydro test, AE signals were generated only during the pressure rise. With increase in pressure, AE signals were generated in newer areas and the areas where AE occurred in the previous pressure steps do not generate AE in the subsequent pressure steps. These signals were attributed to local micro-plastic deformation of the material. A few AE signals were also generated from cracks in concrete columns that were supporting the vessel. AE monitoring during hydro test was useful to confirm the integrity of the vessel.
3. INFRARED THERMOGRAPHY
Infrared thermography is based on the principle of detection, and measurement of infrared radiations QR) arising from the natural or stimulated thermal radiation of an object. All objects around us emit electromagnetic radiations. At ambient temperatures and above, these are predominantly infrared radiations.
Figure 3 (a): setup for infrared testing of lap joints
3.1 INFRARED TESTING OF LAP JOINTS
This image depicts the setup for infrared testing of lap joints. The images below are of a such a lap joint with a three poor spot welds in the middle.
Here is the raw thermo graphic data.
Here is the same data after removing the noise and vertical gradient.
Here is the same data processed for the local gradient in surface temperature.
The above series of images from thermographic data show that sophisticated post processing of the raw data offers advantages in identifying good spot welds from poor ones. Processing the matrix data with FFT algorithms and numerical differentiation brings out important details that are hidden in the raw infrared data. The strong change in the surface temperature gradients at the two spot welds on the outside corresponded to high strength welds. The location of the three spot welds in the middle can be determined and the weak temperature gradients correspond to low strength welds.
4. FERROGRAPHY
Ferrography is a state-of-the-art predictive maintenance technique based on wear debris analysis. It provides a comprehensive non-intrusive evaluation of the health of lubricated components while the equipment is in running condition. In todayâ„¢s modern power generation, manufacturing, refining, transportation, mining and military operations, the cost of equipment maintenance, service, and lubricants are ever increasing. Parts, labor, equipment downtime, lubricant prices and disposal costs are a primary concern in a well run maintenance management program. Machine condition monitoring based on oil analysis has become a prerequisite in comprehensive maintenance programs the ferrography laboratory plays a key role in such programs. It separates and concentrates wear and contaminant particles for microscopic examination. Particle size, surface characteristics and composition are then used to determine wear modes inside a machine so that maintenance recommendations can be made.
4.1. WEAR DEBRIS ANALYSIS
The mechanical systems used in plants have interacting surfaces in relative motion which are lubricated by oil or grease. During operation, there is a steady generation of wear particles at interacting surfaces caused by load and relative motion. These wear debris are carried away by lubricant, which give very useful information regarding the health of the equipment. Over a period of time, various abnormalities, such as, excessive load, fatigue, corrosion, abrasion, misalignment, lubrication starvation and capitation, may arise influencing the wear mechanism and formation of wear debris. The four major findings from ferrography are the mode, rate, severity and location of wear. A particular wear mechanism typically generates a particular type of wear debris. The identified wear modes include abrasion, impact, fatigue, erosion, corrosion, scuffing and severe sliding. The concentration of wear debris indicates the rate of wear and the size of debris indicates the severity of wear. The color of the particles identifies the type of material which pinpoints to the affected component. Thus, an accurate analysis of all these features of 'the wear debris provides a powerful
means of knowing the actual wear mechanism based on which, suitable corrective measures can be taken well in advance.
METAL WEAR POSSIBLE ORIGIN.
ALUMINIUM BEARINGS, BLOCKS, BLOWERS, BUSHINGS, CLUTCHES, PISTONS.
CHROMIUM BEARINGS, PUMPS, RINGS, RODS.
COPPER BEARINGS, BUSHINGS, CLUTHES, WASHERS.
IRON BLOCKS, CRANK SHAFTS, CYLINDERS,DISCS.
SILVER SOLDERS.
TIN PISTONS.
Table 4.1.(a) wear metal origin table
4.2. ANALYSIS OF OIL SAMPLES
Spectrometric analysis is the most commonly used method for trending concentrations of wear metals. Spectrometric analysis determines the elemental concentration of various wear metals, contaminants, and additives present in an used oil sample. But spectroscopy is less sensitive to the larger particles. A spectrometer is an instrument with which one can measure the quantities and types of metallic elements in a sample of oil. The operating principle is as follows. A diluted oil sample is pulverized by an inert gas to form an aerosol, which is magnetically induced to form a plasma at a temperature of about 9000°C. As a result of this high temperature the metal ions take on energy, and release new energy in the form of photons. In this way, a spectrum with different wavelengths is created for each metallic element. The intensities of the emissions are measurable for each such element by virtue of its very specific wavelength, calculated in number of ppm (parts per million). A special spectrometer can detect the very small metal particles in suspension in the oil, i.e. with a size between 0 and 3 microns. Those small particles are a good indication of general wear. The human eye can detect particles of a size starting from 50 microns, which allows them to be
visualized using more conventional means. Complementary analysis of such larger particles can be done by spectrometry, by ferrography or by optical or electronic microscopy.
4.2.1. HOW IS THE TEST PERFORMED
After the laboratory receives the sample, a series of tests are performed. The table below is a listing of each test, and the conditions detected by the test. All tests are not necessarily performed as each laboratory has an established series of tests.
Tests Conditions Detected
# Abnormal
Wear Fuel
Dilution Dirt Water Coolant Incorrect
Oil Oxidation Additive
Depletion
Glycol Test *
Viscosity * *
Appearance/Odor * * * * *
Spectrometric Analysis * * * *
Alkaline Reserve * *
Blotter Spot Test * * *
Water Content *
Distillation *
Flash Point *
Table 4.2.1 (a): Tests on oil samples
4.2.2. SPECTROMETRIC ANALYSIS
In the spectrometer, oil is electrically excited to the point where light is emitted. Each element present in the burning oil emits a light of its own particular color and frequency. The spectrometer translates the intensity of this rainbow of colors into a computerized readout. A typical report from this test would list major wear metals for industrial gear oil and hydraulic oil. The computer compares the amount of wear metals present with a fresh oil sample, and also records of samples from similar equipment. Also, the computer compares the output from previous samples taken from the same piece of equipment to establish wear trends.
5. FIELD SIGNATURE MAPPING
An efficient non-intrusive and rugged method called field signature mapping (FSM) or field signature method has been developed for condition monitoring of the localized corrosion erosion or abrasion and cracking in steel and metal structures, pipelines, pipe bends and vessels. FSM is based on feeding an electric direct current through the selected regions of the structures to be monitored, and sensing the pattern of the electrical field by measuring small potential differences generated on the surface of the monitored structure. The potential difference generated on the surface of the structure is monitored periodically / continuously. Selected area is fitted with a number of sensing electrodes or pins distributed in a matrix with variable spacing. Typical distance between electrodes is 2-3 times wall thickness. By proper interpretation of the changes in the potential differences, information pertaining to wall thinning of the structure or component under investigation can be obtained reliably. In the case of installed components, the measured potentials are compared with those initially measured. These values represent the initial geometry of the component, i.e., it fingerprint, as implied by the name of the method. It has been established that sensitivity for detection of internal corrosion by this method is an order of magnitude higher than that by ultrasonic
5.1 INDUSTRIAL ARRANGEMENT
Fig. 5. 1(a). A typical refinery monitors high temperature areas such as heater bends in the distillation unit
With FSM, you may:
- Reduce life cycle inspection costs
- Dramatically reduce inspection time
- Reduce or eliminate scaffolding costs
- Eliminate costs
- Eliminate unnecessary pipe replacement
- Widely expand possibilities for monitoring

Fig 5.1 (d). FIELD PATTERN
The field proven FSM technique detects metal loss due to corrosion by detecting small changes in the way an induced current flows through a metallic structure. The system presents graphical plots indicating the severity and location of corrosion, and calculates actual corrosion and metal loss. Both sensitivity and accuracy are typically better than 0.5% of remaining wall thickness, but may vary with the application.

6. CONCLUSION
The implementation of condition monitoring methodologies to components and plants is very essential for ensuring safety and reliability and for increasing productivity of plants. Non¬destructive evaluation techniques which aim at detection and characterization of defects, fatigue, stresses, corrosion, dimensional changes and micro structural degradations in materials, bear unique potential for applications related to the condition monitoring of components and plants.

It is probably safe to say that most organizations with a significant capital investment in plant equipment are, these days, employing some form of Condition Monitoring technology in order to predict at least some failures. This is the time from which an incipient failure can first be detected, until functional failure occurs. The primary determinant of frequency of a Condition Monitoring task is the lead time to failure, or PF Interval. For example, the time interval from when overall bearing vibration levels reach an "alarm" limit, until the bearing seizes completely. In order to be completely sure that the failure is detected prior to the functional failure occurring, the bearing must be monitored at a frequency less than the PF Interval. So far so good-in theory. Unfortunately, the practice is that the PF Intervals for sophisticated Condition Monitoring techniques are highly variable. For example, for Vibration Analysis on a bearing, the PF Interval will vary depending on the type of failure detected, the type of bearing installed, the severity of its operating cycle, the type of lubrication applied,
ambient temperature conditions and many other factors. To date, no Condition Monitoring organization can give anything but the most approximate estimate of the PF Interval. Any error tends to be on the conservative (i.e. too frequent) side.
6.1 SUMMARY OF APPLICABILITY AND CAPABILITY OF VARIOUS NDT TECHNIQUES
NDT TECHNIQUE DETECTION CAPABILITY NON CONTACT INSPECTION AUTOMATED INSPECTION DEFECT SIZING
VIBRATION VOLUMETRIC POSSIBLE POSSIBLE POSSIBLE
ACOUSTIC EMISSION VOLUMETRIC POSSIBLE POSSIBLE NOT POSSIBLE
I R THERMO- GRAPHY SURFACE , NEAR SURFACE POSSIBLE POSSIBLE POSSIBLE
FERROGRAPHY VOLUMETRIC POSSIBLE POSSIBLE POSSIBLE
F S M SURFACE, NEAR SURFACE POSSIBLE POSSIBLE POSSIBLE
7. REFERENCES
1. Dr. C K Mukhopadhayay, Dr. T Jayakumar, Dr. Baldev Raj, ËœNon-Destructive Evaluation Techniques for Condition Monitoring of Components and Plantsâ„¢ . Institute of Engineers (India) Journal, vol.15 , 2005, PP 144-155.
2. B C Nakra & K K Choudhry, ËœInstrumentation Measurement and Analysisâ„¢, Tata Mc Graw Hill, 14th reprint, ISBN : 0-07-451791-0, pp 350-366
3. Sushil Kumar Srivastava, ËœIndustrial maintenance managementâ„¢, S.Chand & company Ltd, 2002 Reprint, ISBN : 81-219-1663-1, pp 62-106,202-213.
4. Dr. Baldev Raj, NDT for realising better Quality of Life in Emerging Economies like India, ndtarticle/wcndt00.
5. engr.du.edu/profile/Marvin.htm
6. applied-infrared.com.au/thermography
7. lubricants.s5index.htm\
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CONDITION MONITORING



ABSTRACT

Condition monitoring of components and plants are of great importance for safe and reliable operation and for increasing productivity of plants. The challenges towards the condition monitoring can be successfully met by employing non-destructive evaluation (NDE) techniques. Vibration monitoring techniques are applied for periodic / continuous assessment of machinery parts and plants. Acoustic emission technique is used for leak detection and for structural integrity monitoring applications. Infrared thermographs are employed for condition monitoring applications in steel, electrical and petrochemical industries. Lubricant analysis by ferrography, and filed signature mapping are also used for condition monitoring applications. Here, applications of these NDE techniques could help to properly diagnose faults in plants components, enables taking timely decision about repair / replacement of components / plants, thus ensuring increased safety, reliability and productivity.
INTRODUCTION
The successful operation of structures / components during their entire life requires the implementation of a dedicated programme for condition assessment through in-service inspection (ISI) of all critical components of the plants / structures. The condition assessment through ISI and life prediction approaches enable uninterrupted operation, avoidance of unplanned shutdowns and taking decision on repair, up-gradation, modernization and replacement of necessary components for extension of the overall life of plants beyond their design lives. This is achieved through meticulous planning and incorporation of non-destructive evaluation (NDE) techniques which aims at detection and characterization of defects, stresses, corrosion micro structural degradations and dimensional changes that occur in components during service life, due to exposure to high temperature, pressure, static and dynamic loads, hostile environment etc.

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