Condition Monitoring
Condition based monitoring and its application
In Condition Monitoring system, essential parameters are measured and monitored and if any undesirable deviation is noticed, it is immediately attended and remedial action taken.
It is advisable to have routine monitoring on the functioning of the machinery, in order to find the possible threats that may lead to various system faults. These threats when found in advance, help the concerned person in planning, equipping and implementing proper measures to rectify the problem. This way of resolving the threats is far better than the execution of emergency techniques after the occurrence of system failure.
Parameter measurement during Condition Monitoring The parameters that are measured for analysing the onset and extent of threats in a machine can be classified into two. The first one is the measurement obtained when continuous monitoring is needed and second one is that obtained when just periodic monitoring is required.
Consider the parameter, 'machine housing vibration' obtained during continuous monitoring by using the shaft-observing proximity probes. Analysis of precise - measurements may be needed at that time for observing the condition of the shaft based on the machine housing vibrations from the transducer in various running levels like start-up. In such conditions, the best measurement strategy is by permanently installing a transducer in the machine. This will provide accurate measurements rather than the values obtained from the hand-held transducer during continuous monitoring. The parameter selection is done based on the machine design, mechanical factors and the extent of parameter detailing needed for the particular : monitoring system. In some cases, it is possible to retrieve more than one parameter measurement by using a single transducer. For example: Axial position sensor can measure both axial position and axial vibration of a particular component. Similarly, a radial-vibration proximity probe used for observing the machine vibrations from the shaft can also compute the shaft's radial position that represents the alignment state. Each individual portion of the machinery must be examined separately, for establishing an appropriate protection system to the machinery. Usually, the data obtained for the comprehensive analysis of the machine's varying behaviour under normal and damaged conditions will be inadequate. So it is advisable to make use of the technical knowledge and experience for judging the suitable parameter to be examined. Continuous vibration monitors work according to the overall extent of vibration measurements. These vibration signals will be converted to DC source by means of rectification. In quasistatic and vibration monitors, there are facilities to set the alarm and shutdown time period in advance. Failure Alarms: The power supply to these independent units in a monitoring system can be done either through the common power supply or from an individual power source. Proper signal conditioning such as low level, high level, or by-pass filtering will be done in order to allow only a specific range of frequency band and also to refine the signal-to-noise ratio. By using by-pass filtering technique, the machine vibrations can be monitored for a specific shaft frequency. Periodic monitoring equipment This equipment contains transducers of similar type and location as in the permanent monitoring system. Also by using this equipment, the operator can obtain logging measurements at already set fixed intervals. Periodic monitoring equipment is of different types that from simple reading meters and complex vibration or frequency analysers. Based on the prior working history of the machine and proper time, required data can be gathered by using this equipment. Data can be retrieved on a monthly, weekly, daily, hourly basis and also continuous monitoring can be carried out under crucial conditions. Such computed data can be manually documented, recorded in analogue form or can be digitally manipulated in a computer. Conditional Monitoring technique thus helps in avoiding the expensive maintenance and also reduces the shutting down time of the machinery. Conditional Monitoring can be applied during the pre-installation stage, implementation stage, and also at the time of maintenance stage of a rotating machine for computing its machine vibrations. Additionally, there is one more important role for CM. It is used as a major aid for carrying out Research and development (R&D) related to each and every individual unit in a system. Different models were designed after various research activities on Conditional Monitoring. Online Remote Fault Detection and Analysis using Condition Monitoring Now let ON consider a sample model regarding the R&D on a machine by making use of online Conditional Monitoring (CM) and remote fault detection. After carrying out different tests, a tribological strategy proves to be successful in developing an online remote Conditional Monitoring technique as shown in the figure below. This highly cost effective automatic monitoring system will help in retrieving many minute parameters like surface roughness, moisture content in the oil particles, viscosity, index of the particle covered area (IPCA), etc. In order to gather data, this system includes a remote wear debris analysis system that gets connected with the local monitoring system by using proper telecommunication methods whenever needed. The data collected will contain all the parameter information regarding fault detection and condition monitoring. Also this data collected can be transferred to another remote system for further analysis using internet connection. The data obtained from the wear debris analysis and oil sampling in analysing different working stages of the machine. This tribological system contains two sensors for detecting the required parameters and for sending to proper remote wear debris subsystem for further fault analysis. They are: • Online sensor for graphical representation of the parameters to compute the values of IPCA, number of wear particles, wear type, colour saturation, etc. • Online oil moisture and viscosity detecting sensor The system operations such as data analysis, signal transmission, image processing for wear debris, etc., can also be made simpler by executing them in a remote subsystem. Experimental Analysis: For finding the suitable parameter in order to address the potential fault detection, a machine was taken into consideration. The machine was allowed to work for 100 hours. The two online sensors took samples of lubricant oil every two hours for analysis. For offline oil tests, the lubricant oil was also taken during a period of 12 hours. Then some wear particles were intentionally added to the lubricating system for analysis by making a fault. So, while performing the operations, abrasion occurred and eventually the machine got damaged. All the data retrieved by the machine sensors were transferred to an online remote wear debris subsystem for detailed analysis. Interpretation: Thus from the above experiment, researcher was able to collect samples during the three main phases of the machine. One is the initial running condition of the machine, second is the slightly defective condition and the third is the damaged or non-working condition of the machine. Researches show that the IPCA is the key parameter that immediately responds to the changes when monitored using this method. Conclusion: Hence IPCA value is selected as the main choice to find potential faults in the online remote monitoring system. The following figure is the image processed by the online sensor during the online IPCA monitoring. Thus this tribological strategy of online remote fault detection system will definitely help in improving the usual condition monitoring techniques that makes use of machine vibrations for fault detection. Importance of Condition Monitoring of Diesel Engines It is basically the practice of systematically observing and preferably recording the parameters and condition of the overall performance and the individual components of the engine which is in shipboard use. The parameters / condition are then compared with a laid-down reference / sea / shop trial records and conclusions drawn on the limits, which if overshot, may lead to failure. The basic objective is to get uninterrupted and cost-effective, long-hours of optimum performance-output. Some common condition monitoring requirements for Engines 1. Analysis of lubricating oils 2. Vibration Monitoring 3. Ocular Condition Monitoring Condition Monitoring of marine Diesel Engines Condition Monitoring and Fault diagnosis comprises of a set of methods that are designed to monitor the condition of machines during their lifecycle. Recent technical or computational advancements and environmental regulations have led to the development of many efficient and reliable technologies. In addition' the number of electronic components such as sensors or actuators and the complexity of engine control units (ECUs) are also steadily : increasing. On the other hand, condition monitoring of sensor signals, range of parameter values, open or short circuit detection and verification of control g deviations are carried out by the softwares installed on the main ECU. However, these kinds of condition monitoring systems (CMS) are not 100, reliable to detect and clearly identify different engine failures, sensor drifts and to predict developing failures, (to assess degradation of certain components right in time). Especially the reliable detection and separation of engine malfunctions in order to predict planned maintenance intervals is still a challenge. The common diesel engine faults and their causes are: • Power loss caused by misfire and blow-by • Emission change can be caused due to loss of compression, malfunctioning of the turbocharger, block in the fuel oil filter, wrong timing of the injector, erroneous fuel air ratio, poor quality diesel fuel, block in the air intake filter, faulty piston topping, functional failure of the engine control unit (ECU), etc. • Faults in the lubricating system are due to incorrect values of oil pressure and oil deterioration. • Thermal overload can be caused due to leakage of injection valves, wear or failure of piston ring-cylinder, eroded injector holes, very low level injection pressure, high engine friction, misfire, leaking intake or exhaust manifold or valves, high coolant or lubricant temperatures, etc. • Leaks in the fuel injection system, lubrication system or air intake • Wear of the piston caused by either corrosion or abrasion, or both • Noise and vibration caused by mechanical noise, vibrations resulting from combustion, intake and exhaust noise • Other faults like knocking, filter faults, fuel contamination, aeration, etc. In all the above cases, the condition monitoring algorithm is difficult, since many of them are interconnected. For example: the mechanical failures such as valve closure may result in producing sharp vibration amplitudes. But the faults like gas leakages that appear for a longer time period lead to vibrations with lower amplitudes. These gas leaks are usually affected by changing pressure in the cylinder. Roughness or friction also produces vibration that is characterized by a noisy low amplitude pattern. In marine diesel engines, the most widely used condition monitoring techniques are vibration and acoustic signal analysis. However some notable and economical methods are listed below: 1. Acoustic analysis 2. Vibration analysis 3. Infrared thermography 4. Lubricant analysis 5. Ultrasound emission Till 2008, the Condition Monitoring has been used in isolation, hence could not give the desired results accurately. With the introduction of Distributed Control System, integrated with the Condition Monitoring can handle vibration signal up to 20 KHz. In the present days, the condition monitoring function is fully embedded into the DCS with common hardware, integrated user interface, common trending, alarms, reports and engineering configuration tools. The process components that are controlled by a motor can be installed with multiple vibration sensors and triggers for measuring rotation speed. Figure below shows a typical sensor complement which can be used to deduce misalignment, imbalance, bearing or gearbox faults or other problems with mechanical systems which can be can be fixed or variable speed. The selection of the number and types of sensors required depends on the machine type, its construction, machine size and motor rating. Typical sensor arrangement to deduce misalignment, imbalance, bearing or gearbox faults or other problems with mechanical systems is as shown in the given figure. The drives can be fixed or variable speed. During the extended running period of the machine, the vibration level measurements can be trended and alarmed together with the process data. Engineers can therefore see developing trends and plan maintenance activities. Trends of vibration data can also indicate if the machine can be run safely until the next planned maintenance stop or even for a longer period without encountering any stoppage problems. In many cases, Machine Condition Monitoring enables the detection of mechanical faults like • Bearing wear and instabilities • Unbalance • Misalignment • Thrust bearing wear • Shaft defects • Wear and looseness • Gear mesh problems • Resonances Engine oil analysis includes the analysis of unused or used engine oil samples for inspecting its characteristics and wear debris. This technique is mainly used to monitor the parameters like metallic wear, TAN, TBN, Viscosity, moisture content and contamination level. Engine oil sample have been analyzed by Spectrometric oil analysis program to determine the wear rate, and overall service condition of an engine, along with spotting potential problems and catastrophic failure before it happens. Analysis of Lubricating oil Laboratory analysis & Tests The figure below shows schematically, the appropriate location for collecting L.O. samples for the tests indicated below. Contamination due to ingress of foreign materials or debris of materials from moving components in service. This can be verified by way of laboratory analysis. For laboratory analysis, (around 0.75 litre in a 1 litre jar) sample(s) have to be drawn from the location as suggested above, while the machine is in operation, so that it is truly representative of the LO that is servicing the "relatively moving" parts of the engine. Obviously, if a sample of oil is taken from a tank after the circulating-pump has been shut down for some time, heavier contaminants such as water, dirt, carbonaceous matter, particles from metal wea, etc., will get settle down. The analysis is then liable to show that it is clean LO, which is untrue. If the purpose is to ascertain the effectiveness of the centrifuge itself, samples should be drawn at the inlet and outlet of the centrifuge for a comparative analysis. Some parameters of the LO which need be examined in the laboratory are as follows:- • Viscosity: a very important property of a LO. As per the ISO, various viscosities of LO have been classified. Around 18 viscosity intervals from 1.98 cSt to 1650 cSt 0040°c. have been assigned. Each interval has been given an ISO "viscosity grade" number. If the viscosity of the LO has increased during use, it signifies oxidation / ageing / high carbon-content. This leads to a fall in the lubricating properties of the oil. Dilutions by the contamination of fuel oil or water (unless an emulsion is readily formed) leads to a fall in viscosity. A considerably higher than normal viscosity in cSt at 410°c is liable to have slower oil-flow and, in effect, a reduced cooling capacity. A significantly lower viscosity will cause the oil-film thickness to reduce and lead to excessive wear / scuffing on parts like liners or highly loaded journal bearings. • Flash Point: is the lowest temperature at which the LO emanates a combustible vapour (i.e. concentration of air/ LO vapour mixture) which can just be ignited by a flame or spark. This is estimated by the Pensky - Martin test or by the Cleveland apparatus, having closed and open cups respectively. Most manufacturers mention both the temperatures on account of the following reasons: (a) a LO mixed with FO has a lower flash point when a closed-cup is used and a higher flash point when an open cup is used; (b) both flash points of a cracked LO are considerably low. • TBN (Total Base Number): The TBN is a measure of the reserve alkalinity of a lubricant. The test is relevant to Diesel engines due to the need to neutralize the acidic by-products of combustion generated when Diesel fuel (particularly those with high sulphur content) is burned. These by products, including S0x, NOx etc. enter the crankcase from the blow-past gases leaking past the piston rings. Besides acids entering through blow-past, acids are also generated in other areas of the engine due to heat, oxidation and other chemical changes. With the purpose of countering the corrosive effects of acids on the engine parts, additives (commonly calcium sulfonate, magnesium sulfonate, phenates and salicyclates are used) are added to the LO. Howeve, excessive addition of these additives can lead to unwanted products such as ash, which deposit on the pistons. The amount of these additives is indicated by the TBN value. Usually a TBN of 70 is selected for cylinder oils and up to 30 for system oils, depending on the sulphur content of the fuel being burnt. • Water: Always undesirable since it can lead to unwanted emulsions, corrosion and oil-additive deterioration. With high detergent oils, small amounts of water may be tolerated provided it is dispersed in very small droplets. The water should be preferably removed by centrifuges. Simultaneously, a test is carried out to trace the source of the wate, i.e. is it fresh or sea water. Usually Xylene is added so that a mixture of (xylene + water) is distilled, from which the 80000 00p050000. • Produds of Oxidation: Result from overheated LOs or contamination with partially burned fuel oil. Small amounts may be dispersed without any problem, but large amounts must be removed to prevent gummy deposits on various parts of the engine. Usually, the infra-red analysis method is used to assess the amount of oxidation products in used oil. Shipboard analysis and tests It is not uncommon for situations to arise on board when you may not be able to wait for reports of laboratory analysis to arrive, and immediate decisions may be required to be taken for continued running of the ship. Accordingly, a number of shipboard LO test facilities are available which will enable the on-board personnel to identify problems and take 000000000 action. These simple tests contribute significantly to the preventing of serious breakdowns, which may have very costly repercussions. Flostick for assessing Viscosity The Flostick is used to compare the viscosity of the used LO with that of fresh LO of the same grade. Needless to say, when a LO retains its viscosity, it maintains its ability to form a protective lubricating oil film between metal surfaces having "relative motion". The basic method used is to compare the "rate of running down" of the used oil in relation to the fresh oil, stored in a Flostick. If the used oil does not run-down to a specified mark, its viscosity is higher than acceptable. This may be due to the presence of high insoluble content, products of oxidation or ingress of fuel oil of higher viscosity. If the used oil passes another specified mark, its viscosity is lower than acceptable, probably due to dilution with a lighter distillate fuel. Water Content Test Besides causing corrosion and failure of journals and journal-bearings of Diesel engines and turbines, water contamination can lead to damage to hydraulic systems. Additionally, water encourages bacterial growth in the LO, which in turn promotes corrosion. Water test kits on board enable quick and reliable on-board measurement of the amount of fresh / salt water contamination of the system LO. The test can also be utilized 00 000000 the effectiveness with which, water is removed by a centrifuge. The principle of estimating the amount of water is based on measuring the volume of gas generated (which is directly in proportion to the quantity of water present in the LO sample being tested), by the chemical reaction between calcium hydride and the water that may be present in the LO sample. There are test kits comprising glass indicator-tubes containing a chemical which reacts with the chlorides in the ingressed salt-water. A colour change indicates the presence of chlorides (i.e. salt water). The length of / extent of the discolouration in the indicating-tube, gives a direct and ready indication of the amount of chlorides present. Alkalinity (Minimum TBN) Test Kit The Alkalinity Test Kit is meant for easy determination of the extent of alkalinity (i.e. the Minimum TBN) retained by the LO under test and whether it meets the engine builder's recommendations. The used oil is mixed with a specific amount of a special indicator-solution and an acid-reagent in a test vial. A colour change will take place if the acid reagent neutralizes the alkaline additives in the LO. The achieved colour is checked against a colour comparator to assess if the TBN is above or below a certain level. A purple colour will show that the TBN value is adequate for continued use of the LO. If the colour is green, the TBN value is borderline and about 10% fresh charge of the LO should be added to raise the TBN of the oil to an acceptable level. If the colour is yellow or gold, the TBN value is too low, indicating that the LO is unsuitable for further use and needs to be partly or fully renewed.
Vibration Monitoring
Vibration aspects of a marine Diesel engine are phenomenon that are not only crucial but also, much complex. The hull of the ship is subjected to vibratory influences arising considerably out of the Diesel main propulsion on account of the following:
• Guide Force Moments(transverse reaction forces acting on the X-heads due to the connecting rod / crankshaft mechanism) • Axial vibrations in the shaft system
• Torsional vibrations in the shaft system.
It is not quite the responsibility of shipboard marine engineer to monitor these vibrations with the use of technologically advanced tools. Accordingly, professional vibration monitoring is seldom done by the shipboard engineer. However, the shipboard engineer surely needs to be sensitive to obvious indications of such vibrations (e.g. vibrations arising out of operating the main engine in the critical range) during the operation of the engine or other machines. Sense of touch, or even a screw-driver touching the machine with the other-end to the ear, is usually the best vibration-monitoring resorted to by the shipboard engineer. Besides engines, motors, pumps, compressors, gearboxes, belt-conveyors involve the transmission of rotary mechanisms. Inevitably, these machines require bearings that support the weight of rotating parts and bear the forces arising out of their operations. In fact, large forces are required to be borne by the bearings. Vibration monitoring is therefore normally done at the bearings of these common machines and the accelerometers are positioned near the bearings. Mechanical vibrations originate due to the oscillatory movement of a mass about a reference position. The movement may be depicted in terms of: Displacement: The distance moved by a mass from its natural position. Unit is metres, "m". Velocity: The speed at which the mass moves. Unit is metres/second, "m/s". Acceleration: The rate of change of velocity of the mass. Unit is metres / second2, "m/s2". The decision to monitor one of the three above parameters for assessing the degree of vibration depends on the nature of the vibration and the purpose of the measurement. Displacement is usually preferred for measuring low-frequency vibrations, such as machine imbalance. Acceleration is preferred for measuring shocks and high-frequency vibrations which may give rise to wear and fatigue. Measuring velocity provides a rational indication of the amplitude (i.e. severity) of the vibration and is therefore preferred for monitoring machine conditions. Instruments for Measuring Vibrations • Transient vibration and shock normally warrant the use of "peak indicating instruments". • Continuous vibration measuring requires the usage of RMS (i.e. Effective Value) indicating instruments, with a selectable time-constant for a = steady indication, is usually considered more helpful. The RMS value is usually more informative of the two types. Vibration Transducers The most extensively used vibration transducers is the piezo-electric accelerometer. The accelerometer produces an electrical output which is directly proportional to the acceleration of the vibration in a machine. Its output can also be converted into a velocity or displacement function and thereby a proportional signal, by simple electronic means. Machine Condition Detector is a rugged, handheld analysis tool that captures and displays .mperature, velocity and enveloped acceleration (vibration) and alarms. This is certified Intrinsically Safe (IS) for use in hazardous environments, collects and displays velocity, acceleration enveloping and temperature data with general alarm capabilities.
Ocular Monitoring
Introduction Visual inspections need to be carried out on board without dismantling a machine / component, for the purpose of condition monitoring. The common aids (besides illuminating torches and magnifying glasses, which are used almost daily) for ocular monitoring are:
• Stroboscopes
• Fibrescopes
• Endoscopes
• Mirrors and Light Probes
• Stethoscopes
Stroboscopes
A Stroboscope Stroboscopes uses the phenomenon of persistence of vision to slow-down / stop the motion of a vibrating, rotating or reciprocating parts of a machine, which happen to move too rapidly for the human eye to monitor. The xenon-lamp of the instrument emits a series of brief, intense, short-duration light flashes, at specific intervals. When the frequency of the flashing lights from the stroboscope is adjusted to synchronize with the rotating machine, the later will appear to be at standstill. This illusion is utilized to study the machine, / component's functional-behaviour.
Stroboscopes can be used to determine the correct number of revolutions or frequency of the moving parts of a machine. The frequencies of the flashes are read-off a dial separately.
While carrying out parameter measurements, the sensor signal in the stroboscope generates two vibration frequencies for each measuring point. Velocity vibration detects the faults that are visible in between the frequencies ranging from Its to mid. Hence, it notifies the structural faults such as incorrect alignment, instability, mechanical looseness, etc.
Events that occur in the higher frequency ranges, such as bearing and gear problems, can also be detected with its "acceleration enveloping" capability, a signal processing technique that focuses on enhancing the repetitious vibration signals that characterize such problems. Stroboscopes are portable, compact, and easy-to-use. Bright and powerful flash gives a good target illumination at a distance, with a focused viewing area, Flash rates of up to 300 000 flashes per minute cover most high speed applications. For routine inspections, the powerful lamp mode is useful.
Fibrescopes
A fibrescope is basically a flexible-borescope as explained above. This instrument consists of glass fibre-optic bundles (comprise single fibres finer than human hair combined into bundles joined at the ends and having an outer plastic covering). Each fibre has an eye-piece (magnifies the image carried back by the "imaging bundle" so that the human eye can view it) at one end and a lens at the other that is used to examine difficult-to-reach areas such as cavities, complicated machine structures etc. It consists of:
(i) a high-purity-glass core, surrounded by, (I) a cladding material of a refractive-index that is lower than the refractive-index of the core, for preventing the light from leaking, and,
(iii) a protective, plastic, buffer coating.
A typical fibrescope
The two different types of fibre-optic bundles are Illumination bundle and Imaging bundle.
1. The function of the illumination bundle is sending the light rays to the front portion of the lens.
2. The function of imaging bundle is to transfer the processed image from lens to eyepiece. In this instrument, the light waves are made to move inside the core part of a fibre optic cable by applying the principle of total internal reflection. Light entering the end of the fibre core, will be reflected-off the cladding, right along the length of the fibre. As you know, total-internal-reflection is possible when the following two conditions are met:- (i) The light travels from a more dense medium to a less dense medium; &
(ii) The angle of incidence of the light is greater than the critical angle. To recall your physics, at this angle of incidence, the refracted ray lies along the boundary (i.e. surface of separation between the two mediums), having an angle of refraction of 90°).
In this case, the surface of the core acts like a perfect mirror and the angle of incidence of the light will be larger than the critical angle. Bending the fibre does not affect its performance. This tool is very useful for inspection of cavities where access is difficult, unless long down-times are accepted as a compromise.
Endoscopes
A typical rigid borescope. The utility of this instrument is to examine the internal conditions of a machine / component, which are impossible / difficult to gain access for inspection. Examples of such inspection are: heat exchanger tubes, smaller pressure vessels, gear boxes etc.
Mirrors and Light Probes
Inspection Mirror Probes These mirrors are available in a various forms and are used to make unapproachable corners visible. The utility is enhanced if the mirror is telescopic in design and adjustable to enable the shipboard engineer to put it to various usages. The illumination provided by way of LED lamp(s), will shine on the particular component to be attended-to and the picture-image is transmitted back to the operator of the mirror.
Stethoscopes
Portable Stethoscope Stethoscopes, easily pinpoint bearing and machine noise. An Electronic Stethoscope is a highly sensitive and quickly responding machine that helps in detecting faults in the machine parts by monitoring the machine vibrations or noises. It comprises of a headset having two probes and each probe will be of two different lengths i.e., 70 and 220 mm. In addition to this, a pre-recorded audio CD containing the common faulty machine noises will be provided.
Need for Piston Ring Monitoring and Harmful Effects of blow-by
You are aware of the importance of ensuring that the piston rings succeed to keep the high-pressure combustion gases from leaking past the piston. If the piston ring does not succeed in this sealing, the heat transfer from the combustion gases to the piston and liner takes place because of the high-velocity-blow down of these gases past the liner. The Figure-3 below shows the recording of temperatures in a cylinder liner of a large X-head marine Diesel engine. I Liner temperatures in the order of 300°C at the upper part of the piston ring travel will provide oil-starvation and conditions conducive to piston-ring micro-seizure (also called scuffing). Thermal overload in a unit is the main cause of such gas-leakages.
Harmful Effects of blow-by
• Effects are on the piston, liner and piston rings.
• Overheated piston-rings lose their tension(springiness) and may be deformed by the thermal stresses, aggravating the circumstances prevailing.
• The cylinder lubricating oil on the sides of the piston crown will carbonize and form hard deposits which may lead to abrasion of the liner surface. These deposits in the ring grooves may also obstruct the passage of gas to the inner-side of the rings (i.e. between the thug and groove inner surface), further preventing the sealing of the blow-by of the combustion gases. In effect the rings may collapse or break.
• Blow-by in a cylinder is not continuous, its intensity varies and so does the time interval for which it takes place. Therefore, the temperature - sensors located in the liner for monitoring these anomalies, will not record temperature rises continuously. It records temperature rises from some minutes to a few hours.
• During normal conditions when moderate rates of wear prevail, the wear process is either corrosive or abrasive. However, when abnormal higher wear rates are seen, the cause may be periods with scuffing prevailing.
• The Figure 4 below shows the wear pattern as observed from frequent wear measurements of a cylinder liner of a large marine Diesel engine in service:
Monitoring Temperature in Cylinder Liners
The temperature in the upper part of cylinder liners is quite sensitive to most irregularities / anomalies instancing during combustion inside the temperature is truly representative of the state inside the cylinder unit. Quite obviously, when the temperature remains within prescribed limits and combustion chamber. These may include thermal overloading, blow-by, ring-collapses and heat developed by normal friction / scuffing. The liner remains as stable for reasonably long periods, it may be logically concluded that the cylinder is working in-order.
In order to assess the temperature of cylinder liners, thermocouples are fitted on to the cylinder wall about 5 to 10 mm from the inner (i.e. combustion chamber surface) surface of the liner. This is fitted, as found suitable, between the 1st and 2nd piston rings, when the piston A at the 'TDC'. Usually 2- sensors are positioned diametrically opposite, on to the liner. In fact, since the gas blow-by are liable to happen locally, it may be advantageous to have more number of sensors, say 4, around the liner circumference.
Monitoring Thermal Load This is one of the most important conditions to be monitored for assessing appropriate design and material considerations of a Diesel engine. Success in these aspects gives the shipboard engineer considerable peace in operation. The basic purpose is to measure the temperature which is representative of the thermal load of the engine. The thermal load of an engine is dependent on the engine-load, the fuel / fuel-system and miscellaneous external / ambient parameters. For this purpose, sensor(s) have to be positioned on the cylinder cover. Direct-temperature measurement, is a simple and precise method. Passages are drilled from the outside of the cylinder cover to a point which is 7 mm from the surface exposed to the combustion chamber. Temperature probes are positioned in these holes from the outside. In this method, defective probes are easily replaced from outside.
Complete Survey requirement
The survey requirement for inspection of the propeller, stern tube bearing and tail shafting is:
1. The tail shaft is fitted with either continuous liners, approved oil sealing glands or made of corrosion-resistant material, the survey will be carried out:
• 3 years for single shafting arrangements
• 4 years for multi-shafting arrangements
2. The survey interval may be increased to 5 years in the following categories:
• The propeller is fitted keyless to the shaft taper, the shaft is protected from seawater, the design details are approved and a non-destructive examination of the forward part of the aft shaft taper is performed at each survey by an approved crack-detection method.
• The propeller is fitted to a keyed shaft taper, the design details of which comply with the applicable requirements, and a non-destructive examination of the aft end of the cylindrical part of the shaft i.e from the aft end of the liner, and of about one-third of the length of the taper from the large end is performed at each survey by an approved crack-detection method.
• The propeller is fitted to a solid flange coupling at the aft end of the shaft, the shaft and its fittings are not exposed to corrosion, the design details are approved and a non-destructive examination of the aft flange fillet area of the shaft is performed at each survey by an approved crack-detection method.
3. For other categories, the interval of inspection for the propeller shaft opened up surveys is two years and six months (2.5 years).
Modified Survey requirement
• A modified survey of the tail shaft is an alternate way of examination whose scope is given above. It may be accepted for tail shafts provided that:
• They are fitted with oil-lubricated bearings and approved oil sealing glands
• The shaft and its fittings are not exposed to corrosion
• The design details are approved • The clearances of the aft bearings are found to be in order
• The oil and the oil sealing arrangements prove effective
• Lubricating oil analyses are carried out regularly at intervals not exceeding six months and oil consumption is recorded at the same intervals.
The modified survey is to be carried out five years from the last complete survey with a window period of plus or minus six months.
The next complete survey is to be carried out ten years from the last complete survey.
In Condition Monitoring system, essential parameters are measured and monitored and if any undesirable deviation is noticed, it is immediately attended and remedial action taken.
It is advisable to have routine monitoring on the functioning of the machinery, in order to find the possible threats that may lead to various system faults. These threats when found in advance, help the concerned person in planning, equipping and implementing proper measures to rectify the problem. This way of resolving the threats is far better than the execution of emergency techniques after the occurrence of system failure.
Parameter measurement during Condition Monitoring The parameters that are measured for analysing the onset and extent of threats in a machine can be classified into two. The first one is the measurement obtained when continuous monitoring is needed and second one is that obtained when just periodic monitoring is required.
Consider the parameter, 'machine housing vibration' obtained during continuous monitoring by using the shaft-observing proximity probes. Analysis of precise - measurements may be needed at that time for observing the condition of the shaft based on the machine housing vibrations from the transducer in various running levels like start-up. In such conditions, the best measurement strategy is by permanently installing a transducer in the machine. This will provide accurate measurements rather than the values obtained from the hand-held transducer during continuous monitoring. The parameter selection is done based on the machine design, mechanical factors and the extent of parameter detailing needed for the particular : monitoring system. In some cases, it is possible to retrieve more than one parameter measurement by using a single transducer. For example: Axial position sensor can measure both axial position and axial vibration of a particular component. Similarly, a radial-vibration proximity probe used for observing the machine vibrations from the shaft can also compute the shaft's radial position that represents the alignment state. Each individual portion of the machinery must be examined separately, for establishing an appropriate protection system to the machinery. Usually, the data obtained for the comprehensive analysis of the machine's varying behaviour under normal and damaged conditions will be inadequate. So it is advisable to make use of the technical knowledge and experience for judging the suitable parameter to be examined. Continuous vibration monitors work according to the overall extent of vibration measurements. These vibration signals will be converted to DC source by means of rectification. In quasistatic and vibration monitors, there are facilities to set the alarm and shutdown time period in advance. Failure Alarms: The power supply to these independent units in a monitoring system can be done either through the common power supply or from an individual power source. Proper signal conditioning such as low level, high level, or by-pass filtering will be done in order to allow only a specific range of frequency band and also to refine the signal-to-noise ratio. By using by-pass filtering technique, the machine vibrations can be monitored for a specific shaft frequency. Periodic monitoring equipment This equipment contains transducers of similar type and location as in the permanent monitoring system. Also by using this equipment, the operator can obtain logging measurements at already set fixed intervals. Periodic monitoring equipment is of different types that from simple reading meters and complex vibration or frequency analysers. Based on the prior working history of the machine and proper time, required data can be gathered by using this equipment. Data can be retrieved on a monthly, weekly, daily, hourly basis and also continuous monitoring can be carried out under crucial conditions. Such computed data can be manually documented, recorded in analogue form or can be digitally manipulated in a computer. Conditional Monitoring technique thus helps in avoiding the expensive maintenance and also reduces the shutting down time of the machinery. Conditional Monitoring can be applied during the pre-installation stage, implementation stage, and also at the time of maintenance stage of a rotating machine for computing its machine vibrations. Additionally, there is one more important role for CM. It is used as a major aid for carrying out Research and development (R&D) related to each and every individual unit in a system. Different models were designed after various research activities on Conditional Monitoring. Online Remote Fault Detection and Analysis using Condition Monitoring Now let ON consider a sample model regarding the R&D on a machine by making use of online Conditional Monitoring (CM) and remote fault detection. After carrying out different tests, a tribological strategy proves to be successful in developing an online remote Conditional Monitoring technique as shown in the figure below. This highly cost effective automatic monitoring system will help in retrieving many minute parameters like surface roughness, moisture content in the oil particles, viscosity, index of the particle covered area (IPCA), etc. In order to gather data, this system includes a remote wear debris analysis system that gets connected with the local monitoring system by using proper telecommunication methods whenever needed. The data collected will contain all the parameter information regarding fault detection and condition monitoring. Also this data collected can be transferred to another remote system for further analysis using internet connection. The data obtained from the wear debris analysis and oil sampling in analysing different working stages of the machine. This tribological system contains two sensors for detecting the required parameters and for sending to proper remote wear debris subsystem for further fault analysis. They are: • Online sensor for graphical representation of the parameters to compute the values of IPCA, number of wear particles, wear type, colour saturation, etc. • Online oil moisture and viscosity detecting sensor The system operations such as data analysis, signal transmission, image processing for wear debris, etc., can also be made simpler by executing them in a remote subsystem. Experimental Analysis: For finding the suitable parameter in order to address the potential fault detection, a machine was taken into consideration. The machine was allowed to work for 100 hours. The two online sensors took samples of lubricant oil every two hours for analysis. For offline oil tests, the lubricant oil was also taken during a period of 12 hours. Then some wear particles were intentionally added to the lubricating system for analysis by making a fault. So, while performing the operations, abrasion occurred and eventually the machine got damaged. All the data retrieved by the machine sensors were transferred to an online remote wear debris subsystem for detailed analysis. Interpretation: Thus from the above experiment, researcher was able to collect samples during the three main phases of the machine. One is the initial running condition of the machine, second is the slightly defective condition and the third is the damaged or non-working condition of the machine. Researches show that the IPCA is the key parameter that immediately responds to the changes when monitored using this method. Conclusion: Hence IPCA value is selected as the main choice to find potential faults in the online remote monitoring system. The following figure is the image processed by the online sensor during the online IPCA monitoring. Thus this tribological strategy of online remote fault detection system will definitely help in improving the usual condition monitoring techniques that makes use of machine vibrations for fault detection. Importance of Condition Monitoring of Diesel Engines It is basically the practice of systematically observing and preferably recording the parameters and condition of the overall performance and the individual components of the engine which is in shipboard use. The parameters / condition are then compared with a laid-down reference / sea / shop trial records and conclusions drawn on the limits, which if overshot, may lead to failure. The basic objective is to get uninterrupted and cost-effective, long-hours of optimum performance-output. Some common condition monitoring requirements for Engines 1. Analysis of lubricating oils 2. Vibration Monitoring 3. Ocular Condition Monitoring Condition Monitoring of marine Diesel Engines Condition Monitoring and Fault diagnosis comprises of a set of methods that are designed to monitor the condition of machines during their lifecycle. Recent technical or computational advancements and environmental regulations have led to the development of many efficient and reliable technologies. In addition' the number of electronic components such as sensors or actuators and the complexity of engine control units (ECUs) are also steadily : increasing. On the other hand, condition monitoring of sensor signals, range of parameter values, open or short circuit detection and verification of control g deviations are carried out by the softwares installed on the main ECU. However, these kinds of condition monitoring systems (CMS) are not 100, reliable to detect and clearly identify different engine failures, sensor drifts and to predict developing failures, (to assess degradation of certain components right in time). Especially the reliable detection and separation of engine malfunctions in order to predict planned maintenance intervals is still a challenge. The common diesel engine faults and their causes are: • Power loss caused by misfire and blow-by • Emission change can be caused due to loss of compression, malfunctioning of the turbocharger, block in the fuel oil filter, wrong timing of the injector, erroneous fuel air ratio, poor quality diesel fuel, block in the air intake filter, faulty piston topping, functional failure of the engine control unit (ECU), etc. • Faults in the lubricating system are due to incorrect values of oil pressure and oil deterioration. • Thermal overload can be caused due to leakage of injection valves, wear or failure of piston ring-cylinder, eroded injector holes, very low level injection pressure, high engine friction, misfire, leaking intake or exhaust manifold or valves, high coolant or lubricant temperatures, etc. • Leaks in the fuel injection system, lubrication system or air intake • Wear of the piston caused by either corrosion or abrasion, or both • Noise and vibration caused by mechanical noise, vibrations resulting from combustion, intake and exhaust noise • Other faults like knocking, filter faults, fuel contamination, aeration, etc. In all the above cases, the condition monitoring algorithm is difficult, since many of them are interconnected. For example: the mechanical failures such as valve closure may result in producing sharp vibration amplitudes. But the faults like gas leakages that appear for a longer time period lead to vibrations with lower amplitudes. These gas leaks are usually affected by changing pressure in the cylinder. Roughness or friction also produces vibration that is characterized by a noisy low amplitude pattern. In marine diesel engines, the most widely used condition monitoring techniques are vibration and acoustic signal analysis. However some notable and economical methods are listed below: 1. Acoustic analysis 2. Vibration analysis 3. Infrared thermography 4. Lubricant analysis 5. Ultrasound emission Till 2008, the Condition Monitoring has been used in isolation, hence could not give the desired results accurately. With the introduction of Distributed Control System, integrated with the Condition Monitoring can handle vibration signal up to 20 KHz. In the present days, the condition monitoring function is fully embedded into the DCS with common hardware, integrated user interface, common trending, alarms, reports and engineering configuration tools. The process components that are controlled by a motor can be installed with multiple vibration sensors and triggers for measuring rotation speed. Figure below shows a typical sensor complement which can be used to deduce misalignment, imbalance, bearing or gearbox faults or other problems with mechanical systems which can be can be fixed or variable speed. The selection of the number and types of sensors required depends on the machine type, its construction, machine size and motor rating. Typical sensor arrangement to deduce misalignment, imbalance, bearing or gearbox faults or other problems with mechanical systems is as shown in the given figure. The drives can be fixed or variable speed. During the extended running period of the machine, the vibration level measurements can be trended and alarmed together with the process data. Engineers can therefore see developing trends and plan maintenance activities. Trends of vibration data can also indicate if the machine can be run safely until the next planned maintenance stop or even for a longer period without encountering any stoppage problems. In many cases, Machine Condition Monitoring enables the detection of mechanical faults like • Bearing wear and instabilities • Unbalance • Misalignment • Thrust bearing wear • Shaft defects • Wear and looseness • Gear mesh problems • Resonances Engine oil analysis includes the analysis of unused or used engine oil samples for inspecting its characteristics and wear debris. This technique is mainly used to monitor the parameters like metallic wear, TAN, TBN, Viscosity, moisture content and contamination level. Engine oil sample have been analyzed by Spectrometric oil analysis program to determine the wear rate, and overall service condition of an engine, along with spotting potential problems and catastrophic failure before it happens. Analysis of Lubricating oil Laboratory analysis & Tests The figure below shows schematically, the appropriate location for collecting L.O. samples for the tests indicated below. Contamination due to ingress of foreign materials or debris of materials from moving components in service. This can be verified by way of laboratory analysis. For laboratory analysis, (around 0.75 litre in a 1 litre jar) sample(s) have to be drawn from the location as suggested above, while the machine is in operation, so that it is truly representative of the LO that is servicing the "relatively moving" parts of the engine. Obviously, if a sample of oil is taken from a tank after the circulating-pump has been shut down for some time, heavier contaminants such as water, dirt, carbonaceous matter, particles from metal wea, etc., will get settle down. The analysis is then liable to show that it is clean LO, which is untrue. If the purpose is to ascertain the effectiveness of the centrifuge itself, samples should be drawn at the inlet and outlet of the centrifuge for a comparative analysis. Some parameters of the LO which need be examined in the laboratory are as follows:- • Viscosity: a very important property of a LO. As per the ISO, various viscosities of LO have been classified. Around 18 viscosity intervals from 1.98 cSt to 1650 cSt 0040°c. have been assigned. Each interval has been given an ISO "viscosity grade" number. If the viscosity of the LO has increased during use, it signifies oxidation / ageing / high carbon-content. This leads to a fall in the lubricating properties of the oil. Dilutions by the contamination of fuel oil or water (unless an emulsion is readily formed) leads to a fall in viscosity. A considerably higher than normal viscosity in cSt at 410°c is liable to have slower oil-flow and, in effect, a reduced cooling capacity. A significantly lower viscosity will cause the oil-film thickness to reduce and lead to excessive wear / scuffing on parts like liners or highly loaded journal bearings. • Flash Point: is the lowest temperature at which the LO emanates a combustible vapour (i.e. concentration of air/ LO vapour mixture) which can just be ignited by a flame or spark. This is estimated by the Pensky - Martin test or by the Cleveland apparatus, having closed and open cups respectively. Most manufacturers mention both the temperatures on account of the following reasons: (a) a LO mixed with FO has a lower flash point when a closed-cup is used and a higher flash point when an open cup is used; (b) both flash points of a cracked LO are considerably low. • TBN (Total Base Number): The TBN is a measure of the reserve alkalinity of a lubricant. The test is relevant to Diesel engines due to the need to neutralize the acidic by-products of combustion generated when Diesel fuel (particularly those with high sulphur content) is burned. These by products, including S0x, NOx etc. enter the crankcase from the blow-past gases leaking past the piston rings. Besides acids entering through blow-past, acids are also generated in other areas of the engine due to heat, oxidation and other chemical changes. With the purpose of countering the corrosive effects of acids on the engine parts, additives (commonly calcium sulfonate, magnesium sulfonate, phenates and salicyclates are used) are added to the LO. Howeve, excessive addition of these additives can lead to unwanted products such as ash, which deposit on the pistons. The amount of these additives is indicated by the TBN value. Usually a TBN of 70 is selected for cylinder oils and up to 30 for system oils, depending on the sulphur content of the fuel being burnt. • Water: Always undesirable since it can lead to unwanted emulsions, corrosion and oil-additive deterioration. With high detergent oils, small amounts of water may be tolerated provided it is dispersed in very small droplets. The water should be preferably removed by centrifuges. Simultaneously, a test is carried out to trace the source of the wate, i.e. is it fresh or sea water. Usually Xylene is added so that a mixture of (xylene + water) is distilled, from which the 80000 00p050000. • Produds of Oxidation: Result from overheated LOs or contamination with partially burned fuel oil. Small amounts may be dispersed without any problem, but large amounts must be removed to prevent gummy deposits on various parts of the engine. Usually, the infra-red analysis method is used to assess the amount of oxidation products in used oil. Shipboard analysis and tests It is not uncommon for situations to arise on board when you may not be able to wait for reports of laboratory analysis to arrive, and immediate decisions may be required to be taken for continued running of the ship. Accordingly, a number of shipboard LO test facilities are available which will enable the on-board personnel to identify problems and take 000000000 action. These simple tests contribute significantly to the preventing of serious breakdowns, which may have very costly repercussions. Flostick for assessing Viscosity The Flostick is used to compare the viscosity of the used LO with that of fresh LO of the same grade. Needless to say, when a LO retains its viscosity, it maintains its ability to form a protective lubricating oil film between metal surfaces having "relative motion". The basic method used is to compare the "rate of running down" of the used oil in relation to the fresh oil, stored in a Flostick. If the used oil does not run-down to a specified mark, its viscosity is higher than acceptable. This may be due to the presence of high insoluble content, products of oxidation or ingress of fuel oil of higher viscosity. If the used oil passes another specified mark, its viscosity is lower than acceptable, probably due to dilution with a lighter distillate fuel. Water Content Test Besides causing corrosion and failure of journals and journal-bearings of Diesel engines and turbines, water contamination can lead to damage to hydraulic systems. Additionally, water encourages bacterial growth in the LO, which in turn promotes corrosion. Water test kits on board enable quick and reliable on-board measurement of the amount of fresh / salt water contamination of the system LO. The test can also be utilized 00 000000 the effectiveness with which, water is removed by a centrifuge. The principle of estimating the amount of water is based on measuring the volume of gas generated (which is directly in proportion to the quantity of water present in the LO sample being tested), by the chemical reaction between calcium hydride and the water that may be present in the LO sample. There are test kits comprising glass indicator-tubes containing a chemical which reacts with the chlorides in the ingressed salt-water. A colour change indicates the presence of chlorides (i.e. salt water). The length of / extent of the discolouration in the indicating-tube, gives a direct and ready indication of the amount of chlorides present. Alkalinity (Minimum TBN) Test Kit The Alkalinity Test Kit is meant for easy determination of the extent of alkalinity (i.e. the Minimum TBN) retained by the LO under test and whether it meets the engine builder's recommendations. The used oil is mixed with a specific amount of a special indicator-solution and an acid-reagent in a test vial. A colour change will take place if the acid reagent neutralizes the alkaline additives in the LO. The achieved colour is checked against a colour comparator to assess if the TBN is above or below a certain level. A purple colour will show that the TBN value is adequate for continued use of the LO. If the colour is green, the TBN value is borderline and about 10% fresh charge of the LO should be added to raise the TBN of the oil to an acceptable level. If the colour is yellow or gold, the TBN value is too low, indicating that the LO is unsuitable for further use and needs to be partly or fully renewed.
Vibration Monitoring
Vibration aspects of a marine Diesel engine are phenomenon that are not only crucial but also, much complex. The hull of the ship is subjected to vibratory influences arising considerably out of the Diesel main propulsion on account of the following:
• Guide Force Moments(transverse reaction forces acting on the X-heads due to the connecting rod / crankshaft mechanism) • Axial vibrations in the shaft system
• Torsional vibrations in the shaft system.
It is not quite the responsibility of shipboard marine engineer to monitor these vibrations with the use of technologically advanced tools. Accordingly, professional vibration monitoring is seldom done by the shipboard engineer. However, the shipboard engineer surely needs to be sensitive to obvious indications of such vibrations (e.g. vibrations arising out of operating the main engine in the critical range) during the operation of the engine or other machines. Sense of touch, or even a screw-driver touching the machine with the other-end to the ear, is usually the best vibration-monitoring resorted to by the shipboard engineer. Besides engines, motors, pumps, compressors, gearboxes, belt-conveyors involve the transmission of rotary mechanisms. Inevitably, these machines require bearings that support the weight of rotating parts and bear the forces arising out of their operations. In fact, large forces are required to be borne by the bearings. Vibration monitoring is therefore normally done at the bearings of these common machines and the accelerometers are positioned near the bearings. Mechanical vibrations originate due to the oscillatory movement of a mass about a reference position. The movement may be depicted in terms of: Displacement: The distance moved by a mass from its natural position. Unit is metres, "m". Velocity: The speed at which the mass moves. Unit is metres/second, "m/s". Acceleration: The rate of change of velocity of the mass. Unit is metres / second2, "m/s2". The decision to monitor one of the three above parameters for assessing the degree of vibration depends on the nature of the vibration and the purpose of the measurement. Displacement is usually preferred for measuring low-frequency vibrations, such as machine imbalance. Acceleration is preferred for measuring shocks and high-frequency vibrations which may give rise to wear and fatigue. Measuring velocity provides a rational indication of the amplitude (i.e. severity) of the vibration and is therefore preferred for monitoring machine conditions. Instruments for Measuring Vibrations • Transient vibration and shock normally warrant the use of "peak indicating instruments". • Continuous vibration measuring requires the usage of RMS (i.e. Effective Value) indicating instruments, with a selectable time-constant for a = steady indication, is usually considered more helpful. The RMS value is usually more informative of the two types. Vibration Transducers The most extensively used vibration transducers is the piezo-electric accelerometer. The accelerometer produces an electrical output which is directly proportional to the acceleration of the vibration in a machine. Its output can also be converted into a velocity or displacement function and thereby a proportional signal, by simple electronic means. Machine Condition Detector is a rugged, handheld analysis tool that captures and displays .mperature, velocity and enveloped acceleration (vibration) and alarms. This is certified Intrinsically Safe (IS) for use in hazardous environments, collects and displays velocity, acceleration enveloping and temperature data with general alarm capabilities.
Ocular Monitoring
Introduction Visual inspections need to be carried out on board without dismantling a machine / component, for the purpose of condition monitoring. The common aids (besides illuminating torches and magnifying glasses, which are used almost daily) for ocular monitoring are:
• Stroboscopes
• Fibrescopes
• Endoscopes
• Mirrors and Light Probes
• Stethoscopes
Stroboscopes
A Stroboscope Stroboscopes uses the phenomenon of persistence of vision to slow-down / stop the motion of a vibrating, rotating or reciprocating parts of a machine, which happen to move too rapidly for the human eye to monitor. The xenon-lamp of the instrument emits a series of brief, intense, short-duration light flashes, at specific intervals. When the frequency of the flashing lights from the stroboscope is adjusted to synchronize with the rotating machine, the later will appear to be at standstill. This illusion is utilized to study the machine, / component's functional-behaviour.
Stroboscopes can be used to determine the correct number of revolutions or frequency of the moving parts of a machine. The frequencies of the flashes are read-off a dial separately.
While carrying out parameter measurements, the sensor signal in the stroboscope generates two vibration frequencies for each measuring point. Velocity vibration detects the faults that are visible in between the frequencies ranging from Its to mid. Hence, it notifies the structural faults such as incorrect alignment, instability, mechanical looseness, etc.
Events that occur in the higher frequency ranges, such as bearing and gear problems, can also be detected with its "acceleration enveloping" capability, a signal processing technique that focuses on enhancing the repetitious vibration signals that characterize such problems. Stroboscopes are portable, compact, and easy-to-use. Bright and powerful flash gives a good target illumination at a distance, with a focused viewing area, Flash rates of up to 300 000 flashes per minute cover most high speed applications. For routine inspections, the powerful lamp mode is useful.
Fibrescopes
A fibrescope is basically a flexible-borescope as explained above. This instrument consists of glass fibre-optic bundles (comprise single fibres finer than human hair combined into bundles joined at the ends and having an outer plastic covering). Each fibre has an eye-piece (magnifies the image carried back by the "imaging bundle" so that the human eye can view it) at one end and a lens at the other that is used to examine difficult-to-reach areas such as cavities, complicated machine structures etc. It consists of:
(i) a high-purity-glass core, surrounded by, (I) a cladding material of a refractive-index that is lower than the refractive-index of the core, for preventing the light from leaking, and,
(iii) a protective, plastic, buffer coating.
A typical fibrescope
The two different types of fibre-optic bundles are Illumination bundle and Imaging bundle.
1. The function of the illumination bundle is sending the light rays to the front portion of the lens.
2. The function of imaging bundle is to transfer the processed image from lens to eyepiece. In this instrument, the light waves are made to move inside the core part of a fibre optic cable by applying the principle of total internal reflection. Light entering the end of the fibre core, will be reflected-off the cladding, right along the length of the fibre. As you know, total-internal-reflection is possible when the following two conditions are met:- (i) The light travels from a more dense medium to a less dense medium; &
(ii) The angle of incidence of the light is greater than the critical angle. To recall your physics, at this angle of incidence, the refracted ray lies along the boundary (i.e. surface of separation between the two mediums), having an angle of refraction of 90°).
In this case, the surface of the core acts like a perfect mirror and the angle of incidence of the light will be larger than the critical angle. Bending the fibre does not affect its performance. This tool is very useful for inspection of cavities where access is difficult, unless long down-times are accepted as a compromise.
Endoscopes
A typical rigid borescope. The utility of this instrument is to examine the internal conditions of a machine / component, which are impossible / difficult to gain access for inspection. Examples of such inspection are: heat exchanger tubes, smaller pressure vessels, gear boxes etc.
Mirrors and Light Probes
Inspection Mirror Probes These mirrors are available in a various forms and are used to make unapproachable corners visible. The utility is enhanced if the mirror is telescopic in design and adjustable to enable the shipboard engineer to put it to various usages. The illumination provided by way of LED lamp(s), will shine on the particular component to be attended-to and the picture-image is transmitted back to the operator of the mirror.
Stethoscopes
Portable Stethoscope Stethoscopes, easily pinpoint bearing and machine noise. An Electronic Stethoscope is a highly sensitive and quickly responding machine that helps in detecting faults in the machine parts by monitoring the machine vibrations or noises. It comprises of a headset having two probes and each probe will be of two different lengths i.e., 70 and 220 mm. In addition to this, a pre-recorded audio CD containing the common faulty machine noises will be provided.
Need for Piston Ring Monitoring and Harmful Effects of blow-by
You are aware of the importance of ensuring that the piston rings succeed to keep the high-pressure combustion gases from leaking past the piston. If the piston ring does not succeed in this sealing, the heat transfer from the combustion gases to the piston and liner takes place because of the high-velocity-blow down of these gases past the liner. The Figure-3 below shows the recording of temperatures in a cylinder liner of a large X-head marine Diesel engine. I Liner temperatures in the order of 300°C at the upper part of the piston ring travel will provide oil-starvation and conditions conducive to piston-ring micro-seizure (also called scuffing). Thermal overload in a unit is the main cause of such gas-leakages.
Harmful Effects of blow-by
• Effects are on the piston, liner and piston rings.
• Overheated piston-rings lose their tension(springiness) and may be deformed by the thermal stresses, aggravating the circumstances prevailing.
• The cylinder lubricating oil on the sides of the piston crown will carbonize and form hard deposits which may lead to abrasion of the liner surface. These deposits in the ring grooves may also obstruct the passage of gas to the inner-side of the rings (i.e. between the thug and groove inner surface), further preventing the sealing of the blow-by of the combustion gases. In effect the rings may collapse or break.
• Blow-by in a cylinder is not continuous, its intensity varies and so does the time interval for which it takes place. Therefore, the temperature - sensors located in the liner for monitoring these anomalies, will not record temperature rises continuously. It records temperature rises from some minutes to a few hours.
• During normal conditions when moderate rates of wear prevail, the wear process is either corrosive or abrasive. However, when abnormal higher wear rates are seen, the cause may be periods with scuffing prevailing.
• The Figure 4 below shows the wear pattern as observed from frequent wear measurements of a cylinder liner of a large marine Diesel engine in service:
Monitoring Temperature in Cylinder Liners
The temperature in the upper part of cylinder liners is quite sensitive to most irregularities / anomalies instancing during combustion inside the temperature is truly representative of the state inside the cylinder unit. Quite obviously, when the temperature remains within prescribed limits and combustion chamber. These may include thermal overloading, blow-by, ring-collapses and heat developed by normal friction / scuffing. The liner remains as stable for reasonably long periods, it may be logically concluded that the cylinder is working in-order.
In order to assess the temperature of cylinder liners, thermocouples are fitted on to the cylinder wall about 5 to 10 mm from the inner (i.e. combustion chamber surface) surface of the liner. This is fitted, as found suitable, between the 1st and 2nd piston rings, when the piston A at the 'TDC'. Usually 2- sensors are positioned diametrically opposite, on to the liner. In fact, since the gas blow-by are liable to happen locally, it may be advantageous to have more number of sensors, say 4, around the liner circumference.
Monitoring Thermal Load This is one of the most important conditions to be monitored for assessing appropriate design and material considerations of a Diesel engine. Success in these aspects gives the shipboard engineer considerable peace in operation. The basic purpose is to measure the temperature which is representative of the thermal load of the engine. The thermal load of an engine is dependent on the engine-load, the fuel / fuel-system and miscellaneous external / ambient parameters. For this purpose, sensor(s) have to be positioned on the cylinder cover. Direct-temperature measurement, is a simple and precise method. Passages are drilled from the outside of the cylinder cover to a point which is 7 mm from the surface exposed to the combustion chamber. Temperature probes are positioned in these holes from the outside. In this method, defective probes are easily replaced from outside.
Complete Survey requirement
The survey requirement for inspection of the propeller, stern tube bearing and tail shafting is:
1. The tail shaft is fitted with either continuous liners, approved oil sealing glands or made of corrosion-resistant material, the survey will be carried out:
• 3 years for single shafting arrangements
• 4 years for multi-shafting arrangements
2. The survey interval may be increased to 5 years in the following categories:
• The propeller is fitted keyless to the shaft taper, the shaft is protected from seawater, the design details are approved and a non-destructive examination of the forward part of the aft shaft taper is performed at each survey by an approved crack-detection method.
• The propeller is fitted to a keyed shaft taper, the design details of which comply with the applicable requirements, and a non-destructive examination of the aft end of the cylindrical part of the shaft i.e from the aft end of the liner, and of about one-third of the length of the taper from the large end is performed at each survey by an approved crack-detection method.
• The propeller is fitted to a solid flange coupling at the aft end of the shaft, the shaft and its fittings are not exposed to corrosion, the design details are approved and a non-destructive examination of the aft flange fillet area of the shaft is performed at each survey by an approved crack-detection method.
3. For other categories, the interval of inspection for the propeller shaft opened up surveys is two years and six months (2.5 years).
Modified Survey requirement
• A modified survey of the tail shaft is an alternate way of examination whose scope is given above. It may be accepted for tail shafts provided that:
• They are fitted with oil-lubricated bearings and approved oil sealing glands
• The shaft and its fittings are not exposed to corrosion
• The design details are approved • The clearances of the aft bearings are found to be in order
• The oil and the oil sealing arrangements prove effective
• Lubricating oil analyses are carried out regularly at intervals not exceeding six months and oil consumption is recorded at the same intervals.
The modified survey is to be carried out five years from the last complete survey with a window period of plus or minus six months.
The next complete survey is to be carried out ten years from the last complete survey.
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