vibration characterstics

7/28/2019 Vibration Characterstics

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This paper deals with the current trends in system design and technologies forelectrical systems of large thermalpower plants. Various new systems and technologies which bring about a markedincrease in reliability and maintainability ofthe electrical systems are presented.

Increase in main power plant capacity requires correspondingly larger and moresophisticated auxiliaries / sub-systems. The electrical systems of such large modernpower stations have to possess a high level of reliability for satisfactoryoperation, under both normal and abnormal operating conditions. Emphasis on need forminimum maintenance arises from the necessity of minimizing downtime. Whileinclination isalways there to use only proven products, total insistence on not using new productsand systems would be illogical since rapid growth in technology and knowledge aremaking new, reliable and efficient products and systems available to designers.

However, the stakes involved necessitate a careful evaluation of the new productsand new systems to be in introduced, in order toensure an increase in reliability and maintainability of the electrical systems.This paper presents the trends in systems design and application of new technologies

in the major electrical areas of large thermal power plants.

GENERATOR MAIN CONNECTIONSYSTEMSApplication of Generator Circuit Breaker

The recent trends in thermal power stations having large generating units is tointroduce the generator circuit breaker in the generator main connection, in placeof conventional unit connections with separate start-up transformers. Theadvancement in circuit breaker technology using SF 6 has recently made availablecircuit breakers of higher current and fault level ratings, suitable for applicationas generator circuit breakerat lower costs.

Utility Thermal Power Plant

In a unit connected utility thermal power plant, application of a generator circuitbreaker results in many advantages.GCB scheme eliminates station transformers and associated switchyard equipment,reduces a number of auxiliarybuses and interconnection, and simplifies interlock system Simplifies operations asswitching of power supply from Station to unit sourceduring start up and in reverse during shut down are no more needed As it is dealingwith only one voltagesystem, motors are not subjected to stresses during bus transfer.In a scheme withoutgenerator circuit breaker, the transformer faults continue to be fed by thegenerator until the magnetic field decreases with the de-excitation, which may lastupto 20s depending on theexcitation system and fault location. A generator circuit breaker will interrupt thecurrent within less than 100ms, thus preventing possibility of heavy damages andreduction in lifetime of the transformers.

Layout Aspects

Tubular enclosure for generator circuit breaker matching with the isolated phasebusduct run is already in vogue. Presentlyintegrated version of GCBs are available wherein in addition to the basic CircuitBreaker, all other components of Generator.

Combined Cycle Power Plant

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UntitledCombined Cycle Power Plants ( CCPPs), having combination of gas turbine and steamturbine, benefit from the introductionof generator circuit breaker. Generator circuit breaker helps in eliminating theunit auxiliary transformer and station transformer for thesteam turbo-generator, besides offering the other advantages discussed above.Generator Circuit Breaker also facilitates the starting of the gas turbine generatorby using Static Frequency Converter (SFC). When the gas-turbine is started , thegenerator circuit breaker is kept openinitially , and the static frequency converter is connected to generator busduct.Then the frequency converter together with the static excitation is Switched ON.When the turbine has reached approximately 70 % of its synchronous speed, the SFCand the static excitation are switched off and the turbine accelerate itself to thesynchronous speed. Above 90% of the rated speed, the no-load excitation is released.

Multi-Unit Cogeneration Power Plant

In industries cogeneration method is employed to meet the power as well as steamrequirements for various processes. To have highly reliable and continuous supply, amulti unit system with provision for parallel operation of source is adopted. Theuse of generator circuit breaker in thegenerator main connection, alongwith similar circuit breakers, as bus couplers andtransformers incomers, provides considerable flexibility and reliability if

operation. Cubicle type version of GCB are now available to meet the industrialneeds.

Economic Considerations

While comparing the costs of the start-up Transformers scheme with the generatorcircuit breaker scheme, a number of factorsare to be considered: EHV switchyard, startup transformer, stand by source, increasein the rating of the UAT, generators circuitbreaker , auto changeover scheme, erection and commissioning etc. The introductionof this scheme in power station in India is only a very recent phenomenon, onaccount of the economic consideration . For large sets of 500 MW rating and above,the application of generator circuit breaker is preferably in view of the manyadvantages discussed above. The availability of the generator circuit breaker withSF6 technology for lower rating could make the scheme cost-effective for power unitsof 100 to 210 MW ratings as well.

GENERATOR BUS DUCT

Generator main connections with isolated phase Busducts is a standard practice forunit sizes of 60 MW and above. The practicediffers, however, in methods of maintaining insulation resistance value in thebusducts. Hot air blowing system for smaller ratingsand pressurization of Busducts for higher size (210 MW and above) are almostuniformly accepted practice in India now.In pressurized Busducts, dry and clean air is maintained at a slight over-pressureabove atmosphere (25 to 40 mm of watercolumn) inside the busduct during normal operation of generator. To allow forinevitable leakage of air through joints,inspection cover etc, pressure switches to automatically open and close the airsupply, are provided. For 500 MW generators, natural cooled busducts have been used

in India while in some of the countries, forced air cooled busducts have been used.However , for still higher ratings , forced air cooled busduct, are inevitable.Forced air cooled busduct do not need a separate equipment for maintaining theinsulation resistance value. Experience in power stations in India shows thatpressurized Busducts have been quite successful.

PROTECTION SCHEMEGenerator Protection scheme

In order to achieve maximum utilization of the installed capacity, the generator

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Untitledcircuit protection should be designed in such a waythat damage, when fault occurs, is kept to a minimum, and that machine is notsubjected to abnormal conditions which may behazardous. At the same time, it is important not to overprotect the machine andcause avoidable outage.

Principle of Duplication

The present trend in respect of protection of philosophy for large-size units (500MW and above) is to adopt theprinciple of duplication, employing duplicate protection . Numerical protections arenow preferred. It is necessary to have independent cabling, auxiliary supplies,breaker trip coils for these two groups. Thus, more or less 100% redundancy is builtinto the protectionsystem thereby achieving a higher level of reliability. It may be noted that whenone group is out of commissioning for anyreason whatsoever (like maintenance / servicing defects, non-availability ofauxiliary supply, testing) the other group would be in circuit and the system wouldstill be protected.

Automatic Testing Facility

Automatic testing has been increasingly used in place of manual secondary injectiontesting for relays for protection of largegenerators, in many parts of the world. The automatic testing facility tests all therelays of a group in sequence without requiring any attention from operators, andrelays need not be withdrawn. The testing unit measures the pick-up values and thetime delays, and compares them with stored values of the relay settings. Any valuesoutside the programmed permissible tolerances result in an alarm signal andindication on the paper print-out of the test results. With the aid of theadditional devices , it is also possible to transmit the test results over atelephone or telex line to be recorded or entered into a computer system at someremote point. The testing programs can be initiated by manual local command or byremote signal or by programming the built-in digital lock. If testing is started bythe clock, it is repeated at a set interval, the two groups of relays being testedalternatively. The main advantages of automatic testing,thus are :

Testing can be carried out while the relays are in services;Periodicity of testing can be increased without affecting overall availability.

Microprocessor-Based Protective Relaying

A significant factor in realizing a computer-based protection system was theintroduction of microprocessor in the early1970s. Conventional static relays have excellent track record as far as reliabilityand operating times are concerned. Microprocessors have several inherent advantageslike computing capability, programmability, small size, high reliability,improved maintenance and low cost. Apart from the above advantages, other functionsof importance to the user areaccuracy, selectivity, flexibility and userfriendliness. Self-monitoring is anotherimportant new functions which allows one toextend the intervals between manual functional checking. In a microprocessor based

motor protection relay, a single relay protects the motor against abnormalities likethermal overload, short circuit, earth fault, negative sequence and locked rotor.All current measurement is RMS-based and , hence, take harmonics into consideration.The overload protection prevents overheating ofthe motor while running. To achieve this function, the relay characteristics is madetobe a replica of the thermal model of the motor. The thermal characteristics of therelay can be programmed to match themotor thermal characteristics. This means that a single relay can be interchangeablyused between different motors. Thus, no longer one needs to stock one relay for

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Untitledevery motor as a spare. A substantial reduction in inventory is foreseeing as a bigadvantage.Advancement in microprocessor technology has led to the evolution of Numericalprotection. The protection and control functions are integrated in these relays.Further communication ports are inbuilt to effectively integrate with the plantcontrol system architecture.

GENERATOR EXCITATION SYSTEM

The present trend is to use static as well as brushless excitation systems for allutility thermal sets of ratings 30 MW and above.With design of large-sized generating units having lower inertia and higher per unitreactance, the job of a designing a reliable and stable power system becomes moredifficult . Various methods, including generator excitation parameter control, havebeen adopted by system engineers to improve system reliability and operation.System studies are carried out to verify the suitability of the excitation systemparameters which are significant for powerstations, like responses time, ceiling voltage , loop gain and power systemstabilizer parameters. The optimum choice of the type andparameters of the excitation system of a new generating unit/ proposed addition toan existing power station is absolutely necessary to ensure its satisfactoryperformance under normal and abnormal conditions of the system. Computer programs

are available to study the excitation system parameters, using load flow studies,steady state stability and transient state stability studies. For the above studies,the whole system(grid) needs to be represented in full details, and the study is carried out for thecomputer grid. The computational requirements would be enormously high if the entiresystem is to be represented in all its details. Further, it calls for obtaining andprocessing the complete data of the grid. Currently, reduced order representation ofthe external system using model analysis, such as coherency grouping of generators,is being used in which the proposed generating station and all other generatingstations in the vicinity of it are represented individually in detail alongwiththeir excitation systems. The rest of the generating station systems are groupedinto one or two equivalent machines. Importance of such studies is beingincreasingly, and it is expected that in future it will be become a standardpractice to carry out these studies at the time ofplanning the power plant, or at least prior to ordering out the TG units.

AC AUXILIARY SYSTEM

Main components of AC Auxiliary Systems are MV Switchgear, LV Switchgear andauxiliary service transformers. AC auxiliary design has been given a new dimensionby development and acceptance of generator circuit breaker and variable speed drivesystems. Use of start-uptransformers has been in vogue for many decades, but the concept of start-uptransformers is changing with the introduction of generator circuit breaker. Threewinding transformer are being increasingly employed to reduce fault level. Two levelvoltage system is standard for units upto 210 /250 MW and 500 MW ratings withturbine driven boiler feed pumps. Three-level voltage system is being adopted for500 MW and higher sets, where the boiler feed pumps are motor-driven and of highratings.

Medium Voltage (MV) Circuit Breaker

Steady progress in equipment design and technology has resulted in evolution ofcircuit breakers having superior performance, compact size and reliability . Formedium voltage (3-36kV) systems, the recently reported statistics indicate that,world over, the conventional types like oiland air circuit breakers are totally replaced by the two well establishedtechnologies i.e. vacuum and SF6. Initial apprehensions user had on these twotechnologies are no more there. Development has also taken place in the field ofswitchgear with vacuum bottles in SF6 insulated metal-clad enclosures. Suchswitchgears are now increasingly used in other countries.

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Dry-Type Transformers

In the case of service transformers for stepping down from 11kV or 6.6 kV to 415 V,the recent trend is to specify indoor, dry type design, as this results in compactlayout, avoid length cabling and eliminates fire hazards. Dry-type transformersarrangement is also comparable in cost, as the higher cost of this type oftransformer is offset by reduced length of cabling from / to HT LT switchboards. Theconventional oilfilled transformers must be located outdoors, in transformer yard orat the end of the HT/LT switchgear room with attendant problems of providingadequate fire protection system and routing the cables/busducts. Dry typetransformers, on the other hand, can be located right by sideof HT/LT switchboards, resulting in a neater layout.Cast resin dry type transformers are becoming popular and are replacing theconventional resin-impregnated dry-type transformers due to major advantages suchmoisture-proofness, as the primary and secondary weldings are totally encapsulatedin epoxy resin, improved impulse voltage withstand capability, elimination ofpredrying requirement, freedom from partial discharges, zero maintenance and compactdesign.

Variable Speed Drive Application

A significant challenge facing the electric utility industry today is to generateelectricity in a highly reliable, cost-effective andefficient manner. Recent trend in thermal power stations is towards introduction ofvariable-speed drives for speed control of fan and boilerfeed pump. In the existing practice, speed control of electrically driven boilerfeed pumps and fans is achieved with the use ofhydraulic coupling. The scheme uses the principle of slip regulation, resulting incoupling losses which are removed by a heatexchanger. Additionally, gear boxes are required in the case of boiler feed pumpsdesignated for super synchronous speed.The static speed control methods (variable-speed drives) for such large ratingsemploy a converter-fed synchronous motor.The scheme basically comprises two converters: one on the line side and another onthe machine side. The converter on theline side functions as rectifier and that on the machine side as an inverter feedingthe synchronous motor with variable voltage and

variable frequency, thereby controlling the speed. The high speed motor designinvolves special features to match the staticcontrol system; hence, the motor may get considered more as a part of the totalregulator package than as an associate of the driven equipment. Variable speeddrivers are getting introduced due to several advantages they have over thecurrentlyprevailing technology of hydraulic coupling and fixed speed motor drives: such assmooth control of air and water flow over awider range and absence of limitation on the number of starts, elimination ofvoltage dips in the system due to direct on-linestarting of large-size machines, increased efficiency over the wide operating speedrange, increased life of motor due to softstart,very neat arrangement without any necessity for large cooling equipment forhydraulic coupling, reduction in size of unit/station transformer rating, reductionin switchgear fault level, reduction in the cable size, elimination of requirement

of heavy foundation, totally static (hence less maintenance ) and possibility ofintegration into total automation of power plant. With the variable speed drives, itis possible to maintain the efficiency above 90% for a wide range. As the driverating is decided by test block conditions (normally 10 to 15% higher than testblock rating isselected). It turns out that the MCR point in most of the cases lies at around 60%of the drive rating. The operating point varies, depending on the loading of theboiler, however, experience tells that most of the time the drive has to operate ataround 90% of the MCR point. In comparison, at this operating point, theconventional system using squirrel cage motor and hydraulic coupling offers an

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Untitledefficiency around 67 to 70%. Thus, huge amount of energy saving accrues, andconsequently the pay back periods are very short , in the case of variable-speeddrives. A techno-economic study based on field measurement for a BHEL 210 MW unit IDfan installation reveals that with the use of variable speed drives, energy savingto the extent of 4 million units per boiler can be achieved in a year. In India, thetrends is to use variable-speed drives for ID fan applications for 210 MW and 500 MWsets. It is expected that variable-speed drives for boiler feed pump applicationswill also bewidely adopted in the next 3 to 4 years. Due to energy saving considerations, thereplacement of the existing hydraulic controls by variable speed drives is envisagedas a future trend.

Microprocessor Based Contactor ControlUnits for LV Switchgear

Recently, microprocessor-based contactor control units are also being introduced inthe motor control centres. In the existing practices, individual motor controlcentres are equipped with a multitude of different function-oriented units to beselected on the basis of each driverating and characteristics, resulting in considerable amount of wiring andinventories. In comparison, the microprocessor based contactor control unit providesa multi purpose unit independent of the drive rating and characteristics. The unit

has also self-diagnostic capability integrated within. It is expected thatmicroprocessor based contactor control unit will be replacing the conventionalsystem in the nineties.

Moulded Case Circuit Breaker

The rapid technological advances being made in the design of low voltage switchgearindicate that the moulded case circuit breaker (MCCB) in the primary circuit designis going to be a future trend. This could ultimately replace not only the fuses infeeder and motor starter but also the air circuit breaker for all applicationsincluding incomers and high capacity low voltage motors. MCCB for motor applicationmay replace the contactor and bimetal relay.

High Speed Bus Transfer Scheme

Bus transfer scheme is employed for the purpose of changeover of supplies to

auxiliaries from station transformer to unit transformer during start up and viceversa during shut down. The large number of auxiliaries with high rating requires ahigh speed of changeover to ensure continued boiler operation. Such momentarychangeovers could harm the drive motors if the resultant voltage between the twosystems at the point of changeover is not kept within limits. Now, schemes areavailable wherein the phase angle between the bus voltage and the incoming supply iscontinuously monitored and if the angle exceeds a set limit the transfer is blocked,thereby preventing excessive voltage andtorque on the motor and winding. The scheme involves a check synchronizing phasecomparison relay and could effect transfer within 4-6 cycles. This scheme is mostlyused for transfer of supplies between unit and station busbars However, it could bevery well used for changeover of supplies between the station buses as well, therebyavoiding the paralleling of the high capacity transformers.

ELECTRICAL SYSTEM INTERFACE DESIGN

WITH DISTRIBUTED DIGITAL CONTROL(DDC) SYSTEM

Where distributed digital controls are engineered in the electrical switchgear of apower station, layout schemes and cabling are required to be very closelycoordinated. Control interface such as drive level cards and the associated motorcontrol centres areto be located adjacent to each other, to the maximum extent possible if the cablingis to be reduced to the minimum. The scheme related to motor control centers are tobe engineered to derive the maximum benefits of DDC features, such as system testing

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Untitleddiscrepancy indication in the control room for local intervention. Since the DDCsystem operates on 24 V d.c. suitable interposing relays are to be used in the motorcontrolcentre with terminals separately identified. With the DDC system, it is alsopossible to integrate such function as restarting of essential motors followingmomentary supply failure. By coordinated design of the electrical system with theDDC system, it is possible to re-start motors in a sequence to avoid heavy inrushcurrents. With the availability of data acquisition system, it is now desirable toreduce, to a minimum , theannunciation facias and recorders required for electrical system monitoring, such astransformers and switchgear status and fault conditions, in the control panels asthe operator gets the same information through periodic logs and alarms on CRT. Onlya single feedback for drive status from the motor control centre is required toachieve both control and monitoring functions, thereby reducing the cabling.

D C SYSTEM

The DC system provides reliable supply for emergency drives, controls, lightingcommunication etc. It is the ultimate sourceof power supply for the controls, under both normal and abnormal conditions. Becauseof utmost importance of the DC system, adequate redundancy is still maintained whiledesigning this system despite the tremendous technological improvement effected in

battery and chargercomponents over the past decade. Battery, as a product, has undergone many changes.Lead-acid batteries (both plante and tubular type) of conventional design have beengenerally used in power plants in India. The latest trend is now to go in formaintenance-free type sealed lead-acid batteries because of many advantages offeredby such a design. While the conventional vented type stationary batteries requirewater replenishment due to water decomposition during charging, the newer typeincorporates maintenance free design which eliminates such troublesome \maintenanceas electrolyte-level check, water topping-up, specific gravity measurement andequalizing charge. The other features include absence of gas emission under normalfloating operation, excellent high rate discharge performance, low self discharge,easy handling and compactness in size. The present trend is also to use Nickel-Cadmium batteries for power plant application. The inherent characteristics of thenickel-cadmium couple, using alkaline electrolyte, result in specific advantagessuch as high reliability, exceptionally longlife, excellent high rate performance, simple maintenance, wider operating

temperature range, high rate charge acceptance, low installation costs and all-steelplate construction. These batteries are resistant to mechanical and electricalabuse. However, for the same ampere-hourrating, the new types of batteries definitely do not compare well with theconventional lead-acid batteries cost-wise. But for a given duty cycle, capacityrequirement of nickelcadmium batteries and maintenance free lead-acid batteries arelower, and considering the many other advantages these batteries offer, they arebecoming increasingly popular amongst power plant owners.

PLANT COMMUNICATION SYSTEM

The present practice is to equip the plant with distributed type of Public Address(PA) System and Electronic Private Branch Exchange. The latest trend is to usemicroprocessor based electronic telephonic exchanges which improves the performanceand reliability of the communication system. Another trend which is catching up

fast, is microprocessor-based communication system which integrates the functions ofboth PA system and telephone exchange besides offering many other innovativefacilities made possible by this sophisticated technology. Radio paging system maybecome a trend in the future in power plants in India.

CABLES / CABLING SYSTEMCables

The performance of electrical cables in fire situations is becoming more importantconsidering the loss incurring due to cable

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Untitledfires in large thermal power stations. Investigations and studies done by variousconsultants and users reveal that fire rarely starts from cables due to internaloverheating, as adequate design margins are provided. But the cable insulation andsheath help in propagating the fire particularly at places where cable density ishigh and also generate high corrosive and toxic gases leading to wide-spread damage.With the development of new insulating compounds with superior flame retardant andlow smoke generation properties, the followingtwo type of cables are now being used in thermal power stations in IndiaI. Flame retardant lowsmoke (FRLS) cables.ii. Fire survival (FS) cables rated for 750 Deg C and 3 hours.The economic constraints restrict the use of FS cables to only certain vitalapplications such as DC system, essential tripping circuits andemergency circuits. For all other areas, FRLS cables are used.

Fire Stop/ Fire Seal system

In order to control the propagation of fire and spread of toxic smoke, various cablepenetrations need to be sealed by fire seal systems/fire stops.The desirable characteristics of a firestop/fire seal system are :

Flameproof property ( shall have at least two hours fire rating)

Retention of stability and integrity after application of water jet on theexposed side in order to extinguish fire ;No effect on the current carrying capacity of the cables passing through the

penetration seal.Providing firm grip on the outer surface if the cables, in the event of fire,

and making the system smoke and gas-tight and free from shrinkage or cracking.;Easy to install and mechanically strongAnti-rodent propertiesExcellent dielectric and weather resistant properties

Types of Fire Sealing System

CHARACTERISTICS OF VIBRATIONVibration is simply defined as "the cyclic or oscillating motion of a machine ormachine component fromits position of rest or its 'neutral' position."

Whenever vibration occurs, there are actually four (4) forces involved thatdetermine the characteristics ofthe vibration. These forces are:l. The exciting force, such as unbalance or misalignment.2. The mass of the vibrating system, denoted by the symbol (M).3. The stiffness of the vibrating system, denoted by the symbol (K).4. The damping characteristics of the vibrating system, denoted by the symbol (C).The exciting force is trying to cause vibration, whereas the stiffness, mass anddamping forces are trying tooppose the exciting force and control or minimize the vibration.Perhaps the simplest and easiest way to demonstrate and explain vibration and itsmeasurablecharacteristics is to follow the motion of a weight suspended by a spring. This is avalid analogy since allmachines and their components have weight (mass), spring-like properties (stiffness)

and damping.The motion of the mass from top to bottom range and back to the initial startingposition in the verticaldirection is referred to as one cycle, and it has all the characteristics needed todefine the vibration.Continued motion of the spring-mass system will simply be repeating these measurablecharacteristics.page 9The characteristics needed to define the vibration include:Frequency

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UntitledDisplacementVelocityAccelerationPhaseVibration FrequencyThe amount of time required to complete one full cycle of the vibration is calledthe period of the vibration.If, for example, the machine completes one full cycle of vibration in 1/60th of asecond, the period ofvibration is said to be 1/60th of a second.Although the period of the vibration is a simple and meaningful characteristic, acharacteristic of equalsimplicity but more meaningful is the vibration frequency.Vibration frequency is simply a measure of the number of complete cycles that occurin a specified periodof time such as "cycles-per-second" (CPS) or "cycles-per-minute" (CPM). Frequency isrelated to theperiod of vibration by this simple formula:Frequency = 1/PeriodIn other words, the frequency of a vibration is simply the "inverse" of the periodof the vibration. Thus, ifthe period or time required to complete once cycle is 1/60th of a second, then the

frequency of the vibrationwould be 60 cycles-per-second or 60 CPS.In the real world of vibration detection and analysis, it is not necessary todetermine the frequency ofvibration by observing the vibration time waveform, noting the period of thevibration and then taking andcalculating the inverse of the period to find the frequency - although this can bedone. Nearly all moderndaydata collector instruments and vibration analyzers provide a direct readout of thevibration frequenciesbeing generated by the machine.Although vibration frequency may be expressed in cycles per second or CPS, thecommon practice is touse the term Hertz (abbreviated Hz) in lieu of CPS. This is in honor of HeinrichRudolf Hertz, a 19thcentury German physicist who is credited with discovering electromagnetic radiation.

Thus, a vibrationwith a frequency of 60 CPS would actually be expressed as 60 Hz.Although vibration frequency can be measured and expressed in Hertz (Hz), for mostmachinery vibrationwork, vibration frequency is measured in cycles-per-minute, abbreviated CPM.Expressing vibrationfrequency in terms of CPM makes it much easier to relate this characteristic to therotational speed of themachine that is normally expressed in revolutions- per-minute or RPM. Thus, if amachine operates at3600 RPM, it is much more meaningful to know that a vibration occurs at 3600 CPM (1x RPM) than 60Hz.page 10Of course CPM and Hz can be easily converted to one another as follows:

Given a frequency expressed in Hz, you can convert it to CPM:CPM = Hertz x 60 Seconds/MinuteGiven a frequency expressed in CPM, you can convert it to Hz:Hertz = CPM/60 Seconds/MinuteSignificance of Vibration FrequencyThere are literally hundreds of specific mechanical and operational problems thatcan cause a machine toexhibit excessive vibration. Obviously, when a vibration problem exists, a detailedanalysis of thevibration should be performed to identify or pinpoint the specific cause. This is

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Untitledwhere knowing thefrequency of vibration is most important. Vibration frequency is a very valuableanalysis or diagnostictool.The forces that cause vibration are usually generated through the rotating motion ofthe machine's parts.Because these forces change in direction or amplitude according to the rotationalspeed (RPM) of themachine components, it follows that most vibration problems will have frequenciesthat are directly relatedto the rotational speeds.To illustrate the importance of vibration frequency, assume that a machine,consisting of a fan operating at2400 RPM and belt driven by a motor operating at 3600 RPM, is vibrating excessivelyat a measuredfrequency of 2400 CPM (1 x fan RPM), this clearly indicates that the fan is thesource of the vibration andnot the motor or belts. Knowing this simple fact has eliminated literally hundredsof other possible causesof vibration. Typical 1 x RPM vibration can be attributed to:UnbalanceEccentric Pulley

MisalignmentBent shaftLoosenessDistortion - soft feet or piping strainBad Belts - if belt RPMResonanceReciprocating forcesElectrical problemsDetermining that the frequency of excessive vibration is 2400 CPM (1 x fan RPM) hasreduced the numberof possible causes from literally hundreds to only ten (10) possible causes.A little common sense can reduce this number of possible causes even further. First,since the vibrationfrequency is NOT related to the rotating speed (RPM) of the drive belts, beltproblems can be eliminated asa possible cause. Secondly, since this is not a reciprocating machine such as

reciprocating compressor orengine, the possibility of reciprocating forces can be eliminated from the remaininglist. Finally, since thefrequency is not related to the drive motor in any way, the possibility ofelectrical problems can beeliminated. Now, the number of possible causes of excessive vibration has beenreduced to only seven (7)by simply knowing that the vibration frequency is 1 x RPM of the fan.page 11Vibration analysis is truly a process of elimination. Additional tests andmeasurements can be taken tofurther reduce the number of possible causes of a vibration problem. However, itshould be obvious thatknowing the frequency of vibration and how the frequency relates to the rotatingspeed of the machine

components is truly the first step in the analysis process.Of course, not all machinery problems will generate vibration at a frequency equalto the rotating speed (1x RPM) of the machine. Some problems such as looseness, misalignment, resonance andreciprocatingforces can often generate vibration at frequencies of 2x, 3x and sometimes highermultiples of RPM.Problems with gears usually result in vibration at frequencies related to the "gearmesh" frequency or theproduct of the number of teeth on the gear multiplied by the gear RPM. Aerodynamic

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Untitledand hydraulicproblems with fans and pumps will normally show vibration frequencies that are theproduct of themachine RPM times the number of fan blades or impeller vanes. In addition, not allproblems will result invibration frequencies that are directly related to the rotating speed of themachine. The vibrationfrequencies generated due to flaws or defects in rolling-element bearings is a goodexample.In summary, it is important to realize that different machinery problems causedifferent frequencies ofvibration and that is the significance of knowing the frequency of vibrationVibration AmplitudeAs mentioned earlier, vibration frequency is a diagnostic tool, needed to helpidentify or pinpoint specificmechanical or operational problems. Whether or not a vibration frequency analysis isnecessary, dependson how "rough" the machine is shaking. If the machine is operating smoothly, knowingthe frequency orfrequencies of vibration present is not important. The magnitude of vibration or howrough or smooth themachine vibration is, is expressed by its vibration amplitude. Vibration amplitude

can be measured andexpressed as:DisplacementVelocityAccelerationThe following paragraphs describe each of these units of vibration amplitude, theirsignificance andapplications.Vibration DisplacementThe vibration displacement is simply the total distance traveled by the vibratingpart from one extremelimit of travel to the other extreme limit of travel. This distance is also calledthe "peak-to-peakdisplacement".Peak-to-peak vibration displacement is normally measured in units called mils, whereone mil equals onethousandth

of an inch (1 mil = 0.001 inch). A measured vibration amplitude of 10 mils simplymeans thatthe machine is vibrating a total distance of 0.010 inches peak-to-peak.In Metric units, the peak-to-peak vibration displacement is expressed in micrometers(sometimes calledmicrons), where one micrometer equals one-thousandth of a millimeter (1 micrometer =0.001 millimeter).page 12Electronic instruments for measuring vibration on industrial rotating machinery didnot become readilyavailable until the late 1940's and early 1950's, although the significance andimportance of measuringvibration as an indicator of machinery condition had been well known for decades.Until the introductionof electronic instruments, instruments used to physically measure a machine's

vibration were mechanicaldevices such as seismically-mounted dial indicators, light-beam vibrometers andmechanical linkagedevices that magnified the relatively small amplitudes of machinery vibration tolevels that could bevisually observed. Obviously, with these mechanical devices, the only parameter ofvibration that could bemeasured was the peak-to-peak displacement. As a result, the first guidelines andacceptance standards formachinery vibration were given in units of vibration displacement. A vibration

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Untitledseverity chart based ondisplacement first appeared in an article by Mr. T. C. Rathbone entitled "VibrationTolerances" in theNovember 1939 issue of Power Plant Engineering. While the chart was the first of itskind, it was limitedin its consideration of frequency of vibration.The fact that the severity of a vibration depends not only on displacement butfrequency as well isunderstandable when one realizes that the vast majority of machinery failures causedby problems thatgenerate vibration are FATIGUE problems. To illustrate, consider what happens when apiece of wire isrepeatedly bent back and forth. This repeated bending eventually causes the wire tobreak due to fatigue inthe area of the bend. In many respects, this is exactly the way a machine componentfails - from therepeated cycles of flexing caused by excessive vibratory forces.Considering the example of repeatedly bending a piece of wire, there are two ways toreduce the amount oftime required to achieve fatigue failure. One is to increase the distance(displacement) that the wire is bent.The farther the wire is bent each time, the less time it will take to reach fatigue.

The other is to increase thenumber of times per minute or second (frequency) the wire is bent. The more timesper minute the wire isflexed, the less time it will take to reach fatigue failure. Thus, the severity ofvibration is dependent onboth vibration displacement and frequency.The Problem With DisplacementAlthough measurements of vibration displacement have been used for many years toevaluate machinerycondition, the fact that it is necessary to know the frequency as well, makes theuse of displacementsomewhat cumbersome when dealing with a vibration predictive maintenance programthat may includevirtually hundreds of machines and literally thousands of measurements.. Inaddition, it has already beenshown that machinery vibration is not always simple or occurring at only one

frequency. In many cases,machinery vibration will be complex, consisting of many frequencies. In such cases,it is nearly impossibleto use vibration displacement to judge the "overall" condition of a machine. It mustbe remembered thateach source of vibration contributes to the ultimate fatigue of machine components,and the "overall"condition of the machine can only be determined by an overall measurement ofvibration that takes intoaccount all frequencies of vibration. This is accomplished by measuring VIBRATIONVELOCITY

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vibration characterstics

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