VIBRATION ANALYSIS ATTHERMAL POWER PLANTSSHIVAJI CHOUDHURY
Objective To increase equipment protection; To improve safety for personnel; To improve maintenance procedures; To detect problems early; To avoid catastrophic failures; To extend equipment life; To enhance operations.
Vibration monitoring Vibration monitoring is the technology ofmeasuring vibration characteristics such asamplitude, frequency, and velocity atspecific locations (for example, bearinghousings) to identify abnormal conditionsor faulty components in rotating machinery. Vibration problems can originate fromdesign, installation, set up, in-service wear,or maintenance.
Vibration Analysis(frequency domain) A great deal of vibration analysis is done inthe frequency domain because the varioussources of vibration can usually be isolatedby the frequencies at which they occur. A single channel analysed in the frequencydomain gives a great deal of information,but often it is important to relate vibrationto a second channel as either a phase oramplitude reference, or both.
VIBRATION ANALYSIS(FFT) The basic technique for converting abroadband time trace to discrete frequencies,or frequency bands, is by the application ofthe Fourier transform (FT). This analysis can be realized with the help ofa computer and signal processing software,by special devices (which are usually called aFourier analyser), or by hardware microchips(DSPs). More commonly used in analysers now is amore efficient mathematical routine, the fastFourier transform (FFT).
4.ROTOR RUB Most commonly1X,but alsomultiples of 1X,subsynchronousfrequencies andnaturalfrequencies.
5.JOURNAL BEARING Vibration frequency-1X,2X,3X. Changes in bearingoperating condition orgeometry can causechanges in steady statevibration at 1X andhigher harmonics orcause subsynchronous(oil or steam whirl) ,inlatter case the vibrationis usually unsteady andcan increase with time,often rapidly.
5.1.Oil whirl( journal bearing) Its frequency hasbeen reported to beanywhere fromapproximately 42 toalmost 48 percent ofrpm. Simply view oil whirlvibration frequency as"slightly less than ½ xrpm.
6.GEAR Detection requirestransducers withhigh frequencyresponses. For defect in onetooth: 1x multiples. For worn teeth gearmesh frequency withsidebands andmultiples.
7.ELECTRIC PROBLEMS Stiffness dissymmetry (e g axial windingslot in generator/motor rotor): vibrationpeaks when 2X stimulus is coincident withrotor critical speed .compensating groovesare used as large machine to minimizestimulus. Thermal dissymmetry: can caused bynonuniform rotor ventilation or shortedelectrical winding or nonuniform tightnessof parts ,cause rotor to bow with the samevibration characteristic as unbalance.
8.BEARING Bearing misalignment :1X,2X,or highharmonics. Parallel or angular bearingmisalignment is generally caused byfoundation movement .bearingmisalignment is not direct cause ofvibration excitation but changes thedynamics characteristics of support system. Roller element bearing wear: widebandacceleration at high frequency.
9.Blade pass and vane passvibrations Blade pass or vanepass frequencies arecharacteristics ofpumps and fans. Blade pass frequency(BPF) = number ofblades (or vanes) xrpm This frequency isgenerated mainly dueto the gap problemsbetween the rotor andthe stator.
10.Cavitation in a pumpDue to cavitationin pump
Typical Vibration ComponentsMeasured in Large Centrifugal Pumps
VIBRATION ANALYSIS AT THERMALPOWER PLANT Vibration sensors have been routinely installed onmain turbines, generator and some large pumps tomonitor bearing vibration levels. Main Turbine is the heart of the power plant. Turbineis the most critical part of the plant & it is mandatoryto use maximum protections as well as on linemeasurements of different parameters to avoid anyunexpected failure/shutdown. Power plants have begun to use this technology as apredictive maintenance tool for identifying incipientfailures in many types of rotating equipment such asfans, pumps, and compressors.
Potential failure (P-F) curveVibrationmonitoringThe curve shows that as a failure starts manifesting, the equipmentdeteriorates to the point at which it can possibly be detected (P).If the failure is not detected and corrected, it continues until afailure occurs (F) and repair cost will increases .
VIBRATION ANALYSIS AND DIAGNOSTIC Online expert and diagnostic software are available in mostof thermal power plants, which can indicate machine faultin advance. This software can generates different kind ofplots like Orbit/lissgenus plot ,pattern/bode plot, niquistplot ,polar plot, vector plot ,waveform ,1X,2X,Harmonics . It can give diagnostic results for machine faults like: Unbalance, misalignment, critical speed, permanent bow. Lost rotor parts ,rotor crack, nonsymmetrical rotor. Gear inaccuracy Seal rub Oil whirl Oil whip Steam whirl cavitations
VIBRATION MONITORING OF CRITICALEQUIPMENTS OF POWER PLANTS Boiler feed pumps (TDBFP OR MDBFP) Condensate extraction pumps (CEP) Circulating water pumps Induced draft fans (ID Fan) Force draft fans (FD Fan) Primary air fan Raw water pumps Cooling water pumps Mill motors Coal crushers compressors
On line vibration measurementexample - PA Fanvibration
Vibration measurement monthlyschedule for critical equipmentsS N Equipment 1STWEEK 2NDWEEK 3RDWEEK 4THWEEK1 ID FANS Y2 FD FANS Y3 PA FANS Y Y4 MILL GEAR BOX Y Y5 SEAL AIR FANS Y Y6 BFP Y7 CEP Y8 CW PUMPS Y9 Other Pumps,fans ,compressorY