LESSON
8
TURBINE SUPERVISORY INSTRUMENTATION
LECTURE SUB - OBJECTIVE At the end of the lesson the Tranee !ll "e a"le to de#onstrate an $nderstand the T$r"ne S$%er&sor' Instr$#entaton(
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TURB TURBIN INE E SUP SUPER ERV VISOR ISORY Y INST INSTRU RUME MENT NTA ATION ION Turbine supervisory instruments are used to monitory the operation of the turbine. To use this instrumentation, it is necessary to understand what the instrument is recording so that proper interpretation of the charts and graphs will result. Rega Regard rdle less ss of the the size size of the the turb turbin ine e unit unit,, the the basi basic c idea ideas s unde underl rlyi ying ng turb turbin ine e instrumentation are the same. or purposes of clarity, clarity, the e!amples e!amples discussed here will be ta"en from smaller units #up to $% &' range(.
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The following systems are used for monitoring6 5. 7. 8. 9. :. $. ;.
+ibration hell E!pansion *ifferential E!pansion hell temperature hell 3ressures +alve 3osition Thrust 3osition
VIBRATION )ll rotational euniform heat transfer into the rotor periphery, uneven removal of the heat from windings, and so forth. 0t is important to design turbine casings so that temperature distortion will not occur, and pac"ings with sufficient clearance to minimize the rubbing interference which some times causes very large unbalances to develop in a short period of time. 0t is e
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&inute changes in balance due to e!pansion and contraction of the parts are displayed with great amplification as running vibrations that are easily measured with modern instruments. 0n general, changes in vibration amplitude over the operating ranges should not be considered abnormal when these changes are less than about :% percent of the low base or residual value of vibration. RUBBIN5 There is seldom any value in attempting to balance a unit which is rubbing. There have been many cases where a great deal of effort has been wasted in trying to balance where this was the predominant difficulty. Rubbing by pac"ing or defectors causes localized heating on the shaft surface. Circumferential temperature gradients develop, because the rub is usually more severe on one side of the shaft and the rotor gradually bows toward the hot spot. Rubbing by spill strips on buc"et covers does not produce vibration symptoms, although it can be very destructive. MISALI5NMENT 1alancing is of little value in reducing vibration which is primarily caused by misalignment. &isalignment should be suspected as a cause when there is evidence of oil whipping, vibration instability, apparent change in the critical speed range, unusual and e!ceptionally high critical speed vibration or where the critical speed vibration occurs over a very wide range, say over :%% rpm. &isalignment in itself, produces little vibration stimulus unless it is severe enough to unload bearings to the point of oil whip. 'hen this is the case very large shaft amplitudes will clearly identify the whip. &isalignment may significantly change the vibrational response characteristics to the e!isting unbalance stimulus. 0n some instances, for e!ample, the stationary vibration levels of the last turbine bearing have been mar"edly improved by small alignment changes to the generator which did not affect either the stimulus or the shaft vibration. OIL 64IPPIN5 -il whipping is caused by an unstable oil stimulus in the bearing oil film that develops as a comple! function of =ournal peripheral speed, oil viscosity, bearing shape, radial bearing load, and =ournal attitude angle. &isalignment can contribute to whipping by changing bearing loading. Usually the oil whip will appear and disappear suddenly as operating conditions change. The resulting amplitude of shaft vibration is usually very high, 5% 5: mils, and most of the displacement will occur at near half running speed fre
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T4ERMAL INSTABILITY There are a large number of causes of thermal instability. )mong them are heat sensitivity of the shaft, water or oil in the shaft bore, uneven heat transfer between rotor parts, loose wheels or pin bushings, other loose or poorly fitted parts, un> symmetrical ventilation, and short>circuited turns in field coils. 0n general, it may be said that thermal instability is characterized by mar"ed changes in vibration as operating conditions vary. These vibrations will nearly always e!ist at running speed fregenerator rotors run above the first critical speed, and many run above the second critical ig. 2>;>7. &any times, of course, the rotors are so well balanced the critical speed is not readily noticeable. VIBRATION AS A )UNCTION O) /ISPLACEMENT VS( TIME Turbine vibration is measured in displacement against time as shown in ig. 2>;>8. 'here displacement is the distance between fre
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VIBRATION P4ASE AN5LE The phase angle relationship is the unbalance point on the rotor in reference to some spot on the rotor. 'e can put a magnetic plug or tee piece in the rotor and use an electronic probe to pic" this reference point up. The impulse caused by the plug passing under the probe can be aligned with the highest amplitude or displacement of the vibration probe to give us a phase angle. This phase angle will show the angle or distance in the time that the ?high spot@ and the plug location in the rotor have in relation to one another. ee fig. 2>;>9, which illustrates a phase angle of 7$9A #8$%A > B$A(. Loo"ing at the cross section of the rotor would provide the view shown in ig. 2>;>:.
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)*( 8-+-:( Cross-Setonal anal'ss( The phase angle of vibration meter is used to permit operators to easily monitor the phase angle as well as the amplitude #size( of vibration. uch information is particularly useful for plotting trends in balance and for a more complete understanding of vibration patterns. 0t also improves the accuracy and speed of any balance ad=ustments that must be made. The phase meter compare the phase of the vibration signals from various shaft pic"ups with the angular position of a pulse from the phase reference generator. The meter reads the specific angle between the ?positive direction crossing ? of the pulse and the vibration signals. The phase reference generator is a small magnetic pic"up usually mounted n the first or smallest coupling of the turbine generator unit. This magnetic pic"up produces a pulse each time a pro=ecting bolt on the coupling passes through the air gap. /igh spot readings on the rotor are thus referenced to this bolt. 1oth the phase meter and a filter transformer switch unit are usually located together ig. 2>;>$(. VIBRATION PIC;UPS +ibration pic"ups are made in two types, the shaft riding pic"up and the pro!imity #non>contact( probe. The shaft riding pic"up is shown in fig. 2>;>; as installed through the bearing cap to the turbine shaft. The function of the vibration pic"ups is to measure the magnitude of motion of the turbine shaft in a plane perpendicular to its a!is. +ibration is measured with a detector consisting of a wound coil, which is seismically suspended by a spring in a permanent magnetic field , as shown in ig. 2>;>;. 'hen vibration causes the core and permanent magnet assembly to vibrate, the suspended coil will not follow the vibration motion since it will stay motionless in spaceD thus an alternating current is generated and fed bac" to the recorder.
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The pro!imitor>type probe does not come in contact with the shaft. 0n this system a coil is e!cited near the shaft where the vibrations are to be measured. This probe uses eddy currents to measure the distance from the coil on the probe tip to the shaft surface. The coil in the probe has radio fre;>2. The distance between the probe and the shaft surface is directly proportional. The closer the shaft comes to the probe, the greater will be the loss of the radio fre
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This type of an arrangement can also be used to chec" a shafts eccentricity, thrust position, shaft position, rotor e!pansion, differential e!pansion and shell e!pansion all without physical contact between the reference parts. This ephaser@ and tie to the vibration amplitude recorded as the phase angle. 1entley eveda corporation manufactures Company. ig.2>;>2 shows the system parameters associated with this e
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VIBRATION LIMITS )s a general rule, balancing should be considered at the first opportunity if the vibration of a turbine generator e!ceeds the following limits, when it is operating under load and steady state conditions6 0n most cases, the balance can be considered satisfactory if the vibration is within the following limits6 ) unit have vibration amplitudes e
)*( 8-+-8( Pro>#tor T'%e &"raton /etetor( S4ELL - AN/ /I))ERENTIAL E?PANSION RECOR/IN5 E@UIPMENT This type of e;>B, is used to provide a periodic indication and recording of the e!pansion of the turbine shell, and a practically continuous indication and recording of the difference in e!pansion between the turbine shell and its shaft. The measurement of differential e!pansion not only provides an indication of the operating clearances between the rotating and stationary elements of the turbine, but also provides e!tremely valuable in formation for setting up proper starting and operating procedures. The shell>and differential e!pansion recording e
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The scale has one gree inde! and two red bands. The green inde! is placed on the scale at a point denoting the cold gap setting between the coil at the turbine end and special ring. The red bands are set at the points beyond which any further movement will result in a rubbing between the stationary and moving members of the turbine. The differential e!pansion detector consists of two pairs of stationary coils on laminated cores mounted on the turbine shell in such a position that the special ring moves between the coils. The difference in a!ial motion between the turbine shaft #on which special ring is mounted% and the turbine shell changes the air gap, thus varying the impedance of the coils. E?PANSION /ETECTOR CASE TO )OUN/ATION rom ig. 2>;>B, we can see that the turbine is anchored at the centerline of the condenser flow. The turbine is free to move longitudinal on the steel fle! legs in this case. 0n other e!pansions systems the front standard which carries the high pressure bearing may move on gibbed ways or sliding feet so that the e!pansion of the case in reference to the turbine base or foundation can be measured. -ne way of doing this involves a scale mounted in the foundation at the front standard and a pointer attached to the turbine case. The movement of the case due to e!pansion can then be measured, in inches. 0n another method, a Linear +ariable *ifferential Transformer #L+*T( is mounted on the foundation and the L+*T rod attached to the turbine case. The resulting output of the L+*T would then be transferred to a meter calibrated in inches. #ee ig. 2> ;>5%(. ome units have used straight mounted differential electrical coils with an armature mounted between them. The eddy currents are then read by meter similar to the action noted on the L+*T units.
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)*( 8-+-,,( /fferental E>%anson Meas$re#ent( ROTOR=CASE /I))ERENTIAL E?PANSION 'ith 0nstrumentation of the type shown in igure 2>;>55, two electrical coils are used. The movement detection device is secured to the turbine shell and the gap between a collar on the turbine shaft is measured electrically. This will show if we have a ?rotor long@ or ?rotor short@. These two terms are used to describe the difference between the rotor and the case. 'hen first started up, the rotor will ?see@ the steam first and e!pand
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The smaller area of the rotor is heated as before, and becomes shorter. The case with its mass will lose its heat more slowly. o we have the differential of these two heated elements to consider on startup and load change and shutdown. The operator should not where this difference is on his recorder. The recorder scale is mar"ed to indicate a safe operating range ig. 2>;>55. ECCENTRICITY The purpose of the eccentricity meter is to indicate the straightness of the shaft. This is measured by means of an air gap, usually located in the front of the turbine, ig. 2>;>57. )ny change in this air gap causes a change in the impedance in the pic"up coils. This is recorded on a meter that shows shaft deflection in thousandths of an inch. ) rotor with a .%%$ bow can cause centrifugal unbalance forces of over five or more tons depending on the mass and length of the rotor. haft deflection, or eccentricity, should be less than one thousandth of an inch. #.%%5( before rolling off turning gear. ig. 2>;>58 depicts the eccentricity detector mounted over the shaft.
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)*( 8-+-,( T'%al Eentrt' /etetor Arran*e#ent T4ERMOCOUPLES TURBINE METAL TEMPERATURE T4ERMOCUPLES To assist the operator in controlling shell metal and steam chest temperatures during starting and when applying or reducing load, thermocouples #TCs( are provided. The number and descriptive location of the thermocouples are noted on the turbine wiring diagram for the unit. ) multi>point printing type strip chart recorder for these thermocouples should be located so that it can be referred to during starting and loading of the turbine. The printing type recorder has the distinct advantage over a temperature indicator in that it shows both the difference in temperature across walls, and the change sin temperature which are producing the differentials. The information provided by the recorder is e!cellent for monitoring the boiler and turbine operation to ensure that temperature limitations set forth in the starting and loading instructions are strictly observed. 2>;>59.
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)*( 8-+-,9( T'%al M$lt-%ont Str% Chart Reorder( BEARIN5 METAL T4ERMOCOUPLES 0t has become increasingly evident that bearing metal thermocouples provide a valuable diagnostic tool in evaluating thrust bearing and =ournal performance. Thermocouples are typically provided in the following locations6 5.
Thrust 1earings 5. 7.
7.
&ain Hournal 1earings 5. 7.
8.
3ivoting show type G two TCs per end. Tapered land type G Two TCs per plate
Tilting pad type G -ne TC furnished per bearing. Elliptical bearings G -ne TC furnished per bearing.
-il *rains 5. 7. 8. 9.
-ne TC per main bearing drain. -ne TC per thrust bearing drain. -ne TC per )lterre! bearing drain. -ne TC per 13 bearing drain.
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or all locations #especially for thrust bearings( it is recommended that the bearing metal temperatures be continually monitored on a recorder in order to detect significant temperature changes or trends that may develop. The thermocouples in the thrust bearings are more sensitive to thrust load than is the bearing discharge oil, and they are useful in indicating any changes in thrust. The average of continuous periodic readings should be fairly constant at full load. ) gradual increase in temperature at the same load, over a period of time, may mean that the steam path has become dirty or damaged, or that the bearing has worn. -TE6 *uring initial operation of a new unit, a record should be made of the thrust metal temperature while there is a high load on the machine #preferably a full load(. These readings will serve as a reference point for comparison with future bearing temperatures. )ny subsereturn valves which fail to close or are warped, bac"ed>up water in drains due to plugging or a flooded drain manifold, un>drained pipes, etc. 0t can be seen from this that various comple! station piping systems are involved in the problem, ma"ing identification of the source a difficult tas". 0n order to aid in the detection and correction of the cause of this "ind of mishap, units have been provided with thermocouples intended to aid in the identification of the presence and possibly the source of the water. These inner surface metal thermocouples are arranged in pairs, top and bottom, and located in the various chambers of the outer shell. Under normal conditions, the top and bottom thermocouples will read appro!imately the same. /owever, if water enters, the lower half thermocouple will show a sudden trend downward, thereby creating a temperature differential between top and bottom. These thermocouple signals are recorded on multi>point strip recorders of the type described.
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E?4AUST 4OO/ OVER4EATIN5 PROTECTION *uring starting and low load operation, when the turbine stages remove only little energy from the steam, the e!haust steam is usually at a temperature that is too high for the last stages and the e!haust hood. 0n order to reduce this temperature, water sprays are installed =ust downstream of the last stage wheels. The supply of water to the spray nozzles is ta"en from the condensate from the discharge of the condensate pump at ;: psig minimum at turbine no load. The flow rate of water to the sprays is controlled by a pneumatic control valve that receives its demand signal from one or more pneumatic temperature transmitters located in the e!haust hood or hoods. This temperature control system will start opening the control valve at 57%A and have it fully open at 52%A. ) bypass valve is also provided in case the automatic valve fails to function properly. ) compound pressure gage is used to monitor the water ahead of the spray nozzles. 0t is recommended that the control drawing be chec"ed for the proper rate for the unit of interest. E?4AUST 4OO/ T4ERMOSTATES 0n case the water spray is not capable of holding the e!haust temperature below 5;:A, e!haust thermostats #one in each e!haust hood( will typically close a contact and sound an alarm. ) second set of contacts #set to close at 77:A( should also be present to activate the emergency trip system by energizing the master trip relay. E!act readings in each case should be ta"en from the control drawing for the turbine being chec"ed. )IRST-STA5E PRESSURE Each turbine is supplied with a first>stage pressure curve. irst>stage pressure of a turbine is ta"en after the steam passes through the nozzle and the first stage wheel. #The area around the first>stage wheel is called first>stage pressure.( 0n most cases a turbine casing drain is supplied at this point also. The first stage pressure #3( can be used in operation in many ways. 0t is first an indication of the pressure load on the thrust bearing. 0t can also be used to control the steam producing unit, as the first>stage pressure is proportional to steam flow in pounds per hour versus the power or "ilowatts generated. irst stage pressure and steam flow are plotted in ig. 2>;>5:, which shows that load is directly proportional to steam inlet flow. The first stage pressure and flow charts are considered to be essential tools of the turbine operator.
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)*( 8-+-,:( Throttle )lo! Vs( 5enerator O$t%$t( VALVE POSITION IN/ICATORS 1y means of an L+*T #linear variable differential transformer( attached to the operating rods of the control valves and stop valve, a signal is returned to the E./.C. control board to indicate the position of the valves. ormal operation will soon indicate the proper setting of these valves for each normal load carried. 3ower for the feedbac" from the L+*T is supplied by 8>IC oscillators. CONCLUSION ormal operating records can be used by the operator to chec" normal operation and to note or compare any unusual conditions on the machine. imply recording the readout of the turbine and generator is not ma"ing an analysis of operationD only through an understanding of the tools of turbine supervisory instrumentation can operation be properly monitored. ) typical assortment of supervisory instruments deployed throughout a large unit is shown in ig. 2>;>5$.
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