PLANT/EQUIPMENT RELIABILITY:

Vibration MeasurementOn Turbomachinery

Since most equipment failures occur on startup, provisions should be made to insure that vibrationmonitoring systems function during this critical period.

C. Jackson, Monsanto Co., Texas City, Tex.

ONE OF THE IMPORTANT SYMPTOMS OF Ahealthy centrifugal compressor train is the vibration ofthe main rotors. This vibration can be measured by manydifferent sensors using several different guidelines forseverity, but for workability, sensitivity, and overall ef-fectiveness, Monsanto uses the bearing-mounted proxim-ity probe more than any other protection monitoring sen-sor. Probes might be described as electronic eyeballswhich observe the movement of the rotor shaft with re-spect to its own journal bearings. The probes take manyshapes, as noted in Figure 1. Generally, they are either190- or 300 mils in dia. The manner of holding varies frommachine to machine. The clamped holder is preferred byMonsanto's Mechanical Technology Department ratherthan screw thread locking nuts which can damage the sen-sor if they are over-stretched by an installer.

There are several different kinds of probes; the mag-netic, the capacitive, the photo or light sensing, and theinductive or eddy current types. For the most part, we relyupon the eddy current type (1) since it is least affected byshaft material or the environment around the probe suchas oil, gases, or other liquids. The eddy current probe usesa pancake type coil imbedded in the tip. A high frequencyfield is excited in this coil which has a negative voltagebias, Figure 2. As the distance from the probe coil to theconductive shaft is changed, the output voltage from thedetector-oscillator circuit is also changed, as illustrated in

Figure 1. Proximity probes of various sizes.

Figure 3. This figure also illustrates how the sinusoidalmotion of a shaft in its synchronous whirl would be pro-jected through the calibration curve to a read-out gener-ally mounted several hundred feet away in a controlhouse.

Action pointsIn my opinion, it is important to have two relays or ac-

tion points in vibration measurement. One should be awarning and the other a shutdown point. If a rapid failureis imminent, the warning would allow a few precious sec-onds for operators to get to the board for a crash shut-down. Normally, the warning means that a level abovethe acceptable limit has been reached and the properpersonnel with the proper diagnostic equipment can becalled in to carefully scrutinize the information. In proc-essing the information, extra tools such as oscilloscopes,filters, spectrum analyzers, tapes, and real-time analyzerscan be used to develop a complete array of symptoms vs.causes. Sohre (2) has compiled the best single breakdownof the many vibration responses and their possiblesources, some of which are listed in Table 1.

For every symptom of a problem, there are probablythree exceptions to the rule. The more information andtime one can get to observe a malfunction, the better thediagnosis. Often the machinery will not be patient and aforced shutdown by the second alarm closes out the exam-ination, and rightfully so, if that limit has been properlyselected. Monitoring is not analyzing. Good monitoringwill provide good points and data for analyzing.

Some machines can be successfully analyzed at thebearing caps. Blake (3), Maten (4), and IRD (5) have pub-lished some good guidelines. These are for bearing capreadings and are not to be confused with shaft-to-bearingreadings.

Further, some machines are more sensitive than others,have more mass, and operate at many different speedswith different damping (6). Maten suggests a velocitylimit in in./sec. peak (o/p) for the equipment that hemeasures at Gulf in Canada. I find 0.2- to 0.4 in./sec.(o/p) a practical value before forcing most units off theline. These measures and values are not used when deal-ing with the large centrifugal compressor trains as usedin ammonia, methanol, ethylene, or other hydrocarbonplants. Other operational guidelines must be used forsuch equipment.

Pressure vibration curvesVibration is generally measured in terms of displace-

ment, velocity, or acceleration. In Figure 4 these terms

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are indicated in a sine wave of motion with respect totime. Note that the movement (shaft) at any time, t, andfrequency, co (radian sec.) is, disregarding any phase lag,x = A sin wt. Also, from any calculus or physics text thevelocity would be the first derivative, or x = Aw cos wt., orat the peak x = Aco. The acceleration would be the secondderivative, or x = —Aw2 sin wt., or x — uPx. Most reputa-ble instrument builders can convert any vibration signalwhether it be displacement, velocity, or acceleration, intoany other term by using either differentiating or integra-ting circuits. I have not mentioned that there is a velocity(seismic geophone) type sensor that gives velocity direct-ly, plus the piezoelectric crystal (or strain gauge) acceler-ometers which give "g's" direct. These two types have haddifficulty in the practical measurement of high speed cen-trifugal machinery or its drivers because of size, tempera-

POOR FOR ROTATING PARTS OTHER THAN SPEEDSIGNAL

GOOD FOR AIR/OIL

GOOD FOR AI R /OIL

POOR FOR TWO PHASE ENVIRONMENTS, HIGHESTACCURACY

ture, environment, or other factors. A nomograph, asshown in Figure 5, can be easily used to convert terms.

Sticking with the proximity probes and its circuits, wefind that there are several things to understand aboutthem. First, a material that varies greatly in its electricalproperties will cause an electrical run-out or variable sig-nal output as a shaft is rotated about a true center withthe probe in position sensing this shaft. A material thathas caused some concern is the precipitating hardeningstainless steels, e.g., 17-4 Ph. However, electrical run-outis generally negligible.

Since the surface of the shaft is being observed by theprobe which has a high frequency eddy field actually pen-etrating the shaft's surface, the smoothness of the shaft'ssurface and the mechanical run-out of the surface have abearing on the accuracy of measurement. The probe willsee discontinuities in the surface and will also read anysurface run-out. The rotor has been balanced with themechanical run-out present and, thus, this amount isreally not the vibration signal to be concerned with whenmeasuring a shaft in operation in its synchronous whirlwithin the bearing limits. The mechanical run-out ofmost centrifugal compressors and drivers will fall between

Table 1. Symptoms of, and possible contributors tovibration.

Possible ContributorsRotor unbalance

Figure 2. Displacement proximeters.

Frequency Other Indications1 x speed Phase is steady, increas-(synch) es with speed, clean

signal; no other multiplefrequencies indicated.

2 x speed Changes throughout the Misalignment or stickingday and night, changes couplingin magnitude; correlateswith high axial motionand thrust load or un-load; phase fixed orflipping 180°; orbit iselliptical with loop in orout.

0.4 to 0.45 X Very violent, but also Oil whirlspeed sporatic and unstable,

phase rolls, changeswith oil temperature(maybe); changes withload, dies and comesback; orbit makes loopswhich disappear andreturn.

1/2 speed Steady phase angle, Baseplate resonanceverfiele is more sensitive, (could be loose bearings)case and foot frequency (oil frequency and firstis in phase (seismic critical could have joinedmeasure); both bearing hands)points increase.

2 x speed Phase is unsteady, jerky Loosenessbut in same neighbor-hood; values jump highand low, axial motionlow, shifts with loadchanges, often reducesor stops on slowdown;scratches on orbit andorbit appears to beatlike a heart.

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15 -

14 -

13 .

12 •

10 -

9

o 8& 7

o g ..

LINEAR RANGE

TRANSFER RATIO ORSENSITIVITY ORGAINENGLISH- 20OMV/NILMETRIC- 254 MV/HIIL

10 2O 30 40 5O 60 7O 80 90 100MILS VIBRATION , , (PEAK-TO-PEAKJ

ALARM SET MILS (P/P)

SHOP MECHANICALTEST LIMIT MILS (P/P)

SHUTDOWN SET MILS (P/P)

LFIELD PERFORMANCERUN LIMIT MILS (P/P)

RADIAL VIBRATIONMILS

PEAK-TO-PEAK PROBE LOCATIONS

VERTICAL

(* Horizontal Precedes Vertical In Shaft Rotation")

V^ ^^ -—HORIZONTAL-

VERTICAL

PREFERREDFOR COUNTERCLOCKWISE

PREFERREDFOR CLOCKWISE

* VERTICAL

HORIZONTAL

ALTERNATE ALTERNATE

A Figure 3. Calibration curve for radial vibration measure.

xX = XoSinwt^ For sinusoidal only

Velocity = jf = XowCoscot

d2xAcceleration = dfT = ~ Xow2

Vmax. = Peak Velocity = (Xo)(w) = (§)(2irf) = 52.3D(^Q)xlO

Max. Acceleration = Xoor or in meter readings

G max. = 14.1D(jQoQ)2x 103 = gravity units or (g's)

X = AMPLITUDE AT ANY TIME "t".

Xo = MAX. AMPLITUDE OF THE VIBRATION.

co = CIRCULAR FREQUENCY = 2 TT f.

f = FREQUENCY OF THE VIBRATION.

T = THE PERIOD OF THE VIBRATION.

F = FREQUENCY METER READING, CPM.

D = DISPLACEMENT METER READING, MILS.

Vmax. = PEAK VELOCITY IN INCHES/SEC.

Figure 4. Simple sinusoidal vibration motion.

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ozooUI<ƒ>

V)iüIoz

oo_lUJ

ÜJCL

swttocxi^k mmEXTREMELY ROUGH

.001

.0001 10 100 1000FREQUENCY- CYCLES PER SECOND

i i i i noil I I I HI111 I I I Ml I I I » I HII60 600 6,000

FREQUENCY - CYCLES

60,000

PER MINUTE

600,000

Figure 5. Displacement, velocity, acceleration conversion nomograph.

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0.2- to 0.3 mils TIR (total indicator reading).

Design and maintenanceGood monitoring systems should allow the following

provisions:1. Check for proper probe-to-shaft gap while operat-

ing.2. Monitoring by oscilloscope or other device including

tapes, recorders, filters, and frequency vs. vibration proc-essing analyzers.

3. Continuous monitoring of all points with alarm ortrip relays in service at all times.

4. Display of all points, or frequent scan or review byoperations.

5. Ability to check calibration by introducing a reli-able simulation signal at the machine to check question-able response in the instrumentation and leads.

6. Freedom from false signals introduced from othersources such as RF in the plant or induced electrical pul-ses from switching circuits.

7. A grounded system at low voltage for intrinsic safe-ty against ignition of gases.

8. Maximum resistance to mechanical damage bypeople and weather, particularly rain and steam leakage.

9. Good service by the. builder and adequate training/instructions for the user's plant personnel.

10. High reliability from plant electrical interruptionssuch as power flickers.

11. Provisions to startup equipment and pass throughcriticals without disarming the protection, since mostfailures occur on startups.

12. Provisions for disarming the system safely forplanned troubleshooting of the protection (instrumenta-tion) circuits.

13. Quick identification of an alarmed or tripped chan-nel for response by the operating unit.

14. Back-up spares, standard connections, basic tool-ing, and adequate schematics.

Figure 6 shows a circulator compressor which is pro-tected by probes mounted internally to the bearings ob-serving the shaft motion. This compressor weighs 74,000Ib. The rotor weighs about 500 Ib. With this weight ratioof 147:1, it is conceivable that a high isolation ratio (isola-tion ratio is the ratio of the generated signal or forcingfunction to the received or measured signal) of 75:1 or50:1 could be expected. I do not think that case measuringis practical here.

Figure 7 shows a typical barrel compressor protected bybearing-mounted probes. While the weight ratios mightnarrow down to 20:1, the more sensitive measure is at theshaft where the trouble generally starts in the first place.More normally, a vibration forcing function will originateto, or on, the rotating shaft. Why not start the measure atthat point? If a 4 in. dia. shaft or journal is whirling aboutat a 4 mil peak-to-peak displacement relative to its bear-ing (4 mil dia. orbit), then this bearing, which may have atotal dry clearance of 5- to 6 mils over the journal at am-bient temperature, sets some pretty exacting limits onhow far one should allow the vibration to go. A limit of 8mils peak-to-peak (p/p), for example, would be prettyridiculous it would seem, i.e., 1- to 2 mils of babbitt mayhave to be wiped off to allow the vibration.

If this were a turbine rotor at 5,000 Ib. weight equallysupported with 2,500 Ib. of static weight supported ateach bearing, then API 612 would have called for a bal-ance, minimum, to have residuals not exceeding 10% ofthe journal static weight, or in this case 250 Ib., and calcu-lated at maximum continuous operating speed which wecould say, for illustration, is 10,000 rev./ min. Newtonsays that force equals mass times acceleration, F = M Aor F = (W/g)w2r or converting from pound mass, in./sec. /sec., radian/ sec., inch units to pounds, rev. /min.,oz-in. units we have the force, in pounds, equal to 1.77times the (speed/ 1,000)2 times the oz.-in. unbalance. Or250 Ib. = 1.77 (10,000/1,000)2 (oz.-in.), then the oz.-in.residual unbalance limit would be 1.41 oz.-in. The peak-to-peak measure of this motion would be

1.41 oz. - in.(2,500 Ib.) (16 oz./lb.)

or 70 X 10 e in., or 0.07 mils peak-to-peak. The 4 milsmeasured from above would represent 57 times as muchas was accepted on the balance stand. From anotherstandpoint, and really the one of most interest, the forceon the bearings (assuming zero damping) from the vibra-tions measured represents a cyclic force of 57 X 250 Ib.,or 14,250 Ib. Almost 5 g's at 167 cycles/ sec. which is verydamaging, in my opinion.

This illustration is offered only to attempt a correlationto the work in diagnostics which can be performed withdata, and to show how vibration measure is an importantparameter in the health of a critical large high speed,single line compressor train. The first measure wouldhopefully start at the time the equipment is given themechanical or performance run test at the manufacturer'splant.

Figure 6.Pipeline type booster centrifugal compressor.

Figure 7. Barrel centrifugal compressorplus horizontal split case compressor.

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TRANSFER RATIO—ÛAIN —SENSITIVITY

ENGU =H —«- tOO MV/M L

METRIC —— /27 MV/MIL

SHUTDOWN

J LOAD DEFLECTION . _ . + 20

CALIBRATION .SET POINTAI A A A A

FLOAT ZONEY'Y Y V V

30 l 40 50

MILS GAP

I

BABBITT RE6ION(;60MILS) BABBITT REGION ^60 MILS)

THRUSTMILS-ROTOR MOVEMENT

ABNORMAL-COUNTER THRUST(.—)TOWARDS INLET TURBINE

INACTIVE SIDE OF• THRUST COLLAR

NORMAL THRUST (+)

TOWARDS EXHAUST ON TURBINE

ONE BAR REPRESENTS EITHERACTIVE OR INACTIVE FACE

A C T r v E SIDE OFTHRUST COLLAR

T Y P I C A LTURBINEIllustrated)

T Y P I C A LCOMPRESSOR

COUNTER

EXHAUST^»- ->|v- DISCHARGE SUCTION

NORMALGAP OPENS

COUNTER NORMALGAP CLOSES

Figure 8. Thrust displacement calibration curve.

The same measuring facilities used for vibration meas-ure can be used for thrust displacement which would in-dicate thrust bearing failures. Figure 8 is a typical calibra-tion curve for such a system. Our Mechanical TechnologyDepartment is presently asking for an 80 mil range forsuch a system. Our past practice called for 60 mils ofmeasuring range.

In conclusionI hope that this article has given some insight into the

issue of measuring vibration with proximity probes. Thereare many pitfalls to such a system, e.g., chrome plating ormetalizing a shaft in the region where the probe observesthe shaft. This article does not allow time to discuss thesefactors. Monsanto has over 150 channels at the TexasCity plant alone. We plan to install more. We have start-ed up over 40,000 h.p. of turbomachinery since last winterwith no loss production attributed to machinery, yet wehave corrected two rubs, several misalignments, severaloil whirls, one anti-surge valve spring failure, two sludgedcouplings, two pump vane failures, and one bearing fail-ure, in addition to three miscalculated criticals in themanufacturer's shop during 1970. We use and stronglysupport (7) two probes mounted per bearing, 90° circum-ferentially apart to sense the weakest plane motion. #

Nomenclaturex = amplitude at phase wt.<w = angular velocity, radians/ sec.

t = time in sec.x = velocity, in./sec.x = Acceleration, in./sec.2

g = gravitational acceleration, 386 in./sec.2o/p = zero to peak amplitudep/ p = displacement, peak-to-peak, double amplitude

Literature cited1. Jackson, C., "Monitored Points on Turbomachinery," paper presented at 26th An-

nual Instrument Symposium, ISA, Texas A&M Univ. (January 20,1971).2. Sohre, John S., "Operating Problems with High Speed Turbomachinery," paper

presented at ASME Meeting, Dallas (September23,1968).3. Blake, Michael C., Lovejoy, "New Vibration Standards for Maintenance," Hydro-

carbon Process., 43 (January 1964).4. Maten, Steve, "Vibration Velocity Measuring Program," paper 70-PEM-27 pre-

sented at ASME Meeting, Fort Worth (March 16-18,1970).5. Bernhard, D.L., "Vibration Tolerance for Industry," paper 67-PEM-14 presented

at ASME Meeting, Detroit (April 10-12,1967).6. Ludwig, G.A., "Vibration Analysis of Large High Speed Rotating Equipment,"

paper 65-PET-4 presented at ASME Meeting, Houston (September 19-22,1965).7. Nimitz, W., and J.C. Wachet, "Vibrations in Centrifugal Compressors and Tur-

bines," paper 70-PET-25 presented at ASME Meeting, Denver (September 14,1970).

C. Jackson is a senior engineering specialist ina Mechanical Technology Section working onmechanical and method-oriented problems atthe Monsanto Co. plant, Texas City, Tex. Hehas been with the company for 21 years work-ing in division engineering, production, mainte-nance and plant design, before setting up thepresent department in 1960. Jackson receiveda B.S. Degree in mechanical engineering fromTexas A&M University. He is a registered pro-fessional engineer.

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DISCUSSION

JOHN LIVINGSTONE, ICI, Billingham, England: Welcometo the carbamate failure club, Charlie, but we're still thechampions. I would just like to know if it's possible toknow what sort of conditions you were getting when youstarted to have the failure. Presumably it happened in avery short time. The usual thing is either the CC>2concentration went up or the temperatures went down. Iwonder, have you any information on this at all?JACKSON: I can answer the process questions really easy. Idon't know anything about them. I'm sure the buildupoccurred for some time. Actually, we do have a C02 feedinto the inner stage of that second case, coming fromelsewhere. So it is complicated by that one point. Now asfar as when it started, we really don't know. Of course, theunbalance can jump from 1/2 mil to 2 1/2 mil and whenthis happens some of the material was thrown off andprobably caused the jump in vibration.

We're interested in the fact that location of probes isbeing investigated by turbine manufacturers. We don't wantto locate on a nodal point for if you do, the rotor shaft cango to hell and back but it won't show anything. You'reonly seeing the movement at the node, and that's verysmall.

We just brought down a turbine in ethylene servicewhich was alarming. We were seeing what we would call acoupling critical, but couldn't prove it. We went up to 4mils and shut down at 4.2. We ran somewhere betweenthree and four mils for about three days to try to locate ourrepair sets of blading. Then we brought it down two weeksago and we had a 70% crack through the shaft at thecoupling hub through the keyway. We don't know howmuch the fatigue was going on, but we do feel there was achange in section modulus.ANON: After we repaired a unit we noticed big vibrationsproportional to the frequency of the compressor whenstarting, and it looked like a severely unbalanced system.We investigated very thoroughly and finally we found it wasa misreading due to the rotor shaft having some residualmagnetism. We later found the magnetism was caused by aworkman who had put a magnetic device on the shaftJACKSON: This is a very good point. We had this happenon one test stand. The manufacturer did a magnetic particlecheck and they did not de-gausse the shaft. You get asquare wave and this is really weird. The unit went on thebalance stand three times before the vendor located theproblem.

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