Why Phase Information Is Important For Diagnosing Machinery Problems

 

Why phase information is important for diagnosing machinery problems by Clair Forland Diagnostic Specialist Diagnostic Bently Nevada Corporation e-mail: [email protected]

ccurate diagnosis of  machinery problem problems requires requi res a complete complete set of  machine vi vibrat bratiion inf informa ormati tion. on. That inf nformation ormation comes from from the three priprimary parts parts of the data that we can obtain from a vibration signal: direct ampl pliitude, frequen frequency, and, when there are two signa signals, phase. R Relati elative ve phase is the ti tim ming rel relati ationshi onship p between tween two vibration signals, while absolute phase compares a vibrati vibration on signa signal to a onceperper-turn turn refere reference pulse pulse.. T The he ® K eyphasor signal is a once-per-turn voltage voltage pulse pulse provi provide ded d by a transducer (normally a proxi proxim mity probe). The T he K eyphasor signa signal is used by monitoronitor-

• Shaf Shaftt crack detecti detection on

operation. operati on. S Such uch trends shoul should d be maintained for 1X 1X,, 2X 2X,, an and d slow roll vecvectors. A ny movement of these these vectors vectors outside outsi de of the acceptance boundaries boundaries should be vie viewe wed d wi with th suspi pici cion. on. Correlati Cor relation on of the 1X and 2X vectors vectors with with load, fi f ield cu curr rre ent steamcondi conditions tions and other process process parameters is is used to de determ termine whethe therr vector shi shiffts are caused by a shaft crack crack or by other factors. Changes in the slow roll vector

• Rub de detecti tection on • Shaft balancing

can be caused by shaft cr crack ack and rotor bow.

faults would often often be very dif difff icult, iif  f  not impossible, to diagnose. Why is phase important? Phase information information can be used to detect subtle subtl e changes to the machinery achinery that may otherwise otherwise go unnoticed unnoticed. L isted be bellow are a few of the many phase angl gle e appli pplications: cations: •Trending ffor or Accep Acceptance Regi gions ons

•Shaf Shaft/str t/structural uctural resonance detection ction • Shaf Shaftt mode shape •L ocation of a fluid-i fluid-ind nduce uced instability  Trending  Tre ing amplit litu ude and phase info inforrmati ation on can provide provide earl rly y shaft crack crack detectio de tection n and alert the Operators to other possible possible machine problem problems. Rub Rub conditi condi tions ons can be de detecte tected with wi th the use of phase presented iin n steady state polar polar

Shaft crack detection  The Am  The Amp plitu litude and PH PHa ase versus  Time plot lot (AP (APHT HT plot lot) (Fig (Figu ure 2) shows that 1X vibrat vibratiion ampl pliitude tude is continually increasing. Note that the increase in vibration amplitude level is only half of the pi picture cture. The The continual change in phase shown iin n the plot reveals critical information regarding

plots. Machinery Specialists use phase extensively in their analysis alysis to confi confirm suspe suspected cted machine problem problems.

ing, diagn di agnostic, ostic, and managem gement sy sysstem te ms to genera generate fi filte tered red vibra vibrati tion on ampl pliitude, phase lag, speed, and a varivariety of other infor i nform mation. Ke K eyphasorgene generate rated d iinf nformation ormation can help the Operator or Machinery Specialist identify developing machine problems or distinguish serious problems from less seri rious ous ones. The The Keyphasor signa signal is used to generate more than one third third of  of  the information regarding the condition of the machine. Phas Phase (relative (relative and

 The Ac  The Acc ceptance Regio ion n trend format is a polar or rectangular coordinate plot (Figure 1) produced by sampl pliing vi vibrati bration on vector compone components as a functi tion on of tim ti me. Accepta cceptance

absolute) a critical part ofm this information. Wi Wis ithout thou t phase infor inform ation, overal ralll machine conditi tion on and machine

R egi gion onmo des cribe cri bes s aconsi range of  vector vem ents considered dered Figure 1. Data Ma nager ®  Acceptance Figure Acceptance Region plot of acceptable for normal machine 1X or 2X vibration vectors.

Trending phase i nformation

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the condi conditi tion on of the machine. T The he phase inf nformatio ormation n reduces uces the possibl possible e number ber of machine faults faults to a family of possibilities. Two of the main shaft crack symptoms are 1X vi vibra brati tion on ampl pliitude change and phase shi hifft. With Wi th a knowledge knowledge of other symptoms and a detailed history of the machine, you can analyze nalyze other other plots pl ots and process inf nforma ormati tion on to red reduce the numbe berr of  possible mach machine faults. Withou Withoutt the phase information in this trend plot, the possibl possi ble e se seri riousne ousness of the machine condition could be overlooked. Rub detection Absolute bsolute and rel relati ative ve phase are useful for detecting rub conditions. Changes in steady state phase, reverse components in orbits or full spectrum plots, and abnormal rate of change of phase during transient conditions can be indicators of a rub condition. Figure 3 shows a steady state polar olar plot plot trend during during a rub condition. This polar plot shows the continual conti nual phase changedue to thermal bow modif odif ication to the heavy spot, due to a light seal rub. Without phase information, this would only show as a variatiion in 1X vi at vibra brati tion on amplitud pli tude, and it would wo uld be very diff diffiicu cullt for the the

Machinery Speci Speciali alist st to determine the root cause of the general rallly increa increasing sing vibration amplitude. Phase information considerably reduces the list of possible machine malfunctions. Shaft balancing Absolute phase angl gle e inform nformation iis s crucial cruci al to proper proper rotor balan balanci cing. ng. Phase informati nformation on is is used to compensatefor non-dynamic influences (slow roll vector) and to establish infl influe uence ve vectors ctors for individual machines. Once slow roll, unbalanceresponse, and inf inflluence vectors are establi blishe shed, it i t iis s possible sible ffor or the Machinery Speci Speciali alist st to balance a rotor in a minimal number of runs. Shaft mode shape

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Location of fluid-induced instability

By compari ring ng the absol olute ute or rel relative ative phase between di diffferent measurement pl plane anes, tthe he rotor's rotor's mode shape can be determ determined. ned. In I n the simpl ples est case, we compare the phase of a probe probe signal at the inboard inboard end of the the machi achine ne to that of a probe si signa gnal wi with th the same angular ori orien entation tation at the outboard end of a machine (Fi (Figure 4). If I f the two si signals are in phase, the rotor is operating in the transl translational ational (bo (bow) w) mode. If I f the sigsignals are out of phase, the rotor is oper-

Figure 2. Amplitude and PHase versus versus Time (APHT) trend plot of of 1X vibration due to shaft crack.

ating iin ating n the pi pivotal votal mode. ode. Dep Dependi ding ng on rotor mass, system stif tiffness, ness, and rotor speed, highe hi gher modes of operation tion can be attai ttained. ned. IIff more measurement plane pl anes exist xi st between the inboard and outboard ends, a better rreprese epresentati ntation on of the mode shape can be determined (Fi (Figure 5). IIt's t's iim mportant to re reali alize ze that you could could not determine the rotor's mode shape if you we were re looki looking ng at the ampl pliitude onl only y or jjust ust the frequency of  the vibra vibration tion s siign gna als. M Mode ode identifi ntif ication tion probes can also be useful to determine whether a resonance is struc structural tural or rotor rotor rrelated. elated. Shaft mode shape informa nformation tion can also be used in in determining ning balancing cing strategy gy..

Diff Di ffe erential tial phase is a specif cif ic type of  relati rel ative ve phase measurement that can be used in in determining ning the source of a fluid-i uid-ind nduc uce ed ins i nsta tabil biliity. The T he trans nsdu duccers (which, for best results, should be shaft relative) must be in different plane pl anes along along the rotor rotor and at the same angular orientation. The vibration should shoul d be f iltered to the frequency of  the instabi billity vi vibra brati tion. on. The phase lag increases (Figure 6) as the axial distance between the instabi billity source and the measurement planes planes incr ncrease eases.

Figure 3. Polar vibration trend plot of steady state vibration due to a seal rub.

 

Conclusions Phase information is important for the Operators and M Mac achi hine nery Spe Speci ciali alists. sts.  Tre  Tr ending ing of startu rtup, shutdown, and steady state data provides critical informati tion on for for machinery machinery management.  The  Th e absence of phase info inform rmatio ion n leaves a large gap iin n the usefulne efulness ss of  thi this s information. inf ormation. Chan Changes iin n phase can alert al ert the operator before before costly machine damage occurs. occurs. Bentl Bently y Neva Nevada’s 3500 Machi achine nery Pr Prote otecti ction on System System, the  Tre  Tr endmaster® 2000 System System, and Data Data ® Manager 2000 System can alert alert the operator to changes ges in in 1X and 2X ampl pliitude and phase se.. The T hese Bently Bently Nevada systems were designed designed to present inf informatio ormation n in in a way that is easy for Ope Operators and Mac Machi hine nery Speciali Speci alists sts to retrie retrieve and understand. Di Direct rect ampli plitude, ffreque requency, and phase inf nformation ormation are important in machi chinery nery management and diagnosdiagnostics. ti cs. Lack L ack of phase inform nformati ation can not only lead to an inability to diagnose a machi chine ne problem probl em, but can lea lead to an incorrect diagnosis. That is why Bently Nevada and the Am American erican Petroleu Petroleum I ns nstitute titute,, iin n its spe specif cif ications tions for machinery protection protection systems, recommend install installiing a K eyphasor and redundant K eyphasor transducers for for

Figure 4. Vibration mode shape of rotor rotor can b bee determ ined by ccomparing omparing phase of vvibraibration at each end of the machine. Poor selection of probe location can lead to ambiguity about operating m ode of rotor.

Figure 5. Too few vibration pro probes bes can provide insufficient insufficient phase and amplitude data about the true vibration mode shape of the rotor on a complex machine train.

phase measurement.

Figure 6. Phase lag of the fluid-induced instabili instabili ty vvibration ibration signal increases with distance from the source of the instability. ORBIT

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