ORBIT Second/Third Quarters 1999 29 ccurate diagnosis of m achinery problem s requires a compl ete set of m ach ine vibrat ion informa tion. That inf ormation come s from the three pri- ma r y part s of the da ta that we ca n obtain from a vibration signal: direct am plitude, f requen cy, and , when the re are two signa ls, phase . Relative p has e is thetim ing re lationship be twe en two vibration signals, while absolute phase comp ares a vibration signa l to a once - pe r-turn ref ere nce pulse . The Ke yphas or ® signal is a once-per-turn voltage pul se provide d by a transduce r (norm a lly a pro xim ity probe ). T he Ke ypha sor signa l is use d by m onitor- ing, di agn ostic, and m ana ge m en t sys- te m s to gen era te filte red vibra tion am plitude, phase lag, spee d, an d a vari- e ty of othe r i nform ation. K e ypha sor- ge nerate d information can help the Operator or Machinery Specialist iden- tify developing machine problems or distinguish serious problems from less se rious one s. The Ke ypha sor signa l is used to gene rate more than one thi rd of the information regarding the condition of the m achine. Phas e (relative and absolute) is a critical part of this infor- m ation. Withou t phas e inform ation, ove rall m achine cond ition a nd m achine fau lts wou ld ofte n b e ve ry diff icult, if not impossible, to diagnose. Why is phase important? Phase information can be used to dete ct subtl e chan ges to the m achinery that m a y otherwise go unnoticed. Listed be low are a few of the m an y pha se an gle a pplicati ons: • Trending for Accep tance Re gions • Shaf t crack dete ction •Rub de tecti on • Shaft balancing • Shaf t/structural resona nce d ete ction • Shaf t mode shape • L oca tion of a fluid-i nduce d instability Tr en d in g am p lit ude and p hase info r - m ati on can provide ea rly shaft crack de tection and a lert the Operators to other possible m achine probl em s. Rub conditions can be de tecte d wi th the use of phase presente d in stea dy state polar plots. Machinery Specialists use p has e extens ively in their an alysis to confirm sus pe cte d ma chine probl em s. T rending phase i nformation Th e Ac ce p t ance Re g ion t r end format is a polar or rectangular coordinate plot (Figure 1) pro- duce d b y sam pling vibration vector compone nts as a func- tion of ti m e. Acce pta nce Re gion des cri be s a range of vector movem ents considered acce ptab le for norma l m achine ope ration. Such trend s shoul d be m ain- ta ine d for 1X, 2X, an d slow roll ve c- tors. A ny mo vem en t of the se ve ctors outside of the acceptance boundaries shou ld bevie we d with sus picion. Correlation of the 1X an d 2 X ve ctors with load , f ie ld cu rre nt steamcondi- tions and other process param ete rs is use d to de term ine whe the r vector shifts are cause d by a sha ft cr ack or by other factors. Changes in the slow roll vector can because d by shaft crack and rotor bow. Shaft crack detection Th e Am p lit u d e an d PH ase versus Tim e p lo t (AP HT p lot) (F ig ure 2) sho ws tha t 1X vibrat ion am plitud e is continually increasing. Note that the increase in vibration amplitude level is only half of the picture . The continual cha nge in phase s hown in the plot reveals critical information regarding Why phase information is important for diagnosing machinery problems by Clair Forland Diagn ostic Specialist Bently Nevada Corporation e-mail: [email protected]Figur e 1. Data Ma nager ®Ac ceptance Region plot of 1X or 2X vibration vectors.
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Why phase information is important for diagnosing machinery problems
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7/28/2019 Why phase information is important for diagnosing machinery problems