Lu­bri­ca­tion

DEMM Engineering & Manufacturing - - FRONT PAGE - By AMIN ALMASI

IN­TRO­DUC­TION

Min­eral oils are cost ef­fec­tive and widely used. Hy­dro-treated min­eral oils are em­ployed for their low fluid sol­u­bil­ity (say usu­ally one per­cent to four per­cent). Syn­thetic lubri­cants are used de­pend­ing on the process, tem­per­a­ture and how much di­lu­tion is present. The PAO (Polyal­phaolefin) oils, for ex­am­ple, have ex­cel­lent wa­ter and ox­i­da­tion re­sis­tance. The PAG (Polyal­ka­line Gly­col) oils, which do not read­ily ab­sorb hy­dro­car­bons/flu­ids, are used in ap­pli­ca­tions where the oil is in con­tact with the work­ing flu­ids. One of the main dis­ad­van­tages of us­ing grease over oil is the an­noy­ing ten­dency of grease to cake and dry out. Usu­ally, lu­bri­ca­tion oil is pre­ferred for the lu­bri­ca­tion of a ma­chin­ery.

There are four pri­mary com­po­nents to pre­ci­sion lu­bri­ca­tion oil of ma­chin­ery bear­ings and com­po­nents: 1. The lu­bri­ca­tion oil se­lec­tion. 2. The lu­bri­ca­tion oil sys­tem. 3. The flow, pres­sure and op­er­at­ing

con­di­tions of lu­bri­ca­tion oil. 4. The mon­i­tor­ing, con­trol and con­di­tion mon­i­tor­ing of lu­bri­ca­tion oil and its sys­tem. The con­di­tion-based lu­bri­ca­tion should be part of any world-class op­er­a­tion and main­te­nance pro­gramme, and the sub­ject of con­di­tion-based op­er­a­tion is gain­ing more and more in­ter­est in the ma­chin­ery in­dus­try.

The lu­bri­ca­tion oil con­di­tion mon­i­tor­ing should ex­am­ine the lu­bri­cant prop­er­ties, con­tam­i­nants and var­i­ous kinds of wear de­bris to de­ter­mine ma­chine health; it is com­pa­ra­ble to a blood test on the hu­man body. More than 65 per­cent of ma­chin­ery fail­ures are usu­ally lu­bri­cant-re­lated.

LU­BRI­CA­TION CON­DI­TION MON­I­TOR­ING

The oil con­di­tion mon­i­tor­ing could be in two ma­jor types, the “lab test­ing” and the “on­line test­ing”. For the first op­tion, the “lab test­ing”, lu­bri­ca­tion oil sam­ples are sent to a lab­o­ra­tory, which could be an “out-of-site lab” or an “on-site lab”. The “on­line test­ing” in­stru­ments, which are me­ters, are in­stalled usu­ally in an oil cir­cu­lat­ing sys­tem in or­der to mon­i­tor con­tin­u­ously lu­bri­ca­tion oil con­di­tions. Par­tic­u­larly, “par­ti­cle coun­ters”, “moisture me­ters” and “di­elec­tric in­stru­ments” should be men­tioned as good ex­am­ples for on­line mon­i­tor­ing. Proper lu­bri­ca­tion con­di­tion mon­i­tor­ing should be se­lected for crit­i­cal ma­chin­ery.

Some data pa­ram­e­ters have only up­per lim­its such as par­ti­cle counts or wear de­bris lev­els. A few data pa­ram­e­ters em­ploy lower lim­its like flash point and ox­i­da­tion sta­bil­ity. Many data pa­ram­e­ters like vis­cos­ity and ad­di­tive el­e­ments use both up­per and lower lim­its. Rate-ofchange lim­its are ef­fec­tively ap­plied to some pa­ram­e­ters such as par­ti­cle count­ing, el­e­men­tal wear met­als, fer­rous den­sity, and oth­ers. They also can be ef­fec­tively ap­plied to mon­i­tor ab­nor­mal degra­da­tion of ad­di­tives.

Special care is needed for small ma­chines. For a small ma­chine, the lu­bri­ca­tion oil on­line mon­i­tor­ing is usu­ally not fea­si­ble. The lu­bri­ca­tion oil “lab test­ing” should be per­formed with great care. When sam­pling small reser­voirs, such as those in small ma­chiner­ies be­low 300 kW, fol­low­ing the flush por­tion and then sam­pling, a com­plete oil change would have oc­curred on ev­ery small ma­chine. Con­sid­er­ing the in­creased lu­bri­cant con­sump­tion cou­pled with the ad­di­tional cost of test­ing the oil sam­ples, the over­all costs would be sig­nif­i­cant.

OIL VIS­COS­ITY MON­I­TOR­ING

The con­di­tion of lu­bri­ca­tion oil is a crit­i­cal fac­tor in ex­tend­ing a ma­chin­ery’s bear­ing/com­po­nent life and over­all re­li­a­bil­ity. Mon­i­tor­ing and managing the lu­bri­ca­tion oil vis­cos­ity can pre­vent costly break­downs be­cause of the bear­ing fail­ure. The vis­cos­ity of lu­bri­ca­tion oil also plays a role in the en­ergy ef­fi­ciency, as de­mand for more ef­fi­cient ma­chin­ery is driv­ing the use of lower-vis­cos­ity lubri­cants.

For ma­chiner­ies where lu­bri­ca­tion oil comes in con­tact with light hy­dro­car­bons (for ex­am­ple, com­pres­sors), the lu­bri­ca­tion oil’s vis­cos­ity can break down much more quickly, in­creas­ing the risk of fail­ure. Clearly managing lu­bri­cant vis­cos­ity is crit­i­cal to main­tain­ing ma­chin­ery health. Many op­er­a­tors also have found that real-time tem­per­a­ture mon­i­tor­ing is inad­e­quate to mon­i­tor lu­bri­cant vis­cos­ity. It is a com­mon prac­tice to mon­i­tor lu­bri­cant vis­cos­ity of many medium/large ro­tat­ing ma­chines once a month by send­ing a sample to a lab for test­ing. Rapid changes in vis­cos­ity can oc­cur, and the re­sults can be se­vere. It is rec­om­mended to in­stall the real-time mon­i­tor­ing of lu­bri­ca­tion oil vis­cos­ity (the in­line vis­come­ter) in crit­i­cal ma­chines. Mon­i­tor­ing the lu­bri­ca­tion oil vis­cos­ity is the best way to pre­vent bear­ing wear and ma­chin­ery fail­ure.

Many fac­tors can af­fect lu­bri­ca­tion oil vis­cos­ity. These in­clude ox­i­da­tion, di­lu­tion, con­tam­i­na­tion, bub­bles, tem­per­a­ture changes and oth­ers. The con­tin­u­ous mon­i­tor­ing of the lu­bri­ca­tion oil vis­cos­ity can show traces/ef­fects of all above­men­tioned faults. An op­er­a­tor can look and se­lect cer­tain char­ac­ter­is­tics for an in­line lu­bri­cant vis­cos­ity mea­sure­ment sys­tem, such as menu-driven elec­tronic con­trols, self-clean­ing sen­sors, built-in tem­per­a­ture de­tec­tion, mul­ti­ple out­put sig­nals, au­to­matic vis­cos­ity con­trol,

data log­ging, quick change mem­ory set­tings, se­cu­rity and alerts. A self-clean­ing sen­sor uses the in­line fluid to clean the sen­sor as it is tak­ing mea­sure­ments. For an au­to­matic vis­cos­ity con­trol, a sen­sor that is pre-set but re­con­fig­urable is al­ways pre­ferred.

OIL CON­TAM­I­NA­TION MON­I­TOR­ING

Ma­chin­ery op­er­a­tors are nowa­days em­pow­ered to per­form quick and fre­quent oil anal­y­sis tests par­tic­u­larly tests re­lated to the oil con­tam­i­na­tion. One of the most im­por­tant tests in­cludes the as­sess­ment of the lu­bri­ca­tion oil con­tam­i­nants. There is usu­ally a re­quire­ment for the con­tin­u­ous lu­bri­ca­tion oil con­di­tion feed­back from the lu­bri­ca­tion oil con­tam­i­nant mon­i­tors since the oil con­tam­i­na­tion can be oc­curred sud­denly as re­sult of a mal­func­tion. Also, the oil con­tam­i­na­tion can cause ma­chin­ery dam­ages in a short time. The con­tam­i­nant mon­i­tor­ing in­stru­ments have ad­vanced rapidly in the past decade as their ef­fec­tive­ness and their im­por­tance. There are a va­ri­ety of user-level con­tam­i­nant mon­i­tor­ing in­stru­ments and meth­ods to gain quick read­ings on lu­bri­cant clean­li­ness, dry­ness and other con­tam­i­nant lev­els. An im­por­tant step for a proper oil con­tam­i­na­tion mon­i­tor­ing pro­gram should be iden­ti­fy­ing the points ( ports) for the con­tam­i­nant mon­i­tor­ing on the lu­bri­ca­tion oil cir­cu­lat­ing sys­tem. Two sets of points (ports) need to be iden­ti­fied in­clud­ing the pri­mary (rou­tine anal­y­sis) and se­condary (trou­bleshoot­ing) sam­pling ports.

One of the most im­por­tant fac­tors in the se­lec­tion of lu­bri­ca­tion oil con­tam­i­nant mon­i­tor­ing in­stru­ment is the ease of op­er­a­tion/ap­pli­ca­tion. The “ease of use” is as im­por­tant as the tech­ni­cal spec­i­fi­ca­tions (such as the pre­ci­sion, and other re­quire­ments). The lu­bri­ca­tion oil con­tam­i­nant mon­i­tor­ing in­stru­ment should be fre­quently used, but a mod­er­ate pre­ci­sion is usu­ally suf­fi­cient.

WEAR DE­BRIS ANAL­Y­SIS

The key of a suc­cess­ful con­di­tion mon­i­tor­ing sys­tem is the early de­tec­tion and swift cor­rec­tive mea­sures. One of the im­por­tant as­pects of the “wear de­bris anal­y­sis” is the weak sig­nal ca­pa­bil­ity that can be re­sulted in an early de­tec­tion of a de­vel­op­ing dam­age in a ro­tat­ing ma­chine.

The “wear de­bris anal­y­sis” refers to the ex­ten­sive ar­ray of wear par­ti­cle tech­nolo­gies and tac­tics that can help re­veal the true tri­bo­log­i­cal con­di­tion of a ma­chine. These tech­niques should be used in com­bi­na­tion, since in­di­vid­u­ally they can­not be ef­fec­tive. Even an in­di­vid­ual tech­nique/mea­sure­ment, sometimes, can be mis­lead­ing. There have been many false de­tec­tion re­ports, be­cause peo­ple at­tempt to draw a pre­ma­ture con­clu­sion from an in­com­plete/in­di­vid­ual piece of in­for­ma­tion in the wear de­bris anal­y­sis.

Com­mon im­por­tant tech­nolo­gies used for screen­ing pur­poses in­clude the “fer­rous den­sity anal­y­sis”, the “el­e­men­tal spec­troscopy”, and the “par­ti­cle count­ing and patch test­ing”. Com­ple­men­tary meth­ods used for proper and cor­rect mon­i­tor­ing/ fault-de­tec­tion are the “fil­ter de­bris in­spec­tion”, the “mag­netic plug anal­y­sis”, the “sump sed­i­ment anal­y­sis”, the “fer­rog­ra­phy” meth­ods, the “acid­dis­so­lu­tion spec­troscopy”, the “par­ti­cle heat treat­ment”, the “par­ti­cle im­paction test­ing”, the “chem­i­cal mi­croscopy”, the “dig­i­tal shape pro­fil­ing”, the “rotrode fil­ter spec­troscopy”, the “gravi­met­ric anal­y­sis”, the “ul­tra­cen­trifuge” (sep­a­ra­tion of sol­u­ble metal frac­tion), the “pore block­age par­ti­cle count­ing”, and many more. Again, in­di­vid­u­ally, the above-men­tioned meth­ods might let a dam­age go un­no­ticed or a mis­lead­ing con­clu­sion. How­ever, when two, three or even four trend lines move and show the same re­sult, there can be a re­li­able sig­nal that could be used for a fur­ther mon­i­tor­ing/in­ves­ti­ga­tion or even a cor­rec­tive ac­tion.

The clean oil can strengthen the sig­nal-to-noise ra­tio in a “wear de­bris anal­y­sis”. With­out the back­ground noise of dirty oil, even the weak­est sig­nals sometimes can be de­tected. The cor­rect po­si­tion should be se­lected for “sam­pling” points in a “wear de­bris anal­y­sis”. The par­ti­cle/de­bris gen­er­at­ing sources and the po­ten­tial dam­age lo­ca­tions should be prop­erly iden­ti­fied and cor­rect sam­pling points should be de­fined. By sam­pling down­stream of the wear gen­er­at­ing source and up­stream of fil­ters/reser­voirs, the data is not stripped by fil­tra­tion or muted by di­lu­tion.

The “size” and “shape” of par­ti­cles play an im­por­tant role in the “wear de­bris anal­y­sis”. The most im­por­tant indi­ca­tions are the par­ti­cles in their orig­i­nal “size” and “shape” as pro­duced from their gen­er­at­ing sources such as bear­ings, gears, and other par­ti­cle gen­er­a­tion points. The cir­cu­lated par­ti­cles, which pro­duced some time ago and cir­cu­lated in the lu­bri­ca­tion oil sys­tem, can­not be use­ful. Of­ten, the cir­cu­lated par­ti­cles can­not be prop­erly em­ployed for the mon­i­tor­ing pur­poses be­cause they have been “re­worked” by the ma­chine and its en­vi­ron­ment through crush­ing, lam­i­nat­ing, cor­ro­sive ac­tion or oth­ers and the iden­ti­fi­ca­tion of the wear mode and the lo­ca­tion of the par­ti­cle gen­er­a­tion are nearly im­pos­si­ble. Some good places to find the par­ti­cles in the orig­i­nal “size” and “shape” are fil­ters, sump sed­i­ment, mag­netic plugs, chip col­lec­tors and sim­i­lar sources.

LU­BRI­CA­TION AND RE­LI­A­BIL­ITY

Usu­ally, there is a risk to stop a ma­chine for re­pair (or over­haul) based on only one ab­nor­mal mon­i­tor­ing sig­nal. In other words, with­out hav­ing the ben­e­fit of mul­ti­ple tech­nolo­gies com­ing to the same con­clu­sions, the re­li­a­bil­ity of a ma­chine can­not be prop­erly as­sessed. There are usu­ally the fol­low­ing three mon­i­tor­ing ar­range­ments for a crit­i­cal ma­chine, the “vi­bra­tion anal­y­sis”, the “oil anal­y­sis”, and the “in­frared ther­mog­ra­phy”. At least one tech­nol­ogy should be used on ev­ery ma­jor piece of equip­ment. If an anom­aly is ob­served, other tech­nolo­gies should be ap­plied to eval­u­ate the ma­chine. If at least two mon­i­tor­ing meth­ods show there is an is­sue, a proper ac­tion should be taken. The “vi­bra­tion anal­y­sis” is a com­mon­lyused method and other two can help to con­firm any de­tected is­sue.

For ex­am­ple, the on­line vi­bra­tion mon­i­tor­ing is usu­ally em­ployed for the con­tin­u­ous mon­i­tor­ing of ma­chin­ery’s bear­ings. If there is an ab­nor­mal vi­bra­tion, the “oil anal­y­sis” can be used to fur­ther eval­u­ate the bear­ings. If there is a slight spike in “Lead”, “Tin” or “Alu­minum”, it could be con­cluded that there is a bear­ing is­sue. As an­other ex­am­ple, the on­line vi­bra­tion mon­i­tor­ing of the bear­ing and the cas­ing are em­ployed for many gear units. If there is a high vi­bra­tion record­ing, there could be the “oil anal­y­sis” for fur­ther re­li­a­bil­ity as­sess­ment of the gear unit. A slight in­crease in iron could be con­sid­ered as the con­fir­ma­tion of a prob­lem in the gear unit. With­out hav­ing the ben­e­fit of two or even three tech­nolo­gies that con­firm there is a re­li­a­bil­ity is­sue, there could be a risk to stop and open the ma­chine and find no is­sue.

The fake sig­nals of vi­bra­tions have been re­ported for many ma­chines. There are nu­mer­ous cases that ma­chines have ex­pe­ri­enced a high vi­bra­tion, but still healthy and can work for a rel­a­tively long time with­out any prob­lem. Many ma­chines have ex­pe­ri­enced high vi­bra­tions, even two or three times than their nor­mal (baseline) vi­bra­tion am­pli­tudes, with­out any se­ri­ous prob­lem. A case study is pre­sented here to show the im­por­tance of “oil anal­y­sis” to con­firm a re­li­a­bil­ity is­sue. For a medium size ro­tat­ing ma­chine, the mea­sured vi­bra­tion has been tripled (three times the baseline value) dur­ing four months. Dur­ing this time­frame, the oil anal­y­sis showed sig­nif­i­cant in­creases in bear­ing ma­te­rial and other con­tam­i­nants. The ma­chine was stopped at the first op­por­tu­nity. The de­tailed ob­ser­va­tions showed the seal was dam­aged, al­low­ing con­tam­i­nants to in­vade the bear­ing which caused both high vi­bra­tion (as re­sult of bear­ing dam­ages) and the oil anal­y­sis ab­nor­mal­i­ties.

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Amin Almasi is a lead me­chan­i­cal en­gi­neer in Aus­tralia. He is char­tered pro­fes­sional en­gi­neer of En­gi­neers Aus­tralia ( MIEAust CPEng – Me­chan­i­cal) and IMechE (CEng MIMechE) in ad­di­tion to a M.Sc. and B.Sc. in me­chan­i­cal engi­neer­ing and RPEQ (Reg­is­tered Pro­fes­sional En­gi­neer in Queensland). He spe­cialises in me­chan­i­cal equip­ment and ma­chiner­ies in­clud­ing cen­trifu­gal, screw and re­cip­ro­cat­ing com­pres­sors, gas tur­bines, steam tur­bines, en­gines, pumps, con­di­tion mon­i­tor­ing, re­li­a­bil­ity, as well as fire pro­tec­tion, power gen­er­a­tion, wa­ter treat­ment, ma­te­rial han­dling and oth­ers. Almasi is an ac­tive mem­ber of En­gi­neers Aus­tralia, IMechE, ASME, and SPE. He has au­thored more than 150 papers and ar­ti­cles deal­ing with ro­tat­ing equip­ment, con­di­tion mon­i­tor­ing, fire pro­tec­tion, power gen­er­a­tion, wa­ter treat­ment, ma­te­rial han­dling and re­li­a­bil­ity.

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