In­te­grally-geared com­pres­sors for in­dus­trial and man­u­fac­tur­ing units

DEMM Engineering & Manufacturing - - EQUIPMENT -

IN­TRO­DUC­TION

In­te­grally-geared com­pres­sors are nowa­days used in many dif­fer­ent in­dus­tries and wide ranges of plants, man­u­fac­tur­ing units and fa­cil­i­ties. They have been used in var­i­ous sizes and con­fig­u­ra­tions in a wide range of ser­vices. This ar­ti­cle dis­cusses and re­views these widely used com­pres­sors which un­for­tu­nately were over­looked in many text books and tech­ni­cal ar­ti­cles.

IN­TE­GRALLY- GEARED COM­PRES­SORS

In an in­te­grally-geared com­pres­sor, pin­ion shafts are ar­ranged around a large cen­tral bull-gear. A 3D semi-open im­peller can be mounted on each front side of a pin­ion shaft. There are rel­a­tively large aero­dy­namic ex­cit­ing forces that are gen­er­ated in 3D semiopen im­pellers of in­te­grally-geared com­pres­sors. These forces can act as ex­cit­ing forces that cause high-speed shafts and whole the ma­chine to vi­brate. These forces could also in­duce dy­namic stresses. This is one of the rea­sons that in­te­grally-geared com­pres­sors are some­times (rel­a­tively) less re­li­able and more-noisy.

An­other im­por­tant con­sid­er­a­tion is the oper­a­tion of these com­plex ma­chines at part-load con­di­tions or al­ter­na­tive op­er­at­ing con­di­tions. The aero­dy­namic ex­cit­ing forces in 3D semi-open im­pellers and the com­plex gear unit be­hav­iours would be dif­fer­ent at al­ter­na­tive op­er­at­ing con­di­tions and part-load con­di­tions. Too of­ten, worse cases of dy­namic load­ings of many gear sys­tems are at part-loads. The be­hav­iour of any ro­tor varies sig­nif­i­cantly as the tan­gen­tial forces pro­duced in the gear sys­tem due to op­er­at­ing con­di­tion changes. There­fore, it is im­por­tant to prop­erly an­a­lyse and ver­ify the dy­namic loads and vi­bra­tion of dif­fer­ent parts of these ma­chines, par­tic­u­larly pin­ion shafts, at part-loads. This can af­fect de­sign, se­lec­tion, oper­a­tion and reli­a­bil­ity of the ma­chin­ery.

In 3D semi-open im­pellers of in­te­grally-geared com­pres­sors, the height of each vane is usu­ally about 2550 per­cent greater than the height of vanes on a com­pa­ra­ble con­ven­tional im­peller, in or­der to in­crease the ca­pac­ity and per­for­mance of the im­peller. In ad­di­tion, the num­ber of vanes of an im­peller is in­creased to re­duce the load on each vane by about 30-40 per­cent com­pared to con­ven­tional im­pellers. In this way, the ef­fi­ciency of 3D im­pellers un­der high-pres­sure ra­tios is also im­proved. How­ever, some of these im­pellers can present new chal­lenges and is­sues. Too of­ten, dif­fer­ent op­er­a­tional, me­chan­i­cal and dy­namic be­hav­iours could be ex­pected from 3D semi-open im­pellers used in in­te­grally-geared com­pres­sors com­pared to tra­di­tional 3D im­pellers em­ployed in con­ven­tional com­pres­sors.

For in­te­grally-geared com­pres­sors, in ad­di­tion of com­mon cri­te­ria re­quired for high-speed, high-per­for­mance gear units, the high ax­ial thrusts com­ing from 3D semi-open im­pellers should be taken into con­sid­er­a­tion. These ax­ial forces are di­rectly trans­ferred from the com­pres­sor im­pellers to the pin­ion shafts and can cause tilt­ing of the gears. This in­flu­ences the con­tact pat­tern and there­fore the load rat­ing of gear teeth as well as the mount­ing of the bull-gear shaft. In other words, the ax­ial loads of 3D im­pellers should be man­aged prop­erly oth­er­wise they can cause a tilt of the com­pres­sor shaft on in­te­gral gears which could re­sult in op­er­a­tional prob­lems and reli­a­bil­ity is­sues. Sin­gle he­li­cal gear sys­tems with thrust col­lars are com­monly used. Ax­ial forces from gear tooth sys­tems are usu­ally ab­sorbed by thrust col­lars (or rider rings). The thrust col­lar trans­mits ax­ial forces from 3D im­pellers to an ax­ial bear­ing on the main shaft (bull­gear shaft).

IN­LET GUIDE VANE - IGV

In­te­grally-geared cen­trifu­gal com­pres­sors are con­stant-speed ma­chines be­cause their com­plex dy­namic sit­u­a­tions (ro­tor­dy­nam­ics, lat­eral, tor­sional, etc.) do not al­low vari­able-speed oper­a­tion of these ma­chines. The “In­let Guide Vane” (IGV) is the se­lected ca­pac­ity con­trol method for many in­te­grally-geared com­pres­sors. IGVs usu­ally con­sist of a row of aero­dy­nam­i­cally-shaped blades placed at the in­let of im­peller. The blades can ro­tate around their aero­dy­namic cen­tre in or­der to give the suc­tion flow a pre-swirl that pro­vides an op­ti­mum in­ci­dence an­gle with the im­peller’s lead­ing edge even at re­duced flows.

In an in­te­grally-geared com­pres­sor, im­pellers should achieve high pres­sure ra­tios with good ef­fi­ciency to­gether with op­er­a­tional flex­i­bil­ity, op­er­at­ing at a range. An im­por­tant as­pect is the sup­pres­sion of surge so that the com­pres­sor can op­er­ate safely and ef­fi­ciently at re­duced flow-rates. IGVs gen­er­ate a swirling flow in the direc­tion of ro­ta­tion of the im­peller; this can en­hance the un­der­ly­ing sta­bil­ity of the com­pres­sor. In com­pres­sors with IGVs, the surge usu­ally com­mences at the peak of the pres­sure-ra­tio/mass-flow char­ac­ter­is­tic. The dis­tance be­tween the surge and BEP (Best Ef­fi­cient Point) is also rel­a­tively small.

It is pos­si­ble to in­stall IGV for each 3D im­peller of an in­te­grally-geared cen­trifu­gal com­pres­sor. There are com­pres­sors de­signed with IGV sys­tems at the first two stages or more. How­ever, a com­mon con­fig­u­ra­tion is an IGV only on the first stage. There have been com­pres­sors with an au­to­matic IGV only on the first stage and a man­ual IGV (with a lim­ited range) on the sec­ond stage.

The op­er­at­ing range of a typ­i­cal in­te­grally-geared com­pres­sor might be in­creased by us­ing the vari­able dif­fuser guide vane, but this can in­crease cost, com­plex­ity and main­te­nance. This also re­duces reli­a­bil­ity. This so­lu­tion is not rec­om­mended.

The IGV pre-swirl an­gle has a no­table in­flu­ence on the per­for­mance and oper­a­tion of an in­te­grally-geared com­pres­sor. Neg­a­tive IGV pre-swirl an­gles might be adopted for a flowrate that is slightly larger than the rated flow-rate, and pos­i­tive pre-swirl an­gles un­der a smaller flow-rate or part-loads. There is some­times con­fu­sion about neg­a­tive and pos­i­tive IGV an­gles. A neg­a­tive IGV pre-swirl an­gle is an IGV an­gle in the op­po­site direc­tion of IGV

an­gles that used for part-load (low flow) oper­a­tion.

IGV & RELI­A­BIL­ITY

The dy­namic dis­tur­bances and in­sta­bil­i­ties in in­te­grally-geared com­pres­sors are com­pli­cated due to many rea­sons such as strongly cou­pled un­steady ef­fects and in­ter­ac­tions of IGVs, im­pellers and dif­fusers, par­tic­u­larly be­cause of the reper­cus­sion of dif­fusers.

The blade pass­ing fre­quency is usu­ally the main fre­quency of the un­steady flow, as­so­ci­ated vi­bra­tion and dy­namic forces in an im­peller. Re­gard­ing the IGV re­lated in­sta­bil­i­ties, usu­ally three fre­quen­cies for in­sta­bil­i­ties and dy­namic force peaks have been mea­sured and re­ported which are the im­peller ro­tat­ing fre­quency, blade pass­ing fre­quency and the half of blade pass­ing fre­quency (0.5×blade pass­ing fre­quency). The asym­met­ric flow pat­tern in a com­pres­sor sys­tem usu­ally in­duces ex­ci­ta­tions with the im­peller ro­tat­ing fre­quency. The half of blade pass­ing fre­quency is usu­ally the fre­quency of peak in­sta­bil­ity value. Dou­ble har­monic of the blade pass­ing fre­quency (2×blade pass­ing fre­quency) might also be no­table.

For large pos­i­tive IGV pre-swirl an­gles, in­sta­bil­i­ties are usu­ally very higher com­pared to ones at zero IGV an­gle. For ex­am­ple, in some com­pres­sors, in­sta­bil­ity am­pli­tudes at IGV pre-swirl an­gles of around 40°- 60° could be more than 15 times the val­ues un­der zero IGV pre-swirl an­gle. For a large IGV pre-swirl an­gle, the dom­i­nant fre­quency of un­steady flow is of­ten the half of the blade pass­ing fre­quency.

For typ­i­cal pos­i­tive IGV an­gles (say around +15°, +20° or +30°), the max­i­mum fluc­tu­a­tion of­ten oc­curs near the tip of the IGV vane, and the cir­cum­fer­en­tial dis­tri­bu­tion of pres­sure fluc­tu­a­tion might be rel­a­tively uni­form. On the im­peller in­let plane, the max­i­mum pres­sure fluc­tu­a­tion is most of­ten ap­peared at the pres­sure side of 3D im­peller, and the dis­tri­bu­tion of fluc­tu­a­tion is more or less uni­form along the ra­dial direc­tion. On the im­peller out­let plane, max­i­mum fluc­tu­a­tions of­ten oc­cur in the mid­dle part of the pas­sage, and the dis­tri­bu­tion of pres­sure is of­ten (rel­a­tively) uni­form along the span-wise direc­tion. Larger fluc­tu­a­tions are re­ported at cor­ners of hub/con­cave sur­face and shroud/con­vex sur­face.

When the IGV pre-swirl an­gle is much larger (for ex­am­ple, above +35°), the in­ci­dence an­gle of flow oc­curs at the im­peller in­let, the non-uni­for­mity of im­peller in­let flow be­comes stronger, and the pe­ri­odic in­ci­dence is in­duced on im­peller blades. The pe­ri­odic change of im­peller out­let flow may cause pe­ri­odic fluc­tu­a­tions at the dif­fuser in­let flow. The in­flu­ence of IGV/im­peller wake can also cause pres­sure fluc­tu­a­tions in dif­fusers. Fluc­tu­a­tions of static pres­sure usu­ally de­crease along the flow direc­tion with the de­cay of IGV/im­peller wake. Usu­ally, the mu­tual ef­fect of the IGV wake and the im­peller wake causes max­i­mum pres­sure fluc­tu­a­tions to oc­cur at the in­ter­face be­tween the im­peller and the dif­fuser for the un­steady “IGV–im­peller– dif­fuser” in­ter­ac­tions.

CASE STUDY - SURGE

The case study is about a six-stage in­te­grally-geared cen­trifu­gal com­pres­sor in a large in­dus­trial plant. There was a surge on the 1-stage of this in­te­grally-geared com­pres­sor. This was when the plant was work­ing at 97 per­cent of its rated ca­pac­ity for the first time, around three months after the com­mis­sion­ing. The plant had many prob­lems and is­sues and it could reach at 97 per­cent of its rated ca­pac­ity three months after the com­mis­sion­ing. The surge was trig­gered by a dirty gas fil­ter at the up­stream in the close dis­tance of the ma­chine; it was higher fil­ter dif­fer­en­tial pres­sure than nor­mal. The surge was no­ticed by the op­er­a­tor be­cause the com­pressed gas flow to the unit was de­creased and the anti-surge sys­tems were ac­ti­vated. The dirty fil­ter was changed when the ma­chine was on­line. The gas fil­ter change was not so ef­fec­tive to mit­i­gate the is­sue.

For some in­te­grally-geared com­pres­sors (such as this ma­chine), as flow in­creases, us­ing the first stage IGV, the surge mar­gin de­creases which could re­sult in an un­ex­pected surge be­hav­iour in high flow op­er­at­ing ranges, near the rated flow. In other words, for in­te­grally-geared com­pres­sors us­ing IGV, the peak ef­fi­ciency point and the rated point (as well as the op­er­at­ing point of peak head) could be close to the surge point.

For this surge case, the com­pres­sor was work­ing near the surge line (or prob­a­bly at a stall sit­u­a­tion or a mi­nor in­sta­bil­ity) and the surge was trig­gered by a dirty up­stream fil­ter. The dirty fil­ter (higher pres­sure drop at suc­tion) was just a trig­ger. In this sit­u­a­tion, clean fil­ter might not have a dras­tic ef­fect on the surge sit­u­a­tion. Some in­sta­bil­i­ties con­tin­ued after the fil­ter change. To bet­ter ex­plain, the op­er­at­ing point re­quired (for 97 per­cent plant ca­pac­ity) in those ma­chine set­tings was al­ready close to the surge.

The fo­cus was on the IGV of the 2-stage. In­ves­ti­ga­tions showed that the IGV of the 2-stage was man­u­ally ad­justed at the com­mis­sion­ing when the plant was op­er­at­ing at around 60-70 per­cent of rated ca­pac­ity. The ad­just­ments might be suitable for the com­mis­sion­ing time, but not proper when the plant was op­er­ated at 97 per­cent of rated ca­pac­ity. The man­ual ad­just­ment of the 2-stage IGV from an ini­tial ad­justed-point to fully-open (strait vane po­si­tion) did bring the in­te­grally-geared com­pres­sor back to the nor­mal oper­a­tion.

Based on ex­pe­ri­ences with this ma­chine dur­ing the com­mis­sion­ing and the first year of oper­a­tion, the fol­low­ing im­por­tant notes should also be men­tioned:

A) There is al­ways a po­ten­tial to in­stall an up­stream gas fil­ter in­cor­rectly. This can bring the com­pres­sor to the surge zone and com­plex op­er­a­tional is­sues.

B) An er­ror in in­stru­men­ta­tions might cause surge. For ex­am­ple, a plug­ging flow mea­sure­ment (or dirt in­side flow in­stru­ments) could cause a higher flow mea­sure­ment than the ac­tual flow which may re­sult in an un­de­tected surge.

BY AMIN ALMASI

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