Machinery Components Maintenance And Repair Episode 2 Part 9 pps

Wheellocation should be measured from a thrust collar locating shoulder.There should be a 4–5 mil gap between each component of therotor; i.e., each impeller, each sleeve, etc.. A sketch

• Journal surface—Surfaces that have been scratched, pitted, orscraped to depths of 0.001 in or less are acceptable for use.Deeper imperfections in the range of 0.001 to 0.005 in must

be restored by strapping

• Thrust collar—does it have good finish? Use same guidelines

as for journals Is the locking nut and key tight? If the collar

is removed, is its fit proper? It should have 0.001 to 0.0005 in.interference minimum

3 The journals, coupling fits, overspeed trip, and other highly ished areas should be tightly wrapped and sealed with protectivecloth

pol-4 The rotor should be sandblasted using No 5 grade, 80/120 mesh,polishing compound, silica sand, or aluminum oxide

5 When the rotor is clean, it should be again visually inspected

6 Impellers and shaft sleeve rubs—rubs in excess of 5 mils deep inlabyrinth areas require reclaiming of that area

7 Wheel location—have any wheels shifted out of position? Wheellocation should be measured from a thrust collar locating shoulder.There should be a 4–5 mil gap between each component of therotor; i.e., each impeller, each sleeve, etc

8 On areas suspected of having heat checking or cracks, a dye trant check should be made using standard techniques or “Zyglo”:

pene-a Preparation

Cracks in forgings probably have breathed; that is, they haveopened and closed during heat cycles, drawing in moist air thathas condensed in the cracks, forming oxides and filling crackswith moisture This prevents penetration by crack detectionsolutions To overcome this condition, all areas to be testedshould be heated by a gas torch to about 250°F and allowed tocool before application of the penetrant

These tests require a smooth surface as any irregularities willtrap penetrant and make it difficult to remove, thus giving a falseindication or obscuring a real defect

b Application

The penetrant is applied to the surface and allowed to seep intocracks for 15 to 20 minutes The surface is then cleaned and adeveloper applied The developer acts as a capillary agent (orblotter) and draws the dyed penetrant from surface defects so it

is visible, thus indicating the presence of a discontinuity of thesurface In “Zyglo” an ultraviolet light is used to view thesurface

9 A more precise method of checking for a forging defect wouldrequire magnetic particle check, “Magnaflux” or “Magnaglow.” As

these methods induce a magnetic field in the rotor, care must betaken to ensure that the rotor is degaussed and all residual mag-netism removed.

10 The rotor should be indicated with shaft supported at the journals:

a Shaft run out (packing areas) 0.002 in TIR max

b Impeller wobble—0.010 in TIR—measured near O.D

c Shroud band wobble—0.020 in TIR

d Thrust collar—0.0005 in TIR measured on vertical face

e Vibration probe surfaces 0.0005 in TIR—no chrome plating,metallizing, etc., should be permitted in these areas

f Journal areas—0.0005 in TIR, 20 micro in rms or better

g Gaps between all adjacent shrink fit parts—should be 0.004 to0.005 in

11 If the shaft has a permanent bow in excess of the limit or if there

is evidence of impeller distress, i.e., heavy rubs or wobble, the rotormust be disassembled Similarly, if the journals or seal surfaces onthe shaft are badly scored, disassembly in most cases is indicated

as discussed below

Disassembly of Rotor for Shaft Repair

If disassembly is required the following guidelines will be helpful

1 The centrifugal rotor assembly is made with uniform shrink fitengagement (3/4to 11/2mil/in of shaft diameter), and this requires animpeller heating process or, in extreme cases, a combination process

of heating the impeller and cooling the shaft

2 The shrinks are calculated to be released when the wheel is heated

to 600°F maximum To exceed this figure could result in

metallurgi-cal changes in the wheel Tempil® sticks should be used to ensurethis is not exceeded The entire diameter of an impeller must be uni-formly heated using “Rosebud” tips—two or more at the same time

3 Generally a turbine wheel must be heated so that it expands0.006–0.008 in more than the shaft so that it is free to move on theshaft

4 The important thing to remember when removing impellers is thatthe heat must be applied quickly to the rim section first After therim section has been heated, heat is applied to the hub section, start-

ing at the outside Never apply heat toward the bore with the

remain-der of the impeller cool.

5 To disassemble rotors, naturally the parts should be carefully marked

as taken apart so that identical parts can be replaced in the proper

position A sketch of rotor component position should be made usingthe thrust collar as a reference point Measure and record distancefrom the thrust collar or shoulder to first impeller hub edge Makeand record distance between all impellers.

6 When a multistage compressor is to be disassembled, each peller should be stencilled From thrust end, the first impeller should be stencilled T-1, second wheel T-2, and so on If workingfrom coupling end, stencil first wheel C-1, second wheel C-2, and

im-so on

7 The rotor should be suspended vertically above a sand box to softenthe impact of the impeller as it falls from the shaft It may be nec-essary to tap the heated impeller with a lead hammer in order to get

it moving The weight of the impeller should cause it to move when

it is hot enough

Shaft Design

It is not uncommon to design for short-term loads approaching 80percent of the minimum yield strength at the coupling end of the shaft.The shaft is not exposed to corrosive conditions of the compressed gas atthis point Inside of the casing, the shaft size is fixed by the critical speedrigidity requirements The internal shaft stress is about 5,000–7,000 psi—very low compared to the impellers or at the coupling area With drum-type rotors there is no central portion of the shaft, there are only shaftstubs at each end of the rotor The purpose of the shaft is to carry theimpellers, to bridge the space between the bearings and to transmit thetorque from the coupling to each impeller Another function is to providesurfaces for the bearing journals, thrust collars, and seals

The design of the shaft itself does not present a limiting factor in theturbomachinery design The main problems are to maintain the shaftstraight and in balance, to prevent whipping of overhangs, and to preventfailure which may be caused by lateral or torsional vibration, chafing ofshrunk-on parts, or manufacturing inadequacies The shaft must be accu-rately made, but the limits of technology are not approached as far astheory or manufacturing techniques are concerned A thermally unstableshaft develops a bow as a function of temperature To reduce this bow toacceptable limits requires forgings of a uniformity and quality that canonly be obtained by the most careful manufacturing and metallurgicaltechniques

Rotors made of annealed material are not adequate, because many rials, for example AISI 4140, have a high ductility transition temperature

mate-in the annealed condition This has caused failures, especially of shaft

ends Therefore, it is very important to make sure that the material hasbeen properly heat-treated.

Most compressor shafts are made from AISI 4140 or 4340 AISI 4340

is preferred because the added nickel increases the ductility of the metal.Most of the time the yield strength is over 90,000 psi and the hardness nogreater than 22 Rockwell “C” in order to avoid sulfide stress cracking.While selection of the material is fairly simple, quality control over theactual piece of stock is complicated There are several points to consider

1 Material Quality: Forgings of aircraft quality (= “Magnafluxquality”) are required for all but the simplest machines Bar-stockmay not have sufficient thermal stability, and therefore must beinspected carefully Note that shafts—as well as all other criticalcomponents—must be stress-relieved after rough machining, whichusually leaves 1/16in of material for finishing

2 Testing: Magnaglow of finished shaft Ultrasonic test is desirable for

large shafts Heat indication test is required for critical equipment

3 Shaft Ends: Should be designed to take a moderate amount of

tor-sional vibration, not only the steady operating torque

4 The shaft must be able to withstand the shrink stresses Any mediumstrength steel will do this After some service the impeller hubs coindistinct depressions into such shafts, squeezing the shaft, so to speak.This squeezing process also causes shaft distortion and permanentelongation of the shaft, which can lead to vibration problems or inter-nal rubbing Since part of the initial shrink fit is lost, this may causeother types of problems, such as looseness of impellers, which thencan lead to looseness-excited vibrations such as hysteresis whirl

Rotor Assembly

1 Remove the balanced shaft from the balancing machine, and position it vertically in a holding fixture providing adequate lateralsupport; the stacking step on the shaft should be at the bottom

2 Remove all of the half-keys

3 Assembly of the impellers and spacers on the shaft requires heating,generally in accordance with the procedure previously outlined formandrel balancing The temperature that must be attained to permitassembly is determined by the micrometer measurement of theshaft and bore diameters, and calculation of the temperature dif-ferential needed

4 Due to extreme temperatures, a micrometer cannot be used; fore, a go-no go gauge, 0.006 in to 0.008 in larger than the shaft

there-diameter at the impeller fit, should be available for checking theimpeller bore before any assembly shrinking is attempted.

5 Shrink a ring (0.003 in to 0.004 in tight) on the shaft extendingabout 1/32in past the first impeller location Machine the ring to theexact distance from the machined surface of the impeller to thethrust shoulder, and record it on a sketch This gives a perfect loca-tion and helps make the impeller run true

6 Heating the impeller for assembly is a critical step The importantthing to keep in mind is that the hub bore temperature must not getahead of the rim temperature by more than 10°–15°F The usualgeometry of impellers is such that they will generally be heated sothat the rim will expand slightly ahead of the hub section and tend

to lift the hub section outward With long and heavy hub sections,extreme care must be taken to not attempt too rapid a rate of heatingbecause the bore of the hub can heat up ahead of the hub sectionand result in a permanent inward growth of the bore

Heating of the wheel can be accomplished in three differentways:

a Horizontal furnace: the preferred method of heating the wheelfor assembly because the temperature can be carefully controlled

b Gas ring: The ring should be made with a diameter equal to themass center of the impeller

c “Rosebuds”: The use of two or more large diameter lene torches can be used with good results The impeller should

oxyacety-be supported at three or more points Play the torches over theimpeller so that it is heated evenly, remembering the 600°F limi-tation Tempil®sticks should be used to monitor the temperature

7 The wheel fit of the shaft should be lightly coated with high perature antiseize compound

tem-8 The heated wheel should be bore checked at about the center of the bore fits As soon as a suitable go-no go gauge can be insertedfreely into the impeller fit bore, the impeller should be quicklymoved to the shaft With the keys in place, the impeller bore should

be quickly dropped on the shaft, using the ring added in step 5 as

a locating guide

9 Shim stock, of approximately 0.004–0.006 in thickness, should

be inserted at three equally-spaced radial locations adjacent to theimpeller hubs to provide the axial clearance needed between adja-cent impellers This is necessary to avoid transient thermal bowing

in service

10 Artificial cooling of the impeller during assembly must be used

in order to accurately locate the impeller at a given fixed axial position Compressed air cooling must be immediately applied after

the wheel is in place The side of the impeller where air cooling isapplied is nearest to the fixed locating ring and/or support point Thelocating ring should be removed after the impeller is cooled.

11 Recheck axial position of the impeller If an impeller goes on out

of position and must be moved, thoroughly cool the entire impeller

and shaft before starting the second attempt This may take three

to four hours

12 After the impellers, with their spacers and full-keys, have beenassembled and cooled, the shim stock adjacent to the impeller hubsshould be removed

13 If the rotor has no sleeves, another split ring is needed to locate thesecond impeller This split ring is machined to equal the distancebetween the first and second wheels Then, a split ring is required forthe next impeller, etc Any burrs raised by previously assembled im-pellers should be carefully removed and the surfaces smoothed out

14 Check for shaft warpage and impeller runout as each impeller ismounted It may be necessary to unstack the rotor to correct anydeficiencies

15 The mounting of sleeves and thrust collars requires special tion Sleeves have a lighter shrink than wheels and because of theirlighter cross section can be easily damaged by uneven heating orhigh temperature Thrust collars can be easily warped by heat Thetemperature of the thrust collar and sleeves should be limited toabout 300°– 400°F

atten-16 Mount the rotor, now containing all the impellers, in the balancingmachine, and spin it at the highest possible speed for approximatelyfive minutes

17 Shut down and check the angular position of the high spots andrunout at the three previously selected spacer locations betweenjournals The high spots must be within ±45°, and the radial run-outs within 1/2mil, of the values recorded during bare shaft check-ing If these criteria are not satisfied, it indicates that one or moreelements have been cocked during mounting, thus causing the shaft

to be locked-up in a bow by the interference fits It is then sary to remove the two impellers and spacers from the shaft, and

neces-to repeat the vertical assembly process

18 Install the rotor locknut, being careful not to over tighten it; shaft

bowing can otherwise result If the rotor elements are instead

posi-tioned by a split ring and sleeve configuration, an adjacent spacer

must be machined to a precise length determined by pin ter measurement after all impellers have been mounted.

microme-19 Many compressors are designed to operate between the 1st and 2nd lateral critical speeds Most experts agree that routine check

balance of complete rotors with correction on the first and lastwheels is wrong for rotors with more than two wheels The best

method is to balance the assembled rotor in three planes.

The residual dynamic couple imbalance should be corrected at theends of the rotor, and the remaining residual static (force) imbal-ance should be corrected at about the middle of the rotor

For compressors that operate below the first critical (stiff shaftmachines), two plane balance is satisfactory

20 Install the thrust disc on the rotor; this should require a small

amount of heating It is most important that cold clearances not

exist at the thrust disc bore, since it will permit radial throwout

of a relatively large mass at operating speed Install the bearing spacer, and lightly tighten the thrust-bearing locknut.

thrust-21 Spin the rotor at the highest possible balancing speed, and identifythe correction(s) required at the thrust-bearing location Generally,

a static correction is all that is necessary, and it should be made inthe relief groove at the OD of the thrust disc No correction is permitted at the opposite end of the rotor

22 Check the radial runout of the shaft end where the coupling hubwill mount This runout must not exceed 0.0005 in (TIR), asbefore

Note: Tape sometimes fails during spinning in the balancing

machine It is therefore important that adequate shields beerected on each side of the balancing machine for the pro-tection of personnel against the hazard of flying half-keys

2 Mount the bare shaft, with half-keys in place, in the balancingmachine with the supports at the journal locations Spin the bareshaft at a speed of 300–400 rpm for approximately ten minutes Shutdown, and check the radial runout (TIR) at mid-span using a 1/10mildial indicator; record the angular position of the high spot and runout valve Spin the bare shaft at a speed of 200–300 rpm for an

additional five minutes Shut down, and again check the radial out (TIR) at mid-span; record the angular position of the high spotand runout valve Compare the results obtained after the ten minuteand five minute runs; if they are the same, the bare shaft is ready forfurther checking and balancing If the results are not repetitive, addi-tional spinning is required; this should be continued until two con-secutive five minute runs produce identical results.

run-3 Check the radial run-out (TIR) of the bare shaft in at least threespacer locations, approximately equidistant along the bearing span,and near the shaft ends Record the angular position of the high spotsand the runout values at each location The shaft is generally con-sidered to be satisfactory if both of these conditions are satisfied:

a The radial runout (TIR) at the section of the shaft between nals does not exceed 0.001 in

jour-b The radial runout (TIR) outboard of the journals does not exceed0.0005 in

4 With the balancing machine operating at its pre-determined rpm,make the required dynamic corrections to the bare shaft using wax.When satisfactory balance is reached, start removing material at theface of the step at each end of the center cylindrical section of theshaft Under no circumstances should material be removed from the sections of the shaft outboard of the journal bearings

Rotor Thrust in Centrifugal Compressors

Thrust bearing failure has potentially catastrophic consequences incompressors Almost invariably, failure is due to overloading because ofthe following:

1 Improper calculation of thrust in the design of the compressor

2 Failure to calculate thrust over the entire range of operating conditions

3 A large increase in thrust resulting from “wiping” of impeller andbalance piston labyrinth seals

4 Surging of machine so that rotor “slams” from one side of thrustbearing to the other, and the oil film is destroyed

5 Thrust collar mounting design is inadequate

Rotor Thrust Calculations

Thrust loads in compressors due to aerodynamic forces are affected byimpeller geometry, pressure rise through the compressor, and internal

leakage due to labyrinth clearances The impeller thrust is calculated,using correction factors to account for internal leakage and a balancepiston size selected to compensate for the impeller thrust load Thecommon assumptions made in the calculations are as follows:

1 Radial pressure distribution along the outside of disc cover is tially balanced

essen-2 Only the “eye” area is effective in producing thrust

3 Pressure differential applied to “eye” area is equal to the differencebetween the static pressure at the impeller tip, corrected for the

“pumping action” of the disc, and the total pressure at inlet.These “common assumptions” are grossly erroneous and can be disas-trous when applied to high pressure barrel-type compressors where a largepart of the impeller-generated thrust is compensated by a balance piston.The actual thrust is about 50 percent more than the calculations indicate.The error is less when the thrust is compensated by opposed impellers,because the mistaken assumptions offset each other

Magnitude of the thrust is considerably affected by leakage at impellerlabyrinth seals Increased leakage here produces increased thrust inde-pendent of balancing piston labyrinth seal clearance or leakage A verygood discussion of thrust action is found in Reference 3

The thrust errors are further compounded in the design of the ancing piston, labyrinths, and line API-617, “Centrifugal Compressors,”specifies that a separate pressure tap connection shall be provided to indi-cate the pressure in the balance chamber It also specifies that the balanceline shall be sized to handle balance piston labyrinth gas leakage at twiceinitial clearance without exceeding the load ratings of the thrust bearing,and that thrust bearings for compressors should be selected at no morethan 50 percent of the bearing manufacturer’s rating

bal-Many compressor manufacturers design for a balancing piston leakagerate of about 11/2–2 percent of the total compressor flow Amoco and othersfeel that the average compressor, regardless of vendor, has a leakage rate

of 3–4 percent of the total flow, and the balance line must be sized ingly This design philosophy would dictate a larger balance line to takecare of the increased flow than normally provided The balancing chamber

accord-in some machaccord-ines is extremely small and probably highly susceptible toeductor type action inside the chamber which can increase leakage andincrease thrust action The labyrinth’s leakage should not be permitted toexceed a velocity of 10 ft per second across the drum The short balanc-ing piston design of many designs results in a very high leakage velocityrate Since the thrust-bearing load is represented by the difference betweenthe impeller-generated thrust and the compensating balance piston thrust,

small changes can produce overloading, particularly in high-pressure compressors.

Design Solutions

Many of these problems have been handled at Amoco by retrofitting 34centrifugal compressors (57 percent of the total) with improved bearingdesigns Most of the emphasis has been toward increased thrust capacityvia adoption of a Kingsbury-type design, but journal bearings are alwaysupgraded as part of the package Design features include spray-lubedthrust bearings (about a dozen cases), copper alloy shoes, ball and sockettilting pad journals, pioneered by the Centritech Company of Houston,Texas, and many other advanced state-of-the art concepts

Some of the balancing piston leakage problems have been solved byuse of honeycomb labyrinths The use of honeycomb labyrinths offersbetter control of leakage rates (up to 60 percent reduction of a straightpass-type labyrinth) Honeycomb seals operate at approximately 1/2 theradial clearance of conventional labyrinth seals The honeycomb structure

is composed of stainless steel foil about 10 mils thick Hexagonal-shapedcells make a reinforced structure that provides a larger number of effec-tive throttling points

Compressor shaft failures frequently occur because of loose fit of thethrust collar assembly With no rotor positioning device left, the rotorshifts downstream and wrecks the machine The practice of assemblingthrust collars with a loose fit (1 to 5 mils) is very widespread because itmakes compressor end seal replacements easier The collar is thin (some-times less than 1 in thick) and tends to wobble The shaft diameter is small

in order to maximize thrust bearing area A nut clamps the thrust collaragainst a shoulder Both the shoulder and the nut are points of high stressconcentration With a thrust action of several tons during surging, thecollar can come loose In addition, fretting corrosion between the collarand the shaft can occur

The minimum thrust capacity of a standard 8-in (32.0 square in.)Kingsbury-type bearing with flooded lubrication at 10,000 rpm is well

in excess of 61/2tons The thrust collar and its attachment method must

be designed to accommodate this load In most designs the inboardbearing has a solid base ring and the thrust collar must be installed after this thrust bearing is in place The collar can be checked by revolv-ing the assembled rotor in a lathe The collar is subsequently removed forseal installation and must be checked for true running, i.e., the face isnormal to the axis of the bearing housing again after it is finally fitted tothe shaft

This problem has been addressed at Amoco by redesigning the thrustcollar to incorporate the spacer sleeve as an integral part and have a lightshrink fit (0 to 1 mil tight) A puller is used to remove the collar after asmall amount of heat is applied.

Managing Rotor Repairs at Outside Shops

When it becomes necessary to have rotor repairs performed away fromyour own plant, the outside shop should be required to submit such pro-cedures as are proposed for inspections, disassembly, repair, reassembly,balancing, and even crating and shipping And, while it is beyond the scope

of this text to provide all possible variations of these procedures, two orthree good sample procedures are given for the reader’s information andreview

In the following section, the procedure proposed by a highly enced repair shop for work to be performed on centrifugal compressorrotors is shown

experi-Procedures for Inspection, Disassembly, Stacking, and Balance of

Centrifugal Compressor Rotors*

Incoming Inspection

1 Prepare incoming documentation Note any defects or other damage

on rotor Note any components shipped with rotor, such as couplinghub or thrust collar

2 Clean rotor Protect all bearing, seal, probe, and coupling surfaces.

Blast clean with 200 mesh grit Glass bead, walnut shell, solvent,and aluminum oxide available if requested by equipment owner.After cleaning, coat all surfaces with a light oil

3 Perform non-destructive test Use applicable NDT procedure to

determine existence and location of defects on any components.Record magnitude and location of any defects as indicated in Figure9-6

4 Measure and record all pertinent dimensions of the rotor as shown

in Figures 9-7 and 9-8 Record on a sketch designed for the

par-ticular rotor Record the following dimensions:

• Impeller diameter and suction eye

• Seal sleeves, spacers, and shaft

* Source: Hickham Industries, Inc., La Porte, Texas 77571 Reprinted by permission.

• Journal diameters

• Coupling fits and keyways

• Gaps between adjacent shrunk-on parts

5 Check and record pertinent runouts Rotor is supported at the

bearing journals on “V” blocks Runouts should be phase-relatedusing the coupling (driven end) keyway as the 0° phase reference Ifthe coupling area is double-keyed or has no keyway, the thrust collarkeyway should be used as the zero reference If this is not possible,

an arrow should be stamped on the end of the shaft to indicate plane

of zero-phase reference

6 Check and record electrical runout probe area Use an 8-mm

diam-eter eddy probe Probe should be calibrated to shaft material only.Probe area tolerance should be 0.25 mil maximum

7 Check and record all pertinent axial stack-up dimensions

Re-ferenced from thrust collar shaft shoulder or integral thrust collar

Figure 9-6 Recording rotor imperfections.