Machinery Components Maintenance And Repair Episode 2 Part 7 pdf

For mechanical damage, the depthshould not have reduced the shaft diameter by more than 15 percent, i.e.,depth not to exceed 7.5 percent ¥ D.. wear rings by direct Stellite overlay on th

shaft with only surface-type damage—such as pitting corrosion—thedepth of cut should be between 1/32 and 1/16in (0.8 to 1.6 mm) Theminimum depth of cut is specified to avoid having the fusion line posi-tioned directly on the final machined surface The maximum depth islimited in order to reduce distortion For mechanical damage, the depthshould not have reduced the shaft diameter by more than 15 percent, i.e.,depth not to exceed 7.5 percent ¥ D The edges of all machined areasshould be tapered at 45° to ensure good sidewall fusion.

The area to be welded must be thoroughly cleaned and degreased

2 Welding Procedure

The GTAW (TIG)* process should always be used in order to limit theheat input and reduce the possibility of weld defects The welding currentshould be reduced to where good fusion and adequate bead thickness arestill obtained but without resorting to long dwell times There is a trade-off between current, travel speed, and filler rod diameter These variablesneed to be adjusted to give the lowest heat input in order to control distortion

In general, a current of 100A or less (using a 3/32in EWTh-2 tip) and

a filler rod size of 3/32or 1/8in (2.5 or 3.2 mm) diameter should be used

after completion The rotational speed should be set for the welding speed(2–3 in per minute) This will usually be about 0.1–0.2 revolutions perminute for most large shafts.

The welding is always done in a spiral pattern (Figure 8-15) The undercut depth is limited in order to obtain the required thickness in one thin pass This helps to minimize distortion by limiting the volume

of weld metal and reducing the heat input The maximum bead widthshould be limited to 3/8in (10 mm) As a minimum, one complete cir-cumferential bead should be completed before stopping or interrupting the welding sequence In general, welding is started on the edge to berepaired closest to the middle of the shaft and should proceed toward theshaft end

The maximum interpass temperature is limited to 350°F (175°C) This

is of primary importance since the thermal profile of the heat-affectedzone is a major determinant of residual stress and distortion As heat build-

up occurs, the width of the heat-affected zone increases, which increasesshrinkage

In one case, the shaft runouts were monitored during a portion of thewelding It was found that shaft end deflections (the weld area was 20 in.from the end) of up to 0.015 in (0.38 mm) occurred during the actualwelding but would return to less than 0.005 in (0.13 mm) during coolingperiods

Some cold straightening may be required to correct any residual tortion, but this has not usually been a difficult problem

dis-The finish-machined shaft surface should be completely free of any defects, such as porosity or lack of fusion Other components, such as hard-facing on wear rings or impellers, are not as critical and

an acceptance criterion for rounded indications (porosity) has beenadopted

Figure 8-15 Spiral welding sequence for shafts.

Case Histories

A number of related experiences are summarized below:

Pump shafts, all overlaid with Inconel 625:

1 Water injection pump (Figures 8-16 and 8-17)—Monel K-500 shaft,

5-in dia, approximately 27 in length overlaid on coupling end,

1/16in deep; approximately 21 in length overlaid on thrust end, 1/32–

1/16in, deep Successful

2 Numerous other water injection pumps (identical to 1)—small areas

on shaft ends: Locknut areas, O-ring seal areas, etc All successful

3 Seawater vertical lift pumps shafts—Monel K-500 shaft, 5-in dia.

Overlaid at both ends (coupling and bearing area) and center bearing.All successful

4 Water injection pump—A 638 Gr 660 shaft, 5-in dia Repair of

mechanical damage (3/4in wide, 3/16in deep) Successful

5 Brine injection pump—XM-19 shaft, 5-in dia Numerous areas with

corrosion damage, of which 8 were impeller fit areas; up to 1/8in.deep Unsuccessful

Figure 8-16 General view of repaired shaft during machining.

Since any unsuccessful attempt should generate as much useful mation as a successful result, it is worthwhile to discuss the lessonslearned from this last case:

infor-a The undercut depth may have been excessive (specified at 1/16 to

1/8in.), which when combined with excessive and unnecessary overfill, caused excessive residual stress and distortion

b A large number of separate repairs on the same shaft can createcomplex distortions that are difficult to correct by straightening Asingle repair, even if over a large area, will usually create only asimple bend that can be easily machined and straightened These particular repairs were closely spaced with critical tolerance areasbetween them It was not possible to mechanically straighten theshaft to correct the variety of distortions in these critical areas

Other Components

1 Impellers

Water injection pump impellers (CF8M) are routinely repaired bywelding, such as for cavitation damage and bore dimension buildup

A modification has now been instituted to eliminate the impeller

Figure 8-17 Edge of weld repair area in the rough machined condition.

wear rings by direct Stellite overlay on the impeller If, for example,the pump is a 10-stage design and over 50 pumps are in operation,any potential savings for even one part are well amplified In addi-tion, the elimination of the wear ring also eliminates the problems

of stellited wear ring installation and fracture during operationalupsets

The basic procedure involves building up the impeller shoulderwith E316L electrodes (SMAW* process) to the specified wear ringdiameter, machining 0.060 in undersize on the diameter (0.030-in.cut), Stellite 6 overlay (GTAW process), and final machining to size(Figures 8-18 and 8-19)

Using this procedure, matched spare sets of impellers and casewear rings are produced, which are exchanged as a complete set forthe existing components during a pump rebuild

Figure 8-18 Impeller with direct overlay of Stellite to replace wear rings.

* Submerged Metal Arc Welding.

As mentioned previously, an important part of the procedure is tolimit the heat input, particularly during the buildup of the shoulderusing SMAW electrodes If this is not controlled, distortion of theimpeller shrouds can occur In order to prevent this, 1/8-in, diameterelectrodes, a stringer bead technique, and a maximum interpass tem-perature of 350°F are specified.

2 Water Injection Pump Case

Due to a combination of the water chemistry and the pump design,the carbon steel pump cases were experiencing interstage leakagedue to erosion/corrosion under the case wear rings and along the casesplit line faces The repair procedure developed consists of under-cutting (1/8in deep) the centerline bore and the inner periphery ofthe split line face These areas are overlaid with Inconel 182 (AWSA5.11 ENiCrFe-3) After rough machining, the cases are stressrelieved and then machined to final dimensions (Figures 8-20 and 8-21) The erosion/corrosion problem has been effectively elimi-nated while providing a significant savings compared to the cost of

a stainless or alloy replacement case

Figure 8-19 Impeller with direct Stellite overlay in final machined condition.

Figure 8-20 Pump case with overlay along centerline bore and edge of split line face.

Figure 8-21 Close-up of pump case overlay in the partially machined condition.

3 Seal Flanges

The Monel seal flanges (glands) on a water injection pump wereexperiencing pitting corrosion on the sealing faces A localizedoverlay using Inconel 625 (Figure 8-22) has eliminated the problem

4 Impeller Wear Rings

Prior to the decision to hardface directly on the impeller, attemptswere made to fabricate replacement wear rings The first attemptsused core billets as raw stock, however, it appears easier to use solidbar stock The OD is overlaid before drilling the center bore

Unsolved Problems

1 Split bushings have not yet been successfully overlaid This is due

to the nonuniform stresses that are created The distortion resultingfrom these unbalanced stresses can be enormous These stresses alsochange significantly during machining; thus, it is extremely difficult

to obtain the proper dimensions

2 Materials such as 4140, 4340, and 410 SS have not been included inthis discussion, although some 4140 shafts have been welded foremergency repairs For these materials, the primary concern is thepossibility of cracking in the hard heat-affected zone formed duringwelding Cracking can occur either during (or slightly after) weldingdue to delayed hydrogen cracking or during service If a temper bead

Figure 8-22 Overlaying of seal flange faces.

technique can be effectively developed or if a vertical localized weld heat treatment could be accomplished without shaft distortion,then welded repairs to these materials might also become feasible.

post-Outlook and Conclusions

1 The possibility of using a low temperature stress relief of 600° to800°F (315° to 425°C) for several hours has been considered for theimpeller and wear ring repairs; however, this has not yet been tried

on a controlled basis in order to judge its effectiveness

2 The use of heat absorbing compounds may be tried in order to imize heat buildup for more critical components, such as shaftrepairs

min-• Filler metal selection can provide improved properties, such as rosion and wear resistance, over the original base metal

cor-High Speed Shaft Repair

In the foregoing we saw several successful pump shaft repair techniquesdescribed Quite often the restoration of low speed shafts with less damagethan we saw previously does not represent any problems Flame spraying

by conventional oxyacetylene methods most often will lead to satisfactoryresults The market abounds in a variety of flame spray equipment, andmost in-house process plant maintenance shops have their preferred makesand techniques We would now like to deal with the question of how torepair damaged journals, seal areas, and general geometry of high speedturbomachinery shafts We will mainly focus on centrifugal compressorand turbine rotor shafts in excess of 3,600 rpm

Four repair methods can generally be identified: Two, that result in therestoration of the original diameter, i.e.,

1 Flame spraying—hard surfacing

2 Chemical plating

The other two methods result in a loss of original diameter They are:

1 Polishing

2 Turning down the diameter

Chemical Plating. Later, in Chapter 10, we will discuss the technique ofindustrial hard chrome plating of power engine cylinders Worn bearingjournals, shrink fit areas of impellers and turbine wheels, thrust collarareas and keyed coupling hub tapers have been successfully restored usingindustrial hard chrome We do not see much benefit in describing hardchrome specifications We recommend, however, that our readers alwaysconsult a reputable industrial hard chrome company

Since chrome plating is too hard to be machined, grinding is the onlysuitable finishing process Again, experience and skill of the repair orga-nization is of the utmost importance: Soft or medium grinding wheelsshould be applied at the highest possible, but safe speeds Coolant must be continuous and copious Only light cuts not exceeding 0.0003 in.(7.5mm) should be taken, as heavy cuts can cause cracking and heatchecks

As a rule of thumb, final ground size of a chrome plated shaft areashould not exceed 0.007 to 0.010 in Chrome plating for radial thickness

in excess of these guidelines may require more than one chrome platingoperation coupled with intermediate grinding operations Knowing this, itwould be well to always determine the required time for a shaft chromeplating project before a commitment is made

Flame Spray Coatings. The available flame spray methods will bedescribed later For practical reasons the detonation gun, jet gun, plasmaarc, and other thermal spray processes may suit high speed machinery.There is, however, reason to believe that other attractive techniques willbecome available in the future

We believe that coatings applied by conventional oxyacetyleneprocesses tend to have a weaker bond, lower density, and a poorer finishthan other coatings Further, there are too many things “that can gowrong,” a risk to which we would not want to subject high speed machin-ery components The authors know of an incident where a critical shafthad been allowed to be stored several hours before oxyacetylene metal-lizing Dust and atmospheric humidity subsequently caused a problemwith the coating well after the machine was up and running In conclu-sion, we think that the occasional unavailability of D-gun or plasmacoating facilities and the high cost of these methods far outweigh the riskthat is inherent in applying oxyacetylene flame sprays

Shaft Repair by Diameter Reduction. In polishing up the shaft journal,minor nicks and scratches can be dressed up by light stoning or strapping.

It goes without saying that depth of scratches, affected journal area, ness and taper—or shaft geometry—are factors that should be consideredwhen making the repair decision Generally, scratch depths of 0.001 in orless are acceptable for use A good method is to lightly run the edge of acoin over the affected area in order to obtain a feel for scratch severity.Deeper scratches, from 0.001 in to approximately 0.005 in must bestrapped or stoned Usually scars deeper than 0.005 in should call for aclean-up by machining of the shaft

round-Strapping. This is done with a long narrow strip of #200 grit emery cloth.The strap is first soaked in kerosene and abraded against a steel surface

to remove sharper edges of the abrasive material It is then wrapped aroundthe journal at least two times and pulled back and forth in order to achieve

a circumferential polishing motion This can best be accomplished by twopersons—one on each end of the strap The amount of material removedfrom the journal diameter must not exceed 0.002 in

Stoning. This consists of firm cutting strokes with a fine grit flat oil stonefollowing the journal contour The stone is rinsed frequently in diesel oil

or cleaning solvent to prevent clogging To avoid creating flat spots on thejournal, stoning should be limited to removing any raised material sur-rounding the surface imperfection

If the journal diameter is 0.002 in or more outside of the tolerance, thenjournal, packing ring, and seal surfaces can be refinished to a good surface

by turning down and grinding to the original finish This introduces theneed for special or nonstandard bearings or shaft seals Stocking andfuture spare parts availability become a problem Machining of shaftdiameters for nonstandard final dimensions can therefore only be an emer-gency measure

Generally, the diameters involved should be reduced by the minimumamount required to clean up and restore the shaft surface For this the shaftmust be carefully set up between centers and indicated to avoid eccen-tricity “Standard” undersize dimensions are in 0.010 in increments.The maximum reduction is naturally influenced by a number of factors

It would mainly depend on the original manufacturer’s design tions Nelson1quotes the U.S Navy cautioning against reducing journaldiameters by more than 1/4in., or beyond that diameter which will increasetorsional shear stress 25 percent above the original design, whicheveroccurs first Table 8-2 shows this guideline

assump-Finally, the assembled rotor should be placed in “V” blocks and checkedfor eccentricity Table 8-3 shows suggested guidelines for this check

Shaft Straightening*

Successful straightening of bent rotor shafts that are permanentlywarped has been practiced for the past 40 or more years, the success gen-erally depending on the character of the stresses that caused the shaft tobend

In general, if the stresses causing the bend are caused from improperforging, rolling, heat treating, thermal stress relieving, and/or machiningoperations, then the straightening will usually be temporary in characterand generally unsuccessful

If, however, a bent shaft results from stresses set up by a heavy rub inoperation, by unequal surface stresses set up by heavy shrink fits on theshaft, by stresses set up by misalignment, or by stresses set up by improperhandling, then the straightening will generally have a good chance of per-manent success

Table 8-3 Recommended Eccentricity Limits for High Speed

Turbomachinery Rotors

* From “Repair Techniques for Machinery Rotor and Case Damage,” by H A Erb, Elliott

Co., Greensburg, Pennsylvania Hydrocarbon Processing, January 1975 By permission.

Table 8-2 Limiting High Speed Shaft Journal Reductions 1

Original Design Diameter Minimum Diameter to Which Shaft May Be Reduced

Less than 3.6 inches 93 percent of original design diameter

3.6 inches or greater Original design diameter less 1/4 inch