Machinery Components Maintenance And Repair Episode 2 Part 7 pdf

shaft with only surface-type damage—such as pitting corrosion—the depth of cut should be between 1 / 32 and 1 / 16 in. (0.8 to 1.6 mm). The minimum depth of cut is specified to avoid having the fusion line posi- tioned directly on the final machined surface. The maximum depth is limited in order to reduce distortion. For mechanical damage, the depth should 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 areas should 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 the heat input and reduce the possibility of weld defects. The welding current should be reduced to where good fusion and adequate bead thickness are still obtained but without resorting to long dwell times. There is a trade- off between current, travel speed, and filler rod diameter. These variables need 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 / 32 in. EWTh-2 tip) and a filler rod size of 3 / 32 or 1 / 8 in. (2.5 or 3.2 mm) diameter should be used. 3. Welding Technique The shaft should be well-supported on rollers and mounted in a turner. The shaft is to be rotated at all times during welding and for 30 minutes Repair and Maintenance of Rotating Equipment Components 465 Figure 8-14. Shaft preparation by machining. * Gas-Tungsten-Arc-Welding (Tungsten-Inert-Gas). 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 per minute 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 width should be limited to 3 / 8 in. (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 be repaired closest to the middle of the shaft and should proceed toward the shaft end. The maximum interpass temperature is limited to 350°F (175°C). This is of primary importance since the thermal profile of the heat-affected zone is a major determinant of residual stress and distortion. As heat build- up occurs, the width of the heat-affected zone increases, which increases shrinkage. In one case, the shaft runouts were monitored during a portion of the welding. It was found that shaft end deflections (the weld area was 20in. from the end) of up to 0.015 in. (0.38 mm) occurred during the actual welding but would return to less than 0.005 in. (0.13 mm) during cooling periods. Some cold straightening may be required to correct any residual dis- tortion, but this has not usually been a difficult problem. 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 been adopted. 466 Machinery Component Maintenance and Repair 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 27in. length overlaid on coupling end, 1 / 16 in. deep; approximately 21in. length overlaid on thrust end, 1 / 32 – 1 / 16 in, 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 / 4 in. wide, 3 / 16 in. 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 / 8 in. deep. Unsuccessful. Repair and Maintenance of Rotating Equipment Components 467 Figure 8-16. General view of repaired shaft during machining. Since any unsuccessful attempt should generate as much useful infor- mation as a successful result, it is worthwhile to discuss the lessons learned from this last case: a. The undercut depth may have been excessive (specified at 1 / 16 to 1 / 8 in.), 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 create complex distortions that are difficult to correct by straightening. A single repair, even if over a large area, will usually create only a simple bend that can be easily machined and straightened. These particular repairs were closely spaced with critical tolerance areas between them. It was not possible to mechanically straighten the shaft to correct the variety of distortions in these critical areas. Other Components 1. Impellers Water injection pump impellers (CF8M) are routinely repaired by welding, such as for cavitation damage and bore dimension buildup. A modification has now been instituted to eliminate the impeller 468 Machinery Component Maintenance and Repair 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 operational upsets. The basic procedure involves building up the impeller shoulder with E316L electrodes (SMAW* process) to the specified wear ring diameter, machining 0.060in. 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 case wear rings are produced, which are exchanged as a complete set for the existing components during a pump rebuild. Repair and Maintenance of Rotating Equipment Components 469 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 to limit the heat input, particularly during the buildup of the shoulder using SMAW electrodes. If this is not controlled, distortion of the impeller shrouds can occur. In order to prevent this, 1 / 8 -in, diameter electrodes, 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 leakage due to erosion/corrosion under the case wear rings and along the case split line faces. The repair procedure developed consists of under- cutting ( 1 / 8 in. deep) the centerline bore and the inner periphery of the split line face. These areas are overlaid with Inconel 182 (AWS A5.11 ENiCrFe-3). After rough machining, the cases are stress relieved 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. 470 Machinery Component Maintenance and Repair Figure 8-19. Impeller with direct Stellite overlay in final machined condition. Repair and Maintenance of Rotating Equipment Components 471 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 were experiencing pitting corrosion on the sealing faces. A localized overlay using Inconel 625 (Figure 8-22) has eliminated the problem. 4. Impeller Wear Rings Prior to the decision to hardface directly on the impeller, attempts were made to fabricate replacement wear rings. The first attempts used core billets as raw stock, however, it appears easier to use solid bar 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 resulting from these unbalanced stresses can be enormous. These stresses also change 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 in this discussion, although some 4140 shafts have been welded for emergency repairs. For these materials, the primary concern is the possibility of cracking in the hard heat-affected zone formed during welding. Cracking can occur either during (or slightly after) welding due to delayed hydrogen cracking or during service. If a temper bead 472 Machinery Component Maintenance and Repair Figure 8-22. Overlaying of seal flange faces. technique can be effectively developed or if a vertical localized post- weld heat treatment could be accomplished without shaft distortion, then welded repairs to these materials might also become feasible. Outlook and Conclusions 1. The possibility of using a low temperature stress relief of 600° to 800°F (315° to 425°C) for several hours has been considered for the impeller 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 min- imize heat buildup for more critical components, such as shaft repairs. We conclude: • Experience has shown that welded repairs to shafts and other rotat- ing equipment components can be successfully accomplished. • Welding techniques and procedures must be selected in order to min- imize distortion. This includes the use of low heat inputs and special sequences. • Filler metal selection can provide improved properties, such as cor- rosion and wear resistance, over the original base metal. High Speed Shaft Repair In the foregoing we saw several successful pump shaft repair techniques described. Quite often the restoration of low speed shafts with less damage than we saw previously does not represent any problems. Flame spraying by conventional oxyacetylene methods most often will lead to satisfactory results. The market abounds in a variety of flame spray equipment, and most in-house process plant maintenance shops have their preferred makes and techniques. We would now like to deal with the question of how to repair damaged journals, seal areas, and general geometry of high speed turbomachinery shafts. We will mainly focus on centrifugal compressor and turbine rotor shafts in excess of 3,600rpm. Four repair methods can generally be identified: Two, that result in the restoration of the original diameter, i.e., 1. Flame spraying—hard surfacing. 2. Chemical plating. Repair and Maintenance of Rotating Equipment Components 473 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 of industrial hard chrome plating of power engine cylinders. Worn bearing journals, shrink fit areas of impellers and turbine wheels, thrust collar areas and keyed coupling hub tapers have been successfully restored using industrial hard chrome. We do not see much benefit in describing hard chrome specifications. We recommend, however, that our readers always consult a reputable industrial hard chrome company. Since chrome plating is too hard to be machined, grinding is the only suitable finishing process. Again, experience and skill of the repair orga- nization is of the utmost importance: Soft or medium grinding wheels should be applied at the highest possible, but safe speeds. Coolant must be continuous and copious. Only light cuts not exceeding 0.0003in. (7.5 mm) should be taken, as heavy cuts can cause cracking and heat checks. As a rule of thumb, final ground size of a chrome plated shaft area should not exceed 0.007 to 0.010in. Chrome plating for radial thickness in excess of these guidelines may require more than one chrome plating operation coupled with intermediate grinding operations. Knowing this, it would be well to always determine the required time for a shaft chrome plating project before a commitment is made. Flame Spray Coatings. The available flame spray methods will be described later. For practical reasons the detonation gun, jet gun, plasma arc, and other thermal spray processes may suit high speed machinery. There is, however, reason to believe that other attractive techniques will become available in the future. We believe that coatings applied by conventional oxyacetylene processes tend to have a weaker bond, lower density, and a poorer finish than other coatings. Further, there are too many things “that can go wrong,” a risk to which we would not want to subject high speed machin- ery components. The authors know of an incident where a critical shaft had been allowed to be stored several hours before oxyacetylene metal- lizing. Dust and atmospheric humidity subsequently caused a problem with the coating well after the machine was up and running. In conclu- sion, we think that the occasional unavailability of D-gun or plasma coating facilities and the high cost of these methods far outweigh the risk that is inherent in applying oxyacetylene flame sprays. 474 Machinery Component Maintenance and Repair [...]... 486 Machinery Component Maintenance and Repair Figure 8 -24 Metalstitch® process of casting repair (courtesy In-Place Machining Company, Milwaukee, Wisconsin)8 Repair and Maintenance of Rotating Equipment Components 4 87 (Text continued from page 4 82) be considered It would be advisable to maintain a subscription to at least one used or surplus equipment directory for that purpose The machinery maintenance. .. Locking Honing Grinding Line-Boring Boring Milling Machining ᭹ Straightening Alignment Components Repair and Maintenance of Rotating Equipment Components Machinery Repairs— Field & Shop Work Grouting Repair Operations 484 Table 8-4—cont’d Typical Field and Shop Repair Services Offered by Process Machinery Repair Shops6 ,7 Reciprocating Compressors Power Pumps Foundation Base Frame Crankshaft Main Bearings... Elliott Company Reprint R240, Antonio Casillo, “Twenty Questions About Repairing Machinery, ” reprinted by permission of both the author and Turbomachinery International, ©1990, Business Journals, Inc This material was originally presented at the Fifth Turbomachinery Maintenance Conference in London, U.K., September 1989 488 Machinery Component Maintenance and Repair Figure 8 -25 Gas expander blades of superalloy... about the repairability of a worn or damaged component, and the advice is usually free Fortunately, most turbomachinery components can be repaired at lower cost and shorter lead time than buying new Only in the case of small, inexpensive, mass-produced components is repair not worthwhile Usually repair is considered to reduce delivery time and costs while maintaining product integrity (Figure 8 -25 ) * Elliott... engineering department and know-how of an original equipment manufacturer How to Find Out if the Component Is Repairable A phone call to an expert repairer with a description of the component and of the problem will often result in an answer (Figure 8 -26 ) For bigger problems users can ask the repairer to conduct an inspection of the component at site Repair and Maintenance of Rotating Equipment Components. .. 8 -26 Classical repair problem on all types of rotating machinery is the scoring of shaft journals Welding can be used to repair damage to any depth Formerly journal repair was limited to allowable chrome plating thicknesses Thrust collars can similarly be repaired Quotations for this type of repair can be made quickly What Components Can Be Repaired Practically any part of a rotating machine can be repaired... of parts that are repairable would be lengthy and still be incomplete But just to give an idea, machinery that can be repaired includes pumps, compressors, steam turbines, gas turbines, mixers, and fans Repairs can be effected for breakage, wear, erosion, corrosion, galling, fretting, cracking, bending, and over-temperature, to name but a few types of the many conceivable problems (Figures 8 - 27 and. .. handling, then the straightening will generally have a good chance of permanent success * From Repair Techniques for Machinery Rotor and Case Damage,” by H A Erb, Elliott Co., Greensburg, Pennsylvania Hydrocarbon Processing, January 1 975 By permission Repair and Maintenance of Rotating Equipment Components 477 Before attempting to straighten a shaft, try to determine how the bend was produced If the... an adaptation of a braze -repair method originally developed for jet engine components2 The first step is rebuilding the eroded areas of the impeller blades with an iron-base alloy powder The powder is mixed with an air-hardening plastic binder and used to fill the damaged areas Through-holes are 480 Machinery Component Maintenance and Repair backed up with a temporary support and packed full of the powder/binder... 8 -23 Metal locking a machinery casting4 4 82 Machinery Component Maintenance and Repair erly filled by the lock The result is a joint that lacks strength and from which new cracks may emanate d Assuming the lock is properly fitted, a pinning procedure is now undertaken This consists of mating the lock to the parent metal by drilling holes so that one half of the hole circles are in the parent metal and . equipment may 4 82 Machinery Component Maintenance and Repair (Text continued on page 4 87) Repair and Maintenance of Rotating Equipment Components 483 Table 8-4 Typical Field and Shop Repair Services. Service 484 Machinery Component Maintenance and Repair Table 8-4—cont’d Typical Field and Shop Repair Services Offered by Process Machinery Repair Shops 6 ,7 Repair Operations Machinery Repairs— Field. case. 470 Machinery Component Maintenance and Repair Figure 8-19. Impeller with direct Stellite overlay in final machined condition. Repair and Maintenance of Rotating Equipment Components 471 Figure