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 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, round- ness and taper—or shaft geometry—are factors that should be considered when making the repair decision. Generally, scratch depths of 0.001in. or less are acceptable for use. A good method is to lightly run the edge of a coin 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 be strapped or stoned. Usually scars deeper than 0.005 in. should call for a clean-up by machining of the shaft. 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 around the journal at least two times and pulled back and forth in order to achieve a circumferential polishing motion. This can best be accomplished by two persons—one on each end of the strap. The amount of material removed from the journal diameter must not exceed 0.002 in. Stoning. This consists of firm cutting strokes with a fine grit flat oil stone following the journal contour. The stone is rinsed frequently in diesel oil or cleaning solvent to prevent clogging. To avoid creating flat spots on the journal, 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, then journal, packing ring, and seal surfaces can be refinished to a good surface by turning down and grinding to the original finish. This introduces the need for special or nonstandard bearings or shaft seals. Stocking and future spare parts availability become a problem. Machining of shaft diameters for nonstandard final dimensions can therefore only be an emer- gency measure. Generally, the diameters involved should be reduced by the minimum amount required to clean up and restore the shaft surface. For this the shaft must 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 assump- tions. Nelson 1 quotes the U.S. Navy cautioning against reducing journal diameters by more than 1 / 4 in., or beyond that diameter which will increase torsional shear stress 25 percent above the original design, whichever occurs first. Table 8-2 shows this guideline. Finally, the assembled rotor should be placed in “V” blocks and checked for eccentricity. Table 8-3 shows suggested guidelines for this check. Repair and Maintenance of Rotating Equipment Components 475 Shaft Straightening* Successful straightening of bent rotor shafts that are permanently warped 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 to bend. In general, if the stresses causing the bend are caused from improper forging, rolling, heat treating, thermal stress relieving, and/or machining operations, then the straightening will usually be temporary in character and generally unsuccessful. If, however, a bent shaft results from stresses set up by a heavy rub in operation, by unequal surface stresses set up by heavy shrink fits on the shaft, by stresses set up by misalignment, or by stresses set up by improper handling, then the straightening will generally have a good chance of per- manent success. 476 Machinery Component Maintenance and Repair Table 8-3 Recommended Eccentricity Limits for High Speed Turbomachinery Rotors Surface Tolerance (in.) Impeller eye seal 0.002 Balance piston 0.002 Shaft labyrinth 0.002 Impeller spacer 0.002 All other 0.0005 * 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 Before attempting to straighten a shaft, try to determine how the bend was produced. If the bend was produced by an inherent stress, relieved during the machining operation, during heat proofing, on the first appli- cation of heat during the initial startup, or by vibration during shipment, then straightening should only be attempted as an emergency measure, with the chances of success doubtful. The first thing to do, therefore, is to carefully indicate the shaft and “map” the bend or bends to determine exactly where they occur and their magnitude. In transmitting this information, care should be taken to iden- tify the readings as “actual” or “indicator” values. With this information, plus a knowledge of the shaft material available, the method for straight- ening can be selected. Straightening Carbon Steel Shafts Repair Techniques for Carbon Steel Shafts For medium carbon steel shafts (0.30 to 0.50 carbon), three general methods of straightening the shaft are available. Shafts made of high alloy or stainless steel should not be straightened except on special instructions that can only be given for individual cases. The Peening Method. This consists of peening the concave side of the bend, lightly hitting it at the bend. This method is generally most satis- factory where shafts of small diameters are concerned—say shaft diame- ters of 4 in. (100 mm) or less. It is also the preferred—in many cases, the only—method of straightening shafts that are bent at the point where the shaft section is abruptly changed at fillets, ends of keyways, etc. By using a round end tool ground to about the same radius as the fillet and a 2 1 / 2 - lb machinist’s hammer, shafts that are bent in fillets can be straightened with hardly any marking on the shaft. Peening results in cold working of the metal, elongating the fibers surrounding the spot peened and setting up compression stresses that balance stresses in the opposite side of the shaft, thereby straightening the shaft. The peening method is the preferred method of straightening shafts bent by heavy shrink stresses that some- times occur when shrinking turbine wheels on the shaft. Peening the shaft with a light ( 1 / 2 lb) peening hammer near the wheel will often stress-relieve the shrink stresses causing the bend without setting up balance stresses. The Heating Method. This consists of applying heat to the convex side of the bend. This method is generally the most satisfactory with large- diameter shafts—say 4 1 / 2 in. (~112.5 mm) or more. It is also the preferred Repair and Maintenance of Rotating Equipment Components 477 method of straightening shafts where the bend occurs in a constant diam- eter portion of the shaft—say between wheels. This is generally not applic- able for shafts of small diameter or if the bend occurs at a region of rapidly changing shaft section. Because this method partially utilizes the com- pressive stresses set up by the weight of the rotor, its application is limited and care must be taken to properly support the shaft. The shaft bend should be mapped and the shaft placed horizontally with the convex side of the bend placed on top. The shaft should be supported so that the convex side of the bend will have the maximum possible com- pression stress available from the weight of the rotor. For this reason, shafts having bends beyond the journals should be supported in lathe centers. Shafts with bends between the journals can usually be supported in the journals; however, if the end is close to the journal, it is preferable to support the shaft in centers so as to get the maximum possible com- pression stress at the convex side of the bend. In no event should the shaft be supported horizontally with the high spot on top and the support directly under the bend, since this will put tension stresses at the point to be heated, and heating will generally permanently increase the bend. Shafts can be straightened by not utilizing the compressive stress due to the weight of the rotor, but this method will be described later. To straighten carbon steel shafts using the heating method, the shaft should be placed as just outlined and indicators placed on each side of the point to be heated. Heat should be quickly applied to a spot about two to three in. (~50–75 mm) in diameter, using a welding tip of an oxyacetylene torch. Heat should be applied evenly and steadily. The indicators should be carefully watched until the bend in the shaft has about tripled its pre- vious value. This may only require perhaps 3 to 30 seconds, so it really is very important to observe the indicators. The shaft should then be evenly cooled and indicated. If the bend has been reduced, repeat the procedure until the shaft has been straightened. If, however, no progress has been made, increase the heat bend as determined by the indicators in steps of about 0.010–0.020 in. (0.25–0.50 mm) or until the heated spot approaches a cherry red. If, using heat, results are not obtained on the third or fourth try, a different method must be tried. The action of heat applied to straighten shafts is that the fibers sur- rounding the heated spot are placed in compression by the weight of the rotor, the compression due to expansion of the material diagonally oppo- site, and the resistance of the other fibers in the shaft. As the metal is heated, its compressive strength decreases so that ultimately the metal in the heated spot is given a permanent compression set. This makes the fibers on this side shorter and by tension they counterbalance tension stresses on the opposite side of the shaft, thereby straightening it. 478 Machinery Component Maintenance and Repair The Heating and Cooling Method. This method is especially applicable to large shafts that cannot be supported so as to get appreciable compressive stresses at the point of the bend. It consists of applying extreme cold— using dry ice—on the convex side of the bend and then quickly heating the concave side of the bend. This method is best used for straightening shaft ends beyond the journals or for large vertical shafts that are bent anywhere. Here, the shaft side having the long fibers is artificially contracted by the application of cold. Then this sets up a tensile stress in the fibers on the opposite side which, when heated, lose their strength and are elongated at the point heated. This now sets up compressive stresses in the concave side that balance the compressive stresses in the opposite side. Indicators should also be used for this method of shaft straightening—first bending the shaft in the opposite direction from the initial bend, about twice the amount of the initial bend—by using dry ice on the convex side—and then quickly applying heat with an oxyacetylene torch to a small spot on the concave side. Shafts of turbines and turbine-generator units have been success- fully straightened by various methods. These include several 5,000-kw turbine-generator units, one 6,000-kw unit, and many smaller units. Manufacturers of turbines and other equipment have long used these straightening procedures, which have also been used by the U.S. Navy and others. With sufficient care, a shaft may be straightened to 0.0005in. or less (0.001 in. or 0.025 mm total indicator reading). This is generally satisfactory. Casting Salvaging Methods Repair of Castings. Quite often cast components of process machinery cannot be repaired by welding. We will now deal briefly with these sal- vaging methods: 1. Controlled-atmosphere furnace brazing. 2. Application of molecular metals. 3. Metal stitching of large castings. Braze repair of cavitation damaged pump impellers is an adaptation of a braze-repair method originally developed for jet engine components 2 . 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 Repair and Maintenance of Rotating Equipment Components 479 [...]... 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 are typical examples of new parts deliveries... 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 489 Figure 8-26 Classical repair problem on all types of rotating machinery is... discussion between the owner and the expert repairer Only when the owner is fully satisfied about the security of the repair does the actual repair work commence Repair and Maintenance of Rotating Equipment Components 499 Following the tests, the repairer will present a report of the findings and a proposal for repairs (Figure 8-34) At this point a discussion can be held between repairer and owner so that the... for turbomachinery repair Compressor Rotor Repairs* There are two basic types of compressor rotors: the drum type and the built-up type There are two variations of each style * Material contributed by W E (“Ed”) Nelson (†) and gratefully acknowledged by the authors 501 502 Machinery Component Maintenance and Repair Figure 9-1 Compressor repair and run-out verification at a major independent repair facility... 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 Any list of parts that are repairable would be lengthy and still be incomplete But just to give an idea, machinery. .. consider repair 496 Machinery Component Maintenance and Repair Knowledge Base of Repairers Machines come in a wide variety of shapes, makes, models, and materials Nonetheless, they are all subject to the laws of nature as interpreted through the science and art of engineering Any complex problem or machine can be broken down into its component elements and the laws of engineering applied to it An expert repairer... months Even if the component is a spare, the owner has the security of having the repaired part close at hand sooner By the foregoing discussion it is apparent that repairing is not a hit-ormiss proposition but a controlled science By defining the work scope, the processes, and the specifications, a repairer can absolutely determine and Repair and Maintenance of Rotating Equipment Components 497 guarantee... Metal 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... 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 487 (Text continued from page 482) 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. .. the repair of components, provided (and it’s a very important proviso) that the risks of repair can be assessed and controlled This control can be exercised through the specifications, qualified procedures, and facilities discussed above Initiating the Repair Sequence The repair process can be initiated by simply telephoning an expert repairer, describing the problem, and asking for an opinion If the repairer . 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. equipment may 482 Machinery Component Maintenance and Repair (Text continued on page 487) Repair and Maintenance of Rotating Equipment Components 483 Table 8-4 Typical Field and Shop Repair Services. 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