Guidelines for designing the mandrel are: 1. The mandrel should preferably be made of low alloy steel, i.e., AISI 4140 or AISI 4340, which has been suitably stress-relieved. 2. The journal surfaces should preferably be hardened and ground, with a finish not poorer than 16 rms. 3. All diameters must be concentric within 0.0001 in. TIR. 4. The diameter of the section where the impeller is to be mounted, should be established on the basis of heating the impeller hub to a temperature of approximately 300°F for installation and removal. 5. Keyways are not incorporated in the mandrel. 6. The impeller balancing mandrel should be checked to assure that it is in dynamic balance. Make corrections on the faces, if required. 7. The impeller balancing mandrel should be free of burrs and gouges. 8. Mount each individual impeller, together with its half-key, on the balancing mandrel. A light coating of molybdenum disulfide lubri- cant should be first applied at the fits. Such mounting requires careful and uniform heating of the impeller hub, using a rosebud-tip torch, to a temperature of approximately 300°F; a temperature-indicating Tempil ® stick should be used to monitor the heating operation. Install the mandrel, with the mounted impeller, in the balancing machine. Cool the impeller by directing a flow of shop air against the hub while slowly rotating the assembly by hand or with the balancing machine drive. When the impeller and mandrel have cooled to approximately room temperature, proceed to identify the required dynamic corrections with the balancing machine operating at the highest permissible speed. 9. Make the required dynamic corrections to the impeller by removing material over an extended area, with a relatively fine grade grinding disc, at or near the impeller tip on both the cover and disc surfaces. Other Compressor Balancing 1. Inspect all impeller spacers to assure that they are of uniform radial thickness in any plane perpendicular to their axis; minor axial taper variations are of no concern. The spacers may be mounted on a machining mandrel for concentricity checking. 2. The balance piston is considered to be the equivalent of an impeller. 510 Machinery Component Maintenance and Repair Marking of Impellers Stamping of numbers on impellers should never be allowed. Use a high speed pencil grinder, also called rotating pencil, in low stress areas. Burn marks from the rotating pencil are not as harmful as stamping marks. However, they do create stress raisers, which have occasionally caused fatigue failures. Therefore, even these marks should be used only where absolutely necessary, and in carefully selected locations. Critical Areas Sharp edges on impeller bore: These edges dig into the shaft, causing high stress concentrations and possible starting points for shaft cracks. Polished radii should be provided. Sharp corners in keyways: Keyway radii should be about 1 / 4 of the key- way depth. Two keyways are preferable because single keyways will cause nonuniform heating of the shaft and also shaft warpage because there is no radial shrink stress at the keyway region, while full radial shrink stress acts on the shaft opposite to the keyway. This will cause a shaft bow which changes with shrink stress, i.e., with temperature and speed. The keyway in the impeller should be as shallow as possible, since this key is not meant to transmit torque, but to act only as a positioning and safety feature. Two shallow keys instead of one deep key are expensive, because key- ways must be positioned very accurately, and depth must be equal. But it eliminates a host of problems (cranking effect, contact-pressure effect, component balance problems, thermal bow), and when comparing cost of problems and balancing difficulties against the higher manufacturing cost of double keys, these will probably be more economical, even for initial cost, but certainly when field problems and production loss are included. A makeshift solution can be applied in the field, by relieving the shaft surface opposite the single keyway to get equal contact pattern. The relief need not be deep (say 20 mils or so). This will at least improve the contact pressure and thermal distortion. Another possibility presently gaining favor is to eliminate the keys entirely, depending 100 percent on the shrink fit engagement. The parts are mounted with a very high shrink (2.5 mils/in. or more). Special methods must be used if disassembly becomes necessary, or the parts may be severely damaged. The possibility of stress-corrosion comes to mind with these high shrink stresses (~70,000 psi) but actually the stress in a keyway corner is much higher, usually exceeding yield strength, even with a moderate shrink fit (1 mil/in. or less). Centrifugal Compressor Rotor Repair 511 Rotor Bows in Compressors and Steam Turbines Rotors sometimes will operate very satisfactorily for years, then upon restarting after a shutdown, excessive vibration occurs at the first lateral critical speed. This vibration problem may originate from the following: 1. The unit is tripped at rated speed and the rotative speed abruptly drops. 2. The effective interference fit of the impellers or wheels shrunk on the shaft increases rapidly during this speed reduction. (The bore stress varies with the square of the rotative speed.) 3. At the first critical speed range, maximum vibration amplitudes result; the rotor is then in a simple bow configuration, with the maximum deflection at approximately mid-span. 4. The shrunk-on elements thus literally lock the rotor in this bow shape and the clamping action increases below the critical speed. The out- ermost fibers of the shaft are incapable of sliding axially at the clamp or interference fit areas. 5. At rest, the rotor exhibits a large residual shaft bow, and gross imbal- ance causes high vibration amplitudes upon restarting. 6. Usually, the shaft is bowed elastically; disassembly of the rotor gen- erally results in restoration of the bare shaft to an acceptable run-out condition. Multistage centrifugal compressors tend to be particularly susceptible to the foregoing if they incorporate the same interference fit at each end of the impeller bore. It has been said that rotative speed tends to reduce the interference fit to near zero with the first lateral critical speed at 50 percent of the maximum continuous speed. In some cases the impellers will actually move on the shaft. Susceptibility to this problem increases as the ratio between the maximum continuous speed and the lateral critical speed increases. A very successful method of avoiding this problem is: 1. Making the static interference fit at the impeller toe equal to approxi- mately one-half that at the heel, while maintaining the same land axial length. 2. Making the land axial length of the spacer at the end adjacent to the impeller heel equal to approximately 15–20 percent of the length at the opposite end, while maintaining the same interference fit. 3. A 4–6 mil axial gap between components (i.e., impellers, spacer sleeves, etc.) is provided. 512 Machinery Component Maintenance and Repair Tightness of the impeller and spacer on the shaft at full operating speed is thus assured, while simultaneously providing for controlled axial sliding at the interference fits during deceleration. Some increase in rotor inter- nal friction of course results, with a consequent minor effect on the non- synchronous whirl threshold speed. A transient bow condition similar to the above is sometimes produced in built-up rotors which are subject to rapid starting. This occurs if all shrunk-on elements are fitted tightly together axially during cold assem- bly. In effect, the shrunk-on elements heat up much faster than the bare shaft after startup; the resulting thermal growth of these elements, when combined with a lack of perpendicularity of the vertical mating faces, results in shaft bowing. An axial clearance is usually provided between an impeller or wheel and its adjacent spacer to avoid this. Four to six mils is usually adequate. This discussion is paraphrased from some of the writings of Roy Greene, who has experience in centrifugal compressor design, manufac- ture, and operation for Clark, Cooper-Bessemer, and Ingersoll Rand, and who currently is a consultant 9 . Balancing Since the details of rotor balancing have been fully covered in Chapter 6, we can confine our discussion to a brief recap of this important topic. It is simple to balance a rigid rotor which runs on rigid bearings, so long as neither change shape, or deflect at operating speed. For a high-speed rotor, this is no longer true, because both the rotor and the bearings will deflect as they are exposed to centrifugal forces, and these deflections will result in a very complex unbalance condition that was not there when the rotor was balanced at low speeds in a precision balancing machine. A low- speed balancing machine, no matter how precise and/or sophisticated, cannot detect such conditions: They lie dormant in the machine until high speeds are reached, and only then will the beast show its true nature. There is absolutely no way to accurately predict the behavior of an existing high- speed rotor, except to get it up to speed on its own bearings, in its own casing. Even then it is impossible to determine the precise location of unbalance, and to make definitive and 100 percent effective corrections that will eliminate the vibration throughout the speed range. The best one can hope for is to get a correction, essentially by compromise, that permits smooth operation at one speed. The only way to get a predictable rotor is to maintain perfect balance of each individual component during all operations of machining and assembly and never to disturb this balance by making indiscriminate Centrifugal Compressor Rotor Repair 513 corrections on the finished assembly. To do all this properly is exceedingly difficult, and the methods and accuracies required border on (and often exceed) the limits of available technology of manufacturing and measur- ing. These limits dictate how fast a rotor may run and how many impellers can safely be used at a given speed, both factors being of great economic importance (cost, efficiency). Once a high-speed rotor is assembled and found out of balance at oper- ating speed, the only way to reestablish predictable balance is to com- pletely disassemble the rotor and to start from scratch. Clean Up and Inspection of Rotor Compressor rotors must be carefully inspected for any damage. To accomplish this, these guidelines should be followed: 1. The rotor should rest on the packing area that must be protected by soft packing, annealed copper, or lead to avoid any marring of pol- ished surfaces. Do not use Teflon ® strips since Teflon ® impregna- tion of the metal surfaces can alter the adhesion characteristics of the lubricant in contact with the journal. Lubrication problems could ensue. 2. The rotor should be given an initial inspection for the following: a. Impeller hub, cover, and vane pitting or damage. b. Are there any rubs or metal transfer on the hub or cover indi- cating a shifting of rotor position? All foreign metal should be ground off and the area inspected for heat checking as described in Item 8. c. Journals • Journal diameter—Roundness and taper are the two most crit- ical dimensions associated with a bearing journal. These dimensions are established with a four-point check taken in the vertical and horizontal planes (at 90° to one another) at both the forward and aft edge of the journal. A micrometer is normally used for this purpose. The journal diameter must be subtracted from the liner bore diameter to determine the clear- ance. If the journal diameter is 0.002 in. or more outside of its drawing tolerance, it is necessary to remachine the journal. Another parameter that must be carefully watched is journal taper. Excessive taper produces an increase in the oil flow out one of the ends of the bearing, thereby starving the other end. This can result in excessive babbitt temperatures. Journal tapers greater than about 0.0015 in. require remachining. 514 Machinery Component Maintenance and Repair • Journal surface—Surfaces that have been scratched, pitted, or scraped 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 pol- ished areas should be tightly wrapped and sealed with protective cloth. 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 in labyrinth areas require reclaiming of that area. 7. Wheel location—have any wheels shifted out of position? Wheel location should be measured from a thrust collar locating shoulder. There should be a 4–5 mil gap between each component of the rotor; i.e., each impeller, each sleeve, etc. 8. On areas suspected of having heat checking or cracks, a dye pene- trant check should be made using standard techniques or “Zyglo”: a. Preparation Cracks in forgings probably have breathed; that is, they have opened and closed during heat cycles, drawing in moist air that has condensed in the cracks, forming oxides and filling cracks with moisture. This prevents penetration by crack detection solutions. To overcome this condition, all areas to be tested should be heated by a gas torch to about 250°F and allowed to cool before application of the penetrant. These tests require a smooth surface as any irregularities will trap penetrant and make it difficult to remove, thus giving a false indication or obscuring a real defect. b. Application The penetrant is applied to the surface and allowed to seep into cracks for 15 to 20 minutes. The surface is then cleaned and a developer applied. The developer acts as a capillary agent (or blotter) and draws the dyed penetrant from surface defects so it is visible, thus indicating the presence of a discontinuity of the surface. In “Zyglo” an ultraviolet light is used to view the surface. 9. A more precise method of checking for a forging defect would require magnetic particle check, “Magnaflux” or “Magnaglow.” As Centrifugal Compressor Rotor Repair 515 these methods induce a magnetic field in the rotor, care must be taken 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—0.010in. 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 to 0.005 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 rotor must be disassembled. Similarly, if the journals or seal surfaces on the 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 fit engagement ( 3 / 4 to 1 1 / 2 mil/in. of shaft diameter), and this requires an impeller 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 ensure this 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 expands 0.006–0.008 in. more than the shaft so that it is free to move on the shaft. 4. The important thing to remember when removing impellers is that the heat must be applied quickly to the rim section first. After the rim 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 516 Machinery Component Maintenance and Repair position. A sketch of rotor component position should be made using the thrust collar as a reference point. Measure and record distance from the thrust collar or shoulder to first impeller hub edge. Make and record distance between all impellers. 6. When a multistage compressor is to be disassembled, each im- peller should be stencilled. From thrust end, the first impeller should be stencilled T-1, second wheel T-2, and so on. If working from coupling end, stencil first wheel C-1, second wheel C-2, and so on. 7. The rotor should be suspended vertically above a sand box to soften the 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 80 percent of the minimum yield strength at the coupling end of the shaft. The shaft is not exposed to corrosive conditions of the compressed gas at this point. Inside of the casing, the shaft size is fixed by the critical speed rigidity 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 shaft stubs at each end of the rotor. The purpose of the shaft is to carry the impellers, to bridge the space between the bearings and to transmit the torque from the coupling to each impeller. Another function is to provide surfaces for the bearing journals, thrust collars, and seals. The design of the shaft itself does not present a limiting factor in the turbomachinery design. The main problems are to maintain the shaft straight and in balance, to prevent whipping of overhangs, and to prevent failure which may be caused by lateral or torsional vibration, chafing of shrunk-on parts, or manufacturing inadequacies. The shaft must be accu- rately made, but the limits of technology are not approached as far as theory or manufacturing techniques are concerned. A thermally unstable shaft develops a bow as a function of temperature. To reduce this bow to acceptable limits requires forgings of a uniformity and quality that can only be obtained by the most careful manufacturing and metallurgical techniques. Rotors made of annealed material are not adequate, because many mate- rials, for example AISI 4140, have a high ductility transition temperature in the annealed condition. This has caused failures, especially of shaft Centrifugal Compressor Rotor Repair 517 ends. Therefore, it is very important to make sure that the material has been 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 90,000psi and the hardness no greater than 22 Rockwell “C” in order to avoid sulfide stress cracking. While selection of the material is fairly simple, quality control over the actual piece of stock is complicated. There are several points to consider. 1. Material Quality: Forgings of aircraft quality (= “Magnaflux quality”) are required for all but the simplest machines. Bar-stock may not have sufficient thermal stability, and therefore must be inspected carefully. Note that shafts—as well as all other critical components—must be stress-relieved after rough machining, which usually leaves 1 / 16 in. 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 medium strength steel will do this. After some service the impeller hubs coin distinct depressions into such shafts, squeezing the shaft, so to speak. This squeezing process also causes shaft distortion and permanent elongation of the shaft, which can lead to vibration problems or inter- nal rubbing. Since part of the initial shrink fit is lost, this may cause other types of problems, such as looseness of impellers, which then can 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 lateral support; 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 for mandrel balancing. The temperature that must be attained to permit assembly is determined by the micrometer measurement of the shaft and bore diameters, and calculation of the temperature dif- ferential needed. 4. Due to extreme temperatures, a micrometer cannot be used; there- fore, a go-no go gauge, 0.006 in. to 0.008 in. larger than the shaft 518 Machinery Component Maintenance and Repair diameter at the impeller fit, should be available for checking the impeller bore before any assembly shrinking is attempted. 5. Shrink a ring (0.003 in. to 0.004 in. tight) on the shaft extending about 1 / 32 in. past the first impeller location. Machine the ring to the exact distance from the machined surface of the impeller to the thrust 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 important thing to keep in mind is that the hub bore temperature must not get ahead of the rim temperature by more than 10°–15°F. The usual geometry of impellers is such that they will generally be heated so that 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 heating because the bore of the hub can heat up ahead of the hub section and result in a permanent inward growth of the bore. Heating of the wheel can be accomplished in three different ways: a. Horizontal furnace: the preferred method of heating the wheel for assembly because the temperature can be carefully controlled. b. Gas ring: The ring should be made with a diameter equal to the mass center of the impeller. c. “Rosebuds”: The use of two or more large diameter oxyacety- lene torches can be used with good results. The impeller should be supported at three or more points. Play the torches over the impeller 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 tem- perature antiseize compound. 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 inserted freely into the impeller fit bore, the impeller should be quickly moved 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 the impeller 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 Centrifugal Compressor Rotor Repair 519 [...]... minutes Shut down, and check the radial runout (TIR) at mid-span using a 1/10 mil dial indicator; record the angular position of the high spot and run out valve Spin the bare shaft at a speed of 200–300 rpm for an 522 Machinery Component Maintenance and Repair additional five minutes Shut down, and again check the radial runout (TIR) at mid-span; record the angular position of the high spot and runout valve... in Figure 9-6 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 particular 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 Machinery Component Maintenance and Repair 526 Figure 9-6... 12 13 14 15 16 17 18 19 Machinery Component Maintenance and Repair the wheel is in place The side of the impeller where air cooling is applied is nearest to the fixed locating ring and/ or support point The locating ring should be removed after the impeller is cooled Recheck axial position of the impeller If an impeller goes on out of position and must be moved, thoroughly cool the entire impeller and. .. axial distances for centrifugal compressor rotor Figure 9-8 Dimensional record for compressor rotor sealing areas 527 528 Machinery Component Maintenance and Repair If Disassembly Is Required 1 Visually inspect Visually inspect each part removed Measure and record all pertinent shaft and component dimensions as follows: • Impeller bore sizes—key size where applicable • Shaft sleeve bore sizes • Balance... of particles from one of the surfaces bonding to the other With repeated relative motion between the surfaces, the transferred particles may fracture from the new surface and take on the form of wear debris Adhesive wear is thus analogous to friction, and is present in all sliding systems It can never be eliminated—only reduced Figure 10-1 Wear mechanisms 538 Machinery Component Maintenance and Repair. .. fine grained, and adherent A dye penetrant inspection qualifies the above 530 Machinery Component Maintenance and Repair 8 No chrome plating on top of chromium, unless specified by the repair facility Finish Grinding 1 Chrome coating should be ground to finish dimensions specified Tolerance on OD should be + 0.0005 - 0.0000 unless otherwise specified 2 Grinding should be done with proper coolant and wheel... between the hub and shaft through a shallow circular groove machined either in the hub or in the shaft Install the O-rings toward this groove, the back-up rings away from this groove Do not twist either the O-rings or the back-up rings while Install O-Rings and Back-Up Rings Figure 9-9 Methods of determining and limited hub advance on tapered shaft 532 Machinery Component Maintenance and Repair installing... procedures Use NDT procedures to determine existence and location of any cracks on shaft and component parts Maximum allowable residual magnetism 2.0 gauss 3 Completion of inspection procedures Upon completion of inspection procedures, customer is notified and the results evaluated and discussed The repair scope most advantageous to the customer is confirmed and completed Assembly 1 Check dynamic balance Check... the characteristics of each class 542 Machinery Component Maintenance and Repair Figure 10-3 Comparison of commercial hardness tester scales Figure 10-4 Classification of hard-surfacing materials Protecting Machinery Parts Against Loss of Surface 543 Tool Steels By definition, hard-surfacing is applying a material with properties superior to the basis metal In repair welding of tool steels, a rod is... Cobalt-Base Alloys These materials contain varying amounts of carbon, tungsten, and chromium in addition to cobalt, and provide hardnesses ranging from RC 35 to RC 60 Their wear resistance is derived from complex carbides in a cobalt-chromium matrix The size, distribution and the types of carbides 544 Machinery Component Maintenance and Repair vary with the alloy content The matrix can be harder than the austenitic . naturally the parts should be carefully marked as taken apart so that identical parts can be replaced in the proper 516 Machinery Component Maintenance and Repair position. A sketch of rotor component. A 4–6 mil axial gap between components (i.e., impellers, spacer sleeves, etc.) is provided. 512 Machinery Component Maintenance and Repair Tightness of the impeller and spacer on the shaft at. the 1st and 2nd lateral critical speeds. Most experts agree that routine check 520 Machinery Component Maintenance and Repair balance of complete rotors with correction on the first and last wheels