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AMAZON multi-meters discounts AMAZON oscilloscope discounts Sealing performance, bearing, and seal life will depend to a great extent upon the operating condition of the equipment in which these components are used. Careful inspection of the equipment will do much to minimize component failure and maintenance expenses. Following is a list of the major trouble spots. 1. Seal housing. The seal housing bore and depth dimensions must match those shown on the seal's assembly drawing within ±0.005 in. (±0.13mm). Shaft or sleeve dimensions must be within ±0.001 in. (±0.03mm). See FIG. 1 for complete seal housing requirements. 2. Axial shaft movement. Axial shaft movement (end play) must not exceed 0.010 in. (0.25mm) Total Indicator Reading (T.I.R.) To measure axial movement, install a dial indicator with the stem bearing against the shaft shoulder as shown in FIG. 2. Tap the shaft-first on one end then the other-with a soft hammer or mallet, reading the results. Excessive axial shaft movement can cause the following problems: • Pitting, fretting, or excessive wear at the point of contact between the seal's shaft packing and the shaft (or sleeve) itself. It’s sometimes helpful to replace any PTFE shaft packings or secondary sealing elements with those made of the more resilient elastomer materials to reduce fretting damage. • Spring overloading or under-loading and premature seal failure. • Shock-loaded bearings, which will fail prematurely. • Chipping of seal faces. Carbon and silicon carbide faces are especially vulnerable to axial shaft movement. 3. Radial shaft deflection. Radial shaft deflection at the face of the seal housing must not exceed 0.002 in. (0.05mm) T.I.R. To measure radial movement, install a dial indicator as close to the seal housing face as possible (see FIG. 3). Lift the shaft or exert light pres sure at the impeller end. If movement is excessive, examine for damaged radial bearings and bearing fits-especially the bearing cap bore. Excessive radial shaft movement can cause the following problems: • Fretting of the shaft or sleeve • Excessive leakage at the seal faces • Excessive pump vibration, which can reduce seal life and performance FIG. 1. Seal housing requirements. FIG. 2. Checking for end-play. FIG. 3. Checking for whip or deflection. FIG. 4. Checking for run-out. 4. Shaft sleeve run-out. Shaft run-out (bent shaft) must not exceed 0.003 in. (0.07mm) at the face of the seal housing. Clamp a dial indicator to the pump housing as shown in FIG. 4, and measure shaft run-out at two or more points on the outside dimension of the shaft. Also measure the shaft run-out at the coupling end of the shaft. If run-out is excessive, repair or replace the shaft. Excessive run-out can shorten the life of both the radial and the thrust bearings. A damaged bearing, in turn, will cause pump vibration and reduce the life and performance of the seal. FIG. 5. Checking for seal chamber face run-out. FIG. 6. Checking for seal chamber bore concentricity. 5. Seal chamber face run-out. A seal chamber face which is not perpendicular to the shaft axis can cause a serious malfunction of the mechanical seal. Because the stationary gland plate is bolted to the face of the seal housing, any misalignment will cause the gland to cock, which causes the stationary element to cock and the entire seal to wobble. This condition is a major cause of fretting wear where the mechanical seal shaft packing contacts the shaft or sleeve. A seal that wobbles can also cause wear or fatigue of metal bellows or drive pins, which can cause premature seal failure. To measure seal chamber face squareness, leave the housing bolted in place and clamp the dial indicator to the shaft as shown in FIG. 5, with the stem against the face of the housing. The total indicator run-out should not exceed 0.005 in. (0.13mm) T.I.R. 6. Seal chamber register concentricity. An eccentric chamber bore or gland register can interfere with the piloting and centering of the seal components and alter the hydraulic loading of the seal faces, resulting in reduction of seal life and performance. To measure chamber-bore concentricity, leave the housing bolted in place and insert the dial indicator stem well into the bore of the housing. To measure the gland register concentricity, the indicator stem should bear on the register O.D. The bore or register should be concentric to the shaft within 0.005 in. (0.13mm) of the T.I.R. If the bore or register are eccentric to the shaft, check the slop or looseness in the pump-bracket fits at location "A" as shown in FIG. 6. Corrosion, whether atmospheric or due to leakage at the gaskets, can damage these fits and make concentricity of shaft and housing bore impossible. A remedy can sometimes be obtained by welding the corroded area and re-machining it to proper dimensions, or by replacing the damaged parts. If this is not practicable, it may help to center the entire housing and dowel it in place. 7. Driver alignment and pipe strain. Regularly scheduled inspections are absolutely essential to maintain proper coupling and driver alignment. Follow the recommendation of the coupling manufacturer to check coupling alignment. Because temperature can affect coupling alignment due to thermal growth of pump parts, be sure to check pump coupling alignment at the operating temperature. Pipe strain can cause permanent damage to pumps, bearings, and seals. Many plants customarily blind the suction and discharge flanges of their inactive pumps. These blinds should be removed before aligning the pump driver. After the blinds have been removed, and while the flanges on the suction and discharge are being connected to the piping, read the dial indicator at the O.D. of the coupling half as the flanges are being secured. Any fluctuation indicates that pipe strain is present. Seal Checkpoints Modern process plants use only factory-reconditioned, or brand-new cartridge and/or cassette-type mechanical seals. Therefore, only a few points need to be prechecked on both new and factory-reconditioned assemblies: • Make sure that all parts are clean, especially the mating faces of the assembly • Check the seal rotary unit and make sure it’s free to rotate • Check the setscrews in the rotary unit collar to make sure they are free in the threads. Note: Setscrews should be replaced after each use. • Check the thickness of all accessible gaskets against the dimensions specified in the assembly drawing. An improper gasket thickness may be a safety hazard • Check the fit of the gland ring to the equipment. Make sure there is no interference, binding on the studs or bolts, or other obstructions. Be sure any gland ring pilot has a reasonable guiding fit for proper seal alignment. • Make sure all rotary unit parts of the seal fit over the shaft. Particular care should be given to elastomeric secondaries. Installation of the Seal Many seal failures can be traced to installation errors. Careful installation is a major factor in the life of a seal. 1. Read the instruction booklet and review the drawing that accompanies each cartridge or cassette-type assembly. 2. Remove all burrs and sharp edges from the shaft or shaft sleeve, including sharp edges of keyways and threads. Replace worn shaft or sleeves. 3. Make sure the seal housing bore and face are clean and free of burrs. 4. Prior to seal assembly, lubricate the shaft or sleeve lightly with silicone lubricant. Don’t use oil or silicone at the seal faces. Keep them untouched and don’t disassemble the unit. 5. Flexibly mounted inserts should be lightly oiled and pressed in the gland by hand pressure only. Where an insert has an O-ring mounting on the back shoulder, it’s usually better to nest this O-ring into the gland cavity and then push the insert into the nested O-ring. 6. Strictly follow the manufacturer's installation instructions. They vary for different types of seals. In the rare instances when an old-style, non-cartridge seal is used, and when the seal drawing is not available, the proper seal setting dimension for inside seals can be determined as follows for seals con figured as shown in FIG. 7. FIG. 7. Calculation of seal collar setting. FIG. 8. Calculation of seal collar setting. FIG. 9. Setting for outside seal. FIG. 10. Preset cartridge seal. 1. Establish a reference mark on the shaft or sleeve flush with the face of the stuffing box (point "A" in FIG. 8). 2. With the insert in place, stack up the gland and the rotary unit on a clean bench. 3. Compress the seal rotary unit until the spring gap, dimension "B" in FIG. 8, equals the dimension stamped on the collar for pusher type seals. This can be done by inserting an Allen wrench or piece of tool stock of the proper dimension between the collar and the compression unit before compressing the rotary unit. 4. Measure the distance "C" between the gasket face and the end of the collar. This is the collar-setting dimension. If the seal is con figured as shown in FIG. 8, proceed as follows: 1. Measure the distance from the reference mark "A" to the face of the stationary insert in the gland plate. Be sure to include any gland gasket for proper measurement. This is shown as dimension "B" in FIG. 8. 2. Refer to the manufacturer's instructions for the correct installed length of the rotary unit ( FIG. 8, dimension "G"). 3. Subtract dimension "B" from dimension "G". This is the collar setting dimension. Outside seals (rarely acceptable in modern process plants) should be set with spring gap ("A" in FIG. 9) equal to the dimension stamped on the seal collar. Cartridge seals, FIG. 10, are set at the factory and are installed as complete assemblies. No setting measurements are needed. These assemblies contain centering tabs or spacers that must be removed after the seal assembly is bolted in position and the sleeve collar is locked in place. Retain the tabs to be used when resetting the seal for impeller adjustments or when removing the seal for repairs. Optical Flat Since this text no longer advocates in-house repairs of mechanical seals, optical flats are only considered useful tools for "postmortem" failure identification and troubleshooting (see Volume 2 of this series). An optical flat is a transparent quartz or Pyrex disc having at least one surface flat within 0.00001 in. to 0.00005 in. (0.025 to 0.125 microns) (see FIG. 11). The least expensive optical flats, those in the 0.000005 in. (0.025 microns) accuracy range, are suitable for measuring seal face flatness. Select an optical flat with a diameter at least equal to the diameter of the part being measured. Optical flats in sizes up to 8 in. (200mm) in diameter are available from most seal manufacturers. Care must be taken not to slide or lay the flat side of an optical flat on any rough surface, because it’s easily scratched. Monochromatic Light. White light from the sun is actually a combination of several colors, each of which represents a different wavelength of electromagnetic energy. If sunlight were used to measure seal face flatness, each color would generate its own pattern of bands on the optical flat. A far more practical light source for this purpose is one that provides light of only one color-a monochromatic light source. Helium gas in a tube, when excited by an electrical charge, emits fewer colors than sunlight. One of these colors-yellow orange-is so prominent it overrides all the others. The yellow-orange wavelength is measurable and constant at 23.13 millionths of an inch (0.58 microns). A helium lamp, therefore, is described as emitting a monochromatic light of that wavelength. Flatness Readings. After a seal part has been polished, it’s placed under a monochromatic light, and an optical flat is positioned over its surface. Both the surface of the optical flat and the surface of the part must be absolutely dry and free from any particles of dirt, dust, or lint. FIG. 11. Optical flat. A pattern of light and dark lines appears where the reflection of the surface of the part and the reflection of the surface of the optical flat meet. The dark lines, called "light interference bands," will be visible every half wavelength. Therefore, one interference band equals one-half wavelength or 11.6 micro-inches (0.29 microns). These bands are used to measure the degree of flatness of the surface. The presence either of several straight interference bands, or of a single circular interference band, indicates a surface that is flat within 11.6 micro-inches (0.29 microns). When several curved interference bands appear, the degree of flatness is measured by plotting an imaginary straight-line tangent with one of the curved interference bands. If this straight line intersects only one interference band, the lapped surface is said to be flat within one light band. When two or more curved interference bands are intersected by this line, the degree of flatness is determined by multiplying the number of intersected bands by 11.6. Thus, if three interference bands are intersected by a straight tangent line, the surface is out-of-flat by three light bands or 34.8 micro-inches (0.87 microns). Although a maximum tolerance of three light bands is considered adequate for proper seal face performance, seal faces from experienced manufacturers consistently exhibit a flatness between one and two light bands. Special concave or convex geometries are available if special conditions warrant. A seal face with a diameter greater than 10 in. (250mm) and a thin cross-section is difficult to measure precisely for flatness. See FIG. 12 and 13 for sample light-band readings. Note that when the band pattern is inconsistent or seems to be missing some bands, as in FIG. 13(e), (f), and (g), the tangent line "AB" is drawn to connect the two points at which two imaginary radii-90° apart and perpendicular to the axis of the part-intersect with the outer circumference of the seal face. Once flatness has been achieved, a seal face must be kept clean while being returned to service. FIG. 12. Band patterns that indicate flatness accuracy. FIG. 13. Light-band readings. Installation of Stuffing Box Packing Maintenance of a stuffing box consists primarily of packing replacement. This may sound simple, but certain rules must be followed if the best results are to be obtained from the packing. The following procedure should be preceded by a careful study of the pump manufacturer's instruction manual to determine the correct type and size, number of packing rings, location of lantern ring, and possible special features of construction, operation, or maintenance. 1. Remove gland and packing. If the box contains a lantern ring, make certain that all the packing inboard of it’s also removed. Flexible packing hooks are available. 2. Inspect the shaft or shaft sleeve for score marks or rough spots. A badly worn sleeve or shaft must be replaced; minor wear must be dressed smooth and concentric. Inspect lantern ring to make sure that holes and channels are not plugged up. 3. Clean bore of box thoroughly and be sure sealing-fluid passages are open. 4. Depending on operating conditions, it’s recommended that at least the outside diameter of the replacement packing rings be lightly oiled or greased. Start by installing one end of the first ring in the box and bring the other end around the shaft until it’s completely inserted. Note: Preformed or die-molded rings will ensure an exact fit to the shaft or sleeve and stuffing box core. A uniform packing density is an added benefit since molded rings are partially compressed. If the packing comes in continuous coil form, make sure that the ends are cut square, on a correctly sized mandrel. 5. Push the packing ring to the bottom of the box with the aid of a split bushing. Leave the bushing in place, and replace the gland. Pull up on the gland and seat the ring firmly and squarely. Ideally, the split bushing should be a slip fit over the shaft and in the bore. This precaution will prevent the formation of a lip on the packing being seated. 6. Repeat this method for each ring, staggering the joints 90°. It’s especially important to seat the first few bottom rings firmly, otherwise the rings immediately under the gland will do most of the sealing. 7. The location of the lantern ring, if used, should be predetermined. This can be done via the pump manufacturer's instruction manual or by counting the number of packing rings on the inboard and out board side of it while removing them. Proper location at the point of fluid-seal is necessary. 8. Replace the gland and tighten the gland bolts. Make sure that the gland enters the stuffing box squarely. A cocked gland causes uneven compression and can damage the shaft or sleeve when the pump is placed in operation. 9. Keep the packing under pressure for a short period of time, say 30 seconds, so that it can cold flow and adjust itself. 10. Loosen the gland; allow packing to fully expand with no fluid pres sure in the pump, then tighten the gland and bring it up evenly to the packing, but this time only finger tight. The packing should be loose enough to enable you to turn the pump shaft by hand, assuming that the pump is not too large to turn by hand. 11. Pre-check the lines to the lantern ring or quench gland for flow and proper pressure. 12. Start the pump. The leakage may be excessive, but don’t take up on the gland bolts for the 20-30 minute run-in period. If too tight, shut pump down and repeat steps 8-11 or keep pump running and loosen gland bolts a couple of flats. 13. If leakage is still more than normal after the run-in period, tighten the gland bolts evenly, one flat or a sixth of a turn at a time. This should be done at 20-30 minute intervals until leakage is reduced to normal. This may take several hours, but will pay for itself many times over in maximum packing and sleeve life. Steady Leakage Flow Is a Must Leakage must be sufficient to carry away the packing friction heat. Without sufficient leakage-a steady flow, as opposed to a drip-the packing will burn and the shaft or sleeve will be scored. It follows, then, that a planned maintenance program must be developed around the amount of leakage that is considered normal. Pumps in continuous service may require a daily, or even an hourly inspection. Inspection need be no more than a visual check to determine any deviation from the normal leakage, and make slight corrections as required. Correct procedure and scheduled maintenance will make the repacking of a pump a predictable job rather than an emergency repair. Consider Upgrading to Mechanical Seals It should be pointed out that conversion from packing to well-designed mechanical seal systems is often feasible and has proven to be a cost effective reliability improvement step. We strongly advocate such upgrading wherever feasible; it usually is feasible! Welded Repairs to Pump Shafts and Other Rotating Equipment Components This section will highlight some of the technical aspects of welded repairs to rotating equipment, with particular emphasis on pump shafts. We believe this to be a reasonable alternative to new part installation. Welded repairs to rotating equipment can be an extremely useful maintenance method; however, it should be emphasized that welding is not a panacea for all problems. In fact, many people refuse to recommend or even consider welded repairs on any rotating components. This reluctance is probably due to unsuccessful experiences in controlling distortion. However, other methods of reconditioning, such as sleeving, plating, and spraying also have their limitations. The primary problems with these alternative repair methods are the limitations on allowable coating thickness and lack of bonding with the substrate. For example, sleeving has both minimum and maximum thickness limitations and is also limited to the stepped end areas of a shaft. The advantages of welded repairs are: 1. Full fusion of the overlay with the base material is achieved. 2. Shaft strength is maintained. 3. The corrosion or wear resistance can be improved over the original material. 4. The overlay thickness is essentially unlimited (minimum or maximum). The only real disadvantage of welded repairs is the possibility of distortion. For many engineering applications, this is of negligible concern. However, for most rotating equipment components, this is the major concern and the reason many people recommend against attempting welded repairs. If the distortion problem can be eliminated, then welded repairs can be used with confidence as a standard maintenance procedure. The techniques discussed here are designed primarily to minimize distortion. How to Decide if Welded Repairs Are Feasible These questions must be answered in determining if welded repairs are suitable for a particular situation: 1. What is the damage and what type of repair is required? As previously mentioned, the advantages of welded repairs must be balanced against the increased risk of shaft warpage. However, there are situations where welding is the only possible method of saving a shaft and returning it to service. A good example is damage to a coupling area, where all other repair techniques are unsuitable because of the mechanical strength and bonding requirements. In general, the damaged area should not have reduced the shaft diameter by more than 15 percent and should not exceed 10 percent of the shaft length. The repair should be limited generally to one area or two well separated areas. These figures are only guidelines; there is no hard information on which these values are based. Situations exceeding these restrictions may be repairable but a more careful consideration of shaft strength, possible distortion, and economics needs to be made on an individual basis. 2. What is the shaft material? The shaft material must have good weldability. Some common shaft materials that can be included in this category are: 304 Stainless Steel 316 Stainless Steel Monel 400 Monel K-500 Nitronic 50 (XM-19) Ferralium A 638 Gr 660 Inconel 625 Typical compositions are listed in TBL. 1. TBL. 1 Nominal Composition of Materials Used for Rotating Equipment Weld Repairs Trademarks: Monel and Inconel-International Nickel Co. Nitronic-ARMCO Inc. Ferralium-Langley Alloys Ltd. Stellite-Cabot Corp. All of these materials are very ductile, don’t exhibit hardened heat affected zones, and will usually maintain adequate corrosion resistance in the "as welded" conditions. There does appear to be a common problem with dimensional stability of the Ferralium and the 300 series stainless steel. These materials appear to creep, even under static conditions. The other materials listed don’t seem to be quite as sensitive to this problem. It’s obvious that welded repairs will exacerbate this situation, but it’s not possible to address this problem at this time. Shaft materials that should not normally be considered for welding are: 4140 4340 410 Stainless Steel Although many companies have probably done emergency or spot repairs on shafts of these materials, it should be emphasized that welded repairs are not generally recommended. 3. What is the overlay material? Given the previously mentioned shaft materials that would be considered for welded repairs, Inconel 625 (AWS A5.13 ERNiCrMo-3) would normally be selected as the filler metal. Although other materials could be used, it’s easiest to standardize on one filler metal that gives a deposit of exceptional corrosion resistance. The deposit may or may not match the tensile strength of the original shaft material, but since the deposit is extremely ductile and has complete fusion at the interface, it has only a marginal effect on total shaft strength. It should be remembered that alter native repair methods require undercutting the shaft diameter, thus permanently and perhaps significantly reducing the total shaft strength. For other components, such as wear rings, Stellite 6 (AWS A5.13 RCoCr-A) can be used to give a hard, wear resistant, anti-galling deposit. The Stellite overlay requires a more careful application because the deposit is crack sensitive. As a result, the depth of overlay is restricted and repairs to the overlay may be difficult. Other overlay materials might be required for special cases but these would need to be considered on an individual basis. FIG. 14. Shaft preparation by machining. Repair Techniques 1. Shaft Preparation ( FIG. 14) The damaged area should be undercut by machining the shaft, but the amount of material removed should be limited as much as possible. For a 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 1.6mm). The minimum depth of cut is specified to avoid having the fusion line positioned 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 3.2mm) diameter should be used. FIG. 15. Spiral welding sequence for shafts. 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 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 ( FIG. 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. (10mm). As a minimum, one complete circumferential 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 20 in. from the end) of up to 0.015 in. (0.38mm) occurred during the actual welding but would return to less than 0.005 in. (0.13mm) during cooling periods. Some cold straightening may be required to correct any residual distortion, 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. Case Histories A number of related experiences are summarized below: Pump shafts, all overlaid with Inconel 625: 1. Water injection pump ( FIG. 16 and 8-17)-Monel K-500 shaft, 5-in. dia, approximately 27 in. length overlaid on coupling end, 1/16 in. deep; approximately 21 in. 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. FIG. 16. General view of repaired shaft during machining. FIG. 17. Edge of weld repair area in the rough machined condition. Since any unsuccessful attempt should generate as much useful information as a successful result, it’s 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 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 addition, 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 0.060 in. undersize on the diameter (0.030-in. cut), Stellite 6 overlay (GTAW process), and final machining to size ( FIG. 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. FIG. 18. Impeller with direct overlay of Stellite to replace wear rings. 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 temperature 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 ( FIG. 20 and 8-21). The erosion/corrosion problem has been effectively eliminated while providing a significant savings compared to the cost of a stainless or alloy replacement case. FIG. 19. Impeller with direct Stellite overlay in final machined condition. FIG. 20. Pump case with overlay along centerline bore and edge of split line face. FIG. 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 ( FIG. 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. FIG. 22. Overlaying of seal flange faces. 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’s 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 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 minimize heat buildup for more critical components, such as shaft repairs. We conclude: • Experience has shown that welded repairs to shafts and other rotating equipment components can be successfully accomplished. • Welding techniques and procedures must be selected in order to minimize distortion. This includes the use of low heat inputs and special sequences. • Filler metal selection can provide improved properties, such as corrosion and wear resistance, over the original base metal. Next>> |