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AMAZON multi-meters discounts AMAZON oscilloscope discounts Summary The system of plant maintenance and engineering services outlined has been successful in achieving exceptionally low maintenance costs for a nationwide complex of plants. The overall maintenance system described herein has allowed a steady reduction in plant personnel with corresponding savings. Actual maintenance costs have steadily decreased as a percentage of original investment costs without any allowance for labor and material escalation. With these factors taken into account, the total maintenance cost reductions are indeed significant. The publishing of monitored data on plant performance and preventive maintenance compliance has allowed for prompt management attention to problems and has stimulated a noticeable spirit of competition among the plants. Needless to say, it also serves as a valuable method of evaluating plant manager performance. Although the system described herein may not be directly applicable to every large chemical complex for reasons of size or process type, the general trends toward computerization, sophisticated equipment monitoring methods and processes, cheaper transportation and communication costs, are indicative of increasing advantages obtainable in the future from centralized concepts in plant maintenance and engineering for multi-plant corporations. FIG. 3. Simplified computer program used to evaluate compressor efficiencies. Machinery Maintenance on the Plant Level With this overview behind us, let's get back to the machinery engineer's concerns on an individual plant level. As he of course knows, modern turbo-machines can run reliably for many years if designed, applied, and operated correctly. As of 2004, the periods between inspection and over haul, commonly called "turnaround," on machines in clean, noncorrosive service can exceed eight years. It’s easy to see how plant personnel may have trouble remembering just how much time and effort are required to successfully plan and execute an overhaul of a particular piece of equipment. A proper turnaround involves preplanning and teamwork among plant technical, warehouse, purchasing, safety, operations and maintenance forces, as well as with the original equipment manufacturer and other non-company sources. In the case of sophisticated problems, consultants and laboratories may also come into play to restore machinery to a reliable, smooth-running, and efficient operation. Managing these resources and documenting the results presents a real challenge to those assigned the task of heading up the overhaul effort. This segment of our text deals with turnaround management principles that must be understood and considered by maintenance personnel on the plant level. Assignment of Qualified Personnel Major machinery overhauls require not only early planning input, but also early designation of qualified personnel to execute planning and related tasks. High quality machinery overhauls can be more consistently achieved if machinery expertise is directly applied. Process plants subscribing to this approach define, in specific outline form, the responsibilities of supervisory and staff personnel involved in turnaround (T/A) of major machinery. The outline explains the various job functions involved in T/A activities and identifies the timing and scheduling requirements which precede the actual shutdown. The assignment of qualified personnel starts with the designation of an overall T/A coordinator no later than nine months before the scheduled shutdown. As of this time, the plant's senior machinery specialist is required to maintain formal communications with the T/A coordinator. Among other duties, the T/A coordinator screens and approves the planning and scheduling efforts of maintenance or contractor personnel involved in major machinery T/A's after the senior machinery specialist has had an opportunity to review these efforts. Working hours and team composition are to be determined jointly by the T/A coordinator and senior machinery specialist. Experience shows that a spare parts and materials coordinator should also be designated no later than nine months before scheduled shutdown. This person will generally be responsible for implementing spare parts procurement requested by the senior machinery specialist and mechanical supervisors. He will be required to forward up-to-date listings of parts on hand to personnel requiring this information. The plant senior machinery specialist is generally charged with responsibility and authority to direct planning and execution of the machinery portion of the T/A. His background and experience should make him uniquely qualified for this job, and as senior resident expert he would be thoroughly familiar with all machinery affected by the planned T/A. A key job function is to be fulfilled by turbotrain T/A engineers. On major machinery T/As it was found essential to have one or more of these engineers assigned the responsibility of verifying the quality of execution of all machinery overhaul tasks. If a given plant does not have enough machinery engineers to man the job around the clock, affiliate loan or con tractor engineering personnel should be brought in for the duration. The specific responsibilities of turbotrain T/A engineers have been described as those of a machinery advisor and quality control person who augments the mechanical supervisor and reports to the senior machinery specialist for work direction and guidance. His responsibility and authority extends from machine inlet nozzle to machine outlet nozzle and includes lube and seal oil systems. His work begins after all required blinds have been installed and ends after every item of machinery work is complete. He will then turn over the machinery to the mechanical supervisor for removal of blinds. A turbotrain T/A engineer typically has a degree in an engineering discipline (preferably mechanical engineering) and has had practical machinery engineering experience for a minimum of five years. His past assignments should have included active participation in major machinery erection, commissioning, testing, operation, troubleshooting, and repair. He normally performs work which involves conventional engineering practices but may include a variety of complex features such as resolution of conflicting design requirements, unsuitability of conventional materials, and difficult coordination requirements. His normal sphere of activity requires a broad knowledge of precedents in turbo machinery design and a good knowledge of principles and practices of materials technology, metalworking procedures, instrument-electrical techniques, etc. This person should be a hands-on engineer whose past performance will have established his reputation as a resourceful, highly dependable contributor, a self-starter with sound judgment. Timing and Basic Definition of Critical Pre-Turnaround Tasks Senior Machinery Specialist Immediately following the designation of a T/A coordinator (approximately nine months before T/A), the senior machinery specialist starts to interface with planners, designated turbotrain T/A engineers, maintenance or mechanical supervisors, and the T/A coordinator. From then on, the following action items and timing will be typical of this function: Nine Months Before T/A: • Work list items assembled by maintenance are forwarded to the senior machinery specialist for review purposes. A typical work list page is shown in FIG. 4. • He receives the most probable work zone outline for review and comment. (For description of work zones, refer to Volume I of this series.) • The senior machinery specialist requests up-to-date tabulation of spare parts presently on hand for major machinery trains. The spare parts coordinator must provide this tabulation in a format similar to FIG. 5. Eight Months Before T/A: • The senior machinery specialist determines which replacement parts are required for major machinery T/A • He issues written requests for the spare parts coordinator to place a "hold" on selected parts, locally stocked parts, or to obtain these from the corporate central storage location • He specifies inspection requirements for existing key spare parts and any additional key spares to be procured Six Months Before T/A, the senior machinery specialist must: • Commence refresher training for mechanical supervisors, craftsmen, and designated contract personnel • Arrange for vendor assistance • Review the machinery T/A schedule FIG. 4. Typical work-list page for major compressor turnaround. FIG. 5. Typical spare parts tabulation. Three Months Before T/A: • He meets with designated turbotrain T/A engineers for detailed briefing and solicitation of additional input Two Months Before T/A, the senior machinery specialist should: • Review final (detailed) T/A plan for each train • Verify that work procedures are either available or being produced One Month Before T/A, his tasks include: • Review of detailed information package for each train. This package will have been assembled by the mechanical supervisors and planners, as we will discuss later • Review of bar charts prepared by maintenance, and include these in package Finally, during the actual T/A, the senior machinery specialist must: • Participate in daily T/A meeting • Verify that work procedures are followed • Verify that data are taken and logged in as required • Address deviations from plan • Review test runs • Review updated start-up instructions Turbotrain Turnaround Engineers Approximately three months before the scheduled shutdown, designated turbotrain T/A engineers meet with the senior machinery specialist for detailed briefings and reviews of machinery T/A organization, procedures, and preparations. Additional responsibilities are as follows: Three Months Before T/A: • Prepare detailed information package for each turbotrain. A typical table of contents for one such package is shown in FIG. 6. Sample material making up the package is shown in later illustrations. Two Months Before T/A: • Review final (detailed) T/A plan for each train • Verify that work procedures are satisfactory One Month Before T/A: • Review detailed information package for each train • Review bar charts prepared by and forwarded by the senior machinery specialist FIG. 6. Table of contents for typical compressor turnaround package. During T/A: • Participate in daily T/A meeting • Verify that work procedures are followed • Review condition of used parts as they are removed from the machine • Verify that data are taken and logged in as required • Take photographs and dictate observations into tape recorder • Verify the installation is mechanically correct • Resolve deviations from plan • Verify machinery alignment • Supervise test runs • Restart per startup instructions Mechanical Supervisors/Planners Planners, and also mechanical and maintenance supervisors provide machinery related data and support to the senior machinery specialist and turbotrain T/A engineers involved in planning and execution of turbotrain turnarounds. Again, nine months before the scheduled shutdown for a major machinery T/A, planners and mechanical supervisors will be given initial guidance on anticipated duties and responsibilities prior to and during the actual T/A. From then on, typical action and timing would be: Nine Months Before T/A: • Maintenance personnel forward machinery-related work lists to senior machinery specialist for review • The most probable work zone outline is drawn up and forwarded to the senior machinery specialist • The mechanical supervisors instruct the spare parts coordinator to assemble up-to-date tabulation of spare parts presently on hand for major machinery trains. After review, they forward the tabulation to the senior machinery specialist. Eight Months Before T/A: • The spare parts coordinator and maintenance personnel receive the senior machinery specialist's request to: 1. Place a "hold" on selected parts 2. Order additional spare parts In response, they issue purchase orders for additional replacement parts. • Next, maintenance supervisors commence dimensional checking of selected (existing) spare parts per request made by the senior machinery specialist. The results should be documented within eight days. • Dimensional checking has frequently shown serious discrepancies in parts designation, dimensional configurations, and tolerances. These must be identified early if a smooth turnaround is to result. Six Months Before T/A: • Maintenance personnel arrange for vendor assistance • Maintenance personnel also work up a more definitive work zone arrangement and commence tabulation of detailed work list for each zone • Maintenance or technical department personnel witness check balancing of major turbomachinery rotors • Assemble tools and identify missing tools • Arrange for scaffolding, etc. • Forward data to senior machinery specialist regarding status of spare parts ordered six months earlier • The planner should now provide final work zone arrangement and detailed work list for each One month before T/A, maintenance planners or mechanical supervisors provide bar chart diagrams for machinery-related T/A work. • They participate in a meeting with the senior machinery specialist and designated turbotrain T/A engineers One week before T/A, mechanical supervisors commence meeting with designated turbotrain T/A engineers for briefings on matters relating to machinery work. Specific Preparation and Planning When preparing for an overhaul of a major piece of turbomachinery, it’s important to know as much as possible about the machine and why it needs to be taken out of service. There are several obvious sources of information, including the operating and maintenance personnel, the equipment file folder and the vibration history record. If sufficient information is not found in the file folder, which is all too often the case, this fact should reinforce the resolve to do a proper job of documenting the planned overhaul in a special manual or "machinery T/A package." Before proceeding, one question usually arises: Is a complete overhaul really necessary? To properly answer this question, you will need to evaluate the symptoms. Has the vibration steadily increased over a long period of time or have you witnessed a step change? What does an analysis of the vibration signature reveal? Has the performance gradually fallen off or taken a dramatic drop? Problems such as a locked gear coupling or soluble deposits inside the machine can sometimes be corrected without opening the machine and at a considerable savings of time and effort. Another obvious source of information is the manufacturer's manual. The good ones provide detailed, step-by-step instructions with clear illustrations; others assume prior knowledge or place undue reliance on the manufacturer's service representative. Consequently, it’s prudent to develop procedures, installation instructions, or even detailed commissioning instructions for inclusion in the turnaround package. See FIG. 6 for typical requirements. Since a detailed manual is often too bulky for constant reference, we might reduce portions of it to a critical item list. Certain steps, clearances, and methods are vital to doing a good job. These items should be summarized and kept for ready reference during the course of the overhaul. In fact, one complete turnaround package should be on the compressor platform and should be used while the job is in progress. It’s important to assign the responsibility for the overhaul to one person so that conflicting positions don’t occur. As indicated earlier, we recommend the appointment of a turbotrain T/A engineer to oversee the job and believe that all decisions and compromises should be made by him. He should be responsible for the engineering coverage, interface with the maintenance and operating departments, interface between user company and original equipment manufacturer, and for documenting the overhaul. It’s a responsible assignment, one that requires judgment, maturity, and initiative on the part of the engineer. It’s strongly recommended that the turbotrain T/A engineer assume responsibility for the development of data packages and checklists, some of which are shown later in sample form, but which must of course be adapted to fit a specific machine or turn around situation. Safety Work safely. Be sure all power is off, blinds in, purging procedures followed, etc. A prework safety item checklist is strongly recommended, as is a list of all blind locations. The latter item is important at the beginning of a job to ensure all necessary lines are secure, and, at the end of the job, to check off the removal of all installed blinds. Failure to install or remove a blind at the appropriate time could lead to a disaster. Account for tarps and "welding blankets." These are known to have been left in piping, only to later be ingested into equipment. The consequences of these oversights have ranged from costly to catastrophic. It’s important to establish teamwork and proper communication among the operations, safety, engineering, and maintenance personnel at the start of the job so that each can fulfill his role in the total effort. A list of key players and where or how they can be reached during the overhaul period should be made available at the start of the job. Planning: If this is a planned overhaul, as opposed to a forced outage, so much the better. Take full advantage of the planning period to make a visual inspection of the machine before the shutdown. Pay particular attention to the condition of the foundation, anchor bolts, piping, instruments, and look for leaks. It’s a good idea to keep an "evergreen" list of required maintenance items in the equipment folder. Encourage personnel who frequently go on the machinery deck to make written note of any problems. Take a final check of vibration, performance, alignment, and mechanical health data just prior to the shutdown. A small shirt-pocket size tape recorder or palm-size computer with voice recognition software is particularly useful to record notes; it leaves the hands free to manipulate instruments, etc. The data can then be transferred to spreadsheets back in the office. If you are working a forced outage, the most recent set of data will have to do. Compare the most recent information to previous readings and develop a list of anticipated problems. Translate all of this information into a detailed job plan per FIG. 7, or as available from CMMS software programs. In our experience, machines are normally shut down for overhaul due to fouling (restricted performance); excessive vibration (ingestion of a liquid slug, a loose piece of hardware, the failure of a mechanical component or misalignment); mis-operation (surge, lube oil supply failure, etc.); or when the whole process unit is shut down for a T/A. In general, we don’t open machines that are running satisfactorily just for inspection. At every convenient opportunity one should inspect externally accessible components, such as couplings, and also check items such as rotor float and shaft alignment, and all tripping devices and general instrumentation. In the case of steam turbines, the overspeed trip bolt and the steam trip and throttle valve have proven to be the least reliable-and yet most important-safety devices in the train. A check of these two components is mandatory during major shutdowns, and checks should be made at every other opportunity. We believe these are the most important checks performed during a shutdown. In addition, one should "exercise" the trip and throttle valve weekly by moving the stem in and out manually several turns on the hand wheel to preclude the buildup of deposits that would prevent the machine from tripping during a shutdown condition. FIG. 7. Sample sheet of turbomachinery turnaround job plan. Consider an example of the penalty associated with the failure of a trip circuit at one plant. A 10,000 horsepower, steam turbine-driven compressor train failed to trip during a condition which had caused the compressor to fill with liquid while at full speed. The resulting loads led to catastrophic failure of both the compressor and coupling and allowed the steam turbine to overspeed to destruction. The repair bill for parts and labor came to well over $1,000,000. The cause of the wreck was eventually traced to the buildup of deposits on a brass piston in the hydraulic shutdown system which was in a hot dead-ended oil circuit. The heat caused the oil to decompose over a long period of time, in turn causing the piston to stick. As a result, the plant now checks trip circuits more frequently. They have also installed redundant electronic backup trip devices on large equipment trains. Spare Parts: The time to check spare parts is not in the middle of the night following an emergency phone call from the operating department manager. Most large companies have some degree of computer control on the ware housing and reordering of spare parts. But how many times have you been lied to by a computer? There is no substitute for a hands-on check of parts by a knowledgeable individual. Part numbers must be checked because the item on the shelf is not necessarily the one you expected to find. Many major plants allocate special boxes to major machinery spare parts storage. The boxes have individual compartments for labyrinths, seals, bearings, etc. A list on the lid details all parts inside, their location in the box, the manufacturer's part number and the company's stock number. Once filled, the box is sealed and stored in the usual manner. During an overhaul, the box is taken to the field and some or all of the parts consumed. The box is then returned to the warehouse with a list of consumed parts to be replaced. A computerized call-file system should be used to keep tabs on rotors that are out of the plant for repairs and delivery of other critical spare parts. When checking spare parts, it’s important to recognize that not only must the part be the right size, it also must be in good condition. Handling and improper storage, as well as deterioration with time, are a few of the hazards associated with a warehousing operation. A nicked O-ring or a carbon seal face out of flatness could require a second shutdown to correct the problem. The use of an optical flat, a set of micrometers, and a knowledgeable pair of eyes can be invaluable in detecting a defective part. Also, remember that just because the part came from the factory, doesn't necessarily mean it’s the right one for your machine. While equipment manufacturers have various quality control procedures, they too rely on human beings, and errors do occur. In addition, some parts have a finite shelf life (case split line sealant is an example) and must be fresh when the time comes to use them. This is also the time to check on the availability of special (custom fabricated) tools. These should be kept in a separate box, inventoried at regular intervals, and generally treated as a valuable spare part or essential resource. Delaying an overhaul for several hours to fabricate a special seal nut wrench is time and money wasted. Alignment brackets and coupling "solo" plates fall into this category. FIG. 8. Spare rotors must be removed from storage and cleaned and inspected prior to the turnaround. The Spare Rotor By far the most critical single spare part is the spare rotor. Most companies purchase the spare rotor at the time the machine is purchased and require a four-hour mechanical test to ensure integrity prior to acceptance of the machine. It would be prudent to check the spare rotor after every transport event. This means a runout or rotor bow check upon receipt from the manufacturer, as well as a check of the preservative used for long-term storage. A runout check is also performed at the time the rotor is check balanced and prepared for installation. Be sure to obtain a rotor runout diagram and balance report at that time. Rotors of all sizes are often stored vertically in a remote temperature controlled storage building as shown in FIG. 8. If a user opts for horizontal storage, the rotors must be placed on substantial stands and should be turned 180° two to four times a year. These stands must employ rollers rather than lead or Teflon material at the support points. In cases where sheet Teflon is placed between the storage cradle and the rotor there is some risk of filling the microscopic pores of a shaft journal which could prevent the formation of an adequate oil film on startup and could cause bearing failure. Rotors must, of course, be handled with great care. Nylon slings should be used to prevent damage and all lifts should be made under the watchful eye of a competent individual. Never hesitate to call a halt to a lifting operation if the possibility of damage exists. You are being paid to look out for the company's interests and a rotor worth $200,000 to $1,000,000 or more is well worth a lot of care and concern. The rotor must be slung so that it’s horizontal and its center of gravity is located under the hook, and it must be moved very slowly. Consider your vibration monitoring probes when removing a rotor from storage in preparation for installation. Record the rotor's serial number and verify that it’s not positioned in a way that will interfere with a thrust position eddy current probe. Some users also report success with degaussing and/or micro-peening techniques to minimize electrical runout in the areas viewed by the radial eddy current probes. Others report some success rolling the rotor on a balance stand with the areas under the probes directly on the balance rollers. FIG. 9. Critical dimension diagram for centrifugal compressor. FIG. 10. Critical data tabulation. Diagrams A critical dimension diagram ( FIG. 9) and associated tabular records have proven invaluable in the middle of the night during a complicated overhaul. A critical dimension diagram is a tabulation or sketch recording critical data such as bearing and labyrinth clearances, rotor float, seal clearances, coupling advance, coupling bolt torque, etc. The document must clearly show maximum and minimum values, as well as spaces for "as found" and "as left" conditions. Any warning notes such as internal bolts, left-hand threads, or other critical steps should be clearly flagged on this sheet. Clearances should be properly labeled as to diametral or radial, metric or English units, to avoid confusion. An alignment diagram, as shown in the section on machinery alignment, complete with estimated thermal growth and desired readings, is mandatory. This should be available from previous alignment work. If it’s not, and if reverse dial indicator alignment techniques are not well known and practiced at your plant, we would strongly recommend implementation of such a program. The techniques and procedures have been the subject of many papers. The records and documents described thus far have proven to be time saving and hence, money saving, and are well worth the effort. Another useful item involves preplanning the allowable limits on the desired shaft position. It’s impractical to expect the field crew to place a compressor or turbine in the exact position as shown on the alignment graph. If allow able limits are known in advance (not necessarily by the field crew, but by the engineer in charge of the overhaul), a decision or compromise can be made in a rational manner depending on need for the machine and time available to achieve acceptable alignment. Under no circumstances should alignment be compromised beyond a few thousandths off the desired position nor excessive pipe strain be permitted on the machine. The search for absolute perfection will, however, generally be rewarded with time consuming frustration and an ultimate compromise in any case. FIG. 11. Critical dimension tabulation. FIG. 12. Clearance tabulation. Miscellaneous Items Any good shutdown/overhaul plan should include an inspection of auxiliary components. During the overhaul period is the time to clean lube oil coolers, replace filters, overhaul lube oil pumps, etc. But beware of introducing dirt into the system. Many a clean lube set and newly over hauled machine have been damaged by a few seconds of careless maintenance activity. The instrumentation associated with the machinery train should also be checked and calibrated. Again, a list and adequate record keeping practices are a must. The list should include all set points and complete information on any rebuilt instruments placed back in service. The engineer in charge of the overhaul will normally delegate this task to the instrument group after collaborating on the list with this group and the operating department to pinpoint any troublesome items. Key shutdown instruments such as low oil pressure and high discharge temperature should, of course, receive as accurate a test as practical. The Factory Serviceperson Most machinery manufacturers' manuals recommend the use of a factory serviceperson. It’s most important to know whom you are getting and what his qualifications are. One maintenance manager had the unsettling experience of shaking the hand of a serviceman from a major supplier of gas turbines who then mentioned that he normally worked on steam turbines and this would be his first gas turbine. The point is, after several years in the field, assuming continuity of plant personnel, the user many times will know more about the machine than its manufacturer. Factory field servicemen lead a rough life: 16-20 hour or more shifts are common, as well as frequently being away from home. Attrition is high. You will find some very good and some very bad ones. Keep a list of those to invite back, as well as those you would rather not use again. The Overhaul During the course of the overhaul, it’s very important to keep track of the job on an hour-to-hour, shift-to-shift basis. Companies using a shift log or diary for this purpose have found it to be an invaluable communications tool. The critical dimension diagram and the alignment diagram should likewise be kept available for ready reference, as should the turn around guide or T/A package mentioned earlier. The use of a good quality camera and a capable photographer to document details of the overhaul is strongly recommended. A good T/A package will include a pictorial sequence of assembly and disassembly steps, as we will show later. After all, if you do a good job, it will be five to eight years or longer before anyone sees the inside of the machine again. Be sure to use a digital camera, and keep in mind the merits of video taping to provide training films for the maintenance department. Before the actual shutdown of the machine is the time to take a final set of hot alignment data, if such a program is currently in use at your plant. There are, of course, several accepted methods for checking hot alignment. One is the use of eddy current probes, either inside the coupling guard or on the machine cases. A second is the optical method using a transit and targets on the machine train. A third is the use of a telescoping measuring rod with reference points on each machine and benchmarks on the foundation. A fourth and most up-to-date method may involve laser optics. If you don't currently check running hot alignment (as opposed to the old method requiring a shutdown/alignment check, which has proven to be both inaccurate and unreliable), we would strongly recommend evaluating the various systems to see which one best fits your needs. As with reverse dial indicator alignment, a good hot alignment method can be a real money saver. Several items should be checked after the machine has stopped turning, but before the actual disassembly begins: 1. The coupling: If it’s a gear coupling, is it free to move on the gear teeth? Have you considered upgrading to a contoured diaphragm coupling? 2. Look for broken coupling bolts. Broken bolts can indicate several problems, the most likely being incorrect bolt torque on installation, the wrong bolt material, or mis-machined coupling flanges. 3. Get a sample of coupling grease, if a grease-packed coupling is used. 4. When removing the coupling, remember to turn the nuts and prevent the bolt head from turning so as to avoid wearing the body-fitted bolts. In a double-keyed coupling, be sure to check that the keys are marked as to their location. 5. Be sure to keep the coupling bolts and nuts together as individual assemblies. Don’t plan to reuse the nuts more than twice. If any doubt exists in this area, a new set of match-weighed nut and bolt assemblies is cheap insurance. 6. Check and record rotor float within the thrust bearings, and note also the spacing between shaft ends. 7. Check the total rotor float with the thrust bearing removed, and note the rotor position relative to the machine case. Check nozzle stand off in the case of a steam turbine, or position between diaphragms in the case of a compressor. 8. When removing the thrust bearing, be sure to measure and tag any thrust shims used for thickness and location (inboard or outboard). Opening the Machine Before actually opening a major piece of machinery, take time to review the critical steps in the operation. Attempting to remove an upper half casing without first removing internal (non-exposed) bolting or lifting the casing without using guide pins can result in a much longer and more expensive overhaul. Be especially careful when opening lube oil lines. The loss of a flow control orifice or the introduction of dirt into the system can cause serious problems during the machine startup. As the machine comes apart, take lots of pictures, make written notes, and/or use a handheld computer or tape recorder to document what you see. It's amazing how much detail will be lost and how difficult it’s to accurately reconstruct events hours or days-let alone years-after they have occurred. One major petrochemical company operates four identical 20,000 horsepower steam turbines which, due to a series of blade problems, had to be opened a total of 31 times in an eight-year period. They recognize the importance of rotor charts to keep track of rotor movements and modifications, as shown in FIG. 13. When the first blade in the first rotor failed, it was not apparent that they were in for such a lengthy problem. The rotor movement chart was laboriously constructed from memory when they were halfway into the program and had added a sixth rotor to the system (four installed and two spares). If you plan to remove compressor diaphragms, be sure to match mark them as to their position in the case. Inadvertent mixing of inlet guide vanes could alter machine performance! Be careful to stone down any match marks which are placed in a machined area, such as the casing split line. When the top half of a horizontally split compressor is removed, it’s a good idea to position the rotor with its thrust bearing as it was before shutdown and check to see if the impellers are centered with the diffuser flow passages. FIG. 13. Rotor history chart. Inspection As the machine is being opened, pay particular attention to visible deposits. On machined sealing surfaces you may find telltale tracks of a leak or wire drawing. Such leaks may indicate a need to check the flatness and fit of the surfaces with lead wire or Plasti-Gage, or simply better attention to bolt torquing requirements. Fouling inside the flow passages of the machine will likely not be distributed uniformly from one end to another. In a compressor, the gas will get hotter with each successive stage. With some gases this will bake the deposits in the latter stages; with other gases, heavy, wet deposits will form in the first stages of the machine. Get a sample of the deposits to determine, first, what they are in order to see if they can be eliminated from the process. Failing that, test to see if they can be dissolved in some suitable solvent, for either on-line or off-line washing, in order to delay a subsequent machine over haul. While compressor manufacturers shy away from on-line full-speed washing, knowledgeable users have had very good experiences with both this technique and with off-line washing when the machine is slow rolled while half full of the wash liquid. When choosing a wash fluid be sure it’s compatible with all components in the machine, such as O-rings, as well as the process. On-line abrasive cleaning with walnut hulls, etc., has found wide acceptance with gas turbine users, but is not without its problems. Plugged orifices, airbleed passages, and the like are common. The total subject of on-line or off-line cleaning is beyond the scope of this text, but it’s well worth considering in specific situations as it’s a real time and money saver. The bearings, journals, and seals should be visually checked for signs of distress. One frequent problem has been that of babbitt fatigue. While the aftermarket has been offering bearings with babbitting less than 0.010 in. thick for a number of years, some machinery manufacturers have resisted change in this area. Nevertheless, industry experience with thin babbitt bearings has been excellent to date. Labyrinths can also tell a story which needs to be read and analyzed. Deep grooves in the impellers or shaft spacers are indications of a shaft excursion at some time in the operating cycle. Worn or corroded labyrinths indicate loss of efficiency, and, if found over the balance piston, could lead to a thrust bearing failure. As with bearings, new materials, such as Vespel high-performance graphite-filled polymers able to combat the corrosion problem, are now coming into the after-market. Rubs could indicate mis-operation, such as running at or near a rotor critical speed or in surge; a rotor dynamics problem; a thermal bow, or similar difficulty. Observations regarding location, depth, and distribution of the rubs are the keys to a proper analysis. Cleaning When cleaning fouled components--rotors and diaphragms, etc.-- make sure the work is done in a remote location. Sand or nut hulls used for this purpose will usually find a way of invading the wrong parts of the machine, such as bearings and seals. The rotor should be carefully checked at this time for debris lodged in the gas passages. We know of instances where a rag or piece of metal was jammed in an inaccessible place in an impeller. The use of a small dental mirror and a thorough inspection by hand can reveal much of this debris. It’s fairly common practice to inspect a rotor using magnetic particle or dye penetrant techniques. This is a strongly recommended step; it can turn up defects which could otherwise prove to be highly damaging during a subsequent running period. In one such instance we uncovered an undesirable manufacturing technique which has been practiced for many years. The magnetic particle and subsequent dye penetrant inspections showed several cracks around the eye of the fifth stage impeller in a multistage barrel compressor rotor installed in relatively clean hydrogen service. Up to this point the overhaul had been a routine matter, but now took on far more serious implications. It seems that this particular compressor manufacturer had been in the habit of overspeed testing impellers and then trimming the eye labyrinth area to size, thus weakening the most critical structural area of the impeller. The explanation given (to compensate for bore stretch during overspeed) is, of course, unacceptable. Most manufacturers will now readily guarantee maximum allowable expansion in the diameter of the eye of an impeller as a function of the diameter before the overspeed test. This is the only acceptable way to buy compressor impellers, either as part of a new machine or as a replacement part. Reassembly Once the machine has been opened and all parts cleaned and inspected, the reassembly procedure can begin. There are many critical phases involved with this operation, one of the most important being care in handling the rotor. Large heavy rotors (over approximately 2,500lbs) require special handling and, in some cases, special guide fixtures should be fabricated to avoid damaging components. This is particularly necessary with gas turbines which have many exposed, fragile parts. A solid rotor cradle is also a very necessary item. Don’t jeopardize your most valuable spare part by failing to protect it during the course of an overhaul or during transit to or from the storage warehouse. When fitting housings and other components with multiple O-rings in blind areas, we have found that it’s usually beneficial to first remove the O-rings and fit the housing by hand to check the alignment of the assembly. Blind dowels or concealed shims can be located in this manner with pencil marks. The O-ring fits should be touched lightly with Grade 600 wet or dry emery paper to remove any burrs, and then checked carefully by hand. Lubricate the O-rings with a suitable grease or oil. A cut O-ring, worth very little in itself, can bear heavily on the success of an overhaul. Bearing clearance is one of the most important checks during reassembly. We have found that after several years of operation, the pads of a tilting pad journal bearing will wear small depressions in the support ring or housing which can open the clearance beyond specifications. Also, replacement pads may not be within tolerance. The only proper way to check bearing clearance with this type of bearing is by using a mandrel the size of the journal and a flat plate. The clearance in a sleeve-type journal bearing can be checked with Plasti-Gage. Be sure to torque the bearing cap bolts correctly or you may get a false reading. When assembling bearings be sure the anti-rotation dowels are in place and look to be sure the oil dam (if used in that particular bearing) is in the correct direction of rotation. Some of these steps will sound obvious, but each one results from a problem experienced in the field. It’s also useful to check the alignment of oil supply holes in the housing with oil feed grooves in the bearing. For want of a 3/8-in, groove in the housing of a replacement bearing, one user lost a high speed shaft and impeller assembly in a plant air compressor package. Before the upper half of the casing of a horizontally split machine is bolted in place, a final rotor mid-span bow check is recommended. This is particularly useful if you, as the responsible engineer on an overhaul, have not been able to personally witness all rotor movements during the course of the job. Coupling hub fit is another area requiring consideration. The assumption that the taper is correct provides a false sense of security. By lightly bluing the shaft and transferring the bluing to the coupling bore, the fit can be properly checked. It’s prudent to require at least 85 percent contact. If the contact pattern is not acceptable, the question of whether to lap or not to lap needs to be addressed. We won’t lap using the coupling half for obvious reasons, but will lap using a ring and plug gauge set. The advance of the coupling on the taper must be correct and should be witnessed and recorded by a knowledgeable individual. Coupling bolts must be torqued to the coupling manufacturer's specifications as the clamping force, not the bolt body, is generally the means of transmitting the torque. As the machine goes back together, fill in the information in the critical dimension diagram. Labyrinth and bearing clearances, total rotor float, thrust clearance, coupling advance, bolt torque, etc., should all be measured and logged. Shaft alignment and cold baseline data for comparison with hot growth data taken after startup should also be logged on the appropriate sheet. Remember to check the shaft end gap, as not all rotors are created equal and the wrong dimension could damage your coupling. When leaning into an open machine, it’s well to remember to remove all loose objects from shirt pockets! There are some other checks which may or may not have been incorporated in the critical dimension diagram, most notable of them being whether the rotor is free to turn and whether oil is flowing to and from the proper places. This latter item can be viewed just prior to bolting bearing caps or covers in place, assuming the oil lines have been reconnected. On some machines with internal oil tubing, it’s possible to have oil flow showing in the main oil drain sight flow indicator while no oil is reaching the bearings or seals! Documenting What You've Done Following the overhaul, the startup will need to be monitored. If you don't have a fixed-base monitoring system, use a portable real time analyzer and a modern recorder to obtain baseline vibration data for comparison with previous operating information. Hot alignment readings can usually be taken several hours after startup. Machine performance will normally be checked after the process has stabilized which, on some machines, can be as long as several days after startup. All of this information provides a very useful check on the success of the overhaul and should be taken at the outset of a run and not delayed until a "convenient" time several weeks from startup. As soon as the machine is operating satisfactorily, do the paper work, i.e., update your computer log. Many engineers shy away from this duty and use the excuse of day-to-day business pressures to delay or even forget this very necessary chore. While the events are still fresh in your mind, sit down and finish the job. In documenting an equipment overhaul, consider using the following format: 1. Basic Machine Data-A brief description of the machine, including manufacturer, model number, number of stages and other physical parameters, serial number, date purchased, date of last overhaul and reason for current overhaul. 2. Performance, Vibration, and Mechanical Health Data--A comparison of pre- and post-overhaul levels. Performance and vibration data for the train, including process flow, pressure and temperature, machine case, and eddy current probe vibration levels, as well as oil supply pressure, temperature, and oil return temperature. The performance data should be sufficient to accurately assess the machine's condition. Calibrated instruments are required. 3. Spare Parts--A complete list of spare parts for the machine, as well as a list of parts actually consumed. Include machine manufacturer's part number, as well as company warehouse stock number. 4. Critical Dimension Diagram--Complete with factory specifications, as-found dimensions (logged during disassembly), and as overhauled dimensions. This information must include items such as total rotor float, thrust clearance, rotor position within the total float, labyrinth clearance, radial bearing clearance, nozzle stand off, coupling bluing check, and coupling advance. 5. Rotor run-out diagram and balance report. 6. Shaft Alignment Diagram--A shaft alignment diagram showing desired readings based on anticipated thermal growth data, "as found" readings (prior to overhaul), "as left" readings after over haul, and actual measured thermal growth data. 7. Photographs of the overhaul. 8. A discussion of the overhaul. Refer to appropriate photographs throughout. 9. Recommendations: • For future overhauls • For reconditioning worn but reusable parts • For on-line cleaning, if applicable • For redesigned parts, if applicable 10. Shift logs and backup data as required. In writing a report, decide what went right and what went wrong. Fully identify the causes in each case so that your successor can benefit from your experiences. Send a list of spare parts used in the overhaul to the warehouse controller. While you hope you won't need parts in a hurry, don't bet on it! Decide if you plan to invite the factory serviceperson back for a subsequent overhaul. In either case, put his name on your report so no confusion exists on this point. Go back to the machine manual and make notes in the margin on any errors that may have appeared in the printed material. Nonstandard Parts: Once a new machine has operated for a year, it’s well to remember that the guarantee has probably elapsed. In addition, bear in mind that the original equipment manufacturer's parts were generally a design compromise which took into account a competitive marketplace and existing, available designs in the manufacturer's shop. Any parts that fail to stand up should not necessarily be replaced by standard parts. There are many excellent aftermarket manufacturers of components and many specialized tools such as multi-plane milling machines and overspeed spin pits for individual components. Aerospace technology and materials are beginning to filter down to the aftermarket also. None of the above should be construed as an indictment of the equipment manufacturer, but when his spare part pricing, policies, and failure to solve design problems mount to a point where it becomes necessary to put properly engineered aftermarket components into a machine, don’t hesitate to do what is best for your company. |