Gas Turbine Engineering Handbook 2 Episode 16 pps

G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 735 ± [722±777/56] 1.11.2001 4:00PM c. Basic Machinist Training. Most of the basic training can be devel- oped and conducted by in-plant personnel. This training can be highly detailed and tailored precisely to meet individual plant requirements. Train- ing must be carefully planned and administered to fit the requirements of different machinery in the plant. Many plants have a full-time training program, and personnel for con- ducting training at this basic level. Good maintenance practices should be inculcated into the young machinist from the beginning. He should be taught that all clearances should be carefully checked, and noted both before and after reassembly. He should learn the proper care in the handling of instru- mentation, and the care in placing and removing seals and bearings. A base course on the major turbomachinery principles is a must, so there is basic understanding of what these machines do and how they function. The young machinist should also be exposed to basic machinery-related courses such as: 1. Reverse indicator alignment 2. Gas and steam turbine overhaul 3. Compressor overhaul 4. Mechanical seal maintenance 5. Bearing maintenance 6. Lubrication system maintenance 7. Single plane balancing Tools and Shop Equipment A mechanic must be supplied with the proper tools to facilitate his jobs. Many special tools are required for different machines, so as to ensure proper disassembly and reassembly. Torque wrenches should be an integral part of his tools, as well as of his vocabulary. The concepts of ``finger tight'' and ``hand tight'' can no longer be applied to high-speed, high-pressure machinery. A recent major explosion at an oxygen plant, which resulted in a death, was traced back to gas leakage due to improper torquing. A good dial indicator and special jigs for taking reverse indicator dial readings is a must. The jigs must be specially made for the various compressor and turbine trains. Special gear and wheel pullers are usually necessary. Equipment for heating wheels in the field for assembly and disassembly are needed; specially designed gas rings are often used for this purpose. A maintenance shop should have the traditional horizontal and vertical lathes, mills, drill presses, slotters, bores, grinders, and a good balancing machine. A balancing machine can pay for itself in a very short time in Maintenance Techniques 735 G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 736 ± [722±777/56] 1.11.2001 4:00PM providing a fast turnaround and accurate dynamic balance. Techniques to check the balance of gear-type couplings for the large high-speed compres- sors and turbine drives, as a unit should be developed. This leads to the solving of many vibration related problems. High-speed couplings should be routinely check-balanced. By dynamically balancing most parts, seal life and bearing life is greatly improved, even on smaller equipment. Dynamic balancing is needed on pump impellers, as the practice of static balance is woefully inadequate. Vertical pumps must be dynamically balanced; the long, slender shafts are highly susceptible to any unbalanced-induced vibration. This assembly and disassembly of rotors must be in a clean area. Horses or equivalents should be available to hold the rotor. The rotor should rest on the bearing journals, which must be protected by soft packing, or the equivalent, to avoid any marring of the journals. To accomplish uniform shrink fits, the area should have provisions for heating and/or cooling. A special rotor-testing fixture should be provided; this is very useful in checking for wheel wobbles, wheel roundness, and shaft trueness. Rotors in long-term storage should be stored in a vertical position in temperature- controlled warehouses. Spare Parts Inventory The problem of spare parts is an inherent phase of the maintenance business. The high costs of replacement parts, delivery, and in some instances, poor quality, are problems faced daily by everyone in the main- tenance field. The cost of spare parts for a major power plant or refinery runs into many millions of dollars. The inventory of these plants can run into over 20,000 items, including over 100 complete rotor systems. The field of spare parts is changing rapidly and is much more complex than in the past. A group of plants have gotten together in a given region and formed ``Part Banks.'' Many pieces of equipment are made up of unitized components from several different vendors. The traditional attitude has been to look to the packaging vendor as the source of supply. Many vendors refuse to handle requests for replacement parts on equipment not directly manufactured by them. More and more specialty companies are entering the equipment parts business; some are supplying parts directly to OEM companies for resale as their ``own'' brand. Others supply parts directly to the end user. The end user must develop multiple sources of supply for as many parts as possible. Gaskets, turbine carbon packing, and mechanical seal parts can be pur- chased from local sources. Shafts, sleeves, cast parts can be purchased from 736 Gas Turbine Engineering Handbook G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 737 ± [722±777/56] 1.11.2001 4:00PM local sources. Shafts, sleeves, cast parts such as impellers, are becoming increasingly available from specialty vendors. All this competition is causing the OEM's to alter their spare parts system to improve service and reduce prices, which is definitely a bright spot in the picture. The quality control of both OEM and some specialty houses leaves much to be desired. In turn, this causes many plants to have an in-house quality control person checking all incoming parts, a concept highly recommended. Condition and Life Assessment Condition and life assessment is significant for all types of plants, and especially Combined Cycle Power Plants. The most important aspect of a plant is high availability, and reliability, in some cases even more significant than higher efficiency. The availability of a power plant is the percent of time the plant is available to generate power in any given period. The reliability of the plant is the percentage of time between planed overhauls. The availability of a power plant is defined as A  P ÀS À F P 21-1 where: P Period of time, hours, usually this is assumed as one year, which amounts to 8,760 hours S Scheduled outage hours for planned maintenance F Forced outage hours or unplanned outage due to repair The reliability of a power plant is defined as R  P ÀF P 21-2 Availability and reliability have a very major impact on the plant econ- omy. Reliability is essential in that when the power is needed it must be there. When the power is not available it must be generated or purchased, and can be very costly in the operation of a plant. Planned outages are scheduled for non-peak periods. Peak periods is when the majority of the income is gener- ated as usually there are various tiers of pricing depending on the demand. Many power purchase agreements have clauses, which contain capacity payments, thus making plant availability critical in the economics of the plant. Gas turbines with the new technology, higher pressure ratio and higher firing temperature, has led to the building of large gas turbines producing Maintenance Techniques 737 G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 738 ± [722±777/56] 1.11.2001 4:00PM nearly 300 MW and reaching gas turbine efficiencies in the mid forties. The availability factor for units with mature technology, below 100 MW, are between 94 Â ±97%, while the bigger units above 100 MW have availability factors of 85 Â ±89%. The bigger units produce twice the output, but the avail- ability factor has decreased from 95% to 85%. A decrease of 7 Â ±10 points for all manufacturers. Part of this decrease may be related to larger machinery taking more time to repair. It is also due to the high temperature and pressure. The increase in unit size and complexity together with the higher turbine inlet temperature, and higher pressure ratio has lead to an increase in overall gas turbine efficiency. The increase in efficiency of 7 Â ±10% has in many cases lead to an availability decrease of the same amount or even more as seen in Figure 21-5. A 1% reduction in plant availability could cost $500,000/yr in income on a 100 MW plant, thus in many cases offsetting gains in efficiency. Reliability of a plant depends on many parameters, such as the type of fuel, the preventive maintenance programs, the operating mode, the control systems, and the firing temperatures. Redesign for Higher Machinery Reliability Low reliability of units gives rise to high maintenance costs. Low reli- ability is usually a greater economic factor than the high maintenance costs. 96 35 85 45 0 10 20 30 40 50 60 70 80 90 100 Below 100 MW Above 100 MW Availability Efficiency Figure 21-5. Comparison of availability and efficiency for large frame type gas turbines. 738 Gas Turbine Engineering Handbook G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 739 ± [722±777/56] 1.11.2001 4:00PM In many large power plants, refineries, and petrochemical complexes, about one-third of the failures are due to machinery failure; it is therefore necessary to redesign parts of a machine to improve reliability. The maintenance practice of one large refinery is to replace gas turbine control systems with state-of-the-art electronics and ``plug-in'' concepts for ease of maintenance. These installations have been highly successful in that maintenance has been minimal, and can usually be accomplished on-stream. Another replaces all journal bearings with tilting pad bearings. In addition, the new control systems increase turbine performance, while speed control and flexibility are greatly improved. The original design has been supplemented to include a self-contained alarm system, a semi- automatic sequential start system, and a complete trip and protection system, as well as the electronic controls. The cost of this system is substan- tially less than the cost of a similar device offered by the OEM on new machines. The gas turbines major limitations on the life are the combustor cans, first stage turbine nozzles and first stage turbine blades as seen in Figure 21-6. The effect of dry Low NO x combustors have been very negative on the availability of Combined Cycle Power Plants, especially those with dual fuel capability. Flash back problems are a very major problem as they tend to create burning in the pre-mix section of the combustor, and cause failure of the pre-mix tubes. These pre-mix tubes are also very susceptible to resonance vibrations. Bearing failures are one of the major causes of failures in turbomachinery. The changing of various types of radial bearings from cylindrical and/or 5 27 21 17 15 7 2 2 0 5 10 15 20 25 30 Compressor Blades Combustor Cans First Stage Nozzles First Stage Blades Controls Bearings Seals Couplings Generator 4 Figure 21-6. Contributions of various major components to gas turbine down time. Maintenance Techniques 739 G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 740 ± [722±777/56] 1.11.2001 4:00PM pressure dam babbitted sleeve bearings to tilting pad journal bearings is becoming common in the industry. In most cases, this gives better stability, eliminates oil whirl, and under misalignment condition, is more forgiving. Thrust bearing changes, from the simple, tapered land thrust bearings to tilting pad thrust bearings with leveling links (Kingsbury type), is another area of common change. These types of bearings absorb sudden load surges and liquid slugs. Many users have changed out the inactive thrust bearing to carry the same load as the active thrust bearings. This has been the case in older gas turbines where traditionally the load carrying capacity of the inactive thrust bearing was 1/3 of the active thrust bearing. As gas turbines got older the leakages increased and the thrust forces were altered greatly leading to failures in the inactive thrust bearings. A major plant replaces the entire large journal and thrust bearings in their main machinery to tilting pad bearings in their plant as a matter of practice. Material changes of the babbit are sometimes undertaken. Changing from the more common steel backed babbitted bearings to the copper alloys, with this babbitted pads, conducts surface heat away at a faster rate, thus increas- ing the load carrying capacity. In some instances, a 50 Â ±100% load carrying capacity improvement can be achieved. Some equipment manufacturers are offering bearing-upgrading kits for their machine in service. Design of turbine blades to obtain higher efficiency and damping has been done. In some cases, this has improved efficiency by 8 Â ±10%, and stopped failures in these blades. Steam injection has been utilized in gas turbines to improve efficiency and to increase the power output. Redesign of various bleed-off ports has reduced tip stalls and their accompanying blade failures. Today's machinery, which is pushing the state-of-the-art in design, needs more than ``simple fixes.'' This is one major reason why so much redesign takes place in the field. Maintenance engineers are no longer just required to repair, they are required in many cases to make revisions. Continual improvements and updating of the machinery is required to obtain the long runs and high efficiencies desirable in today's turbomachinery. Maintenance Scheduling The scheduling of maintenance inspections and overhauls is an essential part of the total maintenance philosophy. As we move from ``Breakdown'' or ``Panic'' maintenance towards a performance based total productive maintenance system, total condition monitoring and diagnostics becomes an integral part of both operation and maintenance. Total condition monitoring and diagnostic examines both the mechanical and performance 740 Gas Turbine Engineering Handbook G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 741 ± [722±777/56] 1.11.2001 4:00PM of the machinery and then carries out diagnostics. Condition monitoring systems, which are only mechanical systems without performance inputs give less than half of the picture and can be very unreliable. Unscheduled maintenance is very costly and should be avoided. To properly schedule overhauls, both mechanical and performance data must be gathered and evaluated. As indicated earlier, we want to consider repairs during a planned ``turnaround'' not ``random'' repairs, which are frequently done on an ``emergency'' basis and where, due to time restraints, techniques are sometimes used, which are questionable and should only be used in emergencies. To plan for a ``turnaround,'' one must be guided by the operating history of the given plant and, if it is the first ``turnaround,'' by conditions found in other plants utilizing the same or closely similar process and machinery. This is how the time between subsequent ``turnarounds'' has been extended to three years or more in many instances. By utilizing the operating history and inspection at previous ``turnarounds'' at this or similar installations, one can get a fair idea of what parts are most likely to be found deteriorated and, therefore, must be replaced and/or repaired, and what other work should be done to the unit while it is down. It should be pointed out that, with modern turbomachinery, items such as bearings, seals, filters and certain instrumentation, which are precision made, are seldom, if ever, repaired except in an emergency; such items are replaced with new parts. This means that parts must be ordered in advance for the ``turnaround'' and other work must be planned so that the whole operation may proceed smoothly and without holdups that could have been foreseen. This usually means close collaboration with the manufacturer or consultant and the OEM (or specialty service shop) so that handling facilities, service men, parts, cleaning facilities, inspection facilities, chrome plating and/or metaliz- ing facilities, balancing facilities, and some cases even heat treatment facil- ities, are available and will be open for production at the proper time required. This is the planning, which must be done in detail before the shutdown with sufficient lead-time available in order to have replacement parts available at the job site. The old maxim ``if it ain't broke don't fix it'' is very applicable in today's machinery. A study conducted at a major nuclear power facility found that 35% of the failures occurred after a major turnaround. This is why total condition monitoring is necessary in any performance based total productive maintenance system and leads to overhauls being planned on proper data evaluation of the machinery rather than on a fixed interval. Maintenance Techniques 741 G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 742 ± [722±777/56] 1.11.2001 4:00PM Maintenance Communications It is not uncommon to hear the complaint that the maintenance depart- ment has ``never been informed as to what is happening in the plant.'' If this is a common complaint, the maintenance manager needs to examine the communications in his department. The following are six practical sugges- tions for improving communications: 1. Operation and service manuals 2. Continuous updating of drawing and print files 3. Updating of training materials 4. Pocket guides 5. Written memos, interoffice E-mails 6. Seminars 7. Website postings Each of these items listed, if properly employed, can transmit knowledge to the person who must keep the plant's machinery running. How well the information is transmitted depends entirely on the communication skills applied to the preparation of the materials. Operation and Service Manuals. To be of real value to the mechanic, an operation and service manual must be indexed to permit quick location of needed information. The manual must be written in simple, straightforward language, have illustrations, sketches, or exploded views adjacent to pertin- ent text, and have minimum references to another page or section. Major sections or chapters should be tabbed for quick location. Most often a mechanic or serviceman refers to a manual because of a problem. Problems seem to happen during a production run. It is essential, therefore, that he be able to find the needed information quickly. The mechanic should not be delayed by wordy, irrelevant text. The objective of any manual is to be an effective, immediate source of service informa- tion. The assignment of a nontechnical person to write a manual is shortsighted and more costly in the long run. A well-written manual is continuously in use. Good manuals need not be complicated. In fact, the simpler the better. Manuals should be readable and understandable, whether they are compiled in-house or outside. Drawing and Print File. A good print file is a vital tool for any main- tenance organization. Reference files in a large or multi-plant company can be particularly burdensome for several reasons: 742 Gas Turbine Engineering Handbook G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 743 ± [722±777/56] 1.11.2001 4:00PM 1. Prints are bulky and difficult to store properly 2. Control of use is necessary 3. Files must be kept up to date 4. Handling and distribution of new or revised prints is usually expensive. A practical solution is to digitize the drawings and place them on CD's available to the maintenance and operation department. A good digital file reduces search time and helps the departments do a better job of keeping the machinery operating at their peak efficiency with minimal downtime. Training Materials. Like any other written or audio-visual maintenance tool, training materials of all kinds are basically communication devices, and to be effective, should be presented in a simple straightforward, attractive, and professional manner. Once the need for specific maintenance training has been determined, a program must be developed. If the training need applies to a proprietary machine or one that is unique to a very few industries, it might be necessary to contact companies who specialize in custom digital programs on CD's, slide/tape, movie, videotape, or written training programs. The cost may shock the uninitiated, but after shopping around, the company may find that it can recover far more than the initial cost in tangible benefits over a relatively short period. Pocket Guide. When a new maintenance form or procedure is introduced, a quick reference pocket guide can promote understanding and accuracy. The key to effectiveness is a deliberate design to provide maximum illustrations or examples in simple language. If it cannot be prepared in-house, outside help should be sought. Professionalism is essential to good communications. Written Memos. One of the most effective devices for improving main- tenance communications is a newsletter or internal memo. The memo's success depends heavily on communicating formal tips and techniques in the mechanics language and using photos, sketches, and drawings gener- ously to get the message across. Everyone in the maintenance department should be encouraged to con- tribute ideas on a better way to do a task or a solution to a nagging prob- lem related to the maintenance or operation of production equipment. Each contributor should be given credit by name and location for his or her effort. Very few workers can resist a bit of pride in seeing their names attached to an article that is seen by virtually everyone in the company. Seminars and Workshops. College or industry-sponsored seminars, continuing education courses, and workshops are means of upgrading or Maintenance Techniques 743 G:/GTE/FINAL (26-10-01)/CHAPTER 21.3D ± 744 ± [722±777/56] 1.11.2001 4:00PM sharpening skills of maintenance people. Such an approach serves a twofold purpose. First, it communicates the company's good faith in the person's ability to benefit from the experience, and by acceptance, the worker shows willingness to improve his or her usefulness to the company. The seminars are very useful in disseminating knowledge. They also provide forum for gripes and meaningful solutions. Discussion groups in these seminars and workshops are very important as participants share experiences and solu- tions to problems. The knowledge gained from these seminars is very useful. Inspection As with any power equipment, gas turbines require a program of planned inspections with repair or replacement of damaged components. A properly designed and conducted inspection and preventive maintenance program can do much to increase the availability of gas turbines and reduce unsched- uled maintenance. Inspections and preventive maintenance can be expensive, but not as costly as forced shutdowns. Nearly all manufacturers emphasize and describe preventive maintenance procedures to ensure the reliability of their machinery, and any maintenance program should be based on manu- facturer's recommendations. Inspection and preventive maintenance proce- dures can be tailored to individual equipment application with references such as the manufacturer's instruction book, the operator's manual, and the preventive maintenance checklist. Inspections range from daily checks made while the unit is operating to major inspections that require almost total disassembly of the gas turbine. Daily inspections should include (but are not limited to) the following checks: 1. Lubrication oil level 2. Oil leakage around the engine 3. Loose fasteners, pipe and tube fittings, and electrical connections 4. Inlet filters 5. Exhaust system 6. Control and monitoring system indicator lights The daily inspection should require less than an hour to perform properly and can be made by the operator. The interval between more thorough inspections will depend on the operating conditions of the gas turbine. Manufacturers generally provide guidelines for determining inspection intervals based on exhaust gas temperatures, type and quality of fuel utilized, and number of starts. 744 Gas Turbine Engineering Handbook [...]... contributes to turbine blade degradation during service It is also probable that each alloy will respond differently to a particular temperature/stress combination Figure 21 - 12 shows the typical variation in stress/rupture life determined at 1350  F (375  C) with service time for forged Inconel X-750 blades G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 7 62 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 7 62 Gas Turbine Engineering. .. plotted to provide baseline vibration data for future evaluation of machine performance G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 768 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 768 Gas Turbine Engineering Handbook Typical Problems Encountered in Gas Turbines There are many types of failures associated with a gas turbine, since these units are very complex in their overall makeup The failures in the hot section far... also prevents failures due to abnormal operating modes Fouling of the rotor blades on turbines can cause thrust-bearing failures Deposits on G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 748 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 748 Gas Turbine Engineering Handbook Figure 21 -7 Effect on planned maintenance with usage of borescope turbine governor valves and trip and throttle valves are suspected of causing overspeed... the toughness Another problem with abrasives is what happens to them after they have done the cleaning In a simple-cycle gas turbine they will probably be burnt G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 750 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 750 Gas Turbine Engineering Handbook Figure 21 -8 Effect of cleaning on power output However, in a regenerative unit they can deposit in the regenerator Some regenerator... 1.11 .20 01 4:00PM Maintenance Techniques FPO Figure 21 -15 Burnt first-stage turbine blades Note evenness of burn FPO Figure 21 -16 Burnt first-stage nozzle 769 G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 770 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 770 Gas Turbine Engineering Handbook FPO Figure 21 -17 Damaged crossover tubes NOx emission requirements This injection of steam reduces the temperature in the hot section,... (26 -10-01)/CHAPTER 21 .3D ± 763 ± [ 722 ±777/56] 1.11 .20 01 4:00PM Maintenance Techniques 763 Figure 21 -13 Comparison of stress rupture life at 50 ksi/1350  F (345 MPa/735  C) in service exposed, commercially reheat-treated, laboratory reheat-treated, and HIP reheat-treated used Inconel X-750 turbine blades (Courtesy of Westinghouse Electric Corp., Gas Turbine Div.) G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D... G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 7 52 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 7 52 Gas Turbine Engineering Handbook turbine The combustion chamber positions as well as the actual chambers or baskets should be permanently numbered, and a complete record should be made for each basket regarding hours of service, repairs or replacements made, and their location in the turbine at each inspection date The basket... valves operate satisfactorily Hydraulic control oil pressures changed The turbine governor ``hunts'' Change in sound level of gear boxes G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 746 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 746 12 13 14 15 16 Gas Turbine Engineering Handbook Overspeed devices operate satisfactorily Babbitt or other material found on lubricating oil screens Lube oil analysis shows corrosion factor... by scratches Shoes should be replaced as sets only if: Radial clearance has increased more than 11 2 mils over nominal design clearance b Leading or lagging edges of shoes show signs of wear a G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 756 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 756 4 5 Gas Turbine Engineering Handbook The tilting-pad and support-ball combination spare parts should be lapped together, making... designing and grouting a sole plate: 1 Check to see that the block between the equipment base and the sole plate is adequate to transmit the load G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 766 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 766 2 3 4 Gas Turbine Engineering Handbook Corners on the edges of the sole plate should have at least a two-inch radius to prevent the creation of stress risers and subsequent cracking of . several reasons: 7 42 Gas Turbine Engineering Handbook G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 743 ± [ 722 ±777/56] 1.11 .20 01 4:00PM 1. Prints are bulky and difficult to store properly 2. Control of. utilized, and number of starts. 744 Gas Turbine Engineering Handbook G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 745 ± [ 722 ±777/56] 1.11 .20 01 4:00PM Table 12- 2 shows time intervals for various inspections. removed as well as the top half of the turbine casing. 746 Gas Turbine Engineering Handbook G:/GTE/FINAL (26 -10-01)/CHAPTER 21 .3D ± 747 ± [ 722 ±777/56] 1.11 .20 01 4:00PM Borescope Inspection Borescope