Improving Machinery Reliability 3 Episode 9 pdf

334 Improving Machinery Reliability excited by small, disturbing fluid forces. In addition, the piping loops enhance the internal fluid disturbance by creating cavities and other flow discontinuities associat- ed with excessive pressure drops. A system similar to that shown in Figure 7-5 expe- rienced very severe vibrations in one petrochemical plant. The responsible engineer had to install a large cross beam anchoring all the loops in efforts to reduce vibration to a manageable level. The function of the original loops was lost by the anchoring system. Moreover, the piping still experienced larger than normal vibrations due to flow disturbance caused by a loop which was now structurally fixed, but hydraulical- ly still open to many changes in the direction of flow. Theoretical Restraints A properly designed piping system generally includes restraints to control the movements and to protect sensitive equipment. However, there are also restraints that are placed in desperation by piping engineers trying to meet the allowable load of the equipment. These so-called computer restraints give very good computer analysis results on paper, but are often very ineffective and sometimes even harmful. Figure 7-6 shows some typical situations that work on the computer, but do not work on a real piping system. These pitfalls are caused by the differences between the real system and the computer model. Here are some of the more important discrepancies: Friction is important in the design of the restraint system near the equipment. Fig- ure 7-6 (a) shows a typical stop placed against a long Z-direction line to protect the Figure 7-6. Problems with theoretical restraints. Equipment Reliability Improvement Through Reduced Pipe Stress 335 equipment. If friction is ignored in the design calculations, the calculated reaction at the equipment is often very small. However, in reality, friction acting at the stop surface will prevent the pipe from expanding in the positive X-direction. This fric- tion effect can cause a high X-direction reaction at the equipment. Calculations including the friction will predict this problem beforehand. A proper type of restraint such as a low-friction plate or a strut would then be used. * An ineffective support member is another problem often encountered in the protec- tive restraints. Figure 7-6 (b) shows a popular arrangement to protect the equip- ment. The engineer’s instinct is to always put the fix at the problem location. For instance, if the computer shows that the Z-direction reaction is too high, the natural fix is to place a Z-direction stop near the nozzle connection. This may be all right on the computer, but in reality it is very ineffective. For the support to be effective, the stiffness of support member A has to be at least one order of magnitude higher than the stiffness of the pipe. Here, the pipe stiffness is very high due to the rela- tively short distance from the nozzle to the support. A gap is generally required in the actual installation of a stop. Therefore, if a stop is placed too close to the nozzle connection, its effectiveness is questionable due to the inherent gap. As shown in Figure 7-6 (c), the pipe has to be bent or moved a distance equal to the gap before the stop becomes active. Due to the closeness of the stop to the equipment, nozzle stresses will often reach severe levels even before the pipe reaches the stop. This configuration is not acceptable because the equipment generally can only tolerate a much smaller deformation than the con- struction gap of the stop. e Choking is another problem relating to the gap at the stop. Some engineers are aware of the consequences of the gap at the stop mentioned above and try to solve it by specifying that no gap be allowed at the stop. This gives the appearance of solving the problem, but another problem is actually waiting to occur. As shown in Figure 7-6 (d), when the gap is not provided, the pipe will be choked by the stop as soon as the pipe temperature starts to rise. We generally remember to pay attention TO the longitudinal or axial expansion of a pipe, but we often forget that the pipe expands radially as well. When the temperature rises to a point when the radial expansion is completely choked by the support, the pipe can no longer slide along the stop surface. The axial expansion will then move upward, pushing the entire machine upward. Expansion Joints An alternative solution to keeping allowable nozzle loads in check involves the use of bellows expansion joints. Bellows expansion joints are popular in the exhaust systems of steam turbine drives which typically have extremely low allowable pipe loads for pipes 8 inches and above. Bellows joints are also often used for fitting units coming off a common header, as shown in Figure 7-7 (b). A properly installed and maintained bellows expansion joint should have the same reliability as other compo- nents, such as flanges and valves. However, in real applications, expansion joints are often considered undesirable due to anticipated maintenance problems. For instance, when covered with insulation, the expansion joint looks just like thickly insulated 336 Improving Machinery Reliability piping. Nobody knows exactly what is going on inside the mixed layers of covering. Due to “blindness anxiety,” many installers have resorted to an uninsulated arrange- ment. This not only creates an occupational safety hazard, but can also lead to cracks due to thermal shock from the environment andlor weather changes. In refineries, fires around bellows-type expansion joints have often led to disaster. One important factor often overlooked by engineers in the installation of a bellows expansion joint is the pressure thrust force inside the pipe. The bellows is flexible axially. Therefore, the bellows is not able to transmit or absorb the axial internal pressure end force. This pressure end force has to be resisted either by the anchor at the equipment or by the tie-rod straddling the bellows. With the exception of very low pressure applicators, such as the pipe connected to a storage tank, most equip- ment items are not strong enough to resist a pressure end force equal to the pressure times the bellows cross-sectional area. The pressure thrust force has to be taken by the tie-rod. These facts are not obvious to everyone and may result in some opera- tional difficulties. Figure 7-7 illustrates two actual problems. Figure 7-7 (a) shows I NUTS snoum BE TIGHT (b) Figure 7-7. Tie-rods on expansion joints. Equipment Reliability Improvement Through Reduced Pipe Stress 337 one of many steam turbine exhaust configurations installed in petrochemical plants, The expansion joint layout scheme appears to be sound, but the construction may not be done properly. When the base elbow is anchored, the tie-rod loses its function as soon as the pipe starts to expand. In this case, the pipe expands from the anchor toward the bellow joint, making the tie-rod loose and ineffective. The large pressure thrust force pushes the turbine, often causing shaft misalignment and severe vibra- tions. Figure 7-7 (b) depicts a similar situation. In one plant, the bellow expansion joints were used solely for fitting up the connections. The tie-rods were supposed to be locked; however, before start-up, an engineer had loosened the tie-rod nuts, apparently thinking the tie-rods defeated the purpose of the expansion joint. The tur- bine encountered serious vibration and it took quite a while before it was discovered that the problem was caused by the loose tie-rods. When the nuts are loose, the pres- sure end force simply pushes the machine out of alignment. Other Practical Considerations As can be seen, pipe stress reductions are not always easy to achieve. Especially when dealing with the low allowable nozzle loads specified for some equipment, the technique can become tricky and very often works only on paper. Other practical approaches may have to be explored to further improve overall reliability. One very important resource not to be overlooked is the experience found in operating plants. We have seen good, simple working layouts changed to complicated and question- able layouts only because a computer liked it that way. Undoubtedly, computers are important tools, but they are only as good as the information we give them. Since there are parameters such as friction, anchor flexibility, etc., that cannot be given accurately, computer results need to be interpreted carefully. It is time to realize that if something works well in a plant day in and day out, it should be considered good, regardless of whether or not the computer predicted it to be good. The process of examining and incorporating field experience is very important in designing a good, reliable plant. Other solutions such as the use of sliding supports, spring supports, and more compact in-line arrangements as shown in Figure 7-8 also merit serious considera- tion. It is understood that engineers do not feel too confident about movable assem- blies, but it is important to understand the difference between the movement of the whole assembly and the movement of only the pump or turbine. When the whole assembly moves, shaft alignment can still be maintained, provided the distortion of the equipment is not excessive. This pre-supposes that the piping load is still within the allowable range. It should be noted, however, that movable assemblies are just potential alternatives. One should not be oversold on the idea and blindly use sliding or spring-supported schemes in a plant. To make the sliding base or the spring-sup- port scheme work, an extra strong baseplate is required. Then again, if we have that strong of a baseplate in the first place, it may well be possible to substantially increase the allowable piping load. 338 Improving Machinery Reliability n In-line Pump! E Sliding Base Figure 7-8. Alternative machine assemblies. Bibliography API Standard 610, “Centrifugal Pumps for General Refinery Services,” American Petroleum Institute, Washington, D.C. NEMA SM-23, “Steam Turbines for Mechanical Drive Service,” National Electrical Manufacturers Association, Washington, D.C. API Standard 6 17, “Centrifugal Pumps for General Refinery Services,” American Petroleum Institute, Washington, D.C. ASME B73.1M-1991, “Specification for Horizontal End Suction Centrifugal Pumps for Chemical Process,” American Society of Mechanical Engineers, New York. Chapter 8 Startup Responsibilities Summary of Startup Preparations for Process Plant Machinery A number of key elements are indispensable to ensuring successful commission- ing of rotating machinery in process plants. These include the following mix of mandatory and contingency actions. 1. Develop equipment lists for general reference and progress tracking. 2. Assign mechanics during the erection period to observe or execute: a. Machine assembly: Lifting and handling procedures. Cleaning, inspection, and installation of bearings and seals. Checking and recording critical clearances, and pre-operation settings and Full inspection of most centrifugal pumps. Machinery-related instrument installation and adjustments. adjustments. b. Correct shaft alignment. c. Parallelism and gaskets for major flange connections. d. External flush systems cleanup. e. Piping system cleaning for: Compressors. Turbines. 3. Set up training program for special machinery operation and repair: a. Consider vendor service representatives, as available. b. Use classroom instruction together with practical demonstrations. c. Use contract or on-loan startup advisors as time permits. d. Verify soundness of auxiliary systems, starting interlocks, and shut-down protection alarms, and provide routine checking and trouble-shooting during machinery operations. e. Review vendor’s instruction books, cross-sectional drawings, and inspection procedures. Verify completeness of records. f. Expose mechanics to the same training as operators in such key subjects as: Steam-turbine operation. Centrifugal-compressor shaft seal system. e Recognizing machinery distress. 339 340 Improving Machinery Reliability 4. Assign the plant’s machinery engineers to participate in startup activities full- time. a. Purposes and benefits: Specialized training. Improved communication and implementation of advisor’s recommenda- Permits round-the-clock specialist manning. Assures continuity upon departure of temporary startup advisors. Plant machinery engineers should continue in full-time startup assignment 5. Train the plant’s electricians and instrument mechanics on machinery acces- tions. b. Duration of assignment: for one to two months after the plant goes on stream. sories: a. Oil system. b. Governors. c. Starting interlocks, alarms, and safety shutdowns. a. Machinery auxiliary systems, including interlocks, alarms, and shutdown fea- b. Testing of auxiliaries during operation. c. Machinery conditions requiring emergency shutdown. d. Avoiding the kinds of operating errors that can damage machinery. a. Prepare before run-in, revise before plant startup, finalize after successful startup is completed. b. Integrate vendor’s instructions for driver and compressor into process startup instructions. c. Prepare specific startup and shutdown procedures; checklists and data lists for monitoring: data on interlocks, alarms, and shutdown features; process factors; etc. 6. Train operators in machinery areas: tures. 7. Prepare specific operating instructions for major machinery units: 8. Ascertain availability of test equipment for run-in and operation: a. S troboscope-high-speed coverage. b. Hand-held vibration-measuring equipment. c. Real-time vibration analyzer. d. Computerized data-acquisition systems. a. Investigate nearby facilities for emergency service such as: 9. Identify outside sources for special testing or balancing: Vibration analysis. Dynamic balancing of rotors. Capacity to handle largest rotor. Balance quality achievable with available machines per recent experience. b. Dynamic trim balancing in place: Computerized techniques available. Special equipment required. Skilled technicians required. c. Metallurgical testing laboratory. Startup Responsibilities 341 10. Investigate the availability of expert consultants: a. Vibration. b. Welding. c. Metallurgy. a. Identify possible locations. b. Investigate qualifications. c. Make advance contacts. 12. Investigate local repair facilities: a. Larger machine tools than available in plant shop; special shop facilities or b. Special casting repair techniques (“Metalock@” and “Metalstitch@”) c. Welding and metallurgy. a. Establish procedures for obtaining services of vendor representatives for 11. Investigate plan and facilities for repair of remote vendors’ equipment: tools. 13. Identify machinery vendors’ service personnel: run-in and startup operations. 0 Determine official contact and responsible management. e Also make advance contacts for equipment where service representative is to be on an “on call” only status, such as for governors on steam turbines, materials-handling equipment, gearing, centrifuges, etc. b. Assess qualifications of assigned representatives quickly and obtain replace- c. If the erection advisor continues on as the startup advisor, verify that he is d. Utilize vendor representatives for training plant personnel as time permits. e. Assign qualified plant mechanics to work supervised by vendor representa- ments if qualifications are unsatisfactory. qualified in this area. tives. 14. Arrange preventive maintenance details: a. Prepare equipment: * Portable shelter. 0 Rotor lifting and supporting rig. Verify access path and lifting position of mobile crane. b. Develop pre-planned inspections of critical equipment for execution during c. Develop overall plan for preventive maintenance services on all equipment d. Establish records of inspections, repairs, and part replacements: unplanned, brief plant operation interruptions. items. Separate records for each major machinery item. Use special forms, with sketches for recording vibration and other critical 0 Use vendors’ instruction books and startup advisors for developing forms. Plan overhaul technique of most critical machines in detail (e.& instru- operating parameters. ment air compressor). 15. Understand spare parts situation: a. Status of deliveries. b. Warehouses properly organized for pre-startup period. 342 Improving Machinery Reliability c. Spare rotor storage and protection. d. Review stocking for completeness and adequacy. a. Determine requirements for each machine. b. Verify appropriate product equivalent of vendor-recommended lubricants. c. Procure ample quantity for startup. Stock quantity to replenish possible seal leakage. Allow for discard of initial charge. d. Periodically sample test during operations. 16. Outline lubrication requirements: Machinery Startup Review Tasks The preceding section outlined startup preparation in broad terms. These prepara- tory tasks can be further broken down into completeness reviews, quality assurance tasks, cleanliness checks, etc. The following man-hour percentages could be considered representative for exe- cuting the field construction completeness review tasks associated with the startup of a world-size steam cracker (ethylene plant): Review completeness of installation (“but-list”)-21% Traps Vents Valves Plugs Coupling guards Lube supply lines Review maintainability of equipment-7% Spool pieces Shims Auxiliary line interference Alignment devices Ensure long-term, troublefree operation4% Determine offset values needed to accommodate thermal growth Verify stress-free installation of machinery piping Gear-tooth contact checks Startup Responsibilities 343 Driver solo runs and rotation checks-9% Steam-turbine overspeed trip settings Vibration measurements Cleanliness checks-5% 0 Lube and seal oil systems Steam-turbine inlet piping Pump mechanical seals Autostart simulations (joint mechanical/instrument technician effort)-lS% 0 On lube and seal oil auxiliaries 0 On other autostart drivers Documentation, ix., assembling the following data-3.5% Acceptance forms Equipment record folders 0 Computerized record input data form API data sheets 0 Cross-sectional drawings Spare parts availability Mechanical-seal dimensions Alignment form 0 Startup failure history Startup checklists Lubrication survey Muchineiy repairs-39% 0 Precautionary disassemblies 0 Component (material) modifications Cleaning, reassembling, and general repairs Miscellaneous I 1 % [...]... Manual 4 .3 261 .90 250.15 9/ 2 0000 (91 4) 0100 Air dry-out Utilities Loss of boilers Manual 13. 5 31 0 .90 2 63. 65 91 4 1 430 (91 5) 1510 Air dry-out Machinery Woodward people worked on governor Governor vroblem Manual 0.15 33 5.55 2 63. 80 91 5 1520 2040 Ai dry-out Machinery Manual 0.65 34 0 .9 264.45 Problems with RCQ logic Problems with RCQ logic Tripped by Figure 8-11 Triplshutdown log 35 6 Improving Machinery Reliability. .. and seal oil AP z s+ os s & C s 3 E s Sr $ Date (Hour) Start Trip (Hour) Purpose Discipline Cause of Shutdown Shutdown Time Accumulated Oper Time Accumulated Downtime 8/28 1715 (8 130 ) 14 19 Air dry-out Instruments RCQ 0.4 241 .95 236 .80 8 /30 1445 1640 Air dry-out Instruments RCQ 0.25 2 43. 85 237 .05 8 /30 1650 (91 1) 1042 Air dry-out 9/ 1 1 93 0 194 0 Air dry-out Instruments Machinery Mechanical Problems with... and commissioning oversights ( 0 9 a8wd uo panuyuo3 ixar) ~ I WZO-dl 1 WIO-dl I WSO-dN 1 WEO-dN 2 98 0-dN & 810-dA I WQO-dl I WLO-dl I 91 -dl 1 81 I -dl I 81 I -dl I W IO-dW I 8 IO-dW E 801 -dl 2 8ZO-dW I WZO-dW I a 1odw 1 W IO-dW - f=i 'v D < 2: r P P m €S€ 3 -u Unit Date CT-0 1 Turbine Solo 7 130 198 CT-0 1 Turbine Solo 7 131 /98 CT-02 Turbine Solo 8/02 /98 c-01 Train 8/04 /98 Solo Runs: Description of Run... Process PipeNessel Instrument Electrical Mechanical 1 1 Machinery Technical (Definition) Group: Machinery Mechanical (Implementation) Group: Area Supervisors Records Clerk Lubrication Specialist Machinery Engineers: A 6 Millwrights Mechanics C - Figure 8 -3 Machinery startup sections reporting to the same startup leader 34 8 Improving Machinery Reliability Head Maintenance (or Maintenance/ Construclion)... operation Check: 0Adequate oil in reservoir Oil-mist pressure 20" -30 " H,O 0Oil Temperature-9S"F- 125°F n (checklist continued on nextpage) 35 8 Improving Machinery Reliability 0 Air Initial pressure-gauge outside cabinet approximately 30 psig higher than air inside cabinet A i r temperature 130 °F-155°F 8 Hot-pititip piping strniri check (30 0°F errid nborc) a Placed and adjusted a dial indicator on the... CULUHN 0 NUMBER 5 $1 a REWRRKS UNION L 5XZX10.5 VL MNION 4X6X0.5 YLK UNION 2X3X10.5 YLK UNION 5XZX10.5 E VL PULSRFOR 1120-S-E UNION 3X4XlO.5 VLK 1 UMIUN UHIUN 6X6X8.5 VLK UNION 3X4XI0.5 VLY UNION 3X4XI0.5 UNION 6X6X10.5 VLK YLK N Figure 8-7 Equipment tabulation for major checkout segments at startup 35 2 Improving Machinery Reliability i i I I I I I I i I i i I I I I I I I f I I I I I I I I I I I I... unit 3 2 3 1 - 2 1 unit I unit 3 full set full rotor full set full rotor Gas turbines Combustors Compressor rotor 2 Spare Parts and Their Effect on Service Factors 36 3 Table 9- 1 (Continued) Bearings Turbine rotor Fuel controls Speed controls 2 2 3 2 2 sets full rotor partial set partial set 1 set full rotor partial set partial set Cos expanders See applicable parts of steam turbines Key: No 5 4 3 2... has verified compliance with a “detailed checklist for rotating machinery. ” The reviewer will normally compile such a detailed checklist from the purchaser’s original specification 34 6 Improving Machinery Reliability Completeness Summary Rotating Machinery Rev./Date NOTE:This summary should be used with: Detalled Checkllst for Rotating Machinery Equipment # Baseplate Small-Bore Piping Case Gland Condenser... breakdown 2 Set-ups and changeovers 3 Lack of material or manpower ~ Inefficiencies 1 Inadequate tools or parts to effect repair 2 Equipment design flaw 3 Substandard material input 4 Equipment deterioration 5 Poorly trained workers 6 Blockages 7 Pollution 8 Sporadic usage 36 8 Improving Machinery Reliability Product Defects 1 Poor quality output 2 Decreased yield 3 Excessive scrap 4.Returns, rework,... valves Governor Hydraulic actuator Governor drive gear Turning gear Nozzle-ring bolting Trip throttle valve Electric motors Bearings Oil seals Stator Rotor Fan Frequency 3 2 2 2 1 1 2 2 5 3 2 2 1 1 2 2 2 4 3 3 2 2 5 4 4 4 3 2 2 2 3 2 4 1 2 I SIU Spares Post SIU Spares full set full spare full spare 1 spare 1 set full set full spare full spare 1 spare 1 set - - i set partial 2 sets 1 set 1 set partial . Centrifugal-compressor shaft seal system. e Recognizing machinery distress. 33 9 34 0 Improving Machinery Reliability 4. Assign the plant’s machinery engineers to participate in startup activities. Muchineiy repairs - 39 % 0 Precautionary disassemblies 0 Component (material) modifications Cleaning, reassembling, and general repairs Miscellaneous I 1 % 34 4 Improving Machinery Reliability. Specialist Millwrights Mechanics Figure 8 -3. Machinery startup sections reporting to the same startup leader. 34 8 Improving Machinery Reliability Head, Technical Department Head.