Gas Turbine Engineering Handbook 2 Episode 16 pps

The cost of spare parts for a major power plant or refineryruns into many millions of dollars.. The availability of a power plant is defined as where: P ˆ Period of time, hours, usually

c Basic Machinist Training Most of the basic training can be oped and conducted by in-plant personnel This training can be highlydetailed and tailored precisely to meet individual plant requirements Train-ing must be carefully planned and administered to fit the requirements ofdifferent machinery in the plant.

devel-Many plants have a full-time training program, and personnel for ducting training at this basic level Good maintenance practices should beinculcated into the young machinist from the beginning He should be taughtthat all clearances should be carefully checked, and noted both before andafter reassembly He should learn the proper care in the handling of instru-mentation, and the care in placing and removing seals and bearings A basecourse on the major turbomachinery principles is a must, so there is basicunderstanding of what these machines do and how they function The youngmachinist should also be exposed to basic machinery-related courses such as:

con-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 ensureproper disassembly and reassembly Torque wrenches should be an integralpart 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 anoxygen plant, which resulted in a death, was traced back to gas leakagedue to improper torquing A good dial indicator and special jigs for takingreverse indicator dial readings is a must The jigs must be specially made forthe various compressor and turbine trains Special gear and wheel pullers areusually necessary

Equipment for heating wheels in the field for assembly and disassemblyare needed; specially designed gas rings are often used for this purpose

A maintenance shop should have the traditional horizontal and verticallathes, mills, drill presses, slotters, bores, grinders, and a good balancingmachine A balancing machine can pay for itself in a very short time in

providing a fast turnaround and accurate dynamic balance Techniques tocheck 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 thesolving of many vibration related problems High-speed couplings should

be routinely check-balanced

By dynamically balancing most parts, seal life and bearing life is greatlyimproved, even on smaller equipment Dynamic balancing is needed on pumpimpellers, as the practice of static balance is woefully inadequate Verticalpumps must be dynamically balanced; the long, slender shafts are highlysusceptible to any unbalanced-induced vibration

This assembly and disassembly of rotors must be in a clean area Horses orequivalents should be available to hold the rotor The rotor should rest onthe bearing journals, which must be protected by soft packing, or theequivalent, to avoid any marring of the journals To accomplish uniformshrink fits, the area should have provisions for heating and/or cooling

A special rotor-testing fixture should be provided; this is very useful inchecking for wheel wobbles, wheel roundness, and shaft trueness Rotors

in long-term storage should be stored in a vertical position in controlled warehouses

temperature-Spare Parts Inventory

The problem of spare parts is an inherent phase of the maintenancebusiness The high costs of replacement parts, delivery, and in someinstances, poor quality, are problems faced daily by everyone in the main-tenance field The cost of spare parts for a major power plant or refineryruns into many millions of dollars

The inventory of these plants can run into over 20,000 items, includingover 100 complete rotor systems The field of spare parts is changing rapidlyand is much more complex than in the past A group of plants have gottentogether in a given region and formed ``Part Banks.''

Many pieces of equipment are made up of unitized components fromseveral different vendors The traditional attitude has been to look to thepackaging vendor as the source of supply Many vendors refuse to handlerequests for replacement parts on equipment not directly manufactured bythem More and more specialty companies are entering the equipment partsbusiness; some are supplying parts directly to OEM companies for resale astheir ``own'' brand Others supply parts directly to the end user The enduser 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

local sources Shafts, sleeves, cast parts such as impellers, are becomingincreasingly available from specialty vendors All this competition is causingthe OEM's to alter their spare parts system to improve service and reduceprices, which is definitely a bright spot in the picture The quality control ofboth OEM and some specialty houses leaves much to be desired In turn, thiscauses many plants to have an in-house quality control person checking allincoming parts, a concept highly recommended.

Condition and Life Assessment

Condition and life assessment is significant for all types of plants, andespecially Combined Cycle Power Plants The most important aspect of aplant is high availability, and reliability, in some cases even more significantthan higher efficiency

The availability of a power plant is the percent of time the plant isavailable 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

where:

P ˆ Period of time, hours, usually this is assumed as one year, whichamounts to 8,760 hours

S ˆ Scheduled outage hours for planned maintenance

F ˆ Forced outage hours or unplanned outage due to repairThe reliability of a power plant is defined as

Availability and reliability have a very major impact on the plant 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

econ-be very costly in the operation of a plant Planned outages are scheduled fornon-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 capacitypayments, thus making plant availability critical in the economics of the plant.Gas turbines with the new technology, higher pressure ratio and higherfiring temperature, has led to the building of large gas turbines producing

nearly 300 MW and reaching gas turbine efficiencies in the mid forties Theavailability factor for units with mature technology, below 100 MW, arebetween 94±97%, while the bigger units above 100 MW have availabilityfactors of 85±89% The bigger units produce twice the output, but the avail-ability factor has decreased from 95% to 85% A decrease of 7±10 pointsfor all manufacturers Part of this decrease may be related to larger machinerytaking 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 turbineinlet temperature, and higher pressure ratio has lead to an increase in overallgas turbine efficiency The increase in efficiency of 7±10% has in many caseslead to an availability decrease of the same amount or even more as seen inFigure 21-5 A 1% reduction in plant availability could cost $500,000/yr inincome 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 offuel, the preventive maintenance programs, the operating mode, the controlsystems, and the firing temperatures.

Redesign for Higher Machinery Reliability

Low reliability of units gives rise to high maintenance costs Low ability is usually a greater economic factor than the high maintenance costs

Below 100 MW Above 100 MW

Availability Efficiency

Figure 21-5 Comparison of availability and efficiency for large frame type gasturbines

In many large power plants, refineries, and petrochemical complexes,about one-third of the failures are due to machinery failure; it is thereforenecessary to redesign parts of a machine to improve reliability.

The maintenance practice of one large refinery is to replace gas turbinecontrol systems with state-of-the-art electronics and ``plug-in'' concepts forease of maintenance These installations have been highly successful in thatmaintenance 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 designhas been supplemented to include a self-contained alarm system, a semi-automatic sequential start system, and a complete trip and protectionsystem, 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 newmachines

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 NOx combustors have been very negative on theavailability of Combined Cycle Power Plants, especially those with dualfuel 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 toresonance 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

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Figure 21-6 Contributions of various major components to gas turbine down time

pressure dam babbitted sleeve bearings to tilting pad journal bearings isbecoming 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 totilting pad thrust bearings with leveling links (Kingsbury type), is anotherarea of common change These types of bearings absorb sudden load surgesand liquid slugs Many users have changed out the inactive thrust bearing tocarry the same load as the active thrust bearings This has been the case inolder gas turbines where traditionally the load carrying capacity of theinactive thrust bearing was 1/3of the active thrust bearing As gas turbinesgot older the leakages increased and the thrust forces were altered greatlyleading to failures in the inactive thrust bearings.

A major plant replaces the entire large journal and thrust bearings in theirmain machinery to tilting pad bearings in their plant as a matter of practice.Material changes of the babbit are sometimes undertaken Changing fromthe more common steel backed babbitted bearings to the copper alloys, withthis babbitted pads, conducts surface heat away at a faster rate, thus increas-ing the load carrying capacity In some instances, a 50±100% load carryingcapacity improvement can be achieved Some equipment manufacturers areoffering bearing-upgrading kits for their machine in service

Design of turbine blades to obtain higher efficiency and damping has beendone In some cases, this has improved efficiency by 8±10%, and stoppedfailures in these blades Steam injection has been utilized in gas turbines toimprove efficiency and to increase the power output Redesign of variousbleed-off ports has reduced tip stalls and their accompanying blade failures.Today's machinery, which is pushing the state-of-the-art in design, needsmore than ``simple fixes.'' This is one major reason why so much redesigntakes place in the field Maintenance engineers are no longer just required torepair, they are required in many cases to make revisions Continualimprovements and updating of the machinery is required to obtain the longruns and high efficiencies desirable in today's turbomachinery

of the machinery and then carries out diagnostics Condition monitoringsystems, which are only mechanical systems without performance inputsgive less than half of the picture and can be very unreliable Unscheduledmaintenance is very costly and should be avoided To properly scheduleoverhauls, both mechanical and performance data must be gatheredand 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, techniquesare sometimes used, which are questionable and should only be used inemergencies

To plan for a ``turnaround,'' one must be guided by the operatinghistory of the given plant and, if it is the first ``turnaround,'' by conditionsfound in other plants utilizing the same or closely similar process andmachinery This is how the time between subsequent ``turnarounds'' hasbeen extended to three years or more in many instances By utilizing theoperating history and inspection at previous ``turnarounds'' at this orsimilar 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, areseldom, if ever, repaired except in an emergency; such items are replacedwith 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 proceedsmoothly and without holdups that could have been foreseen This usuallymeans close collaboration with the manufacturer or consultant and theOEM (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 timerequired This is the planning, which must be done in detail before theshutdown with sufficient lead-time available in order to have replacementparts available at the job site

The old maxim ``if it ain't broke don't fix it'' is very applicable intoday's machinery A study conducted at a major nuclear power facilityfound that 35% of the failures occurred after a major turnaround This iswhy total condition monitoring is necessary in any performance basedtotal productive maintenance system and leads to overhauls beingplanned on proper data evaluation of the machinery rather than on afixed interval

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 postingsEach of these items listed, if properly employed, can transmit knowledge

to the person who must keep the plant's machinery running How well theinformation is transmitted depends entirely on the communication skillsapplied 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 ofneeded information The manual must be written in simple, straightforwardlanguage, have illustrations, sketches, or exploded views adjacent to pertin-ent text, and have minimum references to another page or section Majorsections or chapters should be tabbed for quick location

Most often a mechanic or serviceman refers to a manual because of aproblem Problems seem to happen during a production run It is essential,therefore, that he be able to find the needed information quickly Themechanic should not be delayed by wordy, irrelevant text The objective

of any manual is to be an effective, immediate source of service tion

informa-The assignment of a nontechnical person to write a manual is shortsightedand more costly in the long run A well-written manual is continuously inuse Good manuals need not be complicated In fact, the simpler the better.Manuals should be readable and understandable, whether they are compiledin-house or outside

Drawing and Print File A good print file is a vital tool for any tenance organization Reference files in a large or multi-plant company can

main-be particularly burdensome for several reasons:

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'savailable to the maintenance and operation department A good digital filereduces search time and helps the departments do a better job of keeping themachinery operating at their peak efficiency with minimal downtime.Training Materials Like any other written or audio-visual maintenancetool, 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, aprogram must be developed If the training need applies to a proprietarymachine 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 mayshock the uninitiated, but after shopping around, the company may find that

it can recover far more than the initial cost in tangible benefits over arelatively short period

Pocket Guide When a new maintenance form or procedure is introduced,

a quick reference pocket guide can promote understanding and accuracy Thekey to effectiveness is a deliberate design to provide maximum illustrations orexamples in simple language If it cannot be prepared in-house, outside helpshould 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'ssuccess depends heavily on communicating formal tips and techniques inthe mechanics language and using photos, sketches, and drawings gener-ously to get the message across

Everyone in the maintenance department should be encouraged to 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 Eachcontributor 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 anarticle that is seen by virtually everyone in the company

con-Seminars and Workshops College or industry-sponsored seminars,continuing education courses, and workshops are means of upgrading or

sharpening skills of maintenance people Such an approach serves a twofoldpurpose First, it communicates the company's good faith in the person'sability to benefit from the experience, and by acceptance, the worker showswillingness to improve his or her usefulness to the company The seminarsare very useful in disseminating knowledge They also provide forum forgripes and meaningful solutions Discussion groups in these seminars andworkshops 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 plannedinspections with repair or replacement of damaged components A properlydesigned and conducted inspection and preventive maintenance programcan 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 emphasizeand describe preventive maintenance procedures to ensure the reliability oftheir 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 referencessuch as the manufacturer's instruction book, the operator's manual, and thepreventive maintenance checklist

Inspections range from daily checks made while the unit is operating tomajor inspections that require almost total disassembly of the gas turbine.Daily inspections should include (but are not limited to) the followingchecks:

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 lightsThe daily inspection should require less than an hour to perform properlyand can be made by the operator

The interval between more thorough inspections will depend on theoperating conditions of the gas turbine Manufacturers generally provideguidelines for determining inspection intervals based on exhaust gastemperatures, type and quality of fuel utilized, and number of starts

Table 12-2 shows time intervals for various inspections based on fuels andstartups Minor inspections should be performed after about 3000±6000hours of operation, or after approximately 200 starts, whichever comesfirst This inspection requires a shutdown for two to five days, depending

on availability of parts and extent of repair work to be done During thisinspection, the combustion system and turbine should be checked.The first minor inspection or overhaul of a turbine forms the mostimportant datum point in its maintenance history, and it should always bemade under the supervision of an experienced engineer All data should becarefully taken and compared with the turbine erection information toascertain if any setting changes, misalignment, or excessive wear haveoccurred during operation Subsequent inspections are also of great impor-tance, since they verify manufacturers' recommendations or help to establishmaintenance trends for particular operating conditions

When the established time for major maintenance approaches, a ing should be arranged between the operating department and themanufacturer's engineer to discuss and arrange for the date of turbineoutage A short time before taking the turbine out of service a completeoperational test should be made at zero, one-half, and normal maximumloads, preferably in the presence of the manufacturer's engineer Thesetests are for reference temperatures and pressures, which will serve as

meet-a memeet-ans of compmeet-arison with identicmeet-al tests thmeet-at should be mmeet-ade ately after the unit is overhauled The operational tests should end with

immedi-an over-speed trip test to indicate whether attention should be given tothe governor or tripping mechanism during the shutdown These specificdata will also serve together with the logged operational data or casehistory (which should be reviewed with the manufacturer's engineer) todetermine the focal point or items requiring special attention or investiga-tion:

1 Increase or change in vibration

2 Decrease in air compressor discharge pressure

3 Change in lube oil temperatures or pressure

4 Air or combustion gases blowing out at the shaft seals

5 Incorrectly reading thermocouples

6 Change in wheel space temperatures

7 Fuel oil or gas leakage

8 Fuel control valves operate satisfactorily

9 Hydraulic control oil pressures changed

10 The turbine governor ``hunts''

11 Change in sound level of gear boxes

12 Overspeed devices operate satisfactorily

13 Babbitt or other material found on lubricating oil screens

14 Lube oil analysis shows corrosion factor increase

15 Change in pressure drop across heat exchangers

16 Turbogenerator reaches rated load at design ambient and exhausttemperature conditions

Preparation for shutdown should be made as complete as possible toeliminate lost time and confusion at the beginning of the job

A list should be made of all major items that are to be inspected or repairs

to be made if they are known at the time This list should be prepared withthe manufacturer's engineer present A detailed schedule should be formu-lated from this list including the time allotted for the shutdown and themaintenance crew available Plan the work with the expectation of findingthe worst conditionsÐthe unexpected work found after the machine isopened will then be compensated This procedure will greatly reduce thepossible need for costly overtime

Tools on-site should be reviewed by the manufacturer's engineer Allspecial or regular equipment not on hand that is necessary or required to

do any part of the work should be ordered and on-site before shutdown.Exact outage time should be arranged, and the turbine prepared for thecontracting crew or plant maintenance crew All personnel should be on thejob or available to meet the starting date

Facilities, such as convenient air and electrical connections, should beprearranged for operating tools, etc Sufficient hose lengths and connectorsare required as well as electrical extension cords Install air driers or waterseparators in the air system, since dry air is necessary for successful gritblasting of turbine parts

Before removing turbine flange bolts or disturbing the normal turbinesetting, clearance readings between the last row of turbine rotating bladesand their wheel shroud should be made at both horizontal and verticalpositions Evidence of the main turbine flange spreading or warping should

be checked with feeler gauges between each of the flange bolts Elevationchecks at each of the turbine supports should be made for comparison withoriginal readings to determine if there has been movement at these points.When all outside checks have been made, structural beam supports should

be placed under the turbine at the midpoints between the normal turbinesupports Screw jacks must then be used to bring pressure under the turbineuntil a slight deflection on dial has been reached For this purpose, use onlyscrew jacks, not hydraulic or lift jacks Flange bolts can then be removed aswell as the top half of the turbine casing

Borescope Inspection

Borescope inspection is carried out because of the following benefits it canprovide in the maintenance program:

1 Internal on-site visual checks without disassembly

2 External periods between scheduled inspection

3 Allows accurate planning and scheduling of maintenance actions

4 Monitors condition of internal components

5 Increased ability to predict required parts, special tools, and skilledmanpower

Figure 21-7 shows the time savings one may obtain by the proper use ofborescopic inspection for planned maintenance

The borescope contains its own light source throughout the engine nal passages Once inserted, the flexible borescope can be maneuvered toinspect the complete hot-section flow path The results of the visual inspec-tion can be used to assist in planning scheduled disassembly of the gasturbine It must be remembered that a borescope is a monocular device,and it is extremely difficult to estimate size or distance Maintenance per-sonnel should be well trained to use a borescope effectively Photographs,especially colored, can be utilized as a reference on the history of a machine

inter-In addition to performing inspections while the gas turbine is not operating,some research has been conducted to develop methods for inspection duringoperations by providing a film of cooling air around the borescope tube Ifthis system is developed, it will enable visual inspections of the hot sections

up to the first-stage turbine blades without shutting down the unit

Turbomachinery CleaningThere are at least three reasons for ``on-stream'' cleaning The first is torestore the system's capability If the unit is a driver, its maximum horsepowerwill probably drop as it becomes dirty Cleaning will restore this limit If themachine is a dynamic compressor, fouling may reduce its head, and therefore,the maximum gas flow rate Cleaning will restore the capacity limit

The second reason is to increase the machine's efficiency In most cases,fouling will increase the fuel or power required for a certain task Thedeposits change the flow contours Removal of the deposits will restore theoriginal profiles and the efficiency

Cleaning also prevents failures due to abnormal operating modes Fouling

of the rotor blades on turbines can cause thrust-bearing failures Deposits on

turbine governor valves and trip and throttle valves are suspected of causingoverspeed failures Fouling of balance piston labyrinths and balance lineshas caused thrust-bearing failures in centrifugal machines Any rotor depositcan cause vibration from unbalance if it is not laid down uniformly or if itsluffs off nonuniformly There can be other, similar effects which will causefailure of a unit.

Fouling Indicators

A prerequisite of a cleaning program is some kind of fouling detectionsystem Naturally, this system must cover the prime reason for cleaning Ifthe machine is a gas turbine, the prime reason may be horsepower capability,

or it may be efficiency On a centrifugal compressor, the prime reason for

Figure 21-7 Effect on planned maintenance with usage of borescope

cleaning may be to restore capacity, to improve efficiency, or to reducethrust loading.

The selection of a fouling detection system will be strongly influenced

by the safety and complexity of the cleaning procedure For example, theprocedure may be to throw 10 pounds of spent catalyst into the suction of agas turbine Or, it may involve injecting a quart of water into a single-stagemechanical-drive turbine with a 30F superheated inlet In either case therisk of damage and the personnel required are low The cleaning should befrequent and routine

Fouling indicators include:

1 Gas turbine exhaust temperatureknown or is relatively constant

3 The exponent n 1=n in one section of a machine relative to anothersection handling the same gas

4 The pressure ratio in one section of a machine relative to another section

5 Thrust loading or thrust-bearing metal temperature

6 Balance line to suction differential pressure

7 Compressor discharge pressure and temperature

8 High vibration readingsCleaning Techniques

There are two basic approaches to cleaning: abrasive cleaning, and solventcleansing Details of cleaning are given in Chapter 12 Abrasion is the simplest

of the two methods, but it is usually the least effective Figure 21-8 showsthat abrasive cleaning does not bring the unit back to full performance andthat there is a deterioration in the maximum performance after repeatedcleanings The more common abrasives are 1/64-inch nut shells or spentcatalyst The abrasive must have sufficient mass to achieve the momentumrequired to dislodge the dirt However, high-mass particles do not follow thegas stream Also, they are hit by the leading edge of the moving wheels andblades Consequently, the trailing edges are not abraded The closer the dirt

is to the injection point, the less significant the asymmetrical distribution.The abrasive must also be sufficiently tough to resist breakage on impact.Rice is a poor substitute, since it tends to shatter on impact and smallparticles lodge themselves in bearings and seals Again, the closer the injec-tion to the deposit, the less significant the toughness

Another problem with abrasives is what happens to them after they havedone the cleaning In a simple-cycle gas turbine they will probably be burnt

However, in a regenerative unit they can deposit in the regenerator Someregenerator burnouts have been attributed to these deposits In steam sys-tems they will probably plug up traps throughout the system.

During discussions about abrasive cleaning, the possibility of causinglabyrinth damage is always raised In fact, these apprehensions have provengroundless No one knows why, but it could be that the particles are too big

to enter the clearance space On a centrifugal compressor, a typical radialclearance on the interstage shaft labyrinth is 0.008 inches, as compared to aparticle size of 0.060 inches The eye labyrinth has a much greater clearance,but an abrasive particle would have to make an unguided 180turn to reach

it It is unlikely that a particle would do so

How are the abrasives introduced into the machine? With air compressors,the abrasive can be thrown into the open suction If the suction or point ofinjection is pressurized, the abrasives can be introduced with a blow pot Aneductor should be used to put the abrasive leaving the blow pot into afluidized state before introducing it to the main gas stream A good startingpoint for the injection rate is 0.1 weight percent of gas flow

Solvent cleaning is a much more delicate technique than the brute force ofabrasion In reality, there will almost always be some abrasive actioninvolved The idea is to dissolve the deposit in a solvent The solution must

Figure 21-8 Effect of cleaning on power output

then be removed from the system before the solute is redeposited Eachsolvent-cleaning application presents different problems Two methods areoutlined:

1 Water wash This method is used to remove deposits Distilled water issprayed into the air inlet at a specified rate and engine speed Thisspeed is normally at a reduced rpm so that the water will not flash intosteam in the compressor and therefore become ineffective on the latterstages or diffuser

2 Detergent wash This method is used to remove oil and oil-like its A mixture of solvent detergent and water is sprayed into the inletwhile the gas turbine is being rotated by the starter The unit isallowed to remain idle for a period of time to allow the solution todissolve and loosen the deposits The procedure is then repeatedexcept that distilled water is used to flush the deposits off the com-pressor and out the combustor drains.*

depos-Hot-Section MaintenanceCombustion chambers can be removed if integrally arranged with theturbine, or they can be minutely inspected for cracks or burned areas with

a borescope Short, individual cracks are not uncommon and need noimmediate attention However, if the cracks are grouped such that theircontinuance or the beginning of another crack could cause the loss of apiece of metal, then a repair should be made Cracks of this nature normallycan be welded with a type of welding rod recommended by the manufac-turer, depending on the kind of metal involved Burned or warped areas incombustion chambers or baskets can be cut out and new sections welded.However, burned areas should be studied with regard to location, pattern, orrepetition in all chambers to determine the cause of the burning

Individual burned areas may indicate a dirty or faulty fuel burner nozzle

or misalignment of the combustion chamber Similar burned areas in variouschambers may indicate abnormally high firing temperatures during startingdue to excessive fuel use They may also be the result of ``slugs'' of liquidsentering with the fuel gas, excessively rapid starts, or overloading of the

* Solvent wash of the hot section requires the unit to be brought down to idle speed The metal temperatures in the unit should be around 200  F To achieve this temperature in a reasonable time, the unit can be run on the starter rotor For a large turbine, the entire wash cycle will take about 16±20 hours.

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 replacementsmade, and their location in the turbine at each inspection date The basketends, or at places where they are supported, should be inspected for excessivewear from vibration, or expansion and contraction movement Repair ofthese parts should be made by cutting out and welding in new materials orreplacing spring seals if necessary The first-stage turbine stationary blades

or nozzles can be superficially inspected for warps or sags by entering theturbine through the combustion chamber areas or by removing inspectionplates In certain size turbines (and by somewhat difficult maneuvering), thelast row or turbine rotating blades can be inspected by entering through theturbine discharge duct The opportunity should be taken to measure, ifpossible, the blade tip clearance at four points on the circumference Com-parison of these clearance readings with those at installation or at someprevious time will indicate if rubs have occurred and whether or not the sealring is warped and out of round It will also indicate whether or not the rotor

is below its original position and requires further investigation at the haul period

over-As the hot sections become exposed, preliminary inspection for cracks orwarping should be undertaken to estimate work to be done The bearingsrequire inspection for wear and alignment for the same reason The transi-tion pieces should be inspected for cracking and wear at points of contact.Wear usually occurs between the transition piece and the combustion linersleeve, and also at the first-stage nozzle fit The cylindrical section of thetransition piece may be replaced if the wear is excessive; wear at the nozzle-end of the transition piece is more serious because it allows excessive vibra-tion of the transition piece, which might lead to cracking Transition piecesshould be replaced if 50% of the inner or outer seal is reduced to half theoriginal thickness If the transition piece is in otherwise excellent condition,the seals may be ground off and replaced The new floating seals have beenfound to be more reliable than the old fixed seals Transition pieces should

be replaced if cracks are found in the body

Turbine blades should be closely inspected for erosion and cracks.The most critical areas in the turbine rotor are the fir-tree section, where theblades are attached to the rotor, and the trailing edge of the blade near thehub The trailing edge of the turbine blade is usually the hottest section

of the blade These areas should be carefully cleaned and checked for crackswith spray penetrant First-stage inlet vanes and rotating blades should beremoved and blasted clean with a No 200 grit aluminum oxide or otherapproved blasting material They should then be inspected minutely for

cracks by means of red dye or black light The first-stage inlet vanes willprobably need attention, which can be done on the job Vane warpage on thetrailing edge, if any, can be taken out by inserting a spacer piece of correctcross-sectional area between the vanes, heating the top vane to red heat with

a torch, and forging the vane edge flat with a hammer and flatter Thecracks, if less than 1.5 inches long, can be grooved out and welded, providingthe crack does not run under the end-supporting rings In this case, the vanemust be removed and welded or a new vane fitted in place As the vanes arewelded, they must be continually checked for new cracks, which in turn must

be grooved and welded and checked again

While repairing the first-stage inlet vanes, the upper- and lower-vanesection should be bolted or clamped together, and the entire ring should beplaced on a flat, level surface, or sufficiently supported in the horizontalplane to prevent heat warpage of the ring due to heating of the vanes duringtheir repair

After straightening or taking out any warpage in the trailing edges of thevanes (partitions), perpendicular distances between the trailing edges of eachvane and the surface of the next should be carefully measured An average

of these distances should be made and then corrected to a plus or minuspercentage approved by the manufacturer This method will help to assureequal distribution of gas flow to the first-stage rotating blades for elimin-ation of blade vibration

Turbine rotating blades cannot be field-repaired if they are cracked If one

or two blades are damaged mechanically, the manufacturer may recommendfield repair or replacement of the damaged blades However, if several bladesare fatigue cracked, it is recommended that the entire set be replaced, sincethe remaining blades have been exposed to the same operating conditionsand, therefore, have little fatigue life left

Both top and bottom halves of the journal bearings should be inspectedfor misalignment wear as well as excessive ``in-line'' wear, which can occur

in turbines with frequent starts An indication of the condition of thethrust-bearings can be made by removing a small section of the turbineshaft, usually on the governor end, and axially moving or bumping theshaft The amount of axial shaft movement will indicate the thrust clear-ance and, if it is found to be 0.012±0.015 inches, it can be considerednormal

If the turbine is not out of alignment, or the shaft bowed as determined bythe vertical and horizontal clearance checks or the appearance of the bearingsurfaces, it is not recommended that the rotor be removed Some turbinedesigns, however, may require removal of the rotor to facilitate the removal

of some bottom sections of the diaphragms or inlet vanes If the rotor is

removed, special care must be taken when separating the couplings ling flanges must be marked and run-out checks made for alignment so thatthey can be properly reassembled However, the work should always be doneunder the manufacturer's supervision.

Coup-Work should be made to progress strictly in accordance with the plannedflow charts, which must be constantly kept updated Extra work and delayswill probably be encountered; however, a well-planned program will includecertain allowances, and little change should be required If any significantchange is encountered, the program should be revised to show the extraoutage time and possible extra personnel required

Compressor MaintenanceBesides checking the hot section, the compressor blades in axial compres-sors should also be inspected The compressor inspection should be con-ducted to determine the mechanical and aerodynamic condition of thecompressor Most axial-flow compressors have stacked rotors with boltsextending through all the discs The bolts should be inspected and, if anyare loose, the stretch on the bolts should be determined

Axial compressor performance is sensitive to the condition of the rotorblades During a major inspection, all blades should be cleaned and checkedfor cracks with a penetrant test If cracks are found in any blade, that bladeshould be replaced Occasionally, small cracks can be blended out, but thisprocedure should be approved by the manufacturer

The amount of wear on an axial-flow compressor blade is usually afunction of foreign particle ingestion Dust is the most common foreignparticle The maximum and minimum chord lengths should be recordedand reported to the manufacturer, who in turn should be able to reportthe performance loss occasioned by wear and the decrease in structuralstrength

If the air inlet is subjected to saltwater contamination, the rotor and statorblades should be checked for pitting Severe pitting near the blade roots maylead to structural failures The manufacturer should be informed of severepitting

Stator blades are as important as rotor blades All the same cleaning,inspection, and nondestructive test procedures should apply It should benoted that the wear pattern is somewhat different on the stator blades.Again, the manufacturer should be informed of the wear conditions andshould in turn make recommendations concerning continuous operation orreplacement

On completion of required repair and replacement, the gas turbine should

be reassembled This reassembly should be done under careful and enced supervision to ensure all work meets established criteria Blade clear-ances, bearing clearances, and spacing should be checked and recordedduring assembly Special care should be taken to ensure that the machinistuses the proper torque when tightening bolts and nuts There is a very strongtendency for machinists to apply a torque that ``feels'' right rather than using

experi-a torque wrench Torque is experi-a very importexperi-ant experi-aspect of experi-assembly Impropertorqueing can cause component warpage and distortion, especially in thosecomponents subject to high temperatures during operation

Bearing MaintenanceWith high-speed machines, simple bearing failures are rare unless they arecaused by faulty alignment, distortion, wrong clearance, or dirt More com-mon are failures caused by vibrations and rotor whirls Some of these originate

in the bearings, others can be amplified or attenuated by the bearings,the bearing cases, and the bearing support structure

During inspection, all journal bearings should be closely inspected If themachine has not suffered from excessive vibrations or lubrication problems,the bearings can be reinstalled and utilized

Four places should be checked for wear during inspection periods:

1 Babbitted shoe surface

2 Pivoting shoe surface and seat in retaining ring

3 Seal ring bore or end plates

4 The shoe thickness at the pivot point or across ball and socket; allshoes should be within 0.0005% of the same thickness

While being inspected, the following checks should be made:

1 All leading edges of shoes must have a uniform radius for the fulllength across the shoe File the radii if necessary to obtain proper size

2 Light scratches in the babbitt face do not necessarily require shoereplacement If no wear is detected, scrape lightly with a sharp straight-edged scraper (plate type) to remove any upsetting caused by scratches

3 Shoes should be replaced as sets only if:

a Radial clearance has increased more than 11¤2mils over nominaldesign clearance

b Leading or lagging edges of shoes show signs of wear

4 The tilting-pad and support-ball combination spare parts should belapped together, making them an integral unit When a new or usedbearing is disassembled for cleaning and inspection, care should betaken not to mix the tilting-pad and support-ball combinations.

5 On reassembly, care should be taken to return the tilting-pad andsupport-ball combination to the original location in the support ring.Changes in clearance and concentricity can result if the tilting-padand support-ball combination is not returned to the same location

An eccentricity of as little as one mil can cause severe vibrationproblems

Clearance Checks

1 Check housing OD and ID to be sure it is round

2 Check bore and face-end plates for nicked edges, deep scratches, or ring Stone or scrape if necessary, and polish with very fine aluminumoxide polishing paper

sco-3 Check parting-line surfaces for full contact Stone or lap if burrs orraised edges exist

4 Check pivoting surfaces of shoe and housing ring for scratches, ing, or erosion Stone if necessary

scor-5 For tilting-pad bearings, blue-shoe the pivot surface, and check forcontact area and position The contacting surface must be in thecenter only and at the bottom portion of the pivot bore in the retainer

6 Check to be sure that pins do not bottom-out in pads

7 For ball-and-socket designs, check to be sure the ball seats properlyand solidly in the counter bore

8 Check for shaft clearance as follows:

a Select a stub mandrel in which the minimum diameter is thejournal diameter plus minimum desired clearance (about 11¤2 milper inch of shaft diameter) and the larger diameter is journaldiameter plus desired clearance (about 2 mils per inch of shaftdiameter)

b Assemble the bearing halves

c Slip the assembled bearing over the smaller diameter of the drel

man-d Tap the bearing lightly on the back of the housing and slide thebearing down on the next larger diameter

e The mandrel should be rotated and the OD of housing cated

indi-Thrust-Bearing Failure

A thrust-bearing failure is one of the worst things that can happen to amachine, since it often wrecks the machine, sometimes completely Toevaluate the reliability of a thrust-bearing arrangement, we must firstconsider how a failure is initiated and evaluate the merits of the variousdesigns

Failure initiation Failures caused by bearing overload during normaloperation (design error) are rare today, but still far more thrust failuresoccur than one would expect, considering all the precautions taken by thebearing designer The causes in the following list are roughly in sequence ofimportance:

1 Fluid slugging Passing a slug of fluid through a turbine or compressorcan increase the thrust to many times its normal levelÐeven if only afew gallons are involved Instantaneous failures of the downstreambearing may result from fluid slugging

2 Build-up of solids in rotor and/or stator passages (``plugging'' of turbinebuckets) This problem should be noticed from performance or pres-sure distribution in the machine (first-stage pressure) long before thefailure occurs

3 Off-design operation Especially from backpressure (vacuum), inletpressure, extraction pressure, moisture Many failures are caused byoverload, off-design speed

4 Compressor surging Especially in double-flow machines

5 Gear coupling thrust A frequent cause of failure, especially ofupstream thrust bearings Thrust is high when alignment is perfect(friction coefficient 0.4±0.6), decreasing to a minimum when a smallmisalignment is present (about 0.1 at 25 angular misalignment).Friction increases rapidly again to 0.5 or more with an increase inmisalignment (These are rough numbers only, to show basic relation-ships.) The thrust is caused by friction in the loaded teeth that opposesthermal expansion Therefore, thrust can get very high, since it has

no relation to the normal thrust caused by pressure distributioninside the machine (for which the thrust bearing may have beendimensioned) The coupling thrust may act either way, adding to orsubtracting from normal thrust Much depends on tooth geometryand coupling quality A straight-sided tooth can take misalign-ment only when the tooth fit has enough clearance to permit slanting

of the male tooth inside the female teeth For example, with vertical

misalignment, the teeth on both sides will bind when the clearance isinsufficient to allow for slanting This can cause very high thrust,sometimes one can hear a ``metallic sound'' building up until therotors finally slip with a very noticeable ``bump.'' Then the noiseand vibration are gone, at least for a while This phenomenon, ofcourse, is torture for the thrust bearings, and it may cause failure ineither direction Dirt in the coupling can aggravate this situation oreven cause it.

6 Dirt in oil A common cause of failures, especially when combined withother factors The oil film at the end of the oil wedge is only a smallfraction of a thousandths thick If dirt goes through, it can cause thefilm to rupture, and the bearing may burn out Therefore, very finefiltering of the oil is required But the best filter is no good if main-tenance personnel leave the filter or bearing case open after inspection,and the rain and sand blow in, or if they put the wet filter elements onthe sandy floor, or accidentally knock holes in the elements It happensfar too often Once a machine is wrecked, it is difficult to reconstruct

7 Momentary loss of oil pressure Sometimes encountered while ing filters or coolers

switch-Failure protection Fortunately, accurate and reliable instrumentation

is now available to monitor thrust bearings well enough to assure safecontinuous operation and to prevent catastrophic failure in the event of anupset to the system

Temperature sensors, such as RTDs (Resistance Temperature Detectors),thermocouples, and thermistors, can be installed directly in the thrust bear-ing to measure metal temperature The installation shown in Figure 21-9has the RTD embedded in the babbitted surface It is in the most sensitive

Figure 21-9 RTD embedded in bearing surface

zone of the shoeÐ70% from the leading edge and 50% radially The ition of the sensor is critical in establishing the safe operating limits As long

pos-as the probe is generally in the zone of maximum temperature, it will behighly sensitive to load, although the level of temperature may vary con-siderably as can be seen in Figure 21-10 The temperature is also dependent

on the pad-backing material At 500 psi load, the center sensor at A-IIregisters 200F while the sensor at B-I registers 280F in a steel-backedbearing Again, these temperatures are typical and will vary with size, type,speed, and lubrication from bearing to bearing The difference in a copper-backed bearing can be seen to be quite significant, with A-II reading 185Fand B-I reading 205F The position of the sensor with respect to the surface

is less significant in this bearing than in the steel-backed bearing Again,position in the sensitive zone is important in establishing safe operatinglimits with respect to temperature

Axial proximity probes are another means of monitoring rotor positionand the integrity of the thrust bearing A typical installation is shown inFigure 21-11 In this case two positions are being monitored: one at thethrust runner, and one at the end of the shaft near the centerline Thismethod detects thrust-collar runout and also rotor movement In mostcases this ideal positioning of the probes is not possible Many times theprobes are indexed to the rotor or other convenient locations and thus donot truly show the movement of the rotor with respect to the thrustbearing

Figure 21-10 Temperature distribution in bearing surfaces