Process Engineering Equipment Handbook Episode 1 Part 9 potx

Cold end optimization This manufacturer establishes the optimum combination condenser/turbine in eachcase using the following parameters: Cooling water temperature Cooling water flow r

For condenser calculation, the OEM uses standard fouling coefficient h vfor everytube material based on experience.

Layout philosophy

Due to the routing of the steam flow, CB condensers achieve better heat transfervalues (k-values) than some other designs In order to obtain the required condenserpressure up to 20 percent less surface area is required in comparison with someolder designs

Cold end optimization

This manufacturer establishes the optimum combination condenser/turbine in eachcase using the following parameters:

 Cooling water temperature

 Cooling water flow rate

 Power consumption of the cooling water pump

 Space conditions

Type Series (see Figs C-272 and C-273)

In the interests of standardization, a type series has been developed for the CBcondenser that covers the entire power range of industrial turbines and small steamand cogeneration plants This series provides an appropriate link to the large-scalecondensers made by this OEM (type series CM)

Together with the option of varying the tube length it is possible to provide theoptimum condenser for every requirement and plant size

Selection of Material

In most cases the condenser is manufactured from material according to DINstandards If required by the customer, materials can also be used in compliancewith other standards, such as ASTM

Due to the manifold requirements the selection of the tube material is of greatimportance.

The basis of every selection is a sample of the cooling water, which is analyzed

by specialists In cooperation with the end user the appropriate material is thenselected The most important criteria are:

 Corrosion resistance to cooling water

 Sand content of the cooling water

 Cooling water velocity

 Thermal conductivity of the material

 Chemistry of the steam circuit

 Resistance to droplet erosion

The following materials are mainly employed:

Brass. If the quality of the cooling water is good (river water, freshwater lakes)admiralty brass or aluminum brass is a well-proven material, an important featurebeing high thermal conductivity

Copper nickel alloys. If the quality of the water is poor, such as met with in portsand large rivers, CuNi alloys are preferred because they are more resistant thanbrass In cooling tower operation also these alloys are of advantage as the coolingtower water is usually highly concentrated and therefore aggressive

Stainless steels. For special requirements, such as for brackish water or sea water,high-grade steels are suitable These have the advantage that much higher water

FIG C-273 Type series of CB condenser (Source: Alstom.)

velocities are admissible; in the case of nonferrous metals the velocity is limited bywaterside erosion.

Titanium. This material fulfills practically all requirements It is extremelyresistant to corrosion and allows high water velocities as well as offering very goodresistance to steamside droplet erosion The price and the relatively low thermalconductivity can be compensated for, to some extent, by providing thinner tube wallthicknesses

Tubesheet

In general, tubesheets are made of carbon steel with a stainless steel or titaniumcladding on the cooling water side The tubes are roller expanded into the tubesheet.Upon request, the tubes can also be welded into the tubesheet See Figs C-274 andC-275

The tubesheets are welded to the condenser shell, thus ensuring reliabletightness

The waterboxes are also welded to the tubesheet If required, a flange connectioncan also be provided

Venting

Venting the steam shell

The steam shell of a condenser is under vacuum Careful manufacture and the use

of high-grade sealing materials help to reduce the amount of air inleakage to aminimum but it can never be completely eliminated The steam shell of a condensermust therefore be permanently vented

For evacuation purposes, this OEM uses water-jet ejectors or steam-jet ejectorsand, in special cases, also water ring pumps The layout of the suction units employsconcepts in compliance with the German VGB (technical association of large power

FIG C-274 Cladded tubesheet, welded to the condenser housing (Source: Alstom.)

FIG C-275 Example of sacrificial anodes for protection of tubes and waterbox Here the typical shape of the CB air cooler can be seen (Source: Alstom.)

FIG C-276 Determining the air inleakage as a function of the steam flow to the condenser (Source: Alstom.)

utilities) recommendations This is a reliable venting system for all types of loads,requiring minimum equipment and operational outlay With improved ventingcharacteristics, considerable savings in investment can be achieved with this OEM’sdesign (see Fig C-276)

Startup venting

Generally the service ejectors are also used as startup ejectors for generating thenecessary vacuum in the water/steam system before starting up the plant With anend-user request, special hogging vacuum pumps can also be employed to reducethe evacuation time For this purpose usually water ring pumps are provided, asthese have a constant high flow rate over a wide pressure range

Waterbox venting

For economic cooling with fresh water or sea water, the outlet waterbox must have

a slight vacuum due to the geodetic requirements This results in degassing of part

of the cooling water’s dissolved air This degassed air must be constantly removedand for this purpose single-stage water ring pumps are usually employed

Accessories

Basically there are two major accessories: the sponge ball cleaning system and thesteam dump device (SDD)

Sponge ball cleaning system (see Fig C-277)

To a greater or lesser degree all cooling water contains dirt particles that, withoutcountermeasures being taken, adhere to the insides of the condenser tubes thusimpairing the efficiency of the heat transfer

With a continuously operated cleaning system fouling can be reduced to aminimum, the so-called standard fouling This standard fouling also protects thetube material from erosion or corrosion

A cleaning system is also recommended for corrosion-resistant materials, such astitanium or high-grade steel In contrast to alloys containing copper, these materialstend to biofouling, i.e., to forming layers of bacteria This, in contrast to copper, isdue to them being nontoxic to bacteria

FIG C-277 Example of a fouled tube (without cleaning system) and a clean tube (with sponge ball cleaning system) (Source: Alstom.)

Steam dump device (SDD)

End users often need to bypass the turbine during the startup operation or in theevent of load rejections and to route the boiler steam directly into the condenser

A component part of this bypass system is the steam dump device (SDD) into thecondenser See Figs C-278 and C-279 With SDDs the high-energy steam isattemperated with spray water (taken downstream of condensate pumps) andintroduced into the condenser via a perforated cone above the tubes

This SDD system transports the steam smoothly into the condenser It has a lownoise level

FIG C-278 HP/LP bypass system with steam dumping into the condenser (Source: Alstom.)

FIG C-279 Steam dump device (SDD) The high-energy steam is cooled with condensate and led into the condenser via a perforated cone above the tubes (Source: Alstom.)

Design, Manufacture

Manufacturing drawings and tube patterns are raised on CAD systems, enabling direct transfer to numerically controlled tool machines See also Figs C-280 throughC-282

Figure C-283 shows the front end (cooling water inlet and outlet) of a two-pass

CB condenser in the turbine building of a power plant

The water inlet is at the bottom and the water outlet at the top The CB condenseritself is compact and allows simple piping assembly

The space saving contributes to reducing the costs of the turbine building

Choosing a Condenser

The condenser of choice should be an optimum combination of:

 High thermal performance

 Compact design

 Optimum space utilization

 Self-supporting, robust structure without additional internal supports required

FIG C-280 CB condenser during manufacture, ready for tubing (Source: Alstom.)

 Economical manufacture

 Simple transport and assembly

 Extremely low oxygen content in the condensate without any additional measures

 Simple makeup water supply

 No condensate subcooling, resulting in higher efficiency

FIG C-283 CB condenser in the turbine building of a power plant (Source: Alstom.)

Condition Monitoring (CM); Condition-Monitoring System(s) (CMS); Engine

Condition Monitoring (ECM); Engine Condition–Monitoring System(s) (ECMS)*

(see also Measurement)

These four terms are synonyms for the same concept Most commonly, the systemthat can be maintained by a CMS, in a process engineer’s world, is a plant systemconsisting of:

 A driver (gas-turbine, steam-turbine, or electric motor) and a driven component(typically compressor or pump) plus gearbox and coupling and all accessories

 A generating turbine set (many process plants and refineries are becoming smallpower producers or SPPs due to deregulation of the electrical industry worldwide)The CM theory that applies to mechanical (or electrical) drive or generatingpackage is the same The scope of that concept varies, however, among engineers

To some, CM means just the vibration analysis (VA) system that accompanies themachinery system, so it is best to define meaning and intent immediately To others,

CM means anything, including VA, that can be used to determine the health of amachinery system and its components

In modern plants or plants that are being modernized, CMS [together with cycle assessment (LCA)] is a potential basis for some very expensive retrofitengineering or reengineering, especially if major failures or production-loss eventshave occurred, without any warning provided by the existing system Processengineers frequently find themselves being pressured into buying expensive CMS,without being actually convinced of the relevance of the entire system Processengineers generally affirm that rotating machinery is the cause of most of theirproblems Therefore, CM, retrofitted or otherwise, overdesigned or not, the cause

life-of increased “nuisance” trips or otherwise, is high on their list life-of “learn more about.”

If they do design the right system scope, however, they will see:

 A reduction in costs per fired hour

 Reduced incidents of lost time

 Improved environmental performance with a potential for reduced emissions

 An opportunity to debate and reduce the plant’s insurance premiumsNote that the working definition of CM in this book includes LCA and anyassociated performance analysis (PA) (LCA and PA will be dealt with under LCA

in this book.)

Scope and Selection of Condition Monitoring Systems

The troubleshooting process—how easy it is to troubleshoot a machine, how often

it needs to be done, and so forth—depends largely on the type of maintenancephilosophy applied during the machine’s operational life This basic philosophyaffects how often one has to think about the machine

Not surprisingly, the same strategies and philosophy that are behind aturbomachinery item’s selection play a key role in defining maintenancerequirements Unfortunately, a number of operators never link maintenance

* Source: Soares, C M., vibration course notes, 2000.

philosophy and troubleshooting to an appropriate extent They either leavethemselves wide open for disastrous repair bills or spend more than they need to

on maintenance

This is poor risk management Applied to an item of rotating machinery in critical

service it translates into bad business practice This section describes three basicstrategies appropriate for all rotating machinery A choice of these strategies shouldnot be made by the manufacturer, but by the end user It needs to fit the operator’sspecific application and comfort level The manufacturer’s resources and technicalexpertise should be utilized to support the decision process by providing relevantinformation

The operator typically proceeds as follows Basic goals are itemized The highestpriority is to maximize production Optimizing production per unit of energy is part

of that aim Maximum availability and reliability (i.e., no unplanned downtime) arealso critical Operators struggle with financial budgets and therefore pressure tominimize cost They want to minimize the maintenance, service, and repair activity.Too little maintenance results in unexpected failures and consequential major

losses of production and/or customers This impractical approach is termed reactive

strategy, and should be avoided on all important machinery Optimum maintenance

strategy balances reasonable costs with maximum possible availability andreliability The two main maintenance strategies employed by companies today are

labeled predictive strategy and preventive strategy.

Predictive strategy

Predictive maintenance strategies operate without a regular plan for service work

or exchange of parts A maintenance plan is only set up if there’s proof ofdeterioration Consequently, a company with a predictive strategy favorsminimizing cost over maximizing use Annual cost of this strategy may typicallyonly average out to 1 to 2 percent of the prime equipment price

With a predictive maintenance strategy, long-term plans may involve only tworegular procedures:

1 Monitoring of operating data as follows:

 Gas path (mass flow, heed, efficiency)

 Water coolant (differential temperature)

 Oil analysis (water content, deterioration of antiaging additives)

2 Vibration analysis measurements as follows:

 Fast Fourier transfer (FFT) analysis (shaft, pinions), using eddy currentprobes at normal load and turndown

 FFT analysis (all bearing housings), at normal load

Preventive strategy

In contrast with predictive strategy, preventive strategy aims toward maximumsafety against unexpected failures The concept here is to predict the averagelifespan of a part and then replace it before the end of that lifespan Annual cost istherefore higher (anywhere from perhaps 8–10 percent of the prime equipment price

to about 35 percent of the cost of a replacement machine, depending on type)because of the higher numbers of spare parts that need to be purchased andwarehoused

Besides the effects of choice of maintenance strategy on the troubleshooting timeand effort required, the application service the unit is in also has an effect With

increasingly tough environmental legislation that in turn demands maximumenergy usage and recovery, power recovery processes are increasing in number Thederegulation of the power industry, which in turn results in the increase of smallpower producers (such as process plants) also serves to increase this number.

In some installations today, vibration probes and other items used for vibrationanalysis may be installed separately (frequently at different times) than other items used to monitor the health of a machine However, increasingly vibrationmonitoring is part of an overall CMS that then monitors the overall health of allparameters that might indicate the health of a machine The CMS is usuallyprovided by the OEM as part of the machinery purchase price

On occasion, the end user finds that the original CMS is limited in that it cannot

do as much analysis as is desired Certain items on the CMS (such as specific

vibration probes) may be retrofitted Or the OEM or an external vendor supply may

retrofit a different CMS

One key element in a CMS is VA VA can solve up to 85 percent of the problemsfound on rotating machinery As VA instrumentation gets more sophisticated, thenumber of problems it can uncover increases At the same time, however, VAreadings themselves may not finally confirm the existence of a problem Othercorroborating readings from other instrumentation in the CMS are required toconfirm a problem condition

The selection of VA instrumentation therefore should match the accuracy of therest of the CMS system For example, it is pointless having very accurate VA sensorsand crude bearing temperature sensors This is because the problem associatedwith, for instance, high vibration in a certain location and low bearing cavitytemperature may be different from the one that occurs with the same vibrationreading and a slight increase in temperature

Some examples of how temperature readings fit into the overall problemdiagnosis grid follow:

Problem 1. Crossover tube failure:

Fuel pressure: Up or downUnevenness of flame in combustor (sound indication): No changeExhaust temperature spread: Up considerably

Exhaust temperature (average): No change

Problem 2. Cracked combustion liner:

As for Problem 1, except there is audible unevenness (noise) in combustorAlso, vibration readings may be observed to increase

Problem 3. Combustor fouling:

As for Problem 2, except exhaust temperature drops and vibration levels may not indicate any change

Problem 4. Incipient bearing failure:

Differential temperature (bearing): UpBearing pressure: Down

Vibration: Up

Problem 5. Damaged turbine blades:

Vibration increase: LargeExhaust temperature increase

Turbine Diagnosis Table

Problem Efficiency P3/P4 T3/T4 Reading Temperature Temperature Pressure

on starts, fuel consumption, and peak temperatures are among other factors used.Most VA equipment manufacturers now provide the entire CMS and may offerend users the option of selecting different probes to be used in the package Theend user needs to be aware of the VA manufacturer’s strengths and weaknesses in order

to effectively diagnose problems with the CMS or the VA equipment within the CMS.The effectiveness of condition monitoring systems depends on what they consist

of and where, when, and how they are applied

CMS are, deservedly, one of the most controversial and discussed items inturbomachinery technology today The application the system is put together for,the machinery that it has been put on, and the accuracy and positioning of itscomponents may make the same system a great cost saver in one instance and anelaborate waste of money in another

Appropriately used, they are a highly effective means of extending component lifeand time between overhauls (TBOs) Inappropriately used, they can provide a falsesense of security at a very high price and even increase the number of service callsrequired on a system All of these factors are heavily in evidence in global areas ofhigh development activity, which also have the finances to pay (appropriately orotherwise) for these systems

The case for CMS. As long as CMS are not thought of as a substitute for sense or process knowledge, having the appropriate CMS pays dividends Industrymeasurements have provided the following impressive figures (1998 $U.S.):

common-1 From data drawn from the North American power industry:

 Run-to-failure strategy cost was $18/hp

 Planned maintenance (overhaul at specified time intervals) cost was $13/hp

 Cost using a CMS was $9/hp, indicating maintenance cost increase of about

100 percent if no CMS or planned maintenance was done

2 From a U.S nuclear plant’s data:

 Savings of $2 million in one year using CMS and $3.5 million the next

3 From a survey of power, paper, metal, food, and textile producers in the U.S.,Europe, and Australia:

 Savings of 50 to 80 percent in repair and maintenance costs

 Savings of over 30 percent in spares inventories

 Profit margin and revenue up 20 to 60 percentThe only catch is: how much money do you spend on measurement of whatparameters, on what machines, and when

The strongest case for CMS is in prototype applications, design and developmentruns, or where the cost of failure, financial or in terms of human safety, would beunacceptable In some of these cases, it may be appropriate to integrate certaincontrol functions within the CMS

Users purchasing a CMS for a new application or for retrofit would be welladvised to ensure that the designers of that system know the user’s process,application history, global history of the models in question, and the CMS elements(software, hardware, and theory) thoroughly CMS vendors should be invited todiscuss the anticipated life of their proposed components, because particularly inthe case of CMS integrated with controls, the following scenario has occurred CMSvendors who do not manufacture the controls in their system have to buy them offthe shelf It is unlikely that they will be able to purchase the best or the most ruggedcontrols available, as those are made by control manufacturers (who also make andwant to sell CMS) who will not sell controls to a competitor When the CMS vendorsbuy their off-the-shelf components, they may not be able to offer maintenance ofthose model numbers for the life of the plant This means another model numberfor the control in question at some point and all the associated potential problems.Were end users to approach a primarily controls manufacturer for a quote on a

CM controls system, they might find that they are offered a system with fewerfeatures than one quoted by a primarily diagnostics manufacturer This may meanthat the controls manufacturer doesn’t like the additional features Or it may meanthe customer did not need the features in the first place Any potential user of anexpensive CMS should seek the advice of an independent—one that doesn’t sell thesystems, or associated hardware of software—authority on the system scope.Sometimes the “included in purchase price” training that comes with CM systemsachieves only promotion of further sales of such packages The customer avoids this

by taking the “free” training and then using appropriate independent training toenhance critical assessment of the application and equipment

Basic CMS components. A basic CMS consists of VA, temperature monitoring, andsurge control, pressure, and flow measurements It may also include performanceanalysis (PA), LCA, nonintrusive wear monitoring, and a variety of other techniques

Vibration analysis. In the early operational years of the aviation RB 211 (on whichland-based RB211s are based), vibration monitoring (VM) instrumentation helpedavoid fan shaft locating bearing failures These bearings at that point exhibited arapid rate of progression to failure, but the VM provided a few seconds of warning

to allow power reduction and avoid catastrophic failure VM also helped avoidfailure due to:

 Partially missing turbine blades

 Excessive blade tip rubs, spacer ring frettage

 Misplaced or missing locking plates

 Oil migration into compressor drums

 Shaft coupling misalignment

 Compressor stack blot loosening

 Disc frettage

Installed at the outset, VM more than paid for itself in a short while, although onceearly design problems were settled, it revealed fewer problems.

With another fleet operator, of JT9Ds this time, the VM picked up five cases ofhigh engine vibration in 65,000 h (each with a major problem), including a case of

a 150-degree round crack in the high-pressure turbine shaft In three of those fivecases, VM was the only alarm indicator

The above instances all required a tracking filter to properly isolate the flaws inquestion So would similar land-based applications, as aeroderivatives in the land-based gas-turbine mix are becoming more prevalent

How much is too much: which parameters on what machine. Observations of CMSproposals and purchases in large power plants included one case where a highlysophisticated system was being recommended as part of the remodeling for athermal plant The thermal plant had a large number of steam turbines Theirdesign was conservative in terms of peak operating temperatures and they had beenoperating relatively trouble free for about 20 years, as had the associated boilerfeed pumps The OEM-supplied CMS would have trouble tackling the parameteraccuracies a newer turbomachinery equivalent item might require However, it was adequate for the limited temperature ranges and vibration these veryconservatively designed steam turbines and feed pumps had ever seen or will eversee

One such recommended system would have been more appropriate for the testcells of the latest 90,000-lb thrust development aeroengines; it was expensive

“overkill” for the steam turbines Its specification described an expert system thatincluded (many of these items were already measured on the existing system):

 For the turbine generator(s):

 Displacement probes VM

 Velocity probe VM

 Eccentricity monitoring

 Dual thrust position monitoring

 Dual case expansion monitoring

 Differential expansion monitoring

 Dual valve position monitoring

 Rotor speed indicator (all of the above with panel indicators)

 Phase angle transducer

 For the boiler feed water pumps:

 Displacement probes VM

 Accelerometer monitoring for two positions on the hydraulic coupling

 Speed monitor

 Dual thrust position monitor

 Phase angle transducerSuch a system might measure to greater accuracy than the existing system, butgreater accuracy for these parameters in this application might not be necessary inthe first place The expert system then went into specific, in some instances tooconservative, specifications with respect to operating temperature ranges oftransducers, temperature sensitivity of probe and cable, differential pressurewithstanding (of gear oil) potential of transducers, double braiding of shields oncables, transducer protection, electrical isolators, environmental specifications,cable pull strengths, probe mounting potential, transducer frequency responserange, relay configuration and contact ratings, visual displays, calibrationreprogramming or disabling of any monitor, online continuous monitoring, expert

system diagnostics, and a whole host of other factors that could effectively eliminateother vendors that had not suggested the limits specified within the specification.What might have been more to the point was continuous monitoring for creep fatigue degradation, considering the vintage of the units The need for thisexpense would need to be assessed in terms of where the turbomachinerycomponents lay on their stress endurance curves for normal operation andabnormal cyclic operations Earlier methods would have used measurements takenfrom thermocouples and pressure transducers to perform offline calculations, andused conservative design codes The online method system supplied by somevendors has the potential for inputting data from inspections at shutdown,modeling future potential temperature excursions, storing data for trending, andvisual, real-time displays.

Approaching the OEM of the machinery in question may be the cheapest route

to follow, even if the machines are past the warranty cycle Thanks to the advance

of modern electronics, most of the programmable logic controllers (PLCs) that aresupplied as integral to their machinery package could accommodate the additionalreadings that the client may want based on operating experience Sometimes theprovision might already be built in, but often an OEM stays away from giving acustomer too much extraneous data Some OEMs err on the side of providing lessdata, until operations experience warrants otherwise If the OEM proves stubbornabout adding the monitoring facilities, there are now monitoring packages availablethat will work online, real time, remote, take data from marshaling stations, or anycombination thereof Generally they provide the same useful data as a much moreexpensive system that uses pentium equivalents They may not have the samestorage capacity, but current storage capability may be adequate and less expensive.Also, the PLC systems can be designed to allow assessing the machinery situationfrom another remote terminal or computer

Some users prefer the use of nonintrusive wear monitors (in applications wherebearing or other component deterioration might be an issue) to oil debris monitoring(better suited to reciprocating machines because of the lead time) or relying totally

on temperature readings for those indications These monitors work using neutronbombardment techniques and are slowly gaining in exposure

Selection of the overall monitoring package. Selection depends on turbomachineryconditions:

1 Complex or simple (for instance modern gas turbine versus simple pump)

2 Prototype or mature model of machine (age of particular machine(s), modelnumber, history in the world, in specific plant)

3 Prototype application or not

4 Changing or declining process field

5 Estimated time for return on investment

6 Environmental regulations currently in place or newly instituted

7 Changing environmental regulations anticipated (for instance impending CO2tax)

8 Component life requirements (consider life cycle assessment)

9 Changing performance analysis requirements (consider life cycle assessment)

10 What are your retrofit limitations?

Based on the above, a decision is made The most common choices today are:

 To buy an expensive comprehensive system Cost typically in $150 to $250K permachinery train The main advantage is that such a system may pick up flawsthat vibration and performance analysis may not The disadvantages include:

 Capital cost is high

 System may be prone to large amounts of nuisance data that require expensiveanalysis

 High level of operator knowledge may be required

 To buy the best vibration monitoring and PA that is needed and use commonsense Typically vibration monitoring picks up from 60 to 85 percent of theproblems encountered on turbomachinery Performance analysis can pick up 10

to 35 percent of the remaining problems Generally the remaining 5 percent can

be solved with common sense and sufficient expertise Advantages include:

 PA can also pick up areas of

 Operational economic optimization

 Potential to extend component lives

 Environmental optimization

 Personnel tend to develop more expertise than with an “expert” system

Temperature monitoring. Temperature monitoring is very important in determiningthe health of turbomachinery and is particularly effective when used in conjunctionwith other parameters, such as vibration To discuss temperature monitoring,

we shall observe this facet of condition monitoring with specific application to thegas turbine—probably the toughest application of temperature monitoring inturbomachinery

In a gas turbine, exhaust gas temperature is monitored to avoid overheating ofturbine components This is measured with a series of thermocouples in the turbineexhaust Most gas turbines average the readings of the exhaust thermocouples toproduce two single values (If there are eight thermocouples, half of them will beaveraged to give one reading, the other four another reading.) This way if onethermocouple fails, the control can pick the more credible value and avoid a machinetrip This value is one of the inputs fed into the gas turbine’s temperature toppingcontrol, which in turn controls the fuel flow

Note also that some VA probe manufacturers have been able to detect problems

such as combustion liner cracks with VA before temperature indicators picked up

differing spread in adjacent thermocouple readings The temperature monitoringsystem is set up to take such readings in addition to provide an average reading inmore sophisticated gas turbines

Iron/constantan and chromel/alumel provide the best economical compromise andare the most commonly used Thermocouples are sheathed in magnesium oxidesheaths for corrosion protection

Typical effective ranges for thermocouples are as follows:

Iridium/iridium 60% and rhodium 40% 2552 to 3326°F

A gas turbine generally has a protective system completely separate from thecontrol system Another set of temperature sensors record the same or similar

temperatures to the control system sensors The redundancy provides an additionalsafety factor Once the unit is running, at least two flame detectors (many gasturbines only have two) are generally required to indicate flameout for a trip tooccur.

As previously mentioned, some systems, either native to the turbine or as part

of an overall external comprehensive condition monitoring package, take adjacentthermocouple readings This technique has succeeded in pinpointing hot spots inthe combustor section These hot spots may happen as a result of a crackedcombustion liner, a broken or cracked cross tube, or other source of localizedheating

Monitoring of turbine inlet temperature (TIT), the temperature just before thecombustion gases reach the turbine first stage inlet guide vanes, would bepreferable and more accurate than exhaust gas temperature monitoring However,this temperature is not monitored for a practical reason: the damage that wouldoccur if a thermocouple were to break and enter the turbine as FOD (foreign objectdamage-causing material)

Bearing oil temperature is monitored as it leaves the bearing (dischargetemperature) This is the most critical location in the entire oil system If the oiloverheats to the point that appropriate film characteristics are not maintained, thebearing may fail, resulting in overall engine failure To accurately measure bearingtemperature, the thermocouples need to be embedded in the bearing babbitt Thesereadings give early warning of impending failure in journal (sleeve or rollingelement) bearings

Oil temperature monitoring is particularly critical in rotating machinery Oilchemical analysis (generically referred to as SOAP) sample-taking is too slow to beuseful with high-speed rotating machinery in general (It can, however, be veryuseful with reciprocating machinery, where degradation rates are slower.)

Typically, thermocouples can measure from about -200°C to about 2800°C.Thermocouples consist of a bimetallic strip of two dissimilar metals A voltageproportional to the temperatures of the two junctions is developed The temperature

at one junction is known, so the other temperature can be determined withcalibration Since a voltage is generated, no external voltage needs to be created.However, for accuracy, a reference junction is required for each thermocouple type.Resistive thermal detectors (RTDs) determine temperature by measuring thechange in resistance of an element due to temperature Platinum is generally used

in RTDs because it is mechanically stable and chemically inert The useful range

of platinum in terms of measuring temperature is from about -270°C to 1000°C

An electric current must be supplied to the RTD, and the temperature is determined

by the resistance in the element Any type of conducting wires can be used toconnect the element to the measuring device

RTDs are accurate to within 0.01°C, thermocouples to within 1°C

Temperature readings can be used as primary indicators in CM or as backup tovibration and other readings to confirm a condition

Vibration Analysis

Introductory concepts and definitions

Vibration means oscillation in a mechanical system The parameters that measure

vibration are frequency (or frequencies) and magnitude (or amplitude) Withvibration, it is either a physical object or structure, or a force that is oscillating.The history over time of vibration may therefore be considered to be simple

harmonic or sinusoidal; in other words, it follows a wave form.

temperatures to the control system sensors The redundancy provides an additionalsafety factor Once the unit is running, at least two flame detectors (many gasturbines only have two) are generally required to indicate flameout for a trip tooccur.

As previously mentioned, some systems, either native to the turbine or as part

of an overall external comprehensive condition monitoring package, take adjacentthermocouple readings This technique has succeeded in pinpointing hot spots inthe combustor section These hot spots may happen as a result of a crackedcombustion liner, a broken or cracked cross tube, or other source of localizedheating

Monitoring of turbine inlet temperature (TIT), the temperature just before thecombustion gases reach the turbine first stage inlet guide vanes, would bepreferable and more accurate than exhaust gas temperature monitoring However,this temperature is not monitored for a practical reason: the damage that wouldoccur if a thermocouple were to break and enter the turbine as FOD (foreign objectdamage-causing material)

Bearing oil temperature is monitored as it leaves the bearing (dischargetemperature) This is the most critical location in the entire oil system If the oiloverheats to the point that appropriate film characteristics are not maintained, thebearing may fail, resulting in overall engine failure To accurately measure bearingtemperature, the thermocouples need to be embedded in the bearing babbitt Thesereadings give early warning of impending failure in journal (sleeve or rollingelement) bearings

Oil temperature monitoring is particularly critical in rotating machinery Oilchemical analysis (generically referred to as SOAP) sample-taking is too slow to beuseful with high-speed rotating machinery in general (It can, however, be veryuseful with reciprocating machinery, where degradation rates are slower.)

Typically, thermocouples can measure from about -200°C to about 2800°C.Thermocouples consist of a bimetallic strip of two dissimilar metals A voltageproportional to the temperatures of the two junctions is developed The temperature

at one junction is known, so the other temperature can be determined withcalibration Since a voltage is generated, no external voltage needs to be created.However, for accuracy, a reference junction is required for each thermocouple type.Resistive thermal detectors (RTDs) determine temperature by measuring thechange in resistance of an element due to temperature Platinum is generally used

in RTDs because it is mechanically stable and chemically inert The useful range

of platinum in terms of measuring temperature is from about -270°C to 1000°C

An electric current must be supplied to the RTD, and the temperature is determined

by the resistance in the element Any type of conducting wires can be used toconnect the element to the measuring device

RTDs are accurate to within 0.01°C, thermocouples to within 1°C

Temperature readings can be used as primary indicators in CM or as backup tovibration and other readings to confirm a condition

Vibration Analysis

Introductory concepts and definitions

Vibration means oscillation in a mechanical system The parameters that measure

vibration are frequency (or frequencies) and magnitude (or amplitude) Withvibration, it is either a physical object or structure, or a force that is oscillating.The history over time of vibration may therefore be considered to be simple

harmonic or sinusoidal; in other words, it follows a wave form.

Figure C-284 indicates in simple terms how the sinusoidal or wave form isgenerated Assume than 0° and 360° occur at 3 o’clock (Angular position in degreeincreases in the clockwise direction.) Visualize the circle as a long string that onecuts at the 0/360° point and stretches into a long line with 0° at the left end.Assume that the circle represents constant deflection due to, for instance, acentrifugal force resulting from unbalance For the purposes of illustration, assumethat we are considering vertical vibration at one point of the machine only, such as

a particular bearing housing Figure C-285 shows the plot continued for a full cycle

of 360°

Frequency is defined as cycles per unit time To visualize the concept of frequency,

consider the cycle in Fig C-285 takes 1 s to occur, as indicated on the horizontalaxis in Fig C-286

If this cycle took 1/30 s, the rotational speed that the sine wave represents would

be 1800 rpm The force causing the vibration and the vibration itself also occur at

Circle represents constant deflection due to force

START:

At 0 ° vertical deflection equals zero

Reference mark

Vector representing amount of deflection

“Cut” circle stretched out in a line

FIG C-284 A circle represents a constant deflection due to a force vector 18

Deflection vector after

Magnitude, or amplitude, is the maximum value of a sinusoidal quantity (from

the peak of the sine wave to the next peak of the sine wave) This simple model isadequate for theoretical study

In practice, however, vibration is rarely regular or symmetrical It is often acombination of several sinusoidal quantities, each with their own frequency and

amplitude, that gives one total signal If each of these quantities has a frequency

that is a multiple of the lowest frequency, the vibration repeats itself after a specific

period, i.e., it is periodic It is possible to use filtration methods to isolate any one

or more of the total signal components, resulting in a filtered signal If there is no

such relationship between component frequencies, there is no periodicity and the

vibration is then complex vibration.

Vibration may also be called deterministic or random Deterministic vibration is

totally predictable at any point in time based on its past history Most practical

cases of vibration are random; in other words, future vibration is predictable based

on probability that is used to assess past history

Sine wave generated

The model most frequently used for vibration is that of a mass attached to a

spring The vibration of this model may be free or forced In free vibration motion

results from the energy of an initial disturbance and then continues to decay to a

stable state For analysis purposes, in an ideal system there is no damping (or energy dissipation), so the free vibration continues indefinitely In a real system,

damping causes vibration to decay to a negligible value Such free vibration is often

called transient vibration Forced vibration is different from free vibration in that

energy is supplied to the system continuously, which compensates for energy lost

due to damping The forcing frequency of the applied energy appears in the vibration

signature of the system Whether vibration is transient or forced, system vibrationdepends on the relation of the force causing vibration to the rest of the system

In certain machines, fluid flow movement may go from steady (in a well-designed

machine operating near its design point) to erratic or pulsating (a steady back and

forth movement) A typical example is a reciprocating compressor that develops aninadequate inlet flow condition The gas in the compressor starts a pulsatingmovement

In a centrifugal compressor where inlet flow conditions fall below those requiredfor positive forward flow into and through the compressor, the flow goes back onitself, trying to get the pressure at inlet back up to design point When this isreached, the flow can proceed on its normal path until pressure falls again due tothe same causative factors Flow reversal again results This phenomenon is called

surge.

When a similar condition occurs in a centrifugal pump, where the fluid is liquid,bubbles of gas form due to inadequate pressure When the bubbles get to a condition

of sufficient pressure, they collapse This phenomenon is called cavitation Both

these phenomena will be dealt with further when troubleshooting is discussed

Natural frequency is the frequency of free vibration of a system Contrast the

following two responses: a rubber band with a stone attached to the end that ispulled, then released and allowed to oscillate; and the vibration of a tuning forkwhen it is struck It is evident that the more flexible a part, the lower its naturalfrequency Adding weight to the spring system will lower the natural frequency.When the weight’s position is placed so as to increase deflection, this also lowersnatural frequency

Resonance of a system in forced vibration, by definition, exists if any change inthe excitation frequency causes a decrease in the system’s response

To better understand resonance, consider the case of a car that developsmaximum vibration at a speed lower than that of when it is developing maximumpower It may have this maximum vibration at, for instance, idle speed However,the vibration goes away when rpm are increased This happens because at the lowerrpm, the natural frequency of some part of the car’s structure, frame, or wheelassemblies coincided with the rpm at that time Resonance resulted At rpm’s above

or below that value, the vibration lessens

Resonance can magnify the vibration amplitude of systems with low damping by

a large multiple; 10 to 30 times greater for relatively simple systems is notuncommon Damping can reduce the amplitude magnification, but it does noteliminate the potential for component failure

A shaft between bearings can be described as a spring, as can a concrete floor onwhich vibrating machinery is placed, the columns or piles that support the floor,and pipes and tubes in a process plant A spring then is a part that can flex uponhaving a force applied and then tend to return to its former position Each of thesesprings has its own natural frequency that, when matched by a vibration frequency,will resonate Piping systems are then a collection of many springs, the naturalfrequency of which varies according to the span between piping supports and the

rigidity added by hangers For instance, although two pieces of pipe may not beresonant, the connecting elbow between them may cause resonance in the system.Fluid in pipes adds to their weight and lowers the natural frequency The fluid alsoadds damping that lowers vibration levels.

Not all vibrating pipe sections are in resonance A quick check is provided byusing a temporary brace—either a handheld brace for a small gauge that has itsindicator needle vibrating furiously or a pipe length or anchor temporarily used toprovide a brace that changes the natural frequency of a system If there is a majorchange—the gauge needle stops shaking—then the problem was resonance If itcontinues to shake at higher or lower levels, there is another problem, and thesource frequency should be investigated

Sometimes response displacement occurs at a different angle than excitation

force The angle between the response and the excitation is called the phase angle.

Tables C-18 through C-20 and Figs C-289 and C-290 list several definitionscommon to the field of vibration analysis

TABLE C-18 Conversion Factors for Translational Velocity and Acceleration 19

Types of vibration transducers or probes

There are three main kinds of probe that measure:

 Displacement

 Velocity (rate of change of displacement)

 Acceleration (rate of change of velocity)

If one considers the sine wave representation of vibration earlier in these notes, thedifference between these three parameters can be represented mathematically withsuccessive differentiation, as follows:

Displacement x is defined by:

In practice, these are specified thus:

Note that displacement is independent of frequency, velocity is proportional tofrequency, and acceleration is proportional to the square of frequency

A transducer or probe is a device that translates some aspect of vibration into a

voltage varying signal output that can then be depicted graphically on, for instance,

a spectrum analyzer and analyzed for information on the machine’s condition.

Figure C-291 illustrates parameters for:

 Conversion factors for translational velocity and acceleration

 Conversion factors for rotational velocity and acceleration

 Conversion factors for simple harmonic motion

Selection of transducers for specific applications. Displacement probes are sensitive

to external conditions, such as heat Vibration and acceleration probes are direction

A

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value

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