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8 Bolting basicsBy Patrick J. Smith

Bolting is commonly used in machinery and piping systems to fasten parts together. While relatively

simple, bolting technology is commonly misunderstood and problems can develop. This is especiallytrue when bolts are replaced. Problems can easily be avoided by having a better understanding ofboltng technology.

14 Operating experience of a coal-fired boilerretrofit with an advanced hybrid of coalgasification for 502 and NOx emissions controlBy Keith Moore

The recent operating experience of an advanced hybrid of coal-gasification retrofit on an industrial

coal-fired boiler shows promise to reduce operating cost, improve boiler efficiency, and control bothS02 and NOx emissions.

ASME POWER DIVISION SECTION

22 Fuels & Combustion Technologies CommitteeBy Christopher F. Blazek

24 Important concepts of wet-limestoneflue gas desulfurizationBy Brad Buecker

Many utilities in the United States are installng flue gas desulfunzation (FGD) systems to reducesulfur dioxide emissions from coal-fired boilers. The majonty of these systems wil be based onwet-limestone technology due to the reliabilty of the process and the abundance of the scrubbingreagent, limestone, in many areas of the countr.

This artcle outlines a number of fundamental principles of wet-limestone scrubbing, which wilhopefully prove useful to plant personnel who might be asked to plunge into unfamilar territory.

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30 The Machine DoctorVibration due to rotor unbalance

By Bil Hilman

33 Turbine Tech

Turbine inlet cooling with indirect evaporation -with greater density comes more power

By David Voeller and Marcus Bastianen, P.£., S.E

OCTBER 2008

Photo courtesy of iStock Photo.

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ISSUE

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3738

From the Editor

Energy Showcase

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ENERGY-TECH. com - 4

features

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Yie!Poin

Elongation

Figure 2

break. The maximum,Joad that the bolt cansustain is the ultimate tensile point. See thegraph in Figure 2.

Stress is defined as load divided by crosssectional area.

P()=-A

cr = Tensile StressP = LoadA = Cross Sectional Area

The threaded part of a bolt is the weakestpart and it has a complex form. The stressarea is the equivalent area that a round barcould be substituted for the thread. The stressarea for typical bolts can be found in a varietyof references, such as Mark's Handbook forMechanical Engineers. As mentioned earlier,bolts are available in a variety of materials andstrengths. When replacing bolts, it is importantto use bolts of the same strength as the origi-nal bolts.

FatigueA part can fail or break if it is subjected to

repeated loading/unloading cycles, even if theloading is below the yield strength of themateriaL. This type of failure is called fatigue.There is a relationship between the magnitudeof the loading and the number of cycles tofailure and this is typically determined by afatigue test. A specimen is repeatedly loadedand unloaded at a specific stress level and thenumber of cycles to failure is counted. Thistest is repeated at different stress levels. Atypical stress vs. load cycle-to-failure graphcan then be created. See Figure 3 for a typicalgraph for a carbon steel specimen.

As shown, at a stress level correspondingto 107 cycles, the specimen has an infinite life.In other words, the specimen can be subject-

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Number of Cycle to Faiur

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OCTOBER 2008

ed to an infinite number of load/unload cycles at this stresslevel and not fail in fatigue. This stress level is called theendurance limit. For most steels, the endurance limit for anotch-free polished specimen is approximately 50 percent ofthe ultimate tensile strength.

Fatigue failures are fractures that start at defects ormicroscopic cracks in the material surface. These cracksgrow a little with each loading/unloading cycle. Thus, thecondition of the surface, stress concentrations, and otherfactors can affect the endurance limit. Polishing the surface,surface hardening, shot peening, and other surface treat-ments can all improve the endurance limit. As a result, the

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ENERGY-TECH.com - 9

endurance limit is not a true mechanical property like yieldstrength and tensile strength. The estimated endurance limitfor a part is typically determined by de-rating the idealendurance limit for surface finish, size, stress concentration,and other factors.

Bolt threads can be made by rolling or cutting. Threadsthat are cut are formed by cutting or removing materiaL.

Threads that are rolled are formed by extruding. Extrusion isa process where the material is pushed through a die thathas the desired cross section. Extruding results in a workhardened surface that is more fatigue resistant. Thus, theendurance limit for a bolt with rolled threads will be higherthan an equivalent bolt with cut threads. A typical stressconcentration fartor for rolled threads is 3.0, compared witha typical stress concentration factor of 3.8 for cut threads.So, the endurance limit for a bolt with rolled threads will beabout 26 percent higher than the same bolt with cut threads.

Bolts are typically subjected to both constant loads andalternating loads. In this case, neither the yield strength northe endurance limit alone can be used to determine the suit-ability of a particular bolt for this type of application. Thecombined effects of both loads must be considered andthere are several methods available to analyze this. Acommon method is the use the Modified Goodman diagram.The construction of this diagram is shown in Figure 4.

The solid line is the approximate line of failure. If thecombined point is below the line of failure, the fatigue factorof safety is greater than 1.0. If it is above the line, the fatiguefactor of safety is less than 1.0. The equation for the fatiguesafety factor based on the Modified Goodman equation is:

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FS =(Jay' (Jend + (Jmeanl;UTS

FS Factor of Safety(JaIi Alternating Stress

(Jend Endurance Limit

(Jmean Mean Stress(JUTS Ultimate Tensile Strength

Bolt preloadThe bolt in Figure 5 is used to clamp two parts together.When the nut is first installed on the bolt, it is loose. As

the bolt is tightened, the slack is taken up until the bolt issnug against the part. As the bolt is further tightened, thebolt is stretched, or preloaded. The parts also are preloaded,although to a much smaller degree. If a load is then appliedto separate the parts, the parts will not separate until theapplied load exceeds the preload. It is a commonmisconception to assume that the bolt load includes boththe preload and the applied load. Both the bolt and partsact like springs that hold the assembly together. In most

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OCTBER 2008

1008

figure 5

figure 6

applications, the parts are much stiffer than the bolt andmost of the applied load is taken up by the parts. Referringto Figure 6, a preloaded bolt can be thought of as a springthat holds two assemblies together.

As load is applied to separate the parts, the parts willelongate a little, which will stretch the spring a little.However, most of the applied load is taken up by the parts.It isn't until the parts separate that the spring takes the entirepreload and the applied load. In most cases, bolting is notdesigned to handle both loads and the bolt will be over-loaded and faiL.

How much of the applied load is taken up by the partsand how much is taken up by the bolt is a function of therelative stiffness of each component. The equations todetermine the total tensile load in the bolt and the totaltensile load in the parts are:

KbFb = *p + F

Kb + Kp"

KbFb = * P

Kb + Kp

F b = Bolt forceFp = Part force

Kb = Bolt stiffness

Kp = Part stiffness

P = Applied load

F 0 = PreloadIn many cases it is difficult to accurately determine the

stiffness of the parts. In this case, a common assumption tomake is that the parts can be modeled as a hollow cylinderwith an inside diameter the same size as the bolt and anoutside diameter three times the size as the bolt diameter.Assuming the material of the bolt and parts have the samemodulus of elasticity, this works out to the parts being 8times stiffer than the bolt. Thus, the bolt load is only 1j ofthe total applied load. If the bolt and parts are made ofdifferent materials that have different modulus of elasticity,then this stiffness ratio has to be adjusted by the ratio of themodulus of elasticity. The modulus of elasticity is themeasure of the stiffness of a material and is defined as theslope of the stress-strain curve in the elastic region.

Preload is very advantageous in applications where thereare cyclical loads. Sufficient preload reduces the fatigue

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:'.;:i! OCTOBER 2008 ENERGY-TECH.com - 11

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A When you open a turbine for an internalinspection, you can usually see any evidence of

. strctural damage, such as missing blade covers or

a broken blade. What are a little more subtle anddiffcult to evaluate are the many leakage control devicesaround each stage. The auditor knows what the variousclearances are supposed to be and makes careful meas-urements on each stage to determine how the actual val-ues compare with the desired ones.

At Encotech, we have developed a computer programthat we call eSTPE (Steam Turbine PerformanceEvaluation). With eSTPE, we can enter these measuredvalues and the program wil tell us how much loss inoutput and increase in heat rate can be attributed tothe condition of tip spill strips, root spil strips, andinterstage packing seals that are so important toachieving high stage effciency. We also measure thesurface finish of the steam path, quantify the impact ofphysical damage and solid particle erosion, and measureand record the clearances in the shaft end packings toquantify their impact on unit heat rate deficiency.

What makes the steam path audit especially usefulis that it is thorough and follows a well establishedprocedure to be sure that all parts of the turbine thatcan affect performance are inspected, measured, andanalyzed. The data is then entered into the eSTPEprogram which will then produce a report describing allthe contrbutors to unit performance degradation. Thisthen assures the station management that the turbinehas been thoroughly analyzed and also tells them themagnitude of each deficiency so that decisions can bemade quickly as to what is wort fixing.

The Fall outage season is beginning, and the Springoutage season will be here before we know it. For moreinformation on steam path audits, or to request anexample steam path audit report, contact Encotech atthe number or e-mail address below.

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effects of the bolt because the cyclical load on the bolt issignificantly reduced. This is because the parts take most ofthe cyclical load due to the part being much stiffer than thebolt. This can be demonstrated by the following example. A112"x4" 13 UNC steel bolt is used to clamp two steel plates

together as shown in Figure 7.The following values will be used in this example:

· Bolt material has yield strength of 70,000 psi, an ultimatetensile strength of 90.000 psi and an endurance limit of18,700 psi (includes the stress concentration for thethreads). The stress area of the 11z bolt is 0.1419 in2.

Rgure 7

· The steel parts are 8 times stiffer than thebolt.

· A cyclical load between 0-2,400 Ibs isapplied.

· In case 1, the bolt is only snug and thereis no preload.

· In case 2, a 2,500 Ib preload is applied tothe bolt.

In case 1, the load on the bolt is thesame as the load on the part. In thisexample,· Pmin = 0 Ibs

· Pmax = 2,400 Ibs· Pmean = (Pmax+Pmin)/2 = 2,400/2 = 1,200 Ibs

· Palternating = (Pmax - Pmin)/2 = 1,200 Ibs

· Gmean = P mean / StressArea =

1,200/0.1419 = 8,457 psi

· Ga/¡ernaring = Palternaling / StressArea =

1,200/0.1419 = 8,457 psi

Plugging these values into equation (2)yields a fatigue factor safety of 0.85.

In case 2, equation (3A) is used todetermine the bolt load. In this example:· Pmax = 1/9*1,200 + 2,500 = 2,767

· Pmin = 1/9*0 + 2,500 = 2,500

· Pmean = 2,634 Ibs· P alternating = 134 Ibs

· Gmean = Pmean / StressArea =18,558 psi

· GallernalÙig = Pa/¡ernaring / StressArea = 940 psi

Plugging these values into equation (2)yields a fatigue factor of safety of 3.06.

These cases are shown on the modifiedGoodman diagram in Figure 4.

As shown, preload has a dramaticimpact of the fatigue factor of safety of thebolt. This is because the preload significant-ly lowers the cyclical load applied to thebolt. It is important to note that this analysisis not a precise science. There are parts thatoperate at factors of safety less than 1.0and do not fail and there are parts thatoperate at factors of safety greater than 1.0and faiL. This is due to a variety of reasons.First, the fatigue analysis only approximatesthe point of failure. Second, the endurancelimit used in the equation is not a true mate-rial property. It is an estimate based onseveral factors that can vary from one partto another. However, the probability of

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EM = Elongation or stretchE = Material modulus of elasticityL = Length of bolt being stretched

The stretch is the increase in length of the bolt as it istightened. To use this method, there must be a way tomeasure the length of the bolt before and after it is tight-ened. In many cases the bolt is captured and this method

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Ifailure is greater the lower the factor of safety. There is nouniversal minimum factor of safety and it is very much appli-cation dependent. However, a typical minimum factor ofsafety is 2.0.

Comon meods to obtin bolt preloadIn some applications, it is not necessary for a bolt to

be preloaded to a precise value for it to function properlyand not faiL. In these cases, operator "feel" is sufficient toensure a tight joint. However, in many applications it is veryimportant to set an accurate preload. Three commonmethods include the torque wrench, angle torquing, andmeasuring fastener stretch or elongation.

A torque wrench i.s a special wrench with a built-inindicator that measJres the applied torque. The torquerequired to tighten the bolt is used to approximate thepreload. This method is based on the calculated frictionforce between the mating threads and the friction betweenthe bearing surfaces. The torque primarily depends onthe size and type of the bolt and the friction factor. Theequation is:

T = KtdbOltFi

· T = torque in inch-pounds

· Kt = the torque coefficient that depends on the boltgeometry and the friction factors between the threadsand the friction factor at the collar (bearing face)

· dbolt = bolt diameter in inches

· Fi = preload in pounds

If interested, the reader is encouraged to review thereferences for the equation for the torque coefficient. Thereare a lot of assumptions that go into the torque calculation.The friction factor, for example, depends on the lubricant(if used), the materials, surface finish, and the hardness ofthe parts. For example, the friction factors vary dependingon estimates for non-lube, or if lubricated, which type oflube is used. In a given application, the friction factors canchange from 0.4 if no lubricant is used to 0.06 dependingon the type of lubricant. In addition, dirt and debris in thethreads can significantly affect the friction. The torquecalculation also assumes that the mating surfaces areperfectly square, which is not always the case. As aresult of these assumptions and others, the accuracy of atorque wrench in setting the preload is typically about +/-35 percent.

Angle torquing is very similar to the torque wrench. Inthis case, the bolt is turned to some specific angle past thesnugging torque. The accuracy of this method is similar tothat of the torque wrench.

Measuring bolt elongation is a much more precisemethod of obtaining a precise preload. As described earlierin this article, stress is defined as the load (or force)divided by the area. So knowing the required preload andstress area of the bolt, the required stress can be deter-mined from equation (1). Once the stress is known, therequired stretch can be determined from the followingequation:

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008 OCTBER 2008

cannot be used. It also is important to note that the requiredstretch is not based on the entire length of the bolt. It isbased on the length of the bolt that is under load. In general,the accuracy of the preload using this method is +/- 5percent.

ConclusionAlthough bolting technology seems straightforward, there

are a lot of common misconceptions that can lead tomistakes and result in machine failures. When looking at abolting application, it is important to understand the loading,required preload, and method to obtain the preload. Mostbolt failures are due to insufficient preload, not over-tightening. With insufficient preload, bolts can loosen andfail in fatigue. A typical rule of thumb is to pre-load boltsto 75 percent of the yield strength. Readers are encouragedto review the references for additional information.

Reference1.Michael R. Lindenburg, Mechanical Engineering Reference

Manual for the PE Exam, 11th Edition, ProfessionalPublications, Inc., Belmont, CA, 2001

2. Joseph Shigley, Mechanical Engineering Design, 3rdEdition, McGraw Hill, 1977

3. Avallone, E.A.; Baumeister, T., III, Mark's Handbook forMechanical Engineers, 10th Edition, McGraw Hill, 1996

Patrick J. Smitli if lead mac/iinery engineer at AÙ' Products &Cliemicals, Allentown, Po., wlzere Iie provides teclinical machinerysupport to tlie company's operating oil' separation, Iiydrogenprocessing, and cogeneration plants. Contact Pat via e-mail ateditorial(§ WoodwardBizMedia.com.