1988_Kitching_Practical Guidelines for Reducing

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Proceedings of TheSouthAfrican Sugar Technologists' Association - June 1988

PRACTICAL GUIDELINES FOR REDUCING STRESSES ANDLOADS IMPOSED BY THERMAL EXPANSION OF STEAM

PIPINGBy B. H. KITCHING

Technical Management Department, Tongaat-Hulett Sugar Limited

Fy

Resultant Moment .; JMx2 + My2 + MZ2

FIGURE 1 Co-ordinate system showing directions of forces andmovements.

JFx 2 + Fy2 + Fz2esultant Force

of pipe length of growth. However if a turbine runs on lowload, for example if its alternator is not producing powerduring startup, the steam does little work and the exhausttemperature can rise up to 340'C resulting in a growth of4,2mm/metre.The codeANSI B31.1 governingthe designof steam piping limits the maximum resultant combined stress in thepiping to 150MPa at 400'C. This stress takes into account

internal pressure, bending, torsion and shear.Stressintensificationfactorsareapplied to bendsand tees.This stress also provides for a fatigue lifeof 7000cycles.Allowable forces and moments on turbines, pumps andboilers are usuallyavailable from the manufacturersof newequipment but with old plant this is not always the case.Guidelines for the determination of allowable forces andmoments on steam turbines isgivenin the NEMAStandardSM23, and in the API Standard 610 for centrifugal pumps.The limits specified by'manufacturersand theNEMAandAPI standards refer to the resultant forcesand moments onmachine flanges.Forcesand moments at any point in a systemare related

to the 3 co-ordinate axes ie 3 forces and 3 moments areapplied simultaneously, the resultants being given by theroot mean square values as follows:

IntroductionThere are many instances in sugar factories where thedesign, installation and maintenance of high pressuresteamand exhaust piping is neglected. The steam piping codes ofpractice are as stringent as those for a boiler and steampiping deserves the same care and attention as that of aboiler. The expansion of a pipe system subjected to temperature changescreates forces, moments and stresses thatmust be catered for so that the system is safe to man and

machine.Recent design analyses of both old and new steam andexhaust ranges at Tongaat-Hulett Millshave revealedmanyserious deficiencies in these systems. The evolutionary nature of sugar mills .is one of the major causesof these problems. Turbines and vessels are added to the plant and,whereas the new system itself is adequately designed, theeffects on and from the existing systems are often ignored.The common solution for absorbingthermal movementshas been the use of expansion compensators or bellows, butthere have recentlybeen failures in some of these highpressure units, which have caused considerable down time aswell as costs for replacementsor modifications. It has beendemonstrated that the correct application of the appropriate

devices and methods has provided solutions to thermal expansion problems without resorting to the use of bellows.

AbstractThe effects ofthermal expansionoflive and exhaust steampiping, and the imposed load limits on turbine branchesandstress in piping with reference to the codes of practice arepresented.The proceduresto implement and optimise pipingdesignand layoutare examined,and the definitionand applicationof loops, restraints, supports, springhangers, and cold pullsto minimise these thermal effects without the use ofexpan

sion bellows are described.Recommendations on modelling, piping installation andinterpretation of stress analysis results are made.

Limitations of Piping and PlantThe limits of pipe stressand loads applied by the thermalgrowthof the piping to turbines, pumps, boilers and vesselsdictate the needfor careand skillin the designof the system.The most common high pressure (HP) steam generatedin the sugar industry is 3,I MPa(g) at 380 to 400'C. Thepipingavailable in South Africafor this duty is carbon steelpipe to ASTMA 106gradeB,which has an expansion coefficient of 1-2 X 10-6/'C temperature change. This is equivalent to a growth of 4,5 mm for everymetre of pipe length

for a temperature rise of 380'C.Turbineexhaustpressures arecommonly100kPa(g) whichhas a saturated temperature of 120'C,giving 1,3mm/metre99


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Proceedings of The South African Sugar Technologists' Association - June 1988Designof a FlexiblePiping System

The objective of the pipe system designer is to providesufficient flexibility in the system, so that the combined effects of the piping'sownmassand those induced from thermal expansion are contained within the limits of thestandards.Before lookingat the procedure for designing the systeJ?1

one needs to knowthe essentialdevices, methods of application and procedures that are used. The challenge is thento examinethepipe layoutand selectfromthis "set of tools"the items needed to fit into the system to overcome theproblems that have been identified.Listedbelow is a glossary of termsdescribing these"tools".

Isometric layoutThe starting point of the analysis is a d i m e n ~ i o n e d isometric layout of the pipe systemwhich should Include allof the existingpipingfeatures that could affect the new system. This could mean startingat the boiler outlet for a HPsystem.

RestraintsA devicewhichprevents,resistsor limits the f r ~ e t h ~ r m a lmovement of the pipe. Restraints can be either directional,rotational or a combination of both.AnchorsA rigid restraint which provides substantially full fixety,ieencastreor "built in", ideallyallowing neithermovementsnor bendingmoments to pass through them.True anchors are usuallydifficult to achieve. A seeminglysolid gussettedbracket weldedto a house column does notqualifyas an anchorif thecolumndoesnot have thestrengthto resist the loads applied to it.

Expansion LoopApurposedesigned devicewhichabsorbs.thermal growth;usually used in combination with restramts and cold pulls.

Neutral Planes ofMovementThis refersto the planeson the 3 axesof a turbo machineor pump fromwhereexpansionof the machine starts egthefixed end of a turbine casing. This information is normallyprovided by the equipment ~ a n u f a c t u r e r . If n,ot availablefrom this source, the fixed POInts of the machine must bedetermined by inspectionand an estimation of the turbinegrowths calculated.A pipe restraint positioned in line with a n e u t r ~ l planeprevents differential expansion forces between the pipe andthe machine.

Cold pullThis is used to pre-load the piping system in the coldcondition in the oppositedirectionto the expansion, so thatthe effects of expansion are reduced. Cold pull is usually50% of the expansion of the pipe run under consideration.

Spring HangersUsed to support a pipingsystemthat is subjected to verticalthermalmovements.Commercially availablesingle coilspring units are suitable for most applications. Supplier's

catalogues adequately cover the selection of these springs.According to Hooke's law, the spring's supportingcapacitywillvary in directproportion to theamount ofdisplacementthe spring undergoes due to thermal movement. This var-100

iation between coldand hot shouldbe between 25and 50%of the hot loadedcondition.Solid Vertical SupportIn places where vertical thermal movement does not create undesirable effects, or where vertical movement is intentionallypreventedor directed,solidsupports in the form

of rollers, rods or slippers are used.It is important that free horizontalmovement of the pipeisnot impededunless horizontalrestraintisdesired.Slippersand rollers must bewell designed and lubricated.Measurement of actual movement

It is very useful to knowthe movementof an existing pipeat the point at whicha tie-in to a newmachine is intended.This movement is not always predictable, and a physicalcheck .can be made by fixing pointers to a non-movablestructureand measuring themovement of the pipe betweencold and hot.Thiswillrequire a planned shutdownof the plant for thelinesto cool. It is unlikely that the pipingwillreachambienttemperature duringa short shutdown, sothehotand "warm"temperature of the piping should be taken at the time ofmeasurement. The results can be extrapolated to relate toambient and hot conditions.

ProcedureA careful study of the site must be made and particularnote takenof existing supports, restraints, pipemovements,structures that couldbe used fornewsupports, and possiblerouting options for proposednewpipe lines.It is most important that the correct physical aspects areobtained from the site.Forexample, theanalysis resultswillbe significantly affected ifan anchoris assumedwhenin factit is a restraint.The first isometric produced is unlikely to provide thefinal solution. The drawingshouldbe studied and the magnitude and direction of movements determined. Positionsof newrestraints, supports, loops etc. can then be chosen tobest absorband direct forces andmovementsawayfrom theplant item. Close attentionmust begivento adequatelysupporting the pipingwith its valvesand fittings for both coldand hot conditions.The stressanalysis of the systemshould then be undertaken.The resultswill indicate if there are any overstressed

or overloaded conditions. By studying the analysis results,appropriate modifications or additions to the system arechosen to compensate for limits that have been exceeded.This proceduremightrequire repeating severaltimeswithchanges to the pipe run configuration, and the addition ofrestraints or loops. The effects on the existing piping andmachinesmust be consideredwhenmaking these changes.

Examples of improved systemsThe possible combinations and configurations of pipinglayouts are infinite. Examples of two installationmodifications are described.

I. Felixton: Figure 8 shows the beforeand after layouts ofthe 500mm diameter high pressure steam range at Felixton. Repeated failure of the angular compensatorsprompted the Mill and the Tongaat-Hulett TechnicalManagement Department (TMD) to examine ways ofeliminating these weaklinks in the system.


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Proceedings of The South African Sugar Technologists'Association - June1988O ~ E , p a n d e d s h a p e ,r --- - - ---. .,.I

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Expansion ReactionPipe Dia W H L f:::, ForceFmm m m m mm N100 12 3 3150 14,5 2,5 3,8200 18 3 4,5250 21,5 3,5 5,5300 24 4 7,0350 27 4,5 8,5400 30 5 11,0

Piping ASTM A 106 GR. BSteam temperature 400 CFIGURE 4 Typical expansion loops.

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r Growth on legs "LZ,ReSlra;n , Expanded shapeA & B Equal 1r- - - - - - -------,I A II ,I ,I c 0 , ;;,I 0-, '"I Neutralplane oilof Z.Movement , uIL. B r

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FIGURE 5 Use of turbine neutral planes for restraint position selection.102


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Proceedings of The South African Sugar Technologists' Association - June 1988The provision of the vertical loop and the applicationofcold pullswas sufficient to absorb the 260mm ofgrowthbetween the restraints at the boilers and take-off to thepower station.The cost to replace the bellowsunits would beR45 000,and to provide a.permanent solution by installing theloop is estimated at RlOO 000. The cost differencewould

have been marginal if the loop instead of bellows hadbeen installed at the outset.2. Mount Edgecombe: Figure 2 shows the layout of steampiping to a new turbo-alternator. The growth of the linefrom the existing restraint is absorbed by the expansionloop and the restraint opposite the turbine neutral planeprevents differential expansion of the horizontal loop inthe negative X direction. Vertical expansion is taken upin spring hangers.ModelsMovements in a 3-dimensionallayout are difficult to visualise. A simple and effective method of assessing complexthermal movements and to remove doubt as to how cold

pull should be applied, is to make a scalemodel of the pipebranch from light gaugewire or plastic tube. The model canbe manipulated to simulate expansion and a close approximation of the pipes' behaviour can be made.InstallationFew contractors appreciate or understand the importanceof cold pull, supporting of piping and prevention of initialloads onto turbines. Close supervision of these proceduresis advised.The acid test to determine the success of the installationat the turbine is to remove the bolts from the turbine flangeafter completion of the piping. The flanges should then justtouch and be square to each other. If applicable, cold pull

displacements can be measured at these flanges also. Theinitial flange loading can be adjusted to be effectively zeroby means of the spring hangers closest to the machine.A reasonable indication of the stiffness of the piping andforcesimposed at turbine flanges can beobtained by leveringwith a crowbar between the flanges.

Analysis resultsIt is not intended to cover the actual stress analysis of apiping system in this paper. The complexity of a 3-planesystem lends itself to computer analysis and many commercial software packages are available for this purpose.Alternatively these packagescan be hired or a specialist consultant can undertake this work at reasonable fees. Software

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costs between R15000 and R25 000 and a consultant's feewould depend on the size of the analysis and the numberof analysis runs required.The new era analysis packages offer an enormous amountof information. The most significant items are:Validation of input dataMovement and rotation reportForce and Moment reportStress reportSummary of support, restraint, anchor and turbine forcesSummary of maximum system values.

These packagescontain libraries of certain spring hangersand bellows and will automatically select these items if requested. Also they can check that turbine flange forces arein compliance with the NEMA SM23 standard. Some packages will even draw the isometric, indicate deflected shapeand produce an animated graphic display of vibration.Bellows

There are instances where it is not economic to avoid theuse of bellows. Failures on bellows at Darnall and Felixtonwere on HP systems where the Chromemoly and Incalloybellowswere installed in horizontal positions. Pin holes andfractures occured on the bottom of the convolutions. At thetime of writing, tests to determine the cause of the failureswere still being evaluated. However it is significant that, toour knowledge,there has not been a failure of a bellows unitinstalled in the vertical position nor a bellows on exhauststeam duty in either position. This suggeststhat corrosivedeposits and temperature may both have contributed to thefailures.Bellows must be used with caution and conservativelyselected such that they are not deflected to their full limit.Suppliers of bellows and catalogue information can providethe guidelines for the selection and application of units.

ConclusionThe design of steam piping installations requires theknowledgeof the Codes of Practice and manufacturers' loadlimitation for piping and plant.To deal with the effectsof thermal expansion on complexsystems the designer must be familiar with the devices andprocedures that can be applied to absorb, divert and reducethe forces, moments and stresses in the piping and onmachinery.A repetitive process of examination and analysis to identify the mechanisms and then to compensate for the effects,leads to successful solutions for most configurations.

1988_Kitching_Practical Guidelines for Reducing

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