basics of pipe stress

7/28/2019 Basics of Pipe Stress

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Basics of Pipe Stress

1. Introduction:Present day process plant piping systems use various fluids at variousconditions of pressure and temperature. The piping engineer has to designthe systems to ensure reliability and safety throughout designed plant life.The piping systems are subjected to combined effects of fluid internalpressure, its own weight and restrained thermal expansion. The elevatedtemperature also affects the pipe strength adversely. Therefore the task ofthe engineer is:

i) to specify an adequate wall thickness to sustain the internalpressure with safety.

ii) To select a piping layout with an adequate flexibility between pointsof anchorage to absorb its thermal expansion without exceedingallowable material stress levels, also reacting thrusts and momentsat the points of anchorage must be kept below certain limits.

iii) To limit the additional stresses due to the dead weight of the pipingby providing suitable supporting system- effective for cold as wellas hot conditions.

All these objectives are achieved by: -a) Assuming adequate support to prevent excessive sag andstresses in piping system.b) Incorporating sufficient flexibility to accommodate stressresulting from changes in pipe length due to thermal effectsand movement of the connection at the ends of the pipe.

c) Designing the piping system to prevent its exertingexcessive forces and movements on equipment such aspumps and tanks or on other connection and supportpoints.

The stress engineer of a piping design department performs the necessarycalculations to ascertain that the various requirements due to internalpressure, thermal expansion and external weight are satisfied. Variouscomputer packages are available in the market, which perform the requiredrigorous analysis. These analyses are basically static analyses. There aresituations where stresses are introduced into the piping systems due todynamic loading situations like reciprocating compressor vibration, safetyvalve discharge etc. However it is the static analysis which most of the pipestress engineers perform and are acquainted with. Now the present daycomputer packages that are being used (CEASAR-II, CAEPIPE, PIPEPLUSetc.) are quite comprehensive and if the piping configuration and pipe dataare fed properly, comprehensive analysis are done through the computerpackages. This has improved pipe stress analysis job productivity

immensely. However sometimes this has led to a decline in the knowledgeabout the basics of pipe stress analysis especially in situation where thestress analysis engineer after acquiring some sort of skill in the use of theanalysis package does not make effort to learn about the basics of pipestress. Some of the ideas about the basics of pipe stress have beenenumerated herein.


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2. General ideas on failure of materials:Failures of material can occur by:

a) Brittle fractureb) Excessive elastic deformationc) Excessive non-elastic (plastic or viscous) deformationsd) Thermal or mechanical fatigue.

2.1 Brittle Fracture:Steel is generally considered to be a ductile material. However in certaincases steels sometimes rupture without prior evidence of distress. Suchbrittle failures are accompanied by but little plastic deformation, and theenergy required to propagate the fracture appears to be quite low.The three conditions, which control this tendency for steel to behave in abrittle fashion, include (1) high stress concentration; i.e. notches, nickes,scratches, internal flows or sharp edges in geometry (2) a high rate ofstraining and (3) a low temperature.The transition temperature for any steel is the temperature above, whichthe steel behaves in a predominantly ductile manner and below which itbehaves in a predominantly brittle maner. Steel with high transitiontemperature is more likely to behave in a brittle manner during fabrication

or in service. It follows that a steel with low transition temperature is morelikely to behave in a ductile manner and therefore, steel with low transitiontemperature are generally preferred for service involving severe stressconcentrations, impact loading, low temperature or combination of thethree.

2.2 Elastic and non elastic deformation:Elastic deformamations are deformations that disappear when the stress isremoved. Plastic deformation is non-reversible. When the stress isremoved plastic strain approximately remains unaltered. A look at thestress strain diagram of say a carbon steel material will clarify theconcepts. However there is another kind of plastic deformation called creepwhere the deformation increases with time at constant stress. At certaintemperature levels creep, which is the term, used to describe thisprogressive deformation may occur in metals even at stress below theshort time yield strength or proportional limit. Thus the yield strength orproportional limit, which are determined by short time tensile tests do notrepresent satisfactory criteria for the design of piping systems over theentire temperature range CREEP RATE or CREEP LIMIT determinationthrough a large number of long time tensile test of elevated temperaturebecomes necessary.

2.3Thermal and mechanical fatigue:Failure has occurred when the service become more severe than theconditions for which the piping was originally designed. Thermal ormechanical fatigue is usually the most common causes of failures in hightemperature piping systems. Severe localized mechanical stress havecaused or contributed to failures.Thermal fatigue is caused by frequent change in operating temperatures ofpipeline. Thermal expansion and contraction occur in all metal componentsby the change in temperature. Over a long period this results in thermalfatigue. Hence for best metallurgical conditions, the temperature of thehigh temperature piping systems should be maintained continuously anduniformly as far as possible.Mechanical fatigue is caused by pipe movement, vibration, restraintspreventing free movement or other conditions.


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3.0 Allowable stress:From stress strain diagram of a material like carbon steel we know aboutyield strength as also ultimate tensile strength. For our design purpose andallowable stress value is fixed which is based on a certain factor of safety

over the yield strength or ultimate tensile strength. For higher temperatureapplications creep strength also comes in picture. Various codes detail theallowable stress basis. The basis adopted in ANSI B31.3 and IBR aredescribed herein. These two codes have the maximum usage among theIndian pipe stress Engineers for Petrochemical/ Refinery.

3.1 Allowable stress as per ACSI:As per Petroleum refinery piping code ANSI B31.3 the basic allowablestress values are the min. of the following values.a) 1/3 of the minimum tensile strength at room temp.b) 1/3 of tensile strength of design temp.c) 2/3 of Min. yield strength of room temp.d) 2/3 of Min. yield strength at design temp.e) 100% of average stress for creep rate of O/D 1% per 1000 hrs.

3.2 Allowable Stress as per IBR:As pe the Indian Boiler Regulations the allowable working stress iscalculated as shown below:i) For temperatures at or below 454 Deg.C, the allowable stress is the lower of the

following values:Et = 1.5 or R = 2.7

ii) For temperatures above 454 Deg.C the allowable stress is lower of theValues:Et = 1.5 or Sr = 1.5WhereR = Min. tensile strength of the steel at room temp.Et = Yield point (02% proof stress) at the temp.Sr = Average stress to produce rupture in 100,000 hrs. at a temp. and in

No case more than 1.33 times the lowest stress to produce rupture attemp.

Sc = Average stress to produce an elongation of 1% creep in 100,000hrs. All these values have been made available after carrying onrepeated laboratory tests on the specimen.

4.0 Allowable stress range:The stress of a piping system lowers within the elasticity range in

which plastic flow does not occur by self-spring during several initialcycles even if the calculation value exceeds the yield point, andthereafter-steady respective stress is applied. Hence repture in a pipingsystem may be due to low cycle fatigue. It is well known that fatiguestrength usually depends upon the mean stress and the stressamplitude. The mean stress does not always become zero if self springtakes place in piping system but in the ANSI code, the value of themean stress is disregarded while the algebraic difference between themaximum and the minimum stress namely only the stress range SA isemployed as the criterion of the strength against fatigue rupture.The maximum stress range a system could be subjected to withoutproducing flow neither in the cold nor in the hot condition was firstproposed by ARC Mark as follows:a) In cold condition the stress in the pipe material will automatically limit itself

to the yield strength or 8/5 of Sc because Sc is limited to 5/8 th of Y.S.therefore, Ye = 1.6 Sc.


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b) At elevated temperatures at which creep is more likely the stress in thepipe material shall itself to the rupture strength i.e. 8/5th

Sh = 1.6 Sh.Therefore stress range = 1.6f(Sc = Sh)However, the code limits the stress range conservatively as1.25f(Sc + Sh) which includes all stresses i.e. expansion stress,pressure stress, hot stresses and any other stresses inducted byexternal loads such as wind and earthquake, f is the stress rangereduction factor for cyclic conditions as given below:To determine the stress range available for expansion stress alonewe subtract the stresses inducted by pressure stress and weightstress which itself cannot exceed sh.Therefore the range for expansion stress only is

SA = f(1.25 Sc + 0.25 Sh)

VALUES OF FACTOR f Total number of full f factorTemp. Cycles over expected life7,000 and less 1

14,000 and less 0.922,000 and less 0.845,000 and less 0.7100,000 and less 0.6250,000 and less 0.5

5.0 Pressure & Bending Stress& Combination Application:The code confines the stress examination to the most significantstresses created by the diversity of loading to which a pipingsystem is subjected. They are:i) Stress due to the thermal expansion of the line.ii) The longitudinal stresses due to internal or external pressure.iii) The bending stress created by the weight of the pipe and its

insulation, the internal fluid, fittings, valves and external loadingsuch as wind, earthquake etc.

5.1 Stresses due to the thermal expansion of the line:Temperature change in restrained piping cause bending stresses insingle plane systems, and bending and torsional stresses in three-dimensional system. The maximum stress due to thermal, changessolely is called expansion stress SE. This stress must be within theallowable stress range SA.SE = Sb2 + 4St2

Sb = I (Mb / Z) = resulting bending stressMt = (Mt //2Z) = torsional stress

Mb = resulting bending movementMt / = torsional movementZ = section modules of pipei = stress intensification factor

5.2 Longitudinal stress due to internal or externalpressure:The longitudinal stress due to internal/external pressure shall beexpressed as P (Ai / Am)


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Where Ai is inside cross sectional area of pipe, Am is the metalarea, P is the pressure.

5.3 Weight Stress:The stress induced, self weight of pipe, fluid, fittings etc. as givenby SW = M/Z, Where M is bending moment created by the pipe andother fittings, Z is the section modules of the pipe.

The stresses due to internal pressure and weight of the piping arepermanently sustained. They do not participate in stress reductionsdue to relaxation and are excluded from the comparison of which asthe latter has been adjusted to allow for them with the followingprovision.

6.0 Flexibility and stress intensification factor:Some of the piping items (say pipe elbow) show different flexibilitythan predicted by ordinary beam theory. Flexibility factor of a fittingis actually the ratio of rotation per unit length of the fitting inquestion under certain value of moment to the rotation of a straightpipe of same nominal diameter and schedule and under identicalvalue of moment. The pipefitting item, which shows substantialflexibility, is a pipe elbow/bend.

One end is anchored and the other end is attached to a rigid arm towhich a force is applied. The outer fibers of the bend/elbow will beunder tension and the inner fibers will be under compression. Dueto shape of bend both tension and compression will havecomponent in the same direction creating distortion/slottening ofbend. This leads to higher flexibility of the end as there is somedecrease in moment of inertia due to distortion from circular toelliptical shape and also due to fact that the outer layer fibers,which are under tension has to elongate less and the inner layerfibers which are under compression has to contract less toaccommodate the same angular rotation leading to higherflexibility. Piping component used in piping system hasnotches/discontinuities in the piping system, which acts as stressraisers. For example a fabricated tee branch. The concept of stressintensification comes from this and is defined as the ratio of thebending moment producing fatigue failure in a given number ofcycles in straight pipe of nominal dimensions to that producingfailure in the same number of cycles for the part underconsideration. Both flexibility factor and stress intensificationfactors have been described in PROCESS PIPING CODE(ASMEB31.3) and is also included in the various pipe stress analysiscomputer programmes.

7.0 Equipment nozzle loading:As explained earlier pipe stresses are calculated for various type ofloading such as pressure, weight, thermal etc. and it is reviewedwhether the stresses are within allowable limits. However in lot ofcases pipe stress analysis becomes critical and rather complicated

because it is not only stress of piping but the nozzle loading of thevarious equipment which has to be kept within allowable limits.For rotating equipments like steam turbines, compressorscentrifugal pumps, various codes like NEMA SM-23, API-617, API-610 etc. give guidelines regarding the allowable nozzle loading. Forthe analysis of these piping connected with various rotatingequipment, vendor also provide information regarding nozzlemovements and allowable loads. It is the responsibility of theequipment engineer to ensure that the allowable loads as agreed by


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vendors are always equal to or greater the values as per therespective applicable code. Various computer packages now haveequipment nozzle check features. However the pipe stressengineers are advised to study the specific applicable codes alsoas this will give them a further insight for solving specific problemsrelated to equipment nozzle loading.

basics of pipe stress

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