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COADE Pipe Stress Analysis Seminar Notes
A system’s behavior can be quantified through the aggregate values of numerous physical parameters, such as accelerations, velocities, displacements, internaI forces and moments, stresses, and external reactions developed under applied loads. Allowable values for each of the se parameters are set after review of the appropriate failure criteria for the system. System response and failure criteria are dependent on the type of loadings, which can be classified by various distinctions, such as primary vs.
Due to the extensive calculations required during the analysis of a piping system, this field of engineering provides a natural application for computerized calculations, especially during the last two to three decades.
The proliferation of easy-to-use pipe stress software has had a two-fold effect: The intention ofthis course is to provide the appropriate background for engineers entering the world of pipe stress analysis.
The course concentrates on the design requirements particularly from a stress analysis point ofview of the codes, as weIl as the techniques to be applied in order to satisfy those requirements. Although the course is taught using the CAESAR II Pipe Stress Analysis Software, the skills learned here are directly applicable to any means of pipe stress analysis, whether the engineer uses a competing software program or even manual calculational methods.
Why do we Perform Pipe Stress Analysis? There are a number ofreasons for performing stress analysis on a piping system. A few of these foIlow: In order to keep stresses in the pipe and fittings within code allowable levels. In order to determine piping displacements for interference checks. Typical Pipe Stress Documentation Documentation typically associated with stress analysis problems consists of the stress isometric, the stress analysis input echo, and the stress analysis results output.
Examples ofthese documents are shown in Figures through on subsequent pages. The stress isometric Figure is a sketch, drawn in an isometric coordinate system, which gives the viewer a rough 3-D idea of the piping system. The stress isometric often summarizes the piping design data, as gathered from other documents, such as the line list, piping specification, piping drawing, Appendix A Figure of the applicable piping code, etc.
Design data typically required in order to do pipe stress analysis consists of pipe materials and sizes; operating parameters, such as temperature, pressure, and fluid contents; code stress allowables; and loading parameters, such as insulation weight, external equipment movements, and wind and earthquake criteria.
Points of interest on the stress isometric are identified by node points. Node points are required at any location where it is necessary to provide information to, or obtain information from, the pipe stress software. Typically, node points are located as required in order to: The analysis output provides results, such as displacements, internal forces and moments, stresses, and restraint loadings at each node point of the pipe, acting under the specified loading conditions.
The output also provides a code check calculation for the appropriate piping code, from which the analyst can determine which locations are over stressed.
Sifs, interpretations,Coade seminar notes and confussion – Intergraph CADWorx & Analysis
B SH deg. Node is SJI C51 csu 57 U. Since these materials constitute the majority xoade the piping materials in use, and since most cyclic loading events comprise much fewer than 50, cycles, the effects coqde mean stress on fatigue life are negligible nktes piping materials with ultimate strengths belowpsi. For materials with an ultimate strength equal to or greater thanpsi, hotes as high strength bolting, mean stress can have a considerable effect on fatigue strength and should he considered when performing a fatigue analysis.
For a piping application, the implication of the Soderberg line on the fatigue allowable is implemented in a conservative nootes. The sustained stress Ci. Rodabaugh does however suggest that the normal usage whereby the width of the pad is taken to be at least equal to the radius ofthe nozzle should be observed even though not explicitly directed by the code. Nofes the tlI’ ratio gets much smaller than one, the largest stresses shift to the branch.
This finding originally came out of the research for WRC Comparisons ofWRC ‘s proposaIs for stress intensification factors for various types of tees, versus B Cade Loads Markl’s investigation of the fatigue problem, following the earlier recognition of the maximum stress theory offailure, led to identification of the basic problem in the design of piping systems.
Not one, but two different criteria must be satisfied, one for primary loads, which may lead to single application catastrophic failure, and one for cyclic, dis placementdriven loads that may lead to fatigue failure especially in the vicinity offittings and other discontinuities after repeated applications.
The main characteristics ofthese two different types of loads are described below: Once plastic deformation begins it continues unabated until force equilibrium is achieved notws change of the external boundary conditions or through material strain hardeningor until failure of the cross section results. Failure may occur with a single application ofthe load.
Note that failures that occur due to single load applications usually involve pressure hoop stress design failures and are not directly addressed by CAESAR n or seminarr the flexibility stress requirements ofthe codes. Such pressure design requirements are encompassed in the minimum wall thickness requirements discussed in detail in separate sections of the codes. Rather catastrophic failure can occur after some usually high number of applications of the load.
Therefore, even if a system coase been running successfully for many years, it is no evidence that the system has been properly designed for secondary loads. Several examples should help illustrate: Springs were improperly sized to support the weight of the valve operator on a system.
However, heating up the fluid and pipe during startup, the valve sagged and the guardrail was crushed in less than 30 minutes due to the decrease in strength at the operating temperature. The primary stresses were tao high.
After 12 years of successful operation, inspection of the inside surface of a vessel revealed fatigue cracks in the vicinity of a piping nozzle connection. A subsequent analysis showed that a temperature increase in the adjacent ssminar and piping system alongwith a relocation of pipe restraints for the new operating conditions made several years ago caused the stresses to exceed the expansion allowables.
Even though the calculated stress range at the COADE Pipe Stress Analysis Seminar Notes junction was weil over nnotes, psi, thejunction survived several years hecause of the selfrelieving nature of the thermalload, and the fact that the system cycled fewer than a dozen times over the two year period.
Therefore, code compliance requires that the piping system be checked for both types of loading – primary and secondary.
The basic steps involved in doing code compliance are outlined below: Note that due to the shakedown effect, and the fact that the primary and secondary stresses have different failure criteria, these two load types are reviewed in isolation. Therefore, it should he stressed that, as far as most codes are concerned, there is no such thing as “operating stress”.
The stress equations were quite similar throughout the piping codes i. It should be noted that the piping codes exactly calculate the stress intensity twice the maximum shear stress only for the expansion stress, since this load case contains no hoop or radial components, and thus becomes an easy calculation. Including hoop and radial stresses present in sustained loadings only in the stress intensity calculation makes the COADE Pipe Stress Analysis Seminar Notes calculation much more difficult.
When considering the hoop and radial stresses, it is no longer clear which of the principal stresses is the largest and which is the smallest. Additionally, the subtraction of Coace does not produce a simple expression for the stress intensity. As it turns out, the inclusion of the pressure term can be simplified by adding only the longitudinal component of the pressure stress directly to the stress intensity produced by noets loadings only.
This provides an equally easy-to-use equation and sacrifices little as far as accuracy is concerned. Sh is roughly defined as the minimum semiinar This is most commonly interpreted to mean: Sh is defined as the minimum of: The equation for calculating occasional stresses is undefined by B The default interpretation ofthis requirement is to calculate the sustained and occasional stresses independently as per the equation given for sustained stresses above and then to add them absolutely.
Note the differences between these two codes: I intensifies torsion, while B I neglects all forces, while in the default interpretation, B Note that both codes additionally cite a conservative value of SA, f 1. This is a carry over from pre-computer days, when sustained stress calculations were rarely done, so SI was not known explicitly, and conservatively estimated to be at seminwr maximum allowable level of Sh.
Specific requirements of other common codes are shown below as weIl. The occasional stress equations are: For Service Level C Emergency: These alternating stress intensities are designated as SaltlSa1t2, These are designated as NI, N2, Not only are the causes and the failure modes ofthese two loading types quite different, but not surprisingly, the solutions to these two types ofloading are usually different as weIl. Coace fact, the solution to a problem caused by one of the loading types often causes a problem with the other loading type.
Therefore, a compromise must often be reached in order to find the solution to these two types ofloading.
Note that primary loads are usually classified further, according to their duration ofloading. Those primary loads which are nearly always present throughout operation are called sustained loads, while those which occur less frequently are called occasionalloads. The cade ofresisting these two types ofloads are eeminar, with the main difference beingfound in the use of a higher allowable stress for occasionalloads as seen in Section 1. Sustained loads are classified as those caused by mechanical forces which are present throughout the normal operation of the piping system.
Therefore the se loads: Typical sustained loads consist of: Because ofthis, pressure design of components is usually done far before, and therefore in isolation, from the pipe stress analysis phase of piping design. A discussion of pressure design of components is included here for the sake of completeness, and is based upon an amalgam of the requirements of various codes.
Note swminar pressure design of piping components must be done according to the requirements of the user’s specifie code, not to the rules described here! Because the pipe wall is sized for the large hoop stress, this usually provides sufficient margin between the allowable stress and the longitudinal pressure stress to accommodate the weight stresses. sminar
The requirement for the minimum pipe component wall thickness is: Requirements for pressure design of other piping components are described in the following sections. For mitered elbows, the maximum allowable pressure is calculated differently depending on whether the angle of the miter cut is less than or greater than Potential causes of failure are bending stresses in the flange, localized stress concentrations in the hub, yielding of the bolts, or unloading of the gasket, causing leakage.
Instead, the most common piping codes endorse the use offlanges conforming to recognized standards such as ANSI B