Piping CAESAR IIApril 27 2015Muhammed Azeez Sadiq MT-PRJ HO

Why do we perform stress analysis? To keep stresses in pipes within code allowable levelsTo keep nozzle loadings on attached equipment within allowables of manufactures of recognized standards (API 610, API 617)To keep vessel stresses at piping connections within ASME Section VIII allowable levelsTo calculate design loads for sizing supports and restraintsTo determine piping displacements for interfernce checksTo solve dynamic problems in piping, such as pulsation and mechanical vibration

Design DataPipe SizesPipe MaterialsOperating ParametersTemperaturesPressureFluid ContentsCode Stress AllowablesLoading ParametersInsulation Weight Wind Earthquake

Code Stresses vs Real StressesInclusion or exclusion of piping loadsLoading Type Analyzed as if separateCode Committee Tradition Differing Concerns

Types of Stresses in PipingLongitudinal StressNormal force acting parallel to the longitudinal axis

TypesLongitudinal Pressure due to Axial ForceInternal Pressure Bending Stress due to Moments

Types of Stresses in PipingHoop StressesOrthogonal to Axial Direction and Parallel to Pipe Circumference

Types of Stresses in PipingRadial StressesNormal Stress orthogonal to pipe axis and parallel to pipe radiusRadial Stress is zero at the outer radius where bending stresses are maximizedHence radial stress is often ignored in calculations

Types of Stresses in PipingShear StressesApplied in Direction parallel to face of plane of crystal structureCauses adjacent planes to slip past one anotherMaximum at neutral axisZero at maximum distance from neutral axisInsignificant compared to bending stresses hence often ignored

Types of Stresses in PipingShear Stress due to Torsion

Failure TheoriesOctahedral Shear, or von Mises TheoryMaximum Shear, or Tresca TheoryMaximum Stress, or Rankine Theory

Most piping codes including B31.3 use slight modification of Maximum Shear Stress Theory.is similar to

Primary StressesForce DrivenNot Self-limitingTypically not cyclic in natureExcessive load causes failureSecondary StressesDisplacement DrivenSelf-limitingTypically cyclic in natureSingle application of load never causes failure

Code Compliances B31.3Sustained StressesSh =

Code Compliances B31.3Expansion Stresses

Code Compliances B31.3Occasional StressesNot defined explicitlySum of longitudinal stresses not to exceed 1.33 ShCalculate sustained and occasional separately and then add them absolutely.

Loading TypesPrimarySustained Occasional

Secondary

Conflicting Solutions

Sustained LoadsCharacteristicsForce DrivenPresent for relatively long timesExamplesPressureWeight Spring hanger pre-loads

Minimum Pipe Wall Thickness Straight Pipe under Internal PressureEquations in both formsOnly Deals with Pressure EffectsTm=t+c (31.3)

Minimum Pipe Wall Thickness Elbow under Internal Pressure

Minimum Pipe Wall Thickness Mitered Elbows under Internal PressureMultiple Bend EquationSingle Bend Equation

Minimum Thickness of Branched Connection

Types of Stresses