Pipeline Stress Analysis

WHAT IS STRESS ANALYSIS?

Piping stress analysis is a term applied to calculations, which address the static and dynamic loading resulting from the effects of gravity, temperature changes, internal and external pressures, changes in fluid flow rate and seismic activity. Codes and standards establish the minimum requirements of stress analysis.

PURPOSE OF PIPING STRESS ANALYSIS

Purpose of piping stress analysis is to ensure:

·         Safety of piping and piping components.

·         Safety of connected equipment and supporting structure.

·         Piping deflections are within the limits.

HOW PIPING AND COMPONENTS FAIL (MODES OF FAILURES)

There are various failure modes, which could affect a piping system. The piping engineers can provide protection against some of these failure modes by performing stress analysis according to piping codes.

1.  FAILURE BY GENERAL YIELDING : Failure is due to excessive plastic deformation.

·         Yielding at Sub Elevated temperature: Body undergoes plastic deformation under slip action of grains.

·         Yielding at Elevated temperature : After slippage, material re-crystallizes and hence yielding continues without increasing load. This phenomenon is known as creep.

2. FAILURE BY FRACTURE : Body fails without undergoing yielding.

·         Brittle fracture : Occurs in brittle materials.

·         Fatigue: Due to cyclic loading initially a small crack is developed which grows after each cycle and results in sudden failure

WHEN PIPING AND COMPONENTS FAIL (THEORIES OF FAILURE)

Various theories of failure have been proposed, their purpose being to establish the point at which failure will occur under any type of combined loading. The failure theories most commonly used in describing the strength of piping systems are

·         Maximum principal stress theory

This theory states that yielding in a piping component occurs when the magnitude of any of the three mutually perpendicular principle stresses exceeds the yield point strength of the material.

·         Maximum shear stress theory

This theory states that failure of a piping component occurs when the maximum shear stress exceeds the shear stress at the yield point in a tensile test.

In the tensile test, at yield, S1 =Sy (yield stress), S2=S3=0.So yielding in the components occurs when

Maximum Shear stress =τmax=S1-S2/ 2=Sy / 2

The maximum principal stress theory forms the basis for piping systems governed by ASME B31.3.

Note: maximum or minimum normal stress is called principal stress.

Source : http://www.pipingdesign.com/stressanalysis.pdf