Pipe Stress Analysis: Classification of Loads

Introduction

        The system behavior and failure are dependent on the type of loading imposed. These are mainly classified as Primary vs. Secondary or Static vs. dynamic or Sustained vs. Occasional...

        The loads on the piping system are broadly classified into

• Primary Loads
• Secondary Loads

Primary Loads or Primary Stresses.

          Primary loads have their origin in some force acting on the pipe causing tension, compression, torsion, etc. leading to normal & shear stress. The failure of the piping system may be a sudden failure due to onetime loading or fatigue failure due to cyclic loading. The sudden failure is attributed to primary loading. Following are some important points about Primary Loads

1. Primary loads are usually force driven (gravity pressure, spring forces, relief Valve, fluid hammer, etc.)
2.  Primary loads are not self-limiting. Once plastic deformation begins it continues till the failure of the cross-section results.
3. Allowable limits of primary stresses are related to ultimate tensile strength.
4. Primary loads are not cyclic in nature.
5.  Design requirements due to primary loads are encompassed in minimum wall Thickness requirements

          Primary loads are usually classified further, according to their duration of loading as follows;

1. Sustained Load: 

          Those primary loads which are nearly always present thought out the   operation are called Sustained loads. Example. Pipe weight, Insulation weight, etc.

2. Occasional Loads: 

         Those primary loads which occur less frequently are called Occasional loads. Example. Pressure wave generated due to water hammer.

         The deformation due to these loads is limited only if the material shows strain hardening characteristics. If it has no stain hardening property or if the load is so excessive that the plastic instability set in, the system would continue to deform till rupture.

         One says that, primary loads are not self limiting. It means that the stresses continue to exist as long as load persists & deformation does not stop because the system has deformed into a no-stress condition but because strain hardening has come into play.

Secondary Loads or Secondary Stresses

          Secondary loads are caused by the displacement of some kind. The failure of the piping system may be a sudden failure due to onetime loading or fatigue failure due to cyclic loading. Fatigue failure is attributed to secondary loading.  Thermal loads are “secondary loads” because they are self-limiting. That is, yielding or deformation of the part reduces the stress. Following are some important points about Secondary Loads;

1. Secondary loads are usually displacement driven (Thermal expansion, Settlement, Vibration, etc.)
2. Secondary loads are self-limiting i.e. the loads tend to dissipate as the system deforms through yielding.
3. Allowable loads for secondary stresses are based upon fatigue failure modes.
4. Secondary loads are cyclic in nature (expect settlement).
5. A secondary application of load never produces sudden failure and sudden failure occurs after a number of applications of loads.

         The secondary loads are often cyclic but not always. E.g. loads due to tank settlement are not cyclic. Whereas load due to vessel nozzle moving up is cyclic. Failure due to such loads are often due to fatigue & not catastrophic in nature.

Few more piping resources...

1. Stress Analysis Of Column Piping System - 01
2. Stress Analysis Of Column Piping System: PIPING &COLUMN MODELLING IN CAESAR-II 
3. Basics of Pipe analysis: Objectives
4. Basics of Pipe analysis: Principal stresses