BASICS OF PIPE STRESS ANALYSIS: A PRESENTATION-Part 1 of 2

BASICS OF PIPE STRESS ANALYSIS: A PRESENTATION

Contents

Objectives of Pipe Stress Analysis Governing Codes and Standards Stresses in a Piping System Reducing Piping Stresses Basic Allowable Stress Loads on a Piping System Work Flow Diagram Stress Criticality & Analysis Method Piping Stress Analysis using Software Caesar II Type of Supports Questionnaire

Objectives of Pipe Stress Analysis:

Structural Integrity:

Design adequacy for the pressure of the carrying fluid. Failure against various loading in the life cycle . Limiting stresses below code allowable.

Operational Integrity:

Limiting nozzle loads of the connected equipment within allowable values. Avoiding leakage at joints. Limiting sagging & displacement within allowable values.

Optimal Design:

Avoiding excessive flexibility and also high loads on supporting structures. Aim towards an optimal design for both piping and structure.

Governing Codes and Standards:

Codes and Standards specify minimum requirements for safe design and construction (i. e. provides material, design, fabrication, installation and inspection requirements.)

Following are the codes and standards used for Refinery Piping:

1. ASME B31.3: Process piping Code2. ASME B31.1: Power Piping Code3. Centrifugal Pumps: API 6104. Positive Displacement Pumps: API 676

5. Centrifugal Compressors: API 6176. Reciprocating Compressors: API 6187. Steam Turbines: NEMA SM23/ API 6128. Air Cooled Heat Exchanger: API 6619. Fired Heaters: API 56010. Flat Bottom Welded Storage Tanks: API 65011. Heat Exchangers: TEMA/ Vendor Specific.12. Vessel/Column: Vendor Specific

Stresses in a Piping System:

Sources for generation of stress in a Piping System:

1. Weight2. Internal/External Pressure3. Temperature change4. Occasional Loads due to wind, seismic disturbances, PSV discharge etc.5. Forces due to Vibration.

Sustained Stresses are the stresses generated by sustained loads. (e.g. Pressure , Weight). These loads are present continuously throughout plant life.

Resistive force arising out of sustained stresses balance the external forces keeping the system in equilibrium. Exceeding sustain allowable stress value causes catastrophic failure of the system.

As per ASME B 31.3, (clause 302.3.5) “ The sum of the longitudinal stresses, SL, in any component in a piping system, due to sustained loads such as pressure and weight, shall not exceed the product Sh x W ”. Where, Sh=Basic allowable stress at maximum metal temperature expected during the displacement cycle and W=weld joint strength reduction factor.

Pressure Stresses are taken care of by calculating and selecting proper pipe thickness. The pressure thickness (t) of a straight pipe can be obtained as per ASME B31.3 from the equation (Clause 304.1.2) mentioned in Fig.1: