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

Continue from Part 1 of this topic…..

Basic Allowable Stress:

Minimum of (As per ASME B 31.3)

1. 1/3rd of Ultimate Tensile Strength (UTS) of Material at operating temperature.
2. 1/3rd of UTS of material at room temperature.
3. 2/3rd of Yield Tensile Strength (YTS) of material at operating temperature.
4. 2/3rd of YTS of material at room temp.
5. 100% of average stress for a creep rate of 0.01% per 1000 hr.
6. For structural grade materials basic allowable stress=0.92 times the lowest value obtained from 1 through 5 above.

Loads on a Piping System:

There are two types of loads which acts on a piping system: Static loads and Dynamic Loads,Static loads are those loads which acts very slowly and the system gets enough time to react against it. Examples of static loads are shown in Fig.1

Fig.1: Examples of Static Loads

On the other hand dynamic loads acts so quickly that the system does not get enough time to react against it. Examples of dynamic loads are shown in Fig.2

Fig.2: Examples of Dynamic Loads

other disciplines in any organization are shown in Fig. 3:

Fig.3: Inter Departmental Interaction with Stress Team

Stress Criticality and Analysis Methods:

•  Highly Critical Lines (Steam turbine, Compressor connected pipelines): By Computer Analysis
•  Moderately Critical Lines (AFC connected lines): By Computer Analysis
•  Low critical Lines : Visual/Simple Manual Calculation/Computer analysis and
•  Non Critical Lines: Visual Inspection

Stress Analysis using Caesar II :

Inputs:

• Stress Isometric from Layout Group
• LDT And P&ID from Process
• Equipment GA and Other detailed drawings from Mechanical
• Process flow diagram/datasheet if required from process
• Piping Material Specification
• PSV/ Control Valve GA and Datasheet from Instrumentation
• Soil Characteristics from civil for underground analysis
• Nozzle load limiting Standards
• Plot Plan for finding HPP elevation and equipment orientation.
• Governing Code

Analysis:

• Checking the completeness of the piping system received as a stress package.
• Node numbering on stress Iso.
• Filling the design parameters (Design temp, pressure, Ope. Temp, Min. Temp, Fluid density, Material, Line Size and
  thickness, Insulation thk and density, Corrosion allowance etc) on stress Iso.
• Modeling the piping system in Caesar using parameters from stress Iso.
• Analyzing the system and obtaining results.

Conclusion & Recommendation : Whether to accept the system or to suggest necessary changes in layout and supporting to make the system acceptable as per standard requirements.

Output:

•  Final marked up Iso’s to Layout
•  Support Loads to Civil
•  Spring Hanger Datasheets.
•  Datasheets for Special Supports like Sway brace, Struts, Snubbers etc.
•  SPS drawings
•  Stress Package final documentation for records

 Type of Supports:

• Rest
• Guide
• Line Stop
• Anchor
• Variable Spring Hanger
• Constant Spring Hanger
• Rigid Hanger
• Struts
• Snubbers
• Sway Braces etc

Questionnaire:

• What are the various types of loads which cause stresses in the piping system?
• Which code do we refer for Refinery Piping?
• Which standard governs the design of Pumps?
• The coefficient of thermal expansion of a substance is 1.8 mm/m/Deg.F. What is its value in mm/mm/Deg.C.?
• Calculate the minimum pipe thickness of a seamless 10” NB A106- Gr B material with design pressure of 20 bars. (Design Temp= 350 degree C and Corrosion allowance= 1.6 mm)