• When Sh > SL , the allowable stress range is calculated by the following equation (Fig. 3): SL=Longitudinal Stress due to sustained loads.

• Occasional Stresses are generated by the occasional loads such as Wind, seismic, PSV discharge etc.
• This loads act in a piping system for very small period of time, usually less than 10% of total working period.
• As per ASME B31.3 clause 302.3.6 “The sum of the longitudinal stresses, SL, due to sustained loads, such as pressure and weight, and of the stresses produced by occasional loads, such as wind or earthquake should be ≤ 1.33 times the basic allowable stress, Sh”
• Code does not explicitly explain the stresses generated due to vibration.
• The vibration problems are solved by engineering judgment and experience.

Reducing Piping stresses:
• Supports for Weight
• Flexibility for thermal loading Eg. Expansion Loops.

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 act very slowly and the system gets enough time to react against it. Examples of static loads are shown in above figure.

Fig.2

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

Work Flow Diagram:
The interaction of Piping Stress team with other disciplines in any organization is shown in Fig. 3:

Fig.3

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

topic (2)
"Rack Piping for a Piping Stress Engineer"

While designing a pipe rack, there are two main factors which a stress engineer should look into details. Those are:
1. Expansion loop design and placement
2. Pipe rack loading.
The following write up will list few of the considerations while designing pipe loop and rack loading.