Piping Stress Analysis Methods and Applications

▣ Classification of Piping System Stresses

1. Primary Stress
   • Stress induced by forces and moments applied internally and externally to the piping system, including bending stress from internal pressure, self-weight, wind, and other factors, as well as torsional stress.
   • The safety of primary stress is evaluated by comparing it with the allowable stress of the piping material.
2. Secondary Stress
   • Stress caused by thermal expansion due to the temperature of the fluid flowing through the pipeline. Even if this stress exceeds the yield strength of the material, it can enter a safe stress range due to stress relaxation.
   • Unlike primary stress, secondary stress is not compared directly with the allowable stress but rather with the allowable stress range to determine safety.
3. Allowable Stress
   • The stress level that a material can safely withstand under various temperature conditions concerning primary stress.
   • These values are provided in the ANSI Code.

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▣ Types of Stresses

• SI: Longitudinal Stress
• Sc: Circumferential Stress
• Sr: Radial Stress
• Ss: Shear or Torsional Stress

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▣ Static Stress Analysis

1. Sustained Load: Includes dead weight and internal pressure.
2. Occasional Load: Includes wind load and seismic load.
3. Support: Analysis of self-weight, hydrodynamic pressure, and reaction forces.
4. Evaluation of Impact on Connected Equipment Due to Forces & Moments:
   • Includes rotating machinery such as pumps, compressors, turbines, and air fin coolers.
   • Evaluates nozzle load stress for vessel nozzles (cylindrical, spherical) and heaters.
5. Stiffness Ring Design for Vacuum Lines.
6. Underground Stress Analysis: Includes thermal and earth pressure design.
7. Branch Reinforced Pad Design.

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▣ Dynamic Stress Analysis

1. Safety Valve Thrust Calculation.
2. Vibration: Includes considerations for reciprocating compressors and two-phase flow.
3. Seismic Analysis: Includes static method and response spectrum method.
4. Surge Analysis: Involves determining energy absorption devices due to sudden pressure rises in long-distance high-speed fluid pipelines, caused by rapid valve switching or power outages.

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▣ Flexibility Analysis of Piping

• Flexibility analysis involves reviewing whether the piping between fixed points has adequate flexibility to accommodate thermal expansion, ensuring that pipe supports are designed to withstand sustained and occasional loads.
• The flexibility analysis is performed to ensure the proper layout of the piping, and it typically does not require a special calculation procedure or the creation of a calculation report as part of the piping stress analysis documentation.
• It is not necessary to perform flexibility analysis for every piping system.

Cases Where Analysis is Not Required (ASME B31.1):

• The installed piping system is identical to a system with proven usage or is a replacement for such a system.
• The installed piping system is judged to be adequate when compared to a previously stress-analyzed system.
• The installed piping system has a constant diameter, no restraints between two anchors, and the total number of operating cycles is 7,000 or less, satisfying specific equations.

 

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▣ Piping Stress Analysis Codes

• API675: Positive Displacement Pumps Controlled Volume
• API-618: Reciprocating Compressors For General Refinery Services
• NEMA SM23: Steam Turbine For Mechanical Drive Service
• API-560: Fired Heaters For General Refinery Services
• API-610: Centrifugal Pumps For General Refinery Service
• API-611: General-Purpose Steam Turbines For Refinery Service
• API-612: Special-Purpose Steam Turbine For Refinery Service
• API-617: Centrifugal Compressors For General Refinery Service
• API-661: Air-Cooled Heat Exchangers For General Refinery Service
• API-650: Welded Steel Tanks for Oil Storage
• API-1102: Liquid Petroleum Pipelines Crossing Railroads and Highways
• ANSI A58.1: Minimum Design Loads For Buildings and Other Structures
• ANSI B31.3: Chemical Plant and Petroleum Refinery Piping

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▣ Stress Analysis Report

• After performing stress analysis, document the results and retain them for reference.
  • Includes applied codes, computer programs, and general information.
  • Assumptions applied in the design.
  • Hold Item Lists.
  • Isometric Drawings used for piping stress analysis (including input data).
  • Computer input data (design conditions, material properties).
  • Basis for thermal expansion displacement calculations for equipment.
  • Review of nozzle loads based on load combinations.
  • Load Summary Sheets for anchors and supports based on load combinations.
  • Computer-generated results, etc.

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▣ Review Items After Analysis of Self-Weight, Occasional Load, and Thermal Expansion

• Is the sag of the piping due to self-weight within acceptable limits?
• Are the loads on equipment nozzles within allowable limits?
• Is the maximum stress within the allowable stress?
• Are excessive loads generated on the designed anchors?
• Is there an upward load (Up-Lift Load) due to the load?
• Does the thermal expansion displacement cause interference with nearby piping?
• Are there lower points than the drain point due to thermal expansion?
• Are the analysis results within allowable limits for each operating mode?

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▣ Load Combinations and Allowable Stress

1. Design Condition:
   • Considerations include the piping's self-weight (including the weight of the fluid, insulation, concentrated loads such as valves), design pressure, seismic load, etc.
2. Normal Operating Condition During System Operation:
   • Includes piping self-weight, internal pressure, thermal expansion load, and abnormal conditions that may occur during operation.
   • Includes dynamic loads, internal pressure, and thermal expansion loads.
3. Test Condition:
   • Testing considerations.

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▣ Load Combinations (ASME B31.1)

 

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▣ Coordination with Other Disciplines

DisciplineMain Coordination Tasks

Structural	- Transfer of dead load and anchor load, thermal load transmission - Verification of seismic and wind load design criteria
Civil	- Transfer of foundation load for independent supports - Receipt of seismic and wind load design criteria
Equipment	- Transfer of nozzle load (Force & Moment) results - For general vessel nozzles, verification of analysis results by equipment design personnel - For high-temperature, high-pressure vessels, verification by equipment design personnel or manufacturer
Mechanical	- Request for verification of rotating equipment nozzle loads - Confirmation of nozzle integrity by mechanical personnel or manufacturer