Piping Stress Analysis is the most important activity in Piping Design. Once, pipes are routed following design guidelines, they need to be verified by piping stress analysis to ensure they will work smoothly throughout their design life. This article will explain the basic points for Piping Stress Analysis. Piping Stress Analysis is also termed Piping Flexibility Analysis.
What is Pipe Stress Analysis?
Pipe Stress Analysis is an engineering activity that focuses on evaluating the stresses, deformations, and forces within a piping system. It plays a vital role in ensuring the safe and reliable operation of piping systems in various industries, including oil and gas, petrochemical, power generation, and more.
Objectives of Pipe Stress Analysis
Stress Analysis of Critical piping systems is performed to ensure the following objectives.
- Design adequacy for the pressure of the carrying fluid.
- Failure against various loading in the life cycle. Limiting piping stresses below code allowable.
- Limiting nozzle loads of the connected equipment within allowable values.
- Avoiding leakage at flanged joints.
- Limiting sagging & displacement within allowable values.
- Avoiding excessive flexibility and high loads on supporting structures. Aim towards an optimal design for both piping and structure.
Basic Concepts of Piping Stress Analysis
Pipe stress analysis considers various components like pipes, fittings, valves, and supports. Understanding the properties and behavior of these components is crucial for accurate analysis.
- Pipes: Different materials, sizes, and schedules are used for pipes, and they exhibit specific stress-strain behaviors.
- Fittings and Valves: These components introduce stress concentrations and affect the overall behavior of the system.
- Supports: Supports and restraints are essential for controlling pipe movements and distributing loads.
Pipe systems experience several load types, including:
- Static Loads: Steady-state conditions like internal pressure, deadweight, and thermal expansion.
- Dynamic Loads: Transient events such as water hammer, relief valve discharge, and seismic activity.
- Thermal Loads: Temperature variations causing thermal expansion and contraction.
Pipe stress analysis relies on understanding the stress-strain relationship of materials. Key concepts include:
- Elasticity: Materials return to their original shape when the load is removed within their elastic limit.
- Plasticity: Beyond the elastic limit, materials deform irreversibly.
- Creep: Slow, time-dependent deformation under constant load and elevated temperature.
Governing Codes and Standards for Pipe Stress Analysis
Following are the codes and standards used for Piping stress analysis of process piping:
- ASME B31.3: Process Piping Code
- ASME B31.1: Power Piping Code
- Centrifugal Pumps: API 610
- Positive Displacement Pumps: API 676
- Centrifugal Compressors: API 617
- Reciprocating Compressors: API 618
- Steam Turbines: NEMA SM23/ API 612
- Air Cooled Heat Exchanger: API 661
- Fired Heaters: API 560
- Flat Bottom Welded Storage Tanks: API 650
- Heat Exchangers: TEMA/ Vendor-Specific.
- Vessel/Column: Vendor-Specific/ ASME Sec VIII
- ASME B 31.4/ASME B 31.8: Pipeline Stress Analysis
- ISO 14692: GRE/GRP/FRP Piping Stress Analysis
- ASME B31.4: Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids
- ASME B31.8: Gas Transmission and Distribution Piping Systems
- EN 13480: European standard for metallic industrial piping
- API 570: Inspection, repair, alteration, and rerating of in-service piping systems
Stresses in a Piping System
Types of Stress
Pipe stress analysis considers various types of stress, including:
- Axial Stress: Along the length of the pipe.
- Hoop Stress: Circumferential stress due to internal pressure.
- Radial Stress: Stress perpendicular to the longitudinal axis.
- Torsional Stress: Twisting or rotational stress.
- Shear Stress: Stress parallel to the pipe’s cross-section.
- Bending Stress: Stress due to curvature.
Sources for the generation of stress in a Piping System:
- Internal/External Pressure
- Temperature change
- Occasional Loads due to the wind, seismic disturbances, PSV discharge, etc.
- Forces due to Vibration.
Sustained Stresses in Piping System
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 balances the external forces keeping the system in equilibrium. Exceeding sustained 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 “Sh“. Where Sh=Basic allowable stress at the metal temperature for the operating condition being considered.
Pressure Stresses are taken care of by calculating and selecting the 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:
Expansion Stresses in Piping System
- Change in length of a pipe of length L due to temp change (ΔT) is given by ΔL=L α ΔT Here, α =Co efficient of thermal expansion = change in length of unit length element due to unit change in temp.
- Two “α” values (denoted by A and B) in Code (Table C-1 and C-1M in ASME B31.3 Appendix C):
- The thermal Co-efficient “A” of Table C-1 denotes the mean coefficient of linear thermal expansion between 70 degrees F to the indicated temp (μin/in/0F).
- The thermal Co-efficient “B” of table C-1 denotes total linear thermal expansion between 70 degrees F to Indicated temp (unit=in/100ft).
- Table C-1M provides thermal co-efficient values in the metric system.
- Expansion stresses are generated when the free thermal growth due to temperature change is restricted. These are self-limiting or self-relenting.
Stress Intensification Factor in Piping Stress Analysis
SIF( Stress Intensification Factor): This is the ratio of the maximum stress intensity to the nominal Stress. SIF factors for different components can be obtained from Appendix D of ASME B31.3 till edition 2018. From ASME B31.3-2020 onwards Appendix D has been deleted. Now users are required to use ASME B31J or FEA for finding the values of SIF.
Equations for Calculating Expansion Stress Range and Allowable Stress Value
The displacement Stress Range due to thermal expansion is calculated based on equation SE per equation 17 from ASME B31.3( clause 319.4.4).
This SE value shall not exceed the SA value where SA= Allowable Displacement Stress Range.
As per ASME code B 31.3 (Clause 302.3.5) the allowable displacement stress range (SA) can be given by the equation (Fig.2):
Here, f= Stress range reduction factor and Sc=basic allowable stress at minimum metal temp
- When Sh > SL, the allowable stress range is calculated by the following equation (Fig. 3): SL=Longitudinal Stress due to sustained loads.
Occasional Piping Stresses
- Occasional Stresses are generated by occasional loads such as Wind, seismic, PSV discharge, etc.
- These loads act in a piping system for a very short period of time, usually less than 10% of the 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”
- The code does not explicitly explain the stresses generated due to vibration.
- The vibration problems are solved by engineering judgment and experience.
Reducing Piping Stresses
Piping stresses can be reduced by various methods like
- Providing Support at a suitable span to reduce Weight (Sustained) stresses.
- Providing Flexibility to reduce piping expansion stresses generated by thermal loading e.g. Expansion Loops, Offsets, and Inclusion of elbows to change direction.
Flexibility check (as per clause 319.4.1, ASME B 31.3):
Refer Fig. 4
Basic Allowable Stress/ Pipe Material Stress
Pipe materials have defined stress limits to ensure their safety. The basic allowable stress for a pipe material is calculated as follows:
Minimum of (As per ASME B 31.3)
- 1/3rd of the Ultimate Tensile Strength (UTS) of Material at operating temperature.
- 1/3rd of UTS of material at room temperature.
- 2/3rd of Yield Tensile Strength (YTS) of material at operating temperature.
- 2/3rd of YTS of material at room temp.
- 100% of average stress for a creep rate of 0.01% per 1000 hr.
- 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 that act on a piping system: Static loads and Dynamic Loads
Static loads are those loads that act very slowly and the system gets enough time to react against it. Examples of static loads are shown in Fig.5
On the other hand, dynamic loads act so quickly that the system does not get enough time to react against them. Examples of dynamic loads are shown in Fig.6
Work Flow Diagram for Pipe Stress Analysis
The interaction of the Piping Stress team with other disciplines in any organization is shown in Fig. 7:
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
- Input Collection for Piping Stress Analysis
- Performing the stress analysis
- Interpreting the results and suggesting changes if required
- Providing Recommendations Based on Analysis
Inputs required for Piping Stress Analysis:
- Stress Isometric from Layout Group
- Line Designation Table (LDT) or Line List And P&ID from Process
- Equipment GA and Other detailed drawings from Mechanical
- Process flow diagram/datasheet if required from the 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
- Checking the completeness of the piping system received as a stress package.
- Node numbering on stress Iso.
- Filling the design parameters (Design temperature, Design pressure, Operating Temperature, Minimum Design Temperature, Fluid density, Material, Line Size and
thickness, Insulation thickness, density, Corrosion allowance, etc.) on stress Isometric.
- 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 support to make the system acceptable as per standard requirements.
Outputs from Stress Analysis:
- 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
Loads and Load Combinations
Internal and external pressure loads must be accurately analyzed to determine their impact on the piping system.
Temperature fluctuations can cause significant thermal stresses, particularly in large, high-temperature systems.
Deadweight and Operating Loads
The weight of pipes, fittings, valves, and insulation contributes to the system’s overall load.
Wind and Seismic Loads
External forces like wind and seismic events must be considered in high-risk areas.
Water Hammer Effects
Water hammer, or hydraulic shock, occurs when there is a sudden change in fluid flow, resulting in pressure surges that can damage piping systems.
Modifying the piping layout or design to reduce stress concentrations and improve overall system performance.
Adding Supports or Expansion Joints
Introducing additional supports or expansion joints to reduce stress and accommodate thermal expansion.
Strengthening critical areas of the piping system to withstand higher loads and pressures.
Other Piping Stress Analysis Software:
There are a few other pipe stress analysis software available in the market which is used as alternatives to Caesar II software like
- Auto-Pipe by Bentley
- Start-Prof by PASS (Russia)
- Rohr-2 by SIGMA Ingenieurgesellschaft mbH (Germany)
- CAEPIPE by SST Systems Inc (USA)
Type of Pipe Supports
Pipe Stress Analysis will be incomplete without a few words about piping supports. Piping stress analysis, in one way, is the selection of proper supports and placing them in the correct location to avoid detrimental stresses in the piping systems. Various types of supports are used in the piping and pipeline industry like
- Rest Support: Restrict downward movements.
- Guide Support: Arrest lateral movements.
- Line Stop or Axial Stop: Restrict axial or longitudinal movement of the pipe.
- Anchor Support: Completely fixed. Restrict all six degrees of freedom. The pipe at this support point can’t translate or rotate.
- Variable Spring Hanger Support: Flexible support, acting as Resting support with flexibility to thermal movements.
- Constant Spring Hanger: Flexible support, that acts as Rest support allowing thermal displacements.
- Rigid Hanger: Hanging support from the top.
- Struts: Dynamic Restraint
- Snubbers: Dynamic Restraint
- Sway Braces, etc.
In piping stress analysis supports can be classified into two groups
- Uni-Directional Piping Support and
- Bi-Directional Pipe Support.
Unidirectional pipe support is free to move in one direction like +Y, +X, +Z, etc here the supports are free to move in +y, +x, and +z respectively. However, bi-directional piping supports arrests movement in both directions like Y, X, or Z supports.
Basics of Piping Stress Analysis Tutorial Video
To learn the above-mentioned points in detail refer to the following video:
Questionnaire for Piping Stress Analysis
- What are the various types of loads that cause stresses in the piping system?
- Which code do we refer to 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 a design pressure of 20 bars. (Design Temp= 350 degrees C and Corrosion allowance= 1.6 mm)?
Few more piping resources for you..
Basic Piping Stress Analysis Articles
Piping Stress Analysis using Caesar II
Piping Stress Analysis using Start-Prof
Piping Design and Layout Basics
Piping Materials Basics
Tutorials on Piping Design Softwares