Piping Stress Analysis is the most important activity in Piping Design. Once, pipes are routed following design guidelines, those needs to be verified by piping stress analysis to ensure those will work smoothly throughout its design life. This article will explain the basic points for Piping Stress Analysis. Piping Stress Analysis is also termed as Piping Flexibility Analysis.
Objectives of Pipe Stress Analysis
Stress Analysis of Critical piping systems are performed to ensure the following objectives.
- Design adequacy for the pressure of the carrying fluid.
- Failure against various loading in the life cycle. Limiting stresses below code allowable.
- Limiting nozzle loads of the connected equipment within allowable values.
- Avoiding leakage at joints.
- Limiting sagging & displacement within allowable values.
- Avoiding excessive flexibility and also high loads on supporting structures. Aim towards an optimal design for both piping and structure.
Governing Codes and Standards for Stress Analysis
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 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
Stresses in a Piping System
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 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:
- 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 in Code (Table C1 and C3 in ASME B31.3 Appendix C):
- Table C1 denotes total linear thermal expansion between 70 degrees F to Indicated temp (unit=in/100ft).
- Table C3 denotes the mean coefficient of linear thermal expansion between 70 degrees F to indicated temp (μin/in/0F).
- 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.
Equations for Calculating Expansion Stress Range and Allowable Stress Value
Displacement Stress Range due to thermal expansion is calculated based on equation SE = ( Sb^2+4 St^2)^0.5 per equation 17 from ASME B31.3( clause 319.4.4).
This SE value shall not exceed 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 Stresses are generated by occasional loads such as Wind, seismic, PSV discharge, etc.
- These loads act in a piping system for a very small 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
- Supports for Weight
- Flexibility for thermal loading Eg. Expansion Loops, Offsets, Inclusion of elbows to change direction.
Flexibility check (as per clause 319.4.1, ASME B 31.3):
Refer Fig. 4
Basic Allowable Stress
Minimum of (As per ASME B 31.3)
- 1/3rd of 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 which 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.1
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 for Stress Analysis
The interaction of the Piping Stress team with other disciplines in any organization is shown in 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 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 temp, pressure, Ope. Temp, Min. Temp, Fluid density, Material, Line Size and
thickness, Insulation thickness, 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.
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
Type of Pipe Supports
Various types of supports are used in process plants like
- Rest: Restrict downward movements.
- Guide: Arrest lateral movements.
- Line Stop or Axial Stop: Restrict axial or longitudinal movement of the pipe.
- Anchor: Completely fixed. Restrict all six degrees of freedom. Pipe at this support point can’t translate or rotate.
- Variable Spring Hanger: Flexible support, Act as Resting support with flexibility to thermal movements.
- Constant Spring Hanger: Flexible support, Act 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 in two groups
- Uni-Directional Support and
- Bi-Directional Support.
Unidirectional 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 support arrests movement in both direction like Y, X or Z supports.
Questionnaire for Piping Stress Analysis
- What are the various types of loads which 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 degree C and Corrosion allowance= 1.6 mm)?