BASICS OF PIPE STRESS ANALYSIS: A PRESENTATION
Contents
- Objectives of Pipe Stress Analysis
- Governing Codes and Standards
- Stresses in a Piping System
- Reducing Piping Stresses
- Basic Allowable Stress
- Loads on a Piping System
- Work Flow Diagram
- Stress Criticality & Analysis Method
- Piping Stress Analysis using Software Caesar II
- Type of Supports
- Questionnaire
Objectives of Pipe Stress Analysis:
Structural Integrity:
- Design adequacy for the pressure of the carrying fluid.
- Failure against various loading in the life cycle . Limiting stresses below code allowable.
Operational Integrity:
- Limiting nozzle loads of the connected equipment within allowable values.
- Avoiding leakage at joints.
- Limiting sagging & displacement within allowable values.
Optimal Design:
- Avoiding excessive flexibility and also high loads on supporting structures. Aim towards an optimal design for both piping and structure.
Governing Codes and Standards:
- 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 Refinery 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 generation of stress in a Piping System:
- Weight
- Internal/External Pressure
- Temperature change
- Occasional Loads due to 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:
Fig. 1: Equation for Thickness Calculation for Straight Pipe based on ASME B 31.3
- 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 700 F to Indicated temp (unit=in/100ft).
- Table C3 denotes mean coefficient of linear thermal expansion between 700 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 relenting.
- 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.
- 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):
Fig.2: Equation for Displacement Stress Range Allowable
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.
Fig.3: Equation for Liberal Displacement Stress Range Allowable
- 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.
Flexibility check (as per clause 319.4.1, ASME B 31.3): Refer Fig. 4
Fig.4: Flexibility Check Equation for Simple Systems
My proposals for the calculations on the strength of pipelines, tanks and pressure vessels.
Calculation of the overall stress-strain state of piping systems made from both metal and non-metal (including composites) materials, for given values of temperature, pressure, weight of the internal environment, self-weight of the structure, taking into account the kinematic conditions, as well as the friction in the bearings.
Calculation of the strength of the joints, bends, tees, valves elements of pipeline systems involving 3D-geometric and finite element modeling.
Determination of displacements and stresses in the individual fragments of the pipeline circuit different geometry in the presence of poles at a given temperature, pressure, weight of the internal environment on the basis of three-dimensional modeling of cladding and finite elements.
The calculation of the strength of the underground part of the pipeline at a given temperature and pressure of the internal environment, taking into account not elastic properties of the soil (model Mohr-Coulomb, Drucker-Prager, Cam-Clay, Cap-model), the depth of the pipe, soil temperature, pipe friction on the ground.
Determination of pipes, joints, fittings elements with hydraulic shock.
Estimation of seismic stability of pipeline systems as a whole, as well as separate areas and support units.
Calculation of the total strength of the static pressure vessels and tanks made of metal and nonmetal (including composite) materials.
Evaluation of durability and service life of pressure vessels and tanks under cyclic variation of the parameters of internal and / or external environment.
Seismic resistance of tanks and pressure vessels, including partially filled with liquid – allowance for the interaction design with a liquid using “technology” – coupled Eulerian-Lagrangian (CEL) analyses.
Determination hydrodynamic loads “facilities” of fluid flow.
Few nice points has been discussed on this post in linkedin page. Follow this link to access those: https://www.linkedin.com/groups/http-wwwwhatispipingcom-basicsofpipestressanalysis-59422.S.5970562625166127107?view=&item=5970562625166127107&type=member&gid=59422&trk=eml-group_discussion_new_comment-discussion-title-link&midToken=AQH3kT0n6EEJKA&fromEmail=fromEmail&ut=0OXgM9Rbpz56E1.
However for the convenience of my readers i am reproducing those here.
By Mr Rao:
Consultant at DOW chemicals int.pvt.Ltd.
Dear Anup, I have one question ?
In ceaser analysis all supports are considered as point contact which is not true practically.
How does this facilitae accuracy with the effect of friction factor when we are working with srface contact for supports resting flat..
By Anup Kumar Dey:
Sir, In my opinion The point contact and surface contact does not affect the friction factor. Only surface roughness of the contact point does. As for example if the contact is between CS to CS the friction factor is normally 0.3 or 0.4. It does not reduce when used as point contact. But if we can make the surface polished then the same will reduce.
By Mr. SATISH KUMAR
Package Design Engineer at Fichtner Consulting Engineers India P Ltd.
Sir, horizontal force applied will be friction factor * normal reaction force.this horizontal force will be applied against movement of pipe axis.hence place of this force will be different.