Meena Rezkallah, our lead piping stress engineer / Structural Engineer at Little P.Eng. for Engineering Services presents guidelines in pipe stress analysis based on his extensive experience and solid background in multi-disciplinary projects, well recognized by clients and colleagues for his innovative and cost-effective approach to design. He is eagerly involved in the design and supervision of all aspects of Meena Development’s engineering work. Mr. Rezkallah has specialized knowledge in all the piping applications of ASME Codes, API Codes, Structural codes (ASCE, ACI, AISC), Welding CWI, CWB, AWS and other Standards. Also, proficiency at QA/QC of all piping / Structural-related projects.
1.1 Introduction
1.1.1 The Contractor shall ensure that piping systems are safe for all specified design conditions. These guidelines are concerned with the piping flexibility aspect of this function, other related activities include piping design for pressure containment.
1.1.2 This standard, by emphasizing the need for demonstrating engineering integrity, does not seek to restrict the use of sound engineering judgement based on Contractor’s experience. Computer usage should be resorted to where accurate assessment of loading and stresses are essential for line approval or where an alternative (recognized) method of calculation, as identified in Contractor’s approved procedure, would be more costly in terms of time or resources. Where computer calculation is used, consideration shall be given to the form of input data, which must give verifiable results within the completed calculation report.
1.1.3 Whatever the method used for reviewing piping flexibility and stress calculation, the results should be clearly identified and documented for subsequent audit by either Client or by a third party.
1.2 General
1.2.1 Piping systems may be subjected to many diverse load conditions. Stresses induced by pressure, weight of pipe, equipment vibration, fittings and fluids, external loadings such as wind loads, seismic loads, settlement, solar/frost temperature effects and thermal expansion and contraction are significant in the pipe stress analysis of piping systems. Generally most piping movements are due to thermal expansion, but all the above criteria shall be taken into consideration during stress analysis.
1.2.2 Piping systems shall be designed to be adequately flexible and wherever possible this should be achieved by the natural flexibility of the pipework. If necessary the route of the piping should be modified (or expansion loops incorporated) in order to obtain sufficient flexibility. Only in exceptional cases where it is impractical to increase the system flexibility (to reduce the stress range to an acceptable level or to reduce equipment loading) should an expansion joint or similar be considered as for a design solution. The use of expansion joints shall require the prior written approval of Client
1.2.3 When carrying out a flexibility analysis, the worst process design conditions shall be considered, including steam-out. Practical engineering judgement shall be used to decide the worst load case scenario and consideration must be given to running the worst load case. Process conditions which give rise to impulse loading such as pressure surge, relief discharge reaction or two phase flow shall also be taken into account.
1.2.4 Strain sensitive equipment (vessels, heat exchangers, reactors, tanks, pumps and compressors) at which pipe systems terminate shall be considered rigid for pipe stress analysis. The loads shall be calculated at the vessel nozzle/shell interface for all classes of equipment with the exception of rotating machines, where the nozzle is recommended for inclusion within the pipework analysis and treated as rigid.
1.2.4.1. The acceptance or approval of nozzle loads shall be the responsibility of Contractor’s Specialist Equipment Engineer.
1.2.4.2. In the case of tanks, the nozzle loads shall be referred to Contractor’s Specialist Vessel Engineer for review. Design shall follow the requirements of API 650 unless otherwise stated within the listed documents.
1.2.4.3 In all cases Contractor shall be responsible for the structural integrity of piping flanges. Flange leakage calculations shall be performed as required by the relevant Code
1.2.4.4. Piping connected to machinery shall be flexible to ensure that the piping loads transmitted to the machine are acceptable under all design conditions.
1.2.5 Boundary conditions at other Contractor or Client interfaces must be clearly defined before detail commencement of design work. This should include, but is not limited to, the movements, forces and pipe support provisions at boundaries.
2.1 General
All Systems shall be analyzed in accordance with the client's standards. These out-line the methods of examination required for all piping systems covered by this standard and diagrammatically represent the potential need for computer or other analysis. Contractor shall develop these for all cases of pipe schedules and conditions within the detailed design.
2.1.2 Visual inspection or approximate calculation methods may be applied only if they are used in the range of configuration for which their adequacy has been demonstrated. Approximate calculations may include the use of approved charts, nomographs and simplified formulae. The objective of using these methods is to demonstrate that recourse to more precise methods is not required.
2.1.3 Acceptable comprehensive methods of analysis include computer and analytical methods, which include stress intensification and flexibility factors for all components other than pipe, and provide an evaluation of the forces moments and stresses caused by piping displacement.
2.2 Visual, Chart or Informal Checking Methods
2.2.1 Piping flexibility can be checked by visual inspection and the use of approved reference charts e.g. stress nomograph, expansion loop charts, or analogy. Piping loads on equipment shall be avoided by adding suitable guides, restraints and/or anchors.
2.2.2 At pumps having end suction nozzles it may be possible to restrain/anchor the piping close to the pump. However this restraint/anchor should not be more than 1 meter from the fixed point of the pump casing and should be designed such that adjustment can be made when bolting the pipe flange to the pump nozzle. Loads on the restraint/anchor must be minimized by the location of guides further upstream e.g. in the Piperack. This design option should be used with care as poor support specification and installation can generate large forces.
2.2.3 Piping which connects the tube bundles of air cooler exchangers can be checked and the terminal loads evaluated usually by basic hand calculations. Loadings on structures can be estimated by approximate methods. Computer analysis may be necessary in the following cases:
2.2.3.1. Where items of equipment are particularly strain-sensitive, e.g. compressors and very thin walled vessels.
2.2.3.2. For large pipes, diameters greater than 24” NPS.
2.2.3.3. Where movements are large, particularly when due to extraneous causes e.g. differential settlement of foundations, overhead lines rising with tower expansion.
2.3 Comprehensive Methods of Analysis
2.3.1 Before resorting to formal calculations and computer analysis, check that the proposed piping configuration is approximately correct by visual appraisal and by reference to an approximate method. If the stress in the pipe itself is the only criterion, further calculation may be unnecessary, but care must be taken, as the presence of components subject to intensified
Local stresses such as branches, and reduced size piping may cause stresses in excess of those suggested by the nomograph.
In general, computer analysis will be required for that listed below. The listing is not intended to be exhaustive and address every eventuality, however and experienced, engineering judgement shall prevail.
2.3.1.1. Lines connected to strain sensitive equipment to evaluate the magnitude of the terminal loads with sufficient accuracy for final review by the specialist Engineers. Engineering judgement shall be used to decide as to which equipment should be treated as strain sensitive.
2.3.1.2. Rotating equipment with nozzles 2” NPS and above with a design temperature of 80 °C and above.
2.3.1.3. Large diameter (24” NPS and above) or heavy wall pipes, and also lines with design conditions for ferrous and alloy piping above 350 °C and for Stainless Steel above 270 °C.
2.3.1.4. Thin walled vessels (e.g. corroded thickness less than 5mm) with a design temperature of 80 °C and above.
2.3.1.5. Heat exchangers with a design temperature of 80 °C and above.
2.3.1.6. Piping systems operating at temperatures minus 40 °C and below
2.3.1.7. All piping with a wall thickness greater than standard weight, considered on it’s individual case.
2.3.1.8. Where movements are large due to extraneous causes, e.g. differential settlement of foundation, etc.
2.3.1.9. Piping which connects together the tube bundles of air-cooled exchangers.
2.3.1.10. Lines subject to large displacements imposed on them by the movements of other lines or equipment to which they are connected, even though they may be below the limits recommended
2.3.1.11.Jacketed pipework.
2.3.2 Computer Analysis - Normally, the computer input should include the following information:
2.3.2.1. Physical characteristics of the piping and fluid contained.
2.3.2.2. Rate of expansion.
2.3.2.3. Terminal movements.
2.3.2.4. Details of restraints, including fixed supports, which will significantly affect vertical movement of the pipe.
2.3.3 Computer analysis for lines, which are likely to impose significant loads on equipment, are to include all relevant effects (e.g. pressure, weight, and thermal expansion). However, Contractor is cautioned that the computer analysis normally treats nozzle connections as rigid anchors so that very small deflections of pipe between supports may cause indicated bending moments at the nozzle which, in practice, will disappear with very small rotations.
2.3.4 The interpretation of computed forces and moments must take account of the movements from which they originate, and Contractor must make a distinction between loadings which are sustained over a large range of movement and can cause gross distortion, and secondary loading which cause only minor strains.
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