Opensource
Mechanical
- Aug 17, 2008
- 14
Hi
My contractor is facing some issues with the stress calculations due to high seismic acceleration values since they are not able to keep bellow the allowable code limit i.e. 1.33 times the basic allowable stress ( ASME B31.3).
ASME B31.3 in parag. 302.3.6 refers to a Design Earthquake and indicates only occasional load conditions that can be identified with the Operating condition or design basis earthquake condition (OBE) so it does not considers exceptional loads such as the Safe shutdown earthquake condition (SSE) . So according ASME B31.3 only an operating basis earthquake condition (OBE) maintaining operational conditions, can be considered for design.
According ASME B31.3 in parag. 302.3.6 For Elevated Temperature Fluid Service (see definition in para. 300.2) of materials having ductile behavior, as an alternative to the use of 1.33 times the basic allowable stress provided in Table A-1 or Table A-1M, the allowable stress for occasional loads of short duration, e.g., surge, extreme wind, or earthquake, may be taken as the lowest of the following:
(-a) the weld strength reduction factor times 90% of the yield strength at the metal temperature for the occasional condition being considered
(-b) four times the basic allowable stress provided in Appendix A
(-c) for occasional loads that exceed 10 h over the life of the piping system, the stress resulting in a 20% creep usage factor in accordance with Appendix V
ASME B31.3 parag 301.5.3 Earthquake. Indicates “ The effect of earthquake loading shall be taken into account in the design of piping. The analysis considerations and loads may be as described in ASCE 7. Authoritative local seismological data may also be used to define 3or refine the design earthquake loads”. But even with ASCE-07 the idea is avoiding yielding phenomena in the pipes and to maintain operational conditions also in case of strong seismic events so in conclusion yielding is not allowed in ASME B31.3
If we go for the Safe Shutdown Earthquake (SSE) with EN13480-3 an deformation capability of the pipes and fittings (e.g., ovalization) in the plastic range would be necessary. I went through EN13480-3 and this codes accept in pipes a certain level of yielding on the condition that leakage would not occur, and the consequence, for example, to accept a reduction of the thickness will occur. With EN13480-3 allows an exceeding of the basic allowable stress by 80% that is within plastic strain
To my opinion going to EN13480-3 using the 1.8 times the basic allowable stress for me is not accepted because that will reduce the lifetime of the piping. It is possible that a small part of the piping system will undergo considerable inelastic when the rest of the system is almost entirely elastic. This happens when the part concerned is appreciably weaker than the rest, either due to reduced section size, weaker material, or higher temperature. Even if it does not leak immediately after an exceptional load such as for Safe shutdown earthquake condition (SSE) it might happens during the lifetime of the plant which will bring safety issues.
To me this is not an simple answer because ASME B31.3 to my opinion might be too much conservative and does not refers what to be considered in case of exceptional loads. Using also expansion joins and snubbers which also if not well installed and well calculated could also bring to a possible leak and I am comfortable to have this in so many places.
I would like to ask that for lines where the stress cannot always be kept below the allowable code limit and for Elevated Temperature Fluid Service with the weld strength reduction factor times 90% of the yield strength still cannot pass if we can go to the yielding point on condition that the system is later returned to its original based on ASME B31.3 du to parag 319.2.3 ( my understanding is that is allowed on the starting operation the plant) and what you think in design a ammonia plant with ASME B31.3 and using for some lines for stress calculation the code EN13480-3 and if you have faced this what was the solution.
My contractor is facing some issues with the stress calculations due to high seismic acceleration values since they are not able to keep bellow the allowable code limit i.e. 1.33 times the basic allowable stress ( ASME B31.3).
ASME B31.3 in parag. 302.3.6 refers to a Design Earthquake and indicates only occasional load conditions that can be identified with the Operating condition or design basis earthquake condition (OBE) so it does not considers exceptional loads such as the Safe shutdown earthquake condition (SSE) . So according ASME B31.3 only an operating basis earthquake condition (OBE) maintaining operational conditions, can be considered for design.
According ASME B31.3 in parag. 302.3.6 For Elevated Temperature Fluid Service (see definition in para. 300.2) of materials having ductile behavior, as an alternative to the use of 1.33 times the basic allowable stress provided in Table A-1 or Table A-1M, the allowable stress for occasional loads of short duration, e.g., surge, extreme wind, or earthquake, may be taken as the lowest of the following:
(-a) the weld strength reduction factor times 90% of the yield strength at the metal temperature for the occasional condition being considered
(-b) four times the basic allowable stress provided in Appendix A
(-c) for occasional loads that exceed 10 h over the life of the piping system, the stress resulting in a 20% creep usage factor in accordance with Appendix V
ASME B31.3 parag 301.5.3 Earthquake. Indicates “ The effect of earthquake loading shall be taken into account in the design of piping. The analysis considerations and loads may be as described in ASCE 7. Authoritative local seismological data may also be used to define 3or refine the design earthquake loads”. But even with ASCE-07 the idea is avoiding yielding phenomena in the pipes and to maintain operational conditions also in case of strong seismic events so in conclusion yielding is not allowed in ASME B31.3
If we go for the Safe Shutdown Earthquake (SSE) with EN13480-3 an deformation capability of the pipes and fittings (e.g., ovalization) in the plastic range would be necessary. I went through EN13480-3 and this codes accept in pipes a certain level of yielding on the condition that leakage would not occur, and the consequence, for example, to accept a reduction of the thickness will occur. With EN13480-3 allows an exceeding of the basic allowable stress by 80% that is within plastic strain
To my opinion going to EN13480-3 using the 1.8 times the basic allowable stress for me is not accepted because that will reduce the lifetime of the piping. It is possible that a small part of the piping system will undergo considerable inelastic when the rest of the system is almost entirely elastic. This happens when the part concerned is appreciably weaker than the rest, either due to reduced section size, weaker material, or higher temperature. Even if it does not leak immediately after an exceptional load such as for Safe shutdown earthquake condition (SSE) it might happens during the lifetime of the plant which will bring safety issues.
To me this is not an simple answer because ASME B31.3 to my opinion might be too much conservative and does not refers what to be considered in case of exceptional loads. Using also expansion joins and snubbers which also if not well installed and well calculated could also bring to a possible leak and I am comfortable to have this in so many places.
I would like to ask that for lines where the stress cannot always be kept below the allowable code limit and for Elevated Temperature Fluid Service with the weld strength reduction factor times 90% of the yield strength still cannot pass if we can go to the yielding point on condition that the system is later returned to its original based on ASME B31.3 du to parag 319.2.3 ( my understanding is that is allowed on the starting operation the plant) and what you think in design a ammonia plant with ASME B31.3 and using for some lines for stress calculation the code EN13480-3 and if you have faced this what was the solution.
