Whether to use rigid or pipe elements depends on the physical nature of the component that you are modelling. In the Pipe stress programme you are making a mathematical model of a physical system which to give you realistic results , should simulate the actual physical properties of the body.
Coming to modelling of exchangers and Columns,they can be simulated as Pipe bodies as modelling them with rigid elements will make them more stiff than they actually are.
Say a column: It is just like a pipe .However when modelling the column from its centerline to the wall, it is more prudent to model it as rigid element.The rest should be modelled as pipe.
Same logic is valid for modelling of exchangers.
However for a pump, the entire model ( from suction to pump centre and from pump centre to discharge ) should be modelled with rigid elemnts as only then they will have stiffness more than the associated pipe, which is actually the case.
A.BHATTACHARYA
STRESS ANALYST
BECHTEL CORPORATION
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Anindya Bhattacharya