1.)We have modeled the inter stage coolers (Heat Exchangers) as a pipe element...
Generally, this is acceptable for typical pressure vessels, such as shell and tube heat exchangers. However, this is only a partial picture.
...filled with water. Is this procedure correct. If yes / no can you please explain.
Generally, filling a vessel with water is an acceptable practice.
However, your vessel is on springs. If you specify a vessel weight, spring rate, cold load contrary to reality, then the accuracy of your model will suffer.
Your task is to model actual nozzle displacements.
Also in standard EPC practice heat exchangers are modeled as weightless rigid elements. Is there a specific
reason for the same?
When you specify rigid in CAESAR, you're only saying that it's a piping element that's 10x thicker than your inputs, and increasing any insulation weights assigned to that item. Generally, you can get away with assigning rigid status to any equipment so long as the equipment is in reality much stiffer than the piping you connect to it. Regardless how you model the body of the vessel, I recommend modeling the nozzle as pipe.
2.)Our client has commented us to model the Heat Exchanger as a rigid element with weights assigned to those rigid elements. Is this procedure correct. Please provide explanation.
See earlier explanation. To add to that, you generally should model to the vessel centerline. Offset nozzles need a pathway modeled to that centerline, and you generally use rigids to get to the centerline.
3.)The Heat Exchangers are supported using spring hanger support instead of using saddles. Also we do not have any input data for spring hangers. For such situations how can we select a correct spring and provide inputs to get correct spring designed for the system.
Offer to size new spring hangers.
Alternatively, offer to size how the hangers "should" have been originally sized, and use those values, instead.
Either way, you'll need end-user approval on how to proceed.
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