Why does Caesarr II not use flow input?

Side question: What is the difference between "flexibility analysis program vs hydraulics program"?

(suggested brief answer in "general" terms)

What do you mean by flow input?

To answer your second, rather concerning, question...

A hydraulics program evaluates flow rates, friction losses, heat input/output, pumps, etc. They're used to size pumps, control valves, etc.

A stress analysis/flexibility analysis is performed after the system is laid out (all valve, equipment, etc known). It looks at the supports, piping materials, relevant pipe code, loads (equipment nozzle loads, PSV forces, seismic/wind, etc), and ensures the piping is routed/supported correctly so it should not fail.

A flexibility analysis (according to the B31 piping codes) evaluates the mechanical stresses in the piping system assuming a steady-state condition. In most circumstances, the flow in the pipe does not affect the mechanical stresses. (The exception is dynamics.)

For example, the bending stress is computed as iM/Z (where "i" is a Code defined stress intensification factor, "M" is the applied bending moment, and "Z" is the section modulus of the pipe). Where does "M" come from?

"M" comes from the FEA solution of {f} = [K]{x}, where "f" is the applied load vector, usually comprised of system weight, pressure, temperature, predefined displacements, and occasional loads.

You should review the section on flexibility analysis in B31.1 or B31.3 for more details. This short post cannot do the subject of mechanical stress/flexibility analysis justice.

Hydraulic analysis on the other hand (as stated above) evaluates fluid flows and pressures throughout the system. A hydraulic analysis can be used to generated dynamic input data for CAESAR II. These are two completely different fields of Engineering.

One end-game could be to have a software that handles all forms of engineering, from start to finish.

Process/CFD, Mechanical/FEA, Structural/FEA. Let's call it CAESAR++.

That'd be a very large code base. Very expensive. Presently, engineering is multi-discipline. The folks who are qualified to perform all three analyses are a very small minority.

Thus the typical use would be paying for access to a Process Engineer, a Mechanical Engineer, and a Structural Engineer access to 1/3 of CAESAR++.

I'm not sure that's an easy sell.