Calculating Process Line Size

Please provide more information. To what type of process line are you referring? It makes a substantial difference.

line size has to design at bypass of existing line, which has flow rate 34040 Nm3/hr, at 46 deg C containing gas 75 mole% of CO2. What flow rate should be taken for bypass line? and how to calculate the optimum line size?

What is so important about the bypass line? If it's a bypass; just make everything equal!

You need pressure drop allowable. If you have 150 bar available, it might be a 2" line. if only .15 bar, you'll need a 20" line.

You'll never get an answer with a question like that. You need to provide some specifics.

How much i be the specific more than this?

3m/sec can be considered the upper economic limit for liquid velocity in pipes. Take it from there?

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Ask yourself:

Who? What? When? Where? Why?