Criteria for vacuum systems is always allowable pressure drop. How much pressure drop can you afford to lose in the overhead line? There's your answer.

The upper limit is fluid velocity. Gas lines in continuous service should not employ velocities above 15-20 m/sec. For wet steam, reduce that to half of the value.

Dejan IVANOVIC
Process Engineer, MSChE

Vacuum tower tops lines usually run at high velocity, but exceeding 20m/sec as suggested on CS lines can cause excessive erosion corrosion in addition to corrosion from H2S, NH3 etc.

Going up to 30m/sec on corrosion resistant alloy ( CRA) piping would be possible, subject to pressure drop permissible for the operation of the ejectors at the condensors. Use isothermal compressible flow expressions for dp calcs on these lines, if dp > 10% of column top exit nozzle pressure in absolute units ( kpa abs or psia abs)

I found the following rule:
"
dp/100 < 0.035*P^1/2 where P is the pressure of top reciver drum
AND
v < min(100/(dens^1/2); 60-80 ft/s)
"
What do you think about?

These are empirical equations, "rules of thumb", that may or may not be applicable to your case.

Look at what vacuum you need inside the column for the most conservative case, start with the design overhead flow and add some 10-20% margin on top of that, and size the overhead line accordingly. That way you can't do wrong.

Dejan IVANOVIC
Process Engineer, MSChE