We have one newly installed propylene MLA in our product pier. It has both the liquid loading line and vapor return line. After propylene loading, the gas of the vapor line is vented using the vessel's venting system. On the other hand, the liquid fill of liquid line is pushed back to the sphere from shore using the vessel's compressor. The block valve in the pier is closed after the said activity. Line floating is then done again from propylene sphere to the pier block valve before taking final closing gauge of the sphere.
In order to eliminate pushing of the propylene liquid back to sphere, can we just install a tie-in connecting the vapor line to the liquid line? This will be used to push the propylene liquid from the liquid line to the vapor line back to the ship. what do you thick is the minimum size of the tie-in in order to avoid bubbling of the liquid propylene in the stand pipe of vapor line?
I have attached a photo of the setup described. I hope that you can help me.

VRL: 3" dia
Liquid Line: 8" dia
Thanks!