Most of engineer told me that the pressure of cooling water return pressure should not be too low (generally higher than 1BarG).I believe it is correct, but I don't know why, can anyone tell me.
In a broad sense, I can not think of any reason to purposely increase cooling water return pressure.
The impact would be to add restrictions and limit circulation flow.
I once work at a unit that experienced a tube rupture.
The cooling tower basis filled with Styrene.
Someone in the process business may have a practice to elevated return header pressures to direct fluid flow in event of tube failure.
But I have never heard of this practice.
If you are coming back to an evaporative cooling tower, you need enough head to get up the risers and to the flow distribution valves with enough back-pressure so that you can properly balance the flow across the top of the tower.
Besides getting back to the tower, you also want enough pressure so at the highest/hottest CW return point it doesn't boil, go 2 phase, and when it comes to a lower/colder point it water hammers. CW return risers have been known to run at a vacuum!