Mysterious Air Bubbles

Q. I am a manager of finishing operations and have a problem with air bubbles that seem to be coming from an air-piloted fluid regulator. The bubbles start in the air lines after the fluid regulator to the gun. There are no air bubbles at all from the pressure pot to the fluid regulator, but once the fluid leaves the regulator the bubbles start to show up when running at low fluid pressures, below 10 lbs. As we increase the pressure by 1 or 2 lbs at a time the bubbles decrease until they completely disappear at around 12 lbs of pressure (which gives us way too much paint for the product that is being sprayed).

We are using a fluid regulator after a 10-gal pressure pot with the regulator to the pot set at 30 lbs. The fluid lines are 3/8 inch from the pot to the regulator and then ¼ inch to the guns. We also are using an HVLP robotic spray gun with a 0.5-mm tip. The viscosity is roughly as thin as 30 to 34 using a Zahn 1 cup. If we go directly from the regulated pressure pot itself to the guns using a pressure as low as 2 lbs, the air bubbles are not present at all. However, once we introduce the air piloted fluid regulator to the mix, they come back. We need air piloted fluid regulators because our line is automated and has different fluid pressures depending on the product that we are running. Do you have any ideas for solving this issue? D.D.

A. I believe the solvent in the paint is boiling. The solvent is at a higher pressure entering the pressure regulator inlet line and is trying to vaporize when it reaches a lower pressure leaving the outlet of the regulator. Simply stated, going from the higher pressure to a lower pressure causes the paint solvent to boil. If this is the case, you may have to change to a higher-boiling-point solvent blend. I suggest you contact your paint supplier and equipment supplier for their help.