The third loop: pressure, and how loops interact

Switch the gas cap on and a third controller appears: pressure, acting on the gas outlet valve. Gas released by the feed collects above the liquid and raises pressure; opening the valve vents it away.

The first thing to know is the timescale. Pressure responds in seconds where level takes minutes. Gains that feel gentle on a level loop are violent here, which is why this loop starts with a smaller Kp and no derivative at all.

The second thing is that the three loops are not independent. The gas cap is whatever volume the liquid leaves behind, so draining liquid enlarges the gas space and drops the pressure, and a pressure excursion pushes back on how easily the liquid outlets can discharge. Three separate single-loop controllers are each solving part of one coupled problem.

There is a process reason to keep pressure steady beyond the equipment rating. A sharp pressure drop makes gas break out of the liquid inside the vessel, called flashing, which stirs the liquid and corrupts the volumetric measurements the whole well test depends on.