What a three-phase separator does

A separator is a pressure vessel whose only job is to hold the well stream still long enough for gravity to do the work. Gas rises, oil floats, water settles. Everything else in the vessel exists to protect that.

The controlling quantity is residence time: how long fluid stays inside, which is simply volume divided by flow. Light crude in a warm vessel may need three to five minutes; heavy, cold crude can need twenty to thirty. Raise the inlet flow and residence time falls, the dispersed droplets never reach their own phase, and separation efficiency drops sharply.

Between oil and water there is rarely a clean line. Fine solids, surfactants and emulsions can build a stable layer between them, known as a rag layer, that does not obey the settling laws and can grow until it blocks the internal weirs.

Separation fails in two directions, and both are level problems. Let the liquid climb too high and the gas leaving the top drags droplets with it: liquid carry-over. Let it fall too low and gas escapes through the liquid outlet instead: gas carry-under. Level control exists to keep the vessel between those two failures.

Every run here begins the way a real test does: vessel empty, well shut in, valves closed. Pressing Start opens the well. While the vessel fills, the controllers hold their valves shut, since there is nothing to discharge yet, and they begin to open only as the level approaches its setpoint (the target value the loop holds). Watch for that moment; it is the loops taking over from the fill.

During the fill the vessel sits below every low-level limit, and that is not an upset: low-side protection stays bypassed until each level has first reached its normal range, exactly as it does in the field. From then on it is armed, and a falling level is a real event.