Boiler drum level control
Boiler drum level is the level loop that cannot be averaging in the usual sense and cannot be tight in the usual sense either. The measurement lies during a load change: swell raises the indicated level when steam demand rises, just as you need more feedwater, and shrink drops it when demand falls. A single-element PID on drum level therefore moves the feedwater valve the wrong way at the moment it matters.
Three-element control exists because of that lie. Steam flow sets the bulk of the feedwater demand; feedwater flow is the slave that delivers it; drum level trims the balance. Tuning the level PID as if it were a tank on the floor is how drums go empty or carry over.
Single-element fails on the swell
On a load increase the drum pressure falls, water in the drum flashes, and the level indication rises. A level-only controller cuts feedwater. The boiler needed more feedwater, not less. The reverse happens on a load drop. The loop is doing what it was told. The measurement was wrong about the inventory.
What three-element actually is
Steam flow, often with a feedforward gain near one, sets the feedwater flow setpoint. The feedwater flow controller is a normal fast flow loop. Drum level, tuned loosely, adds a trim so the inventory does not walk off over an hour. The level loop is averaging in spirit — it should not fight swell — and integral on it should be slow enough not to chase the false measurement.
Two-element (steam flow plus level, no feedwater slave) is a compromise when there is no feedwater flow meter. It still beats single-element. It still needs the level loop kept slow.
What to check before touching constants
Shrink and swell are physics, not a tuning defect. If the drum still cannot hold through a load change after three-element is in, look at the feedwater valve, the steam-flow measurement, and whether the level transmitter range still matches the glass. A passing feedwater valve and a drifted steam-flow orifice both look like a level loop that will not behave.
Questions that come up
Can I tune drum level like a surge tank?
Only in the sense that the level trim should be slow. You cannot let the drum wander across its range the way a feed tank can — the consequences of empty or full are not the same. Three-element is the structure that lets the level loop stay slow without losing inventory.
Related
Last reviewed 2026-09-10.